Abstract

The large Cerro de Pasco Cordilleran base metal deposit in central Peru is located on the eastern margin of a middle Miocene diatreme-dome complex and comprises two mineralization stages. The first stage consists of a large pyrite-quartz body replacing Lower Mesozoic Pucará carbonate rocks and, to a lesser extent, diatreme breccia. This body is composed of pyrite with pyrrhotite inclusions, quartz, and black and red chalcedony (containing hypogene hematite). At the contact with the pyrite-quartz body, the diatreme breccia is altered to pyrite-quartz-sericite-pyrite. This body was, in part, replaced by pipelike pyrrhotite bodies zoned outward to carbonate-replacement Zn-Pb ores bearing Fe-rich sphalerite (up to 24 mol % FeS).

The second mineralization stage is partly superimposed on the first and consists of zoned east-west–trending Cu-Ag-(Au-Zn-Pb) enargite-pyrite veins hosted in the diatreme breccia in the western part of the deposit and well-zoned Zn-Pb-(Bi-Ag-Cu) carbonate-replacement orebodies; in both cases, sphalerite is Fe poor and the inner parts of the orebodies show typically advanced argillic alteration assemblages, including aluminum phosphate sulfate (APS) minerals. The zoned enargite-pyrite veins display mineral zoning, from a core of enar-gite-pyrite ± alunite with traces of Au, through an intermediate zone of tennantite, chalcopyrite, and Bi minerals to a poorly developed outer zone bearing sphalerite-galena ± kaolinite. The carbonate-hosted replacement ores are controlled along N35°E, N 90° E, N 120° E, and N 170° E faults. They form well-zoned upward-flaring pipelike orebodies with a core of famatinite-pyrite and alunite, an intermediate zone with tetra-hedrite-pyrite, chalcopyrite, matildite, cuprobismutite, emplectite, and other Bi minerals accompanied by APS minerals, kaolinite, and dickite, and an outer zone composed of Fe-poor sphalerite (in the range of 0.05–3.5 mol % FeS) and galena. The outermost zone consists of hematite, magnetite, and Fe-Mn-Zn-Ca-Mg carbonates. Most of the second-stage carbonate-replacement orebodies plunge between 25° and 60° to the west, suggesting that the hydrothermal fluids ascended from deeper levels and that no lateral feeding from the veins to the carbonate-replacement orebodies took place.

In the Venencocha and Santa Rosa areas, located 2.5 km northwest of the Cerro de Pasco open pit and in the southern part of the deposit, respectively, advanced argillic altered dacitic domes and oxidized veins with advanced argillic alteration halos occur. The latter veins are possibly the oxidized equivalent of the second-stage enargite-pyrite veins located in the western part of the deposit.

The alteration assemblage quartz-muscovite-pyrite associated with the pyrite-quartz body suggests that the first stage precipitated at slightly acidic pH. The sulfide mineral assemblages define an evolutionary path close to the pyrite-pyrrhotite boundary and are characteristic of low-sulfidation states; they suggest that the oxidizing, slightly acidic hydrothermal fluid was buffered by phyllite, shale, and carbonate host rock. However, the presence in the pyrite-quartz body of hematite within quartz suggests that, locally, the fluids were less buffered by the host rock. The mineral assemblages of the second mineralization stage are characteristic of high- to intermediate-sulfidation states. High-sulfidation states and oxidizing conditions were achieved and maintained in the cores of the second-stage orebodies, even in those replacing carbonate rocks. The observation that, in places, second-stage mineral assemblages are found in the inner and outer zones is explained in terms of the hydrothermal fluid advancing and waning.

Microthermometric data from fluid inclusions in quartz indicate that the different ores of the first mineralization stage formed at similar temperatures and moderate salinities (200°–275°C and 0.2–6.8 wt % NaCl equiv in the pyrite-quartz body; 192°–250°C and 1.1–4.3 wt % NaCl equiv in the pyrrhotite bodies; and 183°–212°C and 3.2–4.0 wt % NaCl equiv in the Zn-Pb ores). These values are similar to those obtained for fluid inclusions in quartz and sphalerite from the second-stage ores (187°–293°C and 0.2–5.2 wt % NaCl equiv in the enargite-pyrite veins; 178°–265°C and 0.2–7.5 wt % NaCl equiv in quartz of carbonate-replacement orebodies; 168°–222°C and 3–11.8 wt % NaCl equiv in sphalerite of carbonate-replacement orebodies; and 245°–261°C and 3.2–7.7 wt % NaCl equiv in quartz from Venencocha). Oxygen and hydrogen isotope compositions on kaolinite from carbonate-replacement orebodies (δ18O = 5.3–11.5‰, δD = −82 to −114‰) and on alunite from the Venencocha and Santa Rosa areas (δ18O = 1.9–6.9‰, δD = −56 to −73‰). Oxygen isotope compositions of quartz from the first and second stages have δ18O values from 9.1 to 17.8 per mil. Calculated fluids in equilibrium with kaolinite have δ18O values of 2.0 to 8.2 and δD values of −69 to −97 per mil; values in equilibrium with alunite are −1.4 to −6.4 and −62 to −79 per mil. Sulfur isotope compositions of sulfides from both stages have a narrow range of δ34S values, between −3.7 and +4.2 per mil; values for sulfates from the second stage are between 4.2 and 31.2 per mil. These results define two mixing trends for the ore-forming fluids. The first trend reflects mixing between a moderately saline (~10 wt % NaCl equiv) magmatic end member that had degassed (as indicated by the low δD values) and meteoric water. The second mixing indicates condensation of magmatic vapor with HCl and SO2 into meteoric water, which formed alunite.

The hydrothermal system at Cerro de Pasco was emplaced at a shallow depth (~500 m) in the epithermal and upper part of a porphyry environment. The similar temperatures and salinities obtained for the first stage and second stages, together with the stable isotope data, indicate that both stages are linked and represent successive stages of epithermal polymetallic mineralization in the upper part of a porphyry system.

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