Study On Geological Features And Ore-forming Mechanisms Of The Zhengguang Au Deposit In Northern Great Xing’an Range

Zhengguang epithermal Au deposit, located in the northern of Great Xing’an metallogenic belt, was identified as an important deposit in Duobaoshan ore-concentrated area. A systematic study on geological features, geochemistry of whole-rock and pyrite, ore-forming fluids and stable isotopes was performed. Several conclusions have been drawn as follows:(1) Zircon LA-ICP-MS U-Pb dating of previous studies indicate that diorite formed in the Late Jurassic(151Ma). Geochemical data shows that the diorite belongs to metaluminous calc-alkaline series, and is characterized by high iron, calcium, aluminum and magnesium and low alkali, with enrichment in LILE and LREE, depleted in HFSE and HREE, unobvious Eu anomalies. The above characteristics indicate that Zhengguang diorite was formed by partial melting of mantle wedge modified by slab fluids.(2) Only liquid-rich two-phase fluid inclusions were distinguished in Zhengguang Au deposit on the basis of petrographic and microthemometric analysis. Homogenization temperatures of fluid inclusions vary from 116 to 294℃, and salinities w(NaCleq) vary from 0.9% to 13.8%. The δD and δ18O of fluid inclusions in quartz vary from-127‰ to-110‰ and-5.9‰ to 0.6‰. The δ13C and δ18O of calcites vary from-5.3‰ to-2.0‰ and 7.7‰ to 13.5‰, respectively. The 3He/4He, 40Ar/36 Ar and 40Ar*/4He ratios of fluid inclusions extracted from pyrite exhibit a range of 1.75~3.06 Ra, 683~1295 and 0.30~0.63, respectively. Both fluid inclusions and stable isotope compositions suggest that the ore-forming fluids of early metallogenic stage consisted dominantly of meteoric water that evolved by interaction with wall rocks. While along with the proceeding of mineralization, the ore-forming fluids were transformed into the mixture by meteoric water and magmatic water, but still dominated by meteoric water.(3) We determined the type and the occurrence of gold minerals by mineralogical studies, scanning electronic microscope observation and energy dispersive spectrometer analysis. The result indicates that the gold-bearing minerals in the deposit are mainly electrum, küstelite, native gold and petzite. And the occurrence of Au are intergranular gold, fissure gold, inclusion gold and solid solution gold.(4) Electron microprobe and LA-ICP-MS analysis showed that small changes in the pyrite content of Fe and S, but there are differences in Au, Ag, As, Se, Co, Ni content in different types of pyrite. Pyrite in this deposit is a typical hydrothermal pyrite,with Co/Ni range from 0.04 to 318.36(an average of 20) and content of Se range from 0.100 to 395.3ppm(an average of 63). The combined effect of mixing unsaturated ore-forming fluids and poor H2 S fluids may be the main mechanism of gold precipitation in this deposit.(5) The formation of Zhengguang Au deposit is closely linked to diorite. Diorite is not only a major source of ore-forming fluids and ore-forming minerals, but also provides thermal energy for ore-forming fluids.