新元古代兴地II铜镍硫化物矿床成矿机制研究

The Xingdi II ore-bearing intrusion which located in the northeast margin of Tarim Craton is the only Precambrian ore-bearing mafic-ultramafic intrusion found in Xinjiang so far. It shows good prospect for mineral exploration. The previous study by the applicant shows that this intrusion was formed in a subduction environment. The xingdi II deposit thus provides a good opportunity to study the formation of magmatic sulfide deposits in such an unusual tectonic setting for magmatic sulfide deposits. The Xingdi II mafic-ultramafic intrusion was highly differentiated and with almost no alteration, which fit mineral in situ LA-ICP-MS analysis very well. This application is based on major and trace elements analysis of olivine, spinel, clinopyroxene and orthopyroxene, recalculate parental magma composition and the Temperature, Pressure, Oxygen Fugacity, Sulfur Fugacity condition while sulfide accumulating, then constrain the geochemical behavior of chalcophile elements during sulfide segregation. By systematic analyzing of PGE, Ni, Cu, S contents, we try to reveal the process of sulfide accumulation. After all these studies, we hope to improve the understanding of the metallogenic mechanism for Cu-Ni-(PGE) deposit which formed in an active convergent plate margin, and also expect to provide effective guidance for the exploration of Precambrian magmatic sulfide deposits in Xinjiang Province.

塔里木克拉通东北缘的兴地II岩体是至今新疆唯一的前寒武纪铜镍硫化物矿化岩体，具有较好的找矿前景。申请人前期研究发现兴地II岩体形成于俯冲背景，因此提出以俯冲背景下岩浆演化成矿的思路研究兴地II岩体亲铜元素的富集机制和成矿过程。兴地II岩体分异程度较高，各岩相界限清晰，岩体新鲜、基本未遭受热液改造，橄榄石等矿物未受蚀变影响，适合单矿物LA-ICP-MS原位微区分析。本项目拟对不同岩相中尖晶石、橄榄石、辉石等矿物进行主、微量元素分析，查明母岩浆成分；结合矿物温压计、氧逸度计、硫逸度计，还原硫化物富集过程中的物理化学条件；对矿体不同层位的PGE、Ni、Cu、S含量进行分析，配合矿物成分研究来限定硫化物的富集过程，提高对汇聚型板块边界背景下硫化物矿床成矿机制的理解，也期望可以为勘查评价提供有效指导，进而促进新疆前寒武纪铜镍矿床找矿和研究的突破。

世界级超大型铜镍硫化物矿床绝大多数形成于前寒武纪，在我国，甘肃金川是国内最大的铜镍硫化物矿床，形成年龄为820Ma，河南周庵的铂族-铜镍硫化物矿床形成于620Ma，暗示我国此类矿床找矿突破的希望仍然在前寒武纪。新疆库鲁克塔格地区大量发育基性-超基性岩体，伴随重要的铜镍硫化物矿床，是我国重要的铜镍多金属成矿带。目前新疆地区发现的具一定规模的铜镍矿均为显生宙以来形成，例如喀拉通克、黄山、黄山东、图拉尔根、天宇、白石泉等，至今发现确定为前寒武纪铜镍矿床的只有库鲁克塔格地区的兴地II铜镍矿床。. 本项目基于对库鲁克塔格地区前寒武纪兴地II基性-超基性岩体的野外调研、系统采样，综合岩石学、地球化学和地质年代学研究，对岩浆源区、母岩浆性质、PGE亏损的原因进行研究，并结合区域上其他岩体的时空分布特征，对库鲁克塔格地区基性-超基性岩体的形成构造背景进行进一步制约。以期为区域地质及铜镍硫化物矿床研究提供依据，推动我国前寒武纪铜镍硫化物矿床找矿突破。. 兴地II号基性-超基性岩体位于库鲁克塔格地区东北部，主要有辉长岩、辉长苏长岩、二辉辉石岩和二辉橄榄岩等岩相。兴地II 辉长岩锆石U-Pb年龄为744±3Ma，形成于新元古代。岩体适度富集轻稀土元素（LREE），亏损高场强元素（Nb、Ta等）。橄榄石主成分和铂族元素（PGE）研究确定兴地II成矿岩体由高镁玄武岩岩浆演化形成，岩体结晶前的硫化物熔离导致矿石PGE含量较低。硫同位素研究发现地壳混染和地壳硫加入是兴地II岩体硫饱和硫化物熔离的重要原因。岩体Sr-Nd同位素、锆石Hf同位素、Nb等元素地球化学研究证明兴地II岩体源于俯冲流体交代产生的富集地幔，侵位过程经历<20%的地壳混染，岩体形成于新元古代岩浆弧背景，塔里木北缘在新元古代（760-820Ma）是活动大陆边缘。.