大吉山稀有金属花岗岩中富镉闪锌矿的矿物学特征

MINERALOGICAL CHARACTERISTICS OF SPHALERITES WITH HIGH CADMIUM CONTENTS FROM DAJISHAN MINE

电子探针研究表明江西大吉山矿床二云母花岗岩和白云母花岗岩中副矿物闪锌矿为富镉闪锌矿(Cd,4.34—19.89％,wt％,下同),铁含量低(Fe,1.03—4.24),尤以具W、Be、Ta、Nb矿化的白云母花岗岩中闪锌矿镉含量为高,并随岩体由下到上(标高的增大)而增高。矿物中Cd+Fe与Zn之间存在良好的线性负相关关系,指示了主要类质同象替代发生在Cd、Fe与Zn之间。矿物中可能存在不等位替代现象和四面体空位。富镉闪锌矿晶胞参数为0.54250—0.54440nm,随矿物中Cd含量的增加而增大。对照表明,当矿物中镉含量特别高时,Skinner(1961)晶胞参数计算公式估计值偏大,需作重新修正。最后,文章对富镉闪锌矿的成因意义进行了讨论。

Sphalerites with high cadmium contents (great than 4%, wt%)and low iron contents(less than 4%)have been found in muscovite granite(body No. 69, W-, Be-, Ta- and Nb-mineralized)and two-mica granite of Dajishan mine, Jiangxi Province, by the study of EPMA. The content of cadmium in sphalerites which is different in various samples increases from two-mica granite to muscovite granite, and also with the changing of body level from lower to upper. The content of Fe shows a similar tendency as well, but it is not as significat as that of cadmium. Especially in the sample of 467 level for muscovite granite, the content of cadmium is up to 18.24%(average value), which is great out of the approximate limit of Cd substitution in sphalerite determined by B. J. Skinner(1961). A good linear relation between the content of Cd + Fe and Zn in an uint cell is present, slope is -0.675, showing a non-equivalent substitution of Cd + Fe for Zn and implying empty tetrahedron may occur in the sphalerite structure.X-ray diffraction data show an increasing trend of unit cell parameters for sphalerites (from 0.54250 to 0.54440nm)with the increase of Cd and Fe contents. The a0 value is over-estimated according to the equation suggested by B. J. Skinner(1961)for the sample No. 467. More data are needed to modify Skinner's equation.By comparision of the characteristics of sphalerites from two-mica granite, muscovite granite and from quartz-vein tungsten deposit respectively, it is proposed that Cd concentrated fluid should be derived from the magma of two-mica granite and muscovite granite, and the content of Cd and/or the ratio of Cd/Zn in fluid increases with the evolution of magma. The origin of hydrothermal fluid for Dajishan tungsten deposit may have a close relation with the underlying two-mica and muscovite granite or deeper plu-ton.