| During the Neoproterozoic time,a series of major geological events occurred in the world,including super-continental pyrolysis,global glacier deposition,important reproductive stages of multicellular organisms,ancient plate and paleoclimatic,and other major scientific problems.The"Datangpo-type"manganese ore,located in Yu-Xiang-Qian area,is of great concern worldwide.The"Datangpo-type"manganese ore formed during the interglacial period that between Marinoan glacial and Sturtian glaciation."Datangpo-type"manganese mine has been discovered since the last century,it had rapid progress in exploration,and it has gradually become the largest potential manganese in China.The"Datangpo-type"manganese ore is produced in a set of black manganese-bearing rocks at the bottom of the Datangpo Fm.The black rock series consists mainly of black carbon shale,manganese-containing shale odochrosite,tuff.And the thickness of the manganese ore body is positively correlated with the thickness of the black manganese series.The spatial distribution of the manganese-bearing rock series is controlled by the regional structure,especially in the center of the graben basin.The study of"Datangpo-type"manganese ore has mainly focused on the geological characteristics,mineral composition and paleoenvironment of the deposit.In recent years,the geochemical methods have been applied in the research of manganese ore genesis,mineralization environment and material source.The study of the formation of manganese ore has made some more significant results.The controversy over the mechanism of manganese ore has always been the existence of biological genesis,hot water genesis,biochemical genesis,volcanic eruption and natural gas leakage causes which are put forward recently.Based on the comprehensive analysis of the geological survey of the 607geological team in Chongqing and the latest research methods of geological geochemistry of the deposit,this study focuses on the manganese ore and the top and bottom black shale in the Datangpo Fm,including the main elements,trace elements,rare earth elements and isotopic characteristics analysis,to explore the manganese ore mineralization environment,material sources,ore-controlling factors and the relevant evidence of hot water genesis.The main contents of this research include the following aspects:Geological characteristics of manganese deposits:By comparing the profile in the study area drillings and sedimentary basin analysis,that the manganese bearing rock series of things to change,the North-South change obviously,the thickness change is not remarkable,There is a significant positive correlation between the thickness of manganese deposits and the thickness of manganese bearing rocks.Manganese ore is distributed in the center of graben basin with NE/NEE trend,and contains no ore in the uplifted area.?1?The characteristics of the ore bearing strata are located in the lower part of the manganese bearing rock series.Generally,they are composed of 1-6 layers of siderite manganese ore bodies.The ore bodies are bedded in shape,and the lithologic facies of the non ore areas are changed into silty mudstone and sandy dolomite with rhythmic increase.?2?The orebody characteristics,ore deposit type manganese rhodochrosite ore,with lenticular mostly for stratiform,lenticular bodies on both sides of the middle thick thin,the edge is wrapped,outward pinch;single ore body is small,the arrangement in the interior of the main echelon,fish shaped,poor shape,time half overlap shape,shape and imbricate overlap.?3?Characteristics of the ore:the structure is mainly composed of clay crystal structure,micro granular structure,micro scale structure,a small amount of hidden crystal structure,silt structure,metasomatic dissolution structure and inclusion structure.The structures are mainly block structures,layered structures and sand chip structures,and then are banded and lenticular structures.Main element characteristics:The manganese ore in the Datangpo Fm is divided into three groups by means of R-type cluster analysis,The first group is Al2O3?K2O?Fe2O3?SO3?SiO2?TiO2,The second group is Na2O,The third group is CaO?P2O5?MnO?MgO.the MgO/CaO and CIA are used to determine the relative dry heat of the sedimentary environment,and according to the value,we can estimate the seawater deposition environment is shallow sea environment.Through the element correlation diagram,the element correlation matrix table to determine the material source of Mn for the multi-source,mainly to the deep source.?Al2O3+Fe2O3?,Al/?Al+Fe+Mn?and Na2O/K2O-SiO2,Fe/Ti-Al/?Al+Fe+Mn?diagram were used to determine the depositional environment as the continental origin.The elemental correlation matrix was used to determine the source of Mn.The average value of SiO2/Al2O3 ratio is4.67,indicating that there is hot water involved in the material source,Mn/Fe<1indicates the deposition jet type.Trace element characteristics:The content of Ag,Sb,As,Mo,Ba,U,V in the landmark hot water sediments are generally high.The Sr/Ba ratio indicates that the deposition environment of manganese ore may be in a sea-land transition environment,and the terrestrial environment is more obvious.The U/Th ratio,Ni/Co ratio,V/Cr ratio,V/?V+Ni?and V/?Cr+V?ratio of the manganese ore formed in the hypoxic environment,manganese ore Co/Zn ratio close to the proportion of hot water,manganese ore Co/Zn-??Co+Ni+Cu?diagram and?Cu+Ni+Co?×10-Fe-Mn diagram,indicating that the material source has hydrothermal involvement.Rare earth element characteristics:selected North American shale standardized drawing rare earth distribution pattern,?REE average 195.39×10-6;light rare earth LREE average of 171.53×10-6;heavy rare earth HREE average of 23.86×10-6;LREE/The average value of?Ce is 1.06,which indicates that the formation of manganese ore is related to hot water.The La/Ce ratio of La/Ce-Al2O3/?Al2O3+Fe2O3?is higher than and the La/Yb-Ce/La diagram shows that the formation of La/Yb-Ce/La is related to the hot water in the Ce/La reaction depositional environment of the manganese ore.Isotope characteristics:the C isotope of manganese ore has a significant negative bias,the average value of-6.4‰;with the characteristics of submarine hot spring,using O isotope ancient temperature speculation formula to calculate the Datangpo group of manganese ore water temperature of 62.47?,the reaction may have the participation of hot water;rhodochrosite?34S average value of 48.0‰,with a Partial positive anomaly,indicating the formation of manganese ore in the closed-semi-closed confined environment.In summary,through the comprehensive analysis of sedimentary geochemistry,that the source of ore-forming materials in hydrothermal fluid,at the same time,there is a small amount of terrigenous manganese eroded after entering the old sea basin;tectonic environment of manganese bearing rock series of the passive continental margin;manganese ore formed in the bottom of the basin in the anoxic environment.The PAAS REE distribution curve and trace element characteristics of manganese ore samples show that the genesis mechanism of manganese ore is different from that of normal sedimentary marine carbonate.The formation stage can be divided into four stages,The first stage is the enrichment stage of Mn2+in anoxic anoxic water;The second stage,stratified seawater,upper oxidized zone MnO2 supersaturated precipitation stage;the third stage,short-term bottom events will precipitate into MnO2 oxidation at the bottom of the bottom of the basin,with a large number of temporary oxidation enrichment of MnO2 precipitates;the fourth stage,the bottom oxygen is depleted,the reconstruction of seawater redox stratification,water reduction in pelvic H2S rich.The comprehensive effect of organic matter,gas hydrate and anaerobic bacteria to precipitate MnO2 to MnCO3,after the diagenesis,formation of rhodochrosite. |
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