Download La Caridad Porphyry Cu-Mo Deposit: A Complete Hydrothermal

UNIVER SIDAD DE CONCEPCIÓN
DEPARTAMENTO DE CIENCIAS DE LA TIERRA
10° CONGRESO GEOLÓGICO CHILENO 2003
LA CARIDAD PORPHYRY CU-MO DEPOSIT: A PORPHYRYEPITHERMAL TRANSITION IN THE SOUTHWEST NORTH AMERICA
PORPHYRY COPPER PROVINCE.
VALENCIA, V. A. 1, RUIZ J.1, BARRA, F. 1, OCHOA-LANDIN, L.2, PEREZ-SEGURA, E. 2
AND ESPINOZA E.3
1
University of Arizona, Department of Geosciences, 85719 Tucson, AZ., USA
University of Sonora, Departamento de Geologia, Hermosillo, Sonora, Mexico
3
Grupo Mexico, La Caridad Unit, Grupo Mexico, S. A., Nacozari, Sonora, Mexico.
Email: [email protected]
2
RESUMEN
La Caridad porphyry copper-molybdenum deposit, located in northern Mexico, is the largest copper producer in
Mexico (~150,000 tons of Cu in 2001). Regionally, the deposit is located in an untilted and upright block. It is
associated with the emplacement of a Tertiary quartz-monzonitic stock and hosted by Laramide plutonic and
volcanic rocks. Molybdenite Re-Os ages associated to potassic and phyllic alterations suggest that the mineralization
took place in a short period of time. Main stages of alteration and mineralization comprise an early episode with Ksilicate veins with orthoclase-quartz-anhydrite-biotite in the intrusive complex and pervasive biotitization of
andesites-diorites, with a zone of propylitic alteration around this biotitic zone. Early alteration includes weak
mineralization of magnetite-chalcopyrite-molybdenite-sphalerite-pyrite. A second hydrothermal mineralization event
is represented by quartz veins associated with pyrite-sericite-chlorite. Tourmaline occurs as acicular radiating
crystals intergrown with sericite-pyrite-quartz. This alteration type is associated with the main mineralization event
of chalcopyrite-pyrite and lesser molybdenite. Lead-zinc-silver mineralization was emplaced peripherally during the
final stages. High-sulfidation mineralization in the mine is represented by quartz-tennantite-chalcopyrite-pyritequartz-sericite veinlets. La Caridad Antigua, located 3 km east of la Caridad deposit, contains advanced argillic
alteration that locally hosts high-sulfidation epithermal Cu-Ag-Au mineralization. The spatial-temporal association
suggest a genetic link.
INTRODUCTION
Spatial and temporal links between epithermal and porphyry copper deposits have been
documented in many areas in the Circum-Pacific ring (Lepanto, Philippines, Arribas et al, 1995,
Hedenquist et al 1998; Nevados del Famatina and La Mejicana, Argentina, Losada-Calderon and
McPhail, 1996; Maricunga Belt, Chile, Muntean and Eunaudi, 2001). In the Southwest North
America region with its more than 50 porphyry copper deposits (Wilkins and Heidrick, 1995) the
transition between porphyry-epithermal ore deposits in this region has seldom documented. Here
we present a description of La Caridad porphyry Cu-Mo and preliminary data suggesting that La
Caridad porphyry copper-molybdenum deposit and La Caridad Antigua, a high-sulfidation
epithermal system, are related.
La Caridad porphyry Cu-Mo deposit is one of the two Mexican world-class copper-molybdenum
deposits and is the most productive Cu deposit in Mexico (~150,000 ton Cu/y). La Caridad is
located in northeastern Sonora, Mexico, 240 kilometers southeast of Tucson (Fig. 1). Reserves in
2001 were ~4 million tons of Cu and 250,000 tons of Mo (internal report). The deposit was
Todas las contribuciones fueron proporcionados directamente por los autores y su contenido es de su exclusiva responsabilidad.
discovered in 1967 in a program of the Mexican Government in conjunction with the United
Nations and began production in 1978. The geology and history of its discovery and development
have been outlined by Coolbaugh (1979) and Seagart et al. (1974).
La Caridad mine is the only world-class porphyry copper deposit in the Nacozari mining district,
the other minor deposits in the district are Florida-Barrigon (90 million tons with 0.30% Cu and
0.15% Mo), Los Alisos, El Batamote (4.4 million tons 0.36% Cu) and Pilares (Breccia pipe 80
million tons with 0.70 % Cu).
