| The Bainiuchang silver polymetallic deposit is located in central region of Gejiu-Bozhushan-Laojunshan Sn polymetallic metallogenic belt in the southeast Yunnan, it is introduced a typical large-very large silver polymetallic deposit in addition to the Gejiu and Dulong mine field. This paper based on fully absorbing predecessors’ research data, systematically study on the metallogenic background and geological characteristics of southeast Yunnan, and comprehensive research Bainiuchang deposit superimposed mineralization system through the application of rock geochemistry, isotope geochemistry, fluid inclusions, diagenetic chronology, The main results of the thesis are as follows:1. Mining area in the joint parts of Yangtze paraplatform, south China fold system, in where complex geological structure, crustal stability is poorer and mobility is strong. It was developed basement rock series of early Proterozoic of ailaoshan group and fold of yaoshan group in this area. The strata of Sinian, Cambrian, Ordovician, Devonian, Carboniferous, Permian, Triassic and Tertiary compose the sedimentary cover. Honghe、Milei-Shizong、 Nanpanjiang、Wenshan-Malipo deep fault belt restrict and controls the distribution of the magma activity and deposit distribution in this area. The region has experienced tectonic evolution process of caledonian cycle, hercynian tectonic uplift, strong invasion of yanshan period granite, the Himalayan impact extrusion. Occurred hot water sedimentation in chasmic trough of caledonian and yanshanian granitic magma intrusion provides favourable metallogenic dynamics condition for large-superlarge deposit in southeast yunnan..2. The geological structure, magmatism and mineralization alteration are very complex in mining area. Main ore bodies as stratiform and quasi laminate hosted in Tianpeng group of middle Cambrian, Ore-bearing rock are mainly carbonate clastic rock series, and controlled by the F3 interlayer sliding fracture. Under the influence of concealed granite intrusion in yanshan epoch, Hydrothermal sedimentary origin orebody superimposed enriched by magma-hydrothermal, and ore body of Magma-hydrothermal genesis as steep, vein, lenticular occurred in fracture, fracture. Wall rock alteration are greisenization, skarnization, hornfels, marmarization, silicification. Metallogenic epoch is divided into hot water sedimentary age and magmatic hydrothermal ore-forming age, Magma-hydrothermal ore-forming age include skarn mineralization period, mesothermal mineralization period and low temperature hydrothermal metallogenic period.3. This paper find out the superimposed mineralization characteristics of inheritance, new nature and additive. In Space, from the deep to the surface, ore types in the order: magma-hydrothermal type of high temperature liquid ore body (Ⅲ)' superimposed transformation type of layered ore body (Ⅳ)'type hot water deposition and superposition reconstruction complex ore body (Ⅰ)'hot water sedimentary type layer, like bedded ore body (Ⅰ,Ⅱ), and constituting a unique for composite superimposed mineralization system.4.The geochemistry of the deposits shows that:①The range of Σe/Ti、(Fe+Mn)/Ti and Al/(AL+Fe+Mn) ratio of sedimentary samples in Tianpeng group respectively is 0.04-1607.55,0.04-1730,0.06～0.98, this suggested that existence of the hot water sedimentation in in Bainiuchang.②Geochemical analysis of magmatic rock in Bainiuchang shows that the range of SiO2 content is 68.58～76.28%, average 73.11%, total alkalis (K20+Na20) is 4.93 to 8.47%, average 7.57% in Bainiuchang, belong to metaluminous-peraluminous granne. Compared with Bozhushan granite, in the deep mining of granite is similar to the rock composition and quasi-peraluminous, but slightly rich in silicon, suggesting that deep granite has higher evolution degree or belong to the relatively late product of the system of magmatic evolution.③Sediments in Tian Peng formation of Cambrian enrich Ag, Hg, As, Pb, Sb, Sn elements inside and outside the Bainiuchang; granite and granite porphyry contains more W, Bi, As, Co, Ag, Sb, Pb, Zn elements, more than crustal abundance a lot. These elements provides a large number of ore-forming materials for Bainichang..④The cluster analysis and factor analysis of Ore and mineral shows that, the characteristics of multielement composite of ore and mineral are similar both hot water sedimentary rocks and basic-acid magmatic rocks, but higher correlation coefficient of elements combinations tend to magmatic rocks. It is suggest that ore-forming elements come from both formation and magmatic rocks, and magmatic rocks may be provided more ore-forming materials.