ORIGIN AND CHARACTERISTICS OF SEDIMENTARY FACIES OF SINIAN MANGANESE DEPOSITS IN GUIZHOU

Sinian sedimentary manganese carbonate deposits are a main type of manganese deposits in Guizhou.The deposits in stratoid and Ienticular form lie at the bottom of black carbon and silty claystones overlain by the lower Sinian Nantao formation.They are located at the same horizon as that of Xiangtan's manganese deposits. The manganeous horizon is widely distributed R-ith distinct boundary and stable form in the east of Guizhou,which may be compared with the same horizon of neighbouring provinces.It represents a specific geological environment in geological history.Therefore.it may be named" Datangpo Formation".The Datangpo Formation is a series of claystonc rich in organic carbon and fossil algae with distinct rhythmic laminae and tide-beddings.They belong to the sediments formed in the middle stage of the Early Sinian, i.e. sapropel—clay(algae-sheet fades and nearshore environment dominate)—silt. In the geological history there existed a process of development from Dawu glacial age to Datangpo interglacial age in the east of Guizhou during the middle stage of the Earl- Sinian epoch. The general configuration originated by the glacier in the Dawu glacial age,which hadn‘t undergone severe denudation.controls the distribution and character of sedimentary- facies of the Datangpo age.Then its paleogeographic environment may be divided into three facies zones and eight sedimentary facies,that is,continental,transitional and marine facies zones.The continental facies zone,composed of Sichuan一Guizhou old land and characterized by denudation,is mainly distributed in the centre of Guizhou.These sediments occur locally in the depressed basins and valleys and form the sediments of inland lake and alluvial facies.The alternating section of continent and sea with unstable environment forms a narrow transitional fades zone. They are mainly composed of river delta.shoreline sand flats—sand bar and Shoreline mud flats facies and develop in the areas such as Yinjiang. Jiangkou.Yubing. Sansui, Tai jiang and Sandu Counties.Marine widely distributed in the east of Guizhou.The major facies zone of rhodochrosite deposits.in particular the sapropelic algac-sheet facies is the direct sedimentary area of the rhodochrositc deposits.In addition,there are the sediments of shallow sea facies.which may be divided into two kinds restricted nearshore shallow sea facies and open shallow shelf sea facics.According to their characteristics and tectonics,the former occurs in the northeast of Guizhou and the latter the southeast of Guizhou.On the basis of texture and structure of the rhodochrosite ore and characters of associated mineral MnCO3,which may be considered as the immediate deposition of Mn(HCO3)2.Clay mineral associated with rhodochrosite is derived from the denuded areas of old land and is the result of the precipitation of suspended matter.Origin of rhodochrosite ore is w ith close relation to the activities of algae.Because of the specific geological condition in the late Precambrian,marine water rich in CO2 can mane a great deal of Mn2+ exist in it. In the sapropelic algae-sheet environment. a great amount of CO2 are absorbed by the algae photosynthesis.MnCO3 may immediately precipitate in the lower part of the sapropelic algae--sheet facies.After the diagenesis they become the present carbonaceous rhoclochrosite bed.The favourable facies of rhodoc:hrosite deposits is widespread at Xiushan,Songto,Wanshan,Tianzhu,Taijiang and Sandu Counties.