Tucson
USA
La Caridad
Mexico City
Tucson
U. S
MEX . A.
ICO
Mission
Bisbee
Sierrita
Cananea
SONORA
STATE La Caridad
4 Hermanos
f
ul
G
Hermosillo
of
0
100
ia
rn
ifo
al
C
Piedras Verdes
Kilometers
Figure 1.- Localization of La Caridad porphyry copper deposit within the state of Sonora,
Mexico. Also shown are other porphyry copper deposits in the region.
REGIONAL GEOLOGY
The geology of northeastern Sonora is essentially an extension of the geology of southern
Arizona (North America Terrane). The area is dominated by north-trending mountain ranges
consisting of strongly deformed greenschist-grade volcanic and sedimentary rocks of 1.7 Ga
(Anderson et al., 1980) that are intruded by granites emplaced at 1.4 and 1.1 Ga (Anderson and
Schmidt, 1983). Overlying these units are Late Proterozoic and Paleozoic miogeoclinal rocks,
succeding by volcanic and plutonic rocks of Mesozoic and Cenozoic age. Elongate, northtrending plutons of Laramide age intrude the Mesozoic volcanic sequence. The intervening
valleys are filled with Tertiary conglomerates and Quaternary gravels.
The Nacozari mining district lies in the northwestern portion of the Sierra Madre Occidental
physiographic province (Raisz, 1959). The Sierra Madre Occidental physiography in this area
consists of typical Basin and Range structures, with general faulting oriented NW as a response
an old northwestern-oriented structural fabric parallel to the former continental margin, which
became rejuvenated during Laramide time (Titley, 1976). The Nacozari mining district is cut by
two regional structures that divide the district in three blocks. La Caridad and Pilares mines are
located in the central block that is acting as a horst, and the west and the east blocks are grabens.
The Pilares normal fault crops out 7 km from La Caridad deposit; the structure has an orientation
of N40oW dipping S72oW. The La Caridad postmineral normal fault is located at the northeast
side of the ore body and has a general orientation of N45oW dipping N45oE.
The Nacozari district is characterized mainly by extrusive rocks; consisting of a thick series of
andesitic to latitic flows and ignimbrites of Laramide age. The geology from a structural point of
view, can be divided in three blocks:
•
West Block: This block is dominated by a sequence of dacitic to andesitic flows, volcanic
breccias and basaltic dikes (fig. 2). El Batamote and Florida-Barrigon porphyry copper
deposits are located in this block. Locally, a partially metamorphosed sedimentary sequence
composed of limestones and sandstones from the Upper Paleozoic, located in the Copper
Queen and La Cobriza ranges.
•
Central Block: This area is dominated by pseudostratigraphic ignimbrites, andesitic, rhyolitic
and latitic flows (Fig. 2). Reyna and Mayboca (1986) proposed a stratigraphic column of the
following informal units, from older to younger: 1) Esperanza rhyolitic ignimbrite, 2) Lithic
ignimbrite Paulina, 3) Rosario andesite, and 4) Pilares latite. These rocks are intruded by
diorite, granodiorite and quartz monzonite porphyry at La Caridad mine area. La Caridad
porphyry copper deposit, Pilares and the Santo Domingo breccia pipe (Cu-W) are located in
this block.
Rhyolite
Esperanza
and
Ignimbrite
Paulina
Andesite
Porvenir
To Nacozari de
Garcia city and
Copper Queen and
La Cobriza Ranges
Del Valle
Andesites
Central Block
Rosario
Formation
Rhyolite
El Globo
(24Ma)
La Caridad
Antigua
Rhyolite
Carbonera
51.3 ± 1.0
42
Pilares
Falla
Pilares
West Block
Tajo
La Caridad
Latite
Pilares
Falla
La Caridad
Dump
Baucarit
Formation
East
Block
53.1± 0.8
50.4± 1.9
55.2± 1.6
Diorite/Andesites
48.9 ± 1.2
Cretacic Andesite
N
Granodiorite
48.9± 1.9
Santo Domingo
Cretacic Andesite
Andesite Porvenir
Del Valle Andesite
Rhyolite Esperanza
Ignimbrite Paulina
Rhyolite Carbonera
Aplite dikes
Granodiorite
Quartz monzonite
Porphyry
Andesite C.V.
