Sources of ore‐forming material for Pb‐Zn deposits in the Sichuan‐Yunnan‐Guizhou triangle area: Multiple constraints from C‐H‐O‐S‐Pb‐Sr isotopic compositions

Abstract

The Sichuan‐Yunnan‐Guizhou Pb‐Zn triangle area is located in the western margin of the Yangtze Block and comprises more than 400 ore deposits or prospects. This triangular area contains 200 million tons of ores at average grades of 5 wt.% Pb and 10 wt.% Zn. The Pb‐Zn ore bodies are mainly distributed as stratoid and vein types within the Neoproterozoic to Permian carbonate rocks, which are typical epigenetic Pb‐Zn mineralizations. Previous studies have mainly focused on individual deposits. In this study, the compositions of C‐H‐O‐S‐Pb‐Sr isotopes of approximately 20 typical deposits are summarized and studied in detail to reveal the source of the ore‐forming material. δ13CPDB ‐ δ18OSMOW isotopic compositions of the hydrothermal calcite, dolomite, and fluid inclusions show that CO2 in the ore‐forming fluids could have been derived from marine carbonate with subordinate incorporation of organic matter. In addition, δDH2O ‐ δ18OH2O isotopic studies also indicate the multisource features of the ore‐forming fluids, for example, meteoric water and metamorphic water; meanwhile, some deposits are related to organic fluids. The δ34S values of most sulphide minerals range from +9.0‰ to +28.6‰, similar to late Neoproterozoic to Permian seawater sulphate (+8.0‰ to +38.7‰). This similarity suggests that the reduced sulphur in sulphides could have been mainly derived from the thermochemical sulphate reduction of the host strata seawater sulphate. In addition, thermal decomposition of sulphur‐bearing organic matter could have provided some of the reduced sulphur for mineralization in some Pb‐Zn deposits. The Pb isotopic compositions of the sulphides from most deposits are totally homogeneous and located between the crustal Pb and orogenic Pb evolution curves. Comparing these deposits with the regional basement rocks, sedimentary strata, and Permian Emeishan basalts, we propose that the metals for Pb‐Zn mineralization were mainly derived from the basement rocks and sedimentary country rocks. For some deposits (e.g., Jinshachang) with highly radiogenic Pb isotopic compositions, their Pb mainly originated from Precambrian basement carbonaceous slate and Lower Cambrian shales. The (87Sr/86Sr)t ratios of sphalerite, hydrothermal calcite, and fluorite range from 0.7099 to 0.7469, higher than those of the regional sediments and the Emeishan basalts, but lower than those of the Proterozoic basement rocks. Such features also indicate the mixing source for Sr in sulphides and ore‐forming fluids. The comprehensive study of multiple C‐H‐O‐S‐Pb‐Sr isotopes from typical deposits suggests mixed sources for the ore‐forming fluids and materials in this triangular area.