Download Paleogene magmatism and associated skarn

Paleogene magmatism and associated skarn-hydrothermal
mineralization in the central part of Mexico.
L.F. Vassallo1, J.G. Solorio1,M.A. Ortega-Rivera2
J.E. Sousa3, and G. Olalde4
1
Centro de Geociencias, UNAM,
Campus Juriquilla, A.P. 1-742
Querétaro, Qro. 76000, MÉXICO.
[email protected]
2
3
Instituto de Geología, UNAM,
ERNO, Hermosillo, Sonora.
MÉXICO.
Cia. Minera La Negra SA de CV
Maconí, Querétaro, MÉXICO.
4
Cia. Minera Las Cuevas SA de CV
Apdo. Post. 438,
San Luis Potosí, S.L.P. 78090, MÉXICO.
ABSTRACT
Mineralization associated with magmatic rocks occurs at the central part of Mexico. The
mostcharacteristic types of mineralization are hydrothermal Au-Ag deposits, the world class
Fluorite deposits, and Zn-Cu-Pb-Ag skarn deposits. The age of mineralization extends from 43 to
29 Ma and is related to small stocks and dikes of quartz monzonites and porphyritic diorites to
subvolcanic-volcanic felsic rocks.
Keywords: Mexico, age, hydrothermal, skarn deposits.
Introduction
The western North American Cordillera hosts a large number of ore deposits of gold-bearing
quartz veins, silver-bearing quartz veins and Zn-Cu-Pb-Ag skarn systems from the silver belt of
central Mexico, Mother Lode of southern California, through counterparts in British Columbia and
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southeastern Alaska, to the Klondike district in central Yukon. These veins and skarn systems are
structurally controlled by major fault zones or weakness zones, which are often reactivated suture
zones of terranes accreted along the continental margin of North America. Mineralization ages span
mid-Jurassic to early Tertiary and encompass much of the evolution of the Cordilleran orogen.
In this study, we present 14 age K-Ar determinations of rocks related to ore deposits of the
central part of Mexico. The goal of this paper is to determine the space-relation of the age of the ore
deposits, the weakness structures (Vassallo et al 2000a, 2000b, 2000c, Vassallo, 2001) and to set
them in a regional geologic framework of Central Mexico.
Vassallo et al. (2000a) described the general characteristics of several ore deposits of the
central part of Mexico. Here we describe in detail the geology of a small region, between. 99_ and
101_ W and 20_30’ and 22_ N. (fig. 1, 2).
Fig. 1.- Localization of the studied area at the central part of Mexico.
The oldest rocks of the region are upper Jurassic volcanic and volcanic-sedimentary
formations, overlain by early Cretaceous limestones and finally upper Cretaceous terrigenous
rocks. All Mesozoic rocks are folded and covered by volcanics of Oligocene-Miocene age with
rhyolitic, dacitic and andesitic composition and of different depth facies.
2
About 20 small stocks and two big granitic bodies are known, which in their majority are of
alaskitic, monzonitic, dioritic and granitic composition. Additionally there are numerous dikes with
porphyritic texture and rhyolitic to andesitic composition. Often subvolcanic bodies like dikes or
stocks cut the older sequences.
Fig. 2 a. Simplified Geological map of the central part of México, Santa María del RíoZimapán region. 1.- Quaternary layers, 2.- sedimentary miocene. 3.- volcanic andesitic rocks of
oligocene. 4.- volcanic felsic rocks of oligocene. 5.- granodioritic rocks of oligocene age. 6.- Upper
cretaceous sandstones and shales. 7.- Lower cretaceous limestones. 8.- Upper jurassic vulcanosedimentary rocks. 9.- Skarn ore deposits: (1- Zimapán, 2- La Negra, 3- San Rafael, 4- Maravillas,
5- Rio Blanco, 6- La Aurora (Xichú)). 10.- Gold-silver vein deposits: (7- San Martín, 8- Puerto de
Nieto, 9- Santa Catarina, 10- Pozos) 11.- Fluorite deposits: (11- El Sabino, 12- El Capulín, 13- El
Refugio, 14- Rio Verde (El Realito), 15- La Valenciana, 16- La Consentida, 17- Las Cuevas).
