Ore genesis of the Weibao lead–zinc district, Eastern Kunlun Orogen, China: constrains from ore geology, fluid inclusion and isotope geochemistry

Abstract

The Weibao lead–zinc district, composed by two deposits named the East Weibao deposit and the West Weibao deposit, respectively, is located in the Qimantagh area, Eastern Kunlun Orogen. Although both controlled by skarn alternation and experienced similar mineralization processes, the East Weibao deposit is dominated by massive ores, while the West Weibao deposit is characterized by banded ores. The ore-forming processes of both the deposits can be divided into four stages including prograde skarn stage, retrograde skarn stage, sulfide stage and carbonate stage. Three types of fluid inclusions, i.e., pure CO2 fluid inclusion (PC-type), CO2-bearing fluid inclusion (C-type) and aqueous types (W-type), have been identified in quartz, calcite and sphalerite from different stages. Noteworthily, only sulfide stage of the East Weibao deposit contains carbonic fluid inclusions, whereas only aqueous fluid inclusions were observed in sulfide stage of the West Weibao deposit. The CO2-rich and aqueous-type fluid inclusions in sulfide stage of the East Weibao deposit were homogenized at temperatures between 263 and 424 °C, concentrating at 360–400 °C, with salinities ranging between 3.2 and 13.6 wt% NaCl eqv.; the W-type fluid inclusions in sulfide stage of the West Weibao deposit were homogenized at temperatures between 228 and 381 °C, concentrating at 280–340 °C, with salinities between 1.6 and 9.0 wt% NaCl eqv.; and the aqueous-type fluid inclusions in carbonate stage in both deposits were homogenized at temperatures between 115 and 336 °C, concentrating at 200–240 °C, with salinities between 0.2 and 5.9 wt% NaCl eqv. The ore-forming fluid in sulfide stage at the East Weibao deposit yielded calculated $$\updelta^{18} {\text{O}}_{{{\text{H}}_{2} {\text{O}}}}$$ and δ18D values of 4.2 to 5.6 and −88 to −83 ‰, respectively. The results are 8.8 to 10.8, −99 to −92 ‰ for the sulfide stage at the West Weibao deposit and −1.0 to −0.2, −50 to −45 ‰ for carbonate stages in both deposits. The lead isotopic ratios of sulfide samples from the Weibao lead–zinc district range between 18.259 and 18.336 for 206Pb/204Pb, 15.580 and 15.659 for 207Pb/204Pb, and 38.007 and 38.253 for 208Pb/204Pb, respectively, and the corresponding sulfur isotopic values range between −2.2 and 3.6 ‰. The fluid inclusion and isotope studies on the two deposits in the Weibao lead–zinc district suggest that they both have magmatic origin but mineralized by different mechanisms. Mineralization at the East Weibao deposit may be caused by fluid immiscibility and mixing, while at the West Weibao deposit, mineralization may result from water–rock reactions between magmatic fluid and impure carbonate wall rocks. Combined with geological observation, we conclude that the Weibao lead–zinc district as a united magmatic hydrothermal fluid system associated with the Triassic magmatism that widely distributes in the Qimantagh area.