新疆磁海铁矿床Fe-O-S同位素和元素地球化学示踪

Fe-O-S isotopic and geochemical characteristics of the Cihai iron deposit, Xinjiang

新疆磁海铁矿位于北山构造-成矿带西端,以磁海矿段为主要组成部分,铁矿体主要为透镜状、似层状,围岩以辉绿岩为主,二者接触线截然且平直;另外一种矿体产在基性岩体与地层接触带的石榴子石透辉石矽卡岩岩体内,整个矽卡岩岩体即为铁矿体。通过野外矿床特征与矿相学研究,成矿过程可分为:(1)矿浆期,以磁铁矿+单斜辉石+普通角闪石的近同时产出为特征;(2)矽卡岩期,先期以石榴子石+透辉石+磁铁矿为特征,后期以透闪石/阳起石+石英+磁黄铁矿+黄铜矿+黄铁矿的组合为特征。通过对矿浆期及矽卡岩期典型矿物的地球化学及Fe、S、O同位素结果分析,认为矿浆期铁质具有幔源特征,矽卡岩期成矿流体对矿浆期具有继承性。通过与前人研究结果对比分析,认为磁海成矿可能形成于塔里木地区二叠纪地幔柱背景,含矿母岩浆可能为地幔柱的分支。

The Cihai iron deposit, located in the western Beishan tectonic-metallogenic belt, is mainly composed of Cihai ore block, in which the orebodies hosted by diabases in direct contact show stratiform and lentoid forms, whereas other orebodies in Cihai ore block are hosted by garnet-diopside-skarns. Based on the study of field geochemical characteristics and ore microscopy, the authors hold that the formation processes of the Cihai iron deposit involved two periods:(1)ore-magma period characterized by magnetite+clinopyroxene+hornblende formed almost simultaneously, and(2)skarn period which contained garnet+diopside+magnetite formed earlier and tremolite/actinolite+quartz+pyrrhotine+chalcopyrite+pyrite formed at late stage. The geochemical and Fe-S-O isotopic data of main minerals in two periods show that the metallogenic iron was mainly derived from the upper mantle, and the ore-forming fluid in skarn period inherited that of the ore-magma period. In combination with previous studies,it is suggested that the iron mineralization of the Cihai iron deposit occurred in a setting of Permian mantle plum in Tarim. Targeted on iron ores in diabases and based on geological,geochemical and isotopic data and previous researches,the authors hold that the formation process of the Cihai deposit could be divided into immiscibility and injection of iron-rich silicate liquids(primary process)and hydrothermal metasomatic process. The iron mineralization occurred in a setting of Permian mantle plum in Tarim Basin and the metallogenic magma was a branch of mantle plume.