Download Evolution of metallic deposits in time and space in Mexico

Evolution of metallic deposits in time and space in Mexico
KENNETH F. CLARK1 AND DAVID C. FITCH2
1
UNIVERSITY OF TEXAS AT EL PASO, EL PASO, TEXAS, USA
2
GEOLOGICAL CONSULTANT, RENO NEVADA, USA
Abstract
Politically, the development of the Mexican Minerals industry has been divided into six intervals: (1) Pre-Colonial, (Prior to 1521),
(2) Colonial (1521-1821), (3) Post Colonial (1821-1911), (4) Post-Revolution (1917-1961), (5) Mexicanization (1961-1992), and
Post-Mexicanization (1992- onwards). Production of gold and silver, traced from 1500 to the present day, reflects these six intervals
with important variations following Independence in 1821, and the Mexican Revolution (1910-1917) which overlapped World War I.
Another diminishment in production of these two metals occurred during the Mexicanization of the minerals industry during 19611992. From 1521 to 2010 it is estimated that 74.3 million oz of gold and 10.6 billion oz silver have been produced.
Geologically, and using available age determinations made in approximately the last half century, plus compiling stratigraphic
evidence where isotopic determinations are unavailable, six metallizing epochs are recognized: (1) Late Proterozoic (1.1-1.0 Ga), (2)
Early Paleozoic (470-397 Ma), Permo-Triassic (265-228 Ma), (4) Jurassic-Early Cretaceous (190-130 Ma), (5) Late Cretaceousearly Miocene (100-16 Ma), and (6) late Miocene-Present Day (<16 Ma). The areal distribution of these deposits in places is
constrained by individual tectonostratigraphic terranes but more commonly occurs throughout regions covering several terranes that
have been subject to tectonic events along an active margin of western Mexico.
The Late Proterozoic epoch is characterized by Ti, and U, Th and REE deposits in the Oaxaca Complex.
The Early Paleozoic epoch contains few known economic deposits, although pods of Cr and Ni occur in metamorphosed host
rocks, in Southern Mexico.
Permo-Triassic time, that embraces the latter part of compressive tectonism accompanying the formation of Pangea, is similarly
poor in metallic deposits, however polymetallic Pb-Zn-Cu- (Ag-Au) deposits are associated with the Chiapas batholithic complex and
the oldest of the massive sulfides occurs in weakly metamorphosed strata and intercalated basic volcanic rocks of the Sierra Madre
terrane.
The Jurassic-Early Cretaceous epoch contains numerous massive sulfide deposits, contained primarily in the Guerrero terrane of
western mainland Mexico. This terrane has been divided into the Teloloapan (Zn-Pb-Cu) and Zihuatanejo (Zn-Pb-Cu) and Northern
Guerrero (Zn-Cu) subterranes. The oldest porphyry copper deposits occur in this time span, one of which is located in the Maya
terrane of Southern Mexico. Additionally, two red-bed copper deposits are located in Early Cretaceous strata of the Chihuahua
terrane and a world-class manganese deposit is situated in oxidized upper Jurassic strata of the Sierra Madre terrane.
There is a large increase of metallic mineral deposits in the Late Cretaceous-early Miocene epoch, primarily located in the western
margin and also in Central Mexico north of the Trans-Mexican Volcanic Arc. Much of Mexico’s mineral wealth comes from these
ores and can be attributed to hypogene concentration of metals associated with a subduction regime which resulted in orogeny,
ground preparation, intrusive and extrusive magmatism. A porphyry Cu-Mo-(W-Au) belt lines the western margin of mainland
Mexico, covers several terranes, and there is limited spatial overlap with Fe and other metals in Southern Mexico. The largest
deposits occur in the Chihuahua terrane, underlain by cratonic crust. Polymetallic (Pb-Zn-Ag) skarn, replacement, manto and
chimney deposits, are found in several terranes but further inland than the porphyry Cu deposits, and located in the north-central
plateaus. Other Fe and U deposits are found in or near the southern edge of the Chihuahua and Caborca terranes with the
exception of the Cerro de Mercado Fe ores that are located in a caldera environment overlying the Parral terrane. Sandstone-U
mineralization forms a cluster of deposits in the Coahuila terrane adjacent to Texas. There is considerable geographic overlap
between Sn, Hg, and Sb deposits in central Mexico north of the Trans Mexican Volcanic Arc, and Hg and Sb concentrations are
found in additional terranes south of the volcanic arc. Each element occurs in characteristic host rocks. Manganese is widespread
throughout many terranes although few deposits have a specific age assignment. The bulk of W ores are restricted to granitoid
terranes of Northwestern Mexico. The much sought after fissure-vein, epithermal, precious-metal deposits are located primarily in
the occidental volcanic arcs, whereas mesothermal Au-(Ag) deposits occur along the trend of the Mojave-Sonora Megashear that
separates the Chihuahua and Caborca terranes in Northern Sonora, and these have been related to the Laramide Orogeny. A small
number of disseminated Au-(Ag) deposits are found in this region, distributed in sedimentary, volcanic, intrusive, or metamaphosed
host rocks and also in structurally controlled deposits as in southernmost Baja California in the La Paz terrane. The remaining
deposits of this epoch include REE and Th concentrations in carbonatites in the Chihuahua terrane and in an alkaline complex in
southeastern Coahuila terrane. A small number of Mississippi Valley-type deposits also occur in this region.
The youngest metallizing epoch displays porphyry copper and contact metamorphic Fe deposits in southernmost Mexico,
bordering subduction along the Middle America Trench, and stratabound Cu and Mn ores are present in the Vizcaino terrane of the
recently formed Baja California peninsula. The youngest Au-(Au) deposits are located in the widely separated Maya and Cortez
terranes, the latter bearing hot-spring deposits and the Cerro Prieto geothermal field in northernmost Baja California and within the
environment of the San Andreas fault system. Other geothermal fields are located in tectonically active Pliocene and younger
volcanic fields mostly in the Trans-Mexican Volcanic Arc. Placer Au and Sn accumulations in Western Mexico, derived from
hypogene mineralization of earlier epochs are widespread. The youngest and still-ongoing metallization events are associated with
black and white smokers in the East Pacific Rise and Guaymas Basin and near-shore, shallow-water hydrothermal vents in western
Mexico.
Some terranes are characterized by specific metals, whereas other provinces display metal assemblages that straddle several
terranes, especially where subduction and related tectonism have occurred in western Mexico since at least Jurassic time. This
tectonism appears to be the triggering mechanism that spawned tectono-magmatic-hydrothermal processes that produced the bulk
and increasingly diverse metallization as 1.0 Ga of geologic time elapsed.
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deposits, a few of which are mentioned here, had been found,
exploited, studied and documented at least in rudimentary fashion.
According to Bernstein (1964), the quantity of gold and silver
extracted by the Spaniards in Mexico will never be known with
certainty. But during the first two decades of the 18th century,
Mexico’s annual output of gold averaged 524 kg and of silver
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163,800 kg; and in the first decade of the 19 century output
reached 1,763 kg of gold and 553,800 kg silver. However,
following the war of Independence 1810-21, mining and related
activities fell into disrepair as the Spaniards were either expelled
or discriminated from owning land, thus making capital scarce.
In 1823 when legal prohibitions against foreigners owning
mines were reduced, it allowed infusion of British, German, and
United States investments and the post-colonial period saw
continuation of discoveries, several of which were in the more
inaccessible mountain ranges, for example, at Moris (1826) and
the rich Nuestra Señora de Rosario mine in the Guadalupe y
Calvo district (1835), both in Chihuahua. The latter is said to
have values ranging up to 7,910 g/t gold. Other important
districts were identified by Saint Clair DuPort (1843) in his book
entitled La Production de Metaux Précieux au Méxique.
However, war with the United States, terminating with the treaty
of Guadalupe Hidalgo in 1848, the instability of “The Reforma”
and the period of French intervention and Maximillians’s empire,
throttled new investment. Eventually, Benito Juarez came to
power in 1867 and the roots of the Mexican Steel industry were
promoted in Las Truchas, Michoacán, and at Peña Colorada,
Colima. Subsequently, the well known weekly paper El Minero
Mexicano that documented mining activities was published from
1873-1903.
The Diaz era spanned 1876-1911, except for a gap of four
years and produced tremendous growth in European and United
States investments in Mexico, a trend that had been started by
Juarez (Bernstein, 1964). The latter part of the 19th century saw
an increasing participation of foreign companies exploiting
Mexican metallic wealth: the Boleo Company of France operated
the copper mines at, Santa Rosalia, Baja California Sur in 1885;
and the Peñoles Company began operations at Ojuela, Durango
as a subsidiary of the American Metals Co. Ltd. of New York in
1887; the Mazapil Copper Company was organized in 1896 in
England to work the copper mines in Concepcion del Oro in
northern Zacatecas; La Palmarejo and Mexican Gold Fields of
London formed in 1898 to exploit the mines in Chinipas,
Chihuahua; and the Dos Estrellas mine of French capitalization
started operations in the gold and silver lodes at Tlalpujahua in
Michoacán in 1898. At the turn of the century, the Mexico Mines
of El Oro in 1904 in the state of Mexico and the San Francisco
Mines of Mexico which acquired the gold-silver-lead-zinc
properties at San Francisco del Oro, Chihuahua; were both
British ventures. The American presence was felt by the
formation of the Green Gold and Silver Company for exploration
of precious metals and copper deposits in Chihuahua and
Sonora, leading to discovery of the huge copper and
molybdenum deposits at Cananea, Sonora. At Real del Monte
and Pachuca, Hidalgo, following earlier British ownership, the
United States Smelting, Refining and Mining Company initiated
operations in 1906 (Ordoñez, 1986; Fig. 1).
By 1908 there were 1,030 mining companies registered in
Mexico during the latter part of the promotional period of
President Profirio Diaz and some of them were documented in
the book Minas de Mexico by J.R. Southworth (1905), the text
being written in Spanish and also English. Thus by the start of
INTRODUCTION
Historical
Discovery, development, and production of Mexico’s mineral
wealth, particularly the metallic resources can be conveniently
divided into six time intervals each with singular characteristics,
(1) Pre-Colonial (pre-1521), (2) Colonial (1521-1821), (3) Post
colonial (1821-1911) including the Porfirio Diaz era of foreign
investment (1876-1911), (4) Post Revolution with further foreign
participation (1917-1961), (5) Mexicanization of the minerals
industry and cessation of foreign investment (1961-1992), and
(6) Post Mexicanization with renewed foreign investment,
particularly by Canadian companies (1992 onwards).
Mexico’s documented metallic mineral production stems from
the 800’s with the recovery of gold during the classical period at
Monte Alban, Oaxaca, possibly from placer deposits at
Tepeucila (Ordoñez, 1986). Thereafter other metals were
exploited, particularly gold and silver during the colonial period,
and discoveries were made with increasing frequency
throughout what was to become the Republic of Mexico (see
Ordoñez, 2009). The start of the colonial period began with the
smelting of iron ore in the state of Veracruz (Fig. 1) to provide
arms for the newly arrived Spanish conquerors. But the
predominance of silver production was furthered in 1521 with
discoveries at Taxco, Guerrero, a district that is still in
production. Other important discoveries quickly followed at
Sultepec, Mexico state; Pachuca, Hidalgo; Tlalpujahua,
Michoacán; and at Zacatecas, Zacatecas. Further to the north,
and accompanying the Spanish expansion, significant
discoveries of silver-lead ore were made at Santa Barbara, in
1544, and at nearby Parral in 1600, both in Chihauahua.
Elsewhere, the famous Veta Madre, Guanajuato district yielded
the first of its prodigious wealth in 1550. Large iron deposits
were identified at Cerro de Mercado, Durango, in 1551 and the
patio process for the treatment of precious metal ores by
amalgamation with mercury was devised by Bartholome de
Medina in 1554 at Pachuca.
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During the 17 century important discoveries of native silver
were made at Batopilas (1630), and silver and gold at
Cusihuiriachic, again both in Chihuahua; La Plomosa, Sinaloa
(1722); and a bonanza at Taxco, Guerrero (1752) which is said
to have produced 168 kg/t silver. Later, important mining
commenced at Tayolitita, Durango in 1757 and at Catorce, San
Luis Potosi in 1773.
The first of notable scientific enquiries took place in 1570 when
Francisco Hernandez published La Nova Plantarium, Animalium,
et Mineralium Mexicanarum in Rome, Italy, followed by Juan de
Torquemada who produced Veintium Libros Vibrales y
Monarchia Indiana in 1723, a compilation of mines, volcanoes,
earthquakes, geothermal waters, and minerals (Ordoñez, 1986).
Subsequently Baron Von Humbolt in 1804 described 37 mineral
districts in New Spain. Later, in 1823, von Humboldt presented
in Paris, France, one of the first descriptions of rocks in Mexico
in his Essai Georostique Sur le Gisement des Roches dans les
deux Hemispheres. Also, his observations of the veins at
Guanajuato were of sufficient detail and clarity to place them at
the forefront of geologic investigations at that time. The
educational field was enhanced by the foundation of the School
of Mines and Metallurgy in Guanajuato in 1786, and this was
followed by a tax in 1792 on mines in the republic to pay for
construction of a building to house the School of Mines in
Mexico City (Ordonez, 1986). Thus, by the time Mexico was
gaining independence from Spain in 1821, a wealth of metallic
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3
Figure 1. Index to states and deposits in Mexico cited in the historical review (from Clark and Fitch, 2009).
the Mexican revolution in 1910 there were 31,988 mining
concessions covering 450,371 hectares, and operated by
67,987 active mines.
However, in 1914 as the revolution continued, all acts and
decrees of the federal government, including those related to
mining, were annulled and this severely damaged the mining
industry. After the conclusion of the revolution in 1917, Article
27 in the new constitution exercised Mexico’s inalienable rights
to the subsurface, and led to the concept of federal concessions
to obtain mining rights (Bernstein, 1964). Nevertheless,
geological and mining investigations had sporadically continued
and were brought to fruition shortly thereafter in formal
publications, as for example, at El Oro-Tlalpujahua (Flores,
1920; see also Rivera- Ledesma (2009).
The post-revolution period in 1926 was accompanied by a limit
of 30 years life for a mining concession with a minimum
production schedule, which, if not achieved, led to closure of
mines undergoing development. Increase of metal prices at this
time led to a 9.5% contribution in 1930 to the gross domestic
product, (GNP), as the world prepared for WW II. By 1934 the
Comisión de Fometo Minero was established as a Federal
Agency to promote mining. The first large open pit was
developed at Cananea in 1944 and the Instituto Nacional para la
Investigación de Recursos Minerales (later Consejo de
Recursos Minerales and presently Servicio Geologico Mexicano)
was formed in 1949 to explore for mineral deposits.
In 1961 the Mexicanization of the industry, whereby 51 percent
of shares had to be in Mexican hands, private or government,
produced stagnation in investment and consequently
exploration. Mexicanization was applied to the Peñoles, Frisco,
and ASARCO companies, among others. Nevertheless a few
major discoveries were made at this time, including Las Torres
silver property, in the Guanajuato district in 1968; and La
Caridad copper deposit, Sonora, 1975. In 1972 the Hercules
iron mine in Coahuila began production and the open pit silverlead-zinc-(cadmium) mine at Real de Angeles, Zacatecas a
decade later. However by 1970 the Mexican mining industry
generated only 1.5% GNP and 1.3% in 1980, (Ordoñez 1986;
Ordoñez, 2009).
In 1990, new regulations were passed to the existing mining
law. The new regulations provided for greater than 49% foreign
ownership of mining projects. This change sent a clear
message to foreign investors that they would be welcome. In
1992 a new Mining Law was passed as a Constitutional
Amendment to Article 27 (Miranda and Lacy, 1993). The new
Mining Law provided for staking a 6-year exploration concession
that could be converted to a 50-year exploitation concession
renewable for a second term. There was no size limit for a
concession.
The law required annual rentals and work
commitments for a concession. In 1990 New Regulations were
made to conform to the New Mining Law. Then, in 1993, a New
Foreign Investment Law was passed that allowed 100% foreign
ownership of mining projects and immediately exploration
increased together with subsequent development and
production.
Thus, the Mexicanization policy was scrapped and the law
was again amended in 1996 (Torres, 2002). Many of the older
districts that had been idle since the revolution, and, or through
lack of investment and modern technology during the 1961-1992
period, for fear of total nationalization akin to Mexico’s petroleum
industry, came to life again. Gold and silver targets were
predominant, and a few examples illustrate this point: La
Colorada, Sonora, 1993; Ocampo, Chihuahua, (2003) and La
Herradura, Sonora (1998) to name but a few. Simultaneously,
exploitation of micron-size gold deposits by open-pit mining and
heap leach recovery led to the development of new properties in
the Sonora gold belt (Clark, 1998), La Herradura mine being the
largest when discovered, began production in 1998. In the
same belt, the separation of gold particles by a modern air
separation method (Piggot, 2000) at El Boludo a year earlier
revived interest in placer deposits. New discoveries of massive
sulfides were made in Zacatecas, of which the San Francisco I.
Madero deposit, possibly of sedimentary exhalative origin, is the
best known with production starting in 2001 (see GonzalezVilllalvaso and Lopez-Soto, 2009). One of the latest discoveries
brought about by pure geological exploration in virgin territory is
the El Sauzal, micron-size gold deposit in Chihuahua, production
beginning in 2004. Some of the more notable recent discoveries
being brought into production include Los Filos, Guerrero, 2007
(see Rivera Abundis et al., 2009), Peñasquito, Zacatecas (see
Turner, 2009), and Dolores, Chihuahua (see Melendez-Castro,
2009). In short, with significant increases in several metal
prices, exploration, development, and production are enjoying a
new wave of activity in Mexico at least until the global financial
crisis that began in late 2008.
Production History
An estimate of Mexico’s total mine production of gold, silver,
copper, lead and zinc from 1521 through 2010 is shown in Table
1a. The production of gold and silver by year is shown in Figures
2a and 2b. Figure 2a covers the total interval of 1521-2010.
Figure 2b expands Figure. 2a to show the interval 1880-2010.
Sources of the data are Gonzalez-Reyna (1956a), Martino et al.
(1992) and U.S. Geological Survey Minerals Information
Surveys for the period 1955 through 2008. Data for silver
production is reported back to 1521 by Gonzalez-Reyna
(1956a), however data for gold production is fragmented for the
period 1521 to 1880 with totals reported for the period 15211890. Production data for copper and lead are shown as totals
for 1521-1890 and then annually afterward (GonzalezReyna,1956a). Production data for zinc is shown for 1893 and
annually afterward (Gonzalez-Reyna,1956a).
Mexico’s total production of 74.3 million ounces of gold and
10.6 billion ounces of silver places it as the highest of Latin
American countries for both gold and silver production.
An examination of Figures 2a and 2b, including the notation of
events, leads one to the conclusion that political events had a
major effect on production increases and decreases of gold and
silver production throughout the 490-year history represented by
the data. Two periods of tremendous increase in both gold and
silver production are evident. The first period began in 1892 and
the second in 1992 (for silver 1876, 1992), both created by
favorable mining laws. The first period began with the great
railroad boom 1880-1884, and the new Codigo Minero in 1884
with provisions favorable for foreign investors.
This was
followed by the Tax Law of 1887, the 1892 Mining Law, and the
introduction of the cyanidization process in 1892 for the recovery
of gold and silver. The second period of dramatic growth began
with the New Mining Law of 1992, described previously.
Present production
The modem Mexican mining industry produces significant
quantities of 14 metallic commodities. Ten of these are listed
below in tabular form for 2006 through 2010, the latest years for
4
which reliable data are available (Table 1b). Another element,
namely aluminum, is not shown as it is derived from imported
ores. Also, part of the tin production is due to imports. In a later
section, the distribution of deposits will be discussed that
contribute to the individual element totals shown therein.
Thus, Mexico ranked among the top world produces in several
metals (Perez, 2011): it was the world's second leading
producer of silver. In addition, Mexico ranked high in other metal
production including cadmium, lead, zinc, molybdenum,
manganese, copper, and gold. In 2009 the total value of
Mexico's mineral production excluding petroleum and natural
gas was valued at $10.21 billion US of which metals contributed
93% of the total. Considering the total mineral industry
production in Mexico for 2009, gold amounted to a 22.9% share
of the total value followed by silver 18.3%; copper 16.6%, zinc
9.0%; iron 5.1%; molybdenum 3.5%; and lead 2.5% (Perez,
2011).
Mining investment in 2009 was $2.9 B US which was a 22%
decrease compared to 2008 (Perez, 2011). In 2009, a total of
279 mining companies with direct foreign investment were
working on 718 projects (Instituto Nacional de Estadistica y
Geografia, 2010). Mineral commodity prices fell during 20082009 and the mining part of Mexican GDP in 2009 was 1.6%.
More details of Mexico’s mining industry are given in LeeMoreno (2009).
Previous Investigations
Despite the long history of metals production in Mexico plus
the impressive present day production and corresponding
monetary value, relatively few studies have been made of the
distribution of mineral deposits throughout the republic. This in
spite of a sizeable geologic literature in Spanish, English, French
and German, listed in decreasing order of importance. The first
attempt to describe the mineral riches of Mexico, apart from
those mentioned above, was made at length by Ramirez
(1884). Investigations by the Instituto de Geologia at the turn of
the century included the first time-space considerations of
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mineral deposits (Aguilera, 1896, 1901). The 20 century
produced additional research on the distribution of a variety of
individual metallic elements in a series entitled Cartas de la
Republica Mexicana de yacimientos minerals metalicos, a total
Table 1a. Mine production of gold, silver, copper, lead and zinc in Mexico 1521-2010 (after Clark and Fitch, 2009).
Gold (M oz)
Silver (M oz)
Copper (mt)
Lead (mt)
1521-1890
8.8
2,840
80,000
300,000
Zinc (mt)
1891-2010
65.5
7,780
13,567,700
21,245,100
22,546,486
Total
74.3
10,620
13,647,700
21,545,100
22,486,500
-
M – million, mt- metric ton
Table 1b. Important metals production in Mexico 2005-2010 (from Pérez, 2009).
2006
Precious Metals (Kg)
Gold
38,961
Silver
3,028,395
2007
2008
43,710
3,135,430
50,365
3,236,312
2009
2010 est.
51,393
3,553,841
60,000
3,500,000
Industrial Metals (mt)
1
788
414
380
74
2
1,399
1,605
1,550
1,510
1,300
327,536
335,502
268,619
227,748
230,000
10,983,000
10,916,000
11,688,000
11,677,000
12,000,000
135,025
137,133
141,173
143,838
185,000
346,000
423,000
471,964
329,400
-
2,519
6,491
7,811
10,166
8,000
468,924
452,012
453,588
489,766
550,000
Antimony
Cadmium
2
Copper
3
Iron
Lead2
3
Manganese ore
2
Molybdenum
2
Zinc
-
Source 2006-2009: Perez (2011 Minerals Yearbook; 2010: U.S.G.S., 2011, Mineral commodity summaries.
1
Sb content of antimonial lead; 2 Mine output metal content; 3 Mine output, gross weight; est -estimate.
5
Figure 2a. Gold and silver production in Mexico 1521-2010 (from Clark and Fitch, 2009).
Figure 2b. Gold and silver production in Mexico 1880-2010 (from Clark and Fitch, 2009).
6
of 13, which appear in the catalog of publications of the Instituto
de Geologia, Universidad Nacional Autonoma de Mexico
(UNAM), p. 36. They were compiled by Gonzlez-Reyna (1967).
In addition to these maps, tin was also investigated by Foshag
and Fries (1942) and again by Pan (1974). Other individual
element studies were made by Gonzalez-Reyna as follows: for
lead and zinc (1948, 1950); silver (1956), and all mineral
deposits (1947, 1956a). Iron deposits were described by Flores
(1951), Pesquera et al. (1979), and Cabrera et al. (1983).
Manganese was discussed by Trask and Rodriguez-Cabo
(1948) and again in a symposium of the International Geological
Congress held in Mexico by Park (1956). Other papers on
manganese in the same year were authored by Mapes (1956)
and Gonzalez-Reyna (1956a). An important compilation of
epithermal gold and silver deposits in northwestern Mexico was
published by Wisser (1966).
Publications of a more regional nature that considered multiple
metallic elements include Behre (1957), Burnham (1959), Wilson
(1968), Gabelman and Krusiewski (1968), Salas (1970), and de
Cserna (1976).
The latest geologic maps of Mexico include Ortega-Gutierrez
et al. (1992), and Servicio Geológico Mexicano (2007), both at
1:2,000,000 scale.
Salas (1975a) published a 1:3,500.000 scale metallogenetic
map of Mexico using the international code of symbols and
accompanied by a report Salas (1975b). Later he compiled
Geologia Economica de Mexico (Salas, 1988), and this was
translated into English (Salas, 1991). Mexican deposits were
also included in the prodigious compilation of the northeast
quadrant of the Circum-Pacific Region by Guild et al. (1985).
The first introduction of tectonostratigraphic terrane analysis
as it affects the distribution of gold-silver and lead-zinc deposits
in Mexico was made by Campa and Coney (1982, 1983) and
later modified by Sedlook et al. (1993). The distribution of tinbearing rhyolites that straddle several terrane boundaries of
north-central Mexico was depicted by Ruiz (1988).
In comparison, the literature of the precise dating of
metaliferous deposits by isotopic methods on a national scale is
more sparse than the geographic distributions cited above and
in this paper. This is particularly apparent for deposits that are
older or younger than the prolific Late Cretaceous-Early Tertiary
mineralizing epoch. There is no modern study, to date, that has
attempted to deal with the age of all important metallic deposits
in Mexico, although numerous individual deposit descriptions
increasingly provide radiometric data. Nevertheless, a few
studies are worthy of mention here.
The first attempt was made by Sillitoe (1976) with his
description of Mexican porphyry deposits, and this theme was
followed up by Damon et al. (1981, 1983), Barton et al. (1995),
and Valencia-Moreno et al. (2000, 2006). A publication that
dealt with various elements in different classes of deposits for
northern Mexico was compiled by Clark et al. (1979a, 1982). A
volume containing many important Mexican deposits, mostly
metallic, was edited and published by Clark and Salas (1988).
Megaw et al. (1988, 1997) firmly established the age of
carbonate-hosted Pb-Zn-(Ag-Cu-Au) deposits in northern
Mexico. Controls on the formation of low-sulfidation epithermal
deposits using fluid inclusion and stable isotope data were
discussed in detail by Albinson et al. (2001), and this was
followed by Camprubi et al. (2003) who described the ages of
epithermal deposits in Mexico with links to Tertiary volcanism.
The first attempts to recognize provinces and epochs of
mineralization in Mexico as a whole were made by Clark (1989,
1997), and these were followed by Clark and Fitch (2005).
In a volume celebrating the centennial of the Sociedad
Geologica Mexicana, Valencia-Moreno et al. (2007) revisited the
porphyry copper deposits whereas the epithermal deposits were
reclassified by Camprubi and Albinson (2007). The latest deposit
descriptions on a national scale are to be found in the Geology
of Mexico, that also celebrates the Centenary of the Geological
Society of Mexico, and includes descriptions of low-temperature,
carbonate hosted deposits in Mexico by Tritlla et al (2007),
whereas the mineral deposits of the more limited area of the
Mesa Central are described by Nieto-Samaniego et al. (2007).
Finally, Clark and Fitch (2009) addressed the distribution of all
metallic deposits in Mexico in time and space.
Objectives and Methodology
Given the wealth of metallic deposits briefly alluded to above
in terms of longevity of production and present day economic
importance, plus the lack of a comprehensive study that
embraces the distribution of all important metallic commodities in
space and time, this article attempts to provide the foundation
for a compilation of all significant metallic deposits in Mexico in
the modern plate tectonic-tectonostratigraphic terrane context.
This article presents the information by first summarizing the
geological framework leading up to and including each of the six
major time slices considered. This is accomplished by drawing
freely on the published literature of which the prodigious
compilations of Sedlock et al. (1993) and Alaniz-Alvarez and
Nieto Samaniego (2007) are foremost. However, we do not
attempt to examine every possible aspect of this evolution
contained in these and other references, but choose a generally
accepted sequence of events. Finally, as the tectonostratigraphic
terranes of Mexico, first described by Campa and Coney (1982,
1983), and Coney and Campa (1984), were modified and
renamed to a large extent by Sedlock et al. (1993). Where there
is ambiguity in terrane designation the later nomenclature is
given in parentheses.
In this context only two tectonostratigraphic reconstructions
are made: the inclusion of the Mojave-Sonora Megashear (Figs.
5-16) and the formation of the Baja California Peninsula (Fig.
16).
Six time slices that contain the important deposits (Table 2)
will be described whereas the geographic distribution of various
classes of ore will be presented in a series of figures. The choice
of the six time intervals is dictated by consideration of major
tectonic, metamorphic, igneous, and, or dispositional events,
plus the reconstruction of tectonostragraphic terranes. We do
not describe the individual classes of deposit, assuming that the
reader already is aware of their salient characteristics, nor do we
repeat tables of age determinations that have been published
elsewhere. Precious metal grades are given in grams per ton
(g/t), and diagrams showing the distribution of deposits are
indexed by acronyms listed alpahbetically.
In spite of this comprehensive approach, there are limitations
to the data presented, the most obvious of which is there are
more deposits known than there are corresponding data relative
to their age. We limit the deposits included in this report to those
for which age determinations have been presented, or for which
geological evidence of their age is compelling. This is
particularly necessary when considering those deposits that
were exploited and depleted before radiometric dates began to
be widely produced a little more than 50 years ago.
7
8
related to other granulite exposures in Mexico that pertain to the
Oaxaquia microcontinent (Weber and Kőhler, 1999). During
subsequent breakup of Rodinia in Eocambrian-Cambrian time,
Oaxaquia, including the Guichicovi complex, was interpreted as
remaining with Gondwana, but a later collision of Gondwana and
Laurentia led to the present position of the Grenville fragments
in Mexico.
