Economic Geology Vol. 98, 2003, pp. 1575–1605
Porphyry-Style Alteration and Mineralization of the Middle Eocene to Early Oligocene Andahuaylas-Yauri Belt, Cuzco Region, Peru JOSÉ PERELLÓ,† Antofagasta Minerals S.A. Ahumada 11, Oficina 602, Santiago, Chile
VÍCTOR CARLOTTO, Departamento de Geología, Universidad Nacional San Antonio Abad del Cuzco. Avenida de la Cultura, Cuzco, Perú
ALBERTO ZÁRATE, PEDRO RAMOS, HÉCTOR POSSO, CARLOS NEYRA, ALBERTO CABALLERO, Minera Anaconda Perú S.A. Avenida Paseo de la República 3245, Piso 3, San Isidro, Lima 27, Perú
NICOLÁS FUSTER, AND RICARDO MUHR Antofagasta Minerals S.A. Ahumada 11, Oficina 602, Santiago, Chile
Abstract Originally known for its Fe-Cu skarn mineralization, the Andahuaylas-Yauri belt of southeastern Peru is rapidly emerging as an important porphyry copper province. Field work by the authors confirms that mineralization in the belt is spatially and temporally associated with the middle Eocene to early Oligocene (~48–32 Ma), calc-alkaline Andahuaylas-Yauri batholith, a composite body with an areal extent of ~300 × 130 km emplaced into clastic and carbonate strata (e.g., Yura Group and Ferrobamba Formation) of Jurassic to Cretaceous age. Batholith emplacement included early-stage, mafic, cumulate gabbro and diorite between ~48 and 43 Ma, followed by pulses of granodiorite and quartz monzodiorite at ~40 to 32 Ma. Coeval volcanic rocks make up the middle Eocene to early Oligocene Anta Formation, a sequence of >1,000 m of andesite lava flows and dacite pyroclastic flows with interbedded volcaniclastic conglomerate. Sedimentary rocks include the red beds of the Eocene to early Oligocene San Jerónimo Group and the postmineralization late Oligocene to Miocene Punacancha and Paruro formations. Eocene and Oligocene volcanic and sedimentary rocks are interpreted to have accumulated largely in both transtensional and contractional synorogenic basins. New and previously published K-Ar and Re-Os ages show that much of the porphyry-style alteration and mineralization along the belt took place during the middle Eocene to early Oligocene (~42–30 Ma). Thus, batholithic magma emplacement, volcanism, and sedimentation are inferred to have accompanied a period of intense deformation, crustal shortening, and regional surface uplift broadly synchronous with the Incaic orogeny. Supergene mineralization is inferred to have been active since the Pliocene on the basis of geomorphologic evidence and a single K-Ar determination (3.3 ± 0.2 Ma) on supergene alunite. The belt is defined by 31 systems with porphyry-style alteration and mineralization, including 19 systems grouped in 5 main clusters plus 12 separate centers, and by hundreds of occurrences of magnetite-rich, skarntype Fe-Cu mineralization. Porphyry copper stocks are dominated by calc-alkaline, biotite- and amphibolebearing intrusions of granodioritic composition, but monzogranitic, monzonitic, quartz-monzonitic, and monzodioritic stocks occur locally. Hydrothermal alteration includes sericite-clay-chlorite, and potassic, quartz-sericitic, and propylitic assemblages. Calcic-potassic and advanced argillic alteration associations are locally represented, and calc-silicate assemblages with skarn-type mineralization occur where carbonate country rocks predominate. Porphyry copper deposits and prospects of the belt range from gold-rich, molybdenum-poor examples (Cotabambas), through deposits carrying both gold and molybdenum (Tintaya, Los Chancas), to relatively molybdenum-rich, gold-poor end members (Lahuani). Gold-only porphyry systems are also represented (Morosayhuas). Gold-rich porphyry copper systems are rich in hydrothermal magnetite and display a positive correlation between Cu and Au in potassic alteration. The bulk of the hypogene Cu (-Au, -Mo) mineralization occurs in the form of chalcopyrite and bornite, in intimate association with early-stage potassic alteration which, in many deposits and prospects, is variably overprinted by copper-depleting sericite-clay-chlorite alteration. Most porphyry copper systems of the belt lack economically significant zones of supergene chalcocite enrichment. This is due primarily to their relatively low pyrite contents, the restricted development of quartzsericitic alteration, and the high neutralization capacities of both potassic alteration zones and carbonate country rocks as well as geomorphologic factors. Leached cappings are irregular, typically goethitic, and contain copper oxide minerals developed by in situ oxidation of low-pyrite, chalcopyrite (-bornite) mineralization. Porphyry copper-bearing stocks emplaced in the clastic strata of the Yura Group and certain phases of the Andahuaylas-Yauri batholith may develop appreciable supergene chalcocite enrichment in structurally and lithologically favorable zones. †
Corresponding author: e-mail,
[email protected]
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A model for the region suggests that the calc-alkaline magmas of the Andahuaylas-Yauri batholith and subsequent porphyry-style mineralization were generated during an event of subduction flattening which triggered the crustal shortening, tectonism, and uplift assigned to the Incaic orogeny. Shortening of the upper crust would have impeded rapid magma ascent favoring storage of fluid in large chambers which, at the appropriate depth in the uppermost crust, would have promoted large-scale porphyry copper emplacement. Geodynamic reconstructions of the late Eocene to early Oligocene period of flat subduction in the central Andes suggest that emplacement of the Andahuaylas-Yauri batholith took place at an inflection corridor in the subduction zone broadly coincident with the position of the present-day Abancay deflection. Similarly, evidence from southeastern Peru suggests that the Andahuaylas-Yauri belt may be continuous with the late Eocene to early Oligocene porphyry copper belt of northern Chile and that the process of subduction flattening in southern Peru also may have taken place in northern Chile between ~45 and 35 Ma.
Introduction THE ANDAHUAYLAS-YAURI belt (Bellido et al., 1972; Santa Cruz et al., 1979; Noble et al., 1984) covers an area of approximately 25,000 km2 in southern Peru and extends for about 300 km between the localities of Andahuaylas in the northwest and Yauri in the southeast (Fig. 1a). Until the late 1980s, the Andahuaylas-Yauri belt had received only limited geologic scrutiny and was mainly known for its copper-bearing, magnetite skarn deposits (Terrones, 1958; Bellido et al., 1972; Sillitoe, 1976, 1990; Santa Cruz et al., 1979; Einaudi et al., 1981; Aizawa and Tomizawa, 1986), best exemplified by Tintaya, Atalaya, Las Bambas, Katanga, and Quechua. For most researchers, these occurrences were considered to be copper skarns associated with barren intrusions (e.g., Einaudi et al., 1981; Noble et al., 1984), although potassic alteration in host porphyritic stocks had been described and characterized as such (Yoshikawa et al., 1976; MMAJ, 1983; Noble et al., 1984). During the late 1980s, regional work complemented by detailed geologic studies at Tintaya and Katanga (Carlier et al., 1989), followed by grass-roots exploration in the region during the 1990s, confirmed the presence of porphyry-style alteration and mineralization (e.g., Fierro et al., 1997) and resulted in the discovery of additional, potentially economic porphyry copper deposits (Table 1) at Antapaccay (Jones et al., 2000), Los Chancas (Corrales, 2001), and Cotabambas (Perelló et al., 2002), as well as porphyry-skarn mineralization at Coroccohuayco (BHP Company Limited, 1999). Zinc-rich, Mississippi Valley-type mineralization was also discovered in the region (Carman et al., 2000) adding to the metallogenic diversity of the belt. This paper describes the salient geologic features of a number of porphyry Cu (-Au, -Mo) deposits and prospects of the Andahuaylas-Yauri belt that help to define this region as a new porphyry copper province. It also provides new geochronologic data to constrain the age of the porphyry-style alteration and mineralization in the belt and establishes regional correlations and comparisons with nearby porphyry copper provinces. However, the paper is not designed to cover in full the complex geology of this still poorly understood region. Detailed geologic descriptions can be found in Marocco (1978) and Carlotto (1998) for the area under study, and in Clark et al. (1990) and Sandeman et al. (1995) for nearby southeastern Peru transects. The paper focuses on systems for which the bulk of the mineralization is of porphyry type and excludes those deposits in which skarn-type mineralization is the dominant style. Descriptions of the latter can be found elsewhere (Terrones, 1958; Santa Cruz et al., 1979; Aizawa and Tomizawa, 1986; Fierro et al., 1997; Zweng 0361-0128/98/000/000-00 $6.00
et al., 1997). Following a short review of the regional geologic setting of the Andahuaylas-Yauri belt, the main geologic features of several deposits and prospects are described. The paper concludes with a section in which regional metallogenic aspects are reviewed. Methods Except for those deposits and prospects with published descriptions (e.g., Tintaya, Antapaccay, Los Chancas), much of the work represents the product of more than three years of exploration by the authors, including both regional (1:25,000 scale) and detailed (1:5,000 scale) mapping. Field work was complemented by thin section petrographic studies to characterize rock types, alteration assemblages, and dominant vein styles at each prospect. Rock names for the main batholith intrusions and porphyry copper-bearing stocks follow the nomenclature of Streckeisen (1976, 1978) and are based on point counts (1,500 points) for modal proportions of key silicates. Unless otherwise stated, the K-Ar ages reported here were determined at the geochronology laboratory of the Geological Survey of Chile, Santiago and followed standard procedures and techniques (e.g., Dalrymple and Lamphere, 1966; Steiger and Jaeger, 1977; Baksi, 1982). All ages are referred to the geological time table of Haq and van Eysinga (1987). Regional Setting The Andahuaylas-Yauri belt is located at a distance of ~250 to 300 km inland from the present-day Peru-Chile trench (Fig. 1). The region is underlain by thick sialic crust (50 to 60 km; James, 1971), and straddles the transition zone between the southern, normal subduction regime of southern Peru and northern Chile and the northern, flat subduction zone of central and northern Peru (Cahill and Isacks, 1992). It is located immediately southeast of the Abancay Deflection (Marocco, 1978). The region encompasses parts of the intermontane depressions between the Eastern and Western Cordilleras and the northern extremity of the Altiplano (Fig. 1b; Carlier et al., 1996; Chávez et al., 1996). The western part of the belt is characterized by a rugged, mountainous topography where ranges and snow-capped peaks above 4,500 m are incised by deep (>2,000 m), steep-sided canyons. These canyons constitute the main drainage system of the region and include the Santo Tomás, Urubamba, Apurímac, Vilcabamba, Mollebamba, and Antabamba rivers, all of which drain toward the Amazon basin. The eastern and southern parts of the region are characterized by the gently undulating topography of the ~4,000 m-high plateaus that extends into the Altiplano of Bolivia (Fig. 1b).
