微山稀土矿床的钍矿物学特征及其指示意义

Mineralogical characteristics and indicative significance of thorium minerals in Weishan REE deposit

中文关键词:稀土与钍共（伴）生  钍的赋存形式  Th-Pb年龄  微山地区

keywords:REE-Th associated deposit  occurrence from of Thorium  Th-Pb age  Weishan

中文摘要:微山稀土矿床是中国3大轻稀土矿床之一，研究发现矿床中伴生钍矿资源。前人在此开展了大量稀土成矿作用研究，但缺少稀土与钍共（伴）生特征研究。文章通过对矿床中富钍矿石的矿物学、年代学和地球化学的研究，剖析了微山稀土矿床中富钍矿石的类型、钍的赋存形式、钍及含钍矿物的种类、成矿期次和年龄等。独立钍矿物主要为钍石，氟碳铈矿和氟碳钙铈矿是主要的含钍矿物，副矿物烧绿石、独居石和萤石中赋存少量钍。通过矿物学和岩相学分析笔者认为，该矿床存在3期钍矿化：早期形成大颗粒自形钍石代表钍矿化的主成矿期；中期为与多金属矿物共（伴）生的钍石颗粒集合体；晚期发育存在于氟碳钙铈矿、黄铁矿边缘以及裂隙中的不规则状和细脉状钍石。首次通过LA-ICP-MS获得了钍石原位微区的Th-Pb年龄为(104.8±3.0)Ma。富钍矿石中主要矿物的REE元素地球化学特征显示，钍石与萤石均富集中稀土元素，与成矿热液期大量稀土矿物的形成有关，方解石和萤石均显示为热液成因，结合区域岩浆演化特征，认为微山稀土矿中的钍矿化是碱性岩浆侵入后碳酸岩型热液多期作用的产物。

Abstract:The Weishan REE deposit is the third largest LREE-Th associated deposit in China. There are abundant research results on REE mineralization, but there is a lack of research on the characteristics of thorium mineralization associated with rare earths. Through the study of mineralogy, geochronology, and geochemistry of thorium rich ores in the Weishan deposit, the types of thorium rich ores, the occurrence states of thorium, the types and mineralization sequences of thorium and thorium-bearing minerals, as well as the mineralization periods and ages, have been analyzed. Thorite as the independent thorium mineral is the principle ore mineral, while bastnasite and parisite are the main thorium-bearing minerals. Trace amounts of thorium are found in the accessory minerals including pyrochlore, monazite, and fluorite. Mineralogy and petrographic analyses indicate that there are three stages of thorium mineralization. The early stage is dominated by large euhedral thorite, while the middle stage is characterized by aggregates of thorite grains and polymetallic minerals, and the late stage is characterized by irregular and fine vein shaped thorite developed at the edges and fractures of parisite and pyrite. For the first time, the Th-Pb age (104.8±3.0)Ma of thorite is obtained through in-situ La ICP-MS analysis. The REE geochemical characteristics of the main minerals in the thorium rich ore show that thorite and fluorite are enriched in MREE, which are related to the extensive formation of REE minerals in the ore-forming hydrothermal fluid. Calcite and fluorite are both of hydrothermal origin. Combined with regional magma evolution, it is believed that the thorium mineralization in Weishan REE-Th associated deposit is formed as a product of multi-stage development of carbonate type hydrothermal fluid after alkaline magma emplacement.