南秦岭铧厂沟金矿床成矿年龄及黄铁矿微区成分分析

The orogenic-type gold deposit has drawn high attentions from geoscientists and mining professionals due to their huge economic value. The concentration of ore-forming metals has always been the hot and difficult point. Pyrite is the most important gold-bearing mineral in gold deposit. The texture, trace element content and zonation of pyrite could provide important information for metallic mobilization and has becoming the hottest issue of economic geology, due to the rapid development of in-situ microanalysis technique. The Huachanggou gold deposit in South Qinling is located in the shear zone in both spilite and turbidite rocks and therefore is an ideal object for this issue. In this proposal, we systematically analyze the contents and in situ distributions of trace elements in pyrite from spilite and turbidite using LA-ICP-MS technique based on accurate identification of the textures and generations of pyrite from different ore belts, and try to identify some available parameters of and mobilization of the gold deposit. Then, we will further study the ore-forming ages by analyzing SHRIMP U-Pb in-situ ages of hydrothermal xenotime and monazite, the Ar-Ar ages of hydrothermal fuchsite, and attempt to analyze the tectonic setting of mineralization. We aim to reveal the sources and the enrichment process of the ore-forming material, to fulfill the ore-forming process of the shear-zone controlled gold deposit in the orogen, and also to provide guidance to exploration.

造山型金矿极具经济价值，受矿业界及学术界的高度重视，其成矿物质富集过程一直是关注热点和研究难点。黄铁矿是金矿床中最重要的载金矿物，其结构及微区成分特征对成矿物质来源及富集过程具有重要约束作用。近年来，由于原位微区分析技术的快速发展，黄铁矿微量元素地球化学特征研究已成为矿床学领域的新热点。南秦岭铧厂沟金矿床是世界罕见的同时由细碧岩和浊积岩容矿的脉型金矿，是研究金元素富集过程的理想对象。本项目在精确识别黄铁矿精细结构和生成期次的基础上，利用LA-ICP-MS技术进行系统的微量元素原位微区分析，提取指示元素来源和富集过程的精细信息；利用铬云母Ar-Ar定年、磷钇矿及独居石SHRIMP U-Pb原位定年，精确确定成矿年龄；分析其成矿构造背景，探讨造山带演化与成矿作用的耦合关系。预期成果将揭示金等成矿物质的来源及富集过程，可深化对造山带内受构造控制的金矿床成矿过程的认识，为寻找该类矿床提供理论依据。

造山型金矿是极具经济价值的热液型金矿床，受矿业界及学术界的高度重视，其成矿年代学一直是矿床学家关注的热点和研究难点。南秦岭铧厂沟金矿床是上世纪80年代发现的大型剪切带型金矿床，其位于勉略缝合带以南，赋存于新元古代细碧岩和泥盆系灰岩中，受控于近东西向叠瓦状逆冲断层及韧脆性剪切带。本研究利用黄铁矿结构、LA-ICP-MS和EPMA微量元素分析，明确了Au在黄铁矿中的赋存状态以其富集机制。黄铁矿根据其结构可分为早期微细粒状黄铁矿集合体，成矿期自形中粒黄铁矿和粗粒黄铁矿。成矿早期Au和其它微量元素主要以类质同象替换的方式进入微细粒黄铁矿晶格中，且Au含量较高；成矿期Au、Cu、Pb、Zn等元素以包裹体形式进入自形黄铁矿，其晶格中微量元素含量较早期黄铁矿明显降低。说明矿床经历早期Au的预富集作用，在成矿期叠加成矿。为了准确厘定其成矿年龄，本研究创新性地选用与矿石矿物密切伴生的热液磷钇矿、热液独居石进行精确年龄测定。2件铬云母样品的Ar-Ar坪年龄分别为209.37 ± 2.29 Ma和211.54 ± 2.54 Ma，相应的等时线年龄211.43 ± 3.56 Ma和215.27 ± 3.85 Ma，与坪年龄误差范围内一致。蚀变细碧岩矿石及矿化钠长-石英脉中的热液磷钇矿SHRIMP U-Pb测年结果为217 ~ 201 Ma，加权平均年龄为209 ± 5Ma（MACD=0.8）；独居石Pb-U和Pb-Th的加权平均年龄为分别为211 ± 6 Ma和206 ± 6 Ma，总的加权平均年龄为209 ± 5 Ma。铧厂沟金矿床的成矿年龄为211~209Ma，即晚三叠世。铧厂沟金矿床是在碰撞造山过程中区域断裂由低角度逆冲向高角度逆冲剪切兼右行走滑体制转换下形成的造山型金矿床。