华南地区泥盆系MVT铅锌矿床S、Pb同位素特征

The Sulfur-Lead Isotope Geochemistry of MVT Pb-Zn Deposits in Devonian System in South China

华南泥盆系密西西比河谷型(MVT)铅锌矿床,受控于泥盆系海进序列的台地碳酸盐岩,大体可分为以凡口为代表的中低温热液型和以泗顶、北山为代表的低温热液型。在矿床学研究基础上,对研究区内不同类型的铅锌矿、硫铁矿开展系统的硫、铅同位素分析,收集和测定493件S和64件Pb同位素数据,总结硫、铅的来源和硫同位素分馏机制,并初步探讨了成矿机制。硫同位素研究显示,矿石硫有多种来源,主要来自于还原性卤水,部分来自氧化性卤水中ΣSO_4^(2-)的还原,少量硫来自于矿区含矿地层。不同矿床在成矿作用过程中硫同位素的分馏机制不同。在以凡口为代表的中低温热液矿床中,矿石δ^(34)S值高且相对集中,以热力学分馏为主,生物分馏作用较微弱;在以泗顶、北山为代表的低温热液矿床中,矿石δ^(34)S值低且分散,以生物分馏作用为主,仅部分中粗粒铅锌矿石以热力学分馏为主。成矿作用过程中硫同位素分馏远未达到平衡状态。不同矿床的矿石铅同位素组成呈线性分布,反映出不同来源铅的混合。古老铅来自遭剥蚀的古陆,年轻铅代表泥盆系沉积物的普通铅。二者的比例与岩石中陆源物质(Pb)的含量相对应。成矿时的铅直接来自于氧化性卤水,间接来自于卤水对流经的泥盆系含矿层(尤其是底部碎屑岩)的淋滤,更间接地来自古陆剥蚀区以及海相沉积物。金属物质的迅速沉淀成矿作用与两类流体的混合有关,氧化性卤水来自蒸发盐红层盆地,沿泥盆系底部紫色砂岩经区域性迁移,其中富含大量金属成矿元素,并含有少量呈ΣSO_4^(2-)的硫;而还原性流体中富含ΣH2S的硫。流体的混合作用局限于矿区范围内,并不存在区域性的简单大规模流体混合过程。

The Mississippi-valley type （MVT） Pb-Zn deposits, south China, occur in the Devonian platform sandstone-carbonate rocks, are associated with the basin brine in Cretaceous. Two types were recognized, one is middle-low temperature hydrothermal type such as Fankou, and the other is low temperature type such as Siding and Beishan deposits. The studies on geologic setting, fluid inclusion mierothermometry, and δ4S values indicate that sulfur mainly came from reduced fluid, minor reduced from ∑SO42- in oxidized brine, and a few from wall rocks in the deposits. In the middle-low temperature type hydrothermal deposits, δ4S ranges from 12%0 to 22‰, and thermochemical fractionation of sulfur isotope is the principal for metal precipitation, without obvious bacteria fractionation observed. In low temperature type deposits, δ4S value of the ore is lower and in a wide range, bacteria fractionation is the principal, the thermoehemical fractionation only in the forming of the coarse-grain ore. The ore lead isotope composition of the deposits shows a linear array, indicating the mixing sources for lead. The old lead came from the eroded Paleozoic continent, young lead represents the common lead mainly from the Devonian sediments. The proportion of two sources of leads is related to the content of the debris in the Devonian sedimentary rocks. The lead directly precipitated from the oxidized basin brines, ultimately from the Devonian sediments （especially clastic rocks at the Transgression） that fluid flows through, further more ultimately from the Paleozoic continent and marine sediments. It is suggested that the mineralization mechanism is given as follows. The mixing of the two types of fluid leads to the precipitation of sulfide rapidly. The oxidized brines rich in metal-richbasin, and contains a small amount of sulfur of ∑SO4^2-, migrated along the red sandstone of Lower Devonian. The reduced fluid is rich in sulfur of ∑H2S. Fluid mixing function is limited to within the scope of a deposits or ore field, but not the regional.