富镍硫化物成矿机理——以东天山黄山南铜镍矿床为例

Ni tenors in 100% sulfide melt of magmatic sulfide Ni-Cu deposits in the Eastern Xinjiang range from 1 to 10% (Avg. 6%) except that from the newly-found Huangshannan Ni-Cu deposit which is characterized by extremely Ni enrichment in sulfide, ranging from 15 to 18% in 100% sulfide melt. Globally, the metallogenic mechanism and the detail process of enrichment of high Ni tenor Ni-Cu deposits is still unclear up to date. The Huangshannan Ni-Cu deposit in the Eastern Tianshan is one of those deposits characterized by low grade but high Ni tenor, and silicate minerals in lherzolite and olivine websterite have experienced a slight alteration. Thus, it is an ideal object to carry out systematic in-situ investigation of those minerals to constrain the ore genesis and sulfide accumulation of the high Ni tenor Ni-Cu deposits. Specifically, this project focuses on the geochemical behavior of Ni during sulfide segregation and primary sulfide melt composition to offer further constrains on those issues. Firstly, based on field geological observation, sample two typical profiles across the main ore-body. Secondly, carry out systematic petrographic and mineralogical studies. After that, on the one hand, analyze major elements concentrations in olivine, spinel, clinopyroxene and trace elements abundances in olivine, back calculating the parental magma composition of lhzolite. The composition of the mafic dykes around the intrusion will also analyzed to better constrain the parental magma composition. This, combined with the temperature and the pressure as well as the oxygen fugacity indicated by mineral composition, will be used to constrain the geochemical behavior of Ni during sulfide segregation as well as the ore genesis of the high Ni tenor sulfide. On the other hand, stratigraphic variations of minerals as well as platinum group element, Ni, Cu tenors in 100% sulfide melt would be analyzed to reveal the key factor in the enrichment process, such as magma replenishment, parental magma composition and sulfide fractionation. This project is expected to better understand the magmatic evolution and process of sulfide accumulation of high Ni tenor Ni-Cu deposits.

近年探明的黄山南大型铜镍矿床100%硫化物镍含量为15-18%，高于东疆其他铜镍矿床（1-10%，平均6%），指示其硫化物异常富集镍。目前富镍硫化物的形成机制仍存在争论，其富集过程尚不清晰。黄山南以硫化物含量低但硫化物异常富集镍为特征，且岩石蚀变微弱，是开展矿物原位微区分析，研究富镍硫化物形成机制和精细刻画其富集过程的理想对象。本申请以细致野外地质观察为基础，选择典型剖面系统采样，在详细的岩相学、矿相学观测基础上，开展橄榄岩相橄榄石等矿物成分和同期岩墙化学成分分析，厘定母岩浆成分，并结合矿物温压计和橄榄石氧逸度计等还原成矿物理化学条件，从而限定镍在硫化物熔离过程中的地球化学行为，同时开展从矿体底部至顶部的铂族元素、铜、镍、硫等元素系统分析，查明新鲜岩浆补给、岩浆成分变化和硫化物分异作用与富镍硫化物的关联，精细反演富镍硫化物富集过程，揭示其形成机制。该研究可望加深对富镍硫化物成矿机理的认识。

本项目目标为揭示黄山南高镍硫化物形成机制与富集过程，并通过对比其他铜镍矿床阐释造山带高镍铜镍矿床的特性与形成机制。通过黄山南高镍矿床的Sr-Nd-O-Hf同位素分析与模拟得到该矿床成矿岩浆来源于俯冲流体交代的亏损地幔熔融，在上升过程中还经历了15-20%下地壳混染和少量上地壳混染，岩浆硫溶解度计算表明地壳物质混染是黄山南成矿岩浆达到硫饱和的主要控制因素；母岩浆成分反算得到其岩浆相对富镍（450ppm），橄榄石-硫化物-硅酸盐熔体三者之间的Fe-Ni平衡模拟指示橄榄石与硫化物Fe-Ni交换反应使硫化物中的镍含量增加了3-5%。因此，黄山南高镍硫化物由两个因素控制：1）成矿岩浆相对富镍，2）硫化物萃取橄榄石中镍。在黄山南典型矿床研究基础上，还对新疆铜镍矿床硫化物镍含量进行了系统梳理，厘定了新疆铜镍矿床硫化物镍含量的变化范围与控制因素。此外，还与国外实验室合作开展了较高氧逸度条件下橄榄石-硫化物Fe-Ni交换实验，根据新的实验结果，重新拟合了橄榄石-硫化物体系的氧逸度经验公式，改进的公式适用于更氧化的岩浆体系。将该公式应用于新疆铜镍矿床，初步查明了新疆铜镍矿床的氧逸度变化范围与形成机制。通过黄山南铜镍矿床与东天山、东准噶尔、蒙古中部镁铁-超镁铁岩体的系统对比，发现含碳质地壳混染是铜镍矿床形成的重要因素。理论方面，通过黄山南高镍矿床解剖研究，发现了黄山南高镍矿床的特征与形成机制，丰富了造山带铜镍矿床的成矿模型。勘查应用方面，通过全球高镍矿床的系统对比，明确了造山带环境为高镍铜镍矿床产出的有利背景，并系统总结了高镍铜镍矿床的共性与判别标志，为铜镍矿床勘查提供了科学依据；项目开展过程与地勘单位紧密结合，相关成果为矿区深边部找矿、矿床外围含矿岩体优选方面提供了参考。项目负责人多次参与国内、外矿床学会议并交流研究成果，得到同行好评。至此，本项目设定的研究目标和任务都已达成，共发表SCI论文8篇；基于该项目成果，后续研究获得自然科学基金项目支持。