西秦岭夏河-合作地区德乌鲁还原性石英闪长岩的成因机制及其对区域铜金成矿的意义

Reduced Intrusion-related copper-gold deposit is a newly recognized style of magmatic-hydrothermal deposits, which are commonly associated with reduced, ilmenite-series, I type granitoid intrusions, but the genesis of the reduced causative intrusions remains controversial, and the role of the reduced, I-type intermediate to felsic plutons in copper-gold mineralization is still unclear. Recent work has shown that the copper-gold mineralization in Xiahe-Hezuo district, West Qinling is genetically related with the early to middle Triassic ilmenite-series, I-type granitoids. The reduced granitoids show evidence for mixing of mantle- and crustal-derived magmas and are temporal-spatial association with a large variety of mafic dikes, providing an ideal opportunity to better understand the genesis of the ore-forming ilmenite-series granitoid rocks. In this proposal, we propose to take a comprehensive study on the Dewulu ilmenite-series quartz diorite pluton and associated mafic dikes, which genetically related with the Dewulu Au-Cu skarn deposit. The composition of Fe-Ti oxides and zircon from various intrusive rocks will be investigated by integrating electron microprobe and laser ablation ICP-MS analytical technique. The results will be used to constrain oxygen fugacity of the crust-origin and mantle-origin magmas. The trace element composition of sulfide and silicate melt inclusions from above mentioned samples will subsequently be measured by LA-ICP-MS technique. Combined with the magmatic oxygen fugacity data, the variation of ore-forming metallic element contents will play new insights into the role of magmatic oxygen fugacity in the formation of Cu-Au rich magmatic-hydrothermal fluids. This study will yield an improved metallogenic model for the reduced intrusion-related copper-gold deposits in the Xiahe-Hezuo district and provide useful guide for future exploration on the copper-gold deposits in the West Qinling orogen.

与还原性I型中酸性岩有关的铜-金矿床是一种新的矿床成因类型，目前对于其还原性成矿岩体的成因机制及成矿意义仍不清楚。申请人及其他研究者的研究发现，西秦岭夏河-合作地区的铜-金矿床与早-中三叠世还原性I型中酸性岩成因联系紧密，成矿岩体的壳-幔岩浆混合特征明显，并与同时代的中基性岩脉相伴产出，为研究与铜-金成矿有关的还原性I型中酸性岩的成因及其成矿意义提供了理想的条件。本项目以夏河-合作地区的德乌鲁石英闪长岩体和中基性脉岩为研究对象，利用磁铁矿-钛铁矿对氧逸度计和锆石Ce异常氧逸度计等方法，查明壳-幔岩浆混合过程中氧逸度的变化规律。在此基础上，利用 LA-ICP-MS 微区原位分析技术，研究各类岩浆岩中硫化物和硅酸盐熔体包裹体的成矿金属元素含量的变化规律，揭示岩浆氧逸度变化对成矿金属元素迁移、富集的影响。预期成果可以完善夏河-合作地区铜金矿床的成因模式，为西秦岭地区的铜金矿找矿预测提供理论指导。

与还原性侵入岩有关的金铜矿床的矿床成因是一个矿床学前沿热点问题。西秦岭夏河-合作地区发现的早-中三叠世钛铁矿系列I型中酸性侵入岩有关的铜金矿床，是研究还原性成矿岩浆岩的成因机制及其成矿意义的理想对象。本项目对夏河-合作地区的德乌鲁石英闪长岩岩体、中基性脉岩和相关金铜矿床，开展了系统的岩石主微量元素、同位素地球化学，锆石LA-ICP-MS U-Pb、Hf同位素以及流体包裹体成分分析等研究，探讨了岩浆来源以及岩浆混合过程中岩浆氧逸度的演化规律等问题。德乌鲁岩体内发育大量闪长质和辉石闪长质暗色微粒包体，岩石主微量和Sr-Nd-Pb同位素组成指示，德乌鲁岩体由深部幔源岩浆与壳源岩浆混合形成，幔源岩浆可能来源于部分熔融的俯冲沉积物与地幔橄榄岩反应形成的高镁熔体。全岩Fe2O3/FeO数据表明，寄主岩和包体均为中度还原性岩浆。锆石Ce异常氧逸度计结果也显示，寄主岩和辉石闪长质包体的氧逸度均小于ΔFMQ-1，表明幔源端元岩浆具有低氧逸度特征。本次研究发现，德乌鲁岩体内的含电气石伟晶岩脉中含有较多的毒砂、磁黄铁矿、黄铜矿等硫化物以及富CH4流体包裹体，指示岩浆演化末期形成了富集As、Cu等成矿元素的还原性岩浆热液。结合已有资料综合分析，表明德乌鲁岩体形成于早中三叠世弧后伸展背景下，交代富集地幔楔部分熔融形成的富含Au，Cu等成矿元素的还原性幔源岩浆与壳源岩浆发生混合，并且可能与下伏的古生代黑色岩系地层发生了同化混染作用，降低岩浆氧逸度并最终形成钛铁矿系列的中酸性岩浆，同时地层中S，Au等成矿物质的混入可能对岩浆中成矿元素的富集有一定作用。该成因模式进一步完善夏河-合作地区铜金矿床的成因模型，为西秦岭地区的铜金矿找矿和预测提供理论指导。

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