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Corresponding author: e-mail, jperello@aminerals.cl

*
Present address: Cordillera de las Minas S.A., Av. Paseo de la República 3245, Piso 3, San Isidro, Lima 27, Perú.

Abstract

The Cotabambas porphyry copper-gold cluster includes at least four porphyry centers in an area of 5 × 3 km, namely Ccalla, Azulccacca, Huaclle, and Ccarayoc, of which the first two are the best known. The geology of the area is dominated by two large granodiorite and diorite plutons and smaller stocks and dikes of microdiorite and andesite, all of which form part of the middle Eocene to early Oligocene Andahuaylas-Yauri batholith. At Cotabambas, granodiorite (K-Ar age of 39.8 ± 1.5 Ma) intrudes diorite (K-Ar age of 43.2 ± 1.1 Ma) and both, in turn, are intruded by a series of composite, structurally controlled porphyry stocks and dikes of granodioritic to quartz monzodioritic composition associated with porphyry copper-gold mineralization. The main structural features include several intersecting north-northeast- and west-northwest-trending faults, which are interpreted to have controlled the emplacement and architecture of the porphyry centers.

Most of the copper-gold mineralization at Cotabambas is associated with early-stage potassic alteration, including multiphase, magnetite-rich stockworks with quartz, K-feldspar, biotite, chalcopyrite, and bornite. Apatite and anhydrite are common constituents. An early, copper-bearing potassic-calcic assemblage made up of quartz, K-feldspar, biotite, actinolite, hornblende, diopside-hedenbergite, and magnetite is also preserved locally. Early, biotite-rich alteration from Ccalla yielded a K-Ar age of 35.7 ± 0.9 Ma. These mineral associations are overprinted by greenish-colored, intermediate argillic assemblages dominated by quartz, chlorite, illite, smectite, halloysite, and greenish soapy sericite, which have partially to completely destroyed earlier formed chalcopyrite-bornite associations but have contributed pyrite as disseminated grains and veinlets. These associations are spatially related to a series of inter- to late mineral porphyries.

All the previously described rocks and alteration assemblages are cut by a large, late mineral, domelike body of dacitic composition and an associated dike swarm. Both dome and dikes developed incipient alteration to calcite, illite, and chlorite and host a set of centimeter-wide veins with open spaces filled by quartz, calcite, sphalerite, and galena.

Supergene mineralization is present as patchily distributed copper oxides near surface underlain by an irregular supergene enrichment blanket dominated by sooty chalcocite. Supergene kaolinite and alunite are common. A K-Ar age of 3.3 ± 0.2 Ma on alunite shows a late Pliocene age for supergene enrichment and leaching processes in the region.

The mineralization at Cotabambas is assigned to the porphyry copper-gold class because (1) gold grades are typically >0.3 ppm; (2) copper-gold mineralization is accompanied by abundant (>5 vol %) hydrothermal magnetite in potassic alteration; (3) hydrothermal amphibole and pyroxene are present in potassic-calcic alteration; (4) copper and gold display a sympathetic relationship, and all observed Au occurs as micron-sized inclusions in chalcopyrite; (5) intense pyrite-rich intermediate argillic alteration overprints earlier formed potassic and calcic-potassic alteration and associated copper-gold mineralization; and (6) molybdenum contents are low (<0.01%).

The coincident K-Ar and fission-track (apatite) ages (38.6 ± 3.4 and 33.3 ±1.4 Ma) for plutons from the Cotabambas area, together with the late Eocene age of Cotabambas (see above) and regional data, confirm that porphyry copper emplacement in the Andahuaylas-Yauri belt of southeastern Peru took place simultaneously with intense shortening, surface uplift, and rapid exhumation during the middle to late Eocene Incaic orogeny.

Introduction

The Cotabambas porphyry copper-gold deposit (14°10′52″ lat S; 72°20′53″ long W) is located in the vicinity of the town of Cotabambas, approximately 50 km southwest of Cuzco, at an elevation between 3,400 and 3,800 m asl (Fig. 1). The región is characterized by elevations above 4,000 m and by deep (>2,000-m), steep-sided canyons of the Apurímac River and its tributaries. The climate is characterized by dry, cold winters and seasonal summer rains between December and March, with an average annual precipitation of ~1,000 mm and average daily temperatures of 13°C.

Fig. 1.

Location map of the Cotabambas area, Cuzco region, Peru. Main regional physiographic units and tectonic elements after Jaillard et al. (2000).

Fig. 1.

Location map of the Cotabambas area, Cuzco region, Peru. Main regional physiographic units and tectonic elements after Jaillard et al. (2000).

Exploration and discovery

The Cotabambas area has been the site of intermittent exploration and small-scale mining since the beginning of the 20th century. The area was staked in 1911 by a private individual and later, in 1951 and 1962, by Cerro de Pasco Corporation; apparently no exploration work was undertaken.

Anaconda Peru first visited Cotabambas in 1994, carried out reconnaissance geologic work and rock-chip geochemistry with positive results, and claimed mining title to the area (Sutcliffe, 1994). Work in 1995 identified the essential porphyry-type characteristics of the alteration and mineralization, led to the recommendation for immediate detailed geologic mapping and additional geochemistry, and to the conclusion that Cotabambas was a high-priority target worthy of reconnaissance drilling. Mapping conducted in the second half of 1995 (Neyra, 1995) defined the salient features of two main target areas at Ccalla and Azulccacca. Ground magnetic and IP surveys in May 1996 confirmed these targets. Diamond drilling commenced in July 1996 and by December, six holes totaling 1,987 m had been completed (Fig. 2). All holes from this program intersected ore-grade copper mineralization, with the most significant intersections (Zárate, 1996) including CB1 (214–292 m: 0.94% Cu and 0.43 ppm Au), CB2 (0–164 m: 0.87% Cu and 0.26 ppm Au), CB3 (0–226 m: 0.89% Cu and 0.50 ppm Au), CB4 (22–170 m: 0.84% Cu and 0.75 ppm Au), and CB6 (18–128 m: 1.0% Cu and 0.46 ppm Au).

Fig. 2.

Location of discovery drill holes and the main Ccalla and Azulccacca porphyry systems.

Fig. 2.

Location of discovery drill holes and the main Ccalla and Azulccacca porphyry systems.

This program confirmed the copper-gold nature of the porphyry system, its potential, and the many similarities of the mineralization to other gold-rich porphyry systems of the Pacific rim (Sillitoe, 1997). Additional drilling campaigns in 1999 and 2000 contributed to the total of 8,538 m of core drilling which were used to define the alteration and mineralization (Perelló et al., 2001, 2002) and geologic resources for the main target at Ccalla (112 million metric tons (Mt) at 0.62% Cu and 0.36 ppm Au) and the smaller Azulccacca zone (24 Mt at 0.42% Cu and 0.39 ppm Au). Subsequent additional work has identified porphyry-style alteration and mineralization at the Huaclle and Ccarayoc zones (see Fig. 5 below).

Fig. 5.

Geologic map of the Cotabambas porphyry copper-gold cluster. Based on mapping by the authors.

Fig. 5.

Geologic map of the Cotabambas porphyry copper-gold cluster. Based on mapping by the authors.

Regional Setting

Cotabambas belongs to the 300-km-long, middle Eocene to early Oligocene Andahuaylas-Yauri belt of southern Peru, a belt originally defined for its magnetite-rich copper skarns (Bellido et al., 1972; Santa Cruz et al., 1979; Noble et al., 1984) but lately also recognized for its porphyry-style mineralization (Perelló et al., 2002, 2003a).

The regional geology has been described by Carlotto (1998) and Perelló et al. (2003a), whereas the metallogenic relationships have been discussed by Perelló et al. (2003a). A large, multiphase, composite batholith (Andahuaylas-Yauri batholith; Bonhomme and Carlier, 1990) intrudes marine and continental sedimentary and volcanic sequences of Mesozoic to Cenozoic age (Fig. 3a-b). These include clastic rocks of the Jurassic Yura Group; carbonate horizons of the Cretaceous Ferrobamba Formation, which constitutes the main host to skarn-type mineralization in the region; and the fluviodeltaic and lacustrine rocks of the Paleocene Quilque and Chilca Formations (Fig. 4). Cenozoic units include the andesitic volcanic and sedimentary Anta Formation and the red beds of the San Jerónimo Group, both of Eocene to early Oligocene age (Carlotto, 1998; Perelló et al., 2003a; Figs. 34).