PROTEROZOIC
The tectonic evolution of Mexico will be briefly summarized
from Proterozoic through Recent time, giving special attention to
the six principal metallizing epochs for which data is available
In general, we subscribe to the fundamental premise of Coney
et al. (1980), Coney (1981, 1983), and Sedlock et al. (1993) that
the tectonic evolution of Mexico is that of an agglomeration of
terranes accreted to the southern margin of North America.
However, we do not enter into the multitudinous data that
contributes to this evolution, but rather choose a history that best
fits the literature to explain the distribution of deposits within the
geologic framework .
Mineral Deposits in Oaxaca Complex (~1.1-1.0 Ga)
Titanium
The principal metaliferous deposit, for which a Proterozoic age
has been established, occurs at Pluma Hidalgo (Tisur mine),
located in the southernmost part of the complex (Table 2, Fig.
3). The following description of this high grade titanium deposit
is taken from Force (1991), taking into account Paulson (1964)
and an unpublished report (1957) by Thayer. An overlying
gneiss, composed of finely banded quartz-antiperthite-two
pyroxene-garnet-graphite-ilmenite rock assemblage, contains
numerous thin sills of quartz-feldspar. Structurally below, the
gneiss is intruded by concordant and discordant bodies of
impure anorthosite that contain the titanium deposits. These
deposits contain 2 to greater than 50 percent rutile. Rutilebearing intervals average 20 to 40 m in width over a strike length
of 600 m or more. Wall rocks are mostly gneiss, but locally,
impure anorthosite occurs with smaller amounts of rutile.
Ilmenite is also present in the deposit. Force (1991) concludes
that a contact-metasomatic origin is plausible to explain the
deposit, but is probably not the only process of formation. Later,
Schulze et al. (2000) provided dates of 1,010 Ma for anorthositic
gabbro, and 998 Ma for anorthosite at Pluma Hidalgo. For the
latest report on this locality, see McAnulty et al. (2009), who
conclude that rutile occurs interstitially associated with feldspars
in the monzodiorite and alkaline anorthosite and appears to be
the result of late magmatic concentration of residual fluids rich in
titanium.
Another titanium-bearing locality occurs in the northern part of
Oaxaca Complex at Huitzo (Diaz-Tapia, 1988). Disseminated
ilmenite, is concentrated in mantos in foliation planes of gneiss,
termed nelsonite sills by Schmitter-Villada (1970) and CastroMora y Arceo y Cabrilla (1996). The assemblage includes
ilmenite, magnetite, hematite, and apatite distributed in lenses
and mantos. Victoria-Morales (2003) reports that differentiation
during crystallization of magma, due to mineral density
differences, explains the lenses and bands of gabbros and
nelsonites within anorthosite.
Distribution of Proterozoic lithologies
Precambrian rocks are exposed in Mexico in five regions; in
the northwest, north, northeast, south central and southern parts
of the Republic. Thus, in four of these regions metasedimentary
and metaigenous rocks of the Grenville province (~1.0 Ga) are
inferred to continue southwestward into northern Mexico, and
are known to occur in northern Chihuahua, and near Ciudad
Victoria, Molango as well as in the Oaxaca complex.
Immediately the question arises as to whether there is
continuous distribution of the Grenville in the concealed areas
between the widely scattered outcrops (Ruiz et al., 1988), or if
the Grenville terrane occurs as fault-bounded blocks of
basement that are allocthonous with respect to one another and
to the North American craton (Sedlock et al., 1993). This
question does not arise in the case of the Early-to- Middle
Proterozoic rocks of northern Sonora that are closely linked to
terranes in the adjacent North American craton.
Oaxaca terrane
The Oaxaca terrane of Campa and Coney (1983), (Fig. 3) or
Zapoteco terrane of Sedlock et al. (1993), is regarded as a
fragment of Proterozoic continental crust and consists of
crystalline rocks of Grenville age. The oldest unit in this terrane
2
is the Oaxaca Complex (~ 10,000 km outcrop) that consists of
meta-anorsthosite, quartzofeldspathic orthogneiss, paragneiss,
calcsilicate metasedimentary units, and charnockite. It was
formed by metamorphism of miogeoclinal or continental rift
deposits and plutonic rocks (Ortega-Gutiérrez, 1981; 1984).
Pantoja-Alor et al. (1974) have advanced evidence of
Proterozoic aged metamorphic rocks that they interpreted as
evidence of a seaway in adjacent Chiapas, extending
northwards through Oaxaca and beyond. Peak metamorphism
of granulite facies took place at a temperature of 710 ± 50ºC and
pressures of 7 kbar (Mora et al., 1986), and occurred between
1,100-1,000 Ma (Robinson, 1991; Anderson and Silver, 1971,
Ortega-Gutiérrez et al., 1977).
The northern Oaxaca complex has been investigated by Solari
et al. (2004) who recognized metamorphic conditions of 735 ±
5°C and 7.7 ± 0.1 kbar in the granulite facies, which was dated
between ~998 and ~979 Ma using U-Pb isotopic analyses of
zircons. These ages are similar to those found in the southern
Oaxaca Complex, the Guichicovi Complex and the Novillo
Gneiss of east-central Mexico.
The Guihicovi Complex is located near the western edge of
the Maya terrane, and consists of paragneisses and igneous
rocks metamorphosed under granulite facies conditions. It is
separated from the adjacent Juarez (Cuicateco) terrane by the
Vista Hermosa fault. The age of a felsic orthogenesis (1.23 Ga)
dates the primary emplacement of this granite body, and the
2
petrography of the 400 km Guichicovi complex shows that it is
Gold
Elsewhere in Oaxaca, gold has, from to time to time, been
considered to have a Precambrian origin (Capilla, 1909), but to
date, this has not been verified. Among the better known
auriferious localities, is the San Miguel Peras district. It contains
northwest-trending veins inclined subhorizontally to the
southwest, containing milky quartz with sulfides and native gold,
hosted by granulite facies ortho-and parageneiss, pegmatites,
and micaceous marble, part of the Oaxaca Complex (CastroMora and Arceo y Cabrilla,1996; Egüiluz de Atuñano et al., 2004).
Uranium, thorium and rare earth elements (REE)
Uranium, thorium and rare earth elements have a north-south
distribution through the central part of Oaxaca. There is a
lithologic control to these deposits, represented by metamorphic
rocks of the Oaxaca Complex. These elements are contained in
9
10
Figure 3. Tectonostratigraphic terranes of Mexico from Campa and Coney (1983), and Instituto Mexicano del Petroleo (1984). Diagram has been adapted
from Ruiz et al. (1988) and shows lower crustal xenolith localities. Sonoran Proterozoic terranes are taken from Anderson and Silver (1979). Proterozoic
mineral localities are confined to the Oaxaca Complex (after Clark and Fitch, 2009).
allanite and fergusonite, in the Santa Ana and La Crisis
pegmatities in the Telixtlahuaca area (Castro-Mora and Arceo y
Cabrilla, 1996). At this locality, Gomez-Caballero (1990) notes
that the high grade metamorphism in this region was able to
remobilize dispersed incompatible elements (lanthanides) and
deposit them in pegmatites. Another pegmatite in this area that
contains REE is the compositionally zoned, granitic, El Muerto
body that varies from meta-to peraluminous composition. This
pegmatite contains high concentrations of Nb, U, Th, Zr, and
REE (Morales-Alvarado et al., 1999). Additionally, the marbles
of the complex that were mobilized by anatexis (carbonatites?),
see Ortega-Gutiérrez (1977), have been considered as another
lithology potentially favorable for REE concentration.
Permian age (Ramírez and Acevedo, 1957), located in the
Sierra del Cuervo, 30 km NNE of Chihuahua City. Hornblende
from two amphibolite dikes gave ages of 1,024 and 1,037 Ma
(Mauger et al. 1983). These Grenville ages are similar to those
of intrusive granites in the Llano region of Texas (Zartman,
1964), and to hornblende ages in the Van Horn area (Denison,
1970), and Red Bluff Granite of the Franklin Mountains (950 Ma,
Dennison and Hetherington, 1969). To date, no metal deposit is
known at Los Filtros, nor in subsurface lithologies tapped by
PEMEX wells in northern Chihuahua.
Northern Sinaloa
The presence of Grenvillian basement beneath the northern
part of the Guerrero terrane in northernmost Sinaloa has been
invoked by Keppie et al. (2006), and is based on geochemical
data including a recalculated TDM model age of 1238.4 Ma that is
consistent with partial melting of the underlying basement. They
further deduced that migmatized amphibolite-facies gneisses
and amphibolites of the Francisco gneiss resulted from Miocene
exhumation in a core complex of high grade metamorphic rocks.
Molango area, Hidalgo
North of Molango, Hidalgo, the Huiznopala gneiss is exposed,
and also in the core of the nearby Huayacocotla anticline,
Veracruz (Fries and Rincón-Orta, 1965; López-Ramos, 1980),
and a Grenville age has been confirmed by Pachett and Ruiz
(1987). The Huiznopala gneiss, with a total outcrop area of (~25
2
km ) is the smallest of four exposures of Grenvillan granulites in
eastern and southern Mexico, the other localities being the
Novillo gneiss, near Ciudad Victoria, Oaxaca Complex, and the
Guichicovi Gneiss. Lithologically, three units are recognized by
Lawlor et al. (1999): main series orthogenesis, anorthositegabbro complex, and layered paragneiss. The oldest stage
included arc magmatism from ~1,200 to ~1,150 Ma, followed by
granulite facies metamorphism with a peak at ~725 ± 50°C and
7.2 ± 1.0 kbar, and possible emplacement of the anorthositegabbro complex at ~1,000 Ma.
Ductile deformation and
granulite-facies metamorphism in this area closely corresponds
to final thrusting and deformation in the Grenville Province.
However, neither in the Molango area, a small outcrop area of
5 km2, nor in the Huayacocotla anticline, have economic metallic
deposits been found. Nevertheless, a small, decimeter scale
pegmatite that cuts the ductile fabric of the paragneiss has been
found, and it consists of about 65% ilmenite, 20% plagioclase,
10% zircon and the remaining phases are rutile and an altered
maffic mineral (Lawlor et al., 1999).
Northern Sonora
Exposures of Precambrian rocks in northern Sonora were
divided into two major regions by Anderson and Silver (1979),
see Figure 3. The northeastern region was determined to have
a 1.7-1.6 Ga age, whereas the southwestern terrane fell into the
1.8-1.7 Ga range (Paleoproterozoic). The oldest rocks are
found in the Caborca terrane (Campa and Coney, 1983) or the
Seri terrane of Sedlock et al., 1993. In the La Berruga hills,
Anderson et al. (1979) recognized an assemblage of quartzite,
schist, gneiss, and amphibolite, intruded by hornblende diorite
and quartz latite, all overlain discordantly by orthognesis.
Quartz monzonite and granodiorite have suffered metamorphism
and been converted to flaser gneiss and generated segregations
and pegmatites. Mineral assemblages characteristic of the
amphibolite facies are widespread but greenshist grade also
occurs in the southwest part of the Berruga hills.
In northeastern Sonora, lithologies are commonly schistose or
slaty and of greenschist grade and are correlatives of the Pinal
Schist, Arizona (Anderson and Silver, 1979). However, both
terranes have been intruded by anorogenic granites in the
1,500-1,400 Ma interval and by local micrographic granite of
1,100 Ma age, such as the Aibo granite of the Caborca area.
Moreover, the distinction of the two regions based solely on UPb zircon geochronology of igneous rocks has been criticized by
Iriondo et al. (1999). They point to a continuity of Early
Proterozoic magmatism from approximately 1.8 to 1.6 Ga, a
ubiquitous ~1.4 Ga magmatic pulse followed by Grenvilllian age
(1.1 Ga) magmatic pulse in both regions, thus promoting the
concept of a common magmatic evolutionary history during the
Proterozoic.
The two terranes were thought to be separated by the
controversial Mojave-Sonora Megashear (Silver and Anderson,
1974, Anderson and Silver, 1981), along which there is 700-800
km left lateral displacement, which occurred after middle
Jurassic time. Evidence for lateral displacement comes from
similarities between cordilleran miogeoclinical rocks in eastern
California and those in the Caborca region, located on opposite
sides of the megashear (Stewart et al., 1990). At the same time
it can be noted that the younger terrane is continuous with
cratonal rocks of adjacent parts of Arizona (Anderson and Silver,
2005). Recently, Premo et al. (2003) have used U-Pb zircon
Novillo area, Tamaulipas
Further north in the Sierra Madre Oriental, Precambrian rocks
are exposed in the Huizachal-Peregrina anticline. The important
unit is the Novillo Gneiss, composed of garnet and pyroxenebearing granulite, later subject to retrograde reactions (OrtegaGutiérrez, 1978). Sm-Nd dates for the Novillo gneiss are about
900 Ma, but peak metamorphism was likely to have occurred at
about 1,000 Ma (Patchett and Ruiz, 1987). Strongly zoned
garnets yielded higher temperatures and pressures in cores
(729-773°C, 8.9-9.7 kbar) than in the rims (642 ± 33°C, 7.9 ± 0.5
kbar), according to Orozco (1991).
At this locality, native gold occurs in veinlets. Fractures are
oriented N10-12ºW and inclined 25 to 34ºNE, coinciding with the
plane of foliation of the host rock. The gneiss is cut by
numerous sulfide-bearing andesite dikes, striking N80-90ºE,
inclined 80ºSE. The gold is not disseminated in the gneiss
(Eguiluz de Atuñano et al., 2004), and it It is assumed by the
authors that the gold was generated in a post-Grenville phase of
extension.
Los Filtros, Chihuahua
Several tectonic blocks of metagranite and amphibolites and
minor gneisses are found within the Rara Formation of early
11
chronology of the Paleoproterozoic basement of Sonora to show
affinity with pulses of magmatism in the Yavapai and Mazatazal
provinces of central Arizona.
Finally, a complete Late Proterozoic through middle Cambrian
sequence of miogeoclincal strata has been recognized at Cerro
Rajon, southeast of Caborca. It rests unconformably on the
crystalline basement (Arellano, 1956; Stewart et al. 1984).
Within the Proterozoic of Sonora several mineral deposits are
hosted by older crystalline rocks. In particular, and along the
Sonora gold belt (Clark, 1998), several localities are worthy of
mention, including the economic deposits at San Francisco and
La Herradura. But at San Francisco, Pérez-Segura et al. (1996),
40
39
using Ar/ Ar determinations, found that sericite alteration
envelopes gave ages around 40 Ma, and the age of
mineralization at La Herradura is also Tertiary (Pérez-Segura,
2008).
In reviewing fluid inclusion characteristics of several
mesothermal gold veins in the gold belt, including Carina, Mina
Grande, and El Tiro, Albinson (1989) notes the presence of
significant amounts of CO2 (up to 15% molar) which was taken
into account in depth calculations of up to 4 km. In some places,
veins truncate the regional metamorphic foliation, thus
suggesting an age later than peak metamorphism.
Nevertheless, the Sonoran Proterozoic basement is believed to
be either the primary source or indirectly contributing to the CO2
rich mineralizing fluids; gold and other metals being remobilized
by post-Proterozoic metamorphic processes at a depth of 4 km
at the El Tiro and Carina deposits, and consistent with a
mesothermal temperature range. Also, the pH change due to
CO2 loss during boiling is important in mineral deposition
(Hedenquist and Henley, 1985).
Other gold-bearing veins at La Joya, Sierra de Alamo, Cruz de
Pino, and Jenoyema were emplaced in Mesozoic or Cenozoic
host rocks at depths that vary from 2,000-1,500 m due to
repetitive remobilization of CO2-bearing fluids (Albinson, 1989).
Recently, several gold deposits in the Sonoran gold belt have
been dated and fall within the 60-40 Ma range (Pérez-Segura,
2008), and will be discussed later. In reviewing the compilations
of all metallic deposits in Sonora by Pérez-Segura (1985) and
Cendejas-Cruz et al. (1992), no deposits of confirmed
Precambrian age have been identified.
Not found to date, as in adjacent parts of the United States,
are volcanogenic copper-zinc massive sulfide deposits of
Precambrian age, found in the Yavapai series in central Arizona.
These include massive sulfide deposits in the Jerome area
including the United Verde and Iron King deposits, and at
Bagdad, located further to the west (Anderson, 1968). Mauger
et al. (1965) obtained lead isotope model ages of 1,750 Ma and
1,640 Ma for the United Verde and Iron King deposit
respectively. Later, the Old Dick deposit in the Bagdad area
revealed a model age of 1,700 Ma (Doe, 1966).
eastern Canada and its correlatives in Appalachia, Llano and
west Texas, and Oaxaquia. However, in Mexico as noted
earlier, it is still not entirely clear whether the Grenville localities
of northern and eastern Mexico are continuous (Ruiz et al.,
1988) or isolated allocthonous blocks (Sedlock et al., 1993).
Xenoliths associated with basic alkalic volcanic centers of
Quaternary age in San Luis Potosi state, have been derived
from the mantle and base of the crust (Aranda-Gómez, 1993).
The Moho discontinuity in the region was calculated to be
present at a depth between 33 and 46 km. By using isotopic
data obtained from xenoliths in San Luis Potosi and El Toro in
Zacatecas in central Mexico (Fig. 3), Ruiz et al. (1988)
concluded that concealed lower crust exists in central Mexico
that is similar to exposed Grenville localities. Furthermore, they
inferred that phanerozoic crustal genesis must have created part
of the basement under central Mexico, probably of Paleozoic
age. Also, they noted that west of the Precambrian exposures
the basement contains significant amounts of Phanerozoic
(probably Paleozoic) crust. Ruiz et al. (1988) suggested that the
Paleozoic crust may have had it origins as one or more suspect
teranes as suggested by Coney (1981), referred to as the
Cordilleran model. An alternative interpretation is the
Appalachian – Caledonian model that embraces continuity of
Grenville and Paleozoic age rocks though Appalachia and
Texas, southward through eastern Mexico to link with the
Oaxacan and Acatlán complexes respectively, both models
being offset in northern Mexico by the Mojave-Sonora
megashear (see Ruiz et al. 1988, figs. 5a, 5b).
The Cordilleran model was discarded by Yanez et al. (1991)
after assessing isotopic data from the Acatlán complex.
Specifically neodymium model ages are unlike those of accreted
crustal blocks of the Pacific margin.
The existence of a Mesoproterozoic Grenvillian microcontinent
along the axis of the foregoing Appalacian-Caldonian model has
been proposed by Ortega-Gutiérrez et al. (1995). The extent of
this microcontinent is such that it stretches from the Oaxacan
Complex northwards to Molango, Ciudad Victoria and beyond
with possible extensions to the northwest (Solari et al., 2004).
Its origins are considered to be in eastern North America and it
is assumed to have taken up its present geographic location in
late Paleozoic time.
The Proterozoic - Paleozoic lithologic record in southern
Mexico was based on two tectonic cycles. The pre-Mississippian
tectonic evolution of southern Mexico has been elicited as
follows. In the Oaxacan Complex, miogeoclinal sedimentation,
took place during the opening of a seaway in Mesoproterozoic
time. It was followed by the emplacement of anorthosite that
forms the lower part of the complex (Ortega-Gutiérrez, 1981).
Thereafter, these protoliths were subject to the Grenville
orogeny with the production of granulite facies at 1,025 Ma,
resulting in the formation of para- and othogneisses. Later
pegmatites were intruded and uplift took place in Neoproterozoic
time.
In contrast, the adjacent Acatlán Complex protoliths were
developed by sedimentation in Cambro-Ordovician time in a preAtlantic (Iatepus) ocean simultaneously with the appearance of
ophiolite. The consumption of this plate by convergence induced
subduction probably ended in the Devonian as continental
masses collided and produced a suture, today represented by
a dismembered and eclogitized ophiolite of the present day
Acatlán Complex (Ortega-Gutiérrez, 1981). This crustal
thickening resulted in intense metamorphism and plutonism in
Proterozoic-Paleozoic tectonic evolution
Based on paleontological inferences, McMenamin and
McMenamin (1990) postulated a pre-Gondwana supercontinent
and named it Rodinia (also known as Paleopangaea). The
configuration of this supercontinent was ascertained by noting
that at ~1.0 Ga and slightly older, a period of major orogeny
occurred in widely separated parts of the world (Rogers and
Santosh, 2004). Rodinia was broken up at about 0.7 Ga
(Hoffman, 1991), and parts of the original continuous Grenville
orogenic belt became attached to various continents. The
orogenic belts at that time included the Grenville province of
12
the Devonian (Acadian ?) orogenensis. The accreted protoliths
appear to be underlain by Precambrian basement due to the
presence of high-grade xenoliths in Cenozoic stocks up to 400
km to the west of the Caltepec fault zone that separates
Mesoproterozoic rocks of the Oaxacan Complex from Paleozoic
lithologies of the Acatlán Complex (Elías-Herrera and OrtegaGutiérrez, 2002; Keppie et al., 2003). Subsequently, the Acatlán
complex was overlain uncomformably by unmetamorphosed
strata of Mississippian through Permian age (Sedlock et al.
1993, Keppie et al., 2003), but there are deformed,
metamorphosed rocks of late Paleozoic ages at the southern
margin of the Mixteco terrane.
Tectonically, in early to mid-Devonian, the Oaxaca terrane
collided with the Acatlán Complex and basement of the Mixteco
terrane. The age of the collision according to Ortega-Gutiérrez
(1981), Yanez et al. (1991), is indicated by metamorphic and
deformation ages in the Acatlán Complex, ages of granite
intrusion in both terranes, and the presence of clasts of both
terranes in overlying Late Devonian marine strata.
Isotopic studies in the Acatlán Complex reveal three
thermotectonic events, the first being at 400-380 Ma. This
Devonian event has been interpreted as a Laurentia-Gondwana
collision (Yanez et al., 1991). Later in Carboniferous time
another event caused deformation in the complex, plutonic
activity in southern Mexico and deformation in the Ouachita,
Marathon and Appalachian belts, due to a second LaurentianGondwana collision (Yanez et al., 1991).
Subsequent to the foregoing tectonic scenarios OrtegaGutiérrez et al. (1999) produced evidence of an earlier event
based on new age, petrologic and field data. This event, named
the Acatecan orogeny, appears to be the earliest manifestation
of Gondwana-Laurentia collision and has been interpreted to
have taken place in late Ordovician-Early Silurian time as the
Oaxaquia microplate overrode the eastern margin of Laurentia.
Recently, the oblique collision between the Oaxaca and
Acatlán complexes has been described along the Caltepec fault
zone. This fault zone is regarded as a major terrane boundary
and is characterized by dextral transpression (see Elias-Herrera
and Ortega-Gutiérrez, 2002, fig. 11). The anatectonic leucosome
and the resulting syntectonic Cozahuico granite in the fault zone
87
86
yielded Early Permian U-Pb zircon ages. Initial Sr/Sr ratios
and Sm-Nd model ages for the granite and leucosome indicate a
magmatic mixture that originated from melted Proterozoic crust
and a component of depleted mantle (Elías-Herrera et al., 2007).
This event is related to the collision between Gondwana and
Laurentia respectively and relates to the final consolidation of
Pangea. The overlying Matzitzi Formation of Leonardian (Early
Permian) age implies high cooling and uplift rates during the
Permian. There has been tectonic reactivation along the
Caltepec fault during the Early Cretaceous, Paleogene, and Neogene.
To the south of the Acatlán complex is the Xolapa complex
that parallels the Pacific coastline, some 600 km long and 50150 km wide (Ortega-Gutiérrez, 1981). It forms the basement of
the Xolapa (Chatino) terrane.
Essentially it comprises
metasedimentary and metaigneous lithologies exhibiting
amphibolite facies (Were-Keeman and Bustos-Diaz, 2001). The
northern limit is marked by mylonites and cataclastic rocks that
separate migmatities and granites of the complex from the
adjacent Guerrero, Mixteca, and Oaxaca terranes. The age of
the protolith is Proterozoic-Paleozoic (Herrmann, 1994) but the
age of migmatization is Late Cretaceous-Early Tertiary. More
recently, the more limited areal contact between the Xolapa
Complex, which was intruded by granitoid igneous rocks of
Paleogene to Miocene age, and the Oaxacan Complex, has
been described along the Chacalapa fault. The age of the shear
zone is late Oligocene (Tolson, 2007). The complex is overlain
by Tertiary volcanic rocks.
A fourth metamorphic complex in southern Mexico is referred
to as Tierra Caliente (Ortega-Gutiérrez, 1981), and is located in
the Balsas River basin and south of the Trans Mexican Volcanic
Arc, and is part of the extensive Guerrero terrane (OrtegaGutiérrez, 1981). Low grade metamorphism in the complex
permits identification of the original interbedded eugeosynclinal,
sedimentary and volcanic units. Included here are informally
named units such as the Taxco schist, Taxco Viejo greenstone
and the volcano-sedimentary metamorphic sequence of
Teloloapan-Ixtapan de La Sal, the latter being thrust eastwards
over Cretaceous shelf carbonates of the Mixteca terrane
platform. The age of various lithologic units based on
paleontologic and limited radiometric data suggests an evolution
that began in the late Paleozoic and continued through the
Cretaceous (Ortega-Gutiérrez, 1981). The complex may have
been accreted to the continental margin of Mexico by Late
Jurassic time (Sedlock et al., 1993). In contrast, Cabral-Cano et
al. (1993) conclude that thrusting at the eastern margin of the
Guerrero terrane is related to the later Laramide orogeny without
invoking accretion and the allochthonous nature of the Tierra
Caliente metamorphic complex.
Elsewhere in Mexico, marine Paleozoic strata are found in
several basins in Sonora, Chihuahua (Pedregosa), the main
Chihuahua basin, and what Lopez Ramos (1969, 1979), has
called the main geosyncline that stretches southward from
Coahuila and Tamaulipas through Zacateczas, San Luis Potosí,
Querétaro to the Huaycocotla anticline, Veracruz, and the
Tlaxiaco basin in northern Oaxaca. The southernmost outcrops
occur in the Chiapas-Guatemala basin, but their relation to the
main seaway is obscure. This was a narrow seaway, possibly
linked to the Ouachita geosyncline.
The Cambrian section is best developed in the Caborca region
of Sonora. Generally, Ordovician strata are less than 300m in
thickness and have platform characteristics. The Silurian,
according to López Ramos (1969) is poorly represented but is
known at Placer de Guadalupe, Chihuahua (Bridges and Deford,
1961), and in the Ciudad Victoria area of Tamaulipas. The
principal outcrops of Devonian strata occur in Sonora,
Chihuahua and Tamaulipas.
The Mississippian units however, have wider distribution than
any of the previous systems and are recognized from north to
south in Mexico, as are the succeeding Pennsylvanian
sequences. Nevertheless, it is the Permian outcrops that are
the most widespread in Mexico. They are thickest in the main
geosyncline that is bordered to the west by a positive area that
would later be covered by much of the Sierra Madre Occidental
volcanic plateau. To the northeast, there is another widespread
emergent region.
In south-central Mexico, the most complete Paleozoic section
crops out north of Nochixtlán, Oaxaca, (Pantoja-Alor, 1970), and
is unconformably overlain by Cretaceous rocks.
The late Paleozoic history of Mexico has been summarized by
Vachard and Pantoja (1997), and subdivided into five intervals
starting with Early Mississippian strata developed as a
consequence of rifting after the Mixteca and Oaxaca terranes
were joined. Osagean faunas are widespread at many localities
throughout Mexico. The final interval considers a complex
13
paleogeography during late Permian time and the near complete
emergence of Mexico thereafter except for a limited number of
areas.
In northwestern Mexico, a passive margin existed at the
southwestern edge of North America from latest Proterozoic to
middle or late Paleozoic time, and was the site of cratonal and
shelfal miogeoclinal strata. Basinal, eugeoclinal units were
deposited in deeper water to the south and west (Sedlock et
al.,1993). Cratonal sediments are found in northern Chihuahua
and Sonora from Ordovician until the Mississippian and they are
fringed peripherally to the south and west by shelfal strata in
Sonora and Baja California ( Seri terrane). The shelfal strata are
overthrust by basinal rocks of Ordovician to Mississippian age
that had been deposited on the ocean floor further to the south
and west of the North American continent (Stewart et al., 1990;
Sedlock et al., 1993).
Chuacús Group of (?) Early Paleozoic age Castro-Mora, 1999).
They have been subjected to low grade metamorphism by
Permian age (374 Ma) granite bodies of the batholith. In this
mineralized zone, there are outcrops of titaniferous ore
(ilimenite) with trace contents of chromium and nickel, mainly
hosted in anorthositic rocks. These deposits form lenses and
veinlets of ilmenite, the pods being up to 4.4 m long and 1.50 m
wide, with irregular distribution (Castro-Mora, 1999; Castro-Mora
and Ortiz-Hernández, 2000b).
Creation of Pangea.
Due to the convergence between Gondwana (which became
a coherent supercontinent at ~500 Ma (Rogers and Santosh,
2004)), and North America (part of Laurasia), the intervening
oceanic lithosphere was consumed as collision between two
continents took place from the Mississippian to mid-Permian,
resulting in the creation of Pangea at about 250 Ma. This
collision produced the Ouachita-Marathon orogen in
southwestern United States and the Tarahumara terrane that
contains deformed basinal sedimentary rocks that were
obducted onto the North American shelf (Sedlock et al.,1993).
The Tarahumara terrane is not shown in Figure 4, but straddles
the Chihuahua-Coahuila state line. The Ouachitan orogeny in
eastern Mexico is known as the Coahuilan orogeny (de Cerna,
1960). In Pennsylvanian to Permian time, the western edge of
the southern cordillera may have been truncated by an inferred
sinistral fault system, which also terminated the Tarahumara
terrane, its suspected trace being along the line of the future
Mojave-Sonora Megashear (see Sedlock et al., 1993, fig. 32).
The latest reconstruction of the Gondwana-Laurentia collision
is based on the models presented by Dickinson and Lawton
(2001) followed by Elias-Herrera and Ortega-Gutiérrez (2002).
