| Chengkou,located in Qinba graben basin,is controlled by the Wuping fault and the Chengba fault in the south and north direction respectively belonging to the Northwestern-Southeastern extension of the discontinuous manganese-forming basin,and it is an important source of manganese resources in Chongqing.Three manganese deposits are representative for the region,namely they are Gaoyan manganese deposit,Dadu River manganese deposit and Xiuqi manganese deposit.Gaoyan manganese area is particularly prominent.Manganese in Chengkou is Sedimentary type manganese carbonate,with the structure of oolitic,bean,pellet,and showing striped,lamellar,massive structural structure,occurring at the top of the Doushantuo Formation,with continuous deposition of black carbonaceous mud shale and overlying muddy dolomite.Manganese ore layer occurs in the transition horizons where black carbonaceous mud shale transit to the muddy dolomite,clearly controlled by layer.In view of this,this study mainly discuss the manganese-bearing rock series in Gaoyan manganese mine area,Dadu Xi manganese mine area and Xiuqi manganese mine area.Through the field geological survey,sedimentary facies identification and lithofacies palaeogeographic mapping,the analysis of main element of the sample,trace elements and rare earth element,and adding light flake identification and scanning electron microscopy,the study of the source of the manganese ore,the depositional environment and the cause of the manganese ore can to the higher level.Analyzing various sedimentary microfacies and Manganese-forming Conditions in the Manganese-rich Section of Chengkou Mingyue-Gaoyan-Dadu River-Xiuqi Baiguoba area shows that,in early Sinian period,Mingyuexiang mainly deposit consists with manganese dolomite and shale combinations,containing much clay,a small amount of silty and carbon,indicating that Mingyuexiang was close to the offshore,where influenced by the supply of terrestrial material,which represents the limited platform(tide)phase deposition.Under this sedimentary phase control,strip(lenticular)-like mylonite and manganese dolomite can be occasionally found.Meanwhile,Gaoyanxiang deposited manganite and shale assemblage,represents the sedimentary facies of the slopes on the front edge of the platform,and under this facies manganite developed.The characteristics of manganese deposit are highlighted by its gravity flow,positive granular structure,that the texture and structure of the ore is various.Manganese ore has oolitic,lenticular and globular structure and banded,layered and blocky tectonic.The lithologic characteristics of the sedimentary facies are few clay and siltite.This shows that it is affected by terrigenous material supply,that the sea is deep and the slope is steep.Then,the storm-affected oolitic bean deposits under the action of gravity flow.In the same period of time,Gao Pingliang area was deposited with a combination of shale,represents the terrigenous clastic tidal flat facies.Under the control of sedimentary facies,this area is rich in terrigenous materials,and terrigenous materials are deposited here,forming relatively high surrounding environment,deposit of a single sandy or silty shale without manganese.In this period,the Da Duxi and Xiuqi area similar to Gaoyan area manganese rock and shale assemblage,represents the confined platform lagoon facies deposits.Under the control of the sedimentary facies,the manganese rock is also very developed,and the manganese deposits have many types.The structure of manganese ore is similar to that of Gao Yan,but the grain bedding is less developed.The sedimentary environment by terrigenous material supp ly affected small,reflecting stagnant hypoxia,reduction of restrictedplatform lagoon facies.Compared with GaoYan region,the sedimentary facies are wider and distributed along the parallel coastline.In conclusion,the early Sinian Doushantuo stage Chengkou area deposits were NNW-SSE facies of the Mingyue-Gaoyan-Daduxi-Xiuqi area are dominated facies,platform margin slope facies,clastic tidal facies,restrictedplatform lagoon facies.Indicates that the direction of the supply of continental material is from the Gaopingliang to the two estuary that is perpendicular to the coastline.The manganese bearing rocks,polished section and thin sections are identified,and manganese ore generally shows large and small oolitic bean structures.Even dense minerals can be identified at high magnification with ultrafine oolitic beans.The oolitic bean is divided into 5 grades according to the particle size:(1)super microsphere(< 0.01mm);(2)microsphere(0.01-0.05mm);(3)fine chondrules(0.05-0.1mm);(4)medium chondrules(0.1-0.5mm);(5)coarse chondrules(> 0.5mm).Manganese ores are divided into 4 groups according to their ore composition:(1)tight ore;(2)pattern ore;(3)chondrite-belt ore;(4)chondrite.Manganese ores generally have fibrous,radial structures and spurious cross radiation extinction.There are many bacteria and algae in the ore,for example,variation of algal bloom and cyst,but also minerals of iron and silica,which represent microorganisms and play a n important role in the enrichment of manganese ore.In the presence of large amounts of pyrite berries,there is often a production of coarse-grained manganese ore.The organic matter content is high,the sulfate reduction development,shows the anoxic reduction environment.Scanning electron microscopy analysis of manganese ore shows that the integrity of oolitic is good,and it is locally compressed and cracked with concentric circles with 2-3 concentric rings.Bright,dark,and bright changes from the core of the oolitic core to the edge indicate that the core from the core to the edge is siderite,manganese dolomite,and siderite.This indicates that the oolitic is affected by the metasomatism of the outward rich manganese solution.Analysis of element geochemistry of manganese ore and black carbonaceous shale,major elements,trace elements and rare earth elements show black carbonaceous shale,manganese ore reflect more or less hot water or biological indicator.The supply of terrigenous materials is less than that of manganese ore and black carbonaceous shale,which results in a large amount of chalcopyrite,barite,fluorite,hot water mineral assemblage and a large number of organic matter and biological relics.The study area of early Sinian Doushantuoan,manganese manganese source rocks formation is mainly sustained but hot water jet flow and surface runoff intensity change caused by manganese,laid the material foundation for the existence of large concentrations of manganese,scattered,high limits of continental margin basins in seawater.The participation of organisms has resulted in the initial enrichment of a large amount of dispersed and low concentration manganese minerals,and the humus after the death of organisms has been adsorbed and complexed with manganese and enriched again.In a relatively closed basin edge slope and restricted platform lagoon,provides deep water,stagnant,reductive and anoxic environment for the enrichment of manganese.After the climate change and the rise of the sea water the weak oxidation,weak reduction,acidic to alkaline transition environment,for manganese ore deposition created conditions.The manganese rich intergranular water formed by the late diagenesis plays an important role in metasomatism and replacement of the early formed carbonate rocks,thus creating the conditions for the ultimate enrichment of manganese ores.Therefore,the material source area "Gao Yan" Chengkou manganese ore is mainly underground hot water jet along the deep fault and a small amount of manganese in surface runoff into the biological enrichment in precipitation.Then again in late stage of diagenesis of the manganese rich intergranular water replacement,replacement,the final mineralization enrichment. |
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