矿床地球化学国家重点实验室知识库

Polymetallic vein-type Zn-Pb deposits are located in the Xiangxi-Qiandong zinc-lead metallogenic belt (XQMB) of the northwestern margin of the Jiangnan Orogen, South China. Ores are mainly found in fault-bounded quartz veins hosted in the upper part of the Banxi Group that consists of low-grade metamorphic sandstone, siltstone with minor tuff interbeds. The Zn-Pb deposits primarily contain sphalerite, galena, chalcopyrite and pyrite, accompanied by quartz and minor calcite. Zinc, lead, copper, indium and gallium are enriched in these ores. Investigation of the ore fluid reveals low temperature (87-262 C) with scattered salinity (range from 2.73 to 26.64 wt% NaCleqv). Hydrogen and oxygen isotopic compositions of fluid inclusions in quartz indicate mixing of magmatic hydrothermal fluid and meteoric water (delta O-18(H2O SMOW) = 0.2%0 to 4.2%0; delta D-H2O SMOW =-126%, to-80%o). Carbon and oxygen isotopic composition of carbonate samples indicate the magmatic hydrothermal origin of CO3- or CO2 in ore-forming fluid (delta C-13(PDB) =-6,9%0 to-5.7%o, delta O-18 (SMOW) =11.3 parts per thousand to 12.7 parts per thousand). Sulfur and lead isotopic compositions (delta S-34(VCDT) = 8.8-14.2 parts per thousand and (206)pb/(204)pb = 17.156-17.209, Pb-207/Pb-204 = 15.532-15.508, (208)pb/(204)pb = 37.282-37.546) demonstrate that sulfur sources were relatively uniform, and low radiogenic lead isotopic compositions indicate that ore metals were derived from a relatively unradiogenic source, probably by mixing of mantle with crust. Therefore, polymetallic vein-type Zn-Pb mineralization in this area probably arose from a magmatic-related hydrothermal system, and the deposition of sulfides occurred in response to cooling and boiling of magmatic hydrothermal fluids (high salinity, high delta O-18(H2O) and delta D-H2O and metal bearing), and is mainly the result of emplacement into open space and mixing with meteoric water (low salinity, low delta O-18(H2O) and delta D-H2O). This study provides direct evidence that magmatism was involved in the ore-forming processes of the low temperature metallogenic district, South China, and it raises awareness about the presence of polymetallic vein-type Zn-Pb deposits in the northwest margin of Jiangnan Orogen and their potential as a source of zinc, copper, indium and gallium.