Genesis Of Euhedral Pyrites In The Upper Cretaceous Mengyejing Formation In Simao Basin,Yunnan And Its Geological Significance

There are many intra-continental basins with evaporite series in SE Asia,and so many potash salt deposits are located in these basins,such as the Simao Basin.The Simao Basin is located in the southeast part of the Tethyan metallogenic belt.The Upper Cretaceous Mengyejing Formation is widely distributed in this region containing evaporite successions.The Mengyejing Formation is currently the only pre-quartenary solid potash deposit in China.The formation of this group is mainly brownish red and mixed-colored mud conglomerate.Due to the closure of tethy ocean and the impact of complex plate collisions,the Simao basin has a strong tectonic activity and many large metal deposits.The genesis of pyrite can be divided into syngenetic sedimentation pyrite,biological function pyrite and pyrite formed by hydrothermal action.For the genesis of pyrite in the salt series of the formation,many scholars believe that pyrite formed by the formation of pyrite or biological action.Occurrence in background of gypsum and pyrite crystals are currently pyrite octahedron,pentagonal dodecahedron dense output of good crystal shape,such as the phenomenon has not been reported in the Simao basin,the cause of this kind of symbiosis with gypsum pyrite remains to be seen.Therefore,this study uses the geochemical analysis of research methods to explore the problem cause Mengyejing Formation pyrite Simao basin,determining the Simao basin in late Cretaceous Mengyejing Formation major trace elements of the formation of pyrite samples,samples of gypsum and pyrite sulfur isotope,strontium isotope samples distribution,determining the pyrite genetic types,the source of sulfur in pyrite,the pyrite formation mechanism and its effects on potassium ore.Analyses of major and trace elements of euhedral pyrite crystals and sulfur isotopes of gypsum and pyrite of the Mengyejing Formation in the Jiangcheng County were performed by electron probe micro-analyzer?EPMA?and mass spectrometer.The results show that the n?S?/n?Fe?value of pyrite is 2.02,showing that it belongs to sulfur concentration type pyrite.The??Co?/??Ni?values range from 3.01 to 10.74.The?³⁴S_V-CDT-CDT values of gypsum are in the range of 6.6‰¹0.7‰averaging 9.48‰,while the?³⁴S_V-CDT-CDT values of pyrite are in the range of-8.36‰^-3.36‰with the average-6.33‰.The values of S/Fe,Co/Ni,sulfur isotope suggest that the pyrites of the Mengyejing Formation in the Jiangcheng County was formed by hydrothermal processes.The hydrothermal alteration causes the gypsum to be reduced to form S^2-.The S^2-combined with Fe from the hydrothermal fluid and form the granulated pyrite.Through the study of the genetic mechanism of pyrite and gypsum of the Mengyejing Formation,it comes to the conclusion that the Late Cretaceous primary evaporites of the Mengyejing Formation might be experienced obvious alterations by deep hydrothermal fluids accompanied by several kinds of metals,which is controlled by the Indian and Asian collision.The hydrothermal activities in the Simao basin recrystallized the potash minerals of the Mengyejing Formation,but may have favorable contribution to metal deposition.