South China, black organic-rich shales in the lower Cambrian Niutitang Formation host a Ni-Mo polymetallic sulfide layer that discontinuously extends over similar to 1600 km. Seawater and hydrothermal origins are among the many suggested hypotheses and are still under debate. In order to discriminate Zn sources, we report Zn isotopes in Ni-Mo polymetallic sulfide layers and their host shales from the Nayong and Zunyi locations in Guizhou province and the Zhangjiajie section in Hunan province. In each section, host organic-rich shales show homogeneous Zn isotope compositions which likely resulted from quantitative scavenging of dissolved Zn from seawater under euxinic conditions. The difference in the average delta Zn-66 values of organic-rich shales between the two sections in Guizhou (0.76 +/- 0.09%) and one section in Hunan (0.59 +/- 0.10%) might reflect variations of Zn isotope gradient with the depth of seawater. Therefore, the organic-rich sediments need not always represent an isotopically light Zn sink, which is dependent on Zn isotope fractionation in the local basin. However, the delta Zn-66 values in the Ni-Mo polymetallic sulfide layers are different from those of their host shales, indicating that these sulfide layers did not inherit the Zn isotope signal of seawater. Based on the regular increasing trend in delta Zn-66 values from Nayong (0.54 +/- 0.06%) to Zhangjiajie (1.34 +/- 0.09%) and the presence of Pb-Zn mineralization in the Dengying/Doushantuo Formations, we argue that hydrothermal fluids associated with Pb-Zn mineralization could be a major source of Zn in Ni-Mo sulfide layers, especially in the Nayong location. A possible model is that the hydrothermal fluids related to MVT-type mineralization got overprinted on a multiple-sourced synsedimentary sulfide-rich layer. We provide additional evidence that Zn isotopes have great potential as a tracer of metal source and can be applied to similar types of mineralization as e.g., the late Devonian Ni-ZnPGE Nick deposit (Selwyn Basin, Canada) or elsewhere. (C) 2020 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

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