黔东北西溪堡锰矿的沉淀形式与含锰层位中黄铁矿异常高δ^(34)S值的成因

Implication of the Precipitation Mode of Manganese and Ultra-high δ^(34)S Values of Pyrite in Mn-carbonate of Xixibao Mn Ore Deposit in Northeastern Guizhou Province

笔者等对黔东北松桃县的西溪堡锰矿床中锰矿石进行了元素含量分析,对含锰层位中黄铁矿进行了S同位素和微量元素分析。锰矿石稀土元素和微量元素特征表明,Mn是以氧化物或氢氧化物的形式沉淀,锰碳酸岩是在成岩过程中转化而成。黄铁矿形态学、微量元素和稀土元素特征指示黄铁矿形成于强还原、偏碱性的成岩环境。黄铁矿异常高的δ34S值反映了新元古代间冰期海洋深部低硫酸盐浓度和高的硫酸盐细菌还原速率,表明南华纪(成冰纪)大塘坡早期阶段深部海洋并没有被完全氧化。含锰层位中黄铁矿异常高的δ34S值存在两种可能的形成机制:①在极低SO2-4浓度下,通过BSR即可产生δ34SCDT高达58.7‰的黄铁矿;②海洋深部硫酸盐虽然具有很高的δ34S值,但却并没有高达58.7‰,δ34SCDT高达58.7‰的黄铁矿的形成是BSR和H2S与MnO2之间发生厌氧歧化氧化反应两个过程综合作用的结果,即在水体中SO2-4浓度极低的情况下,硫酸盐和还原产物H2S之间硫同位素分馏达到最小,H2S的δ34S值接近母体硫酸盐,BSR产生的H2S被活性铁矿物固定形成的FeS与MnO2之间发生歧化氧化反应所产生的同位素动力学分馏效应使FeS相对硫酸盐富集34S。

Here, in an effort to explore the precipication mode of manganese and the possible cause for ultra- high δ34S values for pyrite in the manganese carbonate series deposited during Nanhuan （Cryogenian） period , we present results for elements analyses of Mn ores, coupled with trace elements and sulfur isotope of pyrite, in the basal of the Neoproterozoic Cryogenian （Nanhuan） Datangpo Formation, deposited between the two major Neoproterozoic glacial episodes （Sturtian and Marinoan）, in South China. REE and trace elements characteristics indicate Mn was precipitated as Mn oxides or hydroxide, following transformation of manganese-oxide or manganese- hydroxide into rhodochrosite during diagenesis is responsible for the formation of Datangpo-type manganese carbonate ore deposit. The morphology, trace elements and REE characteristics of pyrite suggeset pyrite formed during early diagenesis in an anoxic and sub-alkalinity diagenetic environment as suggested by trace elements and REE characteristics of pyrite. Ultra-high δ34S values of pyrite suggest low sulfate concentration and high BSR rates in the deep Cryogenian ocean, which means Cryogenian deep ocean was not ＂oxidized＂ in the early stage of the Datangpo interglacial interval. Two possible mechanism would contribute ultra-high δ34S values for pyrite in the basal of the Datangpo Formation： very low sulfate concentrations in deep water with δ34S values for sulfate as high as 58.7‰ is the most likely reason to cause untra-high δ34S values for pyrite , low sulfate concentrations and high δ34S values, but not as high as 58.7%o, of sulfate in the deep ocean, coupled with the isotopic fractionation during disproportionation reaction between FeS（ the precursor for pyrite formation）and MnO2 is another possible reason.