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Strontium isotope geochemistry of the Xikuangshan antimony deposit, Central Hunan

PENG Jian tang, HU Rui zhong, DENG Hai lin, SU Wen chao(Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550002, China)  
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.
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