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FIG. 1. Sketch maps showing the location of the study area in the context of main geologic, geophysical, topographic, and physiographic features of the Central Andes. a. Area with average elevation >3,000 m and depth contours of the subducted slab after Cahill and Isacks (1992). Oceanic features from Jaillard et al. (2000). b. The study area relative to main regional physiographic provinces (Jaillard et al. 2000), contours of crustal thickness (James, 1971), and main Precambrian basement units (Ramos and Aleman, 2000). 0361-0128/98/000/000-00 $6.00
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PERELLÓ ET AL. TABLE 1. Geologic Resources for Main Deposits of the Andahuaylas-Yauri Belt Tonnage (× 106)
Cu (%)
Au (g/t)
Mo (%)
Tintaya district Antapaccay Coroccohuayco Ccatun Pucara Quechua Tintaya
383 155 24 300 139
0.89 1.57 1.44 0.68 1.39
0.16 0.16 n.a. n.a. 0.23
n.a. n.a. n.a. n.a. n.a.
Cotabambas Area Azulccacca Ccalla
24 112
0.42 0.62
0.39 0.36
1,000 m of volcanic and clastic rocks of the Mitu Group (Permian to Early Triassic; Fig. 2). Mesozoic and Cenozoic stratigraphy The Mesozoic and Cenozoic stratigraphy of the region is chiefly made up of Jurassic and Cretaceous sedimentary sequences deposited in a paleogeographic setting dominated by two main basins (Western and Eastern Peruvian basins) separated by the Cuzco-Puno basement high (Fig. 3; Carlotto et al., 1993; Jaillard and Soler, 1996). The Western basin, also known as the Arequipa basin (Vicente et al., 1982), corresponds to the present-day Western Cordillera. It contains a sedimentary pile (Middle Jurassic to Late Cretaceous) in excess of 4,500 m thick with a lower part dominated by turbidites, a middle part with quartz arenite, and an upper part with abundant limestone (Vicente et al., 1982; Jaillard and Santander, 1992). The northeastern edge of this basin, coincident with the Andahuaylas-Yauri region, includes the Lagunillas and Yura groups (Marocco, 1978), made up of Early Jurassic limestone and Middle to Late Jurassic quartz arenite and shale, with a total thickness of approximately 800 m (Fig. 3). The top of the sequence contains the massive micritic limestone, black shale, and nodular chert of the Ferrobamba Formation (Marocco, 1978; Pecho, 1981). The Cuzco-Puno high includes ~900 m of terrigenous red beds interbedded with shale, limestone, and gypsum (Carlotto et al., 1993; Jaillard et al., 1994). The age of these rocks is Late Jurassic to Paleocene (Fig. 3). The Eastern basin, also known as Putina basin (Jaillard, 1994), is made up of several sequences of Late Cretaceous marine clastic and carbonate rocks, with a total thickness of ~ 2,600 m (Jaillard et al., 1993; Jaillard, 1994; Cárdenas et al., 1997). 0361-0128/98/000/000-00 $6.00
Eocene to early Oligocene stratigraphy Two main units characterize the Eocene to early Oligocene stratigraphy of the region, including the sedimentary San Jerónimo Group and the dominantly volcanic Anta Formation (Figs. 2 and 4). These units unconformably overlie the Mesozoic and early Cenozoic sequences described above. The San Jerónimo Group (Eocene to early Oligocene) consists of two main formations (Kayra and Soncco; Fig. 4), with a total thickness of ~4,500 m, made up of red bed terrigenous (sandstone, shale, pelitic sandstone, and volcanic microconglomerate) strata interbedded with tuffaceous horizons near the top. The age of the San Jerónimo Group is constrained by stratigraphic relations (it unconformably overlies strata with plant fossils of Paleocene to early Eocene age) and on K-Ar and ArAr ages of 29.9 ± 1.4 Ma and 30.84 ± 0.83 Ma, respectively, from the upper tuffaceous horizons of the Soncco Formation (Fig. 4; Carlotto, 1998; Fornari et al., 2002). Sedimentation is interpreted to have taken place initially in a fluvial environment that progressed into structurally controlled, pull-apart basins (Córdova, 1986; Noblet et al., 1987; Marocco and Noblet, 1990; Chávez et al., 1996). Between Cuzco and Sicuani, basal sandstone of the Soncco Formation includes horizons of stratiform copper mineralization, up to several meters thick, with hypogene chalcocite and bornite, and supergene copper oxides (Cárdenas et al., 1999), which have similarities to the red bed deposits from the Bolivian Altiplano (e.g., Corocoro; Sillitoe, 1989) and northern Chile (San Bartolo; Travisany, 1979). The San Jerónimo Group is equivalent to the Puno Group of the Peruvian Altiplano southeast of the study region (Fig. 4), where it is overlain by the volcanic horizons of the Tacaza Group (Klinck et al., 1986; Clark et al., 1990; Jaillard and Santander, 1992). Farther south, sedimentary, conglomeratic, red bed sequences are known in the Altiplano of Bolivia (e.g., the lower horizons of the Tiwanaku Formation and the Berenguela and Turco formations; Hérail et al., 1993), in the Puna of northwestern Argentina (Geste and Quiñoa formations; Alonso, 1992; Kraemer et al., 1999; Coutand et al., 2001), and in the Salar de Atacama area of northern Chile (upper Purilactis Group; Mpodozis et al., 1999). The Anta Formation is a >1,000 m sequence characterized by a lower member with andesite lava flows and dacite pyroclastic flows locally interbedded with alluvial conglomerate, and an upper member of fluvial conglomerate with
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FIG. 2. Geologic map of the study area, modified and greatly simplified after Carlotto (1998), with additions after Pecho (1981) and this study.
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interbedded andesite and basaltic andesite flows (Fig. 4). Its age is constrained to middle Eocene to early Oligocene by stratigraphic relations and K-Ar geochronology (Carlier et al., 1996; Carlotto, 1998). Southwest of Cuzco, two biotite-rich dacitic flows from the middle part of the formation have returned K-Ar ages of 38.4 ± 1.5 and 37.9 ± 1.4 Ma, and a basaltic horizon from the upper part of the unit yields a K-Ar whole rock age of 29.9 ± 1.1 Ma (Carlotto, 1998). The Anta Formation andesites and conglomerates are interpreted to be stratigraphic equivalents of the San Jerónimo Group red beds (Fig. 4), with the erosion products of the Anta Formation feeding the San Jerónimo basin located to the northeast. The coarsening-upward characteristics of the sequence, with alluvial and fluvial conglomerates dominated by volcanic and plutonic clasts at the top, are interpreted to reflect topographic rejuvenation of the source regions in response to increasing regional tectonic uplift, with sedimentation in a piggy-back style basin environment (Carlotto, 1998). Late Oligocene to Miocene stratigraphy The late Oligocene to Miocene sedimentary deposits of the region include the Punacancha (1,500–5,000 m thick) and Paruro (>1,100 m-thick) formations (Fig. 4). They are dominated by coarsening-upward red shale and sandstone, with 0361-0128/98/000/000-00 $6.00
gypsum and conglomerate being characteristic in the upper parts of the sequences. Sedimentation is interpreted to have taken place in a fluvial environment with braided rivers, flood plains and alluvial fans in structurally controlled basins (Carlotto et al., 1996a, 1997; Jaimes et al., 1997; Romero et al., 1997). The age of these sequences is based on stratigraphic relations and fossil flora, as well as a K-Ar age of 10.1 ± 0.5 Ma for a tuffaceous horizon near the base of the Paruro Formation (Carlotto et al., 1997). Oligocene and Miocene volcanic rocks in the region and nearby areas are largely dominated by the calc-alkaline sequences of the Western Cordillera (Inner-Western Cordillera of Sandeman et al., 1995) and Altiplano, and include the Tacaza (Oligocene) and Sillapaca (Miocene) groups. In addition to these, a series of scattered, small shoshonitic volcanic centers of Pliocene to Quaternary age occur in the region (Figs. 2 and 4; Wasteneys, 1990; Carlier et al., 1996; Carlotto, 1998). The Tacaza Group consists dominantly of trachyandesite, andesite, and rhyolite tuff (Klinck et al., 1986; Wasteneys, 1990; Carlotto, 1998), with shoshonitic rocks being important in the Santa Lucía area, southeast of Yauri (Clark et al., 1990; Sandeman et al., 1995). Shoshonitic volcanism in the Santa Lucía area took place between ~32 and 24 Ma (Fig. 4; Clark et al., 1990; Sandeman et al., 1995),
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Dominantly mafic cumulate calc-alkaline intrusions
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Dominantly rhyolitic to rhyodacitic volcanism
Dominantly dacitic volcanism
Dominantly andesitic volcanism
Apparent non-volcanic interval
FIG. 4. Summary stratigraphic columns for representative Eocene to present-day volcanic, sedimentary, and intrusive units of the study area and nearby southeastern Peru transects. Columns A, B, and D simplified after Sandeman et al. (1995) and references therein and A.H. Clark (pers. commun., 2002). Column C for the study area compiled after Carlotto (1998), with additions after Carlier et al. (1989, 1996) and this study. In column C, note the spatial and temporal relationships between batholithic plutons, volcanic rocks of the Anta Formation, and the sedimentary red bed sequences of the San Jerónimo Group.