The tectonic boundary between the Grenville-age Oaxacan
Complex and the Paleozoic Acatlán Complex has been
described above as an Early Permian event as the leading edge
of Gondwana closed on Laurentia along the Marathon-Ouachita
suture to form Pangea. The older Acadian (Early to Middle
Devonian) orogeny previously described, has been discarded by
Elias-Herrera and Ortega-Gutiérrez (2002) because Devonian
deformation, metamorphism and intrusion by syntectonic plutons
in the Acatlán Complex are unknown in the Oaxacan Complex.
Evidence from the Caltepec fault zone suggest oblique
interaction between the Acatlán and Oaxaca blocks outboard of
Gondwana during the final assembly of Pangea in early Permian
time. Other consequences of this assembly include a Late
Mississippian tectonic event with coeval silicic volcanism in the
Ciudad Victoria area and the Late Mississippian-Early Permian
Las Delicias arc in the Coahuila block. A similar reconstruction
by Weber et al. (2006) emphasizes the position of the southern
Maya block, including the Chiapas massif, relative to Gondwana.
Limited metallization in Early Paleozoic time (470-397 Ma)
In the large area of the Paleozoic, that includes
metamorphosed, continental, and marine lithologies from
Cambrian through Permian time, the documentation of metallic
deposits of corresponding age is scant. This statement is based
on an exhaustive search of the literature including the mining
monographs of Oaxaca (Castro-Mora and Arceo y Cabrilla,
1996) and Sonora (Cendejas Cruz et al., 1992). However,
summarizing the tectonic and metalogenetic potential of
ultramafic-mafic complexes in Mexico, Ortiz-Hernández et al.
(2003) note rocks of this affinity at the Novillo Canyon,
Tamaulipas; El Fuerte, Agua Caliente, and Mazatlan, Sinaloa;
and at Tehuitzingo-Tecomatlan, Puebla. At Novillo the values of
Ni, Cr, Co and Pt in serpentinite are in trace amounts , and merit
no further discussion. Of the three localities in Sinaloa, the El
Fuerte and Mazatlán localities, while not dated isotopically,
appear to be of Mesozoic age and will be discussed later. No
mineralization is reported at Agua Caliente.
In southern Mexico, Puebla state, the Tehuitzingo-Tecomatlán
complex is part of the Mixteco terrane characterized by
Paleozoic basement (Acatlán Complex). The underlying
Petlalcingo Group is overthrust by the Piaxtla Group that
contains thrust slices that represent obducted oceanic and
continental lithosphere. The Piaxtla Group represents an
accretionary prism that was possibly assembled in SiluroDevonian time, unconformably overlain by the Tecomate
Formation of Pennsylvanian-Early Permian age, and overprinted
by Permo-Carboniferous deformation (Keppie et al., 2003). The
lower part of the Piaxtla Group is represented by the Xayacatlán
Formation that contains eclogitic mafic and pelitic rocks. The
Xayacatlán Formation is considered to be older than Silurian
(Meza-Figueroa et al., 2003) and has been intruded by
Esperanza granitoids at 440-425 Ma. At Tehuitzingo and
Tecomatlan, Ortiz-Hernandez et al., (2003, 2006) note pods of
chromite in serpentinized host rocks with values of Cr (0.32%)
and Ni (0.19%). Furthermore, the Tehuitzingo chromitites have
low PGE contents, ranging between 102 and 203 ppb, typical of
ophiolitic chromitites (Proenza et al., 2003). Thus the age of
Tehuitzingo and Tecomatlan deposits is uncertain and although
they have been assigned to the Late Paleozoic by OrtizHernandez et al. (2003, 2006), the protolith appears to be Early
Paleozoic.
In the Tacaná region, located in southeastern Chiapas,
adjacent Guatemala, in the Mazapa de Madero mineralized
zone, a metamorphic complex that comprises anorthosites of the
PERMO-TRIASSIC
Lithologies in Southern Mexico
The lithologies associated with metallic mineral deposits of
Permo-Triassic age are primarily located in southern Mexico,
especially in the Maya tectonostratrigraphic terrane (Fig. 4). In
southwestern Chiapas and eastern Oaxaca, the Chiapas massif
comprises metaplutonic and metasedimentary rocks. These
lithologies have been derived from plutonic and sedimentary
protoliths of inferred Neoproterozoic to early Paleozoic age
(Sedlock et al., 1993). This massif contains granitoids of
Permian, Triassic, Jurassic, and Cenozoic age. Pegmatite and
14
15
Figure 4. Paleozoic and Permo-Triassic metallic deposits in Mexico (after Clark and Fitch, 2009).
granitic bodies of latest Proterozoic age have been reported by
Pantoja-Alor et al. (1974), López-Infanzon (1986), and PachecoG. and Barba (1986), whereas Burkart (1990) recognized Late
Cretaceous intrusions. Older units are overlain unconformably
by undeformed Carboniferous to Permian (?) strata according to
Dengo (1985), and are probably correlatives of the Santa Rosa
and Chochal formations respectively. Finally, in the eastern part
of the Sierra de Juarez of northeastern Oaxaca, there are
metamorphic rocks that consist of multiple deformations,
greenschist facies metasedimentary and metaigneous rocks
(Ortega-Gutiérrez et al., 1990) that have yielded late Paleozoic
K-Ar dates.
The Chiapas massif is located in the southern part of a
plutonic arc that has been identified also in eastern and northern
Mexico (Damon et al. 1981; Torres et al., 1993). This is a calcalkaline belt located on the western margin of Pangea in
northwestern South America in Early to late Permian time, being
documented not only in Mexico but also in southwestern United
States during the Late Permian-Early Triassic (Sedlock et al.,
1993, fig. 31). Temporally, this overlaps with the mid-Permian
cessation of Gondwana-North America convergence. The
Chiapas batholith comprises biotite granite, and granodiorite
(Castro-Mora and Ortiz-Hernández, 2000b). Batholith phases
intrude schists and quartzites of the Chuacus Group and
younger phases invade the Santa Rosa Formation.
However, later investigations by Weber et al (2006) show that
2
the Chiapas massif (~20,000 km ) is composed of deformed
granitoids and orthogenesis with inliers of metasedimentary
rocks, and so the term `batholith` is no longer used. Data from
orthogeneiss reveal that the massif was part of an active
continental margin of western Gondwana during the Early
Permian (~272 Ma). Thereafter, latest Permian (254-252 Ma)
metamorphism and deformation affected the whole massif and
resulted in anatexis and intrusion of syntectonic granitoids.
Also, data from a metapelite yielded a Grenvillian source for the
sediments, but para-amphibolite yielded 1.0-1.2 or 1.4-1.5 Ga
sources and suggest South American provenances.
and Tamaulipas (Fig. 1), along the Gulf of Mexico plain and their
ages span 275-219 Ma. Surface outcrops occur in Coahuila in
the Valle de San Marcos (242 Ma) and Potrero de La Mula (213
Ma). According to Sedlock et aI. (1993) this arc, which extends
into southwestern United States and northwestern South
America, probably formed on the western margin of Pangaea
above an east-dripping subduction zone that consumed oceanic
lithosphere located west of Pangea. The Mexican part of the arc
may have been partially disrupted by later tectonic events,
although initially it may have been laterally continuous. In
Coahuila, the Las Delicias arc as portrayed by McKee et aI.
(1999), is known for the debris shed into the Las Delicias basin
and preserved as the Las Delicias Formation. The basin and arc
were active from Mid (?)-Pennsylvanian throughout most of the
Permian (McKee et aI., 1988). Las Delicias arc is also
characterized by shallow, syndepositional plutons as old as the
Rancho Pesuña rhyolite (331 Ma) and the Coyote pluton (220
Ma), the latter possibly representing late arc magmatism, a view
not shared by López (1997).
Absence of Permian-Triassic metallic deposits in Northern
Mexico
The Permo-Triassic metamorphic and metasedimentary rocks
within Coahuila do not contain metallic deposits of economic
interest (Duran-Miramontes, et aI., 1993), and apparently this
also holds for the magmatic arc rocks. In neighboring Chihuahua
at Placer de Guadalupe, Bridges (1964) has described an
overturned sequence of Ordovician through Permian
sedimentary sequence, including a rhyolite flow of early Permian
age (de Cserna et aI., 1968). However, the age of the nearby
Plomosas mine manto deposit (Escandon, 1971) is interpreted
as Oligocene by Megaw et aI. (1988).
Elsewhere in Chihuahua, Clark and de la Fuente (1978), and
Duran-Miramontes et aI. (1994) were unable to successfully
document metallic deposits of Permo-Triassic age or older.
Further west in Sonora, after an hiatus in deposition that
spanned much of the Late Permian to middle Triassic time,
deposition resumed in the Late Triassic (Stewart et aI., 1990),
and consists of two contrasting facies. Marine strata are located
west of Caborca (Gonzalez-Leon, 1980), whereas the Barranca
Group of east-central Sonora has been interpreted as siliciclastic
deposits in a rift basin that contains a few thin layers of tuff
(Stewart and Roldan, 1991). However, there are no known
metallic deposits of Permo-Triassic age in Sonora (see PérezSegura, 1985, p.60).
Thus, there are no known analogous lead-zinc deposits of the
Mississippi Valley-type (MVT) of the United States mid-continent
region, where host rocks are Paleozoic in age, possibly because
of limited intracratonic basins covered with relatively thin
sequences of sedimentary rocks. In southeastern and central
Missouri and also in the Arkansas-Tri State area, MVT ores
have several similarities (Leach and Rowan, 1986), and
subsequently their Permian age of mineralization was
determined by paelomagnetic methods (Symons et al., 1997).
Metallization corresponds to uplift during the Ouachita orogeny,
when the outboard Lanorian plate impacted the North American
plate, a scenario that may have analogous potential in Mexico.
Ophiolitic rocks are found in the Vizcaino peninsula and have
been interpreted as being remnants of island arcs formed on
Late Triassic oceanic crust and subsequently accreted to the
continent in Late Jurassic or Early Cretaceous time (OrtizHernandez, et al., 2003). At El Tigre (Fig. 4), lenses of chromite
Mineral deposits in southern Mexico (265-228 Ma)
The Chiapas batholith complex, (Damon and Montesinos,
1978) is associated with polymetallic deposits, which CastroMora and Ortiz-Hernandez (2,000a) have divided into two
assemblages, although in general, they both contain Pb-Zn-CuAg-Au, for example, at Escuintla-Pijijiapan (Fig. 4) and Las
Palmas (Mapastepec). These intrusion-related deposits have
not been directly dated; thus their inclusion in the PermoTriassic mineralizing event should be treated with caution.
At the Teziutlán, Puebla, massive sulfide deposit (Fig. 4), the
Cu-Zn assemblage is hosted by sericitic schist, phyllites, and
basic metavolcanic rocks. The protolith consisted of Late
Paleozoic (262-207 Ma) mudstones, sandstones, and basic lava
flows (Chavez et al., 1999), as opposed to an Early Paleozoic
age envisaged by Edelen and Lee (1941).
Permo-Triassic rocks of Eastern and Northern Mexico
In synthesizing the distribution of Permo-Triassic magmatic
rocks in eastern and northern Mexico, Sedlock et al. (1993)
concluded that magmatic arc rocks are present in a roughly
linear swath that extends from Coahuila southwards to Chiapas,
and, in fact, continues further south into neighboring Belize and
Guatemala. These rocks have been penetrated by numerous
petroleum wells drilled in the states of Veracruz, Nuevo Leon,
16
contain up to 48.7% Cr2O3 and there are indications of Ru-Os-Ir
(Vatin-Perignon et al., 2003). During 1982-1993, 13,000 t of
chromite ore were extracted. Finally, the ophiolite and volcanic
arc assemblages on the Vizcaino peninsula and nearby Cedros
Island are interpreted by Kimbrough and Moore (2003) as
autochthonous or parautochthonous forearc lithosphere
constructed outboard of the Mesozoic continental margin arc,
that correlate with ophiolitic terranes in western California and
Oregon.
Cretaceous time (Sedlock et al. 1993). Additionally, both the
eastern and western parts of the Vizcaino (Yuma) terrane
contain minor volcanic rocks, and the western region consists
mainly of Late Jurassic and more abundant Early Cretaceous
volcanic rocks. Further south in the southern part of the
Guerrero (Nahuatl) terrane, intermediate composition volcanic
rocks are widespread.
Thus the apparent absence of
continental crust in the Vizcaino (Cochimi and Yuma) terrane, La
Paz (Pericu) and southern Guerrero (Nahuatl) terranes, is
interpreted by Sedlock et al.(1993) as indicating that Jurassic
magmatic rocks in these regions were formed in an island arc
environment. The net result of piecing together these various
Jurassic-Early Cretaceous magmatic locales is to produce a
wide swath of arc rocks that is continuous from the northern
Caborca (Seri) terrane, to the Chiapas Massif (see Sedlock et al.
1993, fig. 36).
JURASSIC-EARLY CRETACEOUS
The breakup of Pangea began in the Late Triassic, although
the formation of the Caribbean basin did not begin until the
Earliest Cretaceous (Sedlock et al., 1993). This was
accomplished by pre-Cretaceous rifting in two stages (Buffler
and Sawyer, 1983). First, Late Triassic to Early Jurassic rifting
took place in the Gulf region of eastern Mexico, and the
depressions were filled with red beds and volcanic rocks. Later,
in middle- to- late Jurassic time, a thin, transitional crust formed
around what would become the Gulf of Mexico with Late
Jurassic spreading in the deeper part of the basin. Whether or
not displacement along the Mojave-Sonora Megashear was
linked to the opening in the Gulf of Mexico, as envisaged by
Pindell and Dewey (1982) and Anderson and Schmidt (1983), or
was due to transpression in the Cordillera, as favored by
Sedlock et al. (1993), has yet to be resolved.
In the cordillera region, Late Triassic to Late Jurassic
eastward subduction generated magmatic arcs that occur in
southwestern North America and northwestern South America
(Sedlock et al., 1993). In Mexico two groups are recognized.
The first group formed Jurassic arc magmatism approximately
concordant with the Permo-Triassic arc in Mexico in the
southern Maya terrane, but was located southwest of the
Permo-Triassic arc in the northern Maya and Coahuila terranes.
A second group formed a continental arc further west or an
island arc west of the continental margin (Sedlock et al., 1993),
as terranes comprising continental blocks and island arcs were
accreted to the western margin of Pangea in the Mexican region.
The first group is part of a Late Triassic-Jurassic arc stretching
from southwestern United Sates to northwestern South America
(Damon et al. 1984). This arc was cut by the Mojave-Sonora
Megashear along the trend of the arc in northern Sonora where
continental volcanic rocks (>180 to 170 Ma) were intruded by
175 to 150 Ma plutons (Anderson and Silver, 1979) and are
correlated with rocks in southern Arizona. Sedlock et al., (1993)
provide evidence of magmatism associated with the first group
of arc rocks in the Acatlán, Chatino, and Yuma terranes.
Plutonic rocks in the northwest and volcanic rocks in the
northeastern parts of their Tepehuano terrane, and middle
Jurassic rocks in the Caborca terrane, are probably parts of the
arc that were displaced southeastward from the Mojave region
or northwestern Sonora in Late Jurassic time. In the eastcentral part of the Guerrero (southernTepehuano) terrane, the
upper Jurassic Chilitos Formation may have been part of a
continental arc. In the northern and southern parts of the Sierra
Madre terrane, volcanic and plutonic rocks, penetrated by wells
(Lopez-Infanzon, 1986), gave Jurassic ages, and in the Chiapas
Massif there are early- to- middle Jurassic granitoids and Late
Jurassic or older andesites.
The second group includes all other Late Triassic to Late
Jurassic magmatic rocks in Mexico. Early Cretaceous plutonic,
volcanic, and volcanoclastic rocks that were probably parts of
oceanic island arcs, later accreted to North America by earliest
Jurassic-Early Cretaceous metallic deposits (191-110 Ma)
Porphyry copper deposits
The number of positively identified metallic deposits of proven
Jurassic-Early Cretaceous age is limited, particularly in
comparison with the succeeding Late Cretaceous-early Miocene
interval. Nevertheless, more mineralization diversity is apparent
including a porphyry copper deposit that has been recognized at
El Arco in Baja California described by Barthelmy (1974, 1975).
This deposit (Fig. 5) was interpreted to be associated with a
granodiorite porphyry dated at 107 Ma, and is located within the
batholithic zone that stretches from southern California and
northern peninsula California in the Guerrero (Yuma) terrane
throughout a large part of western Mexico, a distance of at least
2,700 km as envisaged by Gastil et al. (1976). However, more
recent Re-Os are U-Pb dates of the El Arco deposit give a
Middle Jurassic age of 164.7 ± 6.5 Ma from a granodiorite
porphyry that hosts Cu-Au-(Mo) mineralization (Valencia-Moreno
et al., 2006). Thus the volcanic-plutonic rocks of the El ArcoCalmalli area are probably associated with the San AndresCedros volcanic-plutonic complex (~166 Ma), and likely part of
an intraoceanic arc system which was accreted to the continent
during the Cretaceous (110-98 Ma).
In southern Mexico in the Maya terrane, the San Juan
Mazatlan porphyry copper prospect has been dated at 191 Ma (
Damon et al., 1983a), and in Michoacán, the Inguaran mine,
located near the southern limits of the Trans-Mexican Volcanic
Belt where it overlaps the southern Guerrero (Nahuatl) terrane,
was cited by Sillitoe (1976) as having an age of 100 ± 10 Ma.
But later K-Ar determinations at Inguaran by Damon et al.
(1983a) on biotite and sericite in brecciated rock gave 35.6 and
32.1 Ma respectively. Another porphyry copper deposit worthy of
mention is Bisbee, Arizona, located approximately 10 km north
of the Sonora-Arizona, line (Bryant and Metz, 1966). The age of
the Bisbee deposit, first determined by Creasy and Kistler in
1962 at 178-163 Ma, was assigned to the Jurassic magmatic arc
by Clark et al. (1979a).
There are several deposits in Baja California state that
probably are pre-Laramide in age including from west to east
mesothermal iron and copper related to granite intrusives; gold
in metsedimentary rocks; and tungsten at deep-seated contacts
in carbonate-bearing metasedimentary rocks (Gastil et al.,
1975). Several of these deposits had been reported on from the
18th century onwards, including Wisser (1954) who remarked on
the presence of small, mineable gold deposits within the plutonic
massif (batholith) of the peninsula, and the magnetite-hematite
deposits and the minor stocks with which they are associated
17
18
Figure 5. Volcanic massive sulfide, porphyry copper, red-bed copper, and mafic-ultamafic rock deposits of the Jurassic-Early Cretaceous interval (from Clark
and Fitch, 2009).
that lie in the Alisitos Formation of Albian-Aptian age. At the
south end of the peninsula there are several gold deposits
associated with the batholitic rocks of the La Paz (Pericu) block,
but these are predominantly of Late Cretaceous age (Sedlock et
al., 1993), and will be discussed in a later section.
An Early Cretaceous continental magmatic arc was active
along the southwestern margin of Mexico, stretching from the
Caborca (Seri), La Paz (Pericu), Vizcaino (in part Yuma) and
western and southern parts of the Guerrero (Tahue and Nahuatl)
terranes. This arc includes volcanic and plutonic phases, but in
the Xolapa (Chatino) terrane, orthognessis that yield 160-128 Ma
dates, may represent granitoid intrusions prior to deformation and
metamorphism. In the southern part of this widespread arc,
several volcanogenic massive sulfide deposits occur.
Early Cretaceous. The oldest age was obtained in the Cuale
district (157.4 Ma) and the youngest is recorded from the
Tlanilpa-Azulaquez district (139.7 Ma).
Teloloapan subterrane
The Teloloapan subterrane is characterized by calc-alkali
basalt, andesite, and scarce rhyolite (Talavera 1993), all of which
are interbedded with greywacke, shale, and limestone. The
rocks have been strongly deformed and metamorphosed to
zeolite and greenschist facies. Three lithological facies have
been recognized by Heredia-Barragán y Garcia-Fons (1989): (a)
volcanic, characterized by submarine extrusions with
interfingering subordinate amounts of sediment; (b) pelitic, with
minor amounts of submarine volcanic rocks; and (c) transitional
facies, in which neither rock type predominates. Major volumes
of massive sulfide are linked to possible exhalative sources with
mill rock, a product of explosive phases of siliceous volcanism
and
characterized
by
quartz-sericite-pyrite
alteration
(Heredia,1997). In the Campo Morado group, there are 12
deposits of which La Reforma is the most important, followed by
Suriana and El Naranjo (Miranda-Gasca, 1995; Cluff et al.,
2009). Most of the volcanogenic massive sulfide deposits occur
in a felsic volcanic unit of Early Cretaceous age and calc-alkaline
affinity, near the contact with the stratigraphically overlying, finegrained sedimentary strata. Heterolithic fragmental rocks occur in
the main felsic volcanic unit (Oliver et al. 1997), see also Cluff et
al. (2009). Mortensen et al. (2008) indicate that the Campo
Morado deposits formed between 146.2 and 142.3 Ma.
Other deposits in the Teloloapan subterrane include Rey de
Plata, the Azulaquez group, Santa Rosa, and the Tizapa group.
206
Pb/238U ages in the Tlanilpa-Azulaquiz district are 139.7
The
Ma (Mortensen et al. (2008). These deposits form a northtrending belt some 115 km in length and 50 km wide (Heredia
1997). The deposits contain up to 6 Mt of ore, with Campo
Morado, Tizapa, and Rey de Plata the most significant (MirandaGasca 1995). Overall, along the eastern margin of the subterrane
in Guerrero state, the Teloloapan assemblage is thrust eastward
over Cretaceous shelf carbonates of the adjacent Mixteca terrane
(Campa et. al 1976). Both proximal and distal volcanogenic
deposits occur within an island-arc regime bordering the Morelos
platform (Heredia, 1997).
Volcanogenic massive sulfide deposits
The volcanogenic massive sulfide and sedimentary exhalative
deposits have been exploited in southern Mexico since the 19th
century, (Clark, 1999), with mining initiated at El Rubi in the
Zihuatanejo subterrane in 1850 (Miranda-Gasca, 1995). The
deposits (Fig. 5) are primarily in Guerrero, Jalisco, Mexico,
Michoacán, and Puebla states. The principal group of deposits
containing the Zn-Pb-Cu-(Ag) assemblage is in the southern part
of the Guerrero terrane. Limited gold values are found in the
Campo Morado, (Oliver et al., 1997, 1998), Campo Seco, and
Rey de Plata deposits (Chavez et al., 1999).
In general, this region consists of variably deformed and
metamorphosed sedimentary and volcanic rocks of Jurassic to
Cretaceous age. Mid-Cretaceous and Tertiary plutons intrude the
volcanosedimentary sequences, and Tertiary volcanic rocks
cover parts of the subterranes that host massive sulfide deposits.
Various hypotheses have been advanced for the development
of the southern Guerrero terrane. They include plate-tectonic
scenarios of Campa and Ramirez, 1979; Coney, 1983; and
Ramirez et al., 1991. More recently Centeno-Garcia et al.
(1993), using REE and Nd isotopic data concluded that there
were two stages in the development of the southern Guerero
terrane. The first stage is recorded in the Arteaga Complex
subterrane that may represent an accreted oceanic sequence,
whereas the second stage was the development of an intraoceanic island arc above the Arteaga Complex. Basaltic pillow
lavas of the Arteaga Complex indicate Mid-Oceanic Ridge Basalt
(MORB) affinity, and were considered to be part of an oceanic
basin that received sediments derived from a continental source
(Acatlán and Oxaca Complexes ?). The overlying JurassicCretaceous arc-related rocks have initial ∑Nd and REE patterns
similar to those of evolved intra-oceanic arcs.
The Guerrero terrane has been subdivided into several
subterranes that border the southwestern coastline of Mexico,
and are located to the north and south of the Trans-Mexican
Volcanic Axis. The Teloloapan (Ixtapan), Huetamo, and
Zihuatanejo sub-terranes were first described by Campa and
Coney in 1983, and subsequently other subterranes have been
recognized.
Miranda-Gasca (1995) has described over 60 VMS deposits in
the Guerrero terrane, varying in tonnage from less than 100,000 t
up to 6 Mt. Most of the deposits are of the Kuroko type (Zn-PbCu) and are located in the Zihuatanejo and Teloloapan
subterranes. The Guanajuato and Calmalli deposits contain the
Zn-Cu assemblage.
Recently, Mortensen et al. (2008) have dated several VMS
deposits in the Guerrero terrane by U-Pb methods and find that
they are in a restricted time interval from latest Middle Jurassic to
Zihuatanejo subterrane
The Cuale deposit contains several orebodies that occur in
shale and volcanoclastic rocks of Late Jurassic-Early Cretaceous
age that overly porphyritic rhyolite domes (MirandaGasca,1995).They have been described in detail by MaciasRomo and Solis-Pichardo (1985) and Berrocal and Querol
(1991). The Cuale deposit, that contains high Ag ± Au grades
when compared to similar deposits in western Mexico, has been
dated at 157.2 to 154.0 Ma by Bissig et al. (2008) who employed
zircons and the U-Pb method. The ages of other deposits in the
Cuale region (El Rubi, El Desmoronado, La America, and El
Bramador), are less well constrained than at Cuale. Similarly, a
precise age of the host rocks is unknown at La Minita in
northwestern Michoacán, but may be Early Cretaceous (GaytanRueda et al., 1979). The lead-rich Arroyo Seco deposit, located
in the southern part of Michoacán, was considered to be a sedex
type by Chavez et al. (1999), and is located in the Tepalcatepec
Formation which lies above the contact with the Arteaga
Complex (Mortensen et al., 2008). This formation contains Albian
to Cenomanian fossils which suggest that Arroyo Seco is no
19
older than late Early Cretaceous. At the El Encino iron mine in
southernmost Jalisco, Chavez et al. (1999) list this deposit as a
VMS type, hosted in Early Cretacous rocks.
mineralization, part of which includes sulfides of epigenetic origin.
However, Yta (1993) postulated a high-temperature replacement
origin because of the presence of granite apophyses and the
metamorphic minerals garnet and diopside, that coexisted with
ore minerals. Synsedimentary, non-economic pyrrhotite
mineralization was, nevertheless, recognized. See also
Gonzalez-Villalvaso and Lopez-Soto (2009).
Later, other discoveries in the same volcanosedimentary
sequence have been made at San Jeronimo, which is 20 km
south of the city of Zacatecas ( Western Copper
Holdings,1998a). Two drillholes have resulted in the identification
of massive sulfide and high-grade vein-silver potential (Western
Copper Holdings, 1998a).
Further southeast and to the north of Noria de Angeles, are
the El Salvador and San Nicolas massive sulfide deposits that
contain a Zn-Cu-(Ag-Au) assemblage (Western Copper Holdings,
1998b). The El Salvador Cu-oxide zone, discovered in mid-1996,
is a relatively flat-lying, massive sulfide body that varies from 1 to
13 m in thickness. The San Nicolas deposit described by
Johnson et al (1999), is reported to contain 83.4 Mt of sulfide
resource. These deposits are latest Jurassic in age and are
associated with mafic and felsic volcanic flows. Four U-Pb ages
from the San Nicolas and El Salvador deposits show that felsic
volcanic units adjacent these deposits formed from 151.3 to
147.9 Ma (Mortensen et al., 2008).
Northern Guerrero terrane
Three occurrences of stratiform sulfides have been located
close to Guanajuato city. The San Ignacio, (Cu-Zn), Yolanda,
and Arroyo de Cata deposits are small lenses hosted in black
slates interbedded with propylitized and andesitic and basaltic
flows and rhyolite plugs (Macias-Romo et al., 1991). MirandaGasca 1995) notes the igneous rocks dated by K-Ar methods
have ages from 157-114 Ma. However, precise age
determinations have only been made at El Gordo from a rhyolite
flow in the footwall of the deposit which gave 146.1 Ma
(Mortensen et al., 2008).
In northeastern Baja California, there is a part of the Guerrero
terrane that presumably has been translated to the northwest as
the peninsula developed. The Calmalli deposit is a stratabound
sulfide deposit identified by Echavarri and Rangin (1978). It is
hosted by basaltic flows in thin shales that have been subjected
to greenschist metamorphism (Miranda-Gasca 1995), and like
the deposits at Guanajuato, contains <1.0 Mt of resources.
The Olivos deposit, a small sulfide body in southern
Chihuahua, occurs in a volcanosedimentary sequence near the
junction between the Parral and Cortez terranes (Fig. 5).
Mineralization is associated with carbonaceous shales and
intercalations of andesite and rare felsic beds, all exposed in an
erosional window and metamorphosed to greenschist facies
(Comaduran, 1996; Gastelum-Morales y Comaduran-Ahumada,
1997).
More recently and by far the largest discoveries have been
made in Zacatecas state, and north of the Trans-Mexican
Volcanic Belt. This region, a high plateau (altiplano) to the
southwest of the carbonated-hosted deposits in the Sierra Madre
(Oriental) terrane (Megaw et al., 1996), is separated by a major
tectonostratigraphic terrane boundary (Fig. 5). Traces of Bi, and
possibly also Te, are characteristic metals associated with the
Guerrereo terrane, but are not found in the Sierra Madre Terrane
(Yta and Barbanson 1993). The Zacatecas region is also wellknown for its younger skarn and vein deposits.
In Zacatecas, beneath the wide basins covered by Quaternary
deposits, and in outcrops at other localities, there are Late
Jurassic-Cretaceous sedimentary and volcanic rocks in a lobe of
the Guerrero terrane. This region is underlain in part by Triassic
and possibly Paleozoic crust. The volcanosedimentary sequence
has been recognized from the Fresnillo mining district in the
northwest to just east of Noria de Angeles in the southeast, a
distance of approximately 140 km.
In the Francisco I. Madero area the coexistence of
synsedimentary sulfide deposits of Mesozoic age that host
Tertiary veins that are found between Guanajuato and Fresnillo
was recognized by Gómez-Caballero in 1985 (Arturo GómezCaballero, written personal communication, March, 2009). The
veins are easily identifiable by their 200-600 g/t Ag and up to 6%
Cu content, and include rare minerals containing Cu and Bi.
