西藏甲玛斑岩矿床系统黑云母特征及其地质意义

Characteristics and geological significance of biotites in Jiama porphyry deposit system, Tibet

甲玛矿床位于西藏冈底斯成矿带东段,是公益性研究与商业性勘查结合取得的重要成果。文章以不同产状黑云母为研究对象,采用岩、矿石光薄片显微鉴定、电子探针分析等系统研究矿区黑云母特征。黑云母的成因类型可分为原生黑云母和热液黑云母,原生黑云母和热液黑云母落于黑云母分类图中富镁黑云母和金云母区域,岩体中的黑云母偏向于富镁黑云母,具有富镁特征。与原生黑云母相比,角岩中的热液黑云母具有w(Mg O)、w(Cu O)升高,w(Fe OT)降低,Fe2+和Fe3+分异程度增大的特征;岩体中的热液黑云母w(Ti O2)小于3%,w(Al2O3)大于15%,具有低钛、高铝的特征,w(Mg O)、w(Cu O)升高,w(Fe OT)降低的特点。花岗斑岩中原生黑云母的结晶温度变化在730~750℃,logf(O2)较高(变化范围-13.0^-11.5,logf(O2)>Fe2O3-Fe3O4),属于造山带钙碱性岩系,具壳幔混源的成因特点。与原生黑云母相比,角岩中热液黑云母Fe2+/(Fe2++Mg2+)比值均一程度低,且Fe3+/Fe2+比值显著升高,甚至部分热液黑云母不含Fe2+,指示岩浆-热液流体体系逐渐向氧化态过渡,有利于斑岩铜矿的形成。对于热液黑云母所在的岩矿石样品,矿化作用的强度与黑云母的Cu O呈正相关,对成矿具有一定的指示意义。

The Jiama deposit, located in the eastern Gangdise metallogenic belt of Tibet, has been the important achievement of the combination of nonprofit study with commercial prospecting. In this paper, the authors sys- tematic studied characteristics and geological significance of biotites by using the microscopic identification and electron microprobe analysis. The biotite can be divided into two sorts： protogenic biotite and hydrothermal biotite. Protogenic biotite and hydrothermal biotite belong to eastonite and phlogopite, and biotite in rock mass tends to be eastonite with rich magnesium features. Compared with protogenic biotite, the values of ω（MgO）and w （CuO） increase, Fe^3＋-Fe^2＋ differentiation degree increases, and ω （FeO^T） decreases in hydrothermal biotite of hornfels. In rock mass, compared with protogenic biotite, hydrothermal biotite seems to have characteristics of low titanium, high aluminum （TiO2〈 3 %, Al2O3 〉 15 % ）, the values of Mg and Cu increase, and Fe decreases. The crystallization temperature （T） of the protogenic biotites is 720- 750℃, and oxygen fugacity （logf（O2）） of the protogenic biotites is high （ - 13.0～ - 11.5, logf（O2） 〉Fe2O3-Fe3O4） in granite por- phyry. Granite porphyry, with the genetic characteristics of crust-mantle mixed source, belongs to orogenic calc-alkaline rock series. In hornfels, compared with protogenic biotite, the ratio of Fe^2＋/（Fe^2＋ ＋ Mg^2＋ ） from hydrothermal biotite is of low homogeneous degree, and the ratio of Fe^3＋/Fe^2＋ increases significantly, with some hydrothermal biotite containing no Fe^2＋ , indicating that the magmatic hydrothermal fluid system gradually transformed to oxidation state, which was conductive to the formation of porphyry copper deposit. Mineralization intensity is positively related to the content of CuO in biotite.