3
Fig. 2 b. Geological map of the central part of Mexico, Santa María del Rio-Zimapán
region. Showing metallogenetic belts and K-Ar ages of dated rocks.
In this region ore deposits of skarn silver-polymetallic type, hydrothermal vein Ag-Au, and
the biggest fluorite deposits of the world (fig. 2) are found. It is very interesting that all ore deposits
are located at the lower part of the Cretaceous limestones and the base of the Oligocen acid volcanic
rocks.
Studied Ore deposits
1. Skarn polymetallic deposit La Negra.
La Negra ore deposit (Gaytán-Rueda, 1975; Morrison, 1982; Megaw et al, 1988; Fraga,
1991; Megaw, 1999; Vassallo et al, 2000, 2001, Vassallo et al 2002) is built by lower Cretaceous
limestones of the El Doctor Formation that is cut by several small stocks of dioritic composition.
The limestone is folded to different degrees and the axes of the structures are oriented to the NW.
The dioritic stocks have a skarn aureole (fig. 3a, 3b, 3c), and in the central part of these skarns often
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a rich zone of spurrite is found. In the central part of the La Negra deposit several rhyolitic
porphyry dikes are found, which of course are post-skarn. The relation with the ores is not very
clearly, but they seem to have been emplaced before the mineralization. There also are several dikes
of andesitic-basaltic porphyries related to effusive rocks that are considered post-mineralization.
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Fig. 3a, 3b, 3c.- Geological map sections of La Negra ore deposits.
There are more than 20 ore bodies of polymetallic sufides, but not a single one crops out.
All the ore bodies are within the skarn aureoles. The majority of the bodies have the form of
chimneys with sizes of tens of meters. The size is controlled by faults, in some cases they seem to
be lodes or mounds. The bodies are inclined up to 70-80º and occur close to the contact with the
intrusives for more than hundreds of meters depth.
The useful componets of the ores are galena, chalcopyrite and sphalerite. In some cases
there is also pyrrhotite, but the majority of the sulfides are pyrite. There are some places with rich
ores of arsenopyrite and lellingite. In the deepest horizonts pentlandite is present. We also found
cubanite, freibergite, polybasite, lilianite, heirobsquite, freibergite, and natural bismuth. There are
large amounts of silver-bearing minerals with contents of more than hundreds of grams/ ton.
Bismuth minerals are also present as natural bismuth.
The textures are columnar, although at the early stage of the process linear skarns are
formed by the laminar limestones, and these textures are later inherited to ores by the skarns. The
most common textures are massive and disseminated ores.
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At the first stage of ore formation were intruded several dioritic stocks, which could be
apophyses of a larger intrusive body situated at the deep of 1.5-2 km. At this stage also spurrite
skarns were formed.
After the formation of the marbles the formation of skarns took place, in the following
order: spurrite skarn, rhyolitic porphyries, small narrow skarns and after all five metallic stages of
ores and quartz-calcite veins without metals.
To determine absolute ages two diorite samples were taken, at 2400 masl (N-13 and N-14),
and at the horizont 1990 masl of the Cristo Rey 2 mine two samples of rhyolitic dikes (N-3 and N5) were collected. The results of K-Ar dating are shown in tab. 1.
Table 1
K-Ar age of intrusive rocks at the La Negra ore deposit.
No.
Rock
N-13
N-14
Diorite
Diorite
N-03
N-05
Rhyolitic porphyry
Rhyolitic porphyry
Potassium, %
±!
2.79±0.03
2.88±0.03
4.10±0.04
4.08±0.04
40
Arrad (ng/g) ± !
Age, Ma ± !
