Research On The Source And Metallogenic Mechanism Of Permian Manganese Deposits In Southern China And Northern Guizhou-eastern Yunnan

Manganese(Mn),a strategic key metal element,is one of the most important constituent elements in the Earth's crust.Throughout the world,sedimentary Mn deposits are the most economically valuable and potential Mn deposits.Meanwhile,Mn is a redox-sensitive element with multiple valence states,while the formation of sedimentary Mn carbonate as the result of the interaction of Earth's various spheres,which plays a vital role in the inversion of the evolution of Earth's various spheres and paleoenvironment.The Permian large-scale sedimentary Mn deposits located in the northern Guizhou and eastern Yunnan area,South China,were formed in specific sedimentary backgrounds developed by major geological events at the turn of the Middle and Late Permian.In this paper,we studied the typical Mn ore sections from northern Guizhou to Eastern Yunnan Mn-ore field and presented new sedimentological,mineralogical,whole-rock geochemical,and isotopic data.From the perspective of sedimentary geological characteristics of Mn deposits,deposits genesis,and evolution of the sedimentary environment,as well the dynamic co-evolution relationship between Mn mineralization process and coeval major geological events is established.The aim is to deeply understand the causes and main controlling factors of remarkable mineralization differences in the Mn ore fields of northern Guizhou and eastern Yunnan,and then try to elucidate the mineralization mechanism of various Mn deposits in the region.Based on the above-mentioned studies,the main insights obtained in this paper are as follows:(1)Permian Mn deposits from northern Guizhou to eastern Yunnan have Significant differences and regularities in the spatial-temporal distribution and mineralization types.Specifically,the ore type is characterized by the transition from Mn oxide ores(Gexue area,eastern Yunnan)?Mn oxide ores and Mn-bearing limestone(Shuicheng and Nayong area,western Guizhou)?Mn carbonate ores(Zunyi area,northern Guizhou),and the Mn metallogenic-scale also transition from small-medium to large-medium scale.The Permian Mn deposits from northern Guizhou-Eastern Yunnan are all restricted distributed within the Qianzhong basin,and the Mn ores of northern Guizhou at eastern Qianzhong basin are hosted in the carbon-silica-mud-limestone of third sections of the Maokou Formation,mainly medium-large Mn carbonate deposits with higher ore-grade and thin thickness(2?7.5m).While the Mn ores of western Guizhou-eastern Yunnan at western Qianzhong basin are hosted in the carbon-silica-limestone of second sections of the Maokou Formation,and mainly medium-small Mn oxide ore deposits formed by secondary weathering of Mn-bearing limestones with lower ore-grade and large thickness(10?20m).(2)The Mn ores in the Zunyi area are closely symbiotic with tuffaceous clay rocks,and the Mn ores develop the exhalation sedimentary deposit structures(e.g.,brecciated structures).In Nayong and Zunyi Mn ores,meanwhile,a large number of Mangano calcite and Mn oxide veins(e.g.,veins,stockwork,and disseminated)are filled or crosscutting,and numerous hydrothermal alteration minerals(e.g.,wollastonite and ankerite)are presented in these Mn-bearing veins.In addition,a large number of magmatic-genetic mineral assemblages(e.g.,pyroxene,anorthite,plagioclase,hematite,Johannsenite,etc.)and hydrothermal-genetic mineral assemblages(e.g.,pyrite,pyrrhotite,chalcopyrite,galena,sphalerite,siegenite,molybdenite,barite,rutile,and monazite,etc.)were first found in Mn ores from western and northern Guizhou,and part of the above minerals contain a certain amount of Mn(e.g.,pyroxene,calcium feldspar,plagioclase,etc.).The Mn ores in Zunyi and Nayong Mn deposits are characterized by chalcophile and siderophile element enrichments(e.g.,V,Co,Ni,Cu,Pb,Sb,Sr,Mo,and U,etc.).Both the elemental ratios and discriminant diagrams of Mn ores indicate that the ore-forming materials originates from the contribution of hydrothermal fluids,and the REE differentiation features of Mn ores are similar to those of tuffaceous clay rocks in ore-bed and typical hydrothermal Mn ores.Meanwhile,the initial ⁸⁷Sr/⁸⁶Sr ratios of Mn ores from the Zunyi area indicate that the ore-forming materials are mainly derived from the contribution of Sr in the magma-hydrothermal system.Integrated mineralogical and geochemical signatures indicate that the ore-forming materials were originated from hydrothermal activities related to early-stage Emeishan Large Igneous Province(ELIP)subaqueous volcanism.In addition,the Mn carbonate ores in Zunyi and Nayong Mn deposits have significantly negative?