桂西二叠系铝土矿含矿岩系沉积相特征及其演化

    Characteristics and evolution of sedimentary facies of bauxite-bearing rocks series of Permian bauxite deposits in western Guangxi

    • 摘要: 桂西二叠系铝土矿含矿岩系沉积相研究薄弱,长期未能建立沉积层序。通过线路调查和剖面研究,将桂西二叠系铝土矿含矿岩系厘定为4个沉积相8个亚相,即顶部为沼泽炭泥相,上部为滨海平原泥岩相,中部为准平原铝土岩(矿)相,下部为滨湖−平原湖泊铁铝质岩相。中部准平原相可分为凹地铝土岩夹泥岩亚相、缓坡残坡积铝土矿亚相、凹地沉积铝土矿亚相、斜坡铝土岩亚相。下部滨湖平原湖泊铁铝质岩相可分为滨浅湖铝土质泥岩−铁质泥岩−铁铝岩亚相和滨湖平原古土壤亚相。陆相火山灰在各相中均有残存。含矿岩系中泥岩成分变异指数ICV在1左右,表明存在沉积再循环,泥岩中化学蚀变指数CIA均大于97,显示风化程度高;亲硫元素(Cu、Zn、Co、Ni)和亲铁元素(V)在下部铁铝质岩相中普遍高于中上部铝土岩(矿)−泥岩相,亲氧元素(Th、U、Nb、Ta、Sn、W)则相反;铝土矿相稀土元素总量最高,中上部铝土岩(矿)−泥岩相中的δCe和LREE/HREE均明显高于铁铝质岩相。Fe2O3− Al2O3− SiO2投影表明,季风气候贯穿整个含矿岩系沉积,铝土矿相中的泥岩元素特征值在所有泥岩中均为极值,指示了极端红土化的存在,砾屑及元素地球化学显示含矿岩系具有继承性;对应4个沉积相,可推断3个演化阶段,第一阶段为红土聚集,第二阶段为准平原化(即铝土矿化),第三阶段为平原化;含矿岩系发育期间火山灰持续沉降。铝土矿是第二次风化壳的产物。

       

      Abstract: The study on sedimentary facies of Permian bauxite−bearing rock series in western Guangxi has been rare so that a sediment sequence has not been set up yet. Based on line survey and section study, adding microscopic observation and elemental analysis of rocks and ores, this paper has determined four sedimentary facies and eight subfacies of ore−bearing rock series, the four faces are swamp carbonaceous mud facies on the top, the coastal plain mudstone facies in the upper, the peneplain bauxite facies in the middle and the lakeshore−lake ferric−aluminous rocks facies in the lower, and the peneplain bauxite facies can be divided into subfacies such as valley bauxite laminated mudstone, gentle−slope residual bauxite, valley sedimentary bauxite and slope bauxite subfacies, and the lakeshore−lake ferric−aluminous rocks can be divided into the aluminous mudstone−ferro mudstone−ferroaluminite of the shore−shallow lake and the paleosoil of lakeshore plain. The continental volcanic ash remains in all facies. In bauxite−bearing rock series, the composition variations (ICV) of mudstone fluctuates around 1, indicating the existence of recycling sediments, and chemical index alteration (CIA) of mudstone is greater than 97, indicating a high degree of weathering. The contents of sulfurophilic elements (Cu, Zn, Co, Ni) and ferriophilic elements (V) are generally higher in the lower ferric−aluminous rocks than ones in the bauxite and mudstone, while oxygenophilic elements (Th, U, Nb, Ta, Sn, W) are on the contrary. The values of REE in bauxite phases are the highest, and the δCe and LREE/HREE of bauxite and mudstone are significantly higher than those of ferro−aluminous rocks. The Fe2O3−Al2O3−SiO2 projection shows that monsoon climate persist all the time when the ore−bearing rock series developed, and the special element values of the mudstones in bauxite facies are the extreme ones in all mudstones that mean a period of extreme laterization produced by peneplain, and the gravel and element geochemistry show that the ore−bearing rock series are of inheritance. Corresponding to the four facies the sequence of sedimentary facies can be derived into three stages: the early laterite aggregation, then the peneplain−forming (bauxite mineralization) and the last plain−forming; and volcanic ash kept settling during the development of ore−bearing rock series. The bauxite is a product of secondary weathering crust.

       

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