The Characteristics And Genesis Of Banded Iron Formation From Neoproterozoic Chuos Formation In Damara Belt Of Namibia

Nowadays,the guarantee of mineral sources has become a national strategy in China,and this leads to expanding demands of different mineral resources all over the world among which iron ore as a kind of bulk commodity becomes more and more significant.Therefore,studies on large-super large iron ore deposits have been more and more popular in economic geology researches.Banded iron formation is the dominant resource of iron industry due to its greatest mineral reserve and extensive distribution in the worldwide.Considering the great economic value of BIF and the unique chemical deposit built of early Earth,banded iron formation has been extensively investigated.In this paper,we studied the NIF of Chuos formation formed during glacial period of Neoproterozoic in the northern platform in Namibia,from the aspects of stratigraphic features,tectonic history,texture and structure,mineral compositions,forming environment as well as mineral genesis,using the combined application of mineralogy,petrology,geochemistry and so on.Thus,this study has important theoretical and practical significance.Based on the previous studies about Chuos formation in Kunene,Namibia and other available materials,we carried out investigations on regional geological setting as well as the evolution of Damara orogenic belt and revealed the tectonic setting and deposit environment during the formation of the IF and Chuos formation.Our results suggest that Chuos formation deposited in the margin of Congo Craton shortly after the glacial period,following the intracontinental rift in the southwest region of Africa during Pan Africa orogeny caused by the breakup of Rodinia Supercontinent.The glacial deposit and active rifting in the southwest margin of Congo Craton resulted in great lithologic differences and multiple sedimentary facies,consisting of marginal glacial sedimentary melange,shallow water or shallow sea sandstone,small amounts of IF and basaltic lava flow.After field recording and rock core sampling from Chuos Group in the north of Namibia,we analyzed the petrofacies and mineral compositions of the iron formation rock via electron probe microanalysis and microscopy.By identifying the characters of mineral and chemical compositions,physical-chemical conditions of mineralization,we disclosed the type of the iron deposit and the resource of some substances.We reach the conclusion that there are two types of the iron deposits.One is pure chemical sedimentary origin;siliceous rock containing magnetite and hematite is thin,however,stable and broad under low-grade metamorphism.And the other is mixed sedimentary origin;Hunji Rock and clastic rock were found to be combined by dolomite,flint,sandstone and quartzite,meta-volcanic rock,granite debris from basement.This also proves that the rock came from glaciation.By analyzing trace element geochemical characteristics of stripe magnet quartzite in Chuos Group from borehole core,the distribution patterns of trace rare earth elements were established and the geochemical characteristics of rocks and minerals were also studied.Then we raised the hypothesis of the material resources and sedimentary environment of the ore deposit and further analyzed the ore genesis.We suggested that IF in this area originated from the interaction between oceanic hydrothermal fluid and clastic components,and sea water widely participated in this reaction.By analyzing the ratio of light and heavy rare earth elements,the Y/Ho ratio,and the redox of Ce,we think that NIF in this area would rather be formed in the water with oxidation-reduction environment,than completely anoxic basin.By summarizing related evidences about the earlier snow earth hypothesis,combining field geological investigation and research of type sections,studying stratigraphical characteristics of the Chuos Formation,using sequence stratigraphy as well as the changes or relationship of stratigraphic contact with adjacent strata,all those existing geological phenomena strongly supported the snow earth hypothesis.There are four main pieces of evidence including;(1)widely existent fluviological deposits(peperite);(2)carbonatites in Rasthof Formation covering the Chuos Formation;(3)well-developed NIF iron bands;(4)the wide occurrence of algae and mass limestone in Ombombo.After analyzing the tectonic evolution,stratigraphic sequence,rock mineral and geochemical research results in this work comprehensively,we regarded that the NIF in this region was closely related to the formation of sea basin caused by rifting during the glacial period and the compositions of hydrothermal fluid;The combination of the changeable oxidation-reduction environment of ocean,hydrothermal fluids,weathering of continent,changes of marine biosphere,material-exchanges between atmosphere and ocean,and other factors accounted for the formation of BIF.