华南下寒武统Ni-Mo-Se多金属层S-Se同位素体系

The S and Se isotope of Lower Cambrian Ni-Mo-Se polymetallic layer in South China

为了进一步理解华南下寒武统Ni-Mo-Se多金属层的物质来源及形成环境,文中分析了遵义中南村和张家界后坪两个Ni-Mo-Se矿层及其围岩的黄铁矿硫同位素和全岩的硒同位素组成。硫同位素组成显示两个Ni-Mo-Se矿层形成时的环境存在区域性差别,中南村矿层形成于间歇开放的海洋环境,而后坪矿层形成于封闭的缺氧(静海)环境。较大的硫同位素范围暗示硫酸盐还原菌控制硫同位素的分馏,而热液流体可能提供了大量金属元素,从而导致矿层富集大量的硫化物和稀有金属。硒同位素组成指示牛蹄塘组底部热液流体的Se可能重新经历了氧化还原循环,而Se的富集过程可能受有机质和粘土矿物吸附或类质同象过程控制。因此,控制多金属富集的因素主要为富集金属的热液流体的参与和缺氧环境下的自生沉积。

Sulfur and selenium isotopic composition of two Ni-Mo-Se ore layers from Zunyi and Zhangjiajie were analyzed to understand the oceanic environment and origin of these Lower Cambrian polymetallic layers in South China.Sulfur isotopic difference suggests that there are different environments for these two ore layers.Ni-Mo-Se ore layer of Zunyi was formed under the environment of open ocean in which sulfate was intermittently inputted,and that of Zhangjiajie formed under more depleted oxygen closed environment.Large scale variation of sulfur isotope indicates that bacterial reduction of sulfate played an important role in the formation of these polymetallic layers.Hydrothermal venting into tectonically formed depressions may provide metals that precipitated with bacterially reduced sulfide.Selenium isotopic composition indicates that Se（-II） derived from hydrothermal fluids could have undergone oxidation-reduction recycling at the bottom host black shales and carbonaceous cherts.The parts of Se enriched in Ni-Mo-Se ore layers were controlled by adsorption of organism and clay minerals and isomorphism processes.In a word,metal-rich hydrothermal fluids and their precipitation under anoxic condition control the the enrichment of metals in Ni-Mo-Se polymetallic layers.