| Bainiuchang silver polymetallic deposit located in Laozhai village, Mengzi county of Yunnan province, is ultra-large polymetallic deposit with silver, lead, zinc, tin, and copper. It is an important part of Tin-polymetallic belt in southeast of Yunnan. The famous Gejiu tin-polymetallic deposit is northwest adjacent of Bainiuchang deposit. Together with the Dulong Tin-polymetallic deposit, located in southeast of Bainiuchang deposit, all three constitute the Tin-Lead-Zinc-Silver-Copper-Tungsten polymetallic belt of Southeast of Yunnan.In basis of systematically collecting existing researches and exploration data on Bainiuchang deposit, this paper focused on the geochemistry and mineralogy characteristics of ore by field survey, data analysis and sample testing. In this paper, the deposit metallogenic period of time is divided. Trace elements and rare earth elements characteristics of ore are also discussed. Basic on the mineral indicative significance to metallogenic fluid, the author study on the source, properties, development features of ore-forming fluid, and effect to ore genesis too. The main research results are as follows:(1) Through geological data analysis, field survey, samples identification by hand and microscopic, the author summarized predecessors’ researches, reviewed the ore characteristics, and found out the deposit metallogenic period of time.(2) Through comparative analysis rare earth elements distribution patterns, and ratio of characteristic elements indicating significance (such as the Y/Ho) of different samples, including various ore block, different tectonic ore, rock mass, surrounding rocks, the author came to the conclusion of multi-source ore-forming fluid in Bainiuchang deposit, namely hot solution in exhalative sedimentary period, magmatic hydrothermal,and ore-forming fluid from the surrounding rock in the water-rock reaction leached out of the ore-forming fluid.(3) Owing to abnormal Eu, low∑REE and value of V/(V+Ni) and V/Cr, it can be illustrated the character of the ore-forming fluid showed poor oxygen andreducibility.(4) By using of trace element geochemistry features in ore minerals like sphalerite (Fe, Mn, In, Ga/In, Zn/Fe, Zn/Cd etc.), galena (Ag, Sb, Bi), pyrrhotite (Co/Ni, Fe, and Fe:S), pyrite (Co, Ni, Co/Ni, Se, Te, S, S/Se) and cassiterite (Tin), this figured out that the formation of the deposit is a common effective characteristics of exhalative sedimentation and magmatic hydrothermal. Near the rock mass, magmatic hydrothermal rock plays a major role, while sedimentary exhalative is dominant far from the rock mass. Dominance in the ore-forming process show characteristic of the gradual change.(5) Bainiuchang deposit experienced Middle Cambrian submarine exhalation sedimentation-mineralization and Yanshan Period acidic magmatic hydrothermal superimposed mineralization. The value of Ga/In, Zn/Cd in sphalerite, large number of monoclinic pyrrhotite in magnetic pyrite, and content of In in cassiterite indicate that the formation of the deposit is mainly in medium temperature conditions. So we can get conclusion that Bainiuchang deposit has multi-source, mult-period evolution characteristics. On basis of former exhalative sedimentary material, the late magmatic activity superposition, medium temperature magmatic hydrothermal activity made metallogenic elements been enriched more effectively. In all, Bainiuchang deposit belongs to medium temperature of polygenetic-exhalation stratabound deposit, with multi-source, multi stages evolution characteristics. It is one polygenetic-exhalation stratabound deposit in China. |
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