铬尖晶石原位成分示踪造山带铜镍矿成矿演化-以新疆坡一镍矿为例

Orogenic setting is one of the most important environments for magmatic sulfide deposits in China. Such type of Ni-Cu deposits is related to dynamic magma conduit system. The chemical composition and temperature of parental magma have close relationship with sulfur saturability and sulfide segregation, and the reducing environment is not the only controlling factor of Ni-Cu mineralization, relatively oxidizing environment can also contribute to metallogeny. Meanwhile, water content of parental magma may play multiple roles during ore-forming evolution except for the transporting the chalcophile element. Thus the dynamic changes of parental magma composition, temperature, oxygen fugacity and water content and their genetic link with Ni-Cu deposits, have become significant questions on the further understanding to the magma evolution and ore-forming mechanism of the magmatic sulfide deposits in orogenic setting. In order to answer the questions above, this proposal takes the unique chrome spinel, its melt inclusion and coexisting olivine in the Poyi nickel deposit from the southernmost Central Asian Orogenic Belt as the research object. We plan to perform the in situ analysis of the major-trace element on chrome spinel-olivine mineral pairs and melt inclusion in chrome spinel by high resolution field-emission EPMA and laser ablation ICP-MS, combined with secondary ion mass spectrometry (SIMS) and Fourier transform infrared spectroscopy (FTIR) study on water content of melt inclusion in chrome spinel and coexisting olivine. We aim to obtain the information of primary melt and the compositions of parental magma in different stages, investigate the dynamic changes of temperature, oxygen fugacity and water content in parental magma, reveal the functions of these changes to sulfide saturation and segregation, and finally trace the ore-forming evolution of Ni-Cu deposits in the orogenic setting. This work will complete and advance the metallogenic theory of orogen-related Ni-Cu deposits.

造山带环境岩浆铜镍硫化物矿床是中国铜镍矿的重要类型之一，其形成与动态的岩浆通道系统有关。由于成矿母岩浆成分和温度与硫溶解度密切相关；并且还原环境并非铜镍成矿的必要条件，相对氧化的环境亦可成矿；同时岩浆中水含量可能具备除运移成矿元素外更多元的作用；因此不同阶段成矿母岩浆的成分-温度-氧逸度-水含量动态变化的识别和其对成矿演化的示踪，成为研究造山带铜镍矿成岩-成矿过程的一个重要科学问题。本申请拟以中亚造山带南缘坡一镍矿为研究对象，运用原位分析技术测定不同阶段（橄榄石Fo91-84）特征发育的铬尖晶石-橄榄石矿物对和铬尖晶石熔体包裹体主微量元素组成，查明坡一原始熔体和各阶段母岩浆成分信息，结合铬尖晶石包裹体和共生橄榄石水含量的分析，厘定不同阶段熔体成分、温度、氧逸度和水含量的动态变化，阐释这些变化与硫化物饱和-熔离的成因联系，示踪造山带铜镍成矿演化过程，为进一步认识造山带环境铜镍矿提供理论依据。

近三十年来，基于大量的地质观察和细致的综合研究，造山带环境与岩浆铜镍成矿的时空联系逐渐被揭开，该类矿床基本特点的刻画与形成机制的剖析已具有一定的认识与积累，然而造山带岩浆铜镍矿床的成矿理论研究尚不成熟，成矿母岩浆的成分和氧逸度变化与硫化物成矿的内在关系等关键科学问题仍待深入认识。由于成矿母岩浆成分和温度与硫溶解度密切相关，并且还原环境并非铜镍成矿的必要条件，相对氧化的环境亦可成矿，因此不同阶段成矿母岩浆的成分-氧逸度动态变化的识别和其对成矿演化的示踪，成为研究造山带铜镍矿成岩-成矿过程的一个重要科学问题。本项目以中亚造山带南缘坡一、启鑫矿床为研究对象，获得以下主要科学认识：（1）通过首次设计的监测硫化物饱和时硫的溶解度变化的绝热等焓AFC模拟，将分离结晶和硅铝质地壳混染这两个因素对岩浆体系的影响同时加入到岩浆演化的过程中；本项工作还优化了超镁铁质堆晶岩母岩浆成分的算法，将氧逸度参数有机结合到原有广泛使用的算法中，使模拟结果更为精确。研究结果发现分离结晶+硅铝质地壳混染造成的硫化物饱和在时间上通常较晚，外源硫的加入对于汇聚环境铜镍矿床母岩浆早阶段的大规模硫化物熔离是必要的；（2）利用汇聚环境铜镍矿床的独特性，验证了尖晶石-橄榄石-斜方辉石氧逸度计在低压系统中的适用性，发现广泛应用的橄榄石-尖晶石氧逸度计在低压体系中会有1~2个log单元的高估或低估，利用Wood尖晶石-橄榄石-斜方辉石氧逸度计重新评价了造山带环境矿化超镁铁岩体的氧逸度范围，揭示出这些直接来源于弧下地幔的产物具有相对还原的氧逸度状态，为进一步认识氧逸度变化与造山带铜镍成矿的内在联系奠定了坚实的方法了认识基础。在项目实施过程中，在Geochimica et Cosmochimica Acta、Ore Geology Reviews和矿床地质等刊物上发表了相关的研究成果。在本项目支持下，有一名博士后和两名硕士研究生正开展相关研究工作。