Environment of ore deposition at Cerro de Pasco, Peru

The massive sulfide replacement body in carbonate wall rocks east of the Cerro de Pasco volcanic neck contains an early association of pyrrhotite and FeS-rich sphalerite altered by late fluids to pyrite-marcasite and FeS-poor sphalerite. Rapid sulfide deposition from an ore fluid that fluctuated widely in sulfidation-oxidation state is suggested by: (1) intricate reversals and broad range in X sl FeS ; (2) lack of detailed correlation on a local scale between X sl FeS and iron-sulfide association; (3) evidence from sulfur isotope data for disequilibrium sulfide associations; and (4) superposition and telescoping of mineral stages.The first stage of ore deposition followed the formation of an extensive replacement body of pyrite-quartz in limestone and alteration of adjacent quartz monzonite and volcanic agglomerate to sericite-pyrite-quartz. Upward and outward zoning relative to centrally located pyrrhotite pipes follows the sequence quartz-pyrrhotite-wolframite, pyrrhotite-stannite-chalcopyrite-sphalerite, and pyrrhotite (pyrite)-arsenopyrite-sphalerite. The latter association represents the main stage of Pb-Zn ore formation and extends up to 600 meters from the pyrrhotite pipes, where pyrrhotite disappears and galena becomes increasingly abundant. Polished thin sections and electron microprobe analyses indicate that most sphalerite grains display intricate growth zoning with numerous reversals in FeS content and an overall zoning trend from high FeS cores (X sl FeS = 0.25-0.35) to lower FeS rims (X sl FeS = 0.15). Mean values of X sl FeS : (1) are 0.21 in the pyrrhotite pipes; (2) 0.23 within 50 m of the pipes, where sphalerite rarely contains pyrrhotite blebs and occurs in pyritic ore containing quartz-siderite-chlorite; and (3) zone outward from this point to less than 0.18 in less pyritic fringe ore that replaced limestone. The application of sulfide equilibrium data yields broad limits to depositional temperatures, ranging from a minimum of 254 degrees C for hexagonal pyrrhotite, to maxima of 334 degrees C for pyrrhotite + chalcopyrite and 500 degrees C for pyrite + arsenopyrite. These limits are compatible with sulfur isotopic fractionation temperatures for sphalerite-galena (Austria, 1975), which suggest a thermal decline from about 340 degrees to 230 degrees C with distance from the central pyrrhotite pipes. The compositional data on sphalerite indicate that the ore fluid was not generally in chemical equilibrium with both pyrite and pyrrhotite, a conclusion supported by the lack of isotopic equilibrium between sphalerite-pyrite and galena-pyrite pairs. The overall trend in chemical state of the ore fluid with decreasing temperature may have been buffered by pyrrhotite-siderite-chlorite-quartz toward increasing oxidation-sulfidation states, such that eventually equilibrium with pyrite was reached on the fringes of Pb-Zn ore. Samples of sphalerite from the pyrrhotite pipes that contain local bands of 10 to 15 mole percent FeS may signify brief excursions into the quartz-siderite or quartz-chlorite fields due to influx of oxidized ground water. The lack of sphalerite containing less than 10 mole percent FeS on the fringes of Pb-Zn ore and the presence of arsenopyrite signifies that the sulfidation state never exceeded the arsenopyrite + pyrite + arsenic sulfidation curve. Enargite-luzonite-pyrite veins with advanced argillic alteration envelopes, located near the contact of the vent with the pyrite-quartz body, represent a second stage of ore deposition. Relative to the first stage, the ore fluid of the second stage had evolved to significantly higher oxidation-sulfidation states and lower pH at temperatures around 300 degrees C. Reaction of this fluid with mineral assemblages of the first stage east of the vent resulted in a sharp increase in pH and deposition of sphalerite containing 0 to 10 and averaging 3 mole percent FeS, accompanied by galena, pyrite, marcasite, and minor tennantite. Alteration minerals, which formed by oxidation-sulfidation of original hexagonal pyrrhotite below 254 degrees C, display an increase in S/Fe ratio toward the surface and can be correlated in a general way with the presence of late, low-FeS sphalerite. Residual hexagonal pyrrhotite is associated with monoclinic pyrrhotite on the deepest level of the mine below the bottom of Pb-Zn ore. On higher levels hexagonal pyrrhotite is absent and is replaced by mixtures of monoclinic pyrrhotite and Fe-deficient anomalous pyrrhotite, which in turn are converted to mixtures of pyrite and marcasite with local siderite. The first stage of ore deposition shares some of the features of tin and tungsten deposits of the eastern Andes of Bolivia and thus serves as a metallogenic link between the Cu-Zn-Pb-Ag province of central Peru and the Sn-W-Ag-Bi province of Bolivia. The second stage exhibits mineral assemblages and zonal paragenetic features that are found in numerous other cordilleran base metal sulfide lode deposits, such as Yauricocha and Morococha, Peru; Magma and Bisbee, Arizona; and Butte, Montana. High sulfidation states, above the pyrite-chalcopyrite-bornite buffer, and intense base-leaching of wall rocks are characteristic features of the central copper zone of these deposits. Such an extreme departure of the ore fluid from chemical equilibrium with fresh rocks probably reflects the involvement of oxygenated ground water. Although some lode deposits are associated with an early stage of porphyry copper mineralization representing less of a departure from equilibrium with wall rocks, this is not a general case. It is concluded that the two types represent end members of the process of hydrothermal ore formation associated with subvolcanic calc-alkaline intrusions. The development of dominantly one or the other ore type may ultimately depend on the timing and degree of ground-water involvement.