Geological Characteristics And Volcanic Facies In Copper-tin Polymetallic Deposit Of Kafang, Gejiu, Yunnan

Gejiu,Yunnan, tin-copper polymetallic ore deposit is a tin-copper-based multi-metal mining area, which is one of the most tin-rich areas in the western end of West part of Pacific rim giant tin zone. In the middle stage of Indosinian movement, Gejiu and its adjacent areas were back-arc rift basins, filled with thousands of meters thick carbonate rocks, clastic rocks and basic-ultrabasic rocks, and formed the base of volcano-sedimentary copper-polymetallic deposit. Gejiu Formation in Triassic was mainly comprised of dolomite, limestone and mudstone, belonging to the sedimentary sequences of Gejiu-Napo back-arc rift basin under the regime of thermal subsidence. The volcanic rocks of Kafang Member in Gejiu Formation has a carbonate rock thin interbed layer with stable extension, along with meta-volcanic rocks and sedimentary rocks changing in turn, showing a good relation of both phase transition and facies sequence, which is a excellent study case of volcanic facies. Volcanic facies can be defined that it sumsup all the product of volcano movement in certain environment, it is belongs to Causes phase, sedimentary rock should be studyed as per environment, this essay will take reason and environment both tocompartmentalize magma。Via analyzing the mineralization conditions, such as stratigraphy, lithology, structure, etc, combined with geochemical characteristics of volcanic rocks of this area, finally we restore the volcanic-sedimentary facies, and summarize the intrinsically link among lithofacies, phase sequence and mineralization, then we get the main achievements as follows:1. Kafang Member of Gejiu Formation in middle Triassic Anisian, consists of a set of alkaline volcanic protoliths basic - ultrabasic association, and vertical facies sequence shows the variation from the explosive facies to overflow facies, and then carbonate rock platform facies, and this phase sequence and phase composition reveal the increasing deposition process of continuous change of volcanic activity from the strong to weak. Vertical facie sequence belongs to volcanic -sedimentary facies assemblages of Gejiu-Napo back-arc rift basin in Triassic.2. The volcanic - sedimentary rocks of Kafang Member of Gejiu Formaion in middle Triassic Anisian, can be divided into three cycles withseven periods, with the lower cycle back to the top showing a gradual decrease trend of volcanic activity. The lower part of cycle is composed by the three periods, mainly consist of rock crystal chips to pulp chip tuff breccia, and the outbreak facies is the dominated facies; the middle cycle is consist of one period , composed mainly porphyritic basalt,and the overflow facies is the dominated facied; The upper cycle is consist of three periods, mainly composed of phlogopite hornfels and tuffaceous marble, and the overflow facies and with volcanic-sedimentary facies are the dominated facies.3. The contact zones between superimposed basin fluid facies, with the syngenetic structure breccia facies, skarn facies, volcanic sedimentary facies are the main ore-bearing areas.4.From the viewpoint of facieology in this study area, it proves the existence of submarine basic volcanic Indosinian-sedimentary mineralization, mineralization of Indosinian submarine exhalation, and several stages of mineralization superimposed by late Yanshanian granite.