In the Francisco I. Madero (Madero) deposit, a lower
lithological unit is composed of sericite-chlorite schist overlain by
a pelitic-carbonate sequence, followed upwards by calcareous
units, locally containing lavas, volcaniclastics, and siltstone of
early Cretaceous age (Yta 1993). This deposit has been
interpreted to be of sedimentary exhalative origin, by MirandaGasca (1995) and contains 36 Mt of Zn-Pb-Ag-(Cu)
Other VMS deposits
Of the remaining deposits, Klesse (1970) considered the
copper-rich La Dicha deposit to be Paleozoic in age but later,
Sabanero-Sosa (1990) assigned the Ixcuinatoyac Formation,
which contains La Dicha deposit, to the Late Jurassic-Early
Cretaceous interval, which would be similar to the majority of
VMS deposits described above, albeit that La Dicha occurs in the
Mixteca terrane (Fig. 5).
The Copper King deposit that also contains the Cu-Zn
assemblage is located in a basaltic sequence of Las Ollas
Complex and is thought to be of early to middle Mesozoic age
40
39
(Mortensen et al., 2008), and Ar/ Ar ages of gabbroic dikes
that cut the Las Ollas Complex vary from 112± 3.0 to 96.3 ± 25.5
Ma (Ortiz-Hernandez et al., 2006), and consequently suggest a
pre-late Early Cretaceous age. However, stratigraphically below
the Copper King deposit, intrusions of gabbro and
leucomonzonite yielded a Rb-Sr isochron age of 311 ± 30 Ma
(Pennsylvanian), see de Cserna et al. (1978), which may indicate
a possible older age of the deposit.
Red-bed copper deposits
Stratiform copper deposits in northeastern Chihuahua were first
identified by Weed (1902). Subsequently, they have been studied
by King and Adkins (1946), de la Fuente (1973), Giles and
Garcia, (1973) and Clark and de la Fuente, (1978) who
recognized their distribution relative to other deposits in
Chihuahua. More recently, Price et al. (1988) provided more
details of these deposits, particularly their geochemical
characteristics.
The copper ores occur in fine-grained, marine sandstones of
the Early Cretaceous La Vigas Formation, deposited in the
2
Chihuahua trough over an area of 12,000 km in a sedimentary
basin filled with Jurassic and Cretaceous deposits of evaporites
overlain by clastics and limestones. The copper deposits display
characteristics typical of red-bed copper ores including
discontinuous lenses, with copper sulfides cementing sand
20
grains. De la Fuente (1973), recognized 10 mantos in the Las
Vigas Formation with an average thickness of 1.0-1.5 m,
containing as much as 3 percent Cu, of which 58 percent is
sulfide ore. Oxidation products include malachite, azurite, and
tenorite. Significant alteration halos are absent. Silver values are
less than 24 g/t, the highest of 9 samples, and gold was less than
0.0049 g/t a marked contrast with silver-copper veins in red-bed
sequences in adjacent parts of Texas (Price et al., 1988), that are
notably enriched in silver.
In addition to the deposits in Las Vigas area, a similar
stratiform copper deposit has been described at Samalayuca
(Bruno, 1995), but the age of the deposit is problematical
(Molina, 1997). Finally the arkosic San Marcos Formation of
earliest Cretaceous age, in neighboring Coahuila state contains
disseminated copper (Durán-Miramontes et al., 1993).
is the principle Ni-bearing mineral and selected samples contain
up to 4.9% Ni, and an average value of 20 ppm Pt. CereceroLuna et al., (1984) estimate potential reserves at 200,000 t
grading 0.4-1.0% Co, 2 g/t Au, and 0.5% Ni.
Two outcrops of Jurassic (?) age gabbro were mapped by
Fredrikson (1974) and occur approximately 20 km north of
Mazatlan. Plagioclase – clinopyroxene ± hornblende gabbro is
the dominant rock type in both bodies (Henry et at., 2003).
Cumulus layering parallels contacts, bedding, and schistosity in
the surrounding metasedimentary rocks.
Rodriguez-Torres
(2000) reported on platinum-group metals in this area, and OrtizHernández et al. (2003) note indications of Ni and Cr in
ultramafic phases.
South of the Trans-Mexican Volcanic Belt four serpentinized
mafic-ultramafic rock localities are found along the coast of
Guerrero (Ortiz-Hernández et al., 2006; Fig. 5). At Camalotito,
scarce disseminations of chromite and pentlandite are replaced
by bravoite and violarite (Morales-Velasquez et al., 1985).
The mineralized rocks at Loma Baya occur as a roof pendant
on granitic batholith rocks. Values of Cr are as high as 18%,
whereas Ni content is up to 1400 ppm.
At El Tamarindo, Cr values vary from 2.7-8.0% and Ni varies
from 2,900-3,600 ppm (Ortiz-Hernández et al., 2003).
The fourth locality at Las Ollas, located to the south of the El
Tamarindo occurrence in southernmost Guerrero (Nahautl)
terrane, although barren of mineralization has been dated at 112
± 3.0 – 96.3 ± 2.5 Ma (Early Cretaceous) according to OrtizHernández et al. (2003).
Further inland and in southernmost Mexico State serpentinized
mafic-ultramafic rocks host indications of Cr, Ni, and Co at
Palmar Chico – San Pedro Limon, where Early Cretaceous
isotopic age determinations have been made by Delgado-Argote
et al. (1992).
In central Mexico, in Guanajuato State, at San Juan de Otates,
a complete section of allocthonous oceanic arc rocks (OrtizHernández et al., 1989b; Lapierre et al., 1992) have been dated
at 113-112 ± 7.0 Ma. Values of Cr vary from 105-499 ppm,
whereas Ni varies from 49-135 ppm.
Manganese deposit
Exploration and development of the Molango district ores
began in the 1960's. This deposit is by far the largest in North
America (Okita, 1992, see also Pérez-Tello (2009). Sedimentary
manganese occurs at the base of the Chipoco facies of the
Taman Mixto Formation of Late Jurassic age (Duenas-Garcia et
al., 1992), in the form of rhodochrosite and manganoan calcite.
The deposit formed in a restricted marine environment (Okita,
1992), perhaps related to fluvial sediment loads or sea-floor
hydrothermal activity associated with the development of the Gulf
of Mexico. Manganese oxides were subsequently derived directly
from the carbonate facies by secondary oxidation and leaching to
form weathering products (Alexandri and Martinez, 1991). This
deposit has been assigned to the Jurassic-Early Cretaceous
interval (see Fig. 5), based on the age of the formation.
Mafic-ultamafic rock deposits
As seen in Figure 5, there is a cluster of mafic-ultramafic rocks
in Sinaloa located in part of the Guerrero (Tahue) terrane. These
include, from north to south the Macochin and El Realito
occurrences (Mullan, 1978). The larger Macochin intrusion is
interpreted as a lopolith, 50 km in diameter and at least 1 km
thick. In both intrusions, gravity-layered hornblendite cumulate
forms a basal ultramafic zone, and the opaque phase is ilmenite,
partly altered to sphene.
At Bacubirito, ophiolite (Servais et al., 1986), there are
anomalous values of Pt and Pd (Bustamante-Yanez et al., 1992).
It was first described by Ortega-Gutiérrez et al. (1979). This
locality was further investigated for platinum group minerals by
Consejo de Recursos Minerales, which revealed that
mineralization was a surficial phenomenon and not disseminated
throughout the body (Arturo Gómez-Caballero, written
communication, March, 2009). The relation of this mineralization
to the impact of the largest metallic meteorite in Mexico at
Bacubirito (Ward, 1902; Haro, 1931) remains a possibility.
At the Alisitos locality, the first report of Ni and Au was made by
Kreigner and Hagner (1943), and this was followed by another
report by Clark (1973). Later, Ortiz-Hernandez et al. (2003) report
Ni-Au mineralization in peridotite and latite represented by native
gold, nicolite, maucherite and gersdorfite. Ni values are 0.0960.34% and Co is from 37-45 ppm.
The mina Culiacán locality is located 27 km northeast of the
capital of Sinaloa and comprises a troctolitic roof pendant 4 x 7
km in size above granodiorite of batholithic proportions found
throughout the state (Clark, 1973). Hydrothermal mineralization
is fracture controlled, and the brecciated host rock contains the
assemblage Fe-Ni-Co-As (Ortiz-Hernández et al., 2003). Nicolite
LATE CRETACEOUS – EARLY MIOCENE
Tectonic Evolution
This interval is characterized by the amalgamation of various
terranes along the western flank of Mexico which subsequently
became part of the North American plate (Sedlock et al., 1993).
All these terranes were inboard of a trench along which oceanic
lithosphere of the Farallon plate was subducted during
Cretaceous-Paleogene time. Subduction was accompanied by
magmatism, accretion and the Late Cretaceous-Paleogene
Laramide orogenesis.
In Sonora and Sinaloa the resultant lithologies of the foregoing
tectonic events were the emplacement of granitoid batholiths and
other plutons, and also the formation and widespread distribution
of two major volcanic sequences, all of which became host rocks
for a variety of mineral deposits as described below. The Sierra
Madre Occidental (SMO) continental volcanic arc had been
divided into two great calcalkaline volcanic sequences by
(Wisser, 1966), who referred to them as the lower andesite series
and the overlying rhyolitic sequence (caprock). Later, Clark et al.
(1976, 1979a,b) referred to these two groups of volcanic rocks as
the Lower and Upper Volcanic Series, also known as the Lower
Volcanic Complex and the Upper Volcanic Supergroup of
McDowell and Keizer (1977) and McDowell and Clabaugh
(1979). The latest subdivisions in SMO are taken from Ferrari et
21
al. (2007) who recognize five magmatic and tectonic events, four
of which fall in the time interval considered in this section: (1)
Late Cretaceous to Paleocene plutonic and volcanic rocks; (2)
Eocene andesites and lesser rhyolities of the Lower Volcanic
Complex; (3) Silicic ignimbrites emplaced in two pulses in
Oligocene time (32-28 Ma) and early Miocene (24-20 Ma) which
represent the Upper Volcanic Supergroup; (4) transitional
basaltic – andesitic lavas that were erupted towards the end of
and after each ignimbrite pulse and which have been correlated
with the Southern Cordillera Basaltic Andesite Province of the
Southwestern United States (Cameron et al., 1989), whose ages
cover the interval 32-17 Ma.
Magmatism spread eastward from SMO but batholiths are not
exposed in north-central Mexico, and intrusive events are
represented by individual granitoid stocks, plus correlatives of the
lower andesites and overlying ignimbritic sequences, and are
located in the states of Chihuahua (Alba and Chavez, 1974;
Bagby, et al. 1979; McDowell and Mauger, 1994; Durango,
Ferrari et al., 2007); and in the Mesa Central (Nieto-Samaniego
et al., 2007).
In eastern Mexico, alkaline rocks decrease in age southwards
from Oligocene in Tamaulipas (Bloomfield and Cepeda, 1973) to
San Andres Tuxtla (Cantegral and Robin, 1979), and coincide
with intraplate extension in northern Mexico (James and Henry,
1991). In Veracruz, Hidalgo and Puebla, Miocene and younger
rocks were formed in arc or backarc environments. Mid-Miocene
rocks are calc-alkalic and are probably related to subduction of
the Cocos plate in southern Mexico, whereas late Miocene to
Recent rocks are alkalic and calc-alkalic and were erupted In an
extensional back arc regime (Sedlock et al., 1993).
In southwestern Mexico, as far south as Oaxaca, orogenic
deformation was initiated in the Late Cretaceous. Laramide
deformation included E-W shortening that migrated to the east
with time, generally with easterly vergence (Moran-Zenteno et al.,
2007). Latest Cretaceous to Early Paleocene plutonism in the
Jalisco block is contemporaneous with that in the central part of
Sierra Madre del Sur. Later volcanism, including compositions
from andesite to rhyolite, Paleocene to middle Eocene in age, is
located in the Presa Del Infiernillo area in the states of
Michoacán and Guerrero, Taxco and localities further east in
northwestern Oaxaca.
However, the main axis of magmatism of middle Eocene and
Oligocene age developed along the continental margin from
coastal Michoacán to Oaxaca. Thus, the Pacific coast of
southwestern Mexico is characterized by numerous plutons
including batholiths (Moran-Zenteno et al., 2007), in which
granodioritic and tonalitic compositions predominate.
Sinistral lateral faulting activated NW-SE and N-S faults during
Eocene and early Oligocene time. The NW-SE faults extend as
far south as Oaxaca but the N-S set are located in northern
Sierra Madre del Sur (Moran-Zenteno et al., 2007).
major classes of deposits are porphyry copper, skarns, mantos,
tin in rhyolites, disseminated gold in various host-rocks, and a
wealth of fissure-vein deposits including precious metal,
polymetallic, and manganese; stratabound and breccia pipe
uranium. The majority of these deposits coincide in space and
time with a subduction-related envelope of magmatism that
progressed eastward several hundreds of kilometers as far east
as the meridian of Coahuila, from the trench position paralleling
the west coast of Mexico, and then regressed towards the west
coast by early Miocene time (Clark et al. 1979a; 1982).
Porphyry copper deposits
Individually, porphyry copper deposits were being discovered,
evaluated, and brought into production, primarily in a span of
about 25 years through the early 1960's to the mid-1980's. A few
ore bodies of the larger systems had previously been mined of
which La Colorada and El Capote breccia pipes in the Cananea
district (Kelley, 1935, Perry, 1961) and Pilares breccia pipe
(Emmons, 1906, Wade and Wanke, 1920) in the Nacozari
district, both in Sonora, are examples (Fig. 6). Following the
investigation of numerous deposits along the west coast of
Mexico, several reports were produced that dealt with various
aspects of these deposits. Among these are Sillitoe (1976) who
provided basic information on location, mineralogy and alteration;
Damon et aI. (1983a), who provided K-Ar age determinations for
all deposits; Barton et al. (1995) who added details on petrologic,
mineral assemblage, and geochemical characteristics of these
deposits; and Clark (1996) who related porphyry copper
mineralization to tectonostratigraphic terranes.
The eastward shift in magmatism in northern Mexico has been
documented by Anderson and Silver (1974) and Henry (1975).
This was accompanied by changes in alkalinity. Cameron et al.
(1978, 1980) have shown the variation in alkalinity across
Chihuahua from southwest to northeast from calc alkalic,
moderate-K facies to a high-K facies in the eastern part of the
state. This scenario was further amplified geographically by
McDowell and Clabaugh (1979) who recognized three distinct
alkalinity zones across northern Mexico and adjacent parts of
west Texas.
Porphyry copper deposits have a common association with
Laramide calc-alkaline intrusive centers in northwestern Mexico
and younger intrusions in the southwestern part of the Republic.
Intrusion-related copper deposits are associated with
intermediate and felsic magmas and characteristically contain the
Cu-(Mo-Zn) assemblage. Copper mineralization is closely
associated with strongly porphyritic quartzo-feldspathic
intrusions. Dioritic rocks, according to Barton et al. (1995),
appear to be more closely associated with a Cu-(Fe-Au)
assemblage and alkalic igneous intrusions are not associated
with copper, but with fluorine-rich deposits. Diorite related
porphyry systems are less common in Mexico but are
represented at La Verde, Michoacán, and in other copper
occurrences in southern Mexico. Association with mid-to-Iate
Cretaceous batholiths has been noted in Sinaloa and in the
southern Guerrero terrane in weakly porphyritic granitoid phases.
Breccia pipe mineralization occurs in Guerrrero and Michoacán in
the southern Guerrero terrane at Inguaran and La Verde (SolanoRico, 1995), and also in northern Sonora where the largest
deposits are located in the Chihuahua terrane in the Cananea
and Nacozari districts (see Valencia-Moreno et al., 2007 and
Ayala-Fontes, 2009, respectively).
Barton et aI, (1995) have drawn attention to the role of igneous
rock compositions that exert a basic control on the nature of
Mineral Deposits (100-16 Ma)
From the foregoing summary of tectonic events in late
Cretaceous-Paleogene time, the newly accreted conglomeration
of terranes in Mexico had been structurally prepared on a
widespread scale in northern and southern Mexico for the
mineralizing fluids associated with the accompanying
magmatism. Thus, within a span of approximately 80 million
years, from Late Cretaceous through early Miocene time a
prodigious number of metaliferous deposits of several different
classes were emplaced primarily in northern and central Mexico,
that is, north of the Trans-Mexican Volcanic Belt. Among the
22
23
Figure 6. Porphyry-copper deposits of the Late Cretaceous-early Miocene interval. From Sillitoe (1976), Damon et al. (1983), Valencia-Moreno et al.
(2006), Clark and Fitch, (2009).
related hydrothermal systems through a combination of original
element abundance and chemical equilibria. They conclude that
most metals will become substantially more soluble with
increasing activity of alumina (in metaluminous and
peraluminaous rock). Thus, base metals are commonly
associated with peraluminous and modestly calcic aluminous
systems and there is virtual absence of base metals associated
with peralkaline systems. This, in itself, explains the relative
paucity of predominantly copper-bearing systems in eastern Mexico.
Many characteristics of the Mexican copper belt are also
controlled by basement lithology, subduction regime, and
existence of a protective capping of dominantly volcanic rock,
uplift and subsequent erosion. As the calc-alkalic Occidental
Volcanic Arc, that produced the porphyry copper deposits,
progressed eastward (Clark et aI, 1979a, 1982), associated
magmas became more alkalic and copper poor (Damon et aI.,
1983a). Thus, enrichment to ore grade became increasingly
improbable to the east.
Along the belt, as a whole, Damon et aI. (1983a) showed that
subduction related magmatism varied with latitude. Specifically,
87
86
they contrasted initial strontium isotope ratios ( Sr/ Sr) of
intrusions associated with these deposits, by comparing the low
values that occurred in the accreted Guerrero terrane of Campa
and Coney (1983), with those that occurred in the cratonic
regime in northern Sonora, and adjacent parts of the
southwestern United States. Ratios were plotted against latitude
by Damon et aI.(1983a), and fall in three distance groups, the
highest being in the North America terrane (>0.7063), whereas
mineralized porphyries in the Guerrero terrane of Campa and
Coney (1983) (Tahue, Tepehuano, and Nahuatl terranes of
Sedlock et aI., 1993) have lower values and fall into two distinct
geographic groups. The corresponding interpretation suggests
that magmas originally had low initial 87Sr/86Sr ratios, but the
radiogenic component increased during magma residence and
assimilation in the cratonic crust to the north. In the Cananea
district, Wodzicki (1995) used Nd and Sr isotope data combined
with major and trace element data to conclude that the Laramide
rocks evolved from a mantle-derived parent melt by coupled
assimilation and fractional crystallization.
In this regard, Titley (1992) has presented evidence of the
importance of crustal lithologic contribution to magmas of
Laramide age that relate to porphyry copper formation in Arizona.
Subduction mechanisms in this scenario are regarded as
triggering mechanisms, leading to the evolution of these ores, but
crustal composition was considered to be a more profound
influence on metallogenesis. In contrast, in his detailed synthesis
of giant porphyry copper deposits in the Andes, Clark (1993)
notes there is no anatomical distinction in the unusually large
porphyry copper deposits, and tentatively suggests that very
large deposits may owe their origin to a protracted and consistent
magmatic history in the course of which only limited migration of
the main subduction-related arc occurred.
Copper deposits are located in a zone paralleling the west
coast from Sonora and Baja California (including the older EI
Arco deposit), southwards to Chiapas, a distance of 2,600 km
(Fig. 6), in which they become younger from north to south.
However, the zone becomes wider if other intrusion related
mineral deposits are included, especially those deposits where
copper is only a minor component of the system or in other
deposits where copper is absent (see Barton et aI., 1995, fig. 2
and table 2). As a result, over 600 copper-bearing intrusion
related systems have been recognized, of which about 100 can
be documented with some degree of certainty. But in this report,
we separate those deposits that are skams if they do not contain
disseminations in the intrusive body or breccia pipe
mineralization. Neither do we include those deposits that do not
contain copper. Some deposits predominantly contain
molybdenum (Cerro Colorado, Los Chicharrones, San Judas),
whereas the La Guadalupana locale is rich in tungsten (Schultz,
1953).
Figure 6 shows the location of 37 porphyry copper deposits for
which radiometric age determinations are available. These data
are taken from Sillitoe (1976), and Damon et al.(1983a). But the
older EI Arco and San Juan Mazatlan deposits have been
eliminated from this mineralizing epoch as well as the younger
Toliman and Santa Fe occurrences in Chiapas. Also, removed
from consideration are those deposits that are primarily fissurevein, skarn or replacement deposits which will be considered
separately. One addition, Satevo in Chihuahua, has been added
from Barton et al ( 1995). Considering all deposits, 13 fall in the
Chihuahua (North American) terrane, 7 in the Caborca terrane, 5
in the Cortez terrane, and 11 in the Guerrero terrane. From this
we agree with Damon et aI. (1983a) and Clark (1996), that the
occurrence of these deposits appears to be independent of the
terrane intruded, and in Sonora and Sinaloa several deposits are
associated with batholiths or penecontemporaneous volcanic
rocks. However, the largest and richest deposits are located in
the cratonic Chihuahua terrane.
The most recent age determinations are found in Barra et al.
(2005) and Valencia-Moreno et al. (2007). By using Re-Os
molybdenite ages for nine porphyry copper-molybdenum
deposits from northwestern Mexico, Barra et al. (2005) show that
the ages of this sample of the deposits shown in Figure 6,
stretches from Maria, Sonora in the north to Malpica, Sinaloa in
the south, and spans the interval 61-50 Ma (Paleocene to early
Eocene), and is associated with the Laramide orogeny. The
Tameapa deposit (Chrisinger, 1975), has a protracted
hydrothermal history brought about by four molybdenite
mineralization events between 57 and 50 Ma (Bara et al., 2005).
In contrast, Valencia-Moreno et al. (2007) provide ages of all of
the deposits shown on Figure 6, and many were formed during
the interval 75 and 50 Ma. Metal distribution of these deposits
was separated into three domains according to basement of
emplacement. The northern domain is characterized by
Proterozoic crystalline rocks of North America; the centraldomain is composed of Paleozoic marine strata underlain by
Proterozoic North American terrane, whereas the southern
domain consists of Mesozoic island-arc sequences of the
Guerrero terrane. The northern and central domains include CuMo-W assemblages whereas the southern domain is dominated
by Cu-Au mineralization, and with the exception of El Arco (~3.6
Mt Cu) and Santo Tomas (~1.1 Mt Cu) are relatively small
deposits.
The lessening of age of these deposits from north to south is
striking. Including El Arco, Baja California, and Toliman, Chiapas,
the variation in age is from 165 to approximately 6 Ma, a span of
159 Ma over a distance of 2,600 km or a younging on average of
about 1.0 Ma per 16 km. This rate is much higher than the
convergent rates between the Farallon-North American plates
(Atwater, 1970; Coney, 1976; Clark et al., 1982), and is probably
due to oblique subduction of the Farallon plate under
southwestern North America.
24
Polymetallic skarn, breccia pipe, manto and vein deposits
The geographical and terrane distribution of 22 deposits that
are skarns, replacements, or mantos, or some combination
thereof, for which age determinations are available, is shown in
Figure 7. Many more deposits, less precisely dated, are known
but are not included here. In most cases, the age of the
mineralization is assumed to be approximately the age of the
associated intrusion that varies in composition from diorite to
granite. Using the schematic representation of Megaw et al.
(1988), to describe the spectrum of these deposits, San Martin,
Charcas, Velardena, Providencia, Concepcion del Oro, and
Zimapan are proximal stock contact varieties, whereas Santa
Eulalia, Naica, Mapimi (Ojuela), and La Encantada represent
dike or sill contact skarns, through massive sulfides, to chimneys
and mantos and distal massive sulfide mantos. Of the remaining
Pb-Zn-Ag bearing deposits, La Reforma, Cosala, San Felipe,
Tronco de Peras, Los Reyes, and Dinamita are regarded as
intermediate between the proximal contact skarn and distal
massive sulfides, the latter having no known relation to an
intrusion as at Los Lamentos. In contrast, the Pb-Zn-Ag bearing
skarn at San Carlos in northeastern Chihuahua is related to a
granite pluton emplaced during caldera development at 31 Ma
(Immitt and Kyle, 1981, Immitt, 1987).
Replacement of country rock carbonates is common, and the
high temperature carbonate hosted Ag-Pb-Zn-(Cu) deposits of
northern Mexico, described by Megaw et al. (1988), occupy a
zone running parallel to the paleotrench position but generally
well east of the porphyry copper province referred to above
(Clark et al., 1979a). These deposits are located within the
Caborca (2) Chihuahua (4), Coahuila (3), Sierra Madre (7),
Cortez (1), Parral (3), Toliman (1), and Guerrero (2) terranes, and
thus appear to be independent of terrane lithologies. Two
deposits are located west of the main belt of these ores and
include Cosala (Pb-Zn-Ag), Sinaloa, hosted by Cretaceous
carbonates. The El Tecolote Zn-Cu replacement deposit in
Sonora is also included here rather than under porphyry copper
deposits because of its replacement characteristics (Farias,
1973), and the fact that zinc is the principal metal (CendejasCruz et al., 1992). Thus, the foregoing demonstrates a key
circumstance controlling the formation of these deposits, namely,
the interaction of calcalkaline intrusions developed above a
flattening and eastward migrating subduction zone and
associated magmatic arc and the widespread Meozoic
carbonates, primarily of Cretaceous age, that covered much of
northern Mexico. However, the role of limestone host rocks in
Mexico in this context had been previously noted by Prescot
(1926), and Fletcher (1929), and in North America as a whole by
Callahan (1977).
In general, the age of the skarn, replacement, and manto
deposits is younger than the porphyry copper deposits within a
southwest- northeast transect approximately perpendicular to the
paleotrench position as they are inboard of the porphyry copper
belt. Their age span, as a whole in this mineralizing epoch varies
from about 80 Ma at La Parrilla, Durango to 27 Ma at La
Encantada, Coahuila (Clark et al., 1979a). There appears to be
an overall younging to the south from 59 Ma at La Reforma to 2
Ma at Santa Fe (Damon and Montesinos, 1978), if the latest
mineralizing epoch is taken into account. Likewise, there is a
younging of age from southwest to northeast from 59 Ma at La
Reforma to 27 Ma at La Encantada.
Iron Deposits
Three genetic types of iron deposits were recognized by Flores
(1951), namely, 1) replacement; 2) contact metasomatic; and 3)
residual, totaling 63 localities. Subsequently, an inventory of 134
deposits was made by Pesquera et al. (1979), grouped into four
sizes of reserves with a small number having tonnages greater
than 10 Mt, and a large number with the smallest tonnage of less
than 1 Mt. Of these, 9 deposits were considered to be of
volcanogenic origin, some others were thought to be of magmatic
injection, and the remainder contact metasomatic. Later, the
result of the first symposium on the exploration for iron in Mexico
was held in Monterrey, and a resume of the proceedings was
elaborated by Estrada et al. (1988). Fourteen of the most
significant iron deposits were divided into two groups, nine of
which occur in the Mesozoic island-arc environment in the Sierra
Madre del Sur of southwestern Mexico. Included here are Peña
Colorada and Cerrro Nahualtl, Colima; Las Truchas, Aquila, La
Guayabera and Los Pozos, Michoacán; EI Encino, and La
Huerta, Jalisco; and EI Violin, Guerrero (Fig. 8). These magnetite
rich deposits were thought to be primarily of contact metasomatic
origin the largest of which are situated in calcareous pelitic host
rocks, as at EI Encino, Peña Colorada, and Las Truchas. The
second group of deposits is located in volcanic environments on
the eastern flank of Sierra Madre Occidental in north-central
Mexico, and is described separately below. The possibility of
finding sedimentary origin iron ore deposits of Precambrian
(Bazan-Barron, 1980), Paleozoic, or Jurassic age has not been
confirmed to date.
Contact metasomatic iron deposits
In southwestern Mexico (Fig. 8), several deposits are
considered to have a metasomatic-hydrothermal origin, based on
their association with Tertiary granite intrusives, and the
metamorphic minerals formed at the contact with the host rocks.
However preliminary observations at some of these deposits by
Corona-Esquivel (2000) suggest similarities with the
volcanogenic group, for example Peña Colorada in Colima.
The youngest iron deposit occurs at Cerro Colorado (18 Ma,
Damon and Montesinos, 1978) and is located in the northwestern
part of Chiapas. Although it has characteristics of a typical
porphyry copper contact zone, Castro-Mora (1999) stresses the
importance of iron in this skarn deposit.
Elsewhere, and
further north in Sinaloa, two deposits are worthy of mention,
namely Cerro Mazomique and Los Vasitos. Both are of contact
metasomatic origin, formed by Laramide age granitoids of the
Sinaloa batholith being intruded into early Cretaceous calcareous
rocks (Bustamante-Yañez et al., 1992). At Mazomique, in the
Choix district of northern Sinaloa, Davidge (1973), and Clark et
al. (1979a) note the presence of iron oxide copper sulfide
mineralization in the contact zone of three small mines. Further
south, at Los Vasitos, the granodiorite intrusion was dated at 25
Ma (Clark et al., 1979a).
In Sonora, there are numerous small magnetite-hematitebearing skarn deposits and two more are associated with
volcanic rocks, one of which is located in a volcanic-sedimentary
environment (Pérez-Segura, 1985). The three largest deposits
are 1) San Miguelito dike is associated with volcano-sedimentary
host rock intruded by a monzonite of likely Laramide age, 2) the
San Marcos skarn mineralization which occurs at the contact of
Permian (?) limestone and a Laramide monzonite body, and 3) El
Violin which is located in a subaerial Tertiary rhyolite
25
26
Figure 7. Polymetallic (Pb-Zn-Ag-(Au)) skarn, breccia pipe, manto and vein deposits of the Late Cretaceous-Early Miocene interval. Modified from Megaw
(1988, 1996). From Clark and Fitch (2009).
