Re-Os isotope system of sulfide from the Fule carbonate-hosted Pb-Zn deposit, SW China: Implications for Re-Os dating of Pb-Zn mineralization

The Fule deposit is one of the carbonate-hosted Pb-Zn deposits in southeastern part of the Sichuan-Yunnan Guizhou (SYG) metallogenic province, SW China. This deposit is hosted by dolomitic limestone and dolomite of the Yangxin Formation with Zn-Pb ore reserves of more than 20 million tons (Mt). Ore minerals mainly include sphalerite, galena and pyrite. According to the occurrence of orebodies, ore textures, and mineral assemblages, the hydrothermal Zn-Pb mineralization stage could be subdivided into three generations: (1) light brown sphalerite associated with minor galena in dolomite breccias; (2) dark brown to brown sphalerite associated with galena, dolomite and minor gypsum; (3) disseminated reddish-brown to light yellow sphalerite in calcite vein. Sulfides from three stages have similar major, trace elements, Re-Os and S isotopic compositions. Rhenium and Os in sphalerite are 7-58 ppb and 3-56 ppt, respectively, and in galena of 13-30 ppb and 1-5ppt, respectively. Re-187/Os-188 and Os-187/Os-188 ratios in sphalerite are 387-16193 and 1.29-12.0, respectively, and in galena of 5449-27180 and 0.35-5.0, respectively. Pyrite, galena, and sphalerite have delta S-34 values of +14.69 parts per thousand to +20.64 parts per thousand, +6.26 parts per thousand to +12.39 parts per thousand, and +12.01 parts per thousand to +16.03 parts per thousand, respectively. Re-Os isotopic compositions of sphalerite and galena have Os model ages from 1 Ma to 500 Ma, indicating an unclosed Re-Os system. Different sulfides from the same sample follow the trend of (delta S-34(pyrite) >= ) delta S-34(sphalterite) > delta S-34(galena), indicating that sulfur isotopes between different sulfides have attached equilibrium. Fluid inclusions in sphalerite have homogenization temperatures ranging from 135 degrees C to 235 degrees C with two peaks of 220-235 degrees C and 135-160 degrees C, which agree well with calculated temperatures of sphalerite-galena pairs (99 degrees C-202 degrees C). A relatively high sulfur isotope composition (+6.3 parts per thousand to + 16.6 parts per thousand) in sulfides, indicating that sulfur was mainly derived from the reduction of gypsum in Carboniferous strata. Sphalerite and galena in sulfide ores of carbonate-hosted deposits in SYG metallogenic province have experienced post-ore hydrothermal alteration by oxidizing fluids, forming oxidized minerals including gypsum, smithsonite, and anglesite. The activity of such oxidized fluids might last for a long time. Rhenium in sulfides is more mobile than Os in such an oxidizing environment and thus Re-Os isotope system remains open until the interaction between sulfide and fluids stops. Therefore, redox condition of hosting carbonates can highly influence the closure of Re-Os isotope system of sulfide in carbonate-hosted Pb-Zn deposits. Sulfide in oxidizing mineral bearing carbonate-hosted Pb-Zn deposits is not suitable for Re-Os dating. In contrast, Re-Os isotope system is possible to be used for dating the age of Pb-Zn mineralization within reduced carbonates.