The Sedimentary Environment Of Xiangmeng Formation And The Formation Mechanism Of Xiangtang Manganese Ore Deposits,South China

Datangpo Formation mainly includes manganese carbonate,black shales,gray shales and gray siltstone in the Yangtza platform.Datangpo type manganese ore deposit is hosted in black shales in the basal of Nanhua(Cryogenian)Period Datangpo Formation with rhodochrosite as the main ore-bearing mineral,which is one of the most important manganese resources in China.Well preserved Datangpo Formation makes it record the evolution of the ocean well.In this dissertation,through element geochemistry study of black rock series and rhodochrosite of Datangpo(Xiangmeng)Formation in the Hu'nan,having the following main achievements:1.The redox sensitive trace elements enrichment state,Th/U,V/(V+Ni)and V/Cr ratios of the Xiangmeng Formation black shales indicated the whole stratigraphic is a sub-oxic to oxic depositional environment,compared to the early-to-mid,the late of Xiangmeng Formation environment become relative reduction.2.The CIA of the black shales indicated a medium weathering of warm wet conditions,with a clear change:warm wet(early)-relative cold and dry-warm wet-relative cold and dry.3.The major elements and redox sensitive trace elements couples indicate sub-oxic to oxic depositional environment,unsupported the Mn2+and CO32-direct synthesis of rhodochrosite and precipitation.The positive Ce anomalies,high(REE+Y)contents and PAAS-normalized REE distribution patterns display depletion in light and heavy REE and relative enrichment in middle REE indicate Mn may be in the form of oxide or hydroxide precipitation..4.The inorganic carbon isotope compositions of manganeses ores range-9.3 8‰?-5.79‰with a mean of-7.58‰ in Xiangtan and Minle,indicating part of carbons were of organic orgin.These deposits were initially precipitated as Mn-oxides or hydroxides in the sub-oxic to oxic depositional environment,buried in the environment of the anoxic with organic matter.The Mn-oxides or hydroxides reacted with organic matter and became rhodochrosite.5.The manganese ore deposits mainly distribute at restricted shelf basin,with the marine transgression,Mn aggregated in the basin.Meanwhile,Mn precipitated as Mn-oxides or hydroxides in the sub-oxic to oxic depositional environment of upper water,then buried by organic matter at anoxia diagenetic environment.Reduction of manganese oxides and hydroxides release large amounts of Mn2+,and the degradation of organic matter release a lot CO32-with light carbon isotope.Rhodochrosite with light carbon isotope was formed by Mn2+and CO32-.