The Mineralization Of Cryogenian Datangpo Black Shale-hosted Manganese Deposit In The Adjacent Area Of Guizhou-Hunan-Chongqing

The adjacent area of Guizhou-Hunan-Chongqing province?termed as Qian-Xiang-Yu?is a concentrated region of black shale-hosted manganese in China,where a number of large Mn ore deposits formed during Nanhua Period have been discovered in recent years.The great progresses in exploration provide us an opportunity to study the mineralization processes and its relations with paleo-redox environment of seawater.The Mn mineralization in black shale may genetically associate with global climate change?evolution of glaciation to inter-glaciation?.The Mn deposits in the Qian-Xiang-Yu area occurr in the Nanhua Period Datangpo Formation as stratiform layers,which are enveloped by Sturtian glaciation and Marioan glacial beds.Although many studies have been down with respect to the age of glaciation events,it is still a hot debate for the boundary age of Qingshuixjiang and Tiesi'ao Formation that are underlain the Datangpo Formation.In addition,the origin of manganese,the processes of mineralization and redox conditions of seawater during manganese precipitation still not clear.In this study,we defined the age of tuff layer in Qingshuijiang and Tiesi'ao Formation using zircon LA-ICP-MS U-Pb dating technique,to better constrain the boundary age of these Formations.Analysis results show that the zircon U-Pb age of the Zhailanggou section in Wuluo area and the Yangtou section in Wanshan are752.7±1.8Ma and 746.7±4.9Ma,respectively.We therefore infer that the top boundary age of Qingshuijiang Formation is about 750Ma,which may represents the lower limit of Sturtian glaciation.The U-Pb age of the zircons from Wengnanxi-Xiajiapo section in Wanshan area is 727.1±4.4Ma,representing the bottom boundary age of Tiesi'ao Formation.Our study confirm that the boundary age of Qingshuijiang Formation and Tiesi'ao Formation is about 727Ma⁷50Ma.We present carbon and oxygen isotope data of manganese carbonate and Mn-rich black shales from the Gaodi Mn deposit in Tongren Regin,Guizhou province.Both?¹³C_V-PDB-PDB and?¹⁸O_V-PDB show negative values,from-11.5‰to-6.8‰and form-15‰to-3.9‰,respectively.In combination with previous published data,we conclude that there was a redox stratified structure of the Nanhua Ocean in the Guizhou-Hunan-Chongqing adjacent area.Mn-rich detrital component first fluxed into ocean and precipitated Mn?hydr?oxides under suboxic conditions.Consequently,dues to restricted basin condition of this area,seawater was changed to anoxic and lead to Mn?hydr?oxides converted to manganese carbonate.Such process was catalyzed by decay of organic carbon.