湖北徐家山锑矿床方解石C、O、Sr同位素地球化学

Carbon, oxygen and strontium isotope geochemistry of calcites from Xujiashan antimony deposit, Hubei Province

湖北通山的徐家山锑矿床产在上震旦统陡山沱组和灯影组的海相碳酸盐岩中。对该矿床中成矿前和成矿期方解石进行了较系统的C、O、Sr同位素地球化学研究,结果表明,成矿前方解石具相对较高的δ13CPDB(-0.7‰～+2.0‰)、δ18OSMOW(+18.6‰～+19.6‰)和Sr含量(2645～8174μg/g,平均5656μg/g),及较低的87Sr/86Sr比值(0.7096～0.7097);而成矿期方解石具相对较低的δ13CPDB(-3.9‰～-2.1‰)、δ18OSMOW(+11.5‰～+15.3‰)、Sr含量(785～2563μg/g,平均1571μg/g),和较高的87Sr/86Sr比值(0.7109～0.7154,平均0.7141)。分析认为,成矿前方解石的C、O和绝大部分Sr来源于赋矿围岩———震旦系海相碳酸盐岩;而成矿流体为富H2CO3的溶液,它来自或流经富放射成因87Sr的下伏基底碎屑岩———中元古界冷家溪群浅变质岩;该流体与围岩发生水-岩反应导致成矿期方解石和辉锑矿的沉淀。

The Xujiashan antimony deposit is located at Tongshan County, Hubei Province, and its antimony mineralization occurs in the marine carbonates of Upper Sinian Doushantuo Formation and Dengying Formation. The carbon, oxygen and strontium isotopic compositions of calcites developed during both pre- and syn-mineralization at this deposit are determined in this study. The results show that the pre-mineralization calcites display relatively high δ^13CPDB（-0.7‰＋2.0‰） and δ^18OSMOW values（＋18.6‰- ＋19.6‰） , and low ^87 Sr/^86 Sr ratios （0. 7096 - 0. 7097） with high Sr eoneentrations ranging from 2645 to 8174 μg/g and an average of 5656 μg/g. Whereas the syn＇mineralization ealeites have relatively low δ^13 CPDB（-3.9‰～-2.1‰） and δ^18OSMOW values （＋11.5‰～＋15.3‰）, but high ^87 Sr/^86 Sr ratios （0. 7109-0. 7154, with an average of 0. 7141） with low Sr concentrations varying from 785 to 2563 μg/g and an average of 1571 μg/g. It was revealed that the carbon,oxygen, and predominant Sr of the pre-mineralization calcites might be directly come from the host rocks, the Sinian marine carbonates. The ore-forming fluid characterized by H2CO3-enriched was probably derived from or flowed through the underlying radiogenic Sr-enriched clastic basement, the low-grade metamorphic rocks of Mesoproterozoic Lengjiaxi Group. It is suggested that the precipitation of the syn-mineralization calcite and the intergrown stibnite in the Xujiashan antimony deposit resulted from fluid-rock interaction between host rocks （the Sinian marine carbonates） and fluids from underlying clastic basement （the low-grade metamorphic rocks of Lengjiaxi Group）.