Pb-Zn mineralization formed from coupled hydrocarbon-bearing and hydrothermal fluids in the Ediacaran strata in the central Sichuan Basin, China

Hydrocarbon-bearing fluid plays a crucial role in the Pb-Zn mineralization process. The hydrocarbon role is still the subject of metallogenic research. This study determined the metal and sulfur sources and the hydrocarbon role in the mineralization instigated by mixing of hydrothermal and hydrocarbon-bearing fluids in the central Sichuan Basin based on petrography, fluid inclusion geochemistry, S and Pb isotope compositions, and changes in hydrocarbon phases and components. The results indicated that hydrothermal input significantly improved the salinity and temperature of reservoir fluids. Hydrothermal stages can be divided into three: (I) pyrite and dolomite, (II) quartz and sphalerite, and (III) galena. Solid bitumen was formed at stage II, and the oil inclusions were converted into gas inclusions from stage I to II, suggesting that the hydrothermal fluids caused the oil cracking. The Pb isotope compositions of pyrite, sphalerite, and galena showed that the Pb-Zn deposit had two metal sources from surface karst and hydrothermal fluids. Raman spectroscopy and sulfur isotope compositions implied that H2S formed during hydrothermal input was generated from TSR resulting from hydrothermal sulfate. The H2S produced by TSR preferentially led to the precipitation of metals (Pb, Zn, and Fe) derived from the hydrothermal fluid, followed by the precipitation of metals derived from the karst fluid. After the metal elements in the reservoir were depleted, the remaining H2S was conserved together with the natural gas in the reservoir. This study unravels the transformation of oil to gas and the H2S generation in the reservoir through hydrocarbon inclusion analysis in the metallogenic study, providing new insight into the Pb-Zn mineralization involving hydrocarbon-bearing fluid participation.