Abstract: The Yinmin deposit is an important deposit in the Dongchuan copper ore field in Yunnan Province. It has two mineralization types, one is the FeCu deposit of Xikuangshan type from the Yinmin Formation of the Kunyang Group, the other is the Cu deposit of Dongchuan type from the Luoxue Formation. In this paper, major and trace elemental analyses were combined with thermodynamic calculation to reveal rock characteristics, tectonic setting, sedimentary environment, and mechanism of migration and accumulation of iron and copper. Results show that the Yinmin and Luoxue formations mainly consist of dolomites and siliceous dolomites with dolomitic sandstone as a minor component. Comprehensive analyses of various diagrams, such as 100(Fe2O3/SiO2)100(Al2O3/SiO2), Fe2O3/TiO2Al2O3/(Al2O3+Fe2O3), LaThSc and ThCoZr/10 graphs, indicate that the contents of carbonate samples from the two formations were consistent with an active continental margin sedimentary system. Discrimination diagrams of redox environment, such as V/Cr, V/(V+Ni) and Ce/La, indicate that the Yinmin and Luoxue formations were deposited in a deep water to semideepwater transitional dysaerobic basin. Thermodynamic calculation results show that iron and copper transported mainly in the form of Cl complex. First, temperature, Eh and f(S2) decreased while pH increased in the oreforming system as oreforming fluid transported to the surface and mixed with the reduced water. Then instability occurred in the complexes of copper and iron. Finally, oxides including hematite and magnetite, sulfides including pyrite, chalcopyrite, bornite and chalcocite changed sequentially with changes in the physical and chemical environment, and formed the ferrocopper and copper ore bodies of the Xikuangshan and Dongchuan types, respectively.

Key words: tectonic setting, sedimentary environment, mineralization mechanism, Yinmin copper deposit