与湘南白腊水锡矿床有关的骑田岭花岗岩的绿泥石化研究

Chloritization of Qitianling Granite Related to Bailashui Tin Deposit,Southern Hunan Province

湘南白腊水锡矿床位于骑田岭燕山期黑云母花岗岩中。绿泥石化是这一含矿花岗岩中常见的热液蚀变现象,对采自钻孔的岩石样品研究发现,花岗岩的绿泥石化程度变化较大。本文选择三组绿泥石化蚀变程度显著不同的花岗岩样品进行研究,探讨了与锡成矿密切相关的含矿花岗岩的绿泥石化作用。研究表明,不同程度绿泥石化花岗岩中的绿泥石,在产出形式与化学成分上均表现出一系列明显不同的特征。依据绿泥石产出形式、化学成分、绿泥石化花岗岩地球化学等特征认为,绿泥石对花岗岩造岩矿物的交代顺序为:角闪石、黑云母→斜长石、钾长石→石英;绿泥石形态特征的变化一方面与绿泥石成分有关,另一方面与被其交代的造岩矿物晶体结构有关;花岗岩绿泥石化越强烈,全岩的烧失量和Zn含量就越高,原岩的Cl丢失到蚀变流体中去的量也越多。随着花岗岩绿泥石化程度的加强,其绿泥石的主要成分含量会呈现出规律性的变化。绿泥石的Si含量在绿泥石化过程中逐渐减少;长石遭受破坏溶解后释放出的Al在厘米尺度范围内发生迁移与再分配,引起绿泥石Al含量的升高;Fe从热液中带入,蚀变花岗岩中绿泥石矿物含量的多少与水-岩反应体系中Fe的多少有关;形成绿泥石所需的Mg主要来源于花岗岩暗色矿物内Mg的释放与再分配;Mn则主要由外部流体带入。

The Bailashui tin deposit is located in Qitianling Yanshanian biotite granite,Southern Hunan province. Chloritization is a universal hydrothermal alteration in this ore-bearing granite. Based on the research of the samples collected from the drillhole,we found a notable variation in the chloritized degree of the granite. This study was carried out by selecting three groups of granite samples which displayed notable different degree of chloritization. The chloritization nhich closely related to ore- forming in the Sn-bearing granite had been discussed. The results indicated that,in the granite samples of different degrees of chloritization the petrography and chemical composition of chlorite changed with a series of obviously variable characteristics. According to the analysis of micropetrography and relevant geochemical data,some conclusions could be drew as follows. Themetasomatic sequence of rock-forming minerals in the granite replaced by chlorite is：amphibole and biotite→plagioclase and potassium feldspar→quartz. The variable shape of chlorite was,on the one hand,related to the composition of chlorite;on the other hand,related to the crystal texture of rock-forming minerals which were replaced by chlorite. As the chloritization of granite was stronger,the LOI and Zn content of whole-rock samples would be higher,and the Cl contents losing to hydrothermal liquid from original rock would be more. With the chloritized degree of granite becoming stronger,main compositions of chlorites would present regular variations. The Si content of chlorite decreased in the process of chloritization. Al elements were released after the breakdown and dissolution of feldspar. The mobilization and redistribution of Al on the thin-section scale increased the aluminum content of chlorite. Fe element were brought in to the chrorite by hydrotherm. The amounts of chlorite formed depended on how much Fe had been introduced into the hydrothermal system. The Mg for the formation of chlorite was primarily related to the redistribution of Mg which was released from mafic minerals. Mn element was mainly brought in to the chlorite by the outer hydrotherm.