Fig. 3.

a. Simplified geologic map of the Andahuaylas-Yauri belt (after Perelló et al., 2003a). b. Schematic stratigraphic column of the same belt with emphasis on age relationships between the Andahuaylas-Yauri plutons, the volcanic rocks of the Anta Formation, and porphyry copper mineralization (simplified after Perelló et al., 2003a).

Fig. 3.

a. Simplified geologic map of the Andahuaylas-Yauri belt (after Perelló et al., 2003a). b. Schematic stratigraphic column of the same belt with emphasis on age relationships between the Andahuaylas-Yauri plutons, the volcanic rocks of the Anta Formation, and porphyry copper mineralization (simplified after Perelló et al., 2003a).

Fig. 4.

Simplified geologic map of the Cotabambas quadrangle. Based on work by Carlotto and Cárdenas (2000) and reconnaissance mapping by the authors.

Fig. 4.

Simplified geologic map of the Cotabambas quadrangle. Based on work by Carlotto and Cárdenas (2000) and reconnaissance mapping by the authors.

The Andahuaylas-Yauri batholith comprises a multitude of intrusions that crop out discontinuously for >300 km, including early-stage cumulates (gabbro, troctolite), followed by rocks of intermediate composition (monzodiorite, quartz diorite, granodiorite) that make up the bulk of the batholith. Subvolcanic rocks of dominantly dacitic composition, locally associated with porphyry-style mineralization, represent the terminal stage. The age of the batholith is constrained by isotopic ages (Carlier et al., 1996; Carlotto, 1998; Perelló et al., 2002, 2003a) that cluster between 48 and 32 Ma, with early cumulate phases yielding ages between 48 and 43 Ma and intermediate-stage plutons returning ages of ~40 to 32 Ma (Fig. 3b). The ages also confirm that the batholith was essentially coeval with the Anta Formation and San Jerónimo Group rocks in the middle Eocene to early Oligocene. Andesite and conglomerate of the Anta Formation are interpreted to be stratigraphic equivalents of the San Jerónimo Group red beds, with erosion products of the Anta Formation and the Andahuaylas-Yauri batholith feeding the nearby San Jerónimo basins (Carlotto, 1998; Perelló et al., 2003a; Fig. 3b).

Postbatholith rocks of the región include the late Oligocene to Miocene red bed sedimentary deposits of the Punacancha and Paruro Formations, which are interpreted to have been deposited in a fluvial environment in structurally controlled basins (Carlotto et al., 1996; Carlotto, 1998; Jaimes et al., 1997; Romero et al., 1997). Oligocene to Miocene volcanic rocks of the región are mainly calc-alkaline, rhyolitic, dacitic, and andesitic sequences assigned to the Tacaza (Oligocene) and Sillapaca (Miocene) Groups (Klinck et al., 1986; Clark et al., 1990; Sandeman et al., 1995).

Regional structure is dominated by two main northwest-trending fault systems (Limatambo-Ayaviri and Abancay-Yauri) with exposed lengths of >300 km that show evidence for both high-angle reverse and strike-slip movements (Fig. 3a). These structures transpose Paleozoic plutonic basement rocks over younger cover sequences in the Abancay deflection, west of the study area, and place deep-seated cumulate facies of the Andahuaylas-Yauri batholith on top of either younger intrusions of the same batholith or the Anta Formation (Perelló et al., 2003a). Southeastern extensions of these structures are interpreted to have been associated with major folding and thrusting approximately 300 km from Cotabambas, in the Santa Lucía area (Jaillard and Santander, 1992).

The corridor defined by these major fault systems is occupied by the synorogenic deposits of the Anta Formation and the San Jerónimo Group and both basins and bounding faults are interpreted to have been active during a period of intense regional uplift and erosion associated with the Incaic orogeny (Perelló et al., 2003a). In the area of Figure 4, the Cotabambas fault is part of the same compressive corridor, which is cut and partly offset by a series of north-south- and north-northeast-trending tear structures. These faults also control the shape and extent of the Anta Formation north of Cotabambas, which further suggests simultaneity of sedimentation and faulting during contractional deformation. Examples include the Río Vilcabamba lineament west of Cotabambas and the Huaclle-San José fault corridor that contains the Cotabambas deposit cluster (Fig. 4).

Geology of the Cotabambas Area

The Cotabambas porphyry cluster is dominated by two plutons of diorite and granodiorite, together with numerous dikes and stocks of andesite and microdiorite, all belonging to the Andahuaylas-Yauri batholith. Field evidence indicates that diorite is intruded by andesite and microdiorite and that both, in turn, are intruded by granodiorite (Perelló et al., 2002, 2003a; Fig. 5).

Diorite is characteristically dark gray, with a fine- to medium-grained hypidiomorphic inequigranular texture made up of tabular calcic plagioclase and ferromagnesian minerals, defining an overall mesocumulate texture. Biotite, hornblende, and subordinate augitic pyroxene are the main ferromagnesian components. Accessory minerals include apatite, titanite, and zircon. The unit also includes quartz diorite, tonalite, and monzodiorite, but these are not distinguished in the geologic map (Fig. 5). Microdiorite and andesite stocks and dikes are similar in composition to the main diorite pluton, but grain size and textures differ. Granodiorite is light gray and has a coarse-grained, slightly porphyritic texture with intermediate-composition plagioclase and large hornblende grains in a groundmass consisting mainly of quartz and K-feldspar. Monzogranite has also been identified.

All units described above constitute the country rocks to the porphyry systems. The Cotabambas cluster is composed of four main centers at Ccalla, Azulccacca, Huaclle, and Ccarayoc, plus smaller centers at Ccochapata Norte, Huaclle Este, and Ccalla Sur (Fig. 5). All centers are associated with structurally controlled, multiphase porphyritic intrusions of intermediate composition, with Ccalla and Azulccacca being the best known to date. Much of the description that follows is based on these two systems.

Structure

The structure of the area includes two main northeast-trending fault systems that define a 6-km-long by 2-km-wide corridor that extends beyond the area of study (Fig. 4). The bounding structures include the composite Ccalla and Azulccacca faults on the east and the larger Huaclle-San José fault system on the west. These faults partly truncate and offset the regional, high-angle reverse Asnoccacca fault and the smaller Ccochapata fault that connects Huaclle and Ccalla. To the north of Cotabambas, near the Apurímac River (Figs. 45), the Ccalla and Huaclle-San José systems form a large, horsetail-like structural array.

The Ccalla and Azulccacca faults and related splays exert a strong control on the location and geometry of the Ccalla and Azulccacca porphyry centers. This is particularly apparent at Ccalla, where all porphyry-related intrusions and late mineral dikes and domes are either directly associated with the faults or follow the same structural trend (Fig. 5). Similarly, the Huaclle-San José fault controls the location of Huaclle and, possibly, Ccarayoc. Beyond the Cotabambas area north of the Apurímac River it seems to have controlled the emplacement of both late Eocene porphyry-style mineralization and barren Pliocene volcanic centers at Morosayhuas (Perelló et al., 2003a; Fig. 4). Detailed observation of fault planes from the various splays of the Ccalla and Azulccacca fault in the area of Cotabambas provide evidence for dextral displacement, which is coincident with lateral ramping associated with the shortening proposed above.

Lithologic units

The Ccalla center is composed of several porphyritic dikes and small stocks for which two dominant phases, namely principal and intermineral porphyries, have been mapped. Both, in turn, are intruded and offset by a swarm of late mineral dacite dikes genetically associated with a dome complex (see below).

