北秦岭光石沟大型伟晶岩型铀矿床成矿精细过程：矿物微区原位研究

Granitic pegmatites are identified as one of the important uranium sources in the world. However, only a few pegmatite-type uranium deposits have been discovered in China. Among them, the Guangshigou deposit in the northern Qinling Orogen is currently the most productive pegmatite-type uranium deposit in China. The uraniferous biotite pegmatites and syenogranite have close spatial relationship and consistent crystallization ages. Based on their elemental and isotopic analyses by many studies, genetic relationship between the syenogranite and uraniferous biotite pegmatite can be recognized. Nevertheless, how uranium is enriched during the evolution from the syenogranite to pegmatite and related uranium mineralization? What is the effect of volatile components (F and Cl) on migration behavior of uranium? And What is the effect of temperature, pressure, and oxygen fugacity on uranium mineralization? The key scientific problems related to uranium mineralization have not been well constrained and still need to be solved. This application focuses on the scientific issues. The Guangshigou large-sized uranium deposit is chosen as the research object in this study. We expect to conduct the detailed observation of mineral microstructure for the syenogranite, barren pegmatite developed within the intra-syenogranite, as well as uraniferous biotite pegmatite developed within the adjacent metamorphic strata. A systematic mineralogical study of in situ major and trace element geochemistry on feldspar, biotite, apatite, and monazite from the syenogranite and pegmatites will be carried by EPMA and LA-ICP-MS technology. The magmatic evolution process with different physicochemical conditions from the initial magma, through uraniferous residual magma to barren residual magma can be studied in detail. The aims of this proposal are to constrain the effect on F and Cl on uranium migration behavior in different stages of systematic evolution and to decipher the enrichment mechanism of uranium. In view of these discussions, we try to give a further understanding of metallogenic process. This research will enrich the metallogenic theory about the pegmatite-type uranium deposit.

伟晶岩是国际上铀的重要来源之一，但我国伟晶岩型铀矿目前发现的并不多。北秦岭光石沟铀矿床是目前我国发现的最大的伟晶岩型铀矿床，产铀黑云母伟晶岩与正长花岗岩紧密共生且侵位时代一致，前人通过元素和同位素地球化学研究表明它们之间有成因联系。然而从正长花岗岩到伟晶岩和铀矿化的演化过程中U是如何富集的？挥发分F、Cl对U的迁移起了什么作用？温度、压力和氧逸度等因素对U成矿有什么作用？这些U成矿作用中的关键科学问题仍有待破解。为此，以光石沟铀矿床为研究对象，通过矿物显微结构、EPMA和LA-ICP-MS原位微区分析，对正长花岗岩及其内部的贫矿伟晶岩和外部的产铀伟晶岩不同岩相带的长石、黑云母、磷灰石和独居石进行微区结构和矿物化学研究，精细刻画初始岩浆–含矿残余岩浆–贫矿残余岩浆的分异演化和物化条件变化过程，以阐明不同演化阶段中F、Cl对U迁移行为的作用，揭示U的富集机制，丰富伟晶岩型铀矿成矿理论体系。

本项目在北秦岭造山带分布我国重要伟晶岩型铀成矿带的背景下，聚焦造山带内最大的伟晶岩型铀矿（光石沟铀矿床）。主要研究内容包括矿物组合及显微结构，岩浆-热液演化过程中挥发组分F、Cl、CO2对U的迁移作用，以及U的富集机制。重要结果包括（1）精细反演了岩浆-热液演化与铀成矿过程：从正长花岗岩到产铀黑云母伟晶岩的含矿边部带，岩浆演化经历了同化分离结晶（AFC）过程，而从产铀黑云母伟晶岩的贫矿边部带到非矿内部带，岩浆演化经历了分离结晶（FC）过程；（2）阐明了岩浆-热液演化的不同阶段中F、Cl、CO2 对 U 迁移行的作用；（3）揭示 U 的富集机制，建立了伟晶岩型铀矿成矿模型：早泥盆世（ca. 415-412 Ma）经历了三阶段的铀成矿演化。第一阶段（高温（600-639℃）岩浆阶段）：岩浆中U4+与F-络合进行迁移，在AFC过程中形成黑云母包晶引起体系F-浓度降低，同时fO2降低导致U溶解度降低，引起高Th晶质铀矿沉淀富集；第二阶段（中温（200-400℃）岩浆-热液过渡阶段）：岩浆流体（U6+与Cl-络合进行迁移）交代早阶段独居石，通过溶解-再沉淀形成铀钍石；第三阶段（低温（100-140℃）热液阶段），早阶段高Th晶质铀矿经过流体交代，在形成蚀变的高Th晶质铀矿过程中 U被释放出来，溶解的U6+与CO32-络合形成碳酸铀酰进行迁移，通过溶解-再沉淀形成铀石。晚泥盆世（ca. 373-363 Ma）受热事件（＞350℃）影响，高Th晶质铀矿的局部发生原位溶解-再沉淀形成低Th晶质铀矿。关键数据涉及黑云母、长石、独居石、磷灰石的化学成分，独居石和磷灰石U-Pb年龄，独居石Nd同位素以及群体流体包裹体组分。项目取得的成果丰富了我国伟晶岩型铀矿成矿理论，不仅为区域和深部找矿勘查工作提供了科学的理论依据，而且为北秦岭造山带及我国其他地区同类型矿床的成因研究提供了新的思路或借鉴。