GEOLOGY AND ORIGIN OF TONGCHANG GOLD DEPOSIT IN YUNNAN PROVINCE

The Tongchang gold deposit is tectonically sited in middle south of Jinping Paleozoic cover of Ailaoshan orogenic belt. Gold mineralization occurs within NW-striking fault-fractures between Lower Ordovician flysch sediment and Middle-Upper Silurian dolomite. The gold-bearing minerals are native gold and electrum, which are attached to clay minerals or within crevices of quartz, pyrite and arsenopyrite grains and in between grains of cements, occurring as veinlet, disseminated and smallmassive in ores. Carbonatization, silicification and sericitization develop as wallrock alteration. The δ³⁴S values of pyrite from gold ore range from 0.337‰to 3.113‰. The values of ²⁰⁸Pb/²⁰⁴Pb、²⁰⁷Pb/²⁰⁴Pb and 206 pb/²⁰⁴Pb of pyrite from gold ore range res pectively from 39.3 814 to 40.1504, 15.7093 to 15.7727 and 19.002 to 19.5492. While the δ³⁴S values of pyrite from the wallrock quartz-orthophyry range from -1.118‰to -0.161‰, with ²⁰⁸Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁶pb/²⁰⁴Pb of pyrite from the quartz-orthophyry ranging from 39.0817 to 39.2278, 15.653 to 15.6805 and 18.8186 to 18.8612, respectively. It is indicated that the sulphur of the pyrite from gold ore is not derived from quartz-orthophyry magma fluid but sedimentary wallrock. The lead of pyrite from ore is derived of upper crust and that from the quartz-orthophyry is originated from upper and lower crust or upper mantle. It is inferred that gold in the Tongchang ore was deposited from gold-bearing hydrothermal fluid, which circulated along faults and extracted gold and other ore-forming matters, by infilling and replacing at low pressure and temperature site in the fault system.