Evolution Of Permian Rift Basin And Manganese Mineralization In Zunyi,Guizhou

Zunyi Manganese Deposit in Guizhou is a large-super large deposit formed during the Maokou（middle Permian）.It is one of the important manganese mineral areas and industrial parks in China.It is the first manganese deposit with industrial value found in Guizhou Province.This thesis focused on the Manganese-bearing Maokou Formation in the Zunyi area of Guizhou Province and studied the outcrop sections,drilling cores and typical deposits.By the methods of petrology,ore geology,geochemistry,sedimentology and petrography,fully considering the previous data in the study area,and aiming on the tectonic background of the mineralization,lithofacies paleogeography and tectonic paleogeography,the study identified the synsedimentary faults,reconstructed the evolution of the Northern Guizhou rift basin.With the additional works on the chronology and metallogenesis of the manganese deposits,metallogenic model and prediction model were established.This study is of great significance for the research of metallogenic theory and exploration of the manganese resource.It is a deep integration of the theory and practice.The Northern Guizhou rift basin was formed as a result of the uplift of the Emeishan mantle plume during the late Middle Permian to the early Late Permian.The development of the northern Guizhou rift changed the middle Permian paleogeographic pattern of the Guizhou area.The regional extension background is also an important prerequisite for the development of the rift basins on the previous carbonate platform.The northern Guizhou rift（grade I rift basin）is divided into two grade II（secondary）rift basins,Zunyi and Shuicheng,from NE to SW.In these late Maokou rift basins,synsedimentary faults are widely developed,forming a series of grabensand horst.According to the three kinds of criterias,including the thickness of strata,sedimentary lithofacies and structure of rocks,In this study,six synsedimentary faults controllingⅢbasins and several synsedimentary faults controlling fractional basins were determined in Zunyi area,two grade III horst basins and 13 grade IV graben basins.The activities of the synsedimentary faults largely affected the sedimentation and mineralization of the Maokou strata.The deep sourced hydrotherm fluid rich in Si and Mn rised along the synsedimentary faults,overflowed into the graben basin,and formed the manganese deposits.Therefore,the development of the northern Guizhou rift is an important controlling factor for the deposition of the Zunyi Manganese Deposit.The Tongluojing and Shenxi deposits are typical Maokou sedimentary manganese deposits in the study area,which are representative in strata,tectonics and ore characteristics.In terms of the sedimentary environments,three main types of sedimentary facies,platform,shelf and basin facies,were developed in the study area during the late Maokou.The manganese deposits were formed in the basin facies.The zircon U-Pb chronological analysis on the tuff collected between the manganese ore beds and the underlying carbonaceous siliceous limestone obtained an age of 266.4±3.2 ma.It constraints the metallogenic age of the Permian Zunyi Manganese deposit to the late Maokou period of Permian.The duration of the metallogenesis of the manganese deposits should be 3-5 million years.The formation time of the manganese deposits is about 5-10 Ma earlier than of the eruption of the Emeishan basalt.In fact,the possibility that the ore-forming material originated from the weathering of Emeishan basalt has been ruled out.It is the foundation for further study of basin formation,development and evolution.According to the petrographic paleogeography analysis,the paleogeography evolution of the study area during the Maokou was divided into six stages:the early stage of the rifting（P2m¹）-the rapid settlement period（early P2m²）,the stable settlement period（later P2m² stage）,the re rifting settlement period（early P2m³stage）-the silicification erosion stage（the middle stage of P2m³）-the whole uplift and denudation stage（the late P2m³）.The analysis of mineral characteristics,structures and geochemical characteristics of the manganese ores and the study of C-O isotopic geochemistry indicate that Zunyi Manganese deposit is hydrothermal deposit.In the graben basins of the Zunyi Manganese deposist,at least 5 hydrothermal spouts were found.The spouts consist of the central,transition and edge facies.The identification marks of the three manganese facies were defined,and the hydrothermal exhalative metallogenic model the manganese deposit was established.The manganese ore was mainly formed in a dys oxic environment.Preliminary discussion on the metallogenic mechanism of the manga nese ore supposed that the deep manganese-rich hydrothermal fluids rose along syndepo sitional faults and spilt into the graben basins,and generated the increasing Mn²⁺concen tration in the seawater.The continuous dysoxic conditions created environments for the combination of Mn²⁺and HCO3^-to form rhodochrosite.The cycle of this process result ed in the formation of the giant manganese deposits in Zunyi.The manganese deposits were supposed hydrothermal genetic and their metallogenic model was established.Based on the above research progresses,the prediction model for deep exploration of the Permian Zunyi“hydrothermal exhalative-sedimentary manganese deposit”,and the deep prospecting target area is delineated.In the three manganese ore belts of the Zunyi Manganese ore sub-belt,13 prediction areas can be delineated.Among them,there are 3 class A prediction areas,5 class B prediction areas and 5 class C prediction areas.In the 13 prediction areas.there is a total of 157 million tons of newly added potential resources.In view of the selected target areas,the verification of prospecting target areas has been carried out,and thick and rich manganese ore bodies have been revealed.The theory and technical methods have a better guiding significance for ore prospecting and exploration in the study area.