东秦岭大石沟钼矿床中晶质铀矿年代学研究及其地质意义

The geochronology research on uraninite in the Dashigou carbonate-typemolybdenum deposit of East Qinling area and its geological significance

东秦岭地区是全球最大的钼成矿带,也是我国最重要的钼矿集区。大石沟矿床是东秦岭地区最为典型的碳酸岩型钼矿床之一。该矿床主体发育钼矿化,并伴生有一定规模的铀、稀土、铅等多金属矿化。前人对该矿床中钼和稀土矿化关注较多,但对铀矿化的报道甚少。鉴于此,本文在野外地质调查的基础上,采集典型的铀矿化样品,综合利用TIMA、扫描电镜、电子探针和LA-ICP-MS等测试方法,对矿床内发育的铀矿化开展了详细的矿相学、矿物化学和同位素年代学研究。结果表明,大石沟矿床中铀矿化的主要含铀矿物为钛铀矿、晶质铀矿和铌钛铀矿。晶质铀矿多呈半自形—他形粒状展布,具有高Y、Ce和低Si、Ti、Na、Mg的地球化学组成。晶质铀矿的U/Th、ΣREE、LREE/HREE等参数表明其为岩浆成因,形成于高温高盐度、相对还原环境。晶质铀矿EPMA U-Th-Pb_(Total)化学定年与LA-ICP-MS原位U-Pb定年,结果显示铀矿化年龄为~223 Ma,即晚三叠世,表明大石沟碳酸岩中铀矿化形成于秦岭造山带印支期造山后伸展环境。综合前人和本文研究后认为,大石沟矿床中铀矿化年龄与秦岭造山带内碳酸岩型钼矿床的主体形成时代一致,为同时期岩浆活动的产物,可能形成于同一地质事件。

The East Qinling area is the largest molybdenum metallogenic belt in the world,as well as the most important molybdenum ore-concentrated area in China.The Dashigou deposit is one of the typical carbonatite-type molybdenum deposits in East Qinling area.Molybdenum mineralization is the main mineralization in Dashigou deposit,accompanied by uranium,rare earth,lead and other metal mineralization.Previous studies have paid more attention to molybdenum and rare earth mineralization in this deposit,but only few have researched uranium mineralization.In this study,typical uranium mineralization samples were selected to conduct TIMA,scanning electron microscopy,electron probe microanalyzer and laser ablation inductively coupled plasma mass spectrometry for identifying petrographical,geochemical features of minerals and isotopic geochemical research.The results show that the main uranium minerals of uranium mineralization in Dashigou deposit are brannerite,uraninite and betafite.The uraninite is characterized by high Y,Ce and low Si,Ti,Na and Mg.The U/Th,ΣREE,LREE/HREE of the uraninite indicate a magmatic origin,which formed in high temperature,high salinity and relatively reducing environment.The uraninite EPMA U-Th-Pb_(Total) chemical dating and LA-ICP-MS in-situ U-Pb dating result show the uranium mineralization epoch is ~223 Ma(late Triassic),which indicated that the uranium mineralization in the Dashigou carbonatite was formed in the post-orogenic extensional environment of the Qinling Orogenic Belt during the Indosinian.Based on previous studies and this paper,it is concluded that the uranium mineralization epoch of the Dashigou deposit is consistent with the main carbonatite molybdenum deposit in the Qinling Orogenic Belt.As products of magmatic activity in the same period,the uranium mineralization and molybdenum mineralization may have formed in the same geological event.