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

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 against 1000/T is best explained by the progressive dual decline of the 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 decline at approximately 270°C, with 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.     

Mineralizations of Base Metal Deposits of Acid-Sulfate Type Coexisting with Adularia-Sericite Type

Abstract: Two types of ores are recognized in the Huanzala mine, Peru. Zinc and lead ore of adularia-sericite type, is accompanied by pyrite body and skarn. Orthoclase (adularia), sericite and calcite are found in the skarn and altered country rocks. On the other hand, copper and silver (tin) ore of acid-sulfate type, is characterized by enargite, luzonite and bornite, accompanied by tin, silver and tellurium minerals. Although sericite and calcite are found only at the earlier stage in the mineralization of acid-sulfate type, kaolinite appears in the country rocks at a subsequent stage. It is interpreted that the ore-forming fluid of the acid-sulfate type separated from the adularia-sericite type by boiling in the early stage of mineralization. The separated fluid was nearly neutral just after boiling and changed into acidic by the oxidation of H₂S into HSO₄^-. Both types of mineralizations are studied in terms of fo₂-fs₂-pH-temperature diagrams. The adularia-sericite type might correspond to the hot-water system of a geothermal area, and the acid-sulfate type to the vapor-dominated system.
The copper ores from the Teine mine, Hokkaido, Japan and the Chinkuashih mine, Taiwan are acid-sulfate type, characterized by the occurrence of enargite and luzonite. However, zinc and lead ores of the adularia-sericite type are also present in these deposits. Both types in the Teine and Chinkuashih deposits are supposed to have formed in the similar relation as in the Huanzala deposits. The enargite and luzonite ores in the Hokuetsu mine, Japan are closely associated with native sulfur of solfataric origin.