7.67±0.25
7.81±0.25
39.2±1.3
38.7±1.3
10.9±0.3
11.3±0.3
38.1±1.2
39.6±1.2
Taking into account standard deviations ! = ±1.2-1.3 Ma the ages of all four samples could
be identical and the intrusions were emplaced between 38 and 40 Ma. It is clearly seen that the time
between the intrusion of diorites and the post-ore dikes is very short if any, indicating a very short
period of time for the formation of skarns and mineralization.
2. Skarn-Polymetallic ore deposit Zimapán:
The Zimapán ore deposit (fig. 4a, 4b, 4c) (Vassallo et al, 2000; García y Querol, 1985;
García and Querol, 1991; Megaw, 1999; Megaw, et al, 1988; Miranda-Gasca, 1978; Simons and
Mapes, 1956, 1957; Yta y Moreno-Tovar, 1997) resembles the La Negra deposits, but erosion here
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has taken away the upper part of the Zimapán deposit. In fact, the top of the Zimapán ore deposit is
found at 1400 masl, 1000 m below the top of the La Negra ore deposits.
8
9
Fig. 4a, 4b, 4c .- Geological map and sections of Zimapán ore deposits.
At Zimapán the lower cretaceous El Doctor limestone is intruded by the big body of El
Carrizal quartz-monzonite (fig. 4a, 4b, 4c),. The monzonite contains pyrite, sericite and quartz,
indicting a hydrothermal alteration to beresite, produced by low temperatures fluids.
Around the big dike-stock and parallel to the limestone two 100 m thick vertical zones of
skarns are located. Within the skarn zone several ore bodies with different shape are located. Close
to the ore bodies several dikes of dioritic porphyries were found.
The ore components are sufides of Pb, Cu and Zn. One of the most important metals is
silver. The main ore minerals are chalcopyrite, sphalerite, pyrite, pentlandite, pirrotite, and
arsenopyrite. Minerals containing silver are argentopentlandite, freibergite, tetrahedrite, lillianite,
arsenopolybasite and electrum. The textures are parallel, disseminated, massive and porphyritic.
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The succession of formation of the ore deposit was: Intrusions of quartz-monzonites "
skarn-1 " beresitization of monzonites " dioritic porphyry dikes " skarn-2 (exoskarn) " skarn-3
(endoskarn) " ore mineralization (5 ore stages).
From the Zimapán ore deposit 3 samples were collected from the horizon 1080 masl for
dating: Carrizal monzonite (sample Z-01) and 2 samples from the dioritic porphyries (Z-03 and Z04). The results are summarized in table 2.
Table 2
K-Ar ages of igneous rocks from Zimapán ore deposit
No.
Rock
Z-01
Quartz
monzonites
Porphyritic
diorites
Porphyritic
diorites
Z –3
Z-5
Potassium , %
±!
3.44±0.03
40
Arrad (ng/g) ± !
Age, Ma ± !
10.5±0.3
43.6±1.2
5.42±0.04
15.5±0.3
40.8±1.0
5.15±0.04
15.1±0.3
41.8±1.0
The quartz monzonite is clearly older than the porphyritic diorites, which may be of the
same age if we take into account the standard deviations. It seems that the quartz monzonite is 4-5
Ma older than the dioritic rocks of La Negra, but the dioritic porphyries that are cutting El Carrizal
pluton and skarn may have similar ages as the diorites of La Negra. Their source could be a
common mother pluton for both of them.
3. Gold-silver hydrothermal San Martín deposit
The San Martín ore deposit (Vassallo et al, 2000; Muñoz-Cabral, 1993; Ortiz y Solis, 1986)
is located in Querétaro State, 45 km to the east of Querétaro City. It is a low temperature breccialode type Au-Ag-Qrz-Calcite formation.
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The wall rocks of San Martín are lower Cretaceous-upper Jurassic thin bedded limestone
with minor contents of shale (called by Carrillo-Martínez, 2000, La Peña Azul Formation) which
were strongly folded and faulted during early Tertiary. The deposit is covered by Miocene andesitic
lava domes dated 15.2 Ma (sample SM-10), and products of the Amazcala caldera in the western
part (Aguirre-Díaz and López-Martínez, 2001).
