Proterozoic Fe-Cu-Au-REE Metallogenic System Of"Dongchuan" Group In Central Yunnan Province

Central Yunnan region is well known for its abundant Fe-Cu Resources from Precambrian terrane, in which the Yinmin Fm.-Lvzhijiang Fm.(1.4-1.7Ga) of early mesoproterozoic Dongchuan Group provides the most Fe-Cu-Au-REE deposits. In this study, we combed the entire proterozoic strata in central Yunnan Province, reconstruct the stratigraphic framework and the evolution process of Kunyang Rift in1.7Ga. We also took the Yinachang deposit as an example, systematically studied its geological, geochemical and other characteristics, summed up a mineralization model. According to the comparison between symptoms of deposits in central Yunnan and IOCG deposit, combine with the tectonic-magmatic activity of Kunyang rift, an intraplate extension-related Fe-Cu-Au-REE metallogenic system is established. This study obtained the following points of understanding:1. Reconstruct the stratigraphic framework and the evolution process of Kunyang Rift in central Yunnan. The oldest strata in central Yunnan, the Shaihaiwan Fm.-Pingdingshan Fm.(1.8-2.3Ga), is now nominated as "TangDan Group". Regarding this Group as the metamorphoused basement, a large-scale intraplate extension took place in the central Yunnan Province1.7Ga ago, accompanied by intense mafic-acidic intrusion, generally known as the "Kunyang Rift", which is highly consistent with the breakup of Columbia supercontinent. Different degreed extension in different district of central Yunnan formed several disconnected graben basin. In the epicontinental environment of the basins, several synchronous heteropical strata were sedimented from1.7Ga to1.4Ga, such as the "Dongchuan Group"(consist of the old Dongchuan Group in Dongchuan district, the lower Kunyang Group in central Yunnan district and the Yinachang Fm.), the Hekou Group in LaLa district, the Dahongshan Group in Dahongshan district. The extension ended in the middle of mesoproterozoic(1.4Ga), the strata was uplifted and weathered, and then unconformably overlied by the younger "Kunyang Group"(1.1～1.4Ga, formerly called the Upper Kunyang Group).2. Sumarize the characteristics of the Yinachang deposit, establish the ore-forming model of the deposit. The Yinachang deposit was formed in the Wuding-Lufeng graben basin, ore bodies were controlled both by carbonatite of Yinmin Fm. and the magmatic breccia, accompanied by regional-scale albitization and deposit-scale sim-skarnization and hydrothermal alteration. The ore assemblage of the deposit can be divided into Fe+Cu+Au+REE assemblage and Cu+Au assemblage, the mineralization stage include early stage, main stage Fe oxide-REE substage, main stage Cu-Au substage and post stage. Fe and REE elements were extracted from the wall rock, while the Cu and Au elements were coming from the acidic remelting of lower crust when the deeper mafic magma uplifted in the extensional environment. Researches in microphysiography, composition and isotope of fluid inclusions suggest fluids in Pre-mineralization stage are high temperature, high salinity magma fluid, containing alkalic components, volatile phase and Cu, Au, which formed during the immiscibility effect of felsic magma. This fluid caused a regional albitization of carbonate, and extract Fe and REE elements into the fluid. Metasomatism between magmatic fluid and carbonaceous wallrock caused the precipitation of Fe and REE in the magnetite-REE substage, and the dehydration of wallrock produced the metamorphic hydrothermal for the sulfide-gold substage. In this stage, weteoric water mixed into the hydrothermal, changed the physical and chemical conditions of the system, Cu, Au is no longer stable as clathrate in fluid, but precipitated as sulfide (Cu) and substance (Au). In the stage of post-mineralization, dominant fluid became pure meteoric water, the mineralization was ended. 3. Compare the similar deposits in central Yunnan Province, establish a intraplate rift-related ore system in central Yunnan. Many Fe-Cu-Au-REE deposits are distributed in central Yunnan Province, such as the Xikuangshan deposit, the E’touchang deposit and the Dahongshan deposit. They have many similarities to the Yinachang deposit in geology and geochemistry. Geochronology demonstrates that their dominant mineralization occurred about1.5-1.6Ga, and were closely connected with the forming of Kunyang Rift and related magmatism. They have similar geological/geochemical symptoms and ore-forming mechanism to IOCG deposits, can provide important deposit examples for the IOCG researched worldwide; Above these deposits, there distributed the SEDEX Cu deposits such as the Dongchuan deposit and the Yimen deposit. They were formed during the splitting and sedimentary of Kunyang rift, and were strictly controlled by the anorogenic magmatic-hydrothermal avtivities. It reflects the tectonic-magmatic-metallogenic events in central Yunnan Province in the period of early mesoproterozoic(1.4-1.7Ga), and gives us a new direction on prospecting.