Rosario Formation
Latite Pilares
Breccias
Rhyolite & Latites
Diorite-Andesite
La Caridad
Fanglomerate
Graphic Scale
Andesitic
Conglomerate
Rhyolite
El Globo
0
Baucarit
Formation
Figure 2.- Simplified regional geologic map of the Nacozari mining district
2
4kms
East Block: This block is characterized by premineral and postmineral rocks separated by an
erosional uncomformity. The premineral rocks are andesites and rhyolitic ignimbrites which are
intruded by a quartz monzonite porphyry (mineral phase). These rocks are overlain by an
ferruginous fanglomerate that represents the first phase of postmineral rocks. In the northern area
the fanglomerate is covered by El Globo rhyolite (24 Ma, Livingston, 1973). La Caridad
Antigua, a high sulfidation epithermal deposit, is located in this block (Fig. 2).
Many of the deposits of the southwestern North-America porphyry copper province (e.g. San
Manuel-Kalamazoo, Ajo, Cananea) have been significantly faulted, extended and rotated during
Oligocene and Miocene time. The magnitude of rotation varies from moderate (30o to 60 o) to
severe (60o to 90 o) (Wilkins and Heidrick, 1995). However, the central block of the Nacozari
mining district has been drilled vertically for more than 800 m in the contact between the vertical
breccia Pilares and the host rock, suggesting that the central block is upright and untilted.
DEPOSIT GEOLOGY
Plutonic and volcanic rocks compose the mineralized rock suite of La Caridad porphyry copper
deposit. Approximately 90 percent of the mineralization is contained in the quartz monzonite
porphyry and hydrothermal breccia. Other igneous and volcanic rocks associated with the deposit
are andesites, diorites, granodiorites, aplites and porphyry dikes. The mine scale geologic map is
shown in Figure 3.
Units are described in order of depositation or emplacement. The Oldest rock in the area are
andesites, which form the host rock for the deposit. These rocks are intruded by diorite dikes that
range from fine to coarse grain. Petrographically, the diorite porphyry, has a hypidiomorphicgranular texture and is composed of 40 to 60 % of euhedral plagioclase phenocryst (An 40-45),
clots of biotite (20-30%), quartz (15-20%) and K-feldspar (2-3%). Locally there are breccia of
diorite and andesite. These rocks are located in the southwest and west part of the deposit.
The granodiorite occupies most of the east part of the deposit, plagioclase (An 32-36) appears as
euhedral to subeuhedral crystals (35 –45%) that range in size from 0.5 to 5 mm. This rock is
composed of euhedral K-feldspar phenocrysts (20-25 %), quartz (20-25%) and subeuhedral
biotite (7-10%). Accessory minerals include apatite, rutile, sphene, zircon.
The quartz monzonite porphyry consists of crowded porphyry with 50 % of phenocrysts. The
sizes of the phenocrysts normally range from 1 to 5 mm, locally can reaching 15 mm. The
relative abundance minerals in this rock is plagioclase 30-35%, micropertithic, twinned Kfeldspar phenocryst 5-10%, quartz eyes 5-10% and biotite 5-10%. Accessory minerals include
apatite, rutile, sphene, and zircon. Quartz monzonite porphyry is the productive rock in the
deposit; and intrudes the contact between the granodiorite and the diorite-andesite.
Irregular bodies termed pegmatite (Reyna y Mayboca, 1986) occur in the central and peripheral
portions of La Caridad deposits. These bodies consist mostly of coarse-grained interlocking
biotite crystals with massive quartz and minor K-feldspar.
Barren Tan porphyry dikes intruded the rocks mentioned above. This unit is a crowded
porphyry with an average phenocryst content of 15-20% vol. (quartz eyes and plagioclase).
Narrow aplite dikes 20-120 cm wide are located in the central part of the deposit.
Large bodies of siliceous hydrothermal breccia are located around and in the quartz monzonite
porphyry. The breccia is monolithic to polimictic depending to the adjacent rocks, and is related
to the intrusion of the porphyry complex. The high-grade mineralization is hosted in these rocks.
Although, the La Caridad geology and the intrusive lithologic units have been studied and
established before, it is still debated whether if the quartz monsonite porphyry is the product of
multiple intrusions (Reyna and Mayboca, 1986, Contla, 2002, Pers. Comm.) or a single intrusion
(Seagart et al, 1974, Echevarri, 1971, 1973, Berchenbritter, 1976). Field observation indicates no
cross cutting relations between the intrusive units suggested by Reyna y Mayboca (1986).
