湘中锡矿山锑矿床的Sr同位素地球化学

Strontium isotope geochemistry of the Xikuangshan antimony deposit, Central Hunan

对湘中锡矿山锑矿床围岩灰岩、硅化灰岩、煌斑岩和脉石矿物进行了系统的Sr同位素研究。结果表明,矿区围岩发生了隐性蚀变,灰岩中Sr亏损,而87Sr/86Sr高于同时代的海相碳酸盐,这种隐性蚀变很可能是水/岩反应所致。矿体附近的硅化灰岩中Sr更加亏损,而87Sr/86Sr明显增加。成矿期方解石的87Sr/86Sr较高,成矿体系中变化的W/R比造成了方解石中87Sr/86Sr值的明显波动。成矿流体为一富放射成因87Sr的溶液。成矿流体来自或流经基底地层,流体中的Sr由基底碎屑岩提供,矿质Sb也可能主要来自富Sb的元古宇基底。水/岩反应的理论模拟显示,锡矿山成矿流体中的Sr约为3.0μg/g,87Sr/86Sr为0.717;蚀变成矿体系为一开放体系,矿石的沉淀机制主要为水/岩反应,成矿体系中W/R比较高。

The Xikuangshan deposit located at the Xiangzhong basin, China, is the largest antimony deposit and the only superlarge one so far reported in the world. Despite the previous detailed geological investigation, no research on Sr isotope geochemistry has been reported. The objectives of this study are to investigate the Sr isotope geochemistry of this deposit, and attempt to trace the Sr source in the mineralizing fluid and reveal the mineralization mechanism through the theoretical modeling of fluid rock interaction. Strontium isotopic ratios of wall rock, silicified wall rock, gangue mineral and dyke from the deposit were measured. The limestone in the mining district, exhibits Sr depletion, marked 87Sr enrichment with 87Sr/86Sr value significantly higher than the contemporary marine carbonates, probably experienced cryptic alteration via fluid rock interaction. However, the scale of the cryptic alteration and its effect on mineralization are not clear. Compared with the limestone in the mining district, the silicified limestone hosting ore bodies exhibits more Sr depletion and more radiogenic Sr, indicating the host carbonates interacted with 87Sr enriched fluid. The ore and calcites associated with sulfide mineralization are enriched in radiogenic Sr, and the 87Sr/86Sr ratios of the latter are slightly lower than those of the silicified limestone and ore. Due to the changing W/R ratios in the mineralizing system, the 87Sr/86Sr ratios of the calcites are relatively variable. The mineralizing hydrothermal solution characterized by radiogenic 87Sr acquired the radiogenic nature by interaction with lithologies underlying the carbonate rocks of the Xiangzhong basin. It is revealed that the mineralizing fluid was derived from or flowed through the basement clastic rocks. This observed result didnt conform to the previous opinion that the ore forming materials come from the in situ host carbonates. It has been demonstrated by the previous work that the underlying Proterozoic basement is enriched in the element Sb, therefore, the Proterozoic basement is likely to be responsible for the Sb in the mineralizing fluid. The theoretical modeling of fluid rock interaction indicates that the Sr concentration in the ore forming fluid is about 3.0 μg/g, with a 87Sr/86Sr ratio of 0.717 0, and that the alteration mineralization in the Xikuangshan deposit took place in an open system rather than in a closed system. The W/R ratios in the mineralizing system are generally higher, and the ore precipitation was attributed to the fluid rock interaction. There at least existed twice fluid rock interaction in the Xikuangshan district. The first led to the cryptic alteration of limestone in the mining district. The second was closely associated with the Sb mineralization. The silicification and the quartz stibnite type ores formed at the early stage, at the late stage the calcite stibnite type ores precipitated.