Principal and intermineral phases of the system are similar in texture and composition and are distinguished by the relative intensities of the quartz stockworks. Principal porphyry phases possess moderate to intense, multidirectional veinlet arrays, which are characteristically absent in the late intermineral facies. Coarse-grained (up to 2 cm) porphyritic textures are typically developed, with abundant plagioclase (30–35 vol %) and ferromagnesian minerals (~10 vol %); amphibole predominates over biotite. Groundmass consists of a fine-grained aggregate of quartz and K-feldspar. Plagioclase phenocrysts display subhedral, tabular forms with abundant polysynthetic twinning, whereas amphibole is prismatic and biotite occurs as anhedral grains. Both principal and intermineral phases possess subordinate but conspicuous amounts of sanidine, in subrounded grains with adsorbed edges and millimetric poikilitic inclusions of plagioclase and undifferentiated ferromagnesians. Other facies within the same units display glomeroporphyritic textures, with varied accumulations of plagioclase and K-feldspar, the latter rich in biotite inclusions. Apatite needles and subordinate titanite and zircon are the main accessory minerals.

Several generations of porphyry intrusion are present in all systems of the cluster but are more apparent at Ccalla, Azulccacca, and Ccarayoc, where intermineral dikes with chilled margins, fragments of older porphyry phases with truncated quartz stockworks contained in younger phases, and local development of igneous (contact) breccia are common.

District-wide, end-stage magmatic evolution of the cluster at Cotabambas includes at least one large, composite dome (Ccochapata dome) and an associated dike swarm, both of which intrude all previously described units. Dome and dikes are of dacitic composition and display coarse- to medium-grained porphyritic textures with up to 1-cm-long phenocrysts of plagioclase (35–40 vol %), K-feldspar (1–3 vol %), and hornblende (~5 vol %). Small amounts of biotite are also present. Plagioclase is dominantly of oligoclase composition and K-feldspar is sanidine, the latter typically in large, sub-rounded to subequant crystals with poikilitic inclusions of plagioclase, amphibole, and biotite. Large, well-terminated, prismatic hornblende phenocrysts contain inclusions of quartz and K-feldspar, whereas biotite occurs as fine-grained, anhedral grains. The groundmass has a fine-grained, flow-banded, pilotaxitic texture, with banding imparted by alignment of very fine grained plagioclase, ferromagnesian minerals, and vesicular cavities partly filled by quartz and chlorite. Glomeroporphyritic textures, with plagioclase and sanidine accumulations, are locally present. Accessory minerals are apatite, titanite, and zircon.

Mapping has shown the presence of several generations of dikes. All dikes and the associated Ccochapata dome are late mineral in timing, because they crosscut main and intermineral porphyry phases, lack quartz stockworks, and are barren of copper-gold mineralization. However, they characteristically host one or more veinlet associations of quartz, calcite, sphalerite, and/or galena.

The various phases of principal and intermineral porphyries form irregularly shaped bodies as much as 400 m in diameter, from which smaller dikes radiate (Fig. 1). These dikes vary in width, strike length, and attitude but typically are as much as 50 m wide and are continuous for >500 m along a dominantly northeast direction. Reconstruction of the principal and intermineral porphyry mass prior to the intrusion of the late mineral dacitic dikes at Ccalla shows a main body of triangular form, with dominant northeast- and northwest-controlling intrusion directions (Fig. 6). In cross section (Fig. 7), the principal porphyry is preserved as irregular, funnel-shaped bodies that appear to decrease in volume with depth. Intermineral porphyry intrusions and late mineral dacite dikes, on the contrary, appear to widen at depth.

Fig. 6.

Schematic restoration of principal and intermineral porphyry phases at Ccalla prior to the emplacement of the late mineral Ccochapata dome and dike swarm.

Fig. 6.

Schematic restoration of principal and intermineral porphyry phases at Ccalla prior to the emplacement of the late mineral Ccochapata dome and dike swarm.

Fig. 7.

Schematic cross section, displaying the salient geologic elements of the Ccalla deposit at Cotabambas. See Figure 5 for location.

Fig. 7.

Schematic cross section, displaying the salient geologic elements of the Ccalla deposit at Cotabambas. See Figure 5 for location.

Late mineral dikes range from a few centimeters to 150 m in width and extend for as much as 2.5 km along a preferred northeast trend (Fig. 5). The Ccochapata dome near Ccalla is clearly the source of the dike swarm that cuts through the Ccalla and Azulccacca systems. The main mass of the dome has well-developed columnar jointing and flow foliation, both of which are typical of endogenous dacitic domes. A second major body of dacite, in the area of Huaclle (Fig. 5), displays similar features and constitutes a separate dome center. In conclusion, the domes and dikes define a large, northeast-trending magmatic corridor with a width of between 1 and 3 km and a length of approximately 6 km. This corridor is situated between the Ccalla-Azulccacca and Huaclle-San José fault systems, which is taken as evidence for an intimate relationship between structure and porphyry copper-gold mineralization in the region.

Because of the intense hydrothermal alteration affecting principal and interminerai porphyry phases, their overall composition is difficult to determine. However, the general absence of quartz phenocrysts, the presence of sanidine as an important phenocryst phase, and the presence of subordinate amounts of fine-grained quartz and K-feldspar in the ground-mass of the freshest samples suggest a granodiorite to quartz monzodiorite composition. Moreover, the presence of abundant sanidine phenocrysts in both porphyry-related and late mineral dacitic intrusions suggests emplacement in a shallow subvolcanic environment.

Hydrothermal Alteration

The following description, based primarily on detailed petrographic work at Ccalla and Azulccacca, is considered to be applicable to all systems of the cluster.

Several mineral associations define potassic, potassic-calcic, sericitic, intermediate argillic, and propylitic zones (Fig. 8). They are all indistinctly overprinted by a supergene assemblage of widespread kaolinite and local alunite that defines a ~100- to 150-m-thick blanket that broadly mimics presentday topography. Its thickness is greater where faulting and fracturing are more intense.

Fig. 8.

Schematic cross section, showing the complex array of main alteration zones and assemblages at Ccalla. See Figure 5 for location.

Fig. 8.

Schematic cross section, showing the complex array of main alteration zones and assemblages at Ccalla. See Figure 5 for location.

The hydrothermal alteration assemblages bear a direct relationship to the original host lithology. Potassic alteration is intimately related to the principal porphyry phases and their immediate dioritic country rock and, to a lesser extent, to intermineral porphyry intrusions. The latter typically contain intermediate argillic alteration assemblages. Potassic-calcic alteration is only developed in certain contact zones between principal porphyry and diorite. Propylitic alteration occurs peripherally in diorite, microdiorite, and granodiorite country rocks. Sericitic alteration assemblages are of low intensity in general and are confined to late mineral dacite dikes, except for a small zone of intense quartz-sericitic alteration developed in the upper parts of the Ccalla deposit.

Potassic alteration

Potassic alteration is characterized mainly by the presence of K-feldspar (orthoclase), biotite, and quartz. In general, original plagioclase grains display progressive replacement by K-feldspar, from edges to core, whereas biotite and amphibole are altered to various associations of phlogopitic biotite, K-feldspar, quartz, and chlorite, accompanied by apatite, sphene, and rutile. Augitic pyroxene in the diorite country rock is altered to quartz, K-feldspar, and biotite, with local chlorite, epidote, apatite, rutile, and sphene.

The bulk of the potassic alteration is present as disseminations and in a multitude of millimetric to centimetric veinlet arrays. Veinlets vary from planar to irregular, discontinuous forms, with or without halos, with one or more associations of quartz, K-feldspar, biotite, chlorite, apatite, anhydrite, and calcite, accompanied by chalcopyrite, bornite, magnetite, and trace pyrite (see below). More than 20 veinlet assemblages have been identified, with some of the most important including (1) diffuse veinlets of quartz-K-feldspar-biotite-apatite-(chlorite); (2) K-feldspar hairline fractures with halos of biotite-(chlorite); (3) subparallel veinlets and hairline fractures with quartz-K-feldspar-(chlorite)-(calcite)-(apatite) and alteration halos with abundant apatite and K-feldspar; (4) banded, parallel, ribbonlike veinlets with quartz-K-feldspar-biotite-anhydrite-apatite with abundant magnetite; and (5) planar veinlets with quartz-K-feldspar-chlorite-(biotite) with incipient halos composed of chlorite, apatite, calcite, and K-feldspar. Magnetite is an important component of all the veinlets and dominates in the ribbonlike ones (Table 1). Anhydrite and its hydration product, gypsum, are also common constituents. Significant variations in veinlet mineralogy are apparent throughout the systems, with both abrupt and gradational changes in the mineralogy of single veinlets observable at megascopic and microscopic scales. All veinlet types noted above display broad similarities with the A- and B-type veinlets defined by Gustafson and Hunt (1975) and with the magnetite-rich M-type veinlets of Clark and Arancibia (1995), all of which are also common in other porphyry deposits in the Andahuaylas-Yauri belt (Perelló et al., 2003a).

