东天山东戈壁斑岩钼矿床成矿物质来源及成因研究

The discovery of the giant Donggebi porphyry Mo deposit is an important breakthrough in ore exploration in Tianshan area. The deposit was formed in Triassic. Its orebodies are not located in porphyry stock, but in the stratigraphic rocks intruded by the porphyry, which is unique compared to the well-known porphyry deposits whose orebodies are generally located in the causative porphyries. This out-porphyry mineralization phenomenon is a serious challenge to the traditional porphyry deposit model supposing a source of ore-metals from the causative magmas, and provides a chance to improve the current porphyry deposit model. However, as to the deposit, element and isotope geochemical studies are still lacking, and the source of ore-metals and fluids remain unclear, which definitely hinders the understanding of mineralization. In this project, we will try Mo isotope analysis of molybdenite of diverse stages and forms, and thereby to recover the conditions for Mo-enrichment; through in-situ electron microprobe and LA-ICPMS analysis, to analyze the element and sulfur isotope compositions of sulfides, and thus to investigate the crystallization conditions. We also comparatively study the element (including Mo) and Sr-Nd-Pb-Hf isotope compositions of the orebodies, ore-causative porphyry and ore-hosting strata, and thereby, trace the sources of the ore-forming elements. Finally, integrating the abovementioned results and the previously-obtained isotope ages and fluid inclusion measurements, we unravel the ore genesis and develop a metallogenic model for the deposit, with aims to improve the understandings of genesis of porphyry deposits and future exploration in eastern Tianshan area.

新疆东戈壁超大型斑岩钼矿床的发现是近年东天山地区找矿勘查的重大突破。该矿床形成于三叠纪，其突出的特点是矿体赋存于围岩地层，而非岩体本身，显著不同于著名斑岩矿床，后者矿体主要赋存于成矿岩体内部。这一岩体外成矿的斑岩矿床，研究其成矿物质来源、运移机制和制约条件，具有创新性地研究意义。然而，该矿床缺乏同位素、元素地球化学研究和成矿物质来源探讨，限制了矿床成因认识。因此，本项目拟研究不同阶段和产状的辉钼矿Mo同位素组成，揭示钼富集条件；利用电子探针和LA-ICP-MS技术，原位分析不同阶段硫化物元素和硫同位素组成，探讨矿物结晶条件及其变化；对比研究成矿岩体、围岩地层和矿石S-Sr-Nd-Pb同位素及元素组成，示踪成矿物质的来源；结合已有的年代学和流体包裹体研究成果，综合分析东戈壁钼矿床的成因，建立斑岩矿床岩体外成矿的成矿模式，以期为斑岩矿床成矿理论发展和东天山找矿勘查，提供依据。

东戈壁超大型斑岩型钼矿床位于新疆东天山觉罗塔格成矿带。钼矿体赋存于围岩地层，主要发育钾化，绢英岩化、泥化和青磐岩化。本项目研究获得了成矿斑状花岗岩的年龄为236±2.2Ma，稍早于辉钼矿的Re–Os年龄231–234 Ma，表明东戈壁钼矿形成于三叠纪。斑状花岗岩具有高硅、高碱、富钾，低钛，贫铁、镁、钙的特征，属高钾钙碱性系列的过铝质花岗岩。岩体的轻重稀土元素分馏较强，具有中等的Eu负异常，并富集Rb、U和Pb等大离子亲石元素，而亏损Sr、Ba和Ti等高场强元素，指示了岩浆源于陆壳的部分熔融。Sr-Nd-Pb同位素和锆石Hf同位素研究表明，岩浆来源于新生陆壳的部分熔融，并有地幔物质的参与。成矿过程可分为早期石英-钾长石阶段、主成矿期石英辉钼矿阶段、石英-多金属硫化物阶段和晚期石英-萤石-碳酸盐4个阶段。成矿流体属岩浆热液，具有高温，富CO2、F-、K+的特征。微区原位硫同位素显示成矿物质来源岩浆源区。