The Geology And Genesis Of The Baiyangping Lead-Zinc-Copper-Silver Polymetallic Deposit In Lanping Basin, Yunnan Province, China

Tibetan plateau is the largest continental collision orogen in the world, which is an ideal area for understanding metallogenesis of continent-to-continent collision. Among the deposits occuring in the collision setting of Tibetan plateau, sediment-hosted base metal deposits in "Sanjiang"(Nujiang-Lancangjiang-Jinshajiang) region create an economically significant metallogenic province, but their metallogenic background and mechanism are poorly understood at present. Lanping basin, in the middle part of "Sanjiang" region, hosts many Pb-Zn (Cu-Ag polymetallic) deposits, however, available sediment-hosted base metal deposits’ metallogenic model cannot interpret the basin environment and tectonic location in which these deposits occur, tectonic style and metal combination, so the Lanping basin is the best place for researching the sediment-hosted lead-zinc (-copper-silver polymetallic) deposits in the continental collision orogen. This paper analyzes the regional metallogenic background and Baiyangping Pb-Zn(-Cu-Ag) polymetallic deposits of the Lanping basin in details, employs various tools, such as the mineral deposit geological survey, mines description, microscopic observation, fluid inclusions, deposit geochemistry and isotopic dating, identifies the orebody distribution, host structure, fluid sources, and the potential material sources. Combining with the study on Pb-Zn deposits in the northern segment of "Sanjiang" region, this paper discusses the Pb-Zn(Cu-Ag polymetallic deposits) mineralization mechanism in collision environment, and establishes sediment-hosted Pb-Zn(-Cu-Ag) polymetallic metallogenic model in the collision environment.Observations on the deposit geology reveal that Baiyangping Pb-Zn-Cu-Ag polymetallic deposit in Lanping Basin consists of several ore veins, which can be divided into several ore blocks. The mining area strata mainly are Mesozoic sedimentary rocks and three sets of fault systems are observed in the mine, including nearly north-south-striking set, northwest-west-striking set and northeast-southwest-striking set,. The orebodies are obviously controlled by faults in different ore blocks and have no choice for special strata and lithology, simultaneously, the orebodies primarily occur in the fault zone and some in fracture system, cleavage and space of wall rocks collapse. There are breccia ore, massive ore and vein-like ore which indicate the filling epigenetic mineralization. The ore-hosting rock is sandstone and carbonate rocks. In different ore blocks, mineral composition and association of elements show diversity, such as, the main minerals include sphalerite, gratonite, galena, jordanite, realgar and orpiment, composing Pb-Zn-As-Sb elements assemblage in Liziping and Wudichang ore blocks; the main minerals include sphalerite, jordanite, galena, tetrahedrite series minerals, bournonite, argentite and kongsbergite, composing Pb-Zn-Cu-Ag elements assemblage in Fulongchang ore blocks; in Fulongchang ore blocks, the main minerals include tetrahedrite series minerals, chalcocite, chalcopyrite, jordanite, cobaltine, siegenite, cobalt-bearing arsenopyrite, galena, sphalerite, composing Cu-Co-As- Zn-Pb elements assemblage. The hydrothermal metallogenetic stage can be divided into two stages. Study on typomorphic mineral reveals that the deposits formed at moderate temperature or low temperature ore-forming environment, and it not related with the magmatism. The deposits have the multi-stage metallogenic characteristics, and ore-forming fluid is the arsenic-rich fluid.The fluid inclusions (FI) in Baiyangping deposit are small, and they have the form of round, oval, irregular shape, etc. Their sizes are generally less than 10μm and most of them are gas-liquid two phase inclusions. The ore-forming fluid system is Ca2+-Na+-K+-Mg2+Cl--F--NO3- brine system, mainly is the Ca2+-Na+-Cl-system. One origin of ore-forming fluid is evaporated sea water. The freezing temperature of fluid inclusions in mineral deposits ranges from -26.4 to -0.2℃, average -14.6℃; the homogenization temperature is concentrated in 120～180℃, and the salinity is between 0.35～24.73wt%(NaCleq), average 16.9wt%(NaCleq). The salinity of fluid inclusions in sphalerite ranges from 12.9 to24.2 wt%(NaCleq), average 22.1 wt%(NaCleq).Ore-forming fluid density is between 0.84 and 1.11g/cm3, average 1.04g/cm3. The density of ore-forming fluid in sphalerite FI is between 1.025 and 1.11g/cm3, average 1.09g/cm3; mineralization pressure is between 28.0MPa and 46.9MPa, average 37.6MPa, the corresponding mineralization depths is between 1058m and 2452m, average 1555m, concentrated in 1200m～1800m.Carbon and oxygen isotope data indicate a single carbon source, and the carbon in hydrothermal calcite is derived from the dissolution of carbonate rock strata. The ore-forming fluid belongs to the basin brine fluid system, which is mixed with the precipitate water.S isotope data show that sulfur in sulfides is derived from thermochemical sulfate reduction, or the thermal decomposition of organic matter. Sr isotope、Pb isotope and REE reveal that the mineralization material is from sedimentary strata or basement, and the mineralization has nothing to do with the magmatism. Pb data reveal some mantle source information, however, they are derived from the Triassic basalt or the base Permian basalt.The ore deposit mineralization age of Baiyangping is 30Ma～29Ma through sphalerite Sm/Nd and Rb/Sr dating. Comparing with the age of lead-zinc mineralizations in north section of "Sanjiang" region, because the collision first happen on the cape spot of the two mainland, Yushu area first receives the extrusion to distort, which mineralization age is older than Lanping area and Tuotuohe area. And the Tuotuohe area structure trail distorts weakly, and late, which metallogenetic epoch is the latest.Making research of the relationship between copper and lead-zinc of the mining area, the mining area possibly experiences two mineralization events. One is main copper mineralization event in early Eocene, Another is lead-zinc mineralization event in the early Oligocene, about 30Ma-29Ma. The late fluid of lead-zinc superimposed and reformed the early copper in the Baiyangping ore block which took shape the polymetallic style we can observe now. In entire "Sanjiang" region, except for a more than 1000 kilometer long lead-zinc mineralization belt, there is possibly a copper ore belt possessing similar scale with lead-zinc ore belt. The first stage of copper mineralization event corresponds to the Tibet Plateau main collision stage; the mineralization occurs in short time stress relaxation stage after intense extrusion in Eocene. But the lead-zinc mine occurs in stress relaxation stage after intense extrusion in the early Oligocene, which correspond the late collision transform stage of India-Asia continent-continent collision.Through the comprehensive analysis of all data, the lead-zinc (polymetallic) deposits in "Sanjiang" region are considered as a new type of deposit of sediment-hosted lead-zinc deposit, named as Three River Type Lead-and-Zinc (polymetallic) Deposit(TRT), and also its genetic model is given.