北秦岭蓝田铀矿田产铀岩体的地球化学特征及其对成矿的制约

牧护关岩体西部的二长花岗岩赋存着北秦岭最著名的花岗岩型铀矿田——蓝田铀矿田。笔者对该花岗岩开展了系统的元素地球化学研究, 其结果显示牧护关二长花岗岩为富硅、富钾、富碱和高分异过铝质花岗岩;微量元素特征是花岗岩相对于原始地幔具有富集大离子元素Rb、U、La、Nd、Sm, 亏损Ba、Sr、P、Ti等特征;稀土元素分配模式为轻稀土富集的右倾型。铀矿石与二长花岗岩具有相似的稀土元素特征, 暗示铀矿床的成矿物质可能主要来源于二长花岗岩。尽管牧护关二长花岗岩的U含量远低于一般产铀岩体, 但铀在其中主要以活性的晶质铀矿形式存在, 也能被活化迁移继而成矿。晶质铀矿形式存在的铀可能在花岗岩铀成矿中起着更重要的作用。

Geochemical Characteristics of Uranium-bearing Granite in Lantian Uranium Ore Field of North Qinling and Its Constraints on Uranium Mineralization

Lantian uranium ore field, as the most famous granitic-type uranium ore field in the north Qinling area, lies in the monzogranite of west Muhuguan granitic pluton.Systematic element geochemistry studies were carried out on the Muhuguan monzogranite by the authors. The results show that the Muhuguan monzogranite is highly rich in silicon, potassium, alkaline and differentiation peraluminous. In terms of trace elements, granite is richer in LILE such as Rb, U, La, Nd, Sm and deficient in Ba, Sr, P, Ti compared with primitive mantle. The distribution pattern of rare earth elements are rightward oblique with LREE enrichment. The uranium ore has similar REE characteristics with the monzogranite, suggesting that metallogenic material of the uranium deposit might come from the monzonitic granite. Although uranium content of the Muhuguan monzogranite is much lower than that of the general uranium-bearing granitic pluton, uranium existsin Muhuguan monzogranite in the form of active uraninite, which can be activated to migrate and then mineralize. Uranium, which exists in the form of uraninite, may play a more important role in the mineralization of granitic-type uranium deposits.