Genesis And Sedimentary Environment Of The Santonggou Sedimentary Manganese Deposit,Qinghai Province

The Santonggou manganese deposit is located in the southeast margin of Qaidam Basin in East Kunlun orogenic belt,and in the subduction-accretionary complex belt of South Kunlun Terrane.The discovery of the deposit is an important breakthrough in manganese prospecting in east Kunlun area.In this study,the Santonggou manganese deposit and its ore-bearing rock series are taken as the research object.On the basis of detailed field geological investigation and petrographic study,by using geochronology,mineralogy,geochemistry and isotope methods,the metallogenic age of the Santonggou manganese deposit is defined,the source of the metallogenic material,the paleo-ocean sedimentary environment and the diagenetic transformation process are deeply studied.Futhermore,the relationship between Santonggou manganese deposit and paleo-marine sedimentary environment and its formation mechanism are discussed,so as to establish the metallogenic model of the Santonggou sedimentary manganese deposit.Geochronology study shows that the detrital zircon U-Pb ages are ranging from 1497±17 Ma to 427±4 Ma,with two peaks of 441 Ma and 473 Ma.The youngest detrital zircon age is 427±4Ma,indicate the age of the Santonggou deposit is later than 427 Ma,and indicate that the detritals sources are the middle strata of Naji Tal group.According to the discriminant diagrams of major and trace elements and Mo/U ratio analysis,the Santonggou manganese deposit formed in a backarc restricted basin environment.The rock assemblage of the northern and southern ore block represents a regressive sequence,indicate the extinction of the basin.The rare earth elements（REY）patterns of Santonggou deposit samples characteristic as light REE depletion,positive Ce anomalies and negative Y anomalies（δCe=1.36,δY=0.87）.The whole rock geochemistry and REY results indicate that Santonggou deposit precipitated under an oxic water column.Manganese ore is mainly composed of rhodochroite and manganese calcite,the replacement of pyrolusite by rhodochrosite suggests that the initial Mn sediments was Mn（III/IV）（oxyhydr）oxide.The Mn carbonate ores characteristic significantly negativeδ¹³C(Mn oreδ¹³C=-17.85%,wall rockδ¹³C=-7.10%),suggesting that the Mn carbonates formed as a result of Mn（III/IV）（oxyhydr）oxide reduction by microbial respiration.The studies of diameter size of framboidal pyrites（d=2.5～22μm）indicate that the precipitation of Mn carbonate ore may have occured below sediment-water interface.Based on the results of chronology,petrography,and geochemistry,the mineralizing process of sandonggou manganese deposit is finally discussed:（1）The south Kunlun ocean subducted northward during Ordovician to Silurian,which resulted in the formation of back-arc restricted basins and provided abundant dissolved Mn（II）as ore-forming materials.（2）The continuous subduction resulted in the extinction of the back-arc basin and the lowering of sea level,and changed the redox conditions of the basin which brought out the precipitation of dissolved Mn（II）in the form of Mn（III/IV）oxides.The increase of oxygen content also leads to the explosion of organic matter,the organic matter and Mn（III/IV）oxides are buried together.（3）Microbial reduction of organic matter under the water-rock interface reduced Mn（III/IV）oxide to Mn（II）,which caused the enrichment of dissolved Mn（II）in pore water,finally caused the precipitation of manganese carbonate ore.