Geological And Geochemical Characteristics And Genesis Of The Yinmin Copper Deposit, Yunnan

As an important deposit in the Dongchuan copper ore field, the Yinmin copper depositis located in the Proterozoic rift zone. There are two mineralization types in the Yinmin deposit. One is Dongchuan type copper orebody occured in the Luoxun Formation of Kunyang Group and the other is Xikuangshan type ferro-copper orebody that occured in the Yinmin Formation of the Kunyang Group. Through systematic study of geological and geochemical characteristics of the Yinmin copper deposit, the following conclusions are drawn, regarding the mineral composition, the metalsource, immigration of copper, the genesis and the mineralization mechanism of this deposit.(1) The ore bodies occur mostly in stratiform or stratoid structures and a few in veined structure. The main ore types are hematite type ferro-copper ore, copper-bearing magnetite type ore, dolomite type copper ore, veined rock type copper ore and oxidized ore.There are few types of metallic minerals in the deposit, which are sulfides(pyrite, chalcopyrite, bornite, chalcocite, digenite, covellite, and malachite) and oxides(hematite, magnetite, and malachite). The gangue minerals are dolomite, quartz and calcite. These ores generally show automorphic granular texture, xenomorphic granular texture, separation of solid solution texture, gelatinous texture and metasomatic relict texture. And the horsetail filaments of banded structure, disseminated structure and massive structure are the most commonly observed structures.(2) Based on the geochemical data of the rocks and ores, the variation of trace element contents indicates that the main ore-forming elements have been exchanged during the metallogenic process. The rare earth elements contents of the ore and the host rocks are almost the same. Thus, we conclude that the ore-forming materials originated from both a deep source and the host rocks. The carbon isotopic compositions reveal that the carbon is derived from a deep source in the early stage. While it is mainly derived from the dissolution of the carbonate stratum and a deep source in the late stage. The S-isotopic composition shows that the sulfur has multiple sources that come from both the mantle and the reduced sulfur that resulted from the deduction of the sulfate during chemical and biological process in the seawater. The Pb-isotopic composition shows that the ore-forming materials originate from the mantle and crust.(3) Study of fluid inclusions indicates that the ore-forming fluid is evolved from medium-high temperature, medium-high salinity, and high density to medium-low temperature, low salinity, and medium density. The data of H-O isotope reveal that the fluid is mainly formation water that is mixed with small amount of sea water. The temperature of ore-forming system decrease when ore-forming fluid was transported to the surface and mixed with the reduced water. Then the instability of complex of copper and iron occurred. Finally, ore-forming materials participate while the physical and chemical environment( such as the change of Eh↓, pH↑, fS2↓) change in the formation of Luoxue group, with the mineral formation of oxide( hematite, magnetite et al) and sulfide( chalcopyrite, spotcopper, chalcocite et al).(4) At least two tectonic-magmatic activities occurred in the Yinmin deposit zone( at 1068±5.1Ma and 217.3±1.7Ma, respectively). They represent the Grenville orogeny tectonic-magmatic events in Dongchuan District, and one tectonic activity in the Indosinian, respectively. The copper was locally activated, migrated and enriched in the veined orebody in responding to early tectonic and magmatic events. Then the orebodies of the deposit basically formed in the late tectonic magmatic events.