Table 1.

Summary of Main Alteration and Mineralization Assemblages at Cotabambas

AlterationMineralization
Main productsVeinletsMain mineralogyVeinlets
Potassic (biotite, K-feldspar, quartz, apatite)
Plagioclase→K-feldsparQuartz-K-feldspar-biotite-apatiteChalcopyrite, magnetite, bornite, pyrite1Magnetite-chalcopyrite
Biotite, amphibole→biotite, K-feldspar, chloriteQuartz-K-feldspar with K-feldspar-apatite halosMagnetite-chalcopyrite-bornite
K-feldspar with biotite halosMagnetite-chalcopyrite-pyrite
Pyroxene→quartz, K-feldspar, biotite, chlorite1 epidote1Banded quartz-K-feldspar-biotite-anhydrite-apatiteChalcopyrite-bornite
Quartz-K-feldspar-chlorite-biotite with chloriteapatite-calcite-K-feldspar halos
Potassic-calcic (quartz, actinolite, hornblende, biotite, K-feldspar, pyroxene, apatite)
Plagioclase→K-feldspar, calcite, epidoteActinolite-apatiteChalcopyrite, magnetite, Bornite, pyrite1M agnetite-chalcopyrite
Biotite, amphibole→actinolite, apatiteQuartz-hornblende-actinolite-K-feldspar with actinolite-K-feldspar halos
Pyroxene→actinolite, apatite →actinolite, biotite1Quartz-pyroxene-actinolite-K-feldspar
Quartz-actinolite with biotite halos
Actinolite-K-feldspar-apatite with biotite-epidote halos
Intermediate argillic (quartz, greenish sericite, chlorite, illite-smectite, halloysite, epidote)
Plagioclase→sericite, calcite,1 illite,1 epidote1Quartz-chlorite-calcite with sericite-halloysiteillite-smectite alteration halosPyrite, chalcopyrite, hematitePyrite-chalcopyrite
Amphibole, biotite→chlorite, smectite
Magnetite→martite
Sericitic (white sericite, illite, calcite)
Plagioclase→sericite, illite, calcite1Quartz-calcite with incipient sericite halosPyrite, sphalerite, galena, chalcopyriteChalcopyrite-galena-sphalerite-pyrite
Ferromagnesians→chlorite, calcite, smectite1Calcite-anhydrite-gypsum with sericite-calcite halos
Propylitic (chlorite, epidote, calcite)
Plagioclase→calcite, epidoteCalcite-gypsumPyrite, chalcopyrite1Pyrite
Ferromagnesians→chlorite, epidote, calciteChlorite-epidote-calcite
AlterationMineralization
Main productsVeinletsMain mineralogyVeinlets
Potassic (biotite, K-feldspar, quartz, apatite)
Plagioclase→K-feldsparQuartz-K-feldspar-biotite-apatiteChalcopyrite, magnetite, bornite, pyrite1Magnetite-chalcopyrite
Biotite, amphibole→biotite, K-feldspar, chloriteQuartz-K-feldspar with K-feldspar-apatite halosMagnetite-chalcopyrite-bornite
K-feldspar with biotite halosMagnetite-chalcopyrite-pyrite
Pyroxene→quartz, K-feldspar, biotite, chlorite1 epidote1Banded quartz-K-feldspar-biotite-anhydrite-apatiteChalcopyrite-bornite
Quartz-K-feldspar-chlorite-biotite with chloriteapatite-calcite-K-feldspar halos
Potassic-calcic (quartz, actinolite, hornblende, biotite, K-feldspar, pyroxene, apatite)
Plagioclase→K-feldspar, calcite, epidoteActinolite-apatiteChalcopyrite, magnetite, Bornite, pyrite1M agnetite-chalcopyrite
Biotite, amphibole→actinolite, apatiteQuartz-hornblende-actinolite-K-feldspar with actinolite-K-feldspar halos
Pyroxene→actinolite, apatite →actinolite, biotite1Quartz-pyroxene-actinolite-K-feldspar
Quartz-actinolite with biotite halos
Actinolite-K-feldspar-apatite with biotite-epidote halos
Intermediate argillic (quartz, greenish sericite, chlorite, illite-smectite, halloysite, epidote)
Plagioclase→sericite, calcite,1 illite,1 epidote1Quartz-chlorite-calcite with sericite-halloysiteillite-smectite alteration halosPyrite, chalcopyrite, hematitePyrite-chalcopyrite
Amphibole, biotite→chlorite, smectite
Magnetite→martite
Sericitic (white sericite, illite, calcite)
Plagioclase→sericite, illite, calcite1Quartz-calcite with incipient sericite halosPyrite, sphalerite, galena, chalcopyriteChalcopyrite-galena-sphalerite-pyrite
Ferromagnesians→chlorite, calcite, smectite1Calcite-anhydrite-gypsum with sericite-calcite halos
Propylitic (chlorite, epidote, calcite)
Plagioclase→calcite, epidoteCalcite-gypsumPyrite, chalcopyrite1Pyrite
Ferromagnesians→chlorite, epidote, calciteChlorite-epidote-calcite

Notes: Arrows indicate hydrothermal alteration products

1 Minor component

In addition, potassic alteration at Ccalla and Azulccacca is characterized by a texture-destructive association of quartz and K-feldspar, with development of coarse-grained aggregates with graphic texture and complete destruction of original rock constituents.

Potassic-calcic alteration

Potassic-calcic alteration is characterized principally by the presence of quartz, actinolite, hornblende, and magnetite plus the addition of pyroxene (diopside-hedenbergite), apatite, and calcite (Table 1). Some assemblages, transitional to standard potassic alteration, contain, in addition, appreciable amounts of K-feldspar and biotite. The general features of the potassic-calcic alteration include replacement of calcic plagioclase grains of the diorite by K-feldspar, calcite, and/or epidote, and indistinct replacement of biotite and amphibole by aggregates of acicular actinolite and apatite. Pyroxene phenocrysts are variably replaced by actinolite-apatite or actinolite-phlogopitic biotite.

At least ten types of veinlets belonging to this alteration facies have been recognized, among the most common being the following (Table 1): (1) actinolite-apatite hairline fractures with or without K-feldspar halos; (2) hairline fractures and millimetric veinlets of quartz-hornblende-actinolite-K-feldspar with or without actinolite-(K-feldspar) halos; (3) discontinuous hairline fractures with quartz-pyroxene (diopside/hedenbergite)-actinolite-(K-feldspar); (4) hairline fractures with quartz-actinolite and biotite-(chlorite) halos; and (5) veinlets of actinolite-K-feldspar-apatite with biotite-epidote halos. All of these are rich in magnetite and contain variable amounts of chalcopyrite and pyrite.

Contact relationships mapped in drill core from Ccalla and Azulccacca indicate that the potassic and potassic-calcic alteration assemblages noted above evolved essentially at the same time, judging by the K-feldspar and biotite contained in veinlets dominated by actinolite and pyroxene and by the presence of actinolite-rich veinlets that crosscut biotite-bearing potassic associations.

Intermediate argillic alteration

Intermediate argillic alteration is characterized by a pale, olive-green mixture of chlorite, illite, smectite, halloysite, and green soapy sericite, with subordinate amounts of epidote and calcite (Table 1). Plagioclase is invariably replaced by calcite, sericite, illite, and minor epidote, whereas amphibole and biotite are preferentially altered to assemblages containing chlorite, smectite (vermiculite), illite, and greenish sericite.

