桂西第四系铝土矿成矿中的矿物转化与元素迁移

Complex ore-forming processes make that the mineral forming mechanism and geochemical behavior of elements during the formation of the karstic bauxite are the difficulty and focus of the bauxite investigations. Researchers have conducted detailed investigation concerning the mineral formation and evolution, and element migration, and obtained fruitful achievement; however, the ambiguity still remains. The Quaternary bauxite in western Guangxi is transformed from Permian bauxite via intense weathering. During the formation of the Quaternary bauxite, the dissolution, precipitation and transformation of minerals and geochemical behaviors of elements are still obscure. In previous works, applicant has carefully analyzed the mineral and geochemical composition in the Permian and Quaternary ores, and discovered that the material composition in the Quaternary ore is evidently different from that of Permian ore. This approves that the mineral and element in the ore change greatly during the formation of the Quaternary bauxite; however, the detailed transformed mechanism of minerals and geochemical behaviors of elements still need further investigation. We intend to choose typical bauxite deposits in five bauxite mining area in western Guangxi as studied objects, detailedly study the deposit geology, mineralogy and geochemistry, systematicall collect Permian ore samples from drillings and Quaternary ore samples, precisely find out the differences of the material composition between the Quaternary and Permian ore, determine the physicochemical conditions of each mineral in the ore, comprehensively discuss the dissolutions, precipitation and transformation, and geochemical behaviors of elements in the Quaternary bauxite forming process.

复杂的形成过程导致喀斯特型铝土矿中矿物成因和元素化学活动行为是铝土矿研究中的热点与难点。前人对典型喀斯特型铝土矿矿物形成、演化与元素迁移规律进行了研究，取得丰硕成果，但模糊性仍然存在。桂西第四系铝土矿由二叠系铝土矿经过强烈风化作用形成；其形成过程中，矿物分解、结晶和转化与元素地球化学活动行为是研究中空白部分。申请者在前期工作中对桂西新圩矿床两种类型矿石矿物和元素组成进行了检测，发现两种类型矿石间矿物和元素组成差别明显；证实了矿石转化过程中矿物和元素组成发生了明显改变，但矿物具体转化机制和元素地化活动行为需要继续研究。项目拟选择桂西五个成矿区中典型铝土矿床，详细研究矿床地质、矿石矿物学、元素地球化学，系统采集钻孔深部二叠系原始矿石和第四系矿石样品，准确查明区域内两种类型矿石间物质组成差异，判定矿石矿物形成物理化学条件，综合查清两种矿石转化过程中，矿物分解、结晶和转变及元素地球化学活动行为。

项目以桂西地区德保铝土矿、龙合铝土矿、新圩铝土矿、三合铝土矿、田阳五村铝土矿和教美铝土矿为研究对象，对二叠系铝土矿和第四系铝土矿矿石物质组成与特征进行深入研究，并探索了两种类型矿石转化过程中矿物的形成演化及元素的富集迁移规律。桂西二叠系铝土矿矿物组成主要为硬水铝石、鲕绿泥石/镁绿泥石、锐钛矿、黄铁矿及少量的锆石、金红石、碳氟钙铈矿、氟碳铈矿、针磷钇铒矿，第四系铝土矿矿石主要矿物为硬水铝石、高岭石、赤铁矿、针铁矿、锐钛矿及少量的磷镧镨矿和方铈矿。二叠系铝土矿主量元素以Al、Si、Fe2+、Ti等为主，第四系矿石中主量元素以Al、Si、Fe3+、Ti为主；两种矿石类型微量元素组成差别不明显，都明显富集高场强元素、亏损碱性碱土元素；相对于第四系矿石，二叠系铝土矿明显富集稀土元素。从二叠系铝土矿向第四系铝土矿转化过程中，鲕绿泥石、黄铁矿、碳氟钙铈矿、氟碳铈矿等矿物发生大量溶解，高岭石、针铁矿、赤铁矿等矿物结晶形成。元素Fe2+被大量氧化为Fe3+，大量的稀土元素流失，少部分形成了磷镧镨矿和方铈矿。元素化学行为主要受矿物类型及其演化和元素自身化学性质所制约。矿物组合及成因指示二叠系铝土矿形成于碱性和还原环境，并有微生物大量参与成矿；第四系铝土矿形成环境变化较大，集中于氧化和酸性环境。基于矿床地质、矿物学以及元素地球化学研究，提出桂西铝土矿形成演化包括物源期、表生期、同生期、成岩期、二次表生期五个阶段。