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PERELLÓ ET AL.
whereas farther south, along the arc front, Tacaza-equivalent pyroclastic flows intercalated with the molassic Moquegua Formation commenced at ~26 Ma (A. H. Clark, pers. commun., 2002). In the Andahuaylas-Yauri region, similar age (~29 Ma) shoshonitic rocks have been interpreted to be part of uppermost Anta Formation (see above and Carlotto, 1998), thereby implying some degree of temporal overlap with Tacaza rocks (Fig. 4). Sillapaca Group rocks include mainly dacite flows with subordinate andesite in the southeastern part of the study region (Carlotto, 1998) and subvolcanic dacite plugs and ash-flow tuff in the Santa Lucía area (Fig. 4; Clark et al., 1990). Rocks from Santa Lucía yield ages of between ~22 and 14 Ma (Clark et al., 1990; Sandeman et al., 1995); elsewhere in the Puno region a second effusive event, also assigned to the Sillapaca Group, returns ages of between ~14 to 12 Ma (Klinck et al., 1986). If the correlations above are accepted (Fig. 4), it may be speculated that the temporal overlap of the Oligocene to Miocene volcanism of the Western Cordillera, Altiplano, and Eastern Cordillera (Sandeman et al., 1995) would also apply to the Andahuaylas-Yauri region. This interpretation is consistent with the suggestion by Sandeman et al. (1995) that a >350-km-wide arc was episodically active throughout southern Peru during late Oligocene and Miocene times. In the Andahuaylas-Yauri region, however, volcanism seems to have been intermittently active since the middle Eocene (Carlier et al., 1996, 2000; Carlotto, 1998; Carlotto et al., 1999).
and possess amphibole > biotite as the dominant ferromagnesian phases, with local pyroxene in the more mafic members. They are regularly distributed throughout the region and constitute the main mass of the batholith. Contact aureoles within country rocks are extremely irregular in shape, size, and composition, although garnet skarn is typically formed in calcareous rocks (e.g., Ferrobamba Formation) and biotite and cordierite hornfels are developed where the more pelitic facies of the Mesozoic formations are present (Carlotto, 1998). The age of the batholith is constrained by regional stratigraphic relations and geochronologic data (Table 2; Fig. 5b). Batholith rocks intrude mostly Mesozoic and early Cenozoic marine and continental strata as well as the middle Eocene to early Oligocene Anta Formation (Fig. 4). In addition, several K-Ar ages reported by Carlier et al. (1996), Carlotto (1998), and Perelló et al. (2002), together with a number of ages obtained during the course of the present study, confirm a middle Eocene to early Oligocene age (~48-32 Ma) for the bulk of the batholith (Fig. 5). The geochronologic data support the inference by Bonhomme and Carlier (1990) that cumulate rocks are older (~48-43 Ma) and that intermediate composition rocks are younger (~40-32 Ma), thereby corroborating the concept that batholith emplacement took place in at least two main stages. The data also suggest, however, that considerable time overlap existed between the more mafic and the more felsic intrusions of the younger group (Fig. 5b).
The Andahuaylas-Yauri batholith The northeastern border of the Western Cordillera in the study area is underlain by large bodies of intrusive rocks collectively known as the Andahuaylas-Yauri batholith (Carlier et al., 1989; Bonhomme and Carlier, 1990). It is also known locally as the Abancay (Marocco, 1975, 1978) or Apurímac batholith (Pecho, 1981; Mendívil and Dávila, 1994). The name Andahuaylas-Yauri batholith is used in this paper, following Bonhomme and Carlier (1990). The batholith is composed of a multitude of intrusions that crop out discontinuously for >300 km between the towns of Andahuaylas in the northwest and Yauri in the southeast. Its width varies between ~25 km in the Tintaya area and ~130 km along the Chalhuanca-Abancay transect (Fig. 5a). In general terms, the batholith includes early-stage intrusions of cumulates (gabbro, troctolite, olivine gabbro, gabbrodiorite, and diorite) followed by rocks of intermediate composition (monzodiorite, quartz diorite, quartz monzodiorite, and granodiorite) (Fig. 5c; Carlier et al., 1989; Bonhomme and Carlier, 1990; Carlotto, 1998). Subvolcanic rocks of dominantly granodioritic/dacitic composition, locally associated with porphyry-style mineralization, represent the terminal stage (see below). Early-stage cumulate rocks are exposed mainly along the northern border of the batholith (Fig. 5) between Curahuasi and Limatambo (Carlier et al., 1989; Ligarda et al., 1993), where petrologic work by Carlier et al. (1989, 1996) determined that they constitute typical calcalkaline cumulates crystallized at the bottoms of shallow magma chambers, with temperatures of emplacement of ~1,000°C and pressure conditions of ~2 to 3 kbars. The intrusions of the intermediate stage are lighter gray in color, display mediumto coarse-grained, equigranular to slightly porphyritic textures,
Other intrusions Post-batholith intrusive activity in the region is characterized by a series of small syenitic stocks that have yielded K-Ar ages of ~28 Ma in the Curahuasi area (Carlotto, 1998). These intrusions are part of a larger alkalic magmatic province that also includes the basanites, phonotephrites, and trachytes of the Ayaviri region, with ages between 29 and 26 Ma (Carlier et al., 1996, 2000).
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Structural geology The structure of the region is, in general terms, poorly constrained and understood. Although some exceptions exist (Marocco, 1975; Pecho, 1981; Cabrera et al., 1991; Carlotto et al., 1996b; Carlotto, 1998), regional maps lack the detailed structural data that would help to understand the regional tectonics as a whole. The northeastern border of the Western Cordillera is dominated by Mesozoic to Cenozoic sequences that have been moderately to intensely deformed in large, northwest-trending folds with dominantly northerly vergence (Fig. 2). Intense folding in the region typically involves carbonate and shaly sequences (Ferrobamba Formation and equivalent units) that wrap around cores of quartz arenite of the Yura Group. Low- and high-angle thrusts locally accompany the most intense deformation and folding, particularly in the southern quadrangles of the region (Pecho, 1981), with most of the mapped thrusts displaying northerly vergence. This style has similarities to thin-skinned fold-thrust belts elsewhere (e.g., Benavides-Cáceres, 1999), as no involvement of pre-Mesozoic basement is apparent. The limit between the Western Cordillera and the Altiplano is characterized by two main northwest-trending fault systems (Limatambo-Ayaviri and Abancay-Yauri) with exposed
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OLIGOCENE EARLY LATE
LATE
b
EOCENE MIDDLE
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EARLY
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(1)
(1)
(1)
CURAHUASI
73°00´
35.8±0.9
ABANCAY
37.9±1.4
50 km
72°30´
SANTO TOMAS
34.2 ±0.9
LAS BAMBAS
39.7±1.9
43.2±1.1
39.8±1.5
43.3±1.9(1)
COTABAMBAS
35.1±3.1(1)
MACHU PICHU
(1) K-Ar (Carlotto, 1998)
Batholith Plutons
CHALHUANCA
ANDAHUAYLAS
DIORIT E / GABBRO (CUMULATES)
DIORITE
MONZODIORITE
QUARTZ MONZODIORITE
GRANODIORITE
(1) (1)
73°30´
a
14°30´
14°00´
(1) (1)
(1)
ANDAHUAYLAS-YAURI BATHOLITH
13
72°00´
35.7±0.9
KATANGA
8
9
4
YAURI
71°30´
31.6±0.8
LIVITACA
40.3±1.0
1
15
0
5
2
1
Q
TINTAYA
SICUANI
43.7±1.1(1)
POMACANCHIS
14
3
Monzogranite Granodiorite Tonalite Quartz Monzonite Quartz Monzodiorite Quartz Diorite Monzodiorite Diorite / Gabbro
CUZCO
A
5 6 7 10 11 12 16 17
6
14°30´
14°00´
16
11
7
12 17
Curahuasi Livitaca Pomacanchis Katanga Tintaya Cotabambas Las Bambas
FIG. 5. Distribution and age of the Andahuaylas-Yauri batholith in the study area. a. Displays the main body of the batholith (Fig. 2) and the location of the K-Ar age data from the present study (Table 2) and Carlotto (1998). b. Available K-Ar age data relative to volcanism of the Anta Formation and sedimentation of the San Jerónimo Group as in column C of Figure 4. c. Composition of the main phases of the Andahuaylas-Yauri batholith on a QAP diagram (Streckeisen, 1976), based primarily on work by the writers with additions after Pecho (1981), Carlier et al. (1989), and Carlotto (1998). Main localities studied are identified for better comprehension (see text for descriptions).
