ORIGIN AND CHARACTERISTICS OF SEDIMENTARY FACIES OF SINIAN MANGANESE DEPOSITS IN GUIZHOU
Liu Xunfeng Hu Zhaorong Zeng Lixun Zheng Guangxia Wang Chengyuan ( Institute of Guizhou Geological Bureau )
Sinian sedimentary manganese carbonate deposits are a main type of manganese deposits in Guizhou. The deposits in stratoid and lenticular form lie at the bottom of black carbon and silty claystones overlain by the lover Sinian Nantao formation. They are located at the same horizon as that of Xiangtan's manganese deposits. The manganeous horizon is widely distributed with 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 claystone 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 facies 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 Early 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-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 facies 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. Taijiang and Sandu Counties. Marine facies zone is widely distributed in the east of Guizhou. The major facies zone of rhodochrosite deposits, in particular the sapropelic algae-sheet facies is the direct sedimentary area of the rhodochrosite 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 sen facies. 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 MnCO_3. which may be considered as the immediate deposition of Mn(HCO_3)_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 with close relation to the activities of algae. Because of the specific geological condition in the late Precambrian, marine water rich in CO_2 can make a great deal of Mn_2 exist in it. In the sapropelic algae-sheet environment, a great amount of CO_2 are absorbed by the algae photosynthesis. Mn CO_3 may immediately precipitate in the lower part of the sapropelic algae-sheet facies. After the diagenesis they become the present carbonaceous rhodochrosite bed. The favourable facies of rhodochrosite deposits is widespread at Niushan. Songto. Wanshan, Tianzhu. Taijiang and Sandu Counties.