The Sichuan-Yunnan-Guizhou (SYG) metallogenic province, southwestern China, is one of the largest Zn-Pb producers in the world. The Zn-Pb deposits in this area have been studied over many years and are commonly classified as Mississippi Valley Type (MVT) deposits. We investigated geological and mineralogical features of the Wusihe deposit and found that ores formed at the sedimentary metallogenic stage exhibit banded and normal grading textures and have different mineral assemblages from other deposits in this area. These features have not been reported previously, suggesting special formation mechanisms for these sedimentary ores. In this study, we firstly investigate the isotope geochemistry of Zn and S in syngenetic sphalerite from the Wusihe deposit. We elucidate spatial and temporal variations of Zn and S isotopes to provide a better understanding for Zn and S isotope fractionation during the hydrothermal processes. The delta S-34(CDT) values of microdrilled sphalerite range from 9.4 parts per thousand to 20.9 parts per thousand (mean = 14.3 parts per thousand), which are ca.15 parts per thousand lower than that of sulfate from the Dengying Formation, suggesting that reduced sulfur in sphalerite formed through thermochemical sulfate reduction (TSR) but not microbial sulfate reduction (MSR). At the hand specimen scale, no significant Zn isotopic fractionation was observed from bottom to top of the banded sphalerite ore and we propose a model that Zn isotopes are homogeneous in Zn-bearing hydrothermal fluid, and Zn2+ was rapidly and completely precipitated to form sphalerite. This process results in minor Zn isotopic variations in sphalerite (Delta Zn-66 = 0.08 parts per thousand) and initial hydrothermal fluid composition was calculated to be delta Zn-66 values = 0.37 +/- 0.03 parts per thousand. By contrast, positive linear relationships were observed in the sphalerite ore with normal grading between delta Zn-66 and delta S-34(CDT) values and between delta S-34(CDT) values and Zn/Cd ratios, which are not reported previously. Combined with previous studies, it is proposed that the variations in fluid temperatures may be the key factor that results in such positive correlations.

In addition, this study gives some new insights into geochemical and isotopic behavior of Zn and S in hydrothermal systems, and provides an initial glimpse into the utilization of Zn isotopes as an environmental tracer for reconstruction of Zn isotope compositions in the Ediacaran Ocean.

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