27
Figure 8. Volcanogenic and other iron deposits; volcanogenic, sandstone, and other uranium localities, all of Late Cretaceous-early Miocene age including
the marine phosphate deposits in Baja California Sur. (From Clark and Fitch, 2009).
environment. A fourth deposit, also noted by Pérez-Segura
(1985), is El Volcan, situated some 50 km NE of Ciudad
Obregon. It was brought into production in 2007 by exploiting a
predominantly magnetite-rich ore body within a quartz
monzonite-diorite intrusion near the contact with andesite
volcanic rocks. Its origin is possibly of magmatic injection,
although this interpretation is preliminary (Arroyo-Dominguez,
2009).
In Baja California numerous small iron-copper deposits form a
north-south belt, the majority of which are related to granitoid
bodies of probable mid-to-Iate Cretaceous age that have
intruded volcanic rocks. Mineralization is in tabular bodies, either
veins or mantos (Amaya-Martinez, 1977; Gastil et al., 1975).
This belt is located in the Guerrero terrane of the northern
peninsular region. The Santa Ursula deposit contains about 4 Mt
and is the largest in this region according to Pesquera et al.
(1979).
In Chihuahua three other deposits, for which data are
available, show that contact metasomatic iron deposits also
occur in the north-central part of the country. For example, La
Negra located 20 km southeast of La Perla is associated with a
quartz monzonite stock, and the ore occurs in a lenticular body
surrounded by stockworks and veinlets of iron oxides. This
deposit is assumed to have a similar age as the El Anteojo mine
located 70 km northwest of La Perla, where iron mineralization
occurs in a skarn adjacent a quartz monzonite dated at 35 Ma
(Clark et al., 1979a). In adjacent northeastern Chihuahua, at
San Carlos, magnetite is found in a contact metasomatic deposit
formed where a granitic pluton invades early Cretaceous
limestone. The pluton is related to caldera development at 31
Ma (Immitt and Kyle, 1981). Within the metasomatic zone a PbZn assemblage has also formed, and epithermal veins
containing a zinc-copper-fluorite assemblage occur near the
northwestern margin of the caldera.
Finally, the reader is referred to papers on several of these
deposits by Corona-Esquivel et al. (2009a, c), who also remark
on their genesis.
is lenticular and comprises specular hematite, martite and some
magnetite (Van Allen, 1978), with gradational contact with the
host rock. The various origins that have been suggested for the
deposit have been reviewed by Corona-Esquivel et al. (2009a)
who conclude that the iron is of magmatic origin, promoted by F,
P, and S volatiles that produced the massive iron oxide body by
liquid immiscibility, which partitioned it from the remaining
silicate melt.
Hércules, Coahuila consists of 12 ore bodies in varying stages
of exploitation and 3 areas of geophysical anomalies where ore
awaits development. The published literature includes VelascoHernandez (1964), Prado-Ruiz (1971), Cazares (1973),
Ruvalcaba-Ruiz (1989), Hoyt (1990); and Hernández-Ontiveros
(2009). In addition, a general introduction is given by DuranMiramontes et al. (1993) that show many of the individual
deposits are hosted by igneous rock and that the contact
between ore and host rock is generally sharp but may be
brecciated. The general geology of the area consists of
Cretaceous carbonates overlain by subvolcanic rocks that have
been dated by Ruvalcaba-Ruiz (1989) at 33.7 to 30.7 Ma,
although the age of mineralization may be as young as 28 Ma
(Hernández-Ontiveros, 2009). Ore bodies are located in
andesite and many are fault controlled with metamorphism of
the host rock. Hoyt (1989) produced a conceptual model of
lenticular ore bodies in a fracture at a depth greater than 500 m
containing high temperature (500-400°C) mineralization,
surrounded by metamorphically altered porphyritic syenitic or
trachytic host rock. Lower temperature deposits and altered
host rock occur at shallower depths up to the surface where hot
spring sinters are visualized.
Cerro de Mercado, Durango is an iron deposit that has been
studied for about 150 years. Among the literature are reports by
Weidner (1858); Birkinbine and Palacio y Tebar (1883); Foshag
(1929); Labarthe-Hernández et al. (1987); and Lyons (1975,
1988). Labarthe-Hernández et al. (1987) note that a rhyodacite
flow was intruded by monzonite to quartz monzonite porphyry,
producing a calcic-rich alteration zone, which was replaced by
magnetite, later oxidized to hematite. Iron oxide ore bodies were
controlled by fractures, resulting in subhorizontal ores which at
depth became tabular and steeply inclined. Around the massive
ore are hydrothermal breccia zones. Previously, Lyons (1975)
had recognized the 30 Ma Chupaderos caldera, but stressed
that formation of much of the iron deposits was by subaerial
volcanic processes during an hiatus between two eruptive cycles
of the caldera forming process. The resulting pulverulent
deposits have also been recognized to a minor extent at La
Perla and Hércules, but a subaerial origin has not been invoked.
Lyons (1988) recognized late stage hematite-magnetite dikes
that cut the entire system and fed flows. Both hypotheses entail
the eruption of a magma rich in iron, fluorine and other volatiles.
Peña Colorada, situated like many of the other deposits in
southwestern Mexico in the Guerrero terrane, is formed at the
contact with a 68 Ma diorite body that intrudes mid-Cretaceous
volcano-sedimentary rocks. The volcanic rocks have tholeiitic
affinity and REE values are consistent with a primitive arc
environment (Zurcher et al., 2001). This massive iron oxide
replacement deposit, one of the largest in Mexico, contains 200
Mt with a grade of 60 percent magnetite.
Peña Colorada perhaps has the most complex mineralization.
The middle and upper parts contain mineralogy typical of
replacement including the presence of garnet, and in the highest
part there are veins containing hydrothermal magnetite that cut
part of the volcanic sequence and also the upper conglomerate.
But in the lower part of the deposit, in the area known as “La
Chula”, there are breccias cemented with magnetite, pyroxene,
and apatite, typical of magmatic origin and similar to that of
Cerro de Mercado (Rodolfo Corona-Esquivel, written
communication, March 2009).
Volcanogenic Iron Deposits
Within this group are three deposits in northern Mexico: La
Perla in Chihuahua; Hercules, Coahuila; and Cerro de Mercado,
Durango (Fig. 8), all of which have been assigned to a
continental magmatic arc environment. These deposits are in
contrast to 9 major contact metasomatic deposits of southern
Mexico that are located within an island-arc environment of
Cretaceous age described in the Jurassic-Early Cretaceous
epoch. The southern deposits are Peña Colorada, Las Truchas,
El Encino, La Huerta, Aquila, Cerro Nahuatl, La Guayabera, Los
Pozos, and El Violin.
However, in the last few years, the interpretation of origin of
iron deposits in southwestern Mexico has changed. In previous
years they have been interpreted as metasomatic because of
the presence of intrusives in the region. However, several of
these deposits have geologic and geochemical characteristics
similar to Cerro de Mercado, briefly described below, and to El
Laco in Chile, or Kiruna in Sweden. Thus, Aquila and El Encino
show magmatic characteristics, whereas the Cerro Nahuatl
deposit was formed by metasomatic replacement, and Las
Truchas is mostly of replacement origin (Rodolfo CoronaEsquivel, written communication, March, 2009).
La Perla has been described by several investigators, among
them Crockett (1953); Cardenas-Vargas and Castillo-G. (1964);
Campbell (1977); Van Allen (1978); and Corona-Esquivel et al.
(2003). The deposit is contained in the La Perla Formation, the
lower part of a sequence of rhyodacitic lavas in the Sierra de
Mestenes (Campbell, 1977). K-Ar age determinations of these
volcanic rocks cluster around 30 Ma, and their alkalinity varies
from peralkaline to peraluminous in the host rock. The ore body
28
For the latest descriptions of La Perla, Cerro de Mercado,
Peña Colorada, and many deposits in southwestern Mexico, see
papers by Corona-Esquivel et al. (2009 a, b, c, d).
Other uranium deposits worthy of mention include the ConetoBuenavista district in Durango; and El Chapote - Diana, PresitaTrancas-Peñoles in the state Nuevo Leon (Corona-Esquivel,
written communication, March, 2009).
La Coma, a sandstone-uranium deposit in Nuevo Leon has
been described by Pérez (1973). Other similar deposits located
in the Oligocene Frio No Marino Formation include Buenavista,
El Chapote-Diana and Presita-Trancas-Peñoles, all located in
the Burgos Basin (Salas and Castillo-Nieto, 1991; SanchezRamirez, 2011).
In the Baja California Sur marine phosphate deposits of late
Oligocene-early Miocene age, uranium has been reported in
minor amounts at San Juan de la Costa and San Hilario.
The remaining deposits referred to by Salas and Castillo-Nieto
(1991), are located in the states of Zacatecas, San Luis Potosi,
and Oaxaca (Santa Catarina Tayata), but because their age
assignments are unclear they are not shown in Figure 8.
Finally other localities not mentioned above can be found in
maps published by Instituto de Geologia (M-18) that show
distribution of uranium, berylium, lithium and thalium.
Volcanogenic uranium deposits
Although there has been no commercial production of uranium
in Mexico, exploration for and development of deposits in
Chihuahua took place in the 1970's and 1980's, by the now
defunct Mexican government agency URAMEX and later by
Consejo de Recursos Minerales. Several occurrences of
uranium minerals at the surface and in radioactive anomalies
occur in the Chihuahua City area (Fig. 8), a region that extends
for 200 km in a north-south area around the city, thus leading
Goodell (1983, 1992; Goodell et al., 2009) to refer to this area as
the Chihuahua City uranium province. The most important
locality is Sierra Peña Blanca, located 45 km north of the city
(Fig. 8), where uranium is found in fault zones and stratabound
concentrations in volcanoclastic rocks of a distal facies of a 44 to
35 Ma old ignimbrite sequence (Goodell, 1981, 1982). Uranium
is considered to have originally been dispersed in glassy
tuffaceous rock in the Sierra Del Nido to the west, leached and
then transferred in hydrothermal systems that formed
concentrations in the Sierra Peña Blanca sequence.
The mineralogy of these deposits includes hexavalent
margaritasite, (a Cs-rich analog of carnotite, Wenrich, 1983),
uranophane, betauranophane, carnotite, weeksite, and
metatyuyamunite (Cárdenas-Flores, 1983). Molybdenum is
found in some of the uranium concentrations. In Sierra Peña
Blanca, the deposits are limited to the Nopal Formation (43.8
Ma) and the overlying Escuadra Formation (38.3 Ma), both
having been dated by Alba and Chavez (1974). Important
localities include the Nopal 1 breccia (pipe?), Margaritas
stockwork in the Escuadra Formation, and Puerto 3 stratabound
prospect, mostly in the Escuadra Formation.
Another uranium deposit of probable magmatic origin, located
30 km northwest of Chihuahua City, and closely associated with
a Tertiary caldera, is San Marcos. The Quintas ignimbrite (46
Ma), the result of a second stage of plinian eruptions, is cut by
the two curvilinear rhyolite dikes, the later one is within 200 m of
the uranium prospect (Ferriz, 1981).
The Sierra de Gómez is situated 50 km east of Chihuahua
City, and contains uranium associated with nickel that occurs in
carbonates of Cretaceous age. This low-temperature
mineralization is located in thrust faults and karst fillings
(Goodell, 1992). Some 90 km further east of the Sierra de
Gómez at Placer de Guadalupe, Antunez (1954) noted that
uraninite occurs in hydrothermal veins with quartz and native
gold, associated with dacitic intrusions. This mineralization has
been has been dated at 32 Ma (Goodell, 1992).
Elsewhere in northern Mexico, Yza-Dominguez (1965) has
reported on the Noche Buena uranium mine in Opodepe
municipality, Sonora and Chavez-Aguirre and Cruz-Rios (1987)
have drawn attention to the Los Amoles deposit in Late
Cretaceous trachyandesite in the Sierra Aconchi, Sonora. Some
20 km to the southeast is the San Alejandro prospect, a vein that
cuts Late Cretaceous-Early Tertiary volcanic rocks and contains
uranium and gold values (Cendejas-Cruz et al., 1992).
At Granaditos (Fig. 8), Marquina-Martinez (1981, 1983)
described mineralization in dacitic ignimbrite which included
dissemination and breccia control. In other parts of north-central
Sonora uranium is disseminated in ignimbrite in small veins at
San Pedro de la Cueva; in fractures, disseminations, and
hydrothermal breccia located in ignimbrite at Picacho; and in a
chimney located in a series of pyroclastic and agglomeratic
andesites and latites at Santa Rosalia. Other uranium
occurrences in Sonora reported by Marquina-Martinez (1983)
include localities near Moctezuma and Huasabas.
Volcanogenic tin deposits
There are numerous reports on tin in Mexico, one of the first
being written by Ramirez (1885), followed by Kempton (1896)
who reported occurrences in Durango and Zacatecas
respectively, two of the important former producing states. In
1942, Foshag and Fries reported on tin deposits in Mexico as a
whole and Smith et al. (1950, 1957), supplied more details on
Durango localities. Two of the more recent and complete
geological investigations were made by Lee-Moreno (1972), and
Pan (1974), that dealt with exploration and geochemical
characteristics, of these deposits. Figure 9 shows the deposits
examined by Foshag and Fries in 1942 and these are
presumably the more important localities and in which there has
been some production.
Foshag and Fries (1942) identified tin deposits in 6 states;
Hidalgo, Guanajuato, Aguascalientes, Jalisco, Durango, and
Zacatecas (Fig. 9), and noted that tin had been found in 13 other
states. They divided deposits into three classes, 1) associated
with granite, 2) associated with silver ores, and 3) associated
with extrusive rocks, but only types 2) and 3) have been
economically productive. Another class, which they recognized,
are placer accumulations.
The only district in which tin is associated with granite is
Guadalcazar, San Luis Postosi. At this locality, Ruiz (1988)
remarks on the similar chemistry of the Guadalcazar granite to
the tin-bearing rhyolites in terms of major element
characteristics, age that was determined isotopically at ~28 Ma,
and also an initial strontium isotope ratio of 0.7070. Tin
associated with lead-zinc-silver ores is only known at the San
Antonio mine in the east camp of the Santa Eulalia district,
Chihuahua, (Hewit, 1943, 1951). This mine produced about half
of Mexico's tin production up to 1939, when tin-bearing ore was
exhausted.
There are probably 1,000 tin localities associated with volcanic
rocks, most of them located in small veins and in some placer
deposits in the central plateau, and most importantly in the
states of Durango, Zacatecas, and Guanajuato (Foshag and
Fries, 1942). The common host rock is rhyolite, but a few
deposits occur in underlying latite and andesite. Specularite
commonly accompanies cassiterite or wood tin. Figure 9 shows
the distribution of deposits that were investigated by Foshag and
Fries (1942), with additions from Smith et al. (1957), and Tuta et
al. (1988).
The northern Mexican tin belt contains deposits in 32-30 Ma
age rhyolites flows and domes that are overlain by rhyolytic
ignimbrites (Huspeni et al., 1984). This is a more restricted age
range than that cited by Ruiz (1988) who considered that all
29
30
Figure 9. Late Cretaceous-early Miocene tin deposits, the majority of which are of volcanogenic origin, the exceptions being Santa Eulalia (SE), and
Guadalcazar (Gu). After Clark and Fitch (2009).
Mexican tin-bearing rhyolites were emplaced from 32-26 Ma.
Crystallization
temperatures
obtained
from
feldspar
compositions are 700 ± 50°C in the host rhyolites (>74%Si02),
that are metaluminous and slightly peraluminous. In reviewing
the elemental and trace element content of these rocks, Huspeni
et al. (1984), conclude that the tin-bearing rhyolites were formed
as extreme differentiates in high level magma chambers closely
associated with caldera development. As most tin-bearing
localities in the world are commonly associated with a cratonic
basement, the interpretations of Ruiz et al. (1988), referred to
previously in regard to the likelihood of Precambrian and
Paleozoic crust existing in north-central Mexico, are important.
Ruiz (1988), points out the Mexican tin province coincides where
Bouguer anomalies are greatest (>200 Mgals). The lead-isotope
data of Cumming and Kesler (1979) has also been interpreted
by some as indicating that part of the North American craton
underlies the region of the Mexican tin belt. Thus, tin values in
the rhyolite magma may have a crustal component which was
later followed by differentiation, as indicated by deep negative
europium anomalies (Huspeni et al., 1984). Tin abundances in
the host rocks are enriched with respect to Sierra Madre
Occidental lavas much of which overlies the accreted Guerrero
terrane. Also, initial strontium ratios are more radiogenic than in
equivalent volcanic rocks to the west (Ruiz, 1989), and crustal
xenoliths examined by Ruiz et al. (1988) from the Sierra Madre
terrane have similar isotopic compositions to the tin rhyolites.
Additionally, the timing of tin-bearing rhyolite emplacement may
have coincided with a change of tectonic regime on the eastern
flank of the Sierra Madre Occidental volcanic province from
compression to extension.
Figure 9 shows the location of 22 important tin deposits that
are districts or individual mines. Many mines and deposits are
indicated by Foshag and Fries (1942), and the names of those in
Durango are identified by Smith et al. (1950, 1957). Thus, the
limits of the tin province, as drawn on Figure 9 include only
major deposits for which ages are available. Examination of the
tin province clearly shows that the post-accretion-terranes are
overprinted by Oligocene age volcanic rocks, and those bearing
tin in significant concentration occur either in the Chihuahua
(North American) terrane, or where the largest concentration of
important deposits is clustered in parts of the Parral, Guerrero,
and Toliman terranes, close to the suspected region that may be
underlain by Proterozoic and Paleozoic basement lithologies.
largely hosted by sedimentary rocks (Perez and Gallagher,
1946). The Nuevo Mercurio (San Felipe) deposits were identified
in 1935, and are located in the Indidura Formation of Late
Cretaceous age, that consists of thin-bedded limestone, marl,
and shale. The cinnabar oxides occur in faults and fault-related
breccias located in an overturned anticinal structure, vergent to
the northeast. There are two main fracture systems that were
invaded by hydrothermal solutions that also deposited silica and
siderite (Parga-Pérez et al., 1992). No igneous rock affiliation is
apparent, and the age of the deposits is Late or post-Late
Cretaceous.
There are two important mercury areas in the Sierra Gorda,
Queretaro state. These are the San Joaquin and PlazuelaBucareli localities, each one represented by several former
producing mines (Rodriguez-Medina et al., 1992). At PlazuelaBucareli cinnabar and metacinabar mineralization occurs in
veins, fractures, and cavity fillings. The host rock is the El Doctor
limestone of Early Cretaceous age at Bucareli, whereas the host
rocks are the Soyatal-Mezcala shales of Late Cretaceous age at
Plazuela. Production from the Plazuela area from 1966 to 1984
is reported to total 812,000 kg Hg.
In the San Joaquin area similar mineralization occurs in the El
Doctor Formation, and production from 1970 to 1974 totaled
1,051,000 kg Hg. There are seven mines in this area of which
Calabacillas appears on Figure 10.
In southern Mexico, the Huahuaxtla mercury district is located
in Guerrero state, where cinnabar was discovered in 1923. The
ore is mainly cinnabar that inverted without change of color from
metacinnabar (Gallagher and Pérez 1948). The deposits are
found in limestones of Late Cretaceous age. The principal ore
shoots are found along the Huahuaxtla fault and controlled by
breccia and ribs of limestone.
The quicksilver and antimony deposits of nearby Huitzuco,
Guerrero have been described by McAllister and HernándezOrtiz (1945). The dominant rock is limestone, probably of
Cretaceous age, broken by compressional faults, producing
breccia and isoclinal folding. Small bodies of granite intruded
the limestone and are located in the peripheral parts of the
district. Solutions carrying mercury and antimony invaded the
dolomitized breccia; replacing some of the dolomite with stibnite
and livingstonite (HgS.2Sb2S3). McAllister and Hernández-Ortiz
speculate on a Pleistocene age of mineralization, but this has
not been confirmed and seems unlikely. Weathering of
livingstonite and stibnite produced cinnabar and oxides of
antimony.
The Atargea, San Luis Potosi, and San Joaquin, Queretaro,
deposits, assigned an Oligocene-Pliocene age by Salas (1975a)
are more likely to have a Late Cretaceous-Early Miocene age
based on other mercury deposits where ages are based on
more plausible information.
Mercury deposits
Many mercury deposits of northern Mexico are located near
the contact of Mesozoic sedimentary strata and overlying
volcanic rocks (Tuta et al., 1988). Small deposits are found in
Chihuahua, Durango, Jalisco, and Zacatecas, and the location
of mercury deposits, in general is given in maps published by
Instituto de Geologia (M-7), see also Figure 10.
The geochronology of these deposits has been given by Tuta
et al. (1988) as follows. In the Canoas district, Zacatecas,
mineralization occurs in a latite dome (Gallagher, 1952) and
formed from 27-26 Ma. At Sain Alto, Zacatecas, adjacent the
Sombrerete tin district, deposits are found in rhyolite and the age
is post-37 Ma. At El Cuarenta, Durango, mercury is
disseminated in granite, an overlying conglomerate (Gallagher
and Perez: 1946), and in overlying rhyolites the deposits are
probably in post-rhyolite faults, and their age is post-40 Ma.
Other smaller deposits are found at Cuencame, Durango
(Salas, 1975a), but at Nuevo Mercurio in northeastern
Zacatecas, the most important mercury district, mineralization is
Antimony deposits
There are important deposits of antimony in Sonora,
Queretaro, and San Luis Potosi states. Most of the original
descriptions were made by United States Geological Survey
personnel working with Mexican geologists.
The El Antimonio district of northwestern Sonora (Fig.10),
contains numerous veins hosted by Triassic siltstones and
sandstones that have been thrust faulted and intruded by
igneous rocks that consist of quartz porphyry, diorite and
trachyte, all of which form dikes and sills (White and Guiza,
1949). This district also produced antimony from placer deposits.
Apparently the veins are not productive when hosted by
31
32
Figure 10. Distribution of important mercury and antimony deposits of Late Cretaceous-early Miocene age. From Clark and Fitch, (2009).
limestone. The age of these deposits is possibly Late
Cretaceous or post Cretaceous inasmuch that the Triassic
Antimonio Formation upper member is Early Jurassic age and
overlying it is a thick sequence of volcanic rocks of possible
Cretaceous age (Gonzalez-Leon, 1980). The intrusive rocks,
referred to above, cut the entire sedimentary and volcanic
sequence, and it is with these that the antimony mineralization is
likely to be associated.
Another district containing antimony is Caborca, which is shown
on the Salas (1975a) metallogenic map as consisting of veins
and shears in sedimentary host rocks. Possibly these are the
same deposits noted by Halse (1894), but their assigned age of
Oligocene-Pliocene appears doubtful when compared to other
deposits in Mexico.
Soyatal, Queretaro is the third largest antimony district in
Mexico, and was discovered in 1905. The sedimentary rocks at
Soyatal have been divided into three formations. The lower
formation is mostly limestone, the middle formation is
characterized by cherty limestone and is of Early Cretaceous
age, and the upper formation is conglomerate at its base grading
upwards through limestone and shale (White, 1948). The
mineralogy of the deposits is simple and stibnite is the principle
hypogene mineral, although many of the ore bodies have been
oxidized, and on occasion, there are traces of mercury.
Apparently stibnite was deposited from fluids that invaded faults
and fractures and may have originated from the same source as
the andesite extrusions in the district. The fluids were unable to
penetrate the shale horizons, and White (1948) notes that
antimony ores are preferentially deposited near the crests of
major anticlinal structures. The age of the antimony
mineralization is questionable, but appears to be post-Early
Cretaceous.
The Sierra de Catorce, San Luis Potosi, has three well-known
mining districts: San José (Tierras Negras) or Wadley (Sb); Real
de Catorce (Ag-Pb-Zn); and Santa Maria de La Paz (Cu-Pb-ZnAg-Au). The San José antimony deposits were discovered in
1898 (White and Gonzalez-Reyna, 1946). Mantos are located in
the Santa Emilia Formation of Late Jurassic age. Most of the ore
in mantos occurs near faults and some is localized in anticinal
axes. Migrating mineralizing solutions (Querol-Suné, 1974)
caused selective recrystallization in beds that originally may
have been more permeable. In reviewing these deposits,
Martinez-Ramos and Maldonado-Ramirez (1975), noted that
intrusions of quartz monzonite, granodirite and andesite
porphyry are important in the formation of veins in the Zuloaga
limestone, also of Late Jurassic age. They interpret the
emplacement of the igneous rocks as a Tertiary event in which
the andesite porphyries are youngest. Replacement of limestone
is characteristic of mantos, and cavity filling marks the veins.
The antimony-bearing mantos are associated with mercury,
whereas the veins are associated with silver and lead (DuranMiramontes et al., 1992). More recently, Zarate-Del Valle (1997)
has reiterated his earlier interpretation of the source of the
antimony in these deposits, namely from the underlying TriasJurassic red beds by lixiviation, followed by precipitation in
lagoonal-neritic carbonate facies. This scenario is completed
with remobilization from stratiform deposits and deposition in
anticlinal structures during Miocene tectonic activity.
The La Maroma district discoveries were made in the 1770’s
and reveal an anticlinal structure in which the oldest rocks
exposed comprise the Huizachal Formation, being overlain by
the Zuloaga and La Caja Formations of Jurassic age, and finally
by Lower Cretaceous units of limestone and shale.
All
sedimentary units have been intruded by a granodiorite of
Tertiary age that displays apophyses and dikes up to 2 km
length. Mineralization is in fissure-veins of which the Señor de
la Humildad is 2-5 km in length. As a whole, the district has
produced Pb-Zn-Ag and also Sb the latter being found in the
eastern part of the district and closer to a smaller anticline
formed in the same Mesozoic statigraphic units and intruded by
two small granodiorite stocks. The district is regarded as having
great potential for future exploitation of silver and antimonybearing lodes (Duran-Miramontes et al., 1992).
Los Tejocotes mines, Oaxaca (Fig. 10) were the largest
producers of antimony from 1938 to 1943. Average annual
production was 4,300 t of shipped ores containing 56-58%
metallic antimony (Guiza and White, 1991). Deposits are found
in a mid-Jurassic complexly folded limestone and shale
sequence, near contacts with a porphyry intrusion. Ores occur in
irregular
shaped
bodies,
fracture-filling
veins,
and
disseminations. Stibnite is the main mineral but is oxidized in
near surface deposits.
Of the remaining antimony districts shown on Figure 10, Albino
Zertuche, Caborca, and Villa Hidalgo were assigned an
Oligocene-Pliocene age by Salas (1975a) as were El Antimono
and Soyatal which have now been included in the late
Cretaceous-early Miocene epoch, so it appears these three
deposits are likely to be somewhat older than previously
envisaged.
Manganese deposits
There are several well-known manganese districts in Mexico
and numerous less important localities: the literature that covers
the entire republic is found in Trask and Rodriguez-Cabo (1948),
and the symposium on manganese deposits in Mexico, XX
International Geological Congress, volume III (edited by
Gonzalez-Reyna, 1956b). Several other bulletins and
unpublished reports were made by Consejo de Recursos
Minerales and will be cited below, and an inventory of
manganese deposits has been published by the Instituto de
Geologia ( M-12).
In general, four types of deposits have been described by
Trask and Rodriguez-Cabo (1948), 1) fissure deposits, which
consist of manganese oxides and calcite in fissures in volcanic
rocks, found mainly in Chihuahua and Sonora; 2) silicified
replacement deposits, that contain manganese oxide and
silicates in siliceous replacement zones in fractured tuffaceous
and volcanic rocks, and found mainly in Zacatecas, San Luis
Potosi and the southern part of the state of Mexico; 3) limestone
replacement deposits that comprise manganese silicates and
oxides close to granitoid stocks, found in eastern Durango,
eastern Coahuila, and northern Guerrero; 4) tuff replacement
deposits, that display manganese oxides in bedded tuffs, found
only in one deposit, near Santa Rosalia, Baja California Sur.
This deposit, because it is of early Pliocene age will be
described in the youngest mineralizing epoch below. Most
manganese ore is found in a few large deposits of the Lucifer
(type 4), and stratabound deposits at San Francisco, Jalisco,
and Molango, Hidalgo (previously described). A variant of (type
1) which occurs at Gavilan, Baja California Sur, reveals stringers
in fractured basalt, and is situated in peninsular California.
Presently the Molango deposit, Hidalgo (Pérez-Tello, 2009), is
the largest deposit being mined in Mexico, and for that matter in
33
the whole of North America. The Talamantes (type 1) deposit in
Chihuahua and others have been idle for several years.
Figure 11 is derived from Plate 39 slightly modified, contained
in Trask and Rodriguez-Cabo (1948). Inasmuch that only the
more important deposits, and, or those whose age is reasonably
well known, are mentioned in this report, the reader is referred to
U.S.G.S. Bulletin 954-F for additional descriptions and other
pertinent data. However, we note that up to the mid-1940s,
Trask and Rodriguez-Cabo concluded that manganese had
been reported at 335 deposits in 20 states in Mexico. The
important deposits are designated in Figure 11.
Fissure fillings (type 1) are the most common type in Mexico
and several are found in Chihuahua, of which the Talamantes
deposit (Wilson and Rocha, 1956), is the largest. The
Chihuahua deposits as a whole have been described by Ayub
(1959). Other deposits in Chihuahua that have been described
individually are Terrenates (Wilson and Rocha, 1956; JimenezV. , 1956; and McAnulty 1969); and Borregos (Wilson, 1956).
The age of these deposits is not known with certainty, except for
the Talamantes ores that was dated at post-42.5 Ma (late
Eocene) by Clark et al. (1979). The Gavilan deposit on Punta
Concepción, Baja California Sur is included here, but the basalt
host rock may be of late Miocene age (Hausback, 1984).
Silicified replacement deposits (type 2) are common and in
which rhyolites and trachytic tuffs and agglomerates are
replaced by manganese and silica, in zones along faults and
fractures. The resistant silicified host rock forms a topographic
high relative to the country rock (Trask and Rodriguez-Cabo,
1948). In some deposits, ore bodies form chimneys, and in
others mineralization has spread laterally from fissures,
subparallel to the bedding. Iron is often introduced with the
manganese. Some 20 deposits of this type are known in northcentral Zacatecas and northwestern San Luis Potosi. Important
deposits are the Abundancia mine (Wilson and Rocha, 1956c) in
Zacatecas; Montana de Manganeso (Wilson and Rocha, 1956b)
in San Luis Potosi; Picacho de La Candela, Durango; and a
deposit near Arcelia, Guerrero, in southwestern Mexico state.
