LIU Yuan-xiong 1,2, LI Yong-gang 3, LU Zhi-tong 1,2, ZHANG Jia-wei 4, HAN Tao 1,2*

(1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China;
2. University of Chinese Academy of Sciences, Beijing 100049, China;
3. No. 101 Geological Party, Bureau of Geology and Mineral Exploration and Development of Guizhou Province, Kaili 556000, China;
4. Guizhou Geological Survey, Bureau of Geology and Mineral Exploration and Development of Guizhou Province, Guiyang 550081, China)

Abstract: The lower Cambrian black shale of South China is characterized with the high concentrations of multi-elements which were proposed to be related to the palaeoceanic redox conditions, marine productivity and hydrothermal activity. Previous research results showed that there is obvious zinc (Zn) enrichment in the black shale in South China with Zn contents widely ranging from hundreds to tens of thousands of ppm. In this paper, the origin and precipitation mechanism of zinc have been studied through the analysis of distribution patterns of Zn in different sedimentary facies sections, petrographic observations, and sulfur isotope analyses. The results are shown below. 1) There are significant differences of enriched Zn (10－15000 ppm) in different lower Cambrian sedimentary facies in South China, or even in different facies of the same section. Especially, the Zn enrichment occurred in a short period with the peak at about 520 Ma; 2) Zn mainly occurred in forms of fine and coarse disseminated sphalerite grains which are closely intergrown with hyalophane and coarse-grained pyrite, indicating that the zinc was sourced from the hydrothermal fluid; 3) The wide range of sulfur isotopic compositions (δ34S values varying from -26.5‰ to +44.5‰) suggested that hydrogen sulfide produced by the microbial sulfate reduction was involved in the precipitation of sphalerite and intergrown pyrite. Combined with the knowledge of stratified structure of the redox for the palaeocean in the Ediacaran-Cambrian transition period, namely, the overlying oxic surface water, the anoxic deep water, and the accompanied mid-depth sulfidic water (e.g., euxinia), it is proposed that the Zn enrichment in the lower Cambrian black shale in South China was resulted from the interaction of hydrothermal fluid and commonly existed sulfidic water at about 520 Ma.

Keywords: Niutitang Formation; Liuchapo Formation; hydrothermal fluid; Zn-Pb deposit; hyalophane

ACTA MINERALOGICA SINICA Vol. 43, No. 4, 2023, Page 481