西秦岭大水金矿床成矿物质来源：载金硫化物微区原位分析

The West Qinling orogenic belt is one of the most important gold belts and potential replacement gold resources in China. There is long-term debate on the ore genesis of the gold deposits in West Qinling: orogenic or Carlin-(like) type, with the focus on the source of ore-forming material. Considering the sulfides in the gold deposits are usually fine-grained with complex textures and multiple stages, the traditional isotope tracing could only provide mixed information for the source of ore-forming material; while the in-situ microanalyses of trace elements and sulfur isotope could precisely trace the ore-forming material, constrain the ore-forming processes and determine the ore genesis. The project focuses on the key scientific problem: source of ore-forming material of Dashui gold deposit, which is the largest mined and the most representative gold deposit in West Qinling. Based on detailed observations of field outcrops, handspecimen, samples under microscope and scanning electron microscope, the mineral paragenesis sequence and the microtextures of gold-bearing sulfides (pyrite, arsenopyrite and chalcopyrite) will be determined. The in-situ microanalyses of trace elements and sulfur isotope on sulfides with different paragenetic stage will reveal the characteristics of elemental and sulfur isotope geochemistry. Comparing with the sulfide elemental geochemistry and sulfur isotope from different sources will show light on the source of ore-forming material. Therefore, results of this study will promote the understanding of ore genesis of the gold deposits in West Qinling and similar gold deposits worldwide.

西秦岭是我国最重要的金矿带和最具潜力的金矿接替资源基地之一，对其矿床成因长期有造山型和（类）卡林型之争，其核心焦点是对成矿物质来源的认识不同。传统同位素示踪方法，对于广泛发育的颗粒微细、结构复杂、多成因和多世代共存的硫化物，只能获取混合的组成信息；而微区原位微量元素和硫同位素分析能够有效甄别不同世代和成因的信息，准确示踪成矿物质来源，精细约束成矿过程和矿床成因。因此，本项目拟聚焦西秦岭已开采规模最大和最具代表性的大水金矿床成矿物质来源这一关键科学问题，以载金黄铁矿、毒砂和黄铜矿等硫化物为研究对象，基于详细的野外露头、手标本、显微镜和扫描电镜等多尺度－多方法矿物共生组合研究，厘定矿物共生序列与载金硫化物显微结构特征，开展不同世代硫化物微量元素和硫同位素微区原位分析，查明其元素与硫同位素地球化学特征，并与不同源区数据进行比对，精确示踪成矿物质来源，为区域和该类型金矿床成因模式研究提供理论依据。

大水金矿床是西秦岭已开采规模最大和最具代表性的金矿。本项目大水金矿床成矿物质来源这一关键科学问题，以载金黄铁矿、白铁矿和黄铜矿等硫化物为研究对象，基于多尺度－多方法矿物共生组合研究，厘定了矿物共生序列与载金硫化物显微结构特征，并开展不同世代硫化物微量元素和硫同位素微区原位分析，查明了其元素与硫同位素地球化学特征，并与不同源区数据进行比对。地质地球化学证据表明：西秦岭金矿床受控于地壳尺度断裂带/缝合带，主要位于这些断裂带/缝合带产状发生变化的地方。白龙江成矿带上的金矿床主要受控于近EW向的断裂，且与其他方向断裂交汇部位成矿效应显著。大水—忠曲金成矿带位于白龙江成矿带的西段，多组方向的断裂、岩体和岩脉、及不同岩性接触带均控制了矿化的产出。大水金矿矿物共生组合可分为变质沉积成岩期黄铁矿、成矿期早阶段黄铁矿—石英—方解石、成矿期晚阶段白铁矿—石英—方解石和表生氧化期自然金—铁氧化物。大水金矿金的赋存状态以自然金为主。变质沉积成岩期黄铁矿Fe、S含量较低，含有较高的As、Bi等微量元素。金成矿期黄铁矿/白铁矿具有低含量的Ag、As、Bi、Cd、Co、Cu、Ni、Pb、Sb、Tl、Zn，与岩浆热液期黄铁矿有明显不同。变质沉积成岩期黄铁矿硫同位素δ34S值为9.74-10.08‰，而金成矿期黄铁矿硫同位素δ34S值为-4.5~4.4‰，二者有明显差异。综合分析可得，大水金矿成矿物质最可能来源于深部三叠纪俯冲带之上的含黄铁矿沉积楔，矿床类型为深氧化的浅成造山型金矿，为区域和该类型金矿床成因模式研究提供了理论依据。