The La Guadalupana deposit, 15 km north of Arcelia is the same
(?) as the Tlalchapa deposit (Trask and Rodriguez-Cabo, 1948)
on Figure 11. Deposits in Sonora, Durango, and Zacatecas have
been described in more detail by Ayub (1960). The age of these
deposits is difficult to verify, but at Montaña de Manganeso and
Abundancia, the epigenetic deposits are hosted in Late
Cretaceous sediments.
Limestone-replacement deposits (type 3) occur near intrusive
granitoid bodies. Examples are Dinamita, Durango; Buenavista,
Guerrero; Guadalcazar, San Luis Potosi, and Candela,
Coahuila. The ore is found in recrystallized and also in
unmetamorphosed limestone, and in other places in breccia
zones (Trask and Rodriguez-Cabo, 1948). Other deposits that
are larger than average include Sarnosa and Luz, Durango in
the Dinamita group, and Milagro, Coahuila, part of the Candela
deposits. Mineralization found in brecciated zones in limestone
include those near Muzquiz and Hipolito, Coahuila; and San
Pedro Ocampo, Zacatecas. The age of the deposits at
Guadalcazar are Tertiary, being found at the contact of a
granitoid intrusive of that age within Cretaceous limestone. At
Sarnosa, Early Cretaceous limestone is intruded by a quartz
monzonite stock of likely Tertiary age with manganese being
fround at the periphery of the intrusion.
Tuff replacement deposits (type 4) include one of the largest in
Mexico located at Lucifer, Baja California Sur. The ore is
characterized by siliceous and maganiferrous replacements of
tuff and has been described by Wilson and Veytia (1949).
Because it is Pliocene in age it is considered in the youngest
mineralizing epoch later in this paper.
Other, miscellaneous deposits are discussed by Trask and
Rodriguez-Cabo (1948), but because they collectively amount to
less than five percent of the ore produced during 1942-45 they
are not considered here.
Two of the largest manganese deposits mined in recent years
are San Francisco, Jalisco (Fig.11); and Molango, Hidalgo (Fig.
5). There are 44 prospects in the Autlan-EI Grullo district in
Jalisco, some of which have been classified as hydrothermal in
origin (Rodriguez-Medina et al., 1992b). However, the San
Francisco deposit occurs in tuffs and contains oxides of
manganese and iron that have been deposited in a lacustrine
basin (Zantop, 1981) in a continental environment of early
Tertiary age. However, this age assignment has been
challenged by Zarate-Del Valle (1997) who contends that the
host rock can be correlated with the volcanosedimentary facies
(Hauterivian-Aptian) of the Guerrero-Colima orogenic complex.
Emplacement of the metals is envisaged by chemical
percipitation from volcanically derived solutions that entered the
depositional basin. The deposit was classified as exhalativesedimentary by Zantop (1978). Other smaller manganese
deposits in Jalisco have been described by Echegoyen and
Almanza (1963).
Tungsten deposits
The states that have produced tungsten are located in
northwestern Mexico; Baja California, Sonora, and western
Chihuahua are the principal producers. The lower California
deposits have been described by Fries and Schmitter (1945),
those in Sonora by Weise and Cardenas (1945), and the
Guadalupana deposit in Chihuahua by Schulze (1953). In
general terms, tungsten in these deposits is related to batholitic
rocks in the form of contact replacement, pegmatite, and vein
mineralization.
Scheelite deposits are located in the northern part of the Sierra
2
de Juárez in Baja California in an area that covers 600 km .
These pyrometasomatic-replacement deposits are found in
tactites developed in metamorphosed late Paleozoic sediments
that have been intruded by Mesozoic diorites and pegmatites of
the batholith (Fries and Schmitter, 1945). Metamorphosed rocks
are found in northwest-trending zones and less regular groups of
roof pendants. Of the numerous mineralized localities, EI
Fenomeno mine sustained commercial production during World
War I, and also from 1937-43 (Fries and Schmitter, 1945). The
age of mineralization can be loosely tied to K-Ar determinations
of the batholithic rocks in the area by Gastil et al. (1975), who
concluded that granitoid rock emplacement was probably
complete by 90 Ma, which suggests that the tungsten deposits
41
39
were formed about this time. More recent Ar/ Ar dates in the
Laguna de Juarez and El Topo plutons (Ortega-Rivera, 2003),
within the mineralized region, span 100-79 Ma and 100-83 Ma
respectively, and suggest a larger age span. Figure 12 shows
the location of the El Fenomeno mine and smaller deposits in
the adjacent Corte de Madera and Cerro EI Topo localities.
Included here are Los Cinco Hermanos and Olivia deposits
which have produced little ore.
An early description of producing localities in Sonora was
given by Wiese and Cardenas (1945) who recognized three
mineralization types; 1) contact-metamorphic deposits in
34
35
Figure 11. Manganese deposits of the Late Cretaceous-early Miocene interval are cited by name for those deposits where ages are available. Modified from
Trask and Rodriguez-Cabo (1948). From Clark and Fitch (2009).
36
Figure 12. Tungsten deposits of the Late Cretaceous-early Miocene interval. Modified from Weise and Cárdenas (1945) and Clark and Fitch (2009).
limestone; 2) pegmatite dikes in granite; and 3) quartz veins in
granite. The first recorded production was in 1916, and in 195253, 80% of the national production came from the Baviacora
district, being surpassed in 1960-1962 when 100% of Mexico's
tungsten production came from the same district (Cendejas-Cruz
et al., 1992).
Some of the most important contact-metamorphic deposits are
located near Hermosillo.The host limestones are Permian age
and have been intruded by granitoid plutons forming scheelitebearing tactites. Among the deposits immediately south of
Hermosillo are the San Juan de Dios, Cinco de Mayo, and El
Carmen mines. The Palo Verde deposit was exploited during
World War II. In all, several mines produced ore in the
Hermosillo district, and are collectively designated (Ho) on
Figure 12.
Elsewhere, following discovery and production in the early
1950's, mining was suspended but then renewed in the 1970s in
the Baviacora district, where deposits are related to the El
Jaralito batholith (Damon et al., 1983b, Roldan-Quintana, 1991).
The El Jaralito batholith, an I-type, has been dated at 70-52 Ma.
Important mines and, or mineralized zones, include the San
Antonio, Santa Elena, Bonanza, Los Moros, El Contrabando,
and Cerro de Batamote localities (Cendejas-Cruz et al., 1992).
At the San Antonio mine, Dunn and Burt (1979) note the close
spatial relationship between high-grade scheelite mineralization
and small pegmatite dikes with mineralization in the carbonate
wall rocks. Deposits in the Baviacora district are indicated by
(Bv) on Figure 12. Other deposits of this type, but with more
intermittent production are in the Bacanora area, specifically La
Norteña mine. Other replacement deposits include the Virgen de
Guadalupe mine near Guaymas and deposits near Sahuaripa
plus El Saturno and Picacho. Further east, near Rio Yaqui other
deposits include Tecalote, Extension de Uranio, Cadena de
Cobre, and Coker. In some of these localities the age of the
intrusion is given as Laramide (Pérez-Segura, 1985). El
Nacimiento was described by Rocha-Moreno (1953). Near
Alamos, in the San Alberto prospect, scheelite is located in an
endoskarn below a quartzite of Mesozoic age (Pérez-Segura,
1985).
Pegmatite related tungsten mineralization of importance is
located east of the Yaqui river at Santa Rosa, San Nicolas, and
Santa Ana, all represented by SA on Figure 12, and also at El
Encinal, La Dura (El Tungsteno claim), El Trueno, and Llano
Colorado (Weise and Cardenas, 1945).
Vein-bearing tungsten localities that are hosted in granitoids
include La Paz and El Cobre; the latter was also worked for
molybdenum (Weise and Cardenas, 1945).
Another locality that has produced tungsten in Sonora as byproduct is the Washington mine where breccia pipe, polymetallic
mineralization includes scheelite. Sericite, in the alteration
assemblage, was dated at 46 Ma (Clark et al., 1979a).
In adjacent southwestern Chihuhua, the Guadalupana mine
(Schulze, 1953), tungsten mineralization occurs in quartz veins
and pegmatites that contain sheelite hosted by granodiorite.
There are also minor amounts of copper and molybdenum. The
age of sericite in the alteration envelope is 51 Ma (Clark et al.,
1979a).
In the San José del Desierto, Durango, porphyry copper
deposit, dated at 63.3 Ma (Clark et al., 1979a), tungsten is also
present (Carrasco-Centeño and, Cárdenas-Vargas 1975).
In the Rosario district of southern Sinaloa, tungsten and
molybdenum are found in the Guayabo mine where a quartz
vein contains wolframite and molybdenite, all hosted in
granodiorite (Bustamante-Yañez et al., 1992).
In central Mexico, scheelite was obtained as a by product of
gold and silver ore in fissure veins and replacements at the El
Maguey mine, located in the Comanja granite in the Sierra de
Guanajuato. Bismuth minerals have also been reported at this
locality (Yañez-Mondragon et al., 1992). The Comanja granite
has been dated at 53-51 Ma by Zimmermann et al. (1990).
In the last years of production at the Inguaran porphyry copper
deposit in Michoacán (Fig. 6), tungsten concentrates were
obtained by hydraulic mining and gravimetric concentration from
the 0.02 and 0.04 % W contained in the tailings (Osoria et al.,
1991).
Gold and silver deposits
There has been considerable interest in gold and silver over a
time span of nearly 500 years. Correspondingly, the relative
geological literature is extensive, but only the more modern
reports are mentioned here. In 1966, Edward Wisser
pronounced the Mexican volcanic province from the United
States border southward toward Mexico City, the Sierra Madre
Occidental (SMO), as the greatest epithermal silver province in
the world. Gross production to that time was estimated at $1.5
billion in silver and gold. His classic study explored the
relationship between mineralized structures of SMO, width of
vein deposit, and gold to silver ratios relative to base metals,
and height above the basement (pre-Tertiary volcanic
sequence). Wisser’s work concentrated on the important Au-Ag
deposits on the western flank of SMO. Extrapolation of the
geological sequences to the eastern flank of SMO along the
Altas Llanuras physiographic subprovince was made by Clark et
al. (1979b), who emphasized the presence of the prominent PbZn-Ag-Au-(Cu) assemblage, later acknowledged by Albinson et
al. (2001). Many, but not all fissure-vein deposits have proven to
be epithermal.
In 1991, Clark and Melendez divided all Au-Ag deposits into
11 subclasses including one non–geological type; namely
dumps and tailings that could be reworked by modern methods
of precious metals recovery. The reader is referred to this report
for details that, for space reasons, are not given in the present
analysis. The presently considered sub-classes of Au-Ag
deposits are 1) veins (epithermal, high and low sulfidation); 2)
mesothermal deposits; 3) mantos and chimneys; 4) replacement
(skarn and tactite); 5) by-product in porphyry copper deposits; 6)
by-product in massive sulfides; 7) placer; 8) disseminations in
sedimentary rocks; 9) disseminations in volcanic rocks; 10)
disseminations in intrusive rocks; 11) structurally controlled
disseminated deposits; and 12) hot spring deposits.
Gold and silver in epithermal vein deposits
These are a prodigious number of vein deposits in Mexico,
especially in western Mexico, as previously noted, and
associated with the Occidental arc of Clark et al. (1982), of
which only a small number (48) are shown in Figure 13 whose
age has been established beyond doubt. In a series of papers
Ferrari et al. (1999, 2002, 2007) recognized space-time patterns
of Cenozoic arc volcanism in Sierra made Occidental and the
Trans-Mexican Volcanic Belt (TMVB). Of particular interest
37
in the Late Cretaceous-early Miocene interval under
consideration are the igneous complexes recognized in the Late
Cretaceous-Paleocene plutonic and volcanic rocks; Eocene
andesites and lesser rhyolites of the Lower Volcanic Complex,
and silicic ignimbrites emplaced during two pulses in the
Oligocene (32-28 Ma) and early Miocene (24-20 Ma) and
referred to as the Upper Volcanic Supergroup Ferrari et al.
(2007). These events provide more details than the ignimbrite
flare-up from 40 to ~20 Ma (Coney, 1976) and the continuity of
magmatism and mineralization from ~140 to 16 Ma (Clark et al.,
1979a, 1982) that covered a wider region in northern Mexico.
Precious metal mineralization in the Late Cretaceous-early
Miocene interval (Clark et al. 1979a, 1982) was likewise more
precisely defined by Camprubi et al. (2003) who recognized
three epochs, namely 1) between ~48 and ~40 Ma (Eocene), 2)
between ~36 and ~27 Ma (Late Eocene-Oligocene), and 3)
between ~23 and ~18 Ma (early Miocene) which overlap three
of the main volcanic pulses.
Epithermal deposits are reported from 12 states that span 4845 (?) Ma at Batopilas to 18 Ma at Ixtacamaxtitlán, Puebla
(Tritlla, et al., 2004). Additionally, mineralization is considered to
have occurred within less than 2 Ma after the associated
siliceous volcanic or related intrusive rocks were emplaced. New
ages of early Miocene mineralization in SMO were presented for
7 deposits: Lluvia de Oro, Durango; El Indio, El Zopilote, Santa
Maria de Oro, and La Yesca, in Nayarit; Cinco Minas, Jalisco;
and Mezquital del Oro Zacatecas. These ages, from southern
SMO contrast with late-Eocene-Oligocene ages noted by Clark
et al. (1979a) for vein deposits located further north in SMO. The
new determinations support similar ages found at other localities
at similar latitudes, and north of the TMVB, at El Oro, 27 Ma
(Albinson et al., 2001), state of Mexico; Bolaños, 22 Ma (Lyons,
1988), San Martin de Bolanos, 23-21 Ma (Scheubel et al., 1998),
Jalisco; and at Pachuca-Real del Monte, 20 Ma (Mckee et al.,
1992), Hidalgo. The latter deposit is located well east of SMO
(Geyne et al., 1963; Dreier, 1976), and an early Miocene age of
mineralization was first reported by P. E. Damon (personal
communication, 1991).
Evidence of Oligocene or older epithermal mineralization south
of TMVB was also presented by Camprubi et al. (2003), at the
La Guitarra deposit, Mexico state, which probably occurred after
the emplacement of a rhyolite neck dated at 35 Ma. This
determination supports Oligocene age mineralization in the
southern part of the Occidental arc (south of TMVB) recognized
at Taxco, 38-36 (?) Ma (Alaniz-Alvarez et al., 2002) and at Real
de Guadalupe, 40-37 Ma (Albinson and Parrilla 1988), both in
Guerrero. The reader is referred to Camprubi et al. (2003, table
1) for a complete list of deposits and their ages that are shown
on Figure 13. Subsequently, the age of El Barqueño deposits
has been determined by Camprubi et al. (2006) at ~58 Ma and
as such documents an earlier interval of Paleocene Au-Ag
epithermal-vein mineralization than had been previously
recognized.
Another aspect evident in Camprubi et al. (2003, fig. 3) is that
in southern SMO there is a younging of volcanism from east to
west, together with ages of epithermal deposits, as predicted by
Clark et al. (1979a, figs. 3, 4), during the regressive sweep of
magmatism during late Oligocene-early Miocene time. Of further
interest in fissure-vein deposits is the possibility of encountering
stockworks in the wall rocks, as at Las Torres mine, Guanajuato
district and at Real de Angeles, Zacatecas (Pearson et al., 1988)
and flooding of permeable wall rocks with mineralizing fluids as
seen at Dolores (see Overbay et al., 2001, and MelendezCastro, 2009).
In summary, the unweighted average of 27 major vein
localities for which ages and grades were reported by Albinson
et al. (2001), and shown among others in Figure 13, is 8.29 g/t
Au, and 331 g/t Ag (Clark and Melendez, 1991). The unique,
native-silver deposit at Batopilas, said to carry >2,000 g/t Ag in
some pockets, was not included in the average.
Gold and Silver in Mesothermal deposits
The recognition of mesothermal vein deposits located along
the trace of the Mojave-Sonora megashear in northwestern
Sonora was made by Albinson (1989). Subsequently, Iriondo
and Atkinson (2000) proposed that these deposits were
generated during the Laramide orogeny. Included in this group
from northwest to southeast are Sierra Pinta, Quitovac, La
Herradura. Tajitos, Campó Juarez, San Felix, El Chanate, and
San Francisco (Pérez-Segura, 2008), see also Figure 13. The
age of these deposits falls within the interval 60-40 Ma (late
Paleocene-Eocene). They are further characterized by low
Ag:Au ratios (<10:1), variable age host rocks, and temperatures
of homogenization of fluid inclusions in two populations: a first
stage of 300 ± 50°C and a second stage of 200 ± 50°C among
other features (Pérez-Segura, 1993, 2008).
Of further interest in this belt of gold deposits is the recognition
of thrust faults in the Quitovac area that varies gradually from N
63°W to N23°E and has been interpreted by Iriondo et al (2005)
to represent a clockwise rotation in the direction of Laramide
thrusting through time. The onset of thrusting occurred between
75 and 61 Ma and terminated ~39 Ma. The emplacement of
mesothermal (orogenic) gold mineralization is spatially
associated with thrusting.
Gold and silver in mantos and chimney deposits
Some of the best examples are in Chihuahua at Santa Eulalia
and Naica. Silver and to a lesser extent gold are part of the
polymetallic assemblage. For example at Santa Eulalia, silver
and gold values are 320 g/t Ag and trace gold in the mantos
(Megaw et al., 1988), but there is significantly less silver (250 g/t
Ag) and trace Au in the chimney deposits. At Naica, the chimney
values are 200g/t Ag and 0.5 g/t Au, and at Mapami (Ojuela),
Durango, mantos carry 200 g/t Ag and trace Au, whereas
chimneys average 50 g/t Ag and 3.0 g/t Au. For other manto and
chimney values the reader is referred to Megaw et al. (1988,
table 1), and Megaw et al., (1996, table 1). The distribution of
mantos and chimneys is shown in Figure 7.
Overall, for 8 deposits, using the data of Clark and Melendez
(1991), and the silver values at Santa Eulalia of Megaw et al.
(1988), and the values at Ojuela cited above, but excluding the
very high grades at San Pedro Corralitos, Chihuahua, the
unweighted average is 0.67 g/t Au and 305 g/t Ag.
Gold and silver in skarn deposits
Examples taken from Megaw et al. (1988, table 1), show
skarn at Santa Eulalia carrying less silver than mantos and
chimneys, namely 125g/t Ag and trace Au, and there are similar
silver but higher gold values at Naica; 150 g/t Ag and 0.3 g/t Au.
At La Encantada, Coahuila, skarns, carry 200 g/t Ag and there is
no report of Au. Silver values in this deposit are 250 g/t and 400
g/t in chimneys and mantos respectively. The distribution of AuAg-bearing skams is also shown on Figure 7 and coincides with
38
39
Figure 13. Distribution of gold and silver in epithermal fissure-vein deposits, and gold with minor silver values in mesothermal deposits, Late Cretaceous-early
Miocene (modified after Wisser, 1966; Clark et al., 1979a, b; Albinson et al., 2001; Camprubi et al., 2003; Camprubi and Albinson, 2006, 2007; Pérez-Segura,
2008. From Clark and Fitch 2009).
1989a). For individual ore body values see Rivera-Abundis et al.
(2009c).
Gold and silver disseminated in volcanic rocks
Two large tonnage examples illustrate this subclass (Fig. 14):
Mulatos, Sonora (Staude 2001; Pérez-Segura y Ochoa-Landín,
2009) and El Sauzal, Chihuahua (Charest, 2004; Weiss et al.,
2009). The values used for Mulatos are for the San FranciscoCerro Estrella deposit (40Mt , average 1.79 g/t Au) and for El
Sauzal is 2.0 MT at 3.4 g/t Au. The unweighted average metal
content for these two deposits is 2.2 g/t Au.
the manto and chimney deposits. Gilmer (1987, fig. 95) noted
the strong correlation of these deposits at basin-uplift margins.
The unweighted average grade of 6 skarn deposits cited by
Clark and Melendez (1991), for which chimneys and mantos are
largely absent, is 0.4 g/t Au and 196 g/t Ag.
Los Filos, Guerrero is a complex of three major deposits:
Nukay, Bermejal, and the Los Filos-Aguita open pits. Los Filos
mineralization is associated with a diorite sill and grandiorite
stocks emplaced in calcareous rocks of Late Cretaceous age
with the formation of skarn deposits. Bermejal and Nukay have
similar geologic frameworks. At Los Filos 75% of the gold is
within the diorite sill, whereas Bermejal exhibits a Cu-Pb-Zn-Au
assemblage, and at Nukay gold is accompanied by Ag, Bi, Te,
Cu, and Zn in exoskarn. Total proven and probable reserves
contain 6.555 M oz Au (see Rivera-Abundis et al., 2009a), and
gold recovery is by heap leach methods.
Gold and silver disseminated in intrusive rocks
Four deposits are included in this subclass, all in Sonora (Fig.
14). Cerro Colorado and Magallanes are associated with rhyolite
domes; and Banco de Oro, in the same range as the Tajitos
fissure-vein mineralization, is partially hosted by rhyolite
porphyries and granite Silberman et al. (1988). La Choya is
hosted by granite (Summers et al., 1993, 1998) and is also
considered under structurally-controlled deposits. Silver values
are recorded only at Magallanes (21 g/t Ag), and at Banco de
Oro (1 g/t). The unweighted average gold value for these four
deposits is 0.95 g/t Au.
Gold and silver in porphyry copper deposits
Porphyry deposits with average gold content ≥ 0.4 g/mt may
be defined, albeit arbitrarily, as gold rich (Sillitoe, 1979). In
Mexico, Cananea is cited as carrying 0.1 g/t Au (Barton et al.,
1995), and three other deposits that carry small quantities of
gold and, or silver are Washington, O.17 g/t Au, 16 g/t Ag; La
Verde 0.08 g/t Au, 2 g/t Ag (Clark and Melendez, 1991) and EI
Arco, O.3 g/t Au (Barton et al., 1995). Porphyry deposit locations
are shown in Figure 6.
Thus, for four deposits, the unweighted average grades are
0.16 g/t Au and 4.5 g/t Ag. However, with the huge reserves at
Cananea of 1.8 Gt, it could be considered as a major gold
deposit of 18,000 kg (5.8 M oz) Au.
Gold and silver in structurally controlled deposits
These deposits have been defined by Graubard (1984) for
gold mineralization associated with low-angle shears and faults
related to thrusting or detachments. Two examples are cited that
exemplify this subclass that occur in northwestern Sonora
(Clark, 1998; Pérez-Segura et al., 1998). The La Herradura
deposit (Fig. 14) is structurally controlled and is located in
metamorphic rocks of Precambrian age that includes biotite- and
quartzo- feldspathic gneisses and chlorite schists. These rocks
have been broken by a series of faults due to Tertiary
reactivation of the Jurassic-age Mojave-Sonora megashear
faults (De la Garza et al., 1998). The geology of the oxidized
deposit is dominated by strongly sheared gneisses that are
present in northwest-trending lithotectonic slices, 20-50m wide,
bounded by high-angle faults. La Herradura is further classified
as a mesothermal, low sulfidation deposit. The metamorphosed
and igneous rocks at La Herradura generally resemble those at
both Cargo Muchacho and Mesquite in southern California, and
the structural grain resembles Mesquite, although the La
Herradura gold-bearing quartz veins appear more like Cargo
Muchacho (De la Garza et al., 1998). The deposit contains
approximately 93,330 kg (3 M oz) Au at an average grade of 1.0
g/t Au (de la Torres-Carlos, 2009); and is exploited by open-pit
methods. Estimated geological resources (measured and
indicated) have a cut-off grade of 0.35 g/t Au, and there are
three mineralized zones: Centauro, Yaqui, and Duñas.
Further southeast in the Sonoran disseminated gold belt
(Jacques Ayala and Clark, 1998), a smaller structurally
controlled deposit occurs at Lluvia de Oro where gold is
disseminated in zones of Cretaceous sedimentary rocks
associated with a northeast-trending shear. Reserves before
initiation of open-pit mining were 4 Mt at a grade of 1.0 g/t Au
and 2.95 g/t Ag (Teran-Cruz, 1998). Mineralization is considered
to be Oligocene age (Rothemund, 2000). Regionally, the deposit
is located on the west side of the Magadalena core complex,
resulting in displacement of previously emplaced rocks to the
southwest along the Magadalena detachment fault in late
Gold and silver in massive sulfide deposits
Using 37 deposits in Miranda (1995), the unweighted average
content is 0.93 g/t Au and 189 g/t Ag. However all of these
deposits are older than the late Cretaceous-early Miocene
mineralizing epoch. Their distribution is shown in Figure 5.
Gold and Silver in Placer Deposits
Because of the Recent age of these deposits, discussion of
their gold content is withheld for the latest section of this report.
Gold and silver disseminated in sedimentay rocks
Gold is disseminated in 4 sediment-hosted deposits in Sonora
(Fig. 14): Amelia, Santa Gertrudis, Lluvia de Oro, and El
Chanate (Crespo, 1998; Newell et al., 2004; 2005 and 2009).
Using the low end of the range values cited by Clark and
Melendez (1991), and a revised figure for Santa Gertrudis (AlbaPascoe, 1998), the unweighted average is 1.72 g/t Au and 2 g/t
Ag, with a typically low Ag: Au ratio for these deposits. Silver
values are only recorded at Lluvia de Oro (Rothemund et al.,
2001).
The Real de Angeles, bulk minable deposit carries 75g/t Ag,
1.0 Pb, 0.92 % Zn with recoverable Cd values. Mineralization is
located in Cretaceous age siltstones, in the form of veins and
veinlets, which form a stockwork in the shallow parts of the ore
body, with iron sulfides disseminated along bedding planes
(Pearson et al., 1988). In the San Martin, Queretaro, sediment hosted deposit, gold and silver are found in breccias, 2-5 g/t Au
and 40-80 g/t Ag; in mantos, 3-5 g/t Au and 40-60 g/t Ag; and in
stockworks, I-2 g/t Au and 50 g/t Ag (Ortiz -Hernandez et al.,
40
41
Figure 14. Late Cretaceous-early Miocene gold and silver deposits disseminated in sedimentary, volcanic, and intrusive rocks, plus those that are structurally
controlled by shears and detachments. La Choya and Lluvia del Oro appear in two subclasses and all four subclasses are underlain by Proterozoic terranes in
Sonora. From Clark and Fitch (2009)
Oligocene-early Miocene time (Nourse, 1992). As a
consequence of detachment faulting the gold-bearing host rocks
are bounded on both sides and underlain by lower plate
metamorphic and igneous rocks of the core complex.
Other deposits, in this subclass, include La Choya (Summers,
1993, Summers et al., 1998), San Francisco (Pérez-Segura, et
al., 1996) and Quitovac (Caudillo and Oviedo, 1990; Iriondo et
al., 2005) in Sonora; El Triunfo-San Antonio (tajo San Antonio,
Carrillo-Chavez, 1996), Los Uvares (Carillo-Chavez, 1990;
Carrillo-Chavez et al., 1999), and Paradones Amarillos (Mine
Developoment Associates, 2007), all in Baja California Sur. At
Quitovac (Fig. 14) gold-bearing quartz veins are hosted in
dynamically metamorphosed greenschist facies rocks (Iriondo
and Atkinson, 2000). In Sonora, new H and O isotope data
indicate that water in the mineralizing fluid is of metamorphic
origin. This type of vein formation needs an accompanying
orogeny to dehydrate basement rocks to form the mineralizing
fluids. Consequently, Iriondo and Atkinson (2000) conclude that
the Laramide orogeny (~80-40 Ma) is a preferable mechanism
than the Mojave-Sonora megashear to explain gold
mineralization in northwest Sonora. Additionally, they propose
that the roots of Laramide thrusts and associated structures
acted as the deep conduits for the ascent of metamorphic fluids.
At Paradones Amarillos (Mine Development Associates,
2007), Baja California Sur (Fig. 14), the mineralization occupies
a cataclastic zone of more than 10 km lateral extent between
granodiorite (91 Ma, Echo Bay, 1997) and a diorite complex
(129 Ma). The deformed diorite complex was intruded by
granodiorite and a low-angle fault forms the contact between
these units with the diorite in the hanging wall and grandiorite in
the footwall. Sericite in the cataclasite was dated at 91-79 Ma.
The average grade of the deposit is 1.0 g/t Au (Mine
Development Associates, 2007), and there are similarities with
the cataclastic shear zone at Los Uvares (Bustamante-Garcia,
1999).
Accordingly, for 8 deposits where credible values have been
reported, and using the lower assay values at Quitovac, El
Triunfo-San Antonio, and San Francisco, the average
unweighted value is 1.33 g/t Au, but the large tonnage at La
Herradura skews the average towards 1.0 gt Au. Silver is
recorded only at Lluvia de Oro (2.95 g/t Ag).
Complex. Other possibilities include association with magmatic
segregation iron deposits in Chihuahua, Coahuila, and Durango;
and phosphate deposits in Baja California Sur.
At the Picacho igneous complex in the northwestern part of the
Sierra de Tamaulipas (Fig. 15), REE have been reported by
Elias-Herrera et al. (1990, 1991). The age of this complex is
probably between early Oligocene to mid-Miocene. Nephelinebearing rocks from a stock at the center of the complex host
radioactive veins rich in apatite with REE mineralization from 1.3
to 3.0 percent. The petrogeneis of these rocks have been further
studied by Ramirez-Fernandez (1996) who also recognized a
minor carbonatite phase. The tectonic implications were noted
by Ramirez-Fernandez and Keller (1997, 1998) who recognized
in northeastern Mexico a transition from a subduction regime to
intraplate extension. Subsequently, Ramirez-Fernandez et al.
(2,000) reported three REE-bearing minerals: bastnäsite,
britholite, and cheralite from the carbonatite phase.
