桂西二叠系沉积型铝土矿鲕粒成因机制探讨OACSTPCD

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.