N
55500N
Tan Porphyry
Hydrothermal
Breccia
Quartz-Monzonite
Porphyry
Granodiorite
Diorite
Andesite
54500N
1290 Level
53500N
37500E
38500E
Figure 3.- Geological map of La Caridad mine area, plan view at 1290m level.
STRUCTURE
More than 500 faults were analyzed in La Caridad Pit at benches 1380 and 1305. These data
exhibit two dominant trends: a N31oE/68o NW and a N26oW, 70o SW (Fig. 4). The NE system is
less abundant that the NW orientations. These trends are the same prominent directions as those
reported for Laramide stocks throughout Arizona (Rehrig and Heidrick, 1972; Heidrick and
Titley, 1982). Fracture density varies from 0.2 to 0.4/cm on the upper benches, to 0.2 to 0.3/cm
on the lower benches (Esquivias-Flores, 1998).
ALTERATION AND MINERALIZATION
The main stages of alteration and mineralization comprise an early episode that is represented by
K silicate veins with orthoclase-quartz-anhydrite-biotite in the intrusive complex and a pervasive
biotization of andesites and diorites, with propylithic assemblages around this biotitic zone. This
early alteration includes weak mineralization consisting of magnetite, chalcopyrite, molybdenite,
N
W
E
S
N31O E, 68O NW
N26O W, 70O SW
Figure 4.- Pole projections of 560 faults from benches 1380 and 1305 in La Caridad pit. Lower
hemisphere Schmidt equal area nets.
sphalerite and pyrite. This episode was followed by the emplacement of hydrothermal breccia at
the contact between the andesite-porphyry and porphyry-granodiorite.
A second hydrothermal mineralization event is represented by quartz veins associated with pyrite,
sericite and chlorite, and also occurs as pervasive replacements. Tourmaline occurs as acicular
radiating crystals intergrown with sericite, pyrite and quartz. This event generated the main
mineralization, with mineralization of chalcopyrite, pyrite and lesser molybdenite
A High sulfidation event that represent the latest alteration stages and the collapse of the system,
affected the central part of the pit and is represented by quartz-tennantite-chalcopyrite-pyritesericite veinlets. Lead-zinc-silver mineralization was emplaced as peripheral veins during the
final stages.
Irregular bodies termed pegmatites and veinlets containing coarse molybdenite and pyrite with
minor chalcopyrite cut all the hydrothermal alteration zones. Supergene mineralization, which
has been mined out, was present as a blanket of about 2 km in diameter with an average thickness
of approximately 50 m and ranging from 10 to 230 m (Seagart et al, 1974).
AGES
Available radiometric ages of major intrusive rocks are shown in figure 5 .The K-Ar ages for this
deposit range from 53.5 to 55 Ma (Livingston, 1973, 1974; Damon 1968, Damon et al., 1983).
K-Ar dating shows significant discrepancies with respect to the observed field relationships.
Field mapping indicates de following sequence of intrusion: andesite, diorite, granodiorite, quartz
monzonite porphyry, pegmatite bodies, tan porphyry, aplites, however, K-Ar dates suggest that
the host diorite (49.8± 1.2 Ma) is younger than the quartz monzonite porphyry (53.1± 0.8 Ma)
and the pegmatite (55.2 to 50.4 Ma), both of which are observed to cut the quartz diorite
porphyry in the field. These discrepancies are possibly the result of resetting of the radiometric
clock by a later thermal event ( e.g. intrusion of pegmatites and or quartz monzonite porphyry).