Carbonatites were first recognized in north-central Chihuahua
during 1992-93 and briefly reported on by Comaduran et al.
(1996). Three localities near Villa Ahumada were further
discussed by Nandigam et al. (1997, 1999, 2009). These
localities include from north to south, Mariana, a carbonatite
breccia that intrudes granite porphyry and which was tested by
nine reverse circulation holes. A small carbonatite plug and dike
that cut a Tertiary-age tuff comprise the El Indio locality. Yuca is
a stock-like intrusion, 900 m in diameter displaying roof
pendants at its crest, around which are located smaller bodies of
40
39
breccia, dikes, and sills. Radiometric Ar/ Ar determinations
show that the age of intrusion is 36 Ma (Nandigam, 2000,
Nandigam et al., 2009). Compositional types recognized, from
least to most highly differentiated, are magnesio-, calcio-, and
ferrocarbonatites, and the concentration of Th, U, Nb, Y and
REE increases with differentiation. The Yuca carbonatite is
associated with up to 2,000 times enrichment of La and Ce with
respect to chondrite.
Potential sources of lanthanide concentrations in Mexico,
within the time frame being considered, are iron deposits
containing apatite (and hence Ce), tin-bearing granite with
concentrations of xenotime (Y), syenites and pegmatites that
bear monazite (Ce), and phosphorites that may contain Ln and
Y. For more details on these localities the reader is referred to
Gomez-Caballero (1990, fig. 2).
Gold and silver in hot spring deposits
Because of their late Tertiary-Quaternary age these deposits
will be discussed in a later section, see also Figure 16.
Mississippi Valley type deposits
Sierra Mojada, on the margin of the Sabinas basin, has
already been included in limestone replacement deposits of
Figure 7, based on temperatures of formation greater than
200°C (Megaw et al, 1988). Nevertheless, evidence of Pb and
Zn oxide mineralization south of the Sierra Mojada fault
suggests low temperature, MVT mineralization (Tritlla et al.,
2006, 2007, 2009).
In the Sierra de La Purisima and Sierra San Marcos in the
Mineral de Reforma district, Coahuila (Fig. 15) stratabound
mineralization is controlled by a reef facies of the Early
Cretaceous Cupido Formation, and consists of sphalerite, barite,
and siderite with traces of pyrite and chalcopyrite, all of which
have been affected by supergene alteration. Scarce fluid
inclusion data from the hypogene minerals fall between 100150°C with salinities between 7.5 and 20 wt% eq. NaCl
(Gonzalez-Ramos, 1984).
In northeastern Chihuahua, at the Tres Marias mine, SainEidukat et al. (2008) have identified botryoidal sphalerite with
Rare Earth Elements
Apart from small concentrations of Rare Earth Elements
(lanthanides) in igneous rocks that relate to their petrogenesis,
and in meteorites, there are, to date, no known concentrations of
these metals sufficiently high to warrant extraction in Mexico.
Nevertheless, there are several sub-economic deposits that
merit brief discussion.
In 1990, Gomez-Caballero surveyed the Rare Earth Elements
(REE) localities and divided deposits into primary and secondary
environments. The secondary environment includes placer
deposits that will be described in the latest mineralizing epoch.
In the primary environment, REE are located in Mexico in
carbonatite and alkaline rocks. Alkaline rocks were identified as
the eastern magmatic province, Tamaulipas and Veracruz
states; the southern prolongation of the Rio Grade rift zone in
Chihuahua and Coahuila; and also in the Proterozoic Oaxaca
42
43
Figure 15. Rare earth element, Mississippi Valley type, and mafic-ultramafic rock deposits of the Late-Cretaceous-early Miocene interval. From Clark and Fitch,
2009).
inclusions of bitumen. The authors conclude that the initial
mineralization of Ge-rich sphalerite and galena plus abundant
bitumen could have formed by MVT processes that was later
affected by a higher temperature siliceous alteration that
includes the formation of willemite and hemimorphite.
These three deposits are thought to have formed from marine
water trapped in sediment (connate water) and expulsed during
Laramide orogenesis. As such, their age of formation is
imperfectly known.
The development of the dominantly calc-alkalic Trans-Mexican
Volcanic Belt (Gómez-Tuena et al., 2007), from late Miocene
onwards resulted from subduction of the Cocos and Rivera
plates (Nixon et al., 1987). Near its eastern extremity is the San
Andre Tuxtla center and further south is the Modern
Chiapanecan arc (Damon and Montesinos, 1978). With the
exception of placer and beach deposits, the remaining deposits
discussed in the final 16 Ma interval are related directly or
indirectly to tectonic and magmatic events in the continental and
oceanic environments.
Mafic-ultramafic- rock deposits
Two localities have been identified by Ortiz-Hernández et al.
(2003). In the Yuma terrane (Fig. 15), Ojos Negros, Baja
California Sur comprises metagabbro, and dikes of dolerite and
granophyre. Apart from talc and serpentine and stockworks of
magnesite, there are low contents of Ni (298 ppm) and Cr (140
ppm).
The San Javier, Sinaloa locality is situated at the junction of
Cortez and Guerrero terranes (Fig. 15) and is associated with
several diatremes, 20-600 m width, of micaceous peridotite of
possible kimberlite affinity. These rocks, emplaced in black
slates that exhibit intercalations of limestone and polymictic
conglomerate of late Mesozoic age, have been converted by
metamorphism to greenschist facies (Servais et al, 1982, 1985).
Trace amounts of Au, Pt, Pd, Rh, Ir, Ni, and Co were recorded
by Cerecero-Luna et al. (1984). Additionally, veins and lenses
bearing significant Au-Ag values have been emplaced in the
quartzites of the sedimentary sequence at Real de San Javier.
Details of the San Javier locality are given in Ortiz-Hernández et
al. (2006).
Mineral Deposits (< 16 Ma)
Following the wealth and diversity of mineralization in the late
Cretaceous - early Miocene epoch, the mid-Miocene to Recent
interval reveals still other classes of mineralization due to
changes in tectonic regimes, accompanying magmatism and
sedimentation. These regimes, offer prime exploration
possibilities inasmuch as they appear to have been
underexplored to date. Among the mineralization that occurs in
this interval are porphyry copper and contact replacement,
stratabound copper and manganese, placer gold and tin, blacksand beach, hot-spring gold, geothermal, and black and white
smoker and shallow water hydrothermal vent deposits.
Porphyry copper and contact replacement deposits
The Chiapanecan volcanic arc hosts a porphyry copper
deposit; which is located above the Mayan terrane in Chiapas
(Fig. 16). Among a number of significant mineral deposits
recognized in the Miocene arc, Damon and Montesinos (1978)
refer to Toliman, that occurs in the southeastern corner of the
state, as having all the aspects of a classic disseminated
porphyry copper deposit, whose age is 5.8 Ma.
Across the state line in adjacent Oaxaca, the La Carmen
prospect is a contact metamorphic deposit of granodiorite
intruding limestone. Magnetite is predominant with lesser
amounts of sphalerite (Damon and Montesinos, 1978). Two K-Ar
dates on the granitoid phases fall between 13 and 12 Ma. In
regard to exploration potential, the Chiapas Massif is regarded
by Damon and Montesinos as most probably being an excellent
target for Miocene mineralization associated with the roots of
stratovolcanoes as outlined by Sillitoe (1973).
There is one mineralized locality within the modern
Chiapanecan arc, and this occurs at Santa Fe (Pantoja-Alor,
1991), where gold mining took place before and after the
Revolution, with latest production at La Victoria mine in 1972.
Sulfide minerals are abundant within a stock that intrudes
Oligocene-age limestones. The Santa Fe mine has been
designated as a porphyry copper deposit on Figure 16 but it also
has characteristics of a contact metamorphic deposit, and vein
mineralization has also been exploited. Several K-Ar dates on
intrusive phases in the Santa Fe area consistently fall between 3
and 2 Ma (Damon and Montesinos, 1978).
MID-MIOCENE TO RECENT
This section examines the last epoch, approximately 16-0 Ma,
in which metallic minerals formed and is based on Clark and
Fitch (2005). Metallic mineral deposits that have been
isotopically dated, or those for which there is good geological
reasoning to ascertain their age, are shown on Figure 16.
Lithologic and tectonic framework
Subduction by plate convergence, so prevalent in northern
Mexico during the preceding interval, was replaced by the
Pacific-Cocos-North America triple junction migrating south
along the western edge of the continent. By 16 Ma it was west
of northern Baja California, and by 12 Ma west of southern Baja
California (Lonsdale, 1989, 1991), and from 12 to approximately
5 Ma, the triple junction was south of Cabo San Lucas. Late
Miocene (8 to 5.5 Ma) extension produced an embayment in the
continental margin and this was followed by opening of the Gulf
of California. Subsequent rifting and spreading in the gulf
produced faults connected northward with the San Andreas fault
(Sedlock et al., 1993), see Figure 16.
Accompanying these tectonic events were the development of
several volcanic provinces around the Gulf of California (Gastil
et al., 1979). Extrusions varying in composition from rhyolite to
basalt were deposited on both sides of the northern part of the
gulf, and basaltic rocks are prevalent in the southern part of the
peninsula. Alkalic volcanism occurred in the Baja California
peninsula from 13 Ma onwards and has been dominant since
about 10 Ma (Sawlan, 1991) during rifting and development of
the Gulf of California. Marine strata of late Miocene age are
located in the eastern gulf and nearby coastal areas, in contrast
to marine Pliocene sediments that are found in the western gulf
and coastal Baja California.
Gold and silver deposits
In northern Baja California at San Felipe (Fig. 16), a significant
deposit was exploited by Empresas Frisco S. A. Mineralization
occurs in a low-angle, quartz-filled structure that contains
epithermal veining, breccia, and stockwork. The footwall rocks
are a metamorphic complex of Paleozoic age, an overlying
hanging wall of andesites of Cretaceous age, explosion breccias
and rhyolites dikes of late Tertiary age (Ibarra-Serrano, 1996;
see also Ibarra-Serrano, 2009). Resources/reserves
amounted to 260,000 oz Au and 1,680,000 oz Ag, with average
44
45
Figure 16. Placer, beach, geothermal, stratabound, porphyry copper, hot spring, other Au-Ag, Fe, black smoker, and shallow water hydrothermal vent
deposits of the late Miocene-Present interval (modified from Clark and Fitch, 2005, 2009).
grades of 4.6 g/t Au and 309 g/t Ag. The age of the deposit is 9
Ma or slightly younger, and conforms to Miocene-Pliocene
volcanic rocks in this area (Gastil et al., 1975).
The Ixhuatan deposit is located 5 km south of Santa Fe in
Chiapas. Mineralization contains significant Au-Ag values and
occurs in diatremes areally associated with alkalic basalts and
andesites (2.8 Ma), that have been intruded by diorites and
granodiorites of the same age (see Miranda-Gasca et al., 2009).
of California developed, were points of egress of hydrothermal
fluids, leading to early diagenetic or syngenetic deposition of
metals that invaded the underlying conglomerates, and overlying
slump breccias and mud and fine ash at the base of mantos. A
deep source of hydrothermal brines is consistent with present
day geothermal fields in the region and the high geothermal
gradient of 1°C per 15 m depth in the Boleo district, (see also
Ochoa-Landin et al., 2009).
The Lucifer manganese deposit, situated 17 km northwest of
Santa Rosalia, was developed in 1941, and became a leading
producer in Mexico (Wilson and Veytia, 1949). Ores of
manganese and copper occur in latitic to andesitic tuff of the
Boleo Formation, which is underlain by the Comondú andesitic
and basaltic flows, tuffs, and agglomerates, and overlain by the
Gloria Formation and younger formations referred to above. The
ore at the Lucifer mine forms a gently dipping tabular deposit
enclosed in tuffs, with thickness varying from 1 to 6 meters. A
flat fault lies above the ore zone, and normal faults offset the ore
body by as much as 8 meters. Ore minerals include fine-grained
cryptomelane and pyrolusite, and contain 45 to 50 percent
manganese. Wilson and Veytia (1949) favored a hydrothermal
origin in which solutions rose along faults in the underlying
Comondú volcanics, and then spread laterally in the tuff beds,
with partial replacement. Overall, the origin of the Lucifer
manganese ores was considered to be closely related to the
Boleo copper deposits in which manganese is a common
constituent.
A later study by Freiberg (1983) recognizes textural and
depositional characteristics of the ore deposit and host
sedimentary rocks that imply formation in a shallow marine basin
and whose ore solutions originated from hot springs. Upon
venting, the mineralizing solutions mixed with and impregnated
the matrices of pyroclastic materials of the Boleo Formation. The
contemporaneous deposition of tuffs with manganese suggests
a relation to a magmatic source. Manganese was envisaged as
being scavenged by hot aqueous solutions from the Comondú
volcanics. The hydrothermal solutions were probably derived
from seawater which interacted with the Comondú volcanic
rocks in a convection system similar to Recent mineralization in
spreading environments at mid-ocean ridges (Freiberg, 1983).
Stratabound copper-cobalt-zinc and manganese ores
The Boleo district, at Santa Rosalia, on the east coast of Baja
California Sur (Fig. 16), has been a producer of copper for over
120 years. Estimates are that about 19 million tons of ore at
approximately 4 % copper have been produced during this time
(Escandon-Valle, 1996). Of this, just over 13 million tons
averaging 4.8 percent copper were produced by a French
company between 1886 and 1947. Some of the earliest reports
on the deposit were published by Fuchs (1886) and Saladin
(1892). The district extends along the coast and around Santa
2
Rosalia for over 60 km . Recent measured and indicated
resources are as follows using 3D block models and based on a
copper equivalent (Cu eq) cut-off grade of 0.5% (Cu eq = Cu+
15 Co/1.5 + 1.2 Zn/1.5); 277.2 Mt, 1.77% Cu eq; 0.77% Cu,
0.06% Co, 0.62% Zn, and 3.00% Mn (Albinson and Hodson,
2007).
The principal lithologies at Boleo consist of pre-Tertiary quartz
monozonite, overlain by the Miocene Comondú Formation
andesite tuffs and agglomerates, (24 to 11 Ma), followed by the
late Miocene-early Pliocene Boleo Formation conglomerates,
sands and tuffs of intermediate composition, all overlain by the
Gloria Formation of middle Pliocene age consisting of a
sequence of sands and agglomerates, followed upwards by the
Infiemo and Santa Rosalia Formations and finally by the Tres
Virgenes (Pleistocene-Recent) volcanic rocks and terrace and
alluvial deposits (Wilson and Rocha, 1955). The close of
subduction and the change to extensional tectonics, includes
pre-rift (12.5-10 Ma) high K-basaltic lavas, and synrift (9-1 Ma)
alkalic basaltic andesites (Sawlan and Smith, 1984).
The Boleo deposits occur in the Santa Rosalia basin which
formed as a consequence of late Miocene rifting. In the five
mantos, numbered from 0-4 from the surface downward, the
mineralized beds are flat lying, 1-15 m in thickness and laterally
continuous, being disrupted by faults from a few tens to as much
as 250 m vertical displacement. The ore minerals include
chalcocite, chalcopyrite, digenite, carrolite, pyrolusite, sphalerite,
and pyrite. Some 40 percent of the copper, cobalt and zinc are
found within the molecular structure of montmorillonite.
The origin of the Boleo deposits has been addressed by
Touwaide (1920), who favored a copper source formed by
leaching tuff by connate and ground water, and by EchavarriPérez and Pérez-Segura (1975), who noted the presence of
framboidal pyrite, and the absence of replacement textures.
They concluded the deposit was formed by syngenetic
processes, at an early diagenetic stage in a reducing medium
caused by bacterial action. A recent assessment of the Boleo
ore is by Bailes et al. (2001), who note that many of the pyrite
framboids are rimmed to varying degrees by chalcocite, and
cobalt is associated with several sulfide minerals, including
enrichment rims on chalocite and chalcopyrite, possibly in the
form of carrolite (Cu(CoNi)2 S4). Secondary ore minerals in
mantos are products of chemical breakdown of primary sulfides
above the water table. Bailes et al. (2001) conclude that
mineralized fractures, related to extensional faulting as the Gulf
Placer deposits
The preponderance of fissure-vein gold deposits in the
western part of Sierra Madre Occidental dictates the abundance
of gold placers in the western part of Mexico. In Sonora, West
(1993) has plotted the placer fields that were discovered in the
northwestern part of the state (Fig. 16). From south to north
these are Cieneguila (La Cienega, 1770); Santa Rosa de
Buenavista (1775); San Francisco de Asis (EI Boludo, 1803);
Las Palomas (1935); La Basura (1835); EI Tren (1844); EI Zoni
(1844); Quitovac (1835); and Sonoita (no date). These deposits
are collectively situated on the flanks of ranges in the
subprovince of Buried Ranges (King, 1939) and they are inclined
towards adjacent basins. Individually published reports include a
description of the placers of Sonora (Tyler, 1891), dry placers in
northern Sonora (Merril, 1908), La Cienega (Hill, 1902), Rio
Yaqui (Anonymous, 1994), and El Boludo (Orozco-Fararoni,
1997; Wilson et al., 2009). For other localities in Sonora refer to
Cendejas-Cruz et al. (1992).
The EI Boludo deposit was described in detail by Wilson
(2001), and Wilson et al. (2003, 2009). This is the only locality in
Sonora, and the whole of Mexico, where modern mining is
46
taking place by way of a unique air-separation technology
(Piggott, 1999, 2000). At EI Boludo, gold-bearing veins, found in
the foothills of the adjacent Sierra La Salada are hosted by
Proterozoic quartz-feldspar gneiss. Gold eroded from these
veins has formed an extensive placer in three distinct
sedimentary horizons comprising pediment and alluvial fan
deposits rather than a lacustrine origin, as envisaged by Radelli
and Pérez-Segura (1992). Au/Ag ratios determined by electron
microprobe analyses were used to match placer gold particles
with their primary vein source. The total resource of goldbearing sediments contained in the El Boludo placer field,
amenable to air separation technology, has been estimated at
3
3
25 Mm , grading 0.26 g/m Au (Wilson et al., 2003).
In Sinaloa, the Bacubirito placer has been described by Abel
(1922), Hurst (1922), and Espinoza (1932); the Yecorato placer
by Canon (1971), and Slipp and Stuckenrath (1973); and the
placers of Rio Sinaloa by Cárdenas-Vargas and Camacho-A.
(1973). Other placer deposits in Sinaloa occur at EI Orito, in
terraces of the EI Fuerte river, EI Tambor, and in the Rosario
river (Bustamante-Yanez et aI., 1992).
At Santo Domingo, Chihuahua, on the Conchas River, the
gold-bearing placer has been investigated by Barry (1923), and
Clark (1987a, b). Potential for gold extraction from placers is
also cited at Placer de Guadalupe (Duran-Miramontes et aI.,
1994).
In Jalisco, a placer tin deposit at Rio de Lagos was described
by Pérez-Sodi and Larios (1949). In addition, there are three
other localities: Teocaltiche-Villa Hidalgo, Encarnacion de Diaz,
and Las Latas where placers are associated with primary tin
mineralization (Rodriguez-Medina et aI., 1992b).
In north-central Durango, tin placers have been described at
La Laguna de Santiaguillo and San Bartolo, to the north of the
valley of Canatlán (Rocha-Moreno, 1968). Tin placers also occur
at America-Sapioris, where cassiterite is concentrated in paleochannels and terraces that drain the primary deposits. Gold
placers are found in the western part of the state in the San
Lorenzo and Piaxtla rivers below vein deposits of Tayoltita and
other mines in the San Dimas district, and also in the Rio San
Diego, in the Pueblo Nuevo area (Carrasco-Centeno et aI.,
1993).
In east-central San Luis Potosi, there are metallic deposits in
two alluvial fans in the Guadalazcar district, near the villages of
Guadalazcar and EI Realejo. The sand and gravels have been
derived from the large Tertiary granite intrusion in this area. The
metallic content of gravels in the Las Papas area of the district
resulted in assays of 0.04% Sn, 0.0015% Hg, 9.44 g/t Ag, and
0.06 g/t Au (Fries and Schmitter, 1948).
The Pinzan Morado-Placeres del Oro area in Guerrero and
adjacent Rio del Oro and tributaries has been described by
Were-Keeman and Sanchez-Bautista (1975).
The placer deposits of peninsula California have been referred
to by Oviedo (1895), and in particular, at Calmalli (Bonifont,
1900; Islas-Lopez, 1999), located near EI Arco. The Rosario
gold placer is located southwest of the EI Triunfo-San Antonio
district in Baja California Sur (Bustamante-Garcia, 1999),
adjacent the nearby Juan Marquez deposit (Rocha-Moreno,
1973), and placer La Muela (Altamirano-Ramirez, 1971).
the metamorphic terranes of the Acatlán and Oaxaca
complexes. Fluvial transport was followed by concentration by
waves and currents of the Pacific Ocean. Three types of deposit
are recognized by Martín-Barajas (1987), namely, 1) rivermouth; 2) beaches and bars; and 3) eolian, depending on local
conditions in the littoral environment. The heavy minerals
investigated include magnetite, ilmenite, rutile, zircon, monazite,
and garnet. Localities include Playa de Cayacal, Bahia de Agua
Dulce, Boca del Rio Verde, Boca del Río Colotepec, and Playa
de Ventanilla (Fig. 16).
In the San Antonio del Mar marine terrace and modern beach
deposits in Baja California, magnetite and ilmenite occur with
traces of zircon and leucoxene (Islas-López, 1999).
Disseminated gold and silver in hot-spring deposits
A hot spring, epithermal gold province has been recognized
between Indio, California and San Felipe, Baja California
(Fischer-Watt Gold Co., 1991). Five deposits in this province are
located in Mexico (Clark and Melendez, 1991). The province is
characterized by crustal thinning, volcanism, and high heat flow,
on which was superimposed a northwest trending transcurrent
fault system (Sharp, 1982). Highly permeable fracture zones
formed conduits for hydrothermal activity that resulted in hot
springs and geyser development, and large alteration areas that
display silica veining and flooding, with potentially economic
concentrations of gold and silver. The prospects in Baja
California include Santa Lucia, EI Cerrito, Tina, and Patria, (Fig.
16). The age of the deposits is late Tertiary-Quaternary, and is
clearly associated with ongoing tectonic dislocation in the Gulf of
California and the San Andreas fault system. Because of paucity
of data the nearby Montezuma prospect is placed in the “other
Au-Ag deposits” category. Ignoring the very high values cited in
the Santa Lucia deposit, the average gold content of EI Cerrito,
Tina, and Patria is 1.6 g/t Au and < 56 g/t Ag. No production has
been recorded. These deposits are similar to the gold-bearing
Pliocene, hot-spring system at Modoc, located 24 km southsouthwest of Indio, California, where Fischer-Watt (1991) reports
a substantial deposit.
The polymetallic hot-spring deposits at Comanjilla and
Jacintos (Saldaña, 1990), adjacent the active El Bajio rangefront fault, Sierra de Guanajuato, appear to be the upper part of
the Buchanan (1991) precious metal deposits model, but may
not be directly related to the Guanajuato district mineralization
because of the large intervening time span (Randall, et al.
(1994).
Among the siliceous sinter deposits with a hydrothermal origin
discussed by Camprubi and Albinson (2007) and listed in their
compilation of epithermal deposits with an age of <18 Ma, is
Ixtacamaxtitlán, Puebla (see Fig. 13).
Terrestrial geothermal areas
Active high-temperature geothermal volcanic areas are of
interest in the search for base-metal and precious-metal
deposits, inasmuch as they provide valuable insight to systems
that existed in analogous older terranes (Henley, 1985). For
example porphyry copper-molybdenum deposits are found at
deeper levels within calc-alkaline volcanic systems, and Kurokotype massive sulfides are interpreted as sea-floor, telescoped
equivalents of terrestrial epithermal deposits. Typical models for
geothermal systems in andesite-volcanic terranes and silicicvolcanic environments have also been advanced. Thus, the
present rate of gold deposition at Waiotapa, New Zealand is
Beach deposits
Beach deposits of heavy minerals have been identified in
Guerrero and Oaxaca states (Rocha-Moreno, 1947; MartinBarajas, 1980). These accumulations have been derived from
47
known (Hedenquist and Henley, 1985), and from this, the time
needed to produce a multimillion ounce orebody, under the
same conditions, can be projected. In general, active geothermal
systems may contain subeconornic metallization, as for example
Mo and lesser amounts of Cu, Pb, Zn, and Mn, and anomalous
quantities of Au that have been documented in the Valles
Caldera, New Mexico (Goff and Gardner, 1994).
In Mexico, the relation between ongoing geothermal activity
and the presence of epithermal mineralization in older terranes
in the same locality is scant. In the vertically stacked are bodies
of the Guanajuato district (Echegoyen et al., 1970; Buchanan,
1981), a volcano-plutonic center (Randall et aI., 1994), the main
mineralization in the lower zone took place from 31 to 28 Ma
(mid-Oligocene). Structurally above, in the intermediate zone,
are two geothermal systems, Maren and EI Cubilete that are
peripheral to the deeply eroded caldera. The age of these two
systems is unknown in detail but is considered to be mid-to-Iate
Oligocene. They are 200 m above the lower zone and appear to
represent part of the upper parts of Buchanan's (1981),
epithermal, fissure-vein model. The upper zone, where
geothermal activity is presently active, is on the downthrown
side of the range-front fault of this district. At Jacintos and
Comanjilla, hot springs show values of several metals including
Au, Ag, Pb, Zn, and Cu (Saldaña, 1991). However, although the
mineralized zones are in close proximity, they may be unrelated
because of the large time span between present-day hot springs
and the intermediate and lower zones of mineralization (Randall
et al., 1994).
Throughout Mexico 1,283 thermal shows have been identified
including hot springs, fumaroles, sulfur springs, mud volcanoes,
water wells, or combinations thereof. They are grouped into 515
geothermal zones by assigning them to the same aquifer or by
having a common origin (Razo, and Romero, 1991). They are
further divided into seven provinces. Thus there are four
important zones in the San Andreas and Sonora-Sinaloa
province, closely spaced in northern Baja California, in or near
the Mexicali Valley and the trace of the San Andreas fault.These
include Riito, Guadalupe Victoria, Tulecheck, and Cerro Prieto.
In the latter, where power generation began in 1973, gold was
detected in the scale of two production pipes (Julio AlvarezRosales, personal communication, 2005; Alonso-Espinoza and
Mooser, 1964). Although no gold was reported from the Salton
Sea geothermal system, located 80 km to the north, there were
several sulfophile metals reported in concentrations up to 2,000
ppm (White, 1968). Coincidentally, or not, the hot-spring gold
prospects in northern Baja California, cited above, are located
west and south of Cerro Prieto and adjacent fields, see Figure
16.
In the Trans-Mexican Volcanic Belt the important geothermal
fields from west to east are La Primavera and San Marcos,
Jalisco; Araró and Los Azufres, Michoacán; Pathe, Hidalgo; and
Los Humeros and Las Derrumbadas, Puebla (Fig. 16). Most
thermal spots are closely related to fractures, or are between
acid volcanic structures or calderas (Razo and Romero, 1991),
particularly those associated with Recent fractures. Los Azufres
field has been described by Huitron et a!. (1991); Los Humeros
by Romero (1991); and La Primavera by Venegas-S. et. al.
(1991). At Los Azufres concentrations of Au are known in
alteration silica (Efren Pérez-Segura, written communication,
February, 2009), and the potential for encountering economic
mineralization in the TMVB seems evident. For a description of
the major geothermal fields in Mexico, see Hiriart-LaBert, (2009).
Deep-sea and shallow-water hydrothermal vents
Deep Sea hydrothermal vents (Black and White Smokers)
The final episode of metallization in the mid-Miocene to
Recent epoch is associated with the occurrence of hydrothermal
plumes associated with oceanic spreading centers at latitude
21°N and in the Guaymas Basin. Using a manned diving saucer
(Francheteau et aI., 1979) describe the hydrothermal emissions
that they discovered at latitude 21°N on the East Pacific Rise
(EPR) spreading center, some 250 km south of Cabo San
Lucas, Baja California Sur (see Fig. 16). The expedition was part
of the Rivera-Tamayo (RITA) study of the EPR. Two sites were
sampled on the lightly sedimented flanks of structural
depressions located approximately 600 to 700 m west of the
axis of the extrusion zone in which the youngest lavas occur.
Massive sulfides were sampled from various parts of irregular
columnar edifices, about 10 m high and 5 m wide. The material
consists of friable and porous components that are easily
broken. Ore minerals identified include sphalerite, pyrite,
marcasite, chalcopyrite, iron oxides, and amorphous silica.
Analyses were made by x-ray fluorescence and atomic
absorption spectrometry that also revealed the presence of Co,
Pb, Ag, Cd and Mn. The sulfide phases are mainly sphalerite
and pyrite with minor chalcopyrite and marcasite. The origin of
these sulfides is attributed to chemical precipitates when hot,
convectively circulating seawater leaches metals from the ocean
crust and forms the deposits at the basalt-seawater interface.
The resulting massive sulfides of ore grade can be preserved if
the bottom waters are anoxic or if the deposits become buried
under sediment or lava (Francheteau et aI., 1979).
In general, the active chimneys are classified as black
smokers if the precipitates are dark colored (high temperature),
or white smokers if the precipitates are light colored (lower
temperature). Pyrrhotite is the initial precipitate from the
undiluted hydrothermal fluid and is the dominant particulate in
the dark-colored plume issuing from vents. It may be replaced
by pyrite and marcasite (Zierenberg et aI., 1984).
Cu-Fe sulfides and anhydrite dominate the highest
temperature vents and cubic cubanite has also been identified.
Anhydrite is abundant in active vents and often contains finegrained inclusions of dendritic pyrite and chalcopyrite. Wurzite
predominates in active chimneys and is abundant in basal
mounds, whereas sphalerite and pyrite are more abundant in
inactive mounds. Minor amounts of marcasite occur with pyrite.
Sulfur isotope values of anhydrite and barite indicate
disequilibrium with coexisting sulfides and demonstrate that
seawater sulfate is their sulfur source. Oxygen isotope
geothermometry applied to anhydrite sulfate indicates
temperatures of 194 to 222°C (Zierenberg et al. 1984).
