新疆萨尔托海铬铁矿中铂族矿物及硫化物特征

The characteristics of PGM and BMS in Sartohay chromitites,Xinjiang

新疆萨尔托海高Al型铬铁矿中几乎不含原生的铂族矿物(PGM)和贱金属硫化物(BMS)包体,显示出成矿岩浆贫硫的特征。BMS多产于铬铁矿铬粒间裂隙、基质及蚀变环带中,主要以赫硫镍矿和针镍矿为主,其次为辉铜矿、砷镍矿、硫砷镍矿、毒砂等。PGM以包体产于BMS或铬铁矿粒间缝隙中,以硫钌矿(RuS2)为主,还包括硫锇矿(OsS2)、硫镍锇矿[(Os,Ni)S2]、硫钌锇矿[(Ru,Os)S2],锑钯矿(Pd5Sb2)和少量Cu、Pt、Au的硫化物。铬铁矿全岩ΣPGE含量50.64×10^-9~92.00×10^-9,较世界范围内蛇绿岩型铬铁矿低,且具有IPGE较PPGE富集的特点,PdN/IrN在0.1~0.9之间,具有Os相对Ir富集的特点。铬铁矿主量元素和原位微量元素显示出与菲律宾阿科杰高Al型铬铁矿以及MORB中尖晶石相似的地球化学特征。根据萨尔托海铬铁矿中PGM及BMS的种类、产出特征,结合铬铁矿全岩PGE及单矿物微量元素地球化学特征,认为铬铁矿的形成与贫硫的拉斑玄武质岩浆与地幔橄榄岩的熔体岩石反应有关。铬铁矿形成后的晚期岩浆阶段使得自形程度较高的PGM(如硫锇矿)和BMS(如赫硫镍矿)形成,随后向热液阶段转变的过程中,由于温压条件改变、热液蚀变,形成了萨尔托海铬铁矿中Fe-Ni-As-S和PGM矿物组合。

Sartohay high-Al chromitites in Xinjiang contain no primary platinum group minerals(PGM)and base metal sulfide(BMS)enclaves,which shows the metallogenic magma was depleted in sulphur.The BMS is mainly found in the intergranular cracks,matrix and alteration rim of chromite grains,and the main sulfides are heazlewoodite and millerite,followed by chalcocite,maucherite and arsenopyrite.The PGM occurs in the grains of BMS as inclusions or in the intergranular cracks of chromite grains,and are mainly composed of laurite,followed by erlichmanite,(Os,Ni)S2,stibiopalladinite and few sulfides of Cu,Pt and Au.The whole rock ΣPGE content of chromitites range from 50.64×10^-9 to 92.00×10^-9,lower than worldwide ophiolitic chromitites,and IPGE and Os were more enriched than PPGE and Ir,respectively,the PdN/IrN are between 0.1 to 0.9.Major and trace elements of chromitites display the similar geochemical characteristics as the Philippines Acoje high-Al chromitites and spinels in MORB.Based on the type and occurrence of PGM and BMS in the Sartohay chromitites,it is concluded that the origin of chromitits was related to the melt/rock interaction between S-poor tholeiitic magma and peridotite.The euhedral PGM[e.g.,(Os,Ni)S2]and BMS(e.g.,heazlewoodite)were formed at the late magmatic stage after the formation of chromiteites,and the Fe-Ni-As-S BMS and PGM were formed in Sartohay chromitits due to the change of temperature and pressure,hydrothermal alteration at the subsequent hydrothermal stage.