Intermediate argillic alteration is spatially and temporally associated with intermineral porphyry intrusions and is characteristically superimposed on earlier formed potassic and potassic-calcic assemblages. It occurs as pervasive disseminations with important local structural control, in intimate association with various generations of centimetric to millimetric veinlets dominated by quartz, chlorite, and calcite that typically develop well-defined alteration with sericite, halloysite, illite, and smectite. Partial to complete replacement of original rock textures and components is attained in zones of intense alteration, particularly where vein density is high and alteration halos coalesce. The dominant veinlet style of this alteration is similar, but not identical, to the D-type veins defined by Gustafson and Hunt (1975). Magnetite in this zone is remnant and typically shows evidence of martitization, whereas overall contents of pyrite and chalcopyrite increase and decrease, respectively, relative to the earlier formed potassic and potassic-calcic alteration.

Sericitic alteration

Sericitic alteration is characterized by illite and sericite (fine-grained muscovite), with abundant calcite and subordinate chlorite. Plagioclase is typically altered to sericite, illite, and calcite, whereas ferromagnesians are weakly replaced by chlorite and calcite, with local minor smectite. This alteration style preserves the original rock textures of the host dacitic dikes, in which the assemblage is exclusively developed. The dikes are locally crosscut by isolated veins and weakly developed vein swarms characterized by open-space filling textures with (Table 1) quartz, calcite, and base metal sulfides with incipient sericitic alteration halos and calcite, anhydrite, and gypsum with well-developed halos rich in sericite and calcite. More intense sericitic alteration, accompanied by quartz, is locally present in the shallow central parts of the Ccalla deposit and corresponds to more conventional quartz-sericitic alteration. The assemblage is whitish and composed of flaky sericite, illite, and abundant quartz in micromosaics that completely obliterate original rock texture and composition. This style is also intimately associated with faults and zones of hydrothermal breccia that crosscut and overprint the principal porphyry affected by background potassic alteration.

Propylitic alteration

Propylitic alteration is characterized by chlorite, epidote, and calcite in nontexture destructive assemblages that are distributed around each of the porphyry centers. Important quartz veining is absent but veins of calcite and gypsum are locally present. Magnetite is stable in this environment and pyrite is more abundant than in the central potassic zone.

Mineralization

Copper-gold

Copper-gold mineralization at Ccalla and Azzulccacca can be subdivided into an upper oxide zone and a lower hypogene sulfide zone, separated by an irregular supergene sulfide zone.

The oxide zone has a maximum thickness of approximately 50 m. Copper is contained mainly in chrysocolla, malachite, and neotocite, in association with goethitic and subordinate jarositic limonite and trace amounts of copper sulfides and relict pyrite. The supergene sulfide zone is an irregular, structurally controlled blanket ~20 m thick and containing chalcocite and covellite. Both the form and top of the blanket mimic the present-day topographic features of the area. Sooty chalcocite forms irregular coatings on disseminated pyrite and chalcopyrite as well as on pyrite in D veins and related structures. Chalcocite formation was favored by the presence of pyrite contained in neutral to weakly reactive sericitic and intermediate argillic alteration in intensely fractured zones. Covellite tends to become more common with depth.

The lower hypogene sulfide zone (Fig. 9) is intimately associated with multiphase stockworks and unidirectional arrays of millimetric and centimetric veinlets of A, B, and M types (see above), in potassically altered rock of the principal porphyry and adjacent diorite. Copper-gold mineralization is dominantly contained in steeply inclined, tabular zones of intense quartz veining, typically carrying >30 veinlets per meter. Veinlets are, in general, of varied form, composition, and distribution, with the following types predominanting: (1) quartz-magnetite, (2) quartz-magnetite-chalcopyrite, (3) quartz-magnetite-chalcopyrite-bornite, (4) quartz-magnetite-chalcopyrite-(pyrite), (5) quartz-chalcopyrite-pyrite, (6) quartz-pyrite, and (7) hairline fractures with chalcopyrite-bornite-magnetite.

Fig. 9.

Schematic cross section through the Ccalla deposit at Cotabambas, showing the distribution of selected copper zones. See Figure 5 for location.

Fig. 9.

Schematic cross section through the Ccalla deposit at Cotabambas, showing the distribution of selected copper zones. See Figure 5 for location.

In most of the veinlet assemblages described above, the paragenetic sequence characteristically starts with magnetite, followed by chalcopyrite, and ends with trace pyrite, where the latter is present. Similarly, those veinlets that contain magnetite and chalcopyrite, with or without bornite, tend to be the earliest in the veinlet sequence. Detailed petrographic work has also shown that chalcopyrite and bornite develop complex intergrowths that indicate simultaneity of formation, whereas in those veinlets where magnetite predominates, chalcopyrite tends to occupy interstitial spaces between larger magnetite grains. Locally, chalcopyrite also contains microscopic inclusions of bornite. Actinolite is intergrown with magnetite and chalcopyrite in veinlets of the potassic-calcic zone, whereas intense intermediate argillic-altered rock is rich in pyrite and contains martitized magnetite and specular hematite.

Disseminated mineralization, in general, is consistent with these relationships, with magnetite, chalcopyrite, and trace amounts of pyrite occurring in millimetric accumulations in which chalcopyrite and/or pyrite rim magnetite. Visual estimates of total sulfide contents average 3 to 5 vol percent, whereas magnetite contents range between 5 and 10 vol percent.

Gold possesses a positive, albeit poorly defined, relationship with copper (Fig. 10). This suggests that much of the gold is contained in, or attached to, chalcopyrite and/or bornite, which is supported by the presence of very small (<0.05 mm) inclusions of native gold in chalcopyrite.

Fig. 10.

Plots displaying the overall positive correlation between Cu and Au. a. Ccalla deposit, b. Azzulccacca deposit.

Fig. 10.

Plots displaying the overall positive correlation between Cu and Au. a. Ccalla deposit, b. Azzulccacca deposit.

Lead-zinc

Galena and sphalerite have been recognized only at Ccalla in the vicinity of the Ccochapata dome. Mineralization is spatially and genetically associated with a series of millimetric to decimetric veinlets of quartz and calcite, with open-space filling textures and incipiently to moderately developed sericitic alteration halos (Table 1). Both quartz and calcite typically display comb texture and well-terminated crystals. Voids are filled by various associations of galena, sphalerite, and chalcopyrite in decreasing order of abundance. Locally, anhydrite and/or gypsum are present in minor amounts. These veinlets are dominantly hosted by late mineral dacitic dikes of the Ccochapata dome, where they occur in fault zones intimately associated with small-scale, pyrite-rich pebble dikes with intense sericitic alteration.

Both field relationships and the physical attributes of these veinlets suggest that they constitute part of a final, low-temperature stage of alteration and mineralization, possibly in an intermediate-sulfidation epithermal environment associated with the emplacement of the Ccochapata dome and related dikes. Metal contents are low in general, with the best intervals rarely assaying >0.1 percent Pb + Zn over several meters.

Molybdenum

Molybdenite is a rare constituent of the mineralization at Cotabambas. It has been observed in several veinlet types of the intermediate argillic and sericitic alteration zones at Ccalla and Azulccacca, but its distribution is poorly defined. Molybdenum grades are typically low, <100 ppm, and have not been included in any resource estimate.

Age

The age of the Cotabambas cluster is constrained by regional and district correlations (Perelló et al., 2001, 2002, 2003a), together with a series of K-Ar and apatite fission-track ages obtained on various intrusions and alteration assemblages from the area (Tables 23).

Table 2.

K-Ar Ages from Cotabambas and Adjacent Area1

Sample no.Material datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (Ma, ±2σ)
COTALKAR-02Alunite7.3970.986673.3 ± 0.2
COTKAR-01Magmatic biotite7.5561.3351743.2 ± 1.1
COTKAR-02Magmatic amphibole20.8121.2713439.8 ± 1.1
CCALLAKARHydrothermal biotite7.39310.3751035.7 ± 0.9
Sample no.Material datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (Ma, ±2σ)
COTALKAR-02Alunite7.3970.986673.3 ± 0.2
COTKAR-01Magmatic biotite7.5561.3351743.2 ± 1.1
COTKAR-02Magmatic amphibole20.8121.2713439.8 ± 1.1
CCALLAKARHydrothermal biotite7.39310.3751035.7 ± 0.9

Notes: Constants: λβ = 4.962 × 10−10y−1; λε = 0.581 × 10−10y−1; 40Ar/36Ar = 295.5; 40K = 0.01167 atom %

1 See Figures 4 and 5 for location

2 Some degree of alteration to chlorite present

Table 3.

