The Geochemistry And Geological Significance Of Banded Iron Formations In Guyang Greenstone Belt,Inner Mongolia

Banded iron formations(BIFs)are marine chemical sedimentary rocks and they are widely distributed in the craton all around the world.The BIFs can be divided into two main types according to the different tectonic environment—the Algoma-type and Superior-type.Algoma-type banded iron formations are dominant in China and are generally interlayered with volcanic rocks and/or sedimentary sequences in Archean greenstone belts,mostly followed by metamorphism and deformation.Located in the northern North China Craton(NCC),BIF such as Sanheming,Gongyiming,Dongwufenzi,Gongjucheng etc.,are hosted by Seertengshan Group of the Guyang greenstone belt,which consists of multiple sequences of metavolcanic rocks and metasedimentary rocks.The regional metamorphic grade ranges from upper greenschist-facies to lower amphibolite-facies.The BIF ores are characterized by banded structure and the main mineral compositions of BIF are euhedral-subhedral granular magnetite,xenomorphic granular quartz and only minor amphibole.Chlorite,garnet,plagioclase and other Al-rich minerals can be seen in some BIF ores.Based on the determination of zircon U-Pb isotope data from the surrounding rock on the hanging wall of the Hanhaizi BIF,in the central part of the Guyang greenstone belt,the formation age of BIF in this region could be defined as 2.56~2.57 Ga,combining with previous studies.According to the content of immobile elements,the samples can be subdivided into two types: pure and contaminated.Normalized by post-Archean Australian Shale(PAAS),REE patterns of pure and weakly contaminated BIF are characterized bydepletion of LREE,positive La and Y anomalies,and relatively high Y/Ho ratios(>29),which are typical of modern seawater and some other Archean BIF.Large positive EuPAAS anomalies may indicate that during deposition of BIF,the oceans were greatly influenced by high-temperature(>250?)hydrothermal fluxes which accounts for 0~10%.And compared with modern seawater,a large positive Eu anomaly indicatesthat BIF and Fe in the protolith of BIFin the Guyang greenstone belt is hydrothermally derived.Slightly negative CePAAS anomalies are present in most samples,suggestingits precipitationfrom anoxicwaters.Asmall negative CePAAS anomaly shows partical seawater is in oxidation state,while the value of56 Fe in the BIF in this area is 0.4~0.8‰,which is consistent with that of the typical BIF in Archean.It is also indicated that the BIFis precipitated in a state of oxygen deficiency.However,REE patterns of highly contaminated BIF are obviously distinct.Co/Th ratios in the contaminated BIF indicate that detritus is mainly from basaltic rocks.Disordered destitution of this kind of BIF indicates that deep water in limited regions or time was not quiet before BIF deposited.On the basis of oxygen isotope data of magnetite and quartz in BIF,it is calculated that the temperature reaches 620? during the peak period of metamorphism,which is consistent with the formation temperature(500 ~650?)ofmetamorphic amphibolite facies in this region.In addition,the sequence of mineral formation in BIF in Guyang greenstone belt is similar to that of other BIF which experiencedmedium metamorphism in the world.All these explain the BIF in this regiongenerally experienced metamorphism up to amphibolite facies.