Geochemistry and depositional setting of Banded Iron Formations in Guyang greenstone belt, Inner Mongolia

Algoma-type Banded Iron Formations (BIFs) are marine chemical sedimentary rocks that are generally interlayered with volcanic rocks and/or sedimentary sequences in Archean greenstone belts, mostly followed by metamorphism and deformation. Because rare earth elements (REEs) are usually not fractionated during diagenesis and metamorphism, REE patterns are thus regarded as a powerful tool to understand conditions under which BIF were deposited. Located in the northern North China Craton (NCC), BIFs such as Sanheming, Gongyiming, Dongwufenzi and Gongjucheng are hosted by the Guyang greenstone belt, of Seertengshan Group which consists of multiple sequences of metavolcanic rocks and metasedimentary rocks. The regional metamorphic grade ranges from upper greenschist-to lower amphibolite-facies. The BIFs are composed of laminated magnetite and white to gray quartz associated with amphibole layers. Moreover, minor chlorite, garnet and plagioclase, rich in Al, are observed interbedded with quartz and magnetite. Based on the content of immobile elements, the samples can be subdivided into two types:pure and contaminated. Cr/Th ratios in the contaminated BIF indicate that detritus is mainly from basaltic rocks. Normalized by post -Archean Australian Shale (PAAS), REE patterns of pure and weakly contaminated BIF are characterized by depletion 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. Slightly negative Ce_PAAS anomalies are present in most samples, suggesting its precipitation from anoxic waters. Large positive Eu_PAAS anomalies may indicate that, during deposition of BIF, the oceans were greatly influenced by high-temperature (>250℃) hydrothermal fluxes which accounted for 0-10%. However, REE patterns of highly contaminated BIF are obviously distinct. Disordered destitution of this kind of BIF indicates that deep water in limited regions or time was not quiet before the deposition of BIF.