⑤The rock of close to ore、ore、mineral (Pyrite and pyrrhotite), there has two groups of different distribution patterns curve of rare earth elements, the first group shows the leaning to right characteristics(Chondrite standardized) and leaning to left or horizontal (North American shale standardization) of similar to hot water sediment. The second group showed learning to right that rare earth concentration, heavy rare earth losses of similar to the granite, and Eu losses, Ce normal. In addition, there has the evolution trend of gradual transition of the first group to the second group, and it is explain that ore, mineral(Pyrite and pyrrhotite) are a mix of hot water deposition and magmatic hydrothermalism, and the rock of close to ore were suffered late magmatic hydrothermal modification.5.Study on stable isotope:①Sulfur isotopic composition is divided into two groups, the first group of wide range, range of disulfides δ34S values is -9.4～+23.4‰, the peak range is -6～+6‰, part of the sample δ34S values is+12～+24‰, show that the sulfur source of sulfur to mix, mainly from the magma or deep crust remelting material, followed by marine sulfur, may be disruption of biogenic sulfur. Second groups of narrow range, range of disulfides δ34S values is-3.9～ +5.4%o, the peak range 0～+6‰, for high concentration distribution, shows the characteristics of a single magma sulfur source.②Each kind of ore mineral 206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of Bainiuchang respectively between 17.284-18.842,14.843-15.888,28.91～39.424.206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb of Ore formation carbonate-clastic rock respectively between 21.26～18.26,16.308～15.487,39.96～37.927.206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of granite in Bainiuchang respectively between 17.952～19.52,15.366～15.799,37.724～ 39.956.206Pb/204Pb,207Pb/204Pb and 208Pb/204Pb of granite in Bozhushan respectively between 18.387～18.301,15.655～15.611,38.904～38.677. Pb isotope analysis and graphical display of the mining area with multiple sources of Pb characteristics. Part of the ore and mineral overlap formation, part of the ore and mineral to be in conformity with granite in Bainiuchang, and Pb istope of granite in Bozhushan is almost unanimously in ore, show that lead in ore and mineral are mainly come from granite and the earth’s crust-orogenic belts.③The range of carbon and oxygen isotope of Bainiuchang is larger(δ13CPDB=-8.87～ +1.1‰,δ18OSMOW=11.1～22.2‰). In the illustration shows clear three regions, the first area of CO2 derived from marine carbonate; the second area of CO2 from the deep magma; the third area is the result of marine carbonate and the transitional zone between the magmatic rocks, CO2 can be from salt hot brine and argillaceous rock interaction, or from magmatic rocks and marine carbonate alteration.④The hydrogen and oxygen isotope of Bainiuchang are also showing ore-forming medium water characteristic of mixed with multiple water sources (δDSMOW=-50～-122‰ and δ18OH2O=12～-2.1‰), it not only come form hot brine, magma sources of water, but also mixed with meteoric water.6. The fluid inclusion of hot water sedimentation are mainly Gas-liquid two phase, rich in CH4、H2O gas, medium-low temperature(109.9～372℃, average 216.9℃),low density (0.54～1.01g/cm3,average 0.866g/cm3),low salinity(0.18～16.4%, average 3.79% NaCl. Eqv), medium-low trapping pressures(78-308Mpa,average 156Mpa)。The main types of fluid inclusion of gangue mineral with ore mineral symbiosis of magmatic hydrothermalism, rich in CH4、H2O gas, contains a small amount of CO2 and N2 gas. There was medium-high temperature(125～491℃,average 295℃),low density(0.46～0.98g/cm3,average 0.77g/cm3),low salinity(0.18～16.4%, average 3.79% NaCl. Eqv), medium-high trapping pressures (78～458Mpa,average 225Mpa). Compared two types of fluid inclusions, magma-hydrothermal fluid have higher temperature and capture pressure.7.Ore-forming age is divided into two periods: ①Obtained ores age by lead isotope formed in the medium caledonian period(488.8Ma-432.8Ma); ②The model age of Rb-Sr and Sm-Nd isotope of sphalerite is 126.00±0.41Ma and 79±31Ma, the model age of Sm-Nd of Lead and zinc ore whole rock is 83±16Ma, these age represented the second period of metallogenic age is late yanshan epoch. Among them, The second ore-forming age is similar with diagenetic age of granite (85.34±0.65Ma) and granite porphyry (84.7±1.7Ma)8. The metallogenic model of Bainiuchang deposit should be reduce to "two Metallogenic systems superimposed at different times of one place". The hydrothermal sedimentary metallogenic system was formed in environment of chasmic trough of passive continental margin, and the magma-hydrothermal metallogenic system was formed in environment of collision to intraplate. |
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