Primary mineralogy of banded iron formation:Advances and issues
Precursor mineral information,as the most elementary proposition of Precambrian Banded Iron Formations(BIFs),is fundamental to studying the metallogenetic regularity of BIFs and their implications for the ancient environment on early Earth.Scientists have been always seeking one given BIF,that has undergone low-grade metamorphism with possible traces of primary minerals and sedimentary textures preserved,as a research object for a long time.Important information on primary mineral compositions and their post-depositional evolution could be obtained from this BIF.There are now mainly three views of the original sedimentary products of BIF:ferric(Fe(Ⅲ))oxyhydroxides,iron silicates approximating stilpnomelane and greenalite compositions,and green-rusts.Green rust is a transition chemical compound of Fe(Ⅱ)and Fe(Ⅲ)mostly found in local reducing environments.It remains unknown what specific type of mineral green rust could be transformed into in Precambrian silica-rich seawater and via what specific way green rust is transformed into other iron-bearing minerals.Therefore,green rust is not regarded as the main primary mineral of BIF and whether Fe(Ⅲ)oxyhydroxide or iron silicate is the precursor mineral of BIF remains hotly debated now.This would result in major differences when discussing the origin of BIF and its implications for the ancient environment.In this regard,we provide a detailed review and discussion on recent major advances on the primary minerals of BIF.Based on this,we propose that Fe(Ⅲ)oxyhydroxide is the most dominant primary mineral besides iron silicates.It is possible that both the two processes contributed to the formation of BIF,with one process dominant over the other at various times and in different depositional settings.However,major evidence for primary iron silicate minerals is derived from classic petrographic features so that multiple pieces of evidences such as element and isotope geochemical characteristics are needed.The present iron mineral assemblage of BIF is the product of transformation of the original precipitates of BIF during post-depositional diagenesis and metamorphism.By contrast with early diagenetic siderite,magnetite is the later.Whether magnetite was a product of bacterial iron reduction coupled to oxidation of organic matter or the thermal decomposition of siderite remains to be further explored.The North China is one of the most significant regions around the world hosting BIF.Most BIFs in China are Late Archean in age and have undergone different metamorphic grades.Given these above,bases on previous studies,we should focus on these BIFs and conduct a comprehensive investigation on them,in order to provide further constraints on the genetic evolution of minerals present in BIFs and finally greatly deepen and promote the research on the origin of BIFs in China.
Banded iron formationAdvancesIssuesPrimary mineralogy
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维普
