雅鲁藏布江缝合带西段错不扎铬铁矿特征及形成环境

摘要

Most of chromitite ores produced in China are from the Yarlung Zangbo ophiolite belt (YZSZ), which is divided into three parts: the eastern part (Qu Shui to Mo Tuo), the middle part (Ang Ren to Ren Bu) and the western part (Saga to the Sino-Indian border). Furthermore, the western part contains north sub-belt and south sub-belt. For a long time, most of studies have focused on the eastern part and middle part, with very insufficient studies conducted on the western part, especially on the north subbelt, which inhibits mineral exploration along the YZSZ. Cuobuzha ophiolite is located in the north sub-belt, which occurs along a NW-SE trending belt and is mainly composed of harzburgite with diabase dykes. The field investigation discovered several chromite orebodies 0.5-1 m long and 0.2-0.5 m thick within the ophiolite. All of the ores are massive. The analytical results by electron-probe micro-analyzer (EPMA) show that the Cuobuzha chromitite is of typical high-Cr variety with Cr number [Cr#= 100× Cr/ (Cr + Al) ] ranging from 75 to 78, and Mg number [Mg#= 100 × Mg/ (Mg + Fe2 +) ] ranging from 66 to 69. Calculated results show that the ratio of FeO/MgO ranges from 0.51 to 0.65, and that the values of Al2O3 and TiO2 are 11.27％-12.1 ％ and 0.19％-0.4 ％, respectively, revealing that the Cuobuzha chromitite has boninitic affinities. However, the occurrence of clinopyroxene exsolution indicates that it should have experienced a deep process.%雅鲁藏布江蛇绿岩带是国内铬铁矿床出露点最多,且铬铁矿石储量、产量最大的一个蛇绿岩带.根据空间展布规律,该岩带被划分为东段(曲水-墨脱)、中段(昂仁-仁布)和西段(萨嘎至中印边境)3部分.其中,西段自萨嘎以西分为南、北两支亚带.长期以来的研究工作主要集中在东段和中段,西段的研究程度非常薄弱,尤其是北亚带.不同区段研究程度的不平衡十分不利于雅鲁藏布江蛇绿岩带内铬铁矿找矿工作的开展.错不扎蛇绿岩体位于雅鲁藏布江缝合带西段的北亚带,呈北西-南东向带状产出,主要由方辉橄榄岩组成,并普遍发育基性岩脉.野外地质调查在该蛇绿岩体中发现了多个铬铁矿化点,矿化体呈透镜状产于方辉橄榄岩中,出露地表的长度为0.5～1m,厚为0.2～0.5 m,矿石均为致密块状.电子探针分析结果表明,错不扎铬铁矿属于高铬型铬铁矿,铬尖晶石的Cr#[=100×Cr/(Cr+Al)]为75～78,Mg#[=100×Mg/(Mg+Fe2+)]为66～69.计算结果表明,母岩浆的FeO/MgO比值为0.51～0.65,Al2O3和Ti O2含量分别为11.27％～12.1％和0.19％～0.4％,与玻安质岩浆的化学成分相当.然而,针状单斜辉石出溶体的发现指示错不扎铬铁矿可能还经历了一个深部作用过程.