| The Guizhong area is an important metallogenic area of the Carboniferous manganese deposits in China.In recent years,manganese deposits have acquired important breakthroughs in prospecting,and the hidden manganese ore resources has been newly discovered,exceed 80million tons.However,research on the Carboniferous manganese deposits in Guizhong is lacking,and the key scientific issues involving the genesis,mechanism and mineralization of the deposits need to be further studied.Therefore,this paper takes the Carboniferous Longtou manganese ore in Yizhou,Guangxi as an example.Through detailed sedimentary,mineralogy,and geochemistry of the deposit,the redox conditions and their changes in the formation process of the manganese deposit are revealed,and the sources of manganese are analyzed.Comprehensively discuss the Carboniferous paleoenvironmental evolution,the control of glaciers on sea level,and the impact and control of the paleostructure and paleogeographic evolution of the Youjiang Basin on the manganese ore,and the metallogenic process of the Longtou manganese deposit is described.Provide scientific support for the study of mineralization and prospecting.The Longtou manganese deposit of Yizhou occurred in the Carboniferous Datang Stage.The ore body is in a narrow and long rift basins controlled by synsedimentary faults.The palaeogeography pattern shows carbonate platform marginal deposits.There are four layers of manganese ore in the mining area,of which the first ore layer is mainly gray-black and flesh-red manganese-bearing limestone?Mn ore?,the second ore layer is mainly gray-black manganese-bearing limestone?Mn ore?and a small amount of manganese-bearing muddy ash;the third ore layer is gray-white manganese-bearing limestone?Mn ore?with a small amount of siliceous rock;the fourth ore layer is mainly gray-black manganese-bearing limestone?Mn ore?,and the wall rock is mainly gray-black limestone.The ore structure is dominately massive,and some are band-shaped.Part of the ring structure can be detected in the microstructure.The manganese ores contain mainly calcium rhodochrosite,a small amount of rhodochrosite and pyrolusite.The trace element V/Cr,Th/U,V/Sc ratios of the samples,positive Ce anomalies,and the Mo EFand UEFcharacteristics discrimination diagrams,combining with the carbon isotope composition of the seawater in the Datang period,the paleogeographic background of sea level rise,mainly indicate that the metallogenic environment has successively experienced oxidation to anoxic environment.There are many minerals correlate with hydrothermal solution and their combined characteristics in the ore,such as pyrite,sphalerite,barite,siegenite and scheelite,etc.Combined with Fe-Mn-?Co+Ni+Cu?×10,lg U-lg Th,Co+Ni+Cu-Co/Zn and Cr-Zr genesis discrimination diagram,?La/Ce?Nand Y/Ho ratios,reflecting mineralization is relevant to hydrotherma systems.The 87Sr/86Sr ratios of the ore is slightly higher than that of the seawater of Carboniferous Datang Period,but are lower than the 87Sr/86Sr ratio of terrestrial detrital material,indicating that the strontium may be absorbed more in the formation of manganese ore in seawater.Combining geochemical indicators and paleogeography,the Longtou manganese deposit is located in a secondary faulted basin in the northeast margin of the Youjiang Basin with synsedimentary faults.The ore-bearing hydrothermal fluid carries Mn?II?into the basin through a fault or fissure to mix with seawater.In the early Carboniferous research area,the sea level of the restricted basin rose,the water body appeared redox stratification,and provided the power for the migration and conversion of manganese between the oxidized water body and the reduced water body.The precipitated manganese hydroxide under anoxic conditions experienced the reduction and re-dissolution in the early stage of diagenesis combined with the bicarbonate ion formed by the hydrolysis of marine carbonate rocks and the dissolved carbon dioxide in seawater,and finally the manganese carbonate deposits formed. |
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