Gold, antimony and mercury ore formation and metallogenic link in the Sandu-Danzhai area, Jiangnan Orogen, SW China

The turbidite-hosted Au-Sb-Hg deposits in the Sandu-Danzhai area are distributed along the southwestern marginal fault of the Jiangnan Orogen, yet any genetic link among the Au, Sb and Hg mineralization and their respective ore-forming process remain unclear. Here, we investigate the Paiting Au(-Hg) and Miaolong Au-Sb deposits and present a new metallogenic model based on detailed petrographic observations, and in-situ trace element (EPMA and LA-ICP-MS) and sulfur isotope (fs-LA-MC-ICPMS) analyses on sulfide minerals. Our results show that the ore formation in the Sandu-Danzhai metallogenic belt (SDMB) can be divided into Au (pyrite + arsenopyrite + dolomite), Au-Sb (pyrite + arsenopyrite + stibnite + quartz), Sb (stibnite + native antimony + barite + dolomite + quartz) and Hg (cinnabar + calcite) stages, based on mineral paragenetic and vein crosscutting relationships. Trace element compositions suggest that progressive As enrichment in pyrite is accompanied by increasing Au, Sb and Cu contents, implying that these elements may have been leached from the same source via fluid-rock reactions. Sulfur isotope compositions of the sulfates and sulfides in the Au-Sb-Hg ores reveal that the sulfur was originated from thermal-chemical sulfate reduction (TSR) of marine sulfates in the Cambrian ore-bearing strata. Thus, we propose that deep metamorphic fluids (as evidenced from the high fluid CO2 and CH4 contents) may have promoted ore-material (e.g., Au, Sb, As, Hg) mobilization in the metamorphic basement and/or Cambrian strata. As for the metal transport and precipitation mechanism, fluid-rock interaction may have decreased the ore-fluid As content and caused auriferous pyrite precipitation. Meanwhile, changes in temperature and oxygen-sulfur fugacity may have significantly decreased Sb solubility, leading to extensive stibnite precipitation. Cinnabar associated with calcite was likely deposited via carbonate dissolution in the Cambrian strata in late hydrothermal (waning) stage. Overall, we propose that the Au-Sb-Hg deposits in the SDMB are best classified as orogenic type, and the metallogeny was controlled by boundary faults of the Jiangnan Orogen.