黔西北地区铅锌矿床流体包裹体与硫同位素地球化学研究

Fluid inclusion and sulfur isotopic studies of lead-zinc deposits,northwestern Guizhou,China

黔西北铅锌成矿区位于扬子地块西南缘,矿床产于不同时代的碳酸盐岩建造中,其矿物组合和热液蚀变特征类似于MVT型铅锌矿床。本文通过对该区部分铅锌矿床的矿石显微岩相学结构、矿物学与地球化学、脉石矿物和闪锌矿中流体包裹体岩相学与显微测温学以及硫同位素地球化学等研究,确定了该类型矿床成矿流体的性质及其成矿过程。结果表明,黔西北地区不同赋矿层位铅锌矿床具有相似的成矿流体特征,为中低温(160~260℃)、较高盐度(10%~22%NaCleqv)、含有低密度的CO_2、CH_4和N_2的卤水,不同于MVT型铅锌矿床成矿流体特征;硫同位素地球化学研究揭示,这些铅锌矿床硫化物的δ^(34)S范围均落在对应赋矿地层时代蒸发岩中的硫酸盐和生物成因硫化物的δ^(34)S演化曲线上或其附近,表明这些铅锌矿床硫化物中的S可能主要来源于对应地层本身。结合地球化学热力学分析,认为Pb、Zn等成矿元素的Cl络合物是成矿元素的主要搬运形式,富含Pb、Zn的成矿流体与赋矿地层中富含H_2S还原性流体的混合是铅锌富集成矿的重要机制之一。

Sediment-hosted lead-zinc deposits in northwestern Guizhou, China, are located on the southwest margin of the Yangtze craton and Youjiang orogenic belt. They are hosted by Proterozoic and Paleozoie carbonate rocks, and consist mainly of sphalerite, galena, and pyrite, with quartz, calcite and dolomite veins. Petrography, microthermometry, laser Raman spectroscopy analyses of fluid inclusions in quartz and sphalerite, and sulfur isotopic compositions of sulfides are used to characterize the ore-forming fluids associated with Pb-Zn mineralization in the northwestern Guizhou. The results show that the fluid inclusions in quartz are two-phase, aqueous inclusion and have higher homogenization temperatures （ 160 - 260℃ ） and salinities of 10% - 22% NaCleqv, with low density of C02, CH4 and N2, whereas the homogenization temperatures of two-phase aqueous inclusion in sphalerite range from 160 - 180℃ , with salinities of 3% - 23% NaCleqv. Sulfur isotopic compositions of sulfides indicate that reduced sulfur was mainly derived from the host rocks. We proposed that fluid mixing between metal-rich brines and reduced sulfur-bearing sedimentary formation waters is the most efficient process for Pb-Zn ore mineral precipitation.