STUDY ON THE METALLOGENIC GEOCHEMISTRY OF AXI GOLD DEPOSIT IN WESTERN TIANSHAN MOUNTAINS

One of the most important prospecting achievements in Xinjiang recently is the discovery of the large-scale Axi gold deposit, which occurs in Late Paleozoic continental volcanic rocks in West Tianshan Mountains. The deposit is hosted by the acid-intermediate to intermediate-basic volcanic rocks of Lower Carboniferous Dahalajunshan Formation. The mineralization is closely related to the volcanism and volcano-geothermal system. The hosting volcanic rocks, which belong to calc-alkaline and alkaline series, are characterized by rich silicon and sodium while lacking of potassium, aluminum,magnesium and calcium. The host rocks with medium contents of rare earth elements (∑REE=41.59×10^-6~135.83×10^-6) belong to LREE enriched type (LREE=29.05×10^-6-95.37×10^-6) with fairly high REE fractionation. And the host rocks with features of enriched macro-ion elements such as K, Rb, Ba and Th while lacking of Ti, Y, Yb, Sc etc. are similar to the intraplate transitional basaltic rocks. The low-temperature elements of Au, Ag, As,Sb and Bi are enriched in host rocks, near-ore altered rocks and ore in a gradual enrichment process, showing the genetic relationship between the host rocks and ore bodies. With ²⁰⁶Pb/²⁰⁴Pb of 17.5269-18.6644, ²⁰⁷Pb/²⁰⁴Pb of 15.1073-16.0108 and ²⁰⁸Pb/²⁰⁴Pb of 37.3670-38.1958, the lead-isotope contents of hosting volcanic rocks and ores show their crust-mantle mixed origin. The strontium-isotope initial ratios of Au-bearing quartz vein, hosting volcanic rocks and the metallogenesis-related altered minerals are very closed from 0.705 to 0.709, suggesting the close genetic relation between the metallogenesis and volcanism. The δ³⁴S of pyrite of Au-bearing ore ranges from 0.95‰ to 10.51‰, indicating that the sulfur isotope compositions related to Au ore-forming are mainly from deep volcanism. The ratio ranges of hydrogen and oxygen isotopes are wide (δ¹⁸O from 5‰ to 13‰, δD from -59.504‰ to -115.20‰), suggesting the meteoric character of the ore-forming fluid, which belongs to K⁺Na⁺-SO₄^2-Cl^- type with low-temperature (120-180℃), low-pressure (7.2-21.8 MPa),low-salinity (NaCl,2.4%-4.5%) and enriched CO₂, N₂, O₂ etc. In general,Axi gold deposit has the basic geochemical characters of the classic epithermal ones, but with its own features on ore-forming epoch, background and so on.