桂西二叠系沉积型铝土矿鲕粒成因机制探讨

已有研究表明铝土矿中鲕粒的Al含量普遍高于基质，但鲕粒富集Al过程尚不清楚。本文通过对桂西地区铝土矿进行宏微观观察以及电子探针测试(EPMA)和X粉晶衍射分析，研究了矿石构造和元素地球化学变化规律，试图分析铝土矿中鲕粒生长机制。含矿岩系层序多变特征表明桂西铝土矿为陆相成因；鲕粒内部圈层Al含量大于50%,而Si含量小于1%,Fe含量波动于1%左右，证明矿石中鲕粒Al的含量比基质的高，同时硬水铝石的结晶程度亦较高；与鲕粒生成有密切关系的构造主要为凝胶和裂隙，两者为正反馈关系；胶体在分离过程中出现分凝，主要形成富铝、富硅和富铁凝胶，分凝过程中Fe、Si亲和性强，两者的迁移能力均高于Al。分析认为鲕粒形成于浅埋阶段，是一种凝胶的反复分凝过程，受控于湿润-干热气候，成鲕可大致分为3个阶段，即充水阶段、湿润阶段和干热阶段，成核机制为脱水收缩，圈层增长机制为成胶-凝胶老化交替。铝土矿型风化壳的物质搬运过程使鲕粒的圈层停止生长，形成夭折鲕，若条件有利则形成再生鲕。首次引入磁场的概念解释了鲕粒保持浑圆形习性的原因。

Study of the genetic mechanism of ooids in Permian bauxite ore from western Guangxi

Previous studies have shown that the Al content of ooids in bauxite is generally higher than that of the matrix, but how the ooids enrich Al is still unclear. Basing on the observation of bauxite in western Guangxi, as well as electron probe microanalysis (EPMA) and X-ray powder diffraction, this paper focus on the ore structure and elemental geochemical behave, and try to unravel the growth mechanism of ooids in bauxite.The variable sequences of ore-bearing rock series demonstrate that the bauxite in western Guangxi is formed on continent.The EPMA proves that the Al of the ooids is higher than that of the matrix, such as the Al in the inner layer is greater than 50%, while the Si is less than 1%, and the Fe fluctuates around 1%, meanwhile the degree of crystallization of diaspore is also higher. The structures related to the formation of ooids are mainly gel and fissure, and then a positive feedback was set up between gel and fissure.The colloids grew segregating to form mainly aluminum-rich, silicon-rich and iron-rich gels, and during the segregation process, Fe and Si had strong affinity, and their migration ability was higher than of Al. It can be deduced that the formation of ooids was a repeated process of gel-segregation when weathered materials were buried slightly, and the the humid-dry-hot climate is the drive for the gel-segregation. The formation of ooids can be roughly divided into three stages, namely, water filling stage, humid stage and dry-hot stage, and the nucleation mechanism is dehydration shrinkage, whereas the growth mechanism of the ring layer is the alternation of gelation and gel aging. The transport of matter from bauxite-type weathering crust stopped the growth of ring to form aborted ooids, and when the conditions are favorable the regenerated ooids would formed. The concept of magnetic field is introduced for the first time to explain the reason why the ooids always maintain round shape.