The limestone was intruded by subvolcanic bodies (fig. 5a, 5b). One of them of 39.2 Ma age
is related with the Au-Ag mineralization, in the form of rhyolitic breccias with quartz-calcite-AuAg.
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Fig. 5a, 5b .- Geological map, satellite image and cross sections of San Martin deposit.
The majority of the lode is composed by quartz and calcite. In some places dolomite and
rodocrosite were found. In the zone of argillitization,alunite, montmorillonite, kaolinite, chlorite
and gaylussite were found. Sulfides occur as pyrite and chalcopyrite in small quantity of no more
than 2-3 %. Galena, sphalerite and tennantite-tetrahedrite are very rare. Many silver minerals are
present in the ore: aguilarite, naummanite, freibergite, argentite, acantite, polybasite. A lot of silver
occurs in the form of selenides.
Some geologists believe that the genesis is related to the andesitic porphyries (Ortíz and
Solis, 1986), but our age data suggest that the mineralization rather is related to a 39 Ma event. This
event is close to the Zimapan-La Negra event (39-43 Ma).
After the intrusion of the rhyolitic dike, a breccia-vein body of quartz-calcite gold-silver was
formed. The upward movement of hydrothermal fluids caused a widespread wall rock alteration of
argillic-type.
After 15.2 Ma andesitic porphiries covered the breccia-vein.
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To constrain all ore deposit ages samples were taken from the rhyolites of the automagmatic
breccia (sample SM-6) and from andesitic porphiries that cover the deposit (sample SM-10 from
diamond drills) (table 3).
Table 3
K-Ar ages of igneous rocks of ore deposit San Martín (Querétaro)
No.
Rock
SM-6
SM-10
Rhyolite
_ndesitic porphyr
Potassium , %
±!
0.14±0.01
1.89±0.02
40
Arrad (ng/g) ± !
0.39±0.2
2.00±0.1
Age, Ma ± !
39.2±2.5
15.2±0.8
For the rhyolitic rock we obtained the same age as for the dioritic stocks of La Negra Mine.
Andesitic rocks covering the mineralization have an age older than for the Amazcala caldera
(Aguirre-Díaz et al., 2001) and still belong to Miocene. They are of similar age as andesitic domes
of San Miguel de Allende area (Perez-Venzor et al., 1997).
4. Fluorite deposits Las Cuevas
In the north-western part of the study area several deposits of fluorite are known, among
them the worlds biggest Las Cuevas deposit, (fig. 6a, 6b).
14
Fig. 6a, 6b .- Geological map and regional setting of Las Cuevas fluorite district.
15
All these deposits are found between the lower Cretaceous reef limestone and the
felsicvolcanic rocks of Oligocene age, along the western edge of the Valles-San Luis Potosi
platform.
The strongly folded Cretaceous limestone is in north-west fault-contact with the rhyolites.
This structure presents a magmatic-breccia filled with fluorite, several cavities are filled with
stalactites and columns of fluorite, with crustiform textures representing a cavity-filling, which
some geologists consider as a karst-filling. The mineralogical composition of the ores is very
simply, only fluorite sometimes with a little calcite.
The Valles-San Luis Potosi Island conformed at its western part a reef zone, which controls
the distribution of the fluorite deposits. This western part in conjunction with a wide rhyolite area
called Santa Maria volcanic camp, defined an area of inherited structural weakness which
developed the NW-SE oriented fluorite belt.
The presence of tectonic breccias allows us to say that the structural weakness zones were
active during an extended period, because the andesites also extruded through the same channels.
These zones allowed circulating fluids to deposit through the karst structures and to form big
chimneys like the ore body G which is 250 m wide and up to 750 m long.
Samples were taken from the ore deposit for K-Ar determinations: one of magmatic breccia
of the G body (sample C1/1), another from the open pit La Consentida from the rhyolitic breccia
tuffs (sample C-8). The results are as follow in table 4.
