红旗岭地区镁铁-超镁铁质岩体地幔源区及岩浆氧逸度对铜镍硫化物成矿作用的制约

The oxygen fugacity of magmas controls the sulfur species and solubility in a melt, which is considered as one of the key factors for the formation of Cu-Ni sulfide deposits. The mantle source of mafic-ultramafic intrusions that host Cu-Ni sulfide deposits in convergent margin settings may have undergone metasomatism by subducted plate-derived fluids/melts, upper mantle magma depletion, and/or upwelling of the asthenosphere. Whether these processes could change the oxygen fugacity of the upper mantle and the mantle-derived magmas is key to understand the source of sulfur and the sulfur content in ore-forming magmas. In the Hongqiling area of the central Asian orogenic belt, two stages of mafic-ultramafic intrusions were discovered, which are Middle-Late Permian unmineralized intrusion formed during subduction of oceanic plate and Middle-Late Triassic mineralized intrusion formed in a post-subduction, extensional setting. In this regard, the mafic-ultramafic intrusions in the Hongqiling area provide a good opportunity to investigate the oxygen fugacity of the mantle-derived magmas and it's constraints for the formation of Cu-Ni sulfide deposits in convergent margin settings. The study will apply detailed mineralogical examination and numerical calculation to estimate the mantle source and magma oxygen fugacity of mineralized and unmineralized intrusions in the Hongqiling area, and establish the relationship between the oxygen fugacity of magmas and the sulfur solubility/species in magmas. These results will then be combined with differences in whole-rock S and C isotopic compositions between mineralized and unmineralized intrusions to fully evaluate the controls on Cu-Ni sulfide mineralization in the Hongqiling area.

岩浆氧逸度控制硫在岩浆中的存在形式及溶解度，是影响铜镍硫化物矿床形成的重要因素之一。在汇聚板块边缘发育的铜镍硫化物矿床，其地幔源区往往受到俯冲板片熔/流体的交代、地幔亏损和软流圈上涌等影响，这些因素是否会改变地幔源区的氧逸度并被幔源基性岩浆所继承，对于确定成矿岩浆的硫来源以及硫含量非常关键。中亚造山带红旗岭地区出露有中-晚二叠纪俯冲阶段形成的不含矿镁铁-超镁铁质岩体和中-晚三叠纪碰撞后伸展阶段的含矿岩体，为研究汇聚板块边缘幔源岩浆的氧逸度与铜镍硫化物矿化的关系提供了理想窗口。本申请拟通过精细的矿物学观察以及氧逸度计算，查明该地区含矿与不含矿岩体的地幔源区和岩浆氧逸度范围，建立岩浆的氧逸度与岩浆中硫的溶解度和赋存状态之间的关联，并结合含矿与不含矿岩体中岩石的硫和碳同位素组成，探讨控制红旗岭地区镁铁-超镁铁质岩体铜镍硫化物矿化的主要因素。

区别于经典板内环境产出的岩浆铜镍硫化物矿床，造山带中产出的岩浆铜镍硫化物矿床已经成为国际上该类矿床勘查和研究的新热点。幔源岩浆在地壳浅部达到硫化物饱和是成矿的基础，硫化物饱和的机制也与母岩浆的氧逸度密切相关。但是，对造山带岩浆铜镍硫化物矿床母岩浆及主要控制因素，仅存在定性认识，还缺乏定量评估，对母岩浆硫化物的饱和机制也存在较大争议。针对以上问题，本项目以位于中亚造山带东段的红旗岭赋含铜镍硫化物矿床的镁铁-超镁铁质岩体为对象，开展了系统的全岩微量元素和Sr-Nd-C同位素分析、岩浆氧逸度定量计算、硫化物原位S同位素分析。结果显示，红旗岭岩体具有与岛弧岩浆类似的微量元素分布特征和正的εNd(t)，表明其岩浆来源于交代的、亏损大洋岩石圈地幔的部分熔融。根据红旗岭岩体原生岩浆的V/Sc估算其地幔源区的氧逸度为~FMQ+0.5，较MORB的源区氧逸度（FMQ-1~FMQ）略高，说明古亚洲洋板片俯冲改造过程中可能有部分氧化性物质加入到红旗岭岩体的地幔源区。利用橄榄石-尖晶石氧逸度计并模拟V在橄榄石与熔体间的分配系数，发现红旗岭岩体母岩浆的氧逸度为FMQ+1.0~FMQ+2.1，高于MORB的岩浆氧逸度（FMQ-1~ FMQ+0.5）以及其地幔源区的氧逸度。这说明，红旗岭岩体的母岩浆具有高氧逸度特征，且该特征并不主要继承自氧化的地幔源区，而更可能与交代地幔来源岩浆的结晶演化过程有关。红旗岭岩体中的硫化物的δ34S为在-1.0~1.1‰，与MORB地幔的δ34S（-1.5~0.6‰）相当，说明该岩体的母岩浆具备为形成中小型铜镍硫化物矿床提供足够幔源S的能力。红旗岭岩体具有极负的C同位素组成（-26.7~-24.1‰），说明其母岩浆可能混染了地壳有机碳并导致氧化性岩浆被还原，从而诱发岩浆硫化物饱和并形成铜镍硫化物矿床。本研究首次定量计算了造山带铜镍硫化物矿床母岩浆的氧逸度并建立了岩浆氧逸度与铜镍硫化物成矿间的关系，这对于建立造山带铜镍硫化物矿床的成矿模型以及指导找矿勘查都将提供理论依据。目前为止，发表第一标注SCI文章1篇，第二标注SCI文章2篇。