Geochemistry Of Ore-forming Processes In The Ailaoshan Orogcnic Gold Belt, West Yunnan

The Ailaoshan orogenic belt is an important part of the Sanjiang Tethys. It hasundergone multiple cycles of evolution with innumerable metallic deposits,comprising the most important Himalayan orogenic gold belt. It is also an idealdistrict of investigating the origin of orogenic gold deposits. Based on field geologyand geochemical research of the ore-forming processes of the four typical golddeposits in the Ailaoshan gold belt, the main achievements are summarized below:(1) Based on field geology and petrographical observations, the disseminatedmineralization and quartz-vein mineralization occur commonly in all wall rocks. Theore bodies are controlled by the NW-trending subsidiary structures. The wall-rockalterations occurred at multiple stages with a variety of types, narrow in space with noobvious zoning. Pyrite and quartz are the main gold-hosting minerals.(2) Based on the EMPA and LA-ICP-MS results, the shallow strata provideabundant ore-forming materials for Mojiang and Changan gold deposits, whereas theore-forming materials of the Laowangzhai and Daping deposits are mainly sourcedfrom the deep. The gold-bearing pyrite in the mineralization stage formed frommetamorphic fluids. The high Ni content indicates partial contribution from themantle. The gold and arsenic contents of pyrite from Laowangzhai and Changandeposits show positive correlations and smooth LA-ICP-MS profile, indicating thegold migrates mainly as complexes of [Au, As]2-、[Au(As,S3)]2-in the ore-formingfluids, and as the isomorphism replacing [S2]2-in the lattice. However, gold aremainly in the form of crack gold in Mojiang and Daping gold deposits.(3) Based on the fluid inclusion study, the ore-forming fluids have lowtemperature, low salinity, and CO2-rich characteristics and belong to reducing fluidsof H2O-NaCl-CO2-ΣS system. From Daping, through Laowangzhai, Mojiang toChangan, the main mineralization homogenization temperature of fluid and CO2content decreased. The remarkable decrease of temperature and pressure of theLaowangzhai deposit results in supersaturation of CO2and phase separation. Fluidboiling and vulcanizing are the main precipitation mechanism of gold. (4) The C-H-O-S-Pb isotopes and LA-ICP-MS analyses of pyrite show that theore-forming fluid are mainly made up of deep metamorphic fluid from themedium-lower crust and mantle fluid. Atmospheric water is evolved in the shallowdeposits. The ore-forming materials are multiple sourced, mainly from themedium-lower crust with mixing of the mantle material. The wall rocks also providesome of the ore-forming material.(5) The similarities and differences between regional geology and themineralization of these gold deposits indicate that from Daping through Laowangzhai,Mojiang to Changan deposit, the metallogenic depth becomes shallower in sequence,the proportion of shallow crust material in the metallogenic system graduallyincreased. Based on these, this study established the model of orogenic goldmetallogenesis in the Ailaoshan gold belt.