In situ trace element and sulfur isotope characteristics of pyrite from the Dapingzhang Cu − Au polymetallic deposit: Insights into Au mineralization processes

Abstract

The Dapingzhang Cu − Au polymetallic deposit is a large volcanogenic massive sulfide (VMS) deposit formed during the Proto − Tethyan stage in western Yunnan Province, SW China, and exhibits a stratified distribution with upper massive orebodies (V1) and lower veinlet orebodies (V2). Au − rich orebodies predominantly occur within the massive orebodies near the 16^# exploration line. This study investigates the microstructures, trace elemental, and sulfur isotopic compositions of pyrite, chalcopyrite, and sphalerite from the Au − rich orebodies at 1150 m and 1130 m levels to constrain the Au-bearing hydrothermal fluid sources, migration, and ore genesis. Four pyrite types were identified, representing three mineralization stages: (1) early-stage Py1 − 1 with sedimentary-formed strawberry − like textures; (2) late-stage Py1 − 2 formed through Py1 − 1 aggregation into irregular or euhedral crystals; and (3) Py2 (first-order zoning on Py1 − 2) and Py3 (second-order zoning on Py2). All Py1 − 2, Py2, and Py3 are hydrothermal in origin. Trace elemental composition reveals higher Sb concentrations in pyrite at the 1150 m level compared to those at the 1130 m level, with similar concentrating trends of Cu, Pb, Au, and Se. Evolutionary sequence analysis shows increasing Cu and Au concentrations but decreasing Co and Se concentrations in Py1 − 2, Py2, and Py3 across both the 1150 m and 1130 m levels. All pyrites are enriched in Au, Cu, Pb, Zn, and Sb but are depleted in Co, Ni, Tl, Se, Ti, and Sn. Sulfur isotope values (δ³⁴S =  − 2.63 to + 1.12‰) of pyrite, chalcopyrite, and sphalerite suggest a magmatic sulfur affinity. Gold mineralization is associated with E − W-trending ore-conducting faults. Au − rich fluids migrated upward through fractures, leaching Py1 − 1 to form inclusion textures and causing localized recrystallization that produced Au − enriched Py1 − 2. Continuous fluid replenishment generated Au − rich Py2. Since Py2 − forming fluids were not fully consumed, Py3 − forming fluids added more Au, reaching supersaturation and precipitating native gold and calaverite within Py2 − Py3 intergrowths and fractures. Sustained fluid supply ultimately formed the Au − rich orebodies.