Mineralogy of the Chagangnuoer iron deposit in Western Tianshan Mountains,Xinjiang,and its geological significance

Located in the eastern Awulale metallogenic belt of Western Tianshan Mountains,the large-size Chagangnuoer iron deposit is hosted in andesitic volcaniclastic rocks or andesitic tuff of the Lower Carboniferous Dahalajunshan Formation,with a lenticular marble beneath the main ore body.The ore bodies occur as lamellar,stratoids and lenses,controlled by NW-,NWW-and NEE-striking faults and circular faults.Mineralizations occur along the fissures and fractures in the wall rocks.This ore deposit is composed of two major ore bodies,i.e.,FeⅠ and FeⅡ.The FeI ore body strikes NE-SW,about 2 900 m in length,63 m in average thickness,with Fe average grade of 36.87% and ore reserves of more than 190 million tons.Wall rock alterations include chiefly garnetization,actinolitization,chloritization,epidotization etc.Ore minerals consist mainly of magnetite and subordinately pyrite and chalcopyrite while gangue minerals are composed of garnet,actinolite,chlorite,epidote,tremolite,and calcite.Electron microprobe analyses show that the end member of garnet is andradite(Adr) with an average content of 61.58%,grossularite(Grs) with an average content of 35.45%,and almandite(Alm) with an average content of 2.48%.The composition of pyroxene is dominated by diopside(Di,54.22% on average) and hedenbergite(Hd,44.22% on average),with a small amount of johannsenite(Jo,less than 5.53%).Components of garnet and pyroxene are Adr37.97～97.89Grs0.19～57.21(Alm+Sps)0.84～4.38,Di28.68～87.46Hd10.46～70.13Jo0.24～5.53,which indicates that ① characteristics of these two skarn minerals are quite similar to those in calcic skarn from the major large iron deposits,suggesting that they probably resulted from skarnization;② their wide and different compositions reveal that skarns were not formed under a wholly closed equilibrium condition.Epidote is rich in Al and Ca but poor in Fe and Mg,implying that Fe and Mg were separated from each other at the time when pyroxene was converted into epidote.The chemical compositions of magnetite are rich in Mg,Mn,Ca,Si but poor in Al,Ti,Ni,exhibiting a similar feature to those formed in calcic skarn iron deposits.In the Ca+Al+Mn versus Ti+V discriminant diagram showing spot analyses of magnetite and hematite,almost all data from the Chagangnuoer ore deposit fall into the region of the skarn type iron deposit.In addition,in the ternary plot of TiO2-Al2O3-MgO of magnetite,many data from the Chagangnuoer ore deposit tend to be seated in the sedimentary metamorphogenic and contact meatasomatic region while less data drop into magmatic mafic-ultramafic region.These two diagrams may suggest that the formation mechanism of magnetite from this ore deposit may be similar to that of magnetite from skarn iron deposits.Combining the geological characteristics with the study of mineralogy,the authors hold that iron mineralization was related to skarnization,and that skarnization might have played an quite important role in the formation of the Chagangnuoer iron ore deposit.