Geological And Geochemical Characteristics Of Cold Seep Carbonates Of Neoproterozoic Nanhua Period And Their Significance For Manganese Ore Deposits In East Guizhou

The eastern Guizhou lies in the area between the southeastern Yangtze block and Jiangnan orogenic belt.As the breakup of Rodinia Supercontinent,several rift basins formed in the Neoproterozoic passive continental margin through resultant fracture and extension,which controlled the formation and distribution of cold seep carbonates(rhodochrosite +dolostone)in Lower Nanhua System in the northeastern Guizhou.Cold seep carbonates are present in both of the Liangjiehe and Datangpo formations of the Early Nanhua Period in the eastern Guizhou.The cold seep carbonates of the Liangjiehe Formation occur in the gravel-bearing clastic rocks correlating to the Sturtian glacial period. They forms laminated gravel-bearing dolomite lens or dolomite domes of variable size.Above them are moraines of the Tiesiao Formation.The Datangpo Formation,correlating to period between the Sturtian and Marinoan glaciations,is composed of mainly silt shales with a series of black carbonaceous shales at its bottom.The cold seep carbonates of the Datangpo Formation lies in the black shales.So,the block shales are also called manganese-bearing rock series because the most parts of cold seep carbonates are mainly rhodochrosites,with a few dolostone lens.The shapes of the Liangjiehe cold seep carbonates are lenticular or domed with convex upper boundaries and slightly tip-tilted lateral boundaries.They occur sparsely in gravel-bearing clastics with sharp contacts.The Datangpo cold seep carbonates are lenticular or layer-like.They exhibit concentric distribution in lithological association in the region.In the center are rhodochrosites - carbonaceous shales with the highest manganese content,and well developed sedimentary structures relating to ancient gas seep;Outwards lie dolostone lens,consisting of rhodochrosite- dolostone- carbonaceous shale association.The rhodochrosites are mainly banded with some massive ones.Then rhodochrosites decrease outwards,replacing by dolostonecarbonaceous shales.The peripheral belt is carbonaceous shales - black shales association,with lower organic matter content and thin thickness of the whole black rock series.The Datangpo cold seep carbonate lens(rhodochrosite - dolostone)of manganese-bearing rock series and the Liangjiehe cold seep carbonates(dolostone lens or dolostone domes)in Lower Nanhua system appear genetically associated.That is,if Sturtian cold seep carbonates occur in gravel-bearing clastic rocks,their overlap's black manganese-bearing rock series of the basal Datangpo Formation would contain cold seep carbonates- rhodochrosite deposition;It is also true reversely.Even number of layers of dolostone lens in Sturtian gravel-bearing clastics from the Datangpo of Guizhou,and from the Mingle manganese ore of Hunan are equal.It demonstrates that the two described periods of cold seep carbonates resulted from the same ancient gas seep system,and the system is a complete cold seep carbonate producing system.The occurrence of rhodochrosite in the early Nanhua period thus was an event deposition of magnesium - bearing seep carbonates from one of gas seep systems in eastern Guizhou or even in broad area.And the lenticular seep carbonates appear in Sturtian gravel-bearing clastics may serve as an indicator of rhodochrosite or cold seep carbonates in the Datangpo Formation.Such a conjecture would be significant in scientific research,and has been proved in recent ore exploration in the study area.Similar to the modern cold seep carbonate,typical seep features are common in the cold seep carbonates of early Nanhua age in the study area.They include bubble structures filled by bituminous materials from rhodochrosite ore of the Datangpo area,void structure,diaper structure,seepage tube structure,soft-sediment deformation lamination(inharmonic,radial and recumbent fold,as well as pierced structure),mud volcano structures and so on.Bubble walls of the bubble structure in rhodochrosite are radial arranged chalcedony.As the results of diagenetic compaction,the bubbles were flatten,or even fractured,with long axis parallel to bedding. Typical seep structures are also common in the dolostone domes in the Liangjiehe Formation, including tepee structures,void structures and soft-sediment deformation structures. Mineralogical association of rhodoehrosite deposition is relatively simple,consisting of rhodochrosite,greinerite,greinerite with magnesium alabandite,and minor mangandolomite, manganocaleite,as well as minor clay minerals,organic carbon,pyrite,quartz,apatite,chlorite authigenic mineral;The Liangjiehe dolostone consists of mainly dolomite,which may be grouped into magnesium- and