广西扶绥地区喀斯特型铝土矿矿物学特征及其成因意义

MINERALOGICAL CHARACTERISTICS OF KASTIC BAUXITE IN FUSUI,GUANGXI PROVINCE AND ITS ORIGIN SIGNIFICANCE

扶绥地区铝土矿位于东南地洼区的二级构造单位—黔桂地洼内。通过野外地质调查,结合室内显微镜鉴定,利用X射线衍射分析和电子探针技术对矿床中矿石结构及矿物组成进行深入研究。矿石结构主要为豆鲕粒结构、团粒结构、结核状结构、内碎屑结构、胶状结构、泥晶结构等。矿石中主要的矿物成分为一水硬铝石、高岭石、赤铁矿,次要矿物为勃姆石、针铁矿,并含少量伊利石、叶蜡石、黄铁矿、锐钛矿、石英、方解石等。矿物学研究显示:一水硬铝石具有较高Al2O3含量,SiO2,TiO2,Fe2O3等杂质含量少,且普遍发育重结晶现象;自生高岭石与后生高岭石相比有高Al低Si的特点;赤铁矿和针铁矿含有较多Al,Si,Ti混入物;黄铁矿具有高S、低Co/Ni比的特征。分析表明一水硬铝石主要为三水铝石在地台活化阶段脱水重结晶形成;高岭石的成因分为风化残余、同生及后生三种;叶蜡石的出现反映了铝土矿形成过程中变质作用的存在;赤铁矿和针铁矿是湿热气候条件下岩石风化成土过程的产物;黄铁矿为沉积成因。综合分析,认为本区铝土矿床的形成具有多成因、多阶段的特点,为地台原生沉积+地洼改造+喀斯特化淋滤的多因复成矿床。

The textures and compositions of bauxite ore in Fusui of Guangxi are studied by field investigation, microscope observation, X-ray diffraction and electron microprobe analysis. The ore textures include oolitic texture, crumb texture, nodular texture, intraclastic texture, colloidal texture and micrite texture. Diaspore, kaolinite and hematite are the main minerals and boehmite and goethite are secondary minerals in the deposit with minor illite, pyrophyllite, pyrite, anatase, quartz and calcite. EPMA dates show that diaspore has high Al2O3 content and little SiO2, TiO2, Fe2O3. It is indicated that diaspore is resulted from low-grade metamorphism of gibbsite. Kaolinite aggregates are elastic, oolitic and colloidal. EPMA dates also show that authigenous kaolinite has higher Al content and lower Si content than epigenetie kaolinite. The chemical composition of the latter is close to the theoretical value. It is indicated that the genesis of kaolinite is residul,authigenous and epigenetic. The present of pyrophyllite proves that metamorphism exist in the bauxitizion. Al-substitution in hematite and goethite shows they are resulted from replacement of clay minerals in weathering. High S content and the ratio of Co/Ni（〈1） show pyrite is of sedimentary origin.