Geochemical Characteristics And Development Mechanism Of Gypsum (rock) Minerals In The Jinding Baicaoping Strontium Deposit In Lanping, Northwestern Yunnan

The Baicaoping Sr deposit is located at the SE of the Jinding Pb-Zn ore field in Lanping basin,developing multi-type,multi-layer gypsum rock,while the sulfide mineralization is rare.Due to the complex and diverse sedimentary assemblage,tectonic deformation and polymetallic mineralization patterns,the strata belonging is always in dispute in the area,caused the temporal and spatial relationship between the ore-bearing strata,tectonic deformation and gypsum rock and the polymetallic growth mechanism of the Baicaoping deposit has not yet formed a complete understanding.The Baicaoping area is ancient highland of the Jinding ore field,with the fearture of"west high,east low".There are some records of the synsedimentary thrusts in the period of Jingxing and Yunlong preserved in the region.However,Previous studies seldom contact the development of gypsum rock with the sedimentary strata and tectonic evolution of the basin,and the latest data are not taken into account.These problems strongly restrict the understanding of the Jinding lead-zinc ore field tectonic evolution.The author choose the Baicaoping Sr deposit as the research area,combined with 3 route observation of outside area,geological mapping and exploration drillings observation in the area,carried out detailed studies about the stratigraphic sequence and gypsum rock growth mechanism,has achieved the following new understanding:(1)The formation of the Baicaoping Sr deposit is mainly composed of Jingxing group(K1j),Nanxin group(Kin)and Yunlong group(E1y),there are unconformity contact between the three.The Jingxing group and the lower of the Yunlong group is the host rock of the gypsum rock.(2)Three period synsedimentary faults are reconized in the Baicaoping area:synsedimentary normal fault in the early of Nanxin period,synsedimentary thrust fault in the late of Nanxin period,synsedimentary thrust fault in the late of Yunlong period,respectively.(3)Gypsum can be divided into breccia anhydrite rock,laminated gypsum rock,fibrous gypsum,massive gypsum and selenite according to the.Breccia anhydrite rock formed in the Jingxing deposional period and laminated gypsum rock formed in the Yunlong deposional period,fibrous gypsum formed in the early stage of hydrothermal period and massive gypsum and selenite formed in the late stage of hydrothermal period.(4)The result of XRD indicates that the massive gypsum and breccia anhydrite rock have different phase compositions,and transition between hydration and hardening.(5)The geochemistry of gypsum shows the brine of Yunlong period is the source of the Sr,while the brine of Yunlong period and the limestone is the source of the Ba in the hydrothermal gypsum.(6)The sulfur isotopic geochemistry shows that the sulfur of breccia anhydrite rock in the Jingxing group and the massive gypsum originate from H2S in the upper Triassic strata,the sulfur of the laminated gypsum rock and fibrous gypsum originate froru lower Paleocene strata.(7)The massive gypsum and celestite are formed at the same time,the oxidized brine riching in Ca2+and metal ions mixed the reductive fluid in the giant limestone block and sand-bearing limestone breccia with high porosity and precipitated,indicating the same mechanism in the formation of Pb-Zn deposit in the Jinding.The tensile fracture,ancient cave and collapse breccia body is an important prospecting interface.