YANG Xing-yu1, ZHOU Jia-xi2,3*, An Qi1, REN Hou-zhou1, XU Lei1, LU Mao-da1, WU Cai-jin1
(1. No. 104 Geological Team, Bureau of Geology and Mineral Exploration and Development of Guizhou Province, Duyun 558000, China;
2. School of Resource Environment and Earth Science, Yunnan University, Kunming 650504, China;
3. Stake Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China)
Abstract: Located in the northwestern part of Guizhou Province, the Nayongzhi Pb-Zn deposit is the largest Pb-Zn deposit reported in Guizhou Province, with over 1.3 Mt of proved Pb + Zn metal reserves. It occurs in carbonate rocks of Lower Cambrian Qingxudong Formation and Upper Sinian Dengying Formation, and within ore-controlling structures of reverse fault and anticline. It is controlled by both lithology and structure. Its metallogenic characteristics are relatively similar to those of the typical MVT Pb-Zn deposit. The NanoSIMS in situ analytical results of δ34S values of pyrite and sphalerite grains in the Nayongzhi Pb-Zn deposit show that the δ34S values of pyrite and sphalerite grains range from -16.6‰ to +27.0‰, and from +11.8‰ to +33.0‰, respectively, which are significantly different from the ranges of δ34S values of pyrite separates (δ34S = +4.7‰ to +18.1‰) and sphalerite separates (δ34S = +11.3‰ to +25.22‰) obtained by the conventional bulk-mineral method. Based on mineral assemblage and crystal morphology characteristics of ores, this paper has considered that the reduced S of early colloidal, aggregate-like or replacement remnant pyrites (δ34S = -16.6‰ to -14.9‰) could be formed by bacterial sulfate reduction (BSR), whereas the reduced S of late xenomorphic granular pyrite and sphalerite (δ34S = +11.8‰ to +33.0‰) could be the product of thermochemical reduction (TSR) of marine sulfate. Combined with previous geological and geochemical data, this paper has proposed that the spatial distribution of Pb-Zn deposits in the Wuzhishan area is controlled by local gypsum-bearing evaporated sulfate layers, the BSR occurred before Pb-Zn mineralization and the TSR could be trigged by the input of hydrothermal fluids, and the formation of Pb-Zn deposits could be resulted from the structure-lithology-fluid coupling.
Keywords: NanoSIMS; in situ S isotope analysis; formation mechanism of reduced S; reduction of sulfate; the Nayongzhi Pb-Zn deposit; NW Guizhou