¹³C_V-PDBvalues,and the enrichment degree of Mn O is negatively correlated with?¹³C_V-PDB value and TOC content,indicating that the involvement of organic matter in the diagenetic transformation of Mn oxides is an important mechanism for the formation of Mn carbonate ores.(3)The Qianzhong restricted basin is formed by the tensional differentiation of the earth's crust and constitutes the redox stratified water environment and favorable precipitation site,which required for the formation of sedimentary Mn carbonate ore.However,the crustal uplift and global large-scale regression caused differential evolution of pale-ocean environments(e.g.,the restricted degree of the basin,paleo-depth,and paleo-redox conditions,etc.)at the eastern and western Qianzhong basin,which mainly controlled the precipitation and mineralization of Mn ores at the eastern and western basin.Integrated sedimentary sequence,petrography,sedimentary facies,and redox-proxies evolution of the typical Mn ore sections from northern Guizhou-eastern Yunnan reveal that the Mn-bearing rock series(eastern Yunnan-western Guizhou Mn ore fields)were formed in the oxidized water body with strong circulation in the western basin.The formation of Zunyi Mn ores(northern Guizhou),however,evolved from euxinic-anoxic(footwall deposition),through anoxic-suboxic(Mn mineralization),to oxic(hanging-wall deposition).(4)Mantle uplift caused the formation of Qianzhong basin(restricted basin),and the development of rift system(syn-sedimentary faults)and magmatic-hydrothermal activity in the basin provided the ore-forming materials,migration channel and precipitation site for Mn ore formation,and constituted the"source-migration-sink"system necessary for Mn mineralization.The differential sedimentary mineralization of Permian Mn deposits in northern Guizhou-eastern Yunnan is mainly controlled by the differential evolution of the paleo-oceanic environment at the eastern and western edge of the Qianzhong basin,which is caused by the differential uplift of the crust and the global large-scale regression.(5)The process of Permian Mn mineralization from northern Guizhou to East Yunnan can be divided into three stages:(i)The uplift of the mantle plume led to the formation of the Qianzhong rift basin developed by the fracture system and subaqueous magmatic-hydrothermal activities,and the redox stratified seawater system developed in the basin.The rift basin mainly developed silica-rich magmatic-hydrothermal activity,and formed deposits of carbon-silica limestones and siliceous rock at the base of the Mn ore-bed during the early stage of mineralization.(ii)Under the uplift of the mantle uplift causing differential uplift of the crust and the global large-scale regression,the paleogeography and paleo-ocean environment in the basin has undergone significant changes.Subsequently,a great deal of Mn²⁺released by magmatic-hydrothermal activity enters the basin through the syn-sedimentary faults,which is rapidly precipitated with oxidation(without initial enrichment)in the oxidic water-body at the western basin.Yet the Mn-bearing limestone deposits with thicker layers were formed since the Mn²⁺concentrations could not be reached to the saturation of Mn carbonate in sediments.In contrast,Mn was initially enriched in form of Mn²⁺in the reducing water-body at the eastern basin.(iii)Following the superposition of continuous crustal uplift and the global large-scale regression,the western basin was uplifted to terrestrial and shallow-water platform facies,respectively,and the Mn-bearing limestones underwent weathering and formed secondary Mn oxide ores.Nevertheless,the eastern basin began to be obviously influenced by the continuous decline of redox interface and the enhanced water circulation,which led to the large-scale oxidation of Mn²⁺initially enriched in reduced water-body,thus forming Mn(hydroxide)oxide and organic matter to precipitate and bury,and then transformed into Mn carbonate ore-bed with thin thickness during sediment diagenesis.