The origin of the Maozu carbonate-hosted Pb-Zn deposit, southwest China: Constrained by C-O-S-Pb isotopic compositions and Sm-Nd isotopic age

摘要

The Maozu Pb-Zn deposit, located on the western margin of the Yangtze Block, southwest China, is a typical carbonate-hosted deposit in the Sichuan-Yunnan-Guizhou Pb-Zn metallogenic province with Pb + Zn reserves of about 2.0 million tonnes grading 4.15 wt.% Pb and 7.25 wt.% Zn. Its ore bodies are hosted in Sinian (635-541 Ma) Dengying Formation dolostone and show stratiform, vein and irregular textures. Ores are composed of sphalerite, galena, pyrite, calcite, dolomite, quartz and fluorite with massive, banded, disseminated and veined structures. The C-0-Sm-Nd isotopic compositions of hydrother-mal calcites and S-Pb isotopic compositions of sulfides were analyzed to constrain the origin of the Maozu deposit. δ~(13)C_(PDB) and δ~(18)O_(SMOW) values of hydrothermal calcites range from -3.7‰ to -2.0‰ and +13.8‰ to +17.5‰, respectively, and plot near the marine carbonate rocks field in a plot of δ~(13)C_(PDB) vs. δ~(13)O_(SMOW). with a negative correlation. It suggests that CO_2 in the hydrothermal fluids was mainly originated from marine carbonate rocks, with limited influence from sedimentary organic matter. δ~(34)S_(CDT) values of sulfides range from +9.9‰ to +19.2‰, similar to that of Cambrian to Triassic seawater sulfate (+15‰ to +35‰) and evaporate (+15‰ to +30‰.) in the Cambrian to Triassic sedimentary strata. It suggests that reduced sulfur was derived from evaporate in sedimentary strata by thermo chemical sulfate reduction. Sulfides have low radiogenic Pb isotope compositions (~(206)Pb/~(204)Pb= 18.129-18.375, ~(207)Pb/~(204)Pb=15.640-15.686 and ~(208)Pb/~(204)Pb = 38.220-38.577) that plot in the field between upper crust and the orogenic belt evolution curve in the plot of ~(207)Pb/~(204)Pb vs.~(206)Pb/~(204)Pb, and similar to that of age corrected Proterozoic basement rocks (Dongchuan and Kunyang Groups). This indicates that ore-forming metals were mainly derived from basement rocks. Hydrothermal calcite yields a Sm-Nd isotopic age of 196 ± 13 Ma, possibly reflecting the timing of Pb-Zn mineralization in the SYG province, younger than the Permian Emeishan mantle plume (~260 Ma). All data combined suggests that hydrothermal fluids circulated through basement rocks where they picked up metals and migrated to surface, mixed with reduced sulfur-bearing fluids and precipitated metals. Ore genesis of the Maozu deposit is different from known magmatic-hydrothermal, Sedimentary Exhalative or Mississippi Valley-types, which maybe represent a unique ore deposit type, named as the SYG-type.