Million years
Sample
Post mineralization el Globo Rhyolite
Diorite Porphyry at Caridad Deposit
Granodiorite at La Caridad
58
56
54
52
48
1
24
48.9 +/-1.9
2
50.4 +/- 1.9
Latite near Caridad Antigua1
Esperanza Ignimbrite (Pilares Nacozari)
26
48.9 +/-1.2
1
Esperanza Ignimbrite (Pilares Nacozari)
46
24 +/-0.4
Pegmatite at Santa Rosa Mine at Caridad Pit
51.3 +/- 1
2
52.2 +/- 1.9
3
Quartz Monzonite Porphyry at Caridad area
Mo in Phyllic Alteration
50
1
52.2 +/- 2.5
1
4
53.1 +/- 0.8
53.6 +/- 0.3
4
53.8+/- 0.3
Mo in Potassic Alteration
Pegmatite at Guadalupe mine at Caridad Pit
Pegmatite at Guadalupe mine at Caridad Pit
5
6
54.0 +/-0.8
55.2 +/-1.6
Dates determinated by K-Ar on biotites and sericites
Dates determinated by Re-Os on molibdenites
Figure 5.- Radiometric ages from La Caridad y La Caridad Antigua. In dark bands are the ages
related to the porphyry-high sulfidation epithermal link; note the similar ages. 1 Livingston
(1973), 2 Thoms (1978), 3 Worcester (1976), 4 this study, 5 Damon et al. (1983), 6 Damon
(1968).
On the other hand, two Re/Os ages from molybdenite veins contained in the quartz monzonite
porphyry and within the potassic and phyllic alteration phases at La Caridad Deposit show ages
of 53.6± 0.3 Ma and 53.8 ± 0.3 Ma. These ages are in agreement with the age previously reported
for the quartz monzonite porphyry (53.1± 0.8 Ma, Livingston, 1973). Latite from la Caridad
Antigua (East Block) has a reported K-Ar age of 51.3± 1.0 Ma (Livingston, 1973).
LA CARIDAD ANTIGUA
La Caridad Antigua is located 3 km east of La Caridad ore body in the hanging wall of La
Caridad Fault, and was described by Wandke (1925). This deposit was mined at the beginning of
the past century (1907-1916) and is hosted in latites and rhyolites that are intruded by a small
quartz monzonite porphyry. This deposit has an acid-sulfate epithermal mineral association that
includes pyrophillite, kaolinite, sericite, alunite, quartz and barite as alteration minerals and a
variety of sulfides that include chalcopyrite, pyrite, enargite, bornite and tennantite. The quartz
monzonite porphyry intrusion generated a massive silicic alteration that extends for more than
100 m laterally away from the deposit.
SW
La Caridad Pit
Cu-Mo
La Caridad
Fanglomerate
Fault
NE
La Caridad Antigua
Sulphates, Cu,Ag,Au
Dacite
Diorite-Andesite
Quartz
Monzonite
Porphyry
Granodiorite
Diorite
DCV-4
DCV-2
Graphic Scale
0
500m
Figure 6.- Schematic SW-NE cross section from La Caridad pit to La Caridad Antigua mine.
La Caridad Antigua is believed to represent the high level portion of a porphyry copper system
with a gap between both deposit of ~2-3 km. Given the relatively short period of hydrothermal
activity (53.6 ± 0.3 Ma, Re-Os in molybdenite in K and phyllic alteration), the identical ages for
the host rocks and mineralization, and the intimate spatial association, it is highly improbable that
the two deposits are independent and unrelated. It is more likely that the two deposits were
formed from a single magmatic-hydrothermal system that evolved from porphyry Cu-Mo deposit
at depth to high sulphidation epithermal at shallow levels.
CONCLUSIONS
La Caridad porphyry copper deposit is located at the contact between diorite-andesites and the
granodiorite, which controlled the location of the quartz monzonite porphyry, the mineralization
and the alteration.
Similar Re-Os mineralization ages obtained from the potassic and phyllic alteration zones
suggests a single and short period of mineralization
La Caridad porphyry copper is located in an untilted block and exhibits two dominant trends: a
N31oE/68o NW and a N26oW/ 70o SW, similar to those reported in other North America
porphyry deposits.
The mayor introduction of copper mineralization occurred during phyllic alteration event.
Field relationships, structures and radiometric ages support the general hypothesis of a spatial and
temporal link between La Caridad porphyry Cu-Mo deposit and La Caridad Antigua high
sulphidation epithermal system. This spatial and temporal link between these two deposits has
not been previously described in the North American porphyry copper province.
ACKNOWLEDGEMENTS
We are most grateful to Grupo Mexico for allowing access to the mine operations and logistics
and to the Arizona Geological Society for generous financial support. In particular, we would like
to thank to Ing. Remigio Martinez, Ing. Narcizo Javier Olvera, Ing. Jose Contla, Ing. Marco
Figueroa, Sergio Castro, Dr. John Chesley and Dr Chris Eastoe for their help and for interesting
discussions.
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