Geological and C-O-S-Pb-Sr isotopic constraints on the origin of the Qingshan carbonate-hosted Pb-Zn deposit, Southwest China

摘要

Located in the western Yangtze Block, the Qingshan Pb-Zn deposit, part of the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province, contains 0.3 million tonnes of 9.86 wt.% Pb and 22.27 wt.% Zn. Ore bodies are hosted in Carboniferous and Permian carbonate rocks, structurally controlled by the Weining-Shuicheng anticline and its intrafor-mational faults. Ores composed of sphalerite, galena, pyrite, dolomite, and calcite occur as massive, brecciated, veinlets, and disseminations in dolomitic limestones.The C-0 isotope compositions of hydrothermal calcite and S-Pb-Sr isotope compositions of Qingshan sulphide minerals were analysed in order to trace the sources of reduced sulphur and metals for the Pb-Zn deposit. δ~(13)Cpdb and δ~(18)O_(smow) values of calcite range from -5.0‰ to -3.4‰ and +18.9‰ to +19.6‰, respectively, and fall in the field between mantle and marine carbonate rocks. They display a negative correlation, suggesting that CO_2 in the hydrothermal fluid had a mixed origin of mantle, marine carbonate rocks, and sedimentary organic matter. δ~(34)S values of sulphide minerals range from +l0.7‰ to +19.6‰, similar to Devonian-to-Permian seawater sulphate (+20%o to +35‰) and evaporite rocks (+23‰ to +28‰) in Carboniferous-to-Permian strata, suggesting that the reduced sulphur in hydrothermal fluids was derived from host-strata evaporites. Ores and sulphide minerals have homogeneous and low radiogenic Pb isotope compositions (~(206)Pb/~(204)Pb = 18.561 to 18.768, ~(207)Pb/~(204)Pb = 15.701 to 15.920, and ~(208)Pb/~(204)Pb = 38.831 to 39.641) that plot in the upper crust Pb evolution curve, and are similar to those of Devonian-to-Permian carbonate rocks. Pb isotope compositions suggest derivation of Pb metal from the host rocks. ~(87)Sr/~(86)Sr ratios of sphalerite range from 0.7107 to 0.7136 and (~(87)Sr/~(86)Sr)_(200Ma) ratios range from 0.7099 to 0.7126, higher than Sinian-to-Permian sedimentary rocks and Permian Emeishan flood basalts, but lower than Proterozoic basement rocks. This indicates that the ore strontium has a mixture source of the older basement rocks and the younger cover sequence. C-O-S-Pb-Sr isotope compositions of the Qingshan Pb-Zn deposit indicate a mixed origin of the ore-forming fluids and metals.