calcium - rich dolomites respectively.Because of tha advance of the Sturtian glaciation,terrigenous clastic content in the dolostone lens is relatively high.Also similar to modern cold seep carbonates,the carbon isotope of the early Narthua cold seep carbonates described here shows distinct negative excursion.Theδ¹³C of rhodochrosite ranges from -8.14‰to - 10.38‰,with the lightest in the bubble rhodochrosite,commonly less than -10‰;Lighter values are from the massive rhodochrosite,and then banded rhodochrosite.Theδ¹³C of early Datangpo dolostone lens is -8.19‰～-12.98‰.It further demonstrates the similar origin of the rhodochrosite lens with that of the dolostone from the Datangpo Formation.The negative features ofδ¹³C are also present in the Liangjiehe dolostone.They are -2.19‰～-2.86‰, averaging at -2.65‰,slightly heavier than those in the Datangpo Formation.Theδ¹³C value of organic carbon from the asphalt filled in rhodochrosite bubble is as low as -30.98‰.Theδ¹⁸O‰of rhodochrosite and dolostone are both negative,ranging from -4.72 to -15.32‰.Theδ¹⁸O‰of rhodochrosite is generally lighter than those of dolostone,mostly less than -8‰.Pyrite,common in cold seep carbonates(rhodochrosite)and their host rocks(carbonaceous shale),is highly positive in itsδ³⁴S value.Theδ³⁴S value of the pyrite is generally larger than +4‰,and may up to +60‰,perhaps the heaviest value recorded in Proterozoic.Pyrite from hosting black carbonaceous shale and from underlying gravel-bearing carbonaceous sandstone of the Tiesiao Formation is also highly positive inδ³⁴S value.Although itsδ³⁴S value is lower than that of rhodochrosites,it is still higher than the average value of the coeval marine sulphate. Based on the features of these determinedδ³⁴S value,it is reasonable to infer a non-normal,and highly isolated sedimentary condition.Upsections from carbonaceous shale at the bottom of rhodochrosite ore body to rhodochrosite,and finally to carbonaceous shale of rhodochrosite roof,δ³⁴S varies from +49.30‰,to +52.90‰～+49.5‰in the rhodochrosite ore body,and drop to +39.20‰in the roof rocks.Similar trend were obtained from Datangpo manganese ore district of Songtao and Dawu manganese ore deposits district.The characteristic highly positive value of sulphur isotope of the cold seep carbonates indicates that the cold seep carbonates formed in a kind of gas seep conditions in Neoproterozoic rift basins of the South China.In the eastern Guizhou region,the ratio of Mn to Cr is nearly a constant,around 40,either from the host rocks of the rhodochrosite ore body or from the black carbonaceous shale of early Datangpo stage in Nanhua period.This ratio seems to be another key indicator to tell if there is cold seep carbonates—rhodochrosite in the black series in the horizon.In addition,biomarks from rhodochrosite demonstrated the existence of methane,because such biomarks are believed from typical microorganism living mainly on methane.In another hand,high organic carbon content evinced the seepage of methane,which furnished the flourish of microorganism and in turn led to a negative excursion of ambientδ¹³C.There were at least two phases of volcanic explosion in the study area during the early Nanhua, which is also a possible factor leading to explosive escape of gas.Coincidence of tectonic and volcanic factors might have been the main cause,terminating the anoxic depositional condition of the early Datangpo age,and resultant rapid global variation of environments,and gradually evolved into normal interglacial condition.The activity of magma provided the source of manganese.Around the mouth of cold seep,ancient gas seep leads to the flourish of microorganism,rhodochrosite deposites were formed through anaerobic oxidation of methane.In anaerobic condition,methane was oxidized through sulphate reduction bacteria,resulting in accumulation of rhodochrosite or dolomite and monosulfide(CH₄+SO^2-→HCO₃^-+H₂O). Locally supersaturated condition caused by methanogenetic biomineralisation,resulted in the formation of rhodochrosite or dolomites(2HCO₃^-+Mn²⁺→MnCO₃+CO₂+H₂O;2HCO₃^-+Ca²⁺→CaCO₃+CO₂+H₂O).The oversaturated HS^- may also facilitates the precipitation of pyrite,in the form of strawberry.The theory not only solves the conflicts between inferences from deposit of rhodochrosite and existence of microorganisms,but also born a new frontier science—gas seep and metallogenesis.This theory,focusing on diagenesis and ore-forming process associated with gas seepage,is of creative significance.It not only broadens our viewpoint for gas hydrate as well as its special and temporal distribution,but also sheds new light on metallic mineralization, biological evolution and environmental assessment relating to gas seep.Thus it is of significance in theoretic research and routine practice.