Table 4
K-Ar ages of igneous rocks of fluorite deposit Las Cuevas (San Luis Potosi)
No.
Rock
C – 1/1
Rhyolite of
magmatic breccias
Rhyolitic tuffs
C-8
Potassium , %
±!
0.30±0.015
4.37±0.04
40
Arrad (ng/g) ± !
Age, Ma ± !
0.61±0.03
29.2±2.0
9.13±0.27
29.9±1.0
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Both samples have similar ages but they are 14 Ma younger than the granitic stocks and
rhyolitic rocks from the eastern part of the studied area. Similar ages were reported by Ruiz et al.
(1980) for the called E ore body.
5. Sampling of granitic massives.
At the northern part of the area in fig 1 very good outcrops of granites massives were
described by Labarthe et al. (1984, 1989) and Tristán (1987), about 30-40 km to the south of the
Las Cuevas fluorite deposit.
The central part the granitic massives consist of euhedral grains, at the margin they convert
to porphiries with hight K feldspars up to 3 mm grain size. The matrix is micrograined (0.3-0.5
mm).
It is very difficult to fix the limits between the different facies, and no clear transition to
thevolcanics was seen.
Three samples were taken from these granites (p-201, p-204, p-205) to determinate K-Ar
ages. The results are given in table 5.
Table 5
Ages of granitic rocks from the north western massifs.
No.
Rock
p-201
Porphyritic
granite
Fine grain granite
Aplitic granite
p-204
p-205
Potassium , %
±!
3.93±0.03
3.46±0.03
4.08±0.04
40
Arrad (ng/g) ± !
Age, Ma ± !
8.51±0.27
31.0±1.0
7.52±0.25
9.15±0.30
31.1±1.0
32.0±1.0
All these three ages are very similar and close to the age of the Las Cuevas fluorite deposit.
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Conclusions of the obtained results
1. There is a trend of decreasing rock ages from SE to NW, from several granitic stocks to
sub-volcanic and volcanic rhyolitic rocks. Is this the result of variations of the subduction angle of
the Pacific plates below Mexico?
2. The skarn deposits in the SE are more than 13 Ma older than the hydrothermal fluorite
deposits in the NW.
3. All ore deposits follow a NW-SE trend along the zones of faults or weakness zones,
forming several metallogenetic belts: skarn, fluorite hydrothermal and Au-Ag hydrothermal
deposits. Long living deep structures, produced during Jurassic rifting, channel the flow of
successive batches of ascending magma from the mantle and/or lower crust to the surface or into
high-level subvolcanic composite intrusions that may occur at various crustal levels, producing
three metallogenic belts.
4. The oldest age of stocks that intruded the Cretaceous rocks is 43 Ma. Is this age the real
end of Laramide Orogeny for this part of México?
5. Three metallogenetic NW-SE oriented belts have been recognized: skarn Ag-Cu-Pb-Zn,
hydrothermal fluorite and Au-Ag low temperature hydrothermal deposits.
Acknowledgements
This study was supported by the grant CONACYT-32511-T. V.I. Starostin, N.N. Shatagin of the
Geological Faculty, Moscow State University, and V.A. Lebedev and M.M. Arakeliants of the
Laboratory of Isotopic Geochemistry and Geochronology of IGEM of the Academy of Sciences,
Moscow, RUSSIA helped in many ways. The Mining Companies Peñoles, SANLUIS and Minera
Las Cuevas were very kindfully for giving access to their properties, special thanks to Ing. Javier
García Fons and Ing. Dante Aguilar Casillas. C. Garduño and J.T. Vázquez assisted us with the
preparation of the samples for K–Ar analyses. T. S. Medina Malagón was prompted with the biblio
sources. Dr. Román Pérez Enríquez and Dr. Harald Böhnel reviewed the manuscript and gave smart
suggestions that improved the paper, which we very gratefully acknowledgeReferences
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