北祁连西段小柳沟钨钼矿床矿物学、硫同位素特征及其地质意义

小柳沟钨钼(铜)矿床位于北祁连西段镜铁山_朱龙关裂谷带内,矿化主要产于长城系朱龙关群碳酸盐岩及基性火山岩中,它是一个与花岗质岩石有关的斑岩_矽卡岩_石英脉型矿床。其中,钨矿化主要为矽卡岩型,钼矿化主要为斑岩型。小柳沟花岗质岩石主要由二长花岗岩和花岗闪长岩组成,与成矿作用关系密切的为二长花岗岩。文章以小柳沟花岗质岩石中的主要造岩矿物和矽卡岩中的石榴子石为对象,利用电子探针对其矿物学特征进行研究。结果表明,小柳沟花岗质岩石的成岩物质主要来源于地壳,并有地幔物质的加入,岩体形成于较高氧逸度环境〔(logfO2Ni_NiO〕,是钨钼(铜)矿形成的有利条件。小柳沟矽卡岩中的石榴子石属于钙铝榴石_钙铁榴石系列,以钙铝榴石为主,矽卡岩属于交代成因的钙矽卡岩,主要形成于还原环境。石榴子石环带的核部相对富Al,边部相对富Fe,暗示成矿流体温度降低、氧逸度和p H值升高。矿物成分分析表明,石榴子石的Mn/Fe比值和矿化金属类型之间存在一定联系。硫化物的硫同位素显示,S可能来源于岩浆热液,结合前人的研究成果,认为小柳沟钨钼矿床的钨可能来源于岩浆及地层,钼主要来源于岩浆。

Mineral and sulfide isotopic characteristics of Xiaoliugou W-Mo deposit in Northwest Qilian Mountain and their geological significance

The Xiaoliugou W-Mo (Cu) deposit is located in Jingtieshan-Zhulongguan rift valley in North Qilian Mountains with mineralization occurring in basic volcanics and carbonate rocks of Zhulongguan Group of Changcheng series. The Xiaoliugou deposit is a porphyry-skarn-quartz type deposit related to granitoids in which tungsten was mainly derived from skarn type mineralization and molybdenite came mainly from porphyry-type mineralization. Xiaoliugou granitoids are composed of monzogranite and granodiorite, with the monzogranite closely related to the Xiaoliugou deposit. Taking minerals in granitoids and garnet in skarn as the study objects and using electron microprobe technology,the authors investigated the mineralogical characteristics. Electron microprobe analyses show that the monzogranite and granodiorite were derived chiefly from the crust and its high oxygen fugacities provided suitable conditions for the formation of the W-Mo deposit. Garnets in the skarn are grossula-rite-andradite and the end member is dominated by grossular. The skarn in the Xiaoliugou deposit belongs to calc skarns and was generated in a reducing environment. Garnet of oscillatory zoning shows the concentration of Al decreased and Fe increased gradually during the evolution,indicating the gradually decreasing temperature of the ore-forming fluid together with the gradually increasing oxygen fugacity and pH value. The Mn/Fe ratio of garnet may be related to mineral metals. The sulfide isotope indicates that the sulfide was derived from magmatic hydrothermal solution. Based on this study combined with previous research results,the authors hold that the W was from the monzogranite and strata, whereas the Mo mainly came from the monzogranite.