Precious metal values, with the exception of silver, are low.
Silver content is less than 240 ppm and is associated with zinc
sulfide phases rather than discrete silver-bearing minerals.
Silica, alkali-metals, and manganese are dispersed into the
water by hydrothermal plumes. Gold and platinoid values are
less than 0.2 ppm and 3.0 ppb respectively (Zierenberg et aI.,
1984).
Another locality, but within the central part of the Gulf of
California, where sea floor spreading hydrothermal activity has
been documented, is in the Guaymas basin (Lonsdale et al.,
1980). The basin consists of two northeast-trending grabens,
named the Northern and Southern Troughs (Fig. 16), each
approximately 40 to 20 km long, respectively, and 3 to 4 km
-2
wide. High heat flow (>1.2 Wm ) measurements were the first
48
evidence of hydrothermal activity in the Guaymas Basin. During
1977, ferromanganese, encrusted sulfide and talc deposits were
discovered during submersible dives on the Northern Trough
(Lonsdale, 1978). During leg 64 of the Deep Sea Drilling Project,
three sites were drilled in the Guaymas Basin. Subsequently, in
1980, hydrothermal deposits were mapped by deep-tow, sidescan sonar and photography (Lonsdale, et al., 1980), and
samples of the hydrothermal precipitates were collected by
dredge in 1980 (Peter and Scott, 1988).
In 1980, Lonsdale et al. reported data from the Northern
Trough. Analyses of pyrrhotite and talc revealed minor amounts
of Cu, Co, Zn, Ni, Au, and Ag. Pyrite was probably present in
framboidal sulfide masses. Copper was as high as 3,000 ppm in
one talc sample and 7,000 ppm in pyrhotite, the latter being
disseminated within talc. Oxygen isotope geothermometry gives
a temperature of talc formation at approximately 280°C.
Reduction of seawater sulfate, perhaps by bacterial as well as
hydrothermal reactions within the underlying sediment, is the
source of most of the sulfur. It appears likely that Fe, Mn, and
Cu (and perhaps Zn and P) are partially derived from subjacent
igneous intrusions. The deposits, as a whole, were thought to be
only a few tens of years old; otherwise they would have been
buried by hemipelagic sediment (Lonsdale et aI., 1980).
Submersible dives were made in 1982 in the Southern Trough
in areas of high heat flow, and hydrothermal plumes and
deposits were mapped and sampled. In 1985, additional dives
by ALVIN were accomplished, and hydrothermal spires,
chimneys, and mounds, consisting of carbonates, sulfates,
silicates, and metal sulfides were documented. Among the
sulfides are pyrrhotite, marcasite, pyrite, sphalerite, wurtzite,
isocubanite, and galena. Fluid inclusion trapping temperatures in
calcite from chimneys varies from 150 to 315°C and mean
salinity is 5.4 wt % NaCl. Mineral assemblages are considered
to be due to mixing of hydrothermal fluid with seawater, with
precipitation at the vent largely due to decreasing temperature
due to mixing (Peter and Scott, 1988).
high geothermal gradients, and that there are no obvious links
with volcanic activity.
MINERALIZING EPOCHS
The recognizable mineralizing epochs are shown graphically in
Figure 17. In all, six epochs are distinguished and further
illustrate the same mineralizing intervals summarized in Table 2.
In the following sections metal assemblages, type of deposit,
and the relation to the prevailing tectonic environment are
discussed.
Proterozoic (~1.1-1.0 Ga)
A limited number of deposits are associated with the Oaxaca
terrane (Fig. 3) and these include titanium related to anorthosite
of the Oaxaca Complex; REE related to pegmatites; and
precious metal in veins in gneiss host rocks (Figs. 3 and 17).
As previously discussed, other limited exposures of
Proterozoic lithologies in the Molango, Hidalgo, and Los Filtros,
Chihuahua areas are devoid of metal occurrences, whereas
gold in veinlets at Novillo, Tamaulipas has been assumed to be
of post-Grenville age (Eguiluz de Atuñano, et al., 2004). The
more widespread Proterozoic rocks of northern Sonora, while
host to important deposits, to date, do not reveal metal
concentration of Precambrian age.
Early Paleozoic (470-397 Ma)
In spite of tectonism in southern Mexico in Devonian
(Acadian), Mississipppian and Permian time, as previously
discussed, plus thermal events in the Acatlán Complex
interpreted as part of the Laurentia-Gondwana collision and
subsequent formation of Pangea, metallizing processes are
scant; for example, the Mazapa de Madero Ti-(Cr-Ni)
mineralization pods contained in anorthosites of the Chuacús
Group. Cr and Ni have also been identified in mafic-ultramafic
rocks at Tehuitzingo and Tecomatlán in Puebla (Figs. 4 and 17)
that represent tectonized ophiolite, which formed part of the
oceanic crust on which were deposited sediments of the Acatlán
Complex.
Elsewhere, particularly at Canon Novillo, Tamaluipas, weak
Ni-Cr-Cu-Pt mineralization is related to a possible subduction
complex.
The majority of Mexico was subject to an axis of sedimentation
from Cambro-Ordovician time becoming restricted in the
Silurian-Devonian interval, but broadening in late Paleozoic time,
as indicated by the paleogeographic maps of López-Ramos
(1969). It is in this widespread region that mineralization
appears to be largely absent.
Shallow-water hydrothermal vents
Three shallow-water hydrothermal vents are recognized
offshore in Mexico by Canet and Prol-Ledesma (2007). These
vents are found at less than 200 m below sea level.
At about 500 m offshore from Punta Mita, Nayarit, at the
western end of the Trans-Mexican Volcanic Belt (Fig. 16),
hydrothermal discharge consists of gas and water at 85°C
through a NE-SW fracture in basaltic rocks. The gas comprises
N2 (88%) and CH4 (12%). The vent water is less saline than sea
water and the isotopic characteristics suggest mixing of
seawater with deeply circulating meteoric water. Among the
deposited materials are minute, disseminated grains of cinnabar
associated with layered pyrite aggregates.
At Bahia Concepcion, Baja California, the shallow water
hydrothermal activity extends for 700 m along a faulted section
of the western coast of the bay. Temperature of the escaping
fluids varies from 62 to 87°C and exsolved gas is CO2 (44%)
and N2 (54%). Crusts contain minor amounts of pyrite and
cinnabar.
At Punta Banda, near Ensenada, fluids have a temperature of
102°. N2 and N4 are principal components with CH4 up to 51.4%
of the exsolved gas. Minerals include pyrite and gypsum with
high contents of As, Hg, Sb, and Tl.
Canet and Prol-Ledesma (2007) conclude that the shallowwater vents correspond to continental margin extension with
Permo-Trias (265-228 Ma)
By Permian time polymetallic Pb-Zn-Ag-(Cu-Au) deposits
appear to have formed in the environment of the Chiapas Massif
(Fig. 4), although the mineralization has not specifically been
dated.
The Teziutlán VMS deposit has been interpreted to occur in a
Permo-Traissic protolith or possibly a deformational event of this
age (Figs. 4 and 17).
Mineralization occurs at El Tigre, Baja California Sur, where
the Cr content of chrome spinel is 50% Cr2O3 (BustamanteGarcia, 1999), in an ophiolite of Late Triassic age. While the
ophiolites reflect the convergence of oceanic plates in the west,
to date no mineralization has been registered along the
Tarahuma-Ouachita orogenic trend in northern Mexico.
49
Figure 17. Composite diagram showing six metallic epochs: Proterozoic, Early Paleozoic, Permo-Triassic,
Jurassic-Early Cretaceous, Late Cretaceous-early Miocene, and late Miocene - Present. Distributions are
from SW to NE. Late Cretaceous-early Miocene epoch is modified from Clark et al. (1979, 1982) relative
to the subduction zone. Late Miocene to Present deposits are from Clark and Fitch (2005, 2009).
50
stock-contact skarns to massive-sulfide mantos (Megaw et al.,
1988) is largely confined to Cretaceous carbonate host rocks
and thus appears relatively late within the Late Cretaceous-Early
Tertiary interval as shown by the Dinamita, Naica, Santa Eulalia,
and Providencia deposits in Figure 17.
Jurassic-Early Cretaceous (190-110 Ma)
This interval is characterized by the widespread distribution of
VMS deposits in parts of western and central Mexico as well as
isolated examples in Chihuahua and Baja California (Fig. 5).
These deposits are predominantly associated with submarine
volcano-sedimentary sequences of the Guerrero terrane, itself
divided into three or more subterranes of Late Jurassic-EarlyCretaceous age (Campa and Coney, 1983). Of these, the
Telolapan-terrane exhibits low grade regional metamorphism, is
severely deformed, and is thrust eastward along its eastern
margin. The Zihuatanejo and Huetamo terranes are deformed
but not metamorphosed, but all subterranes have been accreted
subsequently onto the southern part of the North American
continent.
The age assignments of the VMS deposits shown in Figure17
are largely based on the isotopic determinations of Mortensen et
al. (2008). However, for several deposits for which specific ages
have not been determined, it is assumed that their age is similar
to those for which age has been measured, particularly if they
are in the same geographic cluster and in the same subterrane.
Obvious exceptions, based on stratigraphic evidence are Arroyo
Seco (AS) and Teziutlan (Tz).
Also contained in this mineralizing epoch are the important
stratabound manganese ores of Molango, Hidalgo, perhaps
related to depositional and hydrothermal events associated with
the opening of the Gulf of Mexico.
Another deposit type is found at San Juan Mazatlán, Oaxaca
and at El Arco, Baja California, and these represent the oldest
porphyry copper deposits in Mexico, and reflect a subduction
regime.
Finally, two red-bed Cu deposits in Chihuahua of Early
Cretaceous age and numerous occurrences of mafic-ultramafic
rocks, that contain minor contents of Ni, Co, Cr, Au and Ptgroup elements were located along the western margin of
mainland Mexico, complete the diversity of deposit types and
contained metallic elements.
Volcanogenic and other Fe deposits
There are two main clusters of these deposits as noted in
Figure 8, namely (1) eastern Chihuahua and western Coahuila,
and (2) along the coast of four states in southwestern Mexico.
In addition, three other deposits, two of which are currently being
mined, are located in Durango, Sonora, and Sinaloa. Figure 17
shows that there is considerable variation in age, as well as
geographic distribution, with El Encino being the oldest (Corona
et al., 2009a, b, c, d), and El Anteojo representing the youngest.
Volcanogenic and sandstone- U deposits
The well-known Sierra Peña Blanca volcanogenic uranium
province in Chihuahua and other deposits in adjacent Sonora,
appear to be tied to the Chihuahua cratonic terrane. The Sierra
Peña Blanca deposits post-date volcanic units of late Eoceneearly Oligocene age, and Los Amoles deposit in Sonora is
located in Late Cretaceous volcanic rocks as previously noted.
La Coma and adjacent deposits in Nuevo Leon (Fig. 8) are
sandstone uranium deposits and whose origin is related to that
of similar deposits in adjacent South Texas.
Volcanogenic Sn deposits
Among the probable 1,000 tin localities identified by Foshag
and Fries (1942), the majority are associated with Tertiary
volcanic rocks, especially rhyolites of the central plateau, a few
of the important localities having been shown in Figure 9. The
age of these deposits is Oligocene that defines the northern
Mexican tin belt (Huspeni et al., 1984; Ruiz, 1988). Cerro de los
Remedios in Durango, reflects this age in Figure 17.
Mercury and Antimony deposits
For the limited number of mercury deposits for which reliable
data are available (Fig. 10), several deposits in northern Mexico
occur near the contact of underlying Mesozoic sedimentary and
overlying volcanic rocks. In the Canoas district, Zacatecas ores
have an age as young as Oligocene (Gallagher, 1952). Other
deposits are located in Cretaceous host rocks, as at Nuevo
Mercurio, also in Zacatecas; Sierra Gorda, Queretaro; as are
those at Huahuaxtla and probably also at Huitzuco, both in
Guerrero. The Cretaceous sedimentary rock-hosted mercury
deposits in Sierra Gorda, Queretaro, are probably Late
Cretaceous-Tertiary in age (Rodolfo Corona-Esquivel, written
communication, March, 2009).
Canoas and El Cuarenta
deposits are plotted in Figure 17.
As shown in Figure 10, the geographical distribution of the few
antimony deposits for which data are available overlaps that of
mercury in the central part of Mexico. However, the stratigraphic
position of host rocks is more variable than that of mercury and
varies from Triassic at El Antimonio, Sonora, to Late Jurassic
units at Sán José (Wadley), San Luis Potosí. However, at El
Antimonio mineralization may be related to later igneous
intrusions and in the San José district mineralization may be
related to intrusions of Tertiary age where the manto deposits
are also associated with mercury mineralization, but not in the
cavity-filling vein deposits. In the nearby La Maroma district the
fissure-vein deposits may have a similar Tertiary age.
Late Cretaceous-early Miocene (100-16 Ma)
This interval exhibits a great variety of metallic deposits. The
overall distribution is similar to that portrayed by Clark et al.
(1979a, 1982) in which the deposits fall within an envelope of
magmatic activity, subduction related, that swept eastward as
the Farallon plate was consumed. However, in the present
version of this distribution (Fig. 17) a greater variety of metallic
assemblages has been included because more age
determinations have been made in the ensuing 30 years.
Porphyry Cu-Mo-(W-Au) deposits
The diachronous, broad sweep of the porphyry deposits is
clearly identified as before (Fig. 17), and is attributed to a
younging of ages southwards due to oblique subduction along
the western margin of the Mexican mainland during this time as
noted earlier. The largest concentration of these deposits and
those of greatest economic importance are located in the
Caborca and Chihuahua terranes (Fig. 6). Lack of space
prevents inclusion but a few of these deposits in Figure 17.
Nevertheless, Fortuna de Cobre, San José del Desierto,
Cananea, La Caridad, Florida Barrigón, Lucia, Tameapa, La
Guadalupana, Washington, and Inguaran are representative of
porphyry copper deposits.
Polymetallic Pb-Zn-Ag-(Cu) depostis
This distinctive assemblage (Fig. 7), ranging in character from
51
As a consequence of the uncertainty of the age of the
antimony deposits discussed herein, only El Antimonio appears
in Figure 17, and is assigned a possible Late- or postCretaceous age according to White and Guiza (1949).
Gold and Silver in Disseminated and Structurally controlled
deposits
Precious Metal disseminations in sedimentary rocks (Fig. 14)
are characterized by Lluvia de Oro, (~18 Ma, Rothemund,
2000), and Real de Angeles (~45 Ma, Pearson et al., 1988), of
which the latter is shown diagrammatically in Figure 17.
Gold and silver disseminated in volcanic rocks (Fig. 14) is
portrayed at Mulatos (33-24 Ma, Pérez-Segura and OchoaLandín, 2009), and el Sauzal (31-29 Ma, Sellepack, 1997).
Although space limitations in Figure 17 prevent their inclusion,
they clearly overlap the ages of epithermal deposits that contain
precious metals.
An example of gold and silver disseminated in intrusive rocks
(Fig. 14) is exemplified at La Choya where the intrusive has
been dated at 52-48 Ma (Pérez-Segura, 2008).
Finally, gold and silver related to structurally controlled
deposits (Fig. 14) as previously described and where
mineralization age has been determined includes San Francisco
(41-28 Ma, Pérez-Segura, et al., 1996). Again, because of the
profusion of deposits in this mineralizing epoch, lack of space
precludes their inclusion in Figure 17, but clearly they overlap in
time those of epithermal characteristics.
Manganese deposits
There is a widespread distribution of deposits in numerous
terranes along a central axis from north to south in Mexico and
also several deposits along the eastern coast of Baja California
Sur (Fig. 11). A total of 335 deposits in 20 states were identified
by Trask and Rodriguez-Cabo (1948) and were divided into four
types.
Of the fissure-vein deposits, the Talamantes, Chihuahua
mineralization has been determined at post-42.5 Ma (Clark et
al., 1979a). Among several silicified replacement deposits, at
Montaña de Manganeso, San Luis Potosí, the epigenetic
mineralization is hosted in Late Cretaceous sediments. The
third type, limestone replacements, is found near intrusive
granitoids at several localities, and at Guadalcazar, San Luis
Potosi, the age of the intrusion that invades Cretaceous
limestone is Tertiary.
The important San Francisco deposit, Jalisco characterizes a
sedimentary exhalative type (Zantop, 1978) in a continental
environment, and has been assigned an Early Tertiary age. San
Francisco,Talamantes, and Guadalcazar have been assigned
the same Early Tertiary (Eocene) age in Figure 17.
REE, MVT, and Mafic-Ultamafic rock deposits
Only two locales of primary REE deposits are known where
ages have been reasonably well established (Fig.15). In northcentral Chihuahua the Yuca carbonatite has been dated at 36
Ma (Nandigam, 2000), whereas at the El Picacho igneous
complex in the Sierra de Tamaulipas, the age is probably early
Oligocene-mid-Miocene (Elias-Herrera et al., 1990, 1991).
Three localities have been identified by Tritlla et al. (2009)
where base metal mineralization has been characterized as
being of MVT (Mississippi Valley Type), two in Coahuila and one
in adjacent Chihuahua. These three deposits are thought to
have formed from marine water trapped in sediment (connate
water) and expulsed during Laramide orogenesis. As such, their
age of formation is imperfectly known and they do not appear in
Figure 17.
Of the two mafic-ultramafic rock deposits shown in Figure 15,
Ojos Negros in the Vizcaino peninsular of Baja California Sur
has been assigned to the Late Cretaceous-Paleocene by OrtizHernandez et al. (2006), and is shown on Figure 17.
Tungsten deposits
As shown in Figure 12, several important deposits are located
in the batholithic terranes of Sonora and northern Baja
California. Of these, the age of the El Fenomeno deposit is
reasonably well known (~90 Ma) in northern Baja California, and
at the La Guadalupana property (51 Ma) in southwestern
Chihuahua (Clark et al., 1979a). Tungsten is also found in the
ore assemblage in the Washington, Sonora, breccia pipe (46
Ma), designated a porphyry copper deposit in Figures 6 and 17.
Epithermal Ag-Au deposits
In contrast to the porphyry deposits, the epithermal precious
metal deposits, the majority of which are located in Early Tertiary
volcanic rocks, and consequently at a high stratigraphic level
above basement, are tied primarily to the regressive sweep of
volcanism referred to by Clark et al. (1979a). Not all the deposits
shown in Figure 13 whose ages have been isotopically
determined, could be accommodated in Figure 17. Collectively,
epithermal Ag-Au mineralization spans the late Paleocene-early
Miocene interval. Seventeen of these deposits are shown in
Figure 17, varying in age from Mala Noche (48.9 Ma) to Bascis
(27.0 Ma).
Late-Miocene to Recent (< 16 Ma)
As metallic deposits of the youngest mineralizing epoch (Fig.
17) have already been discussed by Clark and Fitch (2005), only
brief mention of their relative ages is made as follows.
Porphyry Cu and contact replacement Fe deposits
One of the youngest of the Mexican porphyry Cu deposits is
located at Toliman (5.8 Ma), Chiapas (Damon and Montesinos,
1978). It and the La Carmen contact metasomatic deposit (1312 Ma) in adjacent Oaxaca (Fig. 16) reflect magmatism
associated with the subduction of the Cocos plate along the
Acapulco trench. Further inboard, the Santa Fe deposit (3-2
Ma) has porphyry copper characteristics, but also exhibits
contact metasomatic and fissure-vein mineralization.
Mesothermal Ag-Au deposits
Mesothermal precious metal deposits, of which gold is the
principal component, are located in veins and other structurally
controlled types and occur in northwestern Sonora being located
along or near the trace of the Mojave-Sonora megashear. As
such, they form a distinct subprovince when compared to the
distribution of Ag-Au epithermal deposits that are related to
volcanism during this epoch. Mesothermal precious metal
deposits were formed in the 60-40 Ma interval during the
Laramide-orogeny (Pérez-Segura, 2008). They vary in
geographic position from Quitovac (65-48 Ma, Iriondo et al.,
2005) in the northwest to San Francisco (~41 Ma Perez-Segura
et al., 1996) in the southeast.
Au-Ag mineralization
Adjacent the Santa Fe deposit (Fig. 16) is the breccia pipe
controlled Ixhuatan Au-Ag mineralization associated with alkalic
basalts and andesites (2.8 Ma), (see Miranda-Gasca et al., 2009).
52
Following the override of the East Pacific Rise and the
separation of Baja California and the related San Andreas fault
system, precious metals have been exploited at San Felipe,
Baja California whose age is 9 Ma or slightly younger. Both
deposits are shown in Figure 17.
SUMMARY AND CONCLUSIONS
A brief historical review shows that the widespread distribution
of precious and other metallic deposits in Mexico has led to an
ongoing, documented quest for their development that exceeds
1,000 years. During this time, and reflecting political pressures
and economic realities, six periods in the development of
Mexico’s mineral wealth can be recognized: Pre-Colonial;
Colonial, Post-Colonial; Post-Revolution; Mexicanization; and
Post-Mexicanization.
Six epochs of metallization are found in Mexico: Proterozoic,
Paleozoic, Permo-Triassic, Jurassic-Early Cretaceous, Late
Cretaceous-early Miocene, and late Miocene-Present. The
Early Paleozoic and Permo-Triassic intervals have few metallic
accumulations and consequently are poorly defined.
The relative scarcity of deposits presently known in the
Proterozoic, Early Paleozoic, and Permo-Triassic epochs stems
from the difficulty of appraising their presence in the older and
often concealed rocks, compared to the surface or near surface
environment where most metallic deposits discoveries are made.
In this study, it is apparent that the diversity of metals
concentrated in a variety of deposit types increases with the
passage of time, and reaches its maximum in the Late
Cretaceous-early-Miocene epoch as illustrated in Figure 17.
The spatial distribution of metallic deposits in each of the
metalizing epochs is portrayed in 13 maps, Figures 3-16. Of
these, 10 maps were employed to show the diversity of
assemblages in the Late Cretaceous-early Miocene metallizing
epoch (Figures 6-15). Each map could be regarded as a sub
province of the Mexican metallic province. It is anticipated that
the time and space distribution of individual metals, or metal
assemblages portrayed in this synthesis will aid in future
exploration for deposits inasmuch that it subscribes to the wellknown exploration philosophy that “elephants are most likely to
be found in elephant country”.
Other important factors contributing to the geographic
distribution of economic metallic ore deposits are rock
composition, as for example the titanium deposits of the Oaxaca
Complex; specific tectonic events such as the Laramide
Orogeny in which deformation and accompanying magmatic
events and associated epigenetic processes of concentration
were so widespread; and syngenetic processes that fostered the
accumulation of red-bed copper deposits, manganese ores, and
possibly the Boleo and Lucifer stratabound deposits.
The spatial distribution of deposits in the six mineralizing
epochs has been shown on tectonostratigraphic terrane maps
and this clearly demonstrates those deposits that are controlled,
or not, by a specific terrane. As previously noted, the Oaxaca
Complex and terrane controls titanium deposits, and the
Guerrero terrane controls the great majority of VMS deposits of
Jurassic-Early Cretaceous age. The cratonic character of the
Chihuahua terrane appears also to dictate the magnitude of
copper and molybdenum concentrated at Cananea and La
Caridad in the Late Cretaceous-early Miocene epoch as
compared with deposits located in non-cratonic terranes to the
south. Major concentrations of uranium appear to be similarly
affected as at Sierra Peña Blanca, and adjacent localities in
Chihuahua and Sonora.
However, the widespread distribution of numerous deposits of
Cu, Pb, Zn, Ag, Au, Sn, Hg, Sb, and Fe, that are found in belts
that straddle several terranes, clearly suggests that they owe
their origin to a considerable extent to first order tectonic events
as is the case of oceanic-plate subduction induced magmatism
Hot-Spring Au-Ag deposits
Immediately to the north of San Felipe and to the west of the
Cerro Prieto geothermal field several hot spring gold deposits
have been found (Fig. 16) and are likely to be no older than
Pliocene. El Cerrito and Tina deposits are shown in Figure 17.
Stratabound Cu-Co-Zn and Mn ores
The Boleo deposits of the Santa Rosalia basin are located in
five mantos within the Boleo Formation of late Miocene- early
Pliocene age. Some 17 km further northwest of Santa Rosalia
(Fig. 16) are the Lucifer manganese deposits that are also
deposited in the Boleo Formation. Both deposits are located to
the west of the spreading center (Fig. 16) and appear on Figure
17.
Placer Au, Sn, deposits
The widespread distribution of these deposits in western
Mexico (Fig. 16) has been previously discussed and no further
comment is made here other than to designate them as having a
Pliocene - Quaternary age. In instances of erosion and
transportation from pre-existing deposits, their concentration is
still ongoing. From northeast to southwest Guadalcazar, Santo
Domingo, America-Sapioris, Laguna de Santiago, Calmalli, and
Juan Marquez are shown in Figure 17.
Beach Deposits
Black sand, Quaternary beach deposits, derived from the
Oaxaca and Acatlán complexes, like some placer deposits, are
still undergoing additional accumulations. Likewise, the blacksand deposits at San Antonio del Mar are derived from intrusive
and metamorphic rocks in the northern part of the Baja
California peninsula. The Bahia de Agua Dulce and Boca del Rio
Colotepec deposits are shown in Figure 17.
Geothermal fields, deep-sea and shallow-water hydrothermal
vents
Active geothermal fields have been included because of their
likely relation to hydrothermal systems, which in places are
related to metal-being systems, past and present, as is the case
of the proximity of hot-spring, precious-metal deposits adjacent
geothermal fields in the Cerro Prieto area (Fig. 16) and the gold
found in scale of two production pipes. Figure 17 shows the
location of Cerro Prieto, La Primavera, Los Azufres, Pathe, and
Los Humeros geothermal fields.
Offshore, ongoing deep-sea hydrothermal activity, commonly
referred to as “Black and White Smokers”, occurs at two locales
where plate tectonic activity has resulted in the accumulation of
metaliferous deposits. While these deposits are not of economic
importance to date, they provide excellent insight to the
formation of some submarine massive sulfides. The Northern
and Southern Troughs of the Guaymas Basin are shown in
Figure 17.
Three shallow-water hydrothermal vents, located at depths of
less than 200 m below surface, are thought to occur in faulted
submarine areas of high heat flow. The Bajia Concepción
locality appears in Figure 17.
53
and related structures, particularly along the tectonically active
western Mexico Margin. This in turn brings into consideration
the three potential magma and metal sources involved:
subducted oceanic lithosphere, mantle, and overlying
continental crust (Clark et al., 1982), and the effect of accreted
terranes (Campa and Coney, 1983).
In the varied and abundant Late Cretaceous-early Miocene
epoch the resulting metallic sub-province for porphyry Cu-Mo(W) deposits forms a well defined belt along the western margin
of Mexico, and there is limited spatial overlap with Fe and other
elements. But more significant overlap occurs with Sn, Hg and
Sb albeit individual localities are restricted to specific host rocks
in northern-central Mexico where also there are numerous PbZn-Ag-(Au) concentrations, many of which are contained in
Cretaceous limestones. The widespread distribution of Mn in
four classes of deposit, of which few have been dated, covers
multiple terranes and consequently overlaps several other
metallic assemblages and reflects the controlling physicalchemical parameters which dictated their deposition. The bulk
of W ores are restricted to granitoid terranes of northwestern
Mexico and fissure-vein, precious-metal deposits, many
important localities of which have been dated, are predominantly
distributed in the occidental, volcanic arcs. In contrast, the
mesothermal Au-(Ag) deposits, located along the trend of
Mojave-Sonora Megashear trend in Sonora have been related to
the Laramide Orogeny. A limited number of other previous
metal deposits are disseminated in sedimentary, volcanic and
intrusive host rocks or controlling structures. The remaining
deposits of this epoch include REE and Th bearing
concentrations located in carbonatite or an alkaline complex in
northern and northeast Mexico respectively and a few examples
of Mississippi Valley type deposits are located in this region.
Additionally, the areal distribution of each metallic assemblage
shown in Figures 6-15 is likely to be modified as the age of more
deposits will be determined by future studies.
The youngest metallizing epoch encompasses the last 16
million years and is dominated by tectonic events and surficial
processes. These include the youngest porphyry Cu, Fe and Au
deposits in Chiapas and Oaxaca, whose origin, again, is related
to subduction related processes. Included in this interval is the
San Felipe gold deposit, and the hot spring gold deposits in
northern Baja California. They and the nearby geothermal fields
in the Cerro Prieto area are within the tectonic regime that led to
the formation of the Gulf of California. An additional region of
high heat flow exists In the volcanic activity of the TransMexican Volcanic Belt, and in locales on the East Pacific Rise
and Gulf of California spreading centers that gave rise to Black
and White Smoker deposits. Shallow water hydrothermal vent
deposits may be independent of magmatic activity.
Another class of deposit is found in the unique tectonic regime
of northwestern Mexico, namely the stratabound Cu-Co-Zn
deposits at Boleo and the Mn-bearing deposit at Lucifer.
Finally, erosion of pre-existing precious metal and tin deposits
in western Mexico, have resulted in numerous placer
accumulations, whereas the erosion of the Oaxaca and Acatlán
complexes has given rise to beach deposits of magnetite,
ilmenite, rutile, zircon, monazite and garnet, and the erosion of
intrusive granitoids and metamorphic rocks in the northern part
of the Baja California state results in the accumulations of
ilmenite, rutile, zircon, hematite, and magnetite at San Antonio
del Mar.
ACKNOWLEDGMENTS
The authors were able to improve the manuscript after
receiving helpful comments from J.A. Gómez-Caballero, Rodolfo
Corona-Esquivel, Fernando Ortega-Gutíerrez, and Efren PérezSegura. We are also grateful to J.A. Gómez-Caballero for his
translation of the original paper into Spanish. Palmira Hart
prepared both versions of the manuscript. To all these
individuals and others not named herein, we further express our
thanks for their timely cooperation in bringing this article to its
final presentation.
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