Mineralogy, Fluid Inclusions, and Sulfur Isotopes of the Huanzala Deposits, Peru: Early Skarn and Late Polymetallic Replacement Style Mineralizations

Abstract

Carbonate-replacement polymetallic mineralization at the Huanzala deposits (9°51′S, 77°00′W) was conducted in two contrasting stages that occurred in almost the same location. Early-stage mineralization has a relation with a granodiorite porphyry stock, whereas the late-stage mineralization is genetically associated with quartz porphyry sills. The early stage involved low to intermediate sulfidation Cu–Zn–(Pb) mineralization associated with metasomatic skarn, and the late stage involved high to intermediate sulfidation Cu–Zn–Pb–(Mn) mineralization associated with hydrothermal alteration characterized by paragonitic sericitization. The orebodies are hosted by steeply dipping (approximately 60°NE) Lower Cretaceous carbonate rocks in a relatively narrow range of approximately 4 km in horizontal extent and less than 1 km in depth. The pathway of the early-stage brine-derived fluids (300–>400°C, >33 wt% NaCl equivalent) along a plot of log fS2 against 1000/T is best explained by the progressive dual decline of the fS2 value and the temperature under rock-buffering conditions; this decline saw the pathway progress through the stability field of pyrrhotite to reach that of pyrite and promoted a decrease in FeS from 14.5 to 1.6 mol% in the sphalerite. In contrast, an explanation for the pathway of the late-stage fluids (140–290°C, 3–13 wt% NaCl equivalent) is given by an almost isothermal fS2 decline at approximately 270°C, with fS2 passing through the stability field of pyrite–bornite to reach that of chalcopyrite, promoting an increase in FeS from 0.1 to 1.6 mol% in the sphalerite, suggesting gas-buffering conditions. The ore formation pressure records in the fluid inclusions illustrate an approximately 2-km erosion during the roughly 2-Myr total lifetime of the hydrothermal system.