Authigenic precipitation of ferric oxyhydroxides and greenalite in Archean oceans

Abstract

The initial mineral phases of banded iron formations (BIFs) were mostly altered during diagenesis and metamorphism. The initial iron mineral, whether it was hydrous ferric oxides (HFO) or hydrous ferrous silicates (HFS) is a matter of debate. Each of those being the primary iron mineral offers us considering completely different biogeochemical conditions of Archean oceans. Some Archean greenstone BIFs of Badampahar (~3.2 Ga), Singhbhum Craton and Nuvvuagittuq (~3.8 Ga), Northern Québec consist alternating grunerite-, magnetite- and quartz-rich authigenic sedimentary bands. The minerals appear to be co-crystallized during metamorphism and show evidence of being originated from different precursor minerals, e.g. magnetite from HFO and grunerite from HFS. The process of such episodic HFO and HFS precipitations apparently related to two different Fe-Si cycles in the Archean oceans is not understood yet. In-situ trace element geochemistry of the minerals proves their authigenic origin and shows some characteristic geochemical differences, e.g. grunerite having higher contents of hydrothermally sourced trace elements, whereas magