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    西藏努日Cu-W-Mo矿床白钨矿原位微量元素、Sr同位素特征及其指示意义

    In situ trace-element and Sr isotopic characteristics of scheelite and their implications for the genesis in the Nuri Cu-W-Mo deposit, Xizang

    • 摘要: 西藏努日Cu-W-Mo矿床是目前冈底斯成矿带上发现的唯一具有Cu-W-Mo矿化组合的大型矿床。然而,自该矿床上世纪发现以来,关于其成因类型,一直存在较大的争议。本次研究根据矿床地质特征对矿床成矿阶段进行了精确的划分,并以氧化物阶段和石英–硫化物阶段的白钨矿(Sch–A和Sch–B)为研究对象,通过LA-ICP-MS微量元素和Sr同位素测试分析,对矿床成矿流体来源、演化过程及成因进行深入探讨。扫描电镜–阴极发光(SEM-CL)图像显示,白钨矿Sch–A具有两个世代,暗色均质Sch–A1被浅色均质Sch–A2不规则交代;而白钨矿Sch–B具有“核–边”结构,核部Sch–B1呈深灰色、具有均匀生长环带,边部为浅灰色、均质的Sch–B2。白钨矿Sch–A的稀土元素球粒陨石标准化分布特征和Sr同位素数据指示,成矿流体早期来源于花岗闪长斑岩,后期因强烈的水–岩反应而有围岩物质混入,而成矿流体与围岩发生的强烈水–岩反应也是白钨矿大量沉淀的重要机制。其高Mo和低Sr含量的特征,也符合岩浆–热液型矿床中白钨矿的特点。因此,综合努日矿床地质特征,白钨矿微量元素、Sr同位素地球化学特征及其对成矿流体来源、演化过程及矿床成因的指示,认为努日矿床属于斑岩–夕卡岩型矿床。

       

      Abstract: The Nuri deposit in Xizang is currently the only large deposit with Cu-W-Mo mineralization discovered in the Gangdese metallogenic belt. However, since its discovery in the last century, its genetic type has been highly controversial. Based on geological features, this study precisely classifies the metallogenic stages of the deposit. As study objects, two types of scheelite (hereinafter referred to as Sch–A and Sch–B) in the oxide stage and quartz-sulfide stage are analyzed to investigate the origins of the metallogenic fluids, the deposit's evolutionary process, and its genesis utilizing Sr isotope and LA-ICP-MS trace element analyses. Scanning electron microscopy-cathodoluminescence (SEM-CL) reveals that Sch–A has two generations, with dark homogeneous Sch–A1 being irregularly replaced by light homogeneous Sch–A2; while Sch–B has a "core-edge" structure, with Sch–B1 possessing dark gray uniform growth rings in the core and light gray homogeneous Sch–B2 in the edge. The distribution pattern of rare earth elements standardized by chondrite in Sch–A and Sr isotope data indicate that the ore-forming fluid originated from granodiorite porphyry in the early stage, and in the late stage, mixed with surrounding rock materials due to strong water-rock interactions, which is a key mechanism for the massive precipitation of scheelite. The scheelite in this study has high Mo and low Sr contents, which is consistent with the characteristics of magmatic-hydrothermal scheelite. Therefore, combining the trace-element and Sr isotope geochemical characteristics exhibited by Nuri scheelite in this study and the indications of the source of ore-forming fluid, the evolutionary process, and the genesis of the deposit, all of these indicate that the Nuri deposit belongs to the porphyry-skarn type deposit.

       

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