The Raul-Condestable Iron Oxide Copper-Gold Deposit, Central Coast of Peru: Ore and Related Hydrothermal Alteration, Sulfur Isotopes, and Thermodynamic Constraints

Abstract

The iron oxide copper-gold (IOCG) Raul-Condestable deposit is located 90 km south of Lima, Peru, and approximately 5 km from the Pacific coast. Mineralization consists mainly of replacement mantos and disseminations within permeable volcaniclastic and carbonate-rich rocks and structurally controlled veins surrounding a coeval and apparently causative intrusion of tonalitic composition emplaced in the core of a dacitic volcano. Potassic (biotite grading upward to sericite-chlorite) alteration and a poorly developed, almost sulfide-free, quartz stockwork closely border the tonalite, affecting the basaltic to dacitic Lower Cretaceous volcano-sedimentary host sequence. Ore is associated with a hydrated calc-silicate (mainly amphiboles) alteration that surrounds the biotite alteration. A hematite-chlorite (albite, epidote, calcite) alteration affects the periphery of the system. The main ore stage is characterized by two end-member mineral associations that were formed according to (1) an oxidized deposition sequence (hematite-magnetite-pyrite-chalcopyrite) occurring in and near feeder structures, and (2) a reduced deposition sequence (pyrrhotite-pyrite-chalcopyrite) found in volcaniclastic rocks and veins. Early specular hematite of the oxidized sequence is transformed to magnetite (“mushketovite”). The main ore-stage mineralization is cut by minor late-stage calcite-sulfide veins. Main ore-stage sulfides have δ34S values asymmetrically distributed from 1.0 to 26.3 per mil, with a median at 6.6 per mil (n = 51). Similar values are observed for pyrrhotite, pyrite, and chalcopyrite. The δ34S values depend on the stratigraphic position, with deep-seated vein samples normally distributed between 1.0 and 6.3 per mil (avg about 3.5‰, n = 13) and shallower samples from 2.7 to 26.3 per mil (median around 7.5‰, n = 39). Sulfides found in late-stage calcite-sulfide veins show strongly negative δ34S values ranging between –32.7 and –22.9 per mil (n = 6), indicating a possible biogenic source. Because no rock unit is known to occur in the internal parts of the deposit that could have oxidized fluids to the point of hematite stability, the oxidized mineral sequence is best explained by magmatic brines following the SO2-H2S gas buffer at high temperature (>350°C) and fluid/rock ratio. This is supported by the close to magmatic δ34S values of sulfides from the deep parts of feeder veins. Mass-balance calculation based on sulfur isotope data suggests that at the deposit scale, the bulk of the sulfides is dominated by magmatic sulfur, with sulfides of the oxidized minerals association having a larger component of magmatic sulfur than those of the reduced mineral association. The deposition sequence from hematite to chalcopyrite reflects the cooling of the magmatic fluid and redox and pH buffering by the basaltic-andesitic volcano-sedimentary host rocks. Thus, the occurrence of magnetite pseudomorphous after early hematite (mushketovite) paragenetically followed by iron-bearing sulfides is interpreted to be direct field evidence for precipitation from oxidized magmatic brines. The same sequence has been described in many IOCG, skarn, and some porphyry copper deposits worldwide. δ34S values of sulfides ranging up to 26.3 per mil are found in what corresponded to a relatively shallow aquifer filled with evolved reduced seawater. Heavy sulfur in H2S was produced through thermochemical reduction of Aptian seawater sulfate (δ34S = 14‰) in the recharge zone, which is interpreted to correspond to the hematitechlorite (albite, epidote, calcite) alteration present at the upper flanks of the hydrothermal system, adjacent to the causative intrusion. Hematitization (through oxidation) resulted from the high fO2 of seawater and from the reduction of its sulfate to H2S by the Fe2+ contained in the rock. In the core of the system, the seawater-derived fluids reached near chemical equilibrium with their actinolitized host rock, at about 300° to 350°C, in reduced, rock-dominated conditions. Mixing of these fluids with magmatic brines, already partially or totally reduced through reaction with wall rock at medium to low magmatic fluid/rock ratio can explain the large positive δ34S scatter observed in sulfides of the reduced mineral association, at stratigraphically shallow positions.