Apatite Fission-Track Ages, Cotabambas Area, Peru1

Sample no.GrainsRhosNsRhoiNiChi-RhodNd SquareAge (Ma, ±2σ)
COTKAR01170.76416954.5834,169951.1325,05833.3 ± 1.4
COTKAR02160.1651610.853832981.1325,05838.6 ± 3.4
Sample no.GrainsRhosNsRhoiNiChi-RhodNd SquareAge (Ma, ±2σ)
COTKAR01170.76416954.5834,169951.1325,05833.3 ± 1.4
COTKAR02160.1651610.853832981.1325,05838.6 ± 3.4

Notes: Ns, Ni, and Nd are the number of spontaneous, induced, and flux dosimeter (CN-5) track, respectively; Rhos, Rhoi, and Rhod are the density of spontaneous, induced, and dosimeter track, respectively (× 106/cm2); a value of 353.5 ±7.1 (CN-5) was used for the zeta factor; age error estimates are at the 67% (1σ) confidence level; analyses at the Fission Track Research Laboratory, Department of Earth Sciences, Dalhousie University, Halifax, N.S, Canada

1 See Figure 4 for location

Magmatic biotite from the mesocumulate diorite pluton near the town of Cotabambas yields a K-Ar age of 43.2 ±1.1 Ma, whereas magmatic hornblende from the main granodiorite pluton located north of Ccalla returns a K-Ar age of 39.8 ± 1.5 Ma. These ages are in agreement with the observation that granodiorite cuts diorite north and west of Ccalla. Hydrothermal biotite from the potassic core at the contact between principal porphyry and diorite at Ccalla yieldes a K-Ar age of 35.7 ± 0.9 Ma (Perelló et al., 2002, 2003a). These ages indicate that the bulk of the magmatic and hydrothermal activity took place during the middle to late Eocene, a period that saw intense magmatism and mineralization associated with the emplacement and evolution of the Andahuaylas-Yauri batholith (Noble et al., 1984; Carlotto, 1998; Perelló et al., 2003a).

In addition, apatite fission-track ages of 33.3 ±1.4 and 38.6 ± 3.4 Ma have been obtained for fresh diorite and granodiorite, respectively, from the Cotabambas area (Table 3; Fig. 11). These ages are broadly similar to their K-Ar counterparts, and the combination of both data sets is interpreted to indicate that these plutons, and therefore porphyry copper mineralization at Cotabambas, were emplaced in a shallow environment (2–3 km) and emplacement was shortly followed by rapid cooling and exhumation (cf. Maksaev, 1990).

Fig. 11.

Schematic cross section between Cotabambas and the Río Apurímac, showing the location and K-Ar and fissiontrack (FT; apatite) ages for selected samples from the Andahuaylas-Yauri batholith. Note the broadly coincident ages of sample COTKAR-02. K-Ar ages taken from Perelló et al. (2003a). See Table 3 for details of fission-track ages.

Fig. 11.

Schematic cross section between Cotabambas and the Río Apurímac, showing the location and K-Ar and fissiontrack (FT; apatite) ages for selected samples from the Andahuaylas-Yauri batholith. Note the broadly coincident ages of sample COTKAR-02. K-Ar ages taken from Perelló et al. (2003a). See Table 3 for details of fission-track ages.

A single K-Ar age of 3.3 ± 0.2 Ma on supergene alunite from Ccalla (Table 2) suggests that supergene chalcocite enrichment took place mainly in the late Pliocene, during the formation of several regional pediplain surfaces mantled by extensive ash-flow sheets (Carlotto, 1998; Perelló et al., 2003a).

Discussion

Evolution of the porphyry systems

Three main stages can be recognized in the evolution of the Ccalla system at Cotabambas:

  1. An early stage, involving emplacement of the principal porphyry, was responsible for the bulk of the potassic and calcic-potassic alteration and associated copper-gold mineralization. This stage included several pulses of porphyry intrusion and evolved through (1) an early potassic event with formation of abundant biotite and K-feldspar in veinlets and as partial to complete replacements of original rock constituents; (2) an intermediate potassic-calcic event with formation of amphibole and pyroxene in veinlets with K-feldspar and conspicuous biotite halos; and (3) a second potassic event involving formation of the main quartz-K-feldspar-biotite stockwork.

  2. An intermediate stage, spatially and temporally associated with the emplacement of intermineral porphyry phases, caused renewed, albeit incipient, potassic alteration but, more importantly, involved formation of most of the intermediate argillic alteration, causing pervasive destruction of previous mineral assemblages. Magnetite underwent martitization under these conditions and bornite and chalcopyrite experienced transformation to more oxidized assemblages containing chalcopyrite and pyrite.

  3. A terminal stage, associated with the emplacement of the Ccochapata dome and related dike swarm, was responsible for the sericite-calcite alteration and involved introduction of an appreciable amount of pyrite and small quantities of base metals in quartz-calcite-galena-sphalerite veins.

The less-studied system at Azzulccacca seems to have evolved similarly, judging by its rock types, multiple intrusive phases, alteration-mineralization assemblages, presence of late mineral dike swarms of dacitic composition, and cross-cutting relationships between different intrusive phases.

Regional considerations

From a regional metallogenic point of view, the late Eocene age of the hydrothermal alteration at Ccalla and, by inference, the rest of the Cotabambas cluster is consistent with the age of other porphyry and porphyry-related skarn systems in the Andahuaylas-Yauri belt (Noble et al., 1984; Bonhomme and Carlier, 1990; Perelló et al., 2002, 2003a). These ages demonstrate the presence of an important, regionally extensive metallogenic event of overall middle Eocene to early Oligocene age in southeastern Peru (Perelló et al., 2003a).

Contrary to previous suggestions (e.g., Santa Cruz et al., 1979; Noble et al., 1984), this belt not only comprises magnetite-rich, skarn-type copper mineralization but also includes numerous porphyry and porphyry-related skarn deposits (Perelló et al., 2003a), of which Los Chancas (Corrales, 2001), Antapaccay (Jones at al., 2000; Fierro et al., 2002), and Cotabambas (Perelló et al., 2002) are recent important discoveries. Furthermore, porphyry-type deposits and occurrences in the belt comprise the whole spectrum of copper, gold, and molybdenum associations, including copper-gold at Cotabambas and Antapaccay, copper-molybdenum-gold at Los Chancas, and copper-poor, gold-only systems at Winicocha and Morosayhuas (Perelló et al., 2003a). In addition, at the Chabuca and Coroccohuayco deposits in the Tintaya cluster, copper-gold-(molybdenum) mineralization is contained in skarns intimately associated with low-grade porphyry copper centers (Fierro et al., 1997; Perelló et al., 2003a).

All regional and district geologic evidence, together with the geochronologic data presented above, supports earlier suggestions (Fig. 3) that batholith emplacement, porphyry copper alteration and mineralization, and sediment accumulation in the Anta and San Jerónimo basins took place during broadly the same time interval in the middle Eocene to early Oligocene (Perelló et al., 2003a). Fission-track ages and geology also combine to suggest that regional surface uplift and sediment accumulation were responses to a widespread event of deformation associated with middle Eocene to early Oligocene Incaic compression.

