Ore genesis of the Xiyi Pb‐Zn deposit, western Yunnan province, China: Geologic, fluid inclusion, and isotopic (C‐H‐O‐S‐Pb) evidence

Abstract

The Xiyi Pb‐Zn deposit, located in the Baoshan Block of the Sanjiang region in western Yunnan, China, contains 1.2 Mt of Pb‐Zn resources. The orebodies are hosted by Devonian‐Carboniferous limestone, structurally controlled by the NE‐trending fractures, and occur in stratiform, vein, and lenticular forms. The mineral assemblage of the ores includes sphalerite, galena, pyrite, arsenopyrite, chalcopyrite, and marcasite. Calcite, barite, and quartz occur as gangue minerals. Three stages of hydrothermal mineralization are recognized based on the ore textures, crosscutting relationship, and mineral assemblages: an early stage of sphalerite + galena + calcite (Stage 1), a middle stage of galena + arsenopyrite + calcite ± sphalerite (Stage 2), and a late stage of calcite + pyrite ± barite ± quartz (Stage 3). Two types of fluid inclusions are associated with the different ore‐forming hydrothermal stages: aqueous inclusions and hydrocarbon‐H2O inclusions. The ore‐forming fluids were characterized by low temperatures (100 ~ 200 °C), medium salinities (8 ~ 20 wt% NaCleq), and medium densities (1.0 ~ 1.1 g/cm3) and contained minor amounts of CH4. The δDH2O and calculated δ18OH2O values of the Stage 1 calcite range from −89‰ to −103‰ and from 1.5‰ to 5.5‰, respectively, indicating that the H2O in the hydrothermal fluids was derived from seawater that had reacted with organic matter. The δ13CPDB and δ18OSMOW values of the calcite associated with Stages 1 and 3 range from −5.2‰ to 3.2‰ and from 12.5‰ to 20.0‰, respectively. These data plot in the field between marine carbonate and granites and are close to the marine carbonate field, suggesting that the CO2 in the ore‐forming fluids was derived from the dissolution of marine carbonate in the sedimentary rocks. The galena, sphalerite, and pyrite in Stages 1 and 3 have a narrow range of δ34S values (0–3‰), which is lower than the δ34S values of early Carboniferous to Middle‐Late Devonian seawater sulfate (+18‰ to +23‰). Therefore, because the mineralization is unrelated to magmatic activity, the sulfur in the hydrothermal sulfides was derived from regional marine sulfates primarily via organic reduction at temperatures of 100 to 200 °C. The lead isotopic compositions in the galena, sphalerite, and pyrite in Stages 1 and 3 are very similar to those in the diagenetic pyrite in the host rocks, plotting in the field of the upper crust Pb evolution curve. Thus, the lead source was likely the Carboniferous and Devonian strata. The geologic, fluid inclusion, and isotopic (C‐H‐O‐S‐Pb) evidence in this study suggests that the Xiyi Pb‐Zn deposit is a Mississippi Valley‐type deposit.