Mineral Geochemical Characteristics And Metallogenic Mechanism Of The Baiyangchang Sandstone Cu-Ag Deposit In Yunlong, Western Yunnan

Baiyangchang sandstone Cu-Ag deposit is located in the southern-middle section of Lanping Mesozoic-Cenozoic basins in Yunlong county,northwest Yunnan.There are a large number of vein type Cu-Ag deposits with different scale.According to previous workers the Baiyangchang deposits,Ore formation,geological and geochemical characteristics and source of mineralization that the Cu-Ag veins are belongs to sedimentary alteration or stratified deposits.The Recent exploration has been a major breakthrough which resulted a large deposit.Systematic observation of open pit slope and combining with the exploration drilling and tunnel geological logging data analysis at Baiyang ore section show that,Cu-Ag ore bodies in the region are disseminated,massive and vein produced in K2h sandstone,the output are strictly related to the same depositional fault-related to lithologic interface control.According to the mineralization level,it can be divided into five types of mineralization zones compareing with the typical SSC type deposits.In addition to I ore body with light purple interaction and weak mineral zoning,the sandstone Cu-Ag deposit is characterized by fault control,mineralization_alteration zonation is weak,but the development of strong baritization which showed significant differences in characteristics.Therefore,it is possible to effectively guide the prospecting and exploration work of this area and the surrounding area by revealing the mechanism of mineralization and enrichment.In this paper,based on the regional geological survey,and detail geological map and exploration history of mining area,the Cu-Ag ore bodies I,II and III were selected for this research study.The detailed study on geology,mineral geochemistry,fluid inclusions and S isotopic geochemical characteristics of Baoyang sandstone Cu-Ag deposit is carried out to identify its metallogenic geological environment,ore-forming fluid characteristics.The sulfur isotopic composition and the variations of fluid inclusions in the metal sulfide and sulfate minerals,which revealedthat,the control mechanism of mineralization,and provide a basis for inversion of ore-forming fluid evolution process and reasonable ore-forming model.The main achievements are the followings.1?The strata exposed in the area are the nanxin formation(K2n),hutousi formation(K2h)to upper Cretaceous and yunlong formation(Ey)of Paleogene.The K2h rich carbon sandstone is the main ore layer,among them,K2n,K2h and Ey are all developed with multiple sedimentary soft deformation structure,including small folds,depositional faults,sandy and muddy and pillow structure;And the sedimentary folds at the top and bottom Ey of K2h show that the fault is synchronized with Cu-Ag mineralization.2?The regional and mining strata and tectonics are the result of the continuous effect of multi-stage extrusion stress,which has resulted the Cretaceous foreland basinfrom the Late Cretaceous to late Paleocene basin(early fold and fault composition of the thrust belt),and the late Paleocene-early Eocene(In late Eocene)the tectonic uplift formed(fold and fault composition).The structure of the Baiyangchang ore section is mainly characterized by the characteristics of closed anticline(local inversion)of the width of slope,and the development of the fault(longitudinal fault)and the secondary strike-slip fault(transverse fault).Taking Baiyangchang ore body as an example,in the lateCretaceous-the early Paleocene period,the continuous squeezing leads to the development of the thrusting belt,In thenorth-south F1 syn-sedimentation fracture(zone),the formation of the northwest to the west side of the closed oblique tilt and the east side of the diagonal parallel to the parallel structure of the grid,which strictly controls the mineralization of the Cu-Ag deposit in the sandstone area and the main mineralization period;The K1-2n and K2h or K2h thrusts over the Ey,resulting the local fractures of both direction the fault zone including FI fault.There is secondary mineralization can be observed.3?The main metal sulfide in the ore has chalcopyrite,bomite,tetrahedrite,pyrite,etc;sulfate minerals are barite and gypsum.4?The characteristics of the sulfur isotopic composition of the metal sulfide show that the sulfur isotope mainly occurs in the BSR process,and at the same time in the metallogenic period also occurred TSR process characteristics.5?Pb isotopes indicate that lead mainly from the crust.but there are characteristics of mantle lead migration.6?The characteristics of the fluid inclusions of copper sulfide symbiotic barite show that the ore-forming fluids are medium-low temperature(119.3-277.8 ?)and low salinity(4.3-15.4wt%NaCl).As the mineralization progresses,there may be new fluids formed in the early stage.7?It is concluded that the Baiyangchang Cu-Ag deposit is a typical SSC type controlled by the trapezoidal structure composed of the faults and folds.It can be classified as a sandstone sub-type in the SSC deposit,Early(E1-2),It is determined that the syn-sedimentation deformation structure is an important ore controlling factor,which can be used as the basis for prospecting and resource evaluation in mining area andsurrounding area.