Comparison with other deposits

The mineralization in the Cotabambas cluster is typical of porphyry copper-gold deposits worldwide (Sillitoe, 2000), including recently discovered examples in the central Andes at La Fortuna (Perelló et al., 1996; Paleczek and Cáceres, 2003), Esperanza (Perelló et al., 2003b, 2004), and several systems in the El Salvador región (Rivera et al., 2003) in Chile; Agua Rica (Perelló et al., 1998) in Argentina; and Minas Conga (Llosa et al., 2000), Antapaccay (Jones et al., 2000; Fierro et al., 2002), and Los Chancas (Corrales, 2001) in Peru. The Chilean examples belong to the late Eocene to early Oligocene porphyry copper belt of northern Chile, of which the Andahuaylas-Yauri mineralization constitutes an extension in Peruvian territory (Perelló et al., 2003a). Also, similarities may be drawn between the Cotabambas porphyries and deposits like Tanamá, Puerto Rico (Cox, 1985), Island Copper, Canada (Clark and Arancibia, 1995; Perelló et al., 1995), and many others (Sillitoe, 2000), with particular regard to the presence of actinolite, hornblende, and pyroxene in early-stage, calcic and potassic-calcic alteration assemblages, with or without associated ore-grade copper-gold mineralization.

Conclusions

The area of Cotabambas hosts a cluster of at least four main porphyry systems at Ccalla, Azulccacca, Huaclle, and Ccarayoc. Additional showings with porphyry-style alteration and mineralization are either separate centers or structurally controlled pieces sliced-off from the main systems. Mineralization is of the copper-gold type and associated with a series of multiphase stocks and dikelike porphyritic intrusions of granodioritic to quartz monzodioritic composition, emplaced into diorite and granodiorite plutons of the regional Andahuaylas-Yauri batholith of middle Eocene to early Oligocene age. Terminal intrusive events in the area include the emplacement of the Ccochapata endogenous dome complex of dacitic composition.

Porphyry mineralization in the cluster is typical of porphyry copper-gold deposits worldwide. Similarities include (1) average gold grades of >0.3 ppm; (2) abundant hydrothermal magnetite, averaging between 5 and 10 vol percent, as an integral component of early-stage potassic alteration; (3) presence of hydrothermal amphibole and pyroxene with biotite defining potassic-calcic alteration; (4) sympathetic relationship between copper and gold grades; (5) gold occurring in native form as micron-sized inclusions in copper-bearing sulfide minerals; and (6) intense overprinting by intermediate argillic assemblages rich in chlorite and smectite.

Apart from displaying a positive correlation, copper and gold contents are intimately related to the intensity of the quartz veining. The latter occurs as either multidirectional arrays of stockwork type or unidirectional swarms, with veins composed of one or more associations of quartz, K-feldspar, biotite, anhydrite, amphibole, pyroxene, magnetite, chal-copyrite, and bornite. Pyrite is a minor constituent of the veinlets, whereas accessory mineralogy is largely dominated by apatite, rutile, and sphene. The intensity of the quartz veining is highest at the contact between the principal porphyry and diorite country rock. The vein intensity decreases both inward and outward from this contact zone. The central parts of principal porphyry intrusions typically display lower copper-gold grades, partly because of decreased vein intensity and partly due to the presence of intermineral porphyry dikes that intruded the cores of the earlier porphyry stocks. Both the composition and textures of these intermineral and late intermineral phases are almost identical to those of the principal porphyry, thus making distinction between them difficult.

The age of the alteration and mineralization is late Eocene, based both on field relationships with country rocks and on a single K-Ar age for hydrothermal biotite of 35.7 ± 0.9 Ma. This age is similar to that for other porphyry and porphyry-related skarn systems of the region, which together constitute the Andahuaylas-Yauri metallogenic belt of southeastern Peru. The southern extension of this belt, in Chilean territory, is represented by the middle Eocene to early Oligocene belt of northern Chile, which includes some of the world’s largest porphyry copper deposits. Apart from age, parallels between the two segments of the belt include the following (Perelló et al., 2003a): (1) tectonomagmatic evolution; (2) presence of large, regional faults; and (3) presence of goldrich porphyry copper systems.

Porphyry emplacement at Cotabambas was spatially and temporally associated with a corridor of northeast-trending tear structures that formed simultaneously with large reverse faults during regional, northeast-directed, middle Eocene to early Oligocene Incaic shortening, uplift, exhumation, and synorogenic sedimentation. A similar structural setting, with syntectonic porphyry copper emplacement in reverse faults associated with shortening and regional transpression, has been established at the Potrerillos deposit east of El Salvador (Tomlinson, 1994) and, more recently, at Esperanza (Perelló et al., 2004), in the middle Eocene to early Oligocene porphyry copper belt of northern Chile.

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Acknowledgments

Geologic discussions with Dick Sillitoe greatly improved our understanding of the geology of Cotabambas. Víctor Carlotto and José Cárdenas are thanked for their help with regional geology and stratigraphy. The authors would like to thank Héctor Poblete for drafting the figures and the staffs of Antofagasta Minerals S.A. and Anaconda Peru S.A. in Santiago and Lima, respectively, for their technical and logistic support. Paula Cornejo conducted petrographic work on selected samples from the project and Carlos Pérez de Arce carried out the geochronologic work at the Geochronology Laboratory of SERNAGEOMIN (Geological Survey of Chile). The manuscript benefited from detailed reviews by Donald C. Noble and Rubén Padilla.

Figures & Tables

Fig. 1.

Location map of the Cotabambas area, Cuzco region, Peru. Main regional physiographic units and tectonic elements after Jaillard et al. (2000).

Fig. 1.

Location map of the Cotabambas area, Cuzco region, Peru. Main regional physiographic units and tectonic elements after Jaillard et al. (2000).

Fig. 2.

Location of discovery drill holes and the main Ccalla and Azulccacca porphyry systems.

Fig. 2.

Location of discovery drill holes and the main Ccalla and Azulccacca porphyry systems.

Fig. 5.

Geologic map of the Cotabambas porphyry copper-gold cluster. Based on mapping by the authors.

Fig. 5.

Geologic map of the Cotabambas porphyry copper-gold cluster. Based on mapping by the authors.

Fig. 3.

a. Simplified geologic map of the Andahuaylas-Yauri belt (after Perelló et al., 2003a). b. Schematic stratigraphic column of the same belt with emphasis on age relationships between the Andahuaylas-Yauri plutons, the volcanic rocks of the Anta Formation, and porphyry copper mineralization (simplified after Perelló et al., 2003a).

Fig. 3.

a. Simplified geologic map of the Andahuaylas-Yauri belt (after Perelló et al., 2003a). b. Schematic stratigraphic column of the same belt with emphasis on age relationships between the Andahuaylas-Yauri plutons, the volcanic rocks of the Anta Formation, and porphyry copper mineralization (simplified after Perelló et al., 2003a).

Fig. 4.

Simplified geologic map of the Cotabambas quadrangle. Based on work by Carlotto and Cárdenas (2000) and reconnaissance mapping by the authors.

Fig. 4.

Simplified geologic map of the Cotabambas quadrangle. Based on work by Carlotto and Cárdenas (2000) and reconnaissance mapping by the authors.

Fig. 6.

Schematic restoration of principal and intermineral porphyry phases at Ccalla prior to the emplacement of the late mineral Ccochapata dome and dike swarm.

Fig. 6.

Schematic restoration of principal and intermineral porphyry phases at Ccalla prior to the emplacement of the late mineral Ccochapata dome and dike swarm.

Fig. 7.

Schematic cross section, displaying the salient geologic elements of the Ccalla deposit at Cotabambas. See Figure 5 for location.

Fig. 7.

Schematic cross section, displaying the salient geologic elements of the Ccalla deposit at Cotabambas. See Figure 5 for location.

Fig. 8.

Schematic cross section, showing the complex array of main alteration zones and assemblages at Ccalla. See Figure 5 for location.

Fig. 8.

Schematic cross section, showing the complex array of main alteration zones and assemblages at Ccalla. See Figure 5 for location.

Fig. 9.

Schematic cross section through the Ccalla deposit at Cotabambas, showing the distribution of selected copper zones. See Figure 5 for location.

Fig. 9.

Schematic cross section through the Ccalla deposit at Cotabambas, showing the distribution of selected copper zones. See Figure 5 for location.

Fig. 10.

Plots displaying the overall positive correlation between Cu and Au. a. Ccalla deposit, b. Azzulccacca deposit.

Fig. 10.

Plots displaying the overall positive correlation between Cu and Au. a. Ccalla deposit, b. Azzulccacca deposit.

Fig. 11.

