A PRELIMINARY ANALYSIS ON THE EPITHERMAL GOLD DEPOSITS IN WEST TIANSHAN MOUNTAINS,XINJIANG: Geochemical Characteristics of Host Rocks and Ore-forming Environments

Axi gold deposit and its peripheral deposits Yiermande,Jingxi,Qiabukanzhuota,etc.,which were found in succession in the Yili Late Paleozoic volcanic rock province in the north section of West Tianshan Mountains,are considered as epithermal mineralizing type,and furtherly divided into adular-sericite sub-type and silicated rock sub-type.The gold deposits are formed in Tulasu-Yelimodun volcanic belt of Yili Late Paleozoic rift, hosted by the acid-intermediate to intermediate-basic volcanic rocks of Lower Carboniferous Dahalajunshan Formation,which belongs to calc-alkalic/alkalic series,characterized by rich silicon and sodium and poor potassium,aluminum,magnesium and calcium.The host rocks with medium contents of rare earth elements (ΣREE=41.59×10^-6-135.83×10^-6) belong to LREE riched type (LREE=29.05×10^-6-95.37×10^-6) with fairly high REE fractionation.And the host rocks enriched with macro ion elements of K,Rb,Ba and Th while lacking of Ti,Y,Yb,Sc etc.are similar to the intraplate transitional basaltic rocks.With ²⁰⁶Pb/²⁰⁴Pb 17.5269-18.6644,²⁰⁷Pb/²⁰⁴Pb 15.1073-16.0108 and ²⁰⁸Pb/²⁰⁴Pb 37.3670-38.1958,the lead isotope contents of host volcanic rocks and ores show their crust mantle mixed origin.The strontium isotope initial ratios of Au-bearing quartz vein,host volcanic rocks and mineralization-related altered minerals are very closed from 0.705 to 0.709,showing the close genetic relation between metallogenism and volcanism.By analyzing the characteristics of metallogeno-geochemistry and the distribution of host volcanic rocks,this paper reveals the internal relations between the metalloginism and the host volcanic rocks,confirms the Au-forming environment of intracontinental rift,sets up the third metallogenitic setting and develops the ore-exploration regions of the epithermal gold deposits.