Genesis of the Qilinchang Pb–Zn deposit, southwestern China: Evidence from mineralogy, trace elements systematics and S–Pb isotopic characteristics of sphalerite

Abstract

The Qilinchang (Pb–Zn)–Ag-Ge deposit of the Yangtze Platform, southwest China are hosted in the Early Carboniferous carbonate rocks of the Baizuo Formation. The deposit is located within the Sichuan-Yunnan-Guizhou (SYG) metallogenic province, which is western bounded by the Yangtze Platform in the southwestern part of China. Ore minerals are sphalerite, galena, pyrite, chalcopyrite, and marmatite, whereas gangue minerals are dolomite, calcite, and quartz. This study highlights the results of sphalerite chemistry analyzed by electron microprobe analyzer (EPMA), the laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) and isotopic analysis. These techniques were used to investigate the major and trace concentrations in sphalerite from the Qilinchang deposit, in an attempt to provide noteworthy discernments into the ore-forming temperature and metal sources of the deposit. The calculated FeS (mole %) results obtained from Fe (wt. %) with the assumption that the sphalerite is stoichiometric in composition range from 0.02 to 13.48 mol % and mode at 3 mol %. This low FeS content falls within the bornite + pyrite + chalcopyrite field on the Fe–Zn–S system suggesting an increase in sulfur activity. The application of Ga/Ge ratios in sphalerite against the formation temperature was used to determine the homogenization temperature of this deposit. This expression is based on the existing Al/Si geothermometers as (Al/Si)total atomic ratio as a function of temperature calculated using available constants of hydrolysis reactions. The log10 (Ga/Ge) ratios calculated for this study range from −2.8 to 0.87, corresponding to formation temperatures of 100–227 °C, with an average of 180.3 °C. Likewise, correlation between PC1* and the homogenization temperature indicated that the calculated formation temperatures for this study ranged from 119.3 to 217.9 °C, which suggests that the deposit is sourced from a low-temperature environment. The δ34S of sphalerite is related to the thermochemical sulfate reduction (TSR) of Carboniferous seawater sulfate and are mainly derived from evaporites in the host strata producing H2S−rich fluid by TSR with restricted inducement from mantle-derived sulfur. The homogenous composition of Pb isotopic data of sphalerite indicates that the metals were predominantly originated from the basement rocks of the Mesoproterozoic-early Neoproterozoic rocks of the Kunyang Group. From these geochemical signatures, it can be inferred that the Qilinchang deposit was not linked to magmatic activities. It is proposed that the ore-forming fluids responsible for the formation of the deposit were generated from a low-temperature environment. Economic potential reveals the most important attributes of this deposit are high Zn and Pb grade (approximately 17.5% Zn and 6.6% Pb, 3.32 Mt of metal). Overall, the mineralization of the Qilinchang Pb–Zn deposit shows it a high-grade remarkable Pb–Zn deposit transiting between MVT and high-temperature carbonate-replacement Pb–Zn deposits. We suggest that the determination of the genetic type of this exceptional deposit is important to the research and exploration of similar Pb–Zn deposits in other metallogenic provinces of China.