诸广南部产铀花岗岩长江岩体中的绿泥石和铀源矿物研究

摘要

Changjiang granite is one of important uranium-ore-bearing granites in the southern Zhuguang granitic composite,northern Guangdong province.In this paper,we present a systematical on mineralogical characteristics of chlorites and accessory minerals and their alteration in the Changjiang granite,by using electron-microprobe analysis and scanning electron-microscope analysis.We found that uranium mineralization was related to the chloritization of biotite by reducing-redox-state hydrothermal fluid in the pre-mineralization stage. The chloritization of biotite altered the original U-bearing accessory minerals of the granite and formed new U-bearing minerals such as coffinite near the chlorite.Uranium in the newly formed U-bearing minerals can be easily released into fluid under circumstance of later hydrothermal fluid being in slightly oxidizing state in the mineralization stage. Accessory minerals in the Changjiang granite include zircon, apatite, coffinite-thorite, uraninite and monazite. Among these accessory minerals, uraninite, uranothorite and coffinite are thought of important uranium-source minerals of the Changjiang granite because of their high uranium content that is readily released during later alterations by oxidizing fluids. Monazite has a moderate quantity of uranium that is potentially able to be liberated when it is altered to synchsite.Such a process only occurs during chloritization of major minerals such as biotite that enclosed the monazite. Therefore monazite is a potential uranium-source mineral. Zircon is not a uranium-source mineral because uranium in zircon remains stable and not released even under hydrothermal alteration.Other minerals like allanite and apatite have little uranium which is even below the detection limit of electron-microprobe, are thus not uranium-source minerals of the Changjiang granite.%长江岩体是诸广南部地区重要的产铀花岗岩体之一,此次研究运用电子探针和扫描电镜对长江岩体新鲜花岗岩和蚀变花岗岩中的绿泥石和有关含铀矿物进行了精细对比,揭示花岗岩中铀的活化与成矿前期或早期致使花岗岩发生绿泥石化的还原性热液蚀变作用关系密切,黑云母等的绿泥石化蚀变,使其中包裹的一些含铀副矿物也发生蚀变,导致原来以类质同象形式存在于副矿物中的惰性铀转变成活性铀,并在绿泥石附近沉淀成铀石等铀含量高且在成矿期低度氧化性热液作用下容易释放铀的矿物.长江岩体中的副矿物有锆石、磷灰石、褐帘石、铀石—钍石、晶质铀矿、独居石等,其中,晶质铀矿、铀石、铀钍石中铀含量高且铀容易释放,是长江岩体的主要铀源矿物;独居石中铀含量较高,当其周围矿物绿泥石化时,独居石蚀变形成直氟碳钙铈矿并释放铀,因而也是长江岩体的潜在铀源矿物;锆石中铀含量虽高,但因其结构稳定,铀难以释放,因此它不是长江岩体中重要的铀源矿物;磷灰石、褐帘石中铀含量均低于检测限,作为铀源矿物的可能性很小.