Schematic cross section between Cotabambas and the Río Apurímac, showing the location and K-Ar and fissiontrack (FT; apatite) ages for selected samples from the Andahuaylas-Yauri batholith. Note the broadly coincident ages of sample COTKAR-02. K-Ar ages taken from Perelló et al. (2003a). See Table 3 for details of fission-track ages.

Fig. 11.

Schematic cross section between Cotabambas and the Río Apurímac, showing the location and K-Ar and fissiontrack (FT; apatite) ages for selected samples from the Andahuaylas-Yauri batholith. Note the broadly coincident ages of sample COTKAR-02. K-Ar ages taken from Perelló et al. (2003a). See Table 3 for details of fission-track ages.

Table 1.

Summary of Main Alteration and Mineralization Assemblages at Cotabambas

AlterationMineralization
Main productsVeinletsMain mineralogyVeinlets
Potassic (biotite, K-feldspar, quartz, apatite)
Plagioclase→K-feldsparQuartz-K-feldspar-biotite-apatiteChalcopyrite, magnetite, bornite, pyrite1Magnetite-chalcopyrite
Biotite, amphibole→biotite, K-feldspar, chloriteQuartz-K-feldspar with K-feldspar-apatite halosMagnetite-chalcopyrite-bornite
K-feldspar with biotite halosMagnetite-chalcopyrite-pyrite
Pyroxene→quartz, K-feldspar, biotite, chlorite1 epidote1Banded quartz-K-feldspar-biotite-anhydrite-apatiteChalcopyrite-bornite
Quartz-K-feldspar-chlorite-biotite with chloriteapatite-calcite-K-feldspar halos
Potassic-calcic (quartz, actinolite, hornblende, biotite, K-feldspar, pyroxene, apatite)
Plagioclase→K-feldspar, calcite, epidoteActinolite-apatiteChalcopyrite, magnetite, Bornite, pyrite1M agnetite-chalcopyrite
Biotite, amphibole→actinolite, apatiteQuartz-hornblende-actinolite-K-feldspar with actinolite-K-feldspar halos
Pyroxene→actinolite, apatite →actinolite, biotite1Quartz-pyroxene-actinolite-K-feldspar
Quartz-actinolite with biotite halos
Actinolite-K-feldspar-apatite with biotite-epidote halos
Intermediate argillic (quartz, greenish sericite, chlorite, illite-smectite, halloysite, epidote)
Plagioclase→sericite, calcite,1 illite,1 epidote1Quartz-chlorite-calcite with sericite-halloysiteillite-smectite alteration halosPyrite, chalcopyrite, hematitePyrite-chalcopyrite
Amphibole, biotite→chlorite, smectite
Magnetite→martite
Sericitic (white sericite, illite, calcite)
Plagioclase→sericite, illite, calcite1Quartz-calcite with incipient sericite halosPyrite, sphalerite, galena, chalcopyriteChalcopyrite-galena-sphalerite-pyrite
Ferromagnesians→chlorite, calcite, smectite1Calcite-anhydrite-gypsum with sericite-calcite halos
Propylitic (chlorite, epidote, calcite)
Plagioclase→calcite, epidoteCalcite-gypsumPyrite, chalcopyrite1Pyrite
Ferromagnesians→chlorite, epidote, calciteChlorite-epidote-calcite
AlterationMineralization
Main productsVeinletsMain mineralogyVeinlets
Potassic (biotite, K-feldspar, quartz, apatite)
Plagioclase→K-feldsparQuartz-K-feldspar-biotite-apatiteChalcopyrite, magnetite, bornite, pyrite1Magnetite-chalcopyrite
Biotite, amphibole→biotite, K-feldspar, chloriteQuartz-K-feldspar with K-feldspar-apatite halosMagnetite-chalcopyrite-bornite
K-feldspar with biotite halosMagnetite-chalcopyrite-pyrite
Pyroxene→quartz, K-feldspar, biotite, chlorite1 epidote1Banded quartz-K-feldspar-biotite-anhydrite-apatiteChalcopyrite-bornite
Quartz-K-feldspar-chlorite-biotite with chloriteapatite-calcite-K-feldspar halos
Potassic-calcic (quartz, actinolite, hornblende, biotite, K-feldspar, pyroxene, apatite)
Plagioclase→K-feldspar, calcite, epidoteActinolite-apatiteChalcopyrite, magnetite, Bornite, pyrite1M agnetite-chalcopyrite
Biotite, amphibole→actinolite, apatiteQuartz-hornblende-actinolite-K-feldspar with actinolite-K-feldspar halos
Pyroxene→actinolite, apatite →actinolite, biotite1Quartz-pyroxene-actinolite-K-feldspar
Quartz-actinolite with biotite halos
Actinolite-K-feldspar-apatite with biotite-epidote halos
Intermediate argillic (quartz, greenish sericite, chlorite, illite-smectite, halloysite, epidote)
Plagioclase→sericite, calcite,1 illite,1 epidote1Quartz-chlorite-calcite with sericite-halloysiteillite-smectite alteration halosPyrite, chalcopyrite, hematitePyrite-chalcopyrite
Amphibole, biotite→chlorite, smectite
Magnetite→martite
Sericitic (white sericite, illite, calcite)
Plagioclase→sericite, illite, calcite1Quartz-calcite with incipient sericite halosPyrite, sphalerite, galena, chalcopyriteChalcopyrite-galena-sphalerite-pyrite
Ferromagnesians→chlorite, calcite, smectite1Calcite-anhydrite-gypsum with sericite-calcite halos
Propylitic (chlorite, epidote, calcite)
Plagioclase→calcite, epidoteCalcite-gypsumPyrite, chalcopyrite1Pyrite
Ferromagnesians→chlorite, epidote, calciteChlorite-epidote-calcite

Notes: Arrows indicate hydrothermal alteration products

1 Minor component

Table 2.

K-Ar Ages from Cotabambas and Adjacent Area1

Sample no.Material datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (Ma, ±2σ)
COTALKAR-02Alunite7.3970.986673.3 ± 0.2
COTKAR-01Magmatic biotite7.5561.3351743.2 ± 1.1
COTKAR-02Magmatic amphibole20.8121.2713439.8 ± 1.1
CCALLAKARHydrothermal biotite7.39310.3751035.7 ± 0.9
Sample no.Material datedK(%)Rad. Ar (nl/g)Atm. Ar (%)Age (Ma, ±2σ)
COTALKAR-02Alunite7.3970.986673.3 ± 0.2
COTKAR-01Magmatic biotite7.5561.3351743.2 ± 1.1
COTKAR-02Magmatic amphibole20.8121.2713439.8 ± 1.1
CCALLAKARHydrothermal biotite7.39310.3751035.7 ± 0.9

Notes: Constants: λβ = 4.962 × 10−10y−1; λε = 0.581 × 10−10y−1; 40Ar/36Ar = 295.5; 40K = 0.01167 atom %

1 See Figures 4 and 5 for location

2 Some degree of alteration to chlorite present

Table 3.

Apatite Fission-Track Ages, Cotabambas Area, Peru1

Sample no.GrainsRhosNsRhoiNiChi-RhodNd SquareAge (Ma, ±2σ)
COTKAR01170.76416954.5834,169951.1325,05833.3 ± 1.4
COTKAR02160.1651610.853832981.1325,05838.6 ± 3.4
Sample no.GrainsRhosNsRhoiNiChi-RhodNd SquareAge (Ma, ±2σ)
COTKAR01170.76416954.5834,169951.1325,05833.3 ± 1.4
COTKAR02160.1651610.853832981.1325,05838.6 ± 3.4

Notes: Ns, Ni, and Nd are the number of spontaneous, induced, and flux dosimeter (CN-5) track, respectively; Rhos, Rhoi, and Rhod are the density of spontaneous, induced, and dosimeter track, respectively (× 106/cm2); a value of 353.5 ±7.1 (CN-5) was used for the zeta factor; age error estimates are at the 67% (1σ) confidence level; analyses at the Fission Track Research Laboratory, Department of Earth Sciences, Dalhousie University, Halifax, N.S, Canada

1 See Figure 4 for location

Contents

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