Triassic multistage antimony-gold mineralization in the Precambrian sedimentary rocks of South China: Insights from structural analysis, paragenesis, 40Ar/39Ar age, in-situ S-Pb isotope and trace elements of the Longwangjiang-Jiangdongwan orefield, Xuefengshan Mountain

Abstract

The Xiangzhong (XZ) Basin and its neighboring Xuefengshan (XFS) Mountain in South China, as one of the world-class antimony (Sb) producers, have provided appropriately 67% of globally exploited Sb metal (0.46 million tons) in human history. The associated gold (Au) with Sb in this region is also of significant economic values. In previous studies, these Sb-Au deposits were debated as a diversity of genetic types such as epithermal, magmatic-hydrothermal, sedimentary exhalative (SEDEX) and orogenic Sb-Au deposits. To evaluate these genesis hypotheses, we select the representative Longwangjiang-Jiangdongwan (LWJ-JDW) orefield to reveal the Sb-Au accumulation process and metal-fluid sources of these deposits. Our orefield-scale structural analysis shows that the localization of principal Sb-Au orebodies is controlled by a NE-striking sinistral shearing system composed by an integrated set of NE-striking and SE-dipping fractures in the pre-ore folded Neoproterozoic Wuqiangxi slate. Two chief ore periods were identified, including the marine sedimentation (SI, pyrite nodular) and tectonic-metamorphic hydrothermal period (SⅡ−SⅣ, Sb-Au veins). The second period is further subdivided into three stages of SⅡ (pre-tectonic deformed pyrite-milky quartz veins), Siii (syn-tectonic pyrite-arsenopyrite-stibnite-quartz veins) and SⅣ (post-tectonic stibnite-calcite veinlets). It is notable that gold precipitation is significantly prior to Sb ores at LWJ-JDW. Three sericite samples throughout SⅡ−SⅣ yield a group of well-defined 40Ar/39Ar plateau ages (253 ± 4, 234 ± 3 and 206 ± 2 Ma, respectively). This indicates the multistage Sb-Au mineralization initiated from Early Triassic and terminated at Late Triassic time. Further trace elements analyses show the earliest nodular PyI is enriched in a list of As, Ni, Sb, Cu, Se, Co, Pb, Zn and Ti that can be analogy to the diagenetic pyrite in the organic-rich marine sedimentary rocks. Extremely high Sb (median, 1107 ppm) and mediate Au (0.07 ppm) are monitored in PyI, indicative of the initial Sb and Au involvement from the Neoproterozoic sedimentation. Comparably, sulfides of the later SⅡ−SⅣ possess a similar spectrum of trace elements but with highly variable concentrations. All the sulfide generations host the wide range of in-situ δ34SV-CDT values (–23.9 to −3.7‰ for SⅠ, −7.6 to −2.9‰ for SⅡ−SⅣ, respectively), indicating sulfur originally sourced from bacterial sulfate reaction (BSR) of the Neoproterozoic seawater sulfates, and then gradually reduced by thermochemical sulfate reaction (TSR) during the subsequent Au-Sb-induced tectonic-metamorphic hydrothermal activity. The lead isotope compositions in accordance with the evolution lines of upper crust and orogen, implying that the Pb were chiefly sourced from host slate of the Wuqiangxi Formation and its underlying basement. Synthesis of these data, we proposed that the Sb-Au accumulation at LWJ-JDW was caused by the multistage mineralization from Neoproterozoic marine sedimentation to Triassic episodic deformation and metamorphic processes (253–206 Ma).