50
40
37
34
30
28.5
24
20
Ma
PUNACANCHA FORMATION
ANTA Fm SAN JERONIMO Gp
STRATIGRAPHY
P
c
PORPHYRY-STYLE ALTERATION AND MINERALIZATION, ANDAHUAYLAS-YAURI BELT, PERU
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PERELLÓ ET AL. TABLE 2. K-Ar Ages of Various Intrusive Phases of the Andahuaylas-Yauri Batholith
Sample no.
Latitude
Longitude
Mineral
K (%)
Radiogenic Ar (nl/g)
Ar (at. %)
Age ± 2σ
LIVIKAR 02 PORKAR 02 KATKAR 05 COTKAR 01 COTKAR 02 PROGKAR 01 PROGKAR 02 LAHUKAR 01 CHALCOKAR 02
14°18'58" 14°29'34" 14°26'38" 13°45'03" 13°41'11" 14°06'02" 14°00'54" 14°25'08" 14°03'37"
71°44'10" 71°56'02" 71°54'30" 72°21'23" 72°21'14" 72°28'35" 72°28'28" 73°00'45" 72°18'21"
Biotite Biotite Biotite Biotite Amphibole1 Biotite1 Amphibole Biotite Amphibole
7.041 7.212 5.066 7.556 0.812 6.833 0.331 7.493 0.504
11.155 10.100 6.278 1.335 1.271 9.171 0.516 10.544 0.751
31 25 13 17 34 26 53 27 36
40.3 ± 1.0 35.7 ± 0.9 31.6 ± 0.8 43.2 ± 1.1 39.8 ± 1.5 34.2 ± 0.9 39.7 ± 1.9 35.8 ± 0.9 37.9 ± 1.4
Constants: λβ = 4.962 × 10-10y–1; λε = 0.581 × 10–10y–1; 40Ar/36Ar = 295.5; 40K = 0.01167 at. percent See Figure 5 for sample location 1 Some degree of alteration to chlorite present
lengths of >300 km (Fig. 2). Both are made up of several segments or smaller faults with individual continuous runs of >50 km that display high-angle reverse and strike-slip movements. In the vicinity of the Abancay deflection (Marocco, 1978), these structures transpose Paleozoic plutonic rocks over younger cover sequences. Farther east, near Curahuasi, they place deep cumulate facies of the Andahuaylas-Yauri batholith on top of either younger intrusions of the same batholith or over volcanic horizons of the Anta Formation (Carlotto, 1998). Farther southeast, in the area of Santa Lucía, high-angle reverse structures belonging to the southeastern extension of the Abancay-Yauri fault are interpreted to have been associated with a major fold-thrust deformation event (Jaillard and Santander, 1992). The ~300-km-long, 10 to 50-km-wide corridor defined by the Limatambo-Ayaviri and Abancay-Yauri fault systems is occupied by the synorogenic rocks of the Anta Formation and the San Jerónimo Group. The two main fault systems are inferred to have been active during Mesozoic time and to have largely controlled the shape and extension of the Cuzco-Puno high in the region (Carlotto, 1998); they would therefore constitute structures reactivated during Andean deformation (Jaillard and Santander, 1992; Benavides-Cáceres, 1999). The Altiplano is characterized by the synorogenic sequences that filled the basins of the San Jerónimo Group and the Punacancha and Paruro formations. These sequences display intense synsedimentary deformation structures including tight folding and fault-controlled progressive unconformities (Carlotto, 1998). Tectono-Magmatic Synthesis The main part of the region under consideration seems to have been affected by several Late Cretaceous to Pliocene tectonic events (Marocco, 1975; Pecho, 1981; Cabrera et al., 1991; Carlotto et al., 1996b) of which the Eocene to early Oligocene (Incaic) and Oligocene to Miocene (Quechua) pulses are the most important. Important sedimentary, tectonic, and magmatic activity occurred in the Eocene and Oligocene. The red beds of the San Jerónimo Group were deposited in structurally controlled, northeast-trending synorogenic basins localized at the boundary between the Eastern and Western Cordillera. Fluvial sedimentation is thought to have progressed from south to north (Fig. 6). The presence of several progressive unconformities in the sedimentary se0361-0128/98/000/000-00 $6.00
quences is interpreted to indicate that successive compressive events were modifying the original pull-apart transtensional architecture into contractional basins (Córdova, 1986; Noblet et al., 1987; Marocco and Noblet, 1990; Chávez et al., 1996; Carlotto, 1998). Locally, important volcanism (Anta Formation) accompanied deposition of the San Jerónimo red bed sequences. The bulk of the deformation, interpreted to have begun at ~42 Ma (Carlotto, 1998) and thus broadly synchronous with the Incaic orogeny of central Peru (Noble et al., 1974, 1979; Mégard et al., 1984; Mégard, 1987; Farrar et al., 1988; Sébrier et al., 1988; Sébrier and Soler 1991), is also thought to have been the most important event of compressive deformation in the region. Paleogeographic reconstructions (Fig. 6) suggest that this northeast-directed deformation was responsible for the development of the basins that accommodated middle Eocene to early Oligocene volcanism and sedimentation. Reactivation of older, basin-bounding structures (e.g., Cuzco-Puno high) into major high-angle reverse faults that favored the uplift of the Andahuaylas-Yauri batholith, also took place during late Eocene to early Oligocene time (~40–32 Ma; Carlier et al., 1996; Carlotto, 1998; see below). This summary suggests that the Incaic orogeny in the study region constitutes a long-lived period of semicontinuous deformation of ~20 to 15 m.y., between the middle Eocene and the early Oligocene. Several distinct deformation events and associated shortening and uplift are, however, apparent and may have accompanied emplacement of the various phases of the Andahuaylas-Yauri batholith in at least two main events, at ~48 to 43 and ~40 to 32 Ma (Bonhomme and Carlier, 1990; Carlier et al., 1996). Evidence from outside the study region (Portugal, 1974; Jaillard and Santander, 1992; Carlotto et al., 1996b; Carlotto, 1998) strongly suggests that magmatism, folding, uplift, and erosion were all integral components of the Incaic orogeny in the Western Cordillera and Altiplano, a contrasting view to that of Clark et al. (1990) and Sandeman et al. (1995) for a contiguous transect (13–20°S) farther southeast. Porphyry Copper Geology Distribution The Andahuaylas-Yauri belt extends for ~300 km and is defined by 31 prospects and deposits with porphyry-style
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73° MACHU PICHU
Ri
o A
p
ur
Ab ANDAHUAYLAS
an
ca y
im
Fau
ac
Fault
ault Limatambo F CUZCO
lt ABANCAY
C
ot
ab
URCOS
am
ba
s F a
ult
14°
14°
Po
CHALHUANCA
SICUANI
ma
LIVITACA
ca
s
Fa
av
ir
ult
i Fa
ul
t
a: CONGLOMERATES b: VOLCANIC ROCKS
MAIN PLUTONS OF ANDAHUAYLAS-YAURI BATHOLITH
hi
ault uri F
b ANTA FORMATION
Ay
nc
a
Ya
SANTO TOMAS
SAN JERONIMO GROUP BASINS
YAURI
MESOZOIC ROCKS
50 km PALEOZOIC BASEMENT SEDIMENT PROVENANCE
71°
73°
FIG. 6. Schematic paleogeographic reconstruction of the study area during late Eocene to early Oligocene time. Note the intimate spatial relationship between batholithic plutons, volcanic rocks of the Anta Formation, and the sedimentary basins of the San Jerónimo Group in this reconstruction. Also shown are the main fault systems and high-angle thrusts that are interpreted to have controlled both uplift of the batholith at the deformation front south of Cuzco (the Cuzco-Puno high) and the San Jerónimo basins. Main localities are shown for reference. Modified after Carlotto (1998).
alteration and mineralization, including 19 systems grouped in 5 main clusters plus 12 separate porphyry centers (Fig. 7a). Of these, 3 systems (Leonor, Leticia, and Aceropata; Fig. 7a) are excluded from this review due to a lack of data. The belt has a known maximum width of 130 km in a northeast-southwest direction, with deposits exposed at elevations between 3,400 and 4,700 m (Fig. 7b). However, beyond the limits of the belt in the Eastern Cordillera, porphyry Cu-Mo mineralization is exposed at elevations as low as 2,800 m, as at Aurora (Fig. 7b). A salient feature of the belt is the spatial distribution of porphyry copper stocks around the edges of the main intrusions that make up the Andahuaylas-Yauri batholith, as exemplified by the Katanga, Morosayhuas, and Las Bambas clusters and by the Peña Alta, Lahuani, Alicia, Leonor, and Winicocha systems. Only at Panchita, Portada, Leticia, Cotabambas, and Antapaccay was the earlier plutonic phase effectively penetrated by the later porphyry stocks. Isolated systems, such as those of the Tintaya cluster and Trapiche, Los Chancas, and Chaccaro, far from the main batholithic 0361-0128/98/000/000-00 $6.00
intrusions, are nevertheless related to smaller plutons and outliers of the same batholith. Host rocks Country rocks for selected porphyry systems of the belt are indicated in Figure 8 and Table 3. Wall rocks to the Las Bambas, Katanga, and Tintaya clusters include either intrusive batholithic rocks or sedimentary rocks preserved as roof-pendants in the batholith. Within the Mesozoic units, it is apparent that lower Ferrobamba Formation horizons, near their contact with the Mara Formation, are the preferred host rocks (Tintaya deposits; Fierro et al., 1997; Zweng et al., 1997; Jones et al., 2000), whereas the Chuquibambilla and Soraya formations constitute common host rocks where the Yura Group predominates (Fig. 8), in the southwestern part of the belt (Los Chancas; Corrales, 2001). The Cotabambas cluster is restricted to diorite and granodiorite of the AndahuaylasYauri batholith (Perelló et al., 2002), whereas both batholith intrusions and Anta Formation volcanic horizons make up the host rocks of the Morosayhuas cluster farther north (Fig. 8).
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PERELLÓ ET AL.
71°
A´
72°
73°
AURORA (4)
13° Elevation meters a.s.l. IN SE
T
5000
A
A´
R
17 25
ON
FO
MOROSAYHUAS CLUSTER
9
29
30
CT I
4500
SE
6 3 24
4000 32
LLOCLLACSA (21)
18
19
2 23
31 14 16 26
28 13
33
11
5
27
22
7
3500
MAKI (22)
COTABAMBAS CLUSTER
10
20
QENCO (27)
CHA-CHA (11)
12 21
8 1
15 CHILCACCASA (12) HUACLLE (15) CCALLA (7)
LETICIA (20)
3000
ACEROPATA (1)
b
AZULCCACCA (5)
CCARAYOC (8)
4
50 km
2500 CHACCAR O (10)
CHALCOBAMBA (9)
FERROBAMBA (14)
14° ALICIA (2) LOS CHANCAS (18) PEÑA ALTA (24)
SULFOBAMBA (29)
LAS BAMBAS CLUSTE R
LEONOR (19)
CRISTO DE LOS ANDES (6)
PANCHITA (25)
KATANGA CLUSTER
LAHUANI (17)
A
WINICOCHA (33)
MONTE ROJO (23)
PORTADA (26)
SAN JOSE (30)
TRAPICHE (32) KATANGA (16)
TINTAYA (31)
50 km
a
TINTAYA CLUSTER
ANTAPACCAY (3)
15°
COROCCOHUAYCO (13) QUECHUA (28)
FIG. 7. Distribution of the porphyry copper deposits and prospects referred to in this study. a. Illustrates the location of the main clusters at Morosayhuas, Katanga, Cotabambas, Las Bambas, and Tintaya, together with other separate deposits and prospects. Numbers in parentheses are keyed to the section of Figure 7b. The Aurora prospect is also shown for reference. b. Simplified section A-A´ displaying the distribution of the systems relative to present-day elevation above sea level.
Geometry Porphyry copper-bearing stocks of the Andahuaylas-Yauri belt are centered on multiple-pulse porphyritic intrusions (Table 3) that commonly display both dike- and cylinder-like geometries. In plan view, the stocks generally range from ~0.25 to 0.6 km2, but also include the much smaller examples of the Morosayhuas cluster where the stocks can be as small as 150 × 50m, as at Qenqo. In general, the form of the stocks 0361-0128/98/000/000-00 $6.00
is difficult to determine, owing to (1) post-mineralization moraine cover (Katanga), (2) late mineral dikes (Cotabambas and Tintaya; Fig. 9), (3) syn- to post-mineralization faults (Cotabambas and Antapaccay; Fig. 9), and (4) textural and compositional similarities between porphyry copperbearing stocks and wall rocks (Winicocha, Morosayhuas, Chalcobamba, and Panchita). Moreover, other systems seem to have developed complex geometries from the outset, such as the “rootless” nature of the stocks at Chabuca and
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PORPHYRY-STYLE ALTERATION AND MINERALIZATION, ANDAHUAYLAS-YAURI BELT, PERU
LABRA/ CHUQUIBAMBILLA Fms
WINICOCHA CRISTO DE LOS ANDES
PEÑA ALTA
COTABAMBAS
ALICIA
LAHUANI
YURA Gp
SORAYA/ HUALHUANI Fms
LOS CHANCAS
MURCO/MARA/ HUAMBO Fms
QUECHUA
FERROBAMBA/ ARCURQUINA/ AYAVACAS Fms
TINTAYA
ANTAPACCAY CHACCARO
NEOCOMIAN - TURONIAN
PUQUIN/ ANTA-ANTA Fms
LAS BAMBAS
MOROSAYHUAS
ANTA/ KAYRA/ SONCCO Fms
CHILCA/ QUILQUE Fms
MAASTRICHTIAN
BAJOCIAN - TITHONIAN
ANDAHUAYLAS - YAURI BATHOLIT H
KATANGA
SAN JERONIMO Gp
PALEOCENE
EOCENE-EARL Y OLIGOCENE
PUNACANCHA Fm
CACHIOS/ PISTE Fms
PELITE LAGUNILLAS Gp
SANDSTONE
CONGLOMERATE
QUARTZITE
GYPSUM
LIMESTONE
ANDESITE
FIG. 8. Schematic diagram illustrating the tentative location of selected porphyry systems of the belt relative to the main Mesozoic to Cenozoic stratigraphic units of the region and the Andahuaylas-Yauri batholith.
Coroccohuaycco (Fierro et al., 1997; Zweng et al., 1997) and the bedding-parallel attitude of the multiple sills at Quechua (J. Perelló, unpub. data, 2003), in the Tintaya cluster. Systems dominated by tabular, dike-like geometries include the Cotabambas (Fig. 9c), Las Bambas, and Morosayhuas clusters, and the Peña Alta and Quechua composite stocks. Typical cylindrical host intrusions include Los Chancas (Corrales, 2001), Cristo de los Andes, Chaccaro, and Alicia (Fig. 9a), with plan dimensions of between 300 and 600 m. Composition Porphyry copper-bearing stocks of the belt are, like the major intrusions of the Andahuaylas-Yauri batholith, typically calc-alkaline in composition (Carlier et al., 1989; Bonhomme and Carlier, 1990). Although it is difficult to assign precise petrographic names to altered and mineralized intrusive rocks, most of the porphyry-related intrusions studied possess an intermediate composition, with dacite and/or granodiorite predominanting (Fig. 10c). Exceptions include some quartz monzodioritic stocks at Cotabambas (Perelló et al., 2002; Fig. 10c), the monzogranitic composition of Panchita, the monzonitic nature of some stocks in the Katanga cluster (MMAJ, 1983), and the dominantly quartz monzonitic to monzonitic composition of the Tintaya cluster, including Quechua (Fierro et al., 1997; Zweng et al., 1997; Jones et al., 2000; Fierro et al., 2002). 0361-0128/98/000/000-00 $6.00
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Biotite and amphibole are by far the most abundant ferromagnesian phenocrysts in all the porphyries studied, although pyroxene is also present locally at Katanga. Proportions of biotite to amphibole vary greatly among the different systems, with biotite being more abundant at Alicia, Los Chancas, Peña Alta, Panchita, Trapiche, and in the Las Bambas and Katanga clusters. Amphibole dominates at Cristo de los Andes, Portada, Chaccaro, and in the Cotabambas and Tintaya clusters. Other important phenocryst populations are largely dominated by plagioclase (30 to 80 vol %) and subordinate quartz “eyes” and orthoclase (~10 vol % each) (Fig. 10c). Groundmass mineralogy is dominated by quartz, plagioclase and orthoclase in microfelsitic aggregates, which locally contain interstitial biotite. In most deposits, the bulk of the mineralization seems to be genetically associated with one single phase of intrusion, as at Alicia, Cristo de los Andes, Peña Alta, Portada, and the Morosayhuas and Cotabambas clusters (Table 3). Two phases are apparent at Los Chancas and Lahuani, and up to six phases have been described at Antapaccay (Jones et al., 2000). Similarly, inter- to late-mineral porphyry intrusions constitute integral parts of all systems in the belt. These later intrusions vary from one central phase at Alicia (Fig. 9), through two at Ccalla (Cotabambas) and Chabuca (Tintaya), to at least three at Antapaccay. Earliest inter-mineral porphyries exhibit similar textures, compositions, and alteration products to the main intrusions, making distinction between them difficult. However, they tend to possess weaker versions of the same alteration and mineralization types. Later inter-mineral and younger phases display different compositions and textures, and are characterized by much weaker alteration, in addition to lacking significant hydrofracturing. Inter-mineral dikes of roughly the same composition as that of the main stocks are present at Cotabambas, Alicia, Chaccaro, Antapaccay, Quechua, Katanga, Lahuani, Las Bambas, Morosayhuas, and Tintaya, whereas younger, compositionally and texturally distinct phases occur at Tintaya, Antapaccay, Los Chancas, Katanga, Lahuani, and Peña Alta. These younger phases, which may or may not include postmineral intrusions, are commonly dominated by andesitic, dacitic, and microdioritic dikes. Postmineral mafic dikes are common at Tintaya (Fierro et al., 1997; Zweng et al., 1997). The size, shape and location of inter- to late-mineral intrusions, with respect to the main stock, exert a marked influence on the geometry of both the main stocks and the mineralized zones in porphyry deposits of the belt. In most cases, inter- to late-mineral phases are dike-like in form, as at Lahuani and Peña Alta, and in the Tintaya, Katanga, and Cotabambas clusters, but are cylindrical in shape at Alicia and irregular at Morosayhuas and Chaccaro. In some deposits, as at Ccalla at Cotabambas, the earliest inter-mineral phases are centrally located with respect to the main-phase stock, whereas later-mineral intrusions occupy peripheral positions in association with late-stage dome and dike swarms of dacitic composition (Fig. 9c). At Alicia, however, the cylinder-like, inter-mineral intrusion occupies a central position (Fig. 9a) and at Tintaya (e.g., the Chabuca deposit) the dikes cut the early-stage porphyry stock and related mineralization almost at a 90-degree angle and extend far beyond mineralized zones (Zweng et al., 1997). Modification of
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Six phases: monzonite to quartz monzonite One phase?: quartz monzonite One phase: granodiorite to quartz monzodiorite Several phases: dacite Several phases: dacite and/or granodiorite
Antappacay (Tintaya)
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One phase: dacite
Winicocha
Cristo de los Andes Peña Alta
Lahuani
Chaccaro
Alicia
Two phases: granodiorite to quartz monzonite One phase: granodiorite and/or dacite One phase: dacite One phase: rhyodacite to dacite One phase: dacite One phase: dacite/ rhyodacite
Los Chancas
Qenco/Maki One phase: (Morosayhuas) diorite and quartz diorite
San José (Katanga)
Katanga (Katanga)
Ferrobamba (Las Bambas) Chalcobamba (Las Bambas)
Ccalla (Cotabambas)
Several phases: monzonite and dacite Two main phases: dacite
Two phases: monzonite
Chabuca (Tintaya)
Quechua (Tintaya)
Mineralized intrusion(s)
Deposit or prospect (cluster)
N.A.
Several phases: andesite and microdiorite dikes Several phases: dacite and andesite dikes Several phases: andesite dikes Several phases: andesite dikes
One central phase: dacite
N.A.
Several phases: dacite and andesite dikes Several phases: dacite and andesite dikes; granodiorite stock Several phases: dacite and andesite dikes Two phases: dacite and andesite dikes Several phases: andesite dikes
Several phases: andesite and quartz monzonite dikes Two phases: granodiorite and dacite dikes
Several phases: diorite, dacite an mafic dikes Three phases: andesite dikes
Mid- and late mineral intrusions
Microdiorite and dacite stocks
Upper Soraya Fm
Soraya Fm
Upper Ferrobamba Fm Chuquibambilla Fm
Upper Ferrobamba Fm
Chuquibambilla and Soraya Fms
Lower Anta Fm; diorite pluton
Undifferentiated Ferrobamba Fm
Undifferentiated Ferrobamba Fm
Lower Ferrobamba Fm Lower Ferrobamba Fm
Lower Ferrobamba, Mara, and upper Soraya Fms Diorite and granodiorite plutons
Lower Ferrobamba Fm; diorite pluton
Lower Ferrobamba Fm
Wall rocks
Early potassic overprinted by quartz-sericitic Early potassic and calcicpotassic overprinted by sericite-clay-chlorite and peripheral quartz-sericitic Early potassic? overprinted by quartz-sericitic and local advanced argillic
Early potassic overprinted by sericite-clay-chlorite Early potassic overprinted by quartz –sericitic
Early potassic overprinted by sericite-clay-chlorite
Early potassic overprinted by intense quartz-sericitic and local advanced argillic Early potassic and calcicpotassic overprinted by sericite-clay-chlorite and local quartz-sericitic and advanced argillic Early potassic with peripheral quartz-sericitic
Early potassic
Early potassic and calcicpotassic overprinted by sericite-clay-chlorite and local quartz-sericitic Early potassic overprinted by sericite-clay-chlorite Early potassic overprinted by sericite-clay-chlorite
Early potassic overprinted by quartz-sericitic
Early potassic overprinted by albite-rich sericiteclay-chlorite Early potassic overprinted by sericite-clay-chlorite
Ore-related hydrothermal alteration
Trace chalcopyrite
Chalcopyrite
Chalcopyrite
Chalcopyrite, molybdenite
Chalcopyrite
Chalcopyrite ~ bornite
Chalcopyrite > bornite
Trace chalcopyrite
Chalcopyrite
Chalcopyrite
Chalcopyrite > bornite Chalcopyrite ~ bornite
Chalcopyrite > bornite
Chalcopyrite
Bornite ~ chalcopyrite
Chalcopyrite > bornite
Ore mineralogy
Absent
Copper oxides and chalcocite blanket Minor copper oxides
Minor copper oxides Minor copper oxides
Minor copper oxides
Copper oxides and chalcocite blanket
Copper oxides and chalcocite blanket Absent
Abundant copper oxides
Irregular copper oxides Irregular copper oxides
Irregular copper oxides and chalcocite blanket Irregular copper oxides and chalcocite
N.A.
Irregular copper oxides
Supergene mineralization
TABLE 3. Geological Features of Selected Porphyry Systems of the Andahuaylas-Yauri Belt
Inter- to latemineral; sericite-rich
Inter-mineral; sericite-rich N.A.
Inter-mineral; magnetite-rich N.A.
N.A.
N.A.
Large intermineral; sericite-rich Local tourmalinerich dikes
N.A.
Contact breccias N.A.
Contact breccias and pebble dikes
Pebble dikes
Large postmineral diatreme
Pebble dikes
Hydrothermal breccias
Distal skarn at 1–2 km
Minor exoskarn; distal jasperoids at ~2 km Distal skarn at 2–3 km Distal skarn at 1.5 km
Minor exoskarn
Dominant: exoskarn
N.A.
Distal skarn at 1–2 km
Present: exo>endoskarn
Dominant: exo>endoskarn
Important: exo>endoskarn Dominant: exo>endoskarn
Distal skarn at 2–3 km
Locally important exoskarn
Minor exoskarn
Dominant: exo>endoskarn
Skarn mineralization
1588 PERELLÓ ET AL.
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PORPHYRY-STYLE ALTERATION AND MINERALIZATION, ANDAHUAYLAS-YAURI BELT, PERU
a
b
ALICIA
ANTAPACCAY 71°20´ ANTAPACCAY NORTH
71°59´
14°05´
15°00´
Late-mineral Diatreme Late Porphyry
200 m
Late Porphyry
Main Porphyry
Main Porphyry
Pre-mineral Diorite
Skarn
Skarn
Reverse Fault
Zone of Intense Stockwork Veining
Anticline
c
ANTAPACCAY SOUTH
d
COTABAMBAS
500 m
SAN JOSE
72°22´
Cerro Saiwa
4400m HUACLLE
4300m
?
4200m
4100m
13°44´
CCALLA
200 m
CCARAYOC
AZULCCACCA
Late Dome and Dikes
1 km
a b
a b
Porphyry-related Intrusions Andahuaylas-Yauri Batholith a: Diorite b: Granodiorite Skarn
Leached Capping
Late Andesite Dike
Supergene Chalcocite Blanket a: Intersected by Drilling b: Projected
Hydrothermal Breccia Main Porphyry Drill Hole
FIG. 9. Main geologic attributes of selected porphyry copper systems of the Andahuaylas-Yauri belt. a. Displays the cylindrical form of the porphyry copper-bearing stock at Alicia and the central location of the late-mineral porphyry dike, as mapped by the writers. b. Illustrates the structurally controlled nature of the porphyry copper systems at Antapaccay and the large postmineral diatreme breccia (simplified after Jones et al., 2000 and Fierro et al., 2002). c. Displays the cluster at Cotabambas and the structurally controlled nature of the stocks at Ccalla and Azulccacca, together with the peripheral, latemineral dome and its dike swarm (simplified after Perelló et al., 2002). d. Schematic cross section through the San José porphyry system at Katanga displaying the distribution of the main geologic units and the location of the supergene enrichment zone. Based on data provided by the Metal and Mining Agency of Japan (1983) and mapping by the writers. 0361-0128/98/000/000-00 $6.00
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b
?
?
1590
(1) (1)
(1)
(1)
14°30´
14°00´
(1)
LOS CHANCAS PANCHITA
WNW
LAHUANI
TRAPICH E
a
73°30´
200 km
CHALHUANCA
50 km
CRISTO DE LOS ANDES
SANTO TOMAS
SULFOBAMBA
KATANGA
ALICIA
72°00´
PORTADA
CHACCARO
CHALCOBAMBA FERROBAMBA
LAS BAMBAS CLUSTER
72°30´
CHILCACCASA
CUZCO
A
5 10
20
35
71°30´
ANTAPACCAY
KATANGA CLUSTER
SAN JOSE
90
Q
7
4
6
90
65
11
10 2 3 8 5 1
9
DACITE
60
90
QUECHUA
COROCCOHUAYCO
TINTAYA CLUSTER
SICUANI
TINTAYA
YAURI
RHYOLITE
WINICOCHA
LIVITACA
MONTE ROJO
60
ALICIA CRISTO DE LOS ANDES CHACCARO KATANGA CLUSTER PORTADA PEÑA ALTA PANCHITA WINICOCHA LAS BAMBAS CLUSTER TRAPICHE COTABAMBAS CLUSTER
13°30´
c
1 2 3 4 5 6 7 8 9 10 11
COTABAMBAS CLUSTER
AZULCCACCA
CCALLA
CCARAYOC
HUACLLE
MAKI
QENCO
LLOCLLACSA
MOROSAYHUAS CLUSTER CHA-CHA
MACHU PICHU
LETICI A
ACEROPATA
SSE
Skarn Fe-(Cu, Au)
TRAPICHE
LEONOR
PEÑA ALTA
73°00´
LAHUANI
PANCHITA
ABANCAY
73°00´
PORPHYR Y COPPER ALTERATION-MINERALIZ ATION
CRISTO DE ANDES SULFOBAMBA CHALCOBAMBA FERROBAMBA
LOS CHANCAS
73°30´ ANDAHUAYLAS
ANDAHUAYLAS-YAURI BATHOLITH
(1) (1)
(1)
QUECHUA
20
FIG. 10. a. Distribution of porphyry copper clusters and systems relative to the Andahuaylas-Yauri batholith and Fe-Cu skarn occurrences. Note the preferred location of the porphyry clusters along the edges of main batholithic bodies. b. Age distribution of selected porphyry copper deposits and prospects of the belt (Table 4) relative to the Andahuaylas-Yauri batholith and the volcanic and sedimentary stratigraphy of the region. c. Dominant composition of selected porphyry copper-bearing stocks of the belt according to their modal mineral contents on a QAP diagram (Streckeisen, 1978).
STRATIGRAPHY
50
40
37
34
30
28.5
PUNACANCHA FORMATION
ANTA Fm SAN JERONIMO Gp
24 ALICIA CCALLA CHILCACCASA CHACCARO
20 SAN JOSE MONTE ROJO WINICOCHA
OTHER INTRUSION INTER-LATE PHASE PORPHYR Y AND/OR ALTERATION MAIN PHASE ALTERATION (2) Re-Os (Mathur et al., 2001) (3) K-Ar (Noble et al., 1984)
KATANGA
GRANODIORITE QUARTZ MONZODIORITE MONZODIORITE DIORITE DIORIT E / GABBRO (CUMULATES) (1) K-AR (Carlotto, 1998)
PORTADA
MIOCENE EARLY
OLIGOCENE EARLY LATE
LATE
EOCENE MIDDLE
EARLY
0361-0128/98/000/000-00 $6.00 PEÑA ALTA
TINTAYA (3) TINTAYA (2)
Ma
P
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ore zones at Cotabambas, Tintaya, and Las Bambas by intrusion of the inter- to late-mineral bodies is appreciable. Hydrothermal alteration and mineralization Six distinct types of alteration-mineralization are recognizable in porphyry systems of the Andahuaylas-Yauri belt. These make up the potassic, propylitic, sericitic (phyllic), advanced argillic, and calc-silicate types of Meyer and Hemley (1967) and subsequent investigators (Lowell and Guilbert, 1970; Guilbert and Lowell, 1974), as well as an alteration type characterized by sericite, chlorite, and clays (illite-smectite). The latter was originally termed SCC-type by Sillitoe and Gappe (1984) in the Philippines porphyry copper deposits and is now referred to as intermediate argillic alteration by Sillitoe (2000). An additional, less widespread alteration-mineralization type includes the mixed calcic-potassic assemblages observed at several deposits and prospects. Potassic alteration: With a few exceptions (Morosayhuas, Winicocha), potassic alteration is the principal alteration type directly associated with mineralization in Andahuaylas-Yauri porphyry systems (Table 3). In all cases, potassic alteration occurs early in the evolution of each system and consists of quartz, biotite, and K-feldspar. Hydrothermal biotite replaces ferromagnesian components, typically magmatic hornblende and, less commonly, magmatic biotite. It also occurs in the groundmass of porphyry stocks and in veinlets, either alone or accompanied by other silicate phases. Early, typically barren biotite seams and veinlets occcur at several systems, including Peña Alta and Cotabambas, and can be compared with the early biotite veins described by Gustafson and Quiroga (1995) at El Salvador, Chile. In most deposits and prospects, including the Cotabambas, Tintaya, Las Bambas, and Katanga clusters, and at Lahuani, Alicia, and Los Chancas, biotite is accompanied by K-feldspar, which at Cotabambas and Tintaya constitutes a volumetrically significant alteration mineral. For example, the most intense potassic alteration at Ccalla is dominated by aggregates of quartz and K-feldspar, with local development of graphic textures and complete destruction of original rock textures. K-feldspar also occurs in a variety of veinlet types with quartz and biotite, within the veinlets or as alteration halos, and as partial replacements of original plagioclase sites. Calcite, apatite, anhydrite, and magnetite are additional minerals in potassic alteration assemblages and are also common constituents of veinlet assemblages. Conspicuous magnetite accompanies potassic alteration in gold-rich porphyry systems of the belt, and at Cotabambas attains >5 vol percent (Perelló et al., 2002). Major quantities of quartz were introduced as either uni- or multidirectional veinlets during potassic alteration in all deposits and prospects, but most characteristically at Cotabambas, Antappaccay, San José, Ferrobamba, Chalcobamba, Maki, Llocllasca, and Winicocha. In at least five systems, including Maki at Morosayhuas, San José at Katanga, Ccalla and Azulccacca at Cotabambas, and Winicocha, the quartz veinlets coalesce to form massive bodies of nearly pure quartz. Where overprinting by either quartz-sericitic or sericite-clay-chlorite alteration is intense, these massive bodies are the only remnants of the early-stage potassic alteration. In common with porphyry systems elsewhere (e.g., Gustafson and Hunt, 1975; Gustafson and Quiroga, 1995; Sillitoe, 2000), a variety of 0361-0128/98/000/000-00 $6.00
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quartz veinlets, introduced in several generations, characterize potassic alteration in porphyry deposits and prospects of the Andahuaylas-Yauri belt. Typical assemblages and textures compare closely with the A- and B-type veinlets described by Gustafson and Hunt (1975) from El Salvador porphyry copper deposit, Chile. A-type veinlets carry significant mineralization in the form of chalcopyrite and/or bornite at a number of deposits, including the Ccalla and Azulccacca centers at Cotabambas, Peña Alta, Ferrobamba, and Chalcobamba at Las Bambas, and Antapaccay at Tintaya. At Alicia and Chalcobamba, B-type veinlets are characterized by semicontinous centerlines filled by millimeter- to centimeter-sized grains of bornite and chalcopyrite, whereas at Peña Alta, Lahuani, Los Chancas, and Quechua, they are dominated by chalcopyrite and molybdenite. A-veinlets also occur in copper-poor, goldbearing porphyry systems, such as those from the Morosayhuas cluster (see below), where they contribute minor amounts of chalcopyrite. Gold-rich porphyry copper deposits of the belt, such as the Ccalla and Azulccacca centers at Cotabambas, contain appreciable amounts of magnetitebearing veinlets that are similar to the M-type veinlets of Clark and Arancibia (1995) and to the A- and C-type veinlets described by Cox (1985) at Tanamá, Puerto Rico. Calcic-potassic alteration: Calcic-potassic alteration is represented at Cotabambas, Morosayhuas, and Peña Alta (Table 3). The assemblage is characterized by veinlets of quartz, actinolite, and hornblende, with important K-feldspar, biotite, apatite and calcite, and volumetrically minor amounts of clinopyroxene and epidote. In general, plagioclase is variably altered to K-feldspar, calcite, and/or epidote, whereas magmatic biotite and amphibole are selectively replaced by needles of actinolite, commonly intergrown with apatite. Magmatic pyroxene is altered to aggregates of actinolite-apatite and actinolite-biotite, and magmatic hornblende is converted to mixtures of clinopyroxene, biotite, and hornblende as in several systems of the Morosayhuas cluster. Alteration halos to various veinlet sets include K-feldspar, actinolite, biotite, and chlorite. Magnetite is a common constituent, and chalcopyrite, as part of this association, gives rise to ore-grade CuAu mineralization. Sericite-clay-chlorite alteration: Several deposits and prospects of the belt, including Alicia, Chaccaro, Peña Alta, Las Bambas, Morosayhuas, Cotabambas, and Tintaya, possess significant sericite-clay-chlorite alteration as part of their ore zones (Table 3). This assemblage imparts a pale-green overprint to potassic alteration and gives a soft aspect to the rock (cf. Sillitoe and Gappe, 1984). It generally modifies, but with some degree of preservation, original rock textures. Sericiteclay-chlorite alteration varies in both intensity and mineralogy, although assemblages defined for systems of the belt always include one or more associations of sericite (finegrained muscovite), illite, smectite, chlorite, calcite, quartz, and varied proportions of epidote, halloysite, and albite. Plagioclase (both phenocrysts and groundmass) is replaced by a pale-green, greasy sericite assemblage which also includes illite and, locally, smectite. Amphibole and biotite, the latter of magmatic and/or hydrothermal origin, are characteristically replaced by chlorite. Calcite is common as a replacement of plagioclase, and in some deposits and prospects, including Cotabambas and Chaccaro, it is a major constituent of the
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PERELLÓ ET AL.
assemblage. Albite is locally important as a replacement of plagioclase, as at Morosayhuas and at the Chabuca Este deposit at Tintaya (Zweng et al., 1997). Quartz veinlets include various associations with chlorite and calcite, and halos of green sericite, halloysite, and mixedlayer illite-smectite are common (Cotabambas, Chaccaro). Where quartz veining is intense and halos coalesce, the rock is completely replaced by fine- to very fine-grained mosaics of quartz, sericite, and mixed-layer illite-smectite that obliterate original host rock textures, as at Ccalla, San José, Maki, Llocllacsa, and Winicocha. Chalcopyrite is locally present in some veinlet assemblages and may constitute monomineralic veinlets with chlorite and quartz, but in general, chalcopyrite contents are lower than in earlier potassic alteration-mineralization. Bornite, if present in earlier assemblages, rarely survives sericite-clay chlorite alteration. Pyrite is typically present in the form of veinlets and disseminations (Chaccaro, Maki), and is volumetrically important at Cotabambas. Magnetite is variably transformed to martite, and specular hematite is a characteristic constituent of the assemblage. Quartz-sericitic alteration: Well-defined zones of quartzsericitic alteration accompany ore in several systems of the belt at San José, Cristo de Los Andes, Quechua and, possibly, at Winicocha and Chilcaccasa. Moderate amounts are also present at Cotabambas (Ccalla), Los Chancas, Chaccaro, Lahuani, Peña Alta, and Morosayhuas (Table 3). The quartzsericitic assemblages typically comprise white, texturally destructive aggregates of quartz, sericite (fine-grained muscovite), and illite, accompanied by several percent pyrite. In all systems mentioned above, quartz-sericitic alteration typically overprints earlier-formed potassic or, as at Cotabambas, sericite-clay-chlorite alteration. Broad quartz-sericitic alteration halos around potassic cores, common in many porphyry Cu-Mo deposits worldwide (e.g., Lowell and Guilbert, 1970), are not widely developed in systems of the Andahuaylas-Yauri belt, although they are inferred at Los Chancas (Corrales, 2001). At the Ccalla deposit in Cotabambas, structurally controlled patches of quartz-sericitic alteration abut intermediate argillic assemblages in the upper parts of the system (Perelló et al., 2002) and contributed to the formation of an irregular chalcocite blanket. A similar situation is also observed at Cristo de los Andes, Quechua and at the San José deposit in the Katanga cluster (MMAJ, 1983). D-type veinlets (Gustafson and Hunt, 1975) are typically associated with overprinting quartz-sericitic alteration in most systems of the belt where this style of alteration occurs. D veinlets fill planar, continuous, centimeter-wide structures with pyrite and quartz, which develop quartz-sericitic halos. Tourmaline, a common constituent of sericitic alteration in many parts of the world, is rarely developed in AndahuaylasYauri porphyry systems, occurring only at Trapiche and Morosayhuas. Advanced argillic alteration: Hypogene advanced argillic alteration is not recognized as a common assemblage in porphyry systems of the Andahuaylas-Yauri belt. However, sericite-rich alteration that also contains pyrophyllite and kaolinite-group minerals is present at San José, Winicocha, and Maki (Table 3). At San José, advanced argillic alteration is intimately associated with transgressive structures, whereas at Maki it is controlled by permeability contrasts at the 0361-0128/98/000/000-00 $6.00
contact between quartz diorite and volcanosedimentary country rocks. At Maki and San José, advanced argillic alteration is superimposed on the porphyry stocks and associated potassic and sericite-clay-chlorite alteration, whereas at Winicocha it is developed at higher elevations and constitutes the roots of a porphyry copper lithocap. Propylitic alteration: Propylitic alteration in AndahuaylasYauri belt porphyry systems (chlorite, epidote, and calcite) is found mainly as part of the outer halo confined to noncarbonate wall rocks. In other systems, as at Cotabambas, Chaccaro, Lahuani, and Las Bambas, propylitic alteration occurs within porphyry copper ore zones in late-mineral stocks and dikes. In both cases, disseminated and veinlet pyrite, in amounts of ~1 vol percent, is common. Calc-silicate alteration: Calc-silicate alteration is represented in many deposits and prospects of the belt. Indeed, associated mineralization has constituted the main source of Cu-Au ore at the Tintaya (Terrones, 1958; Santa Cruz et al. 1979; Noble et al., 1984; Zweng et al., 1997) and Katanga (MMAJ, 1983) mines, and it is an important contributor to mineralization at the Las Bambas skarn-porphyry cluster and the Quechua deposit (E. Tejada, pers. commun., 2003). In addition, proximal calc-silicate assemblages and associated skarn-type mineralization are present in most systems of the belt, excluding Cotabambas, Cristo de los Andes, Peña Alta, and Morosayhuas. However, at distances of ~3 km, all of the deposits have distal skarn-type assemblages in roof-pendants of Ferrobamba Formation and equivalent units. Garnet, diopside, epidote, and actinolite are the characteristic calc-silicate assemblages (Terrones, 1958; Santa Cruz et al., 1979). At Tintaya (Fierro et al., 1997; Zweng et al., 1997); calc-silicate alteration and mineralization occur in endoskarn and exoskarn facies, and as products of prograde (anhydrous) and retrograde (hydrous) events (Table 3). The bulk of the Cu (-Au, -Mo) mineralization at Tintaya and Las Bambas was introduced during prograde events, typically as chalcopyrite and, less commonly, bornite, whereas at the smaller Alicia system, bornite, with or without chalcopyrite, is the dominant Cu and Au contributor. Distal skarn mineralization in porphyry-centered systems of the belt is similar to that elsewhere (Einaudi et al., 1981), in that it is richer in Pb and Zn (e.g., Morosayhuas). Another expression of the distal environment is the structurally and lithologically controlled, yellow-brown jasperoid developed in limestone beyond the skarn front at Tintaya, Las Bambas, Katanga, and Lahuani, which at Tintaya is reported to contain up to 1 ppm Au (Zweng et al., 1997). The presence of jasperoid in several deposits and prospects is evidence that they constitute integral parts of porphyry-centered systems in the region. Moreover, at Lahuani (Table 3), distal replacement of calcareous shale by structurally controlled, As-anomalous jasperoidal mantos resembles the Carlin-style gold environment described around some porphyry centers (Sillitoe and Bonham, 1990). Hydrothermal breccias Hydrothermal breccias are poorly documented in Andahuaylas-Yauri porphyry systems (Table 3). During this study, they were identified at most deposits and prospects, an observation supported by descriptions of Antapaccay (Jones et al., 2000; Fierro et al., 2002) and Tintaya (Fierro et al.,
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PORPHYRY-STYLE ALTERATION AND MINERALIZATION, ANDAHUAYLAS-YAURI BELT, PERU
1997; Zweng et al., 1997). Most of the breccias are volumetrically small, with that at Antapaccay probably constituting the largest single mass in any porphyry system in the belt (Fig. 9). As at Antapaccay, all the observed breccias postdate main-stage mineralization, although contact (igneous) breccias associated with the emplacement of early and intermineral porphyry stocks are clearly intermineral in timing. This is particularly evident where the breccias are cut by mineralized veinlets, as at Cotabambas and San José. Most mapped hydrothermal breccias are either dike-like in form or occur as narrow zones at intrusive contacts. Dikelike breccias possess strong structural control and conform to pebble dikes as is common in porphyry systems worldwide. They typically consist of centimeter-sized, subrounded lithic clasts supported by volumetrically important matrices of finely comminuted (rock flour) material. Illite, chlorite, and fine-grained (dusty) pyrite are typical constituents of the matrices. Larger expressions of a similar style of brecciation, as at Winicocha, include rounded to subrounded, exfoliated clasts, tens of centimeters in size, in a sericitic matrix. Ore zone geometry Most Andahuaylas-Yauri porphyry deposits and prospects possess mineralization that is variably hosted by porphyry stocks and their immediate country rocks. The following examples document the variety observed in the belt. 1. Mineralized skarns are present in country rocks where porphyry stocks intrude carbonate rocks of the Ferrobamba Formation and equivalent units, as at Tintaya, Alicia, and Chalcobamba. Significant mineralization, however, is also hosted by both porphyry stocks and wall rocks at Ferrobamba and San José, despite the fact that country rocks there are dominated by carbonate horizons of the Ferrobamba Formation. 2. Porphyry stocks constitute the main host to ore where country rocks are dominated by the terrigenous facies of the Yura Group and equivalent units, as at Lahuani, Cristo de los Andes, Los Chancas and Quechua, or by volcaniclastic and red bed horizons of the Anta Formation, as in the Morosayhuas cluster. 3. Ore seems to be evenly distributed between porphyry stocks and wall rocks where intrusions of the AndahuaylasYauri batholith constitute the dominant country rock, as at Cotabambas. The Tintaya cluster further exemplifies the diversity of mineralization styles and ore hosts, including (1) skarns at the various Chabuca deposits and Coroccohuayco, and associated with low-grade porphyry-style mineralization, (2) porphyry stocks, with minor skarn mineralization at Quechua, and (3) porphyry stocks and dioritic country rocks with small amounts of skarn at Antappaccay (Jones et al., 2000; Fierro et al., 2002). Metal contents Porphyry copper deposits and prospects of the Andahuaylas-Yauri belt range from gold-rich, molybdenum-poor examples (Cotabambas), through deposits carrying both gold and molybdenum (Tintaya, Los Chancas), to relatively 0361-0128/98/000/000-00 $6.00
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molybdenum-rich, gold-poor end-members (Lahuani). Goldonly porphyry systems, although poorly explored, are present in at least two areas at Morosayhuas and Winicocha. The Ccalla and Azulccacca centers at Cotabambas are the best examples of the gold-rich category of porphyry copper deposits (Perelló et al., 2002) of the belt, with an average Au grade >0.3 ppm and appreciable volumes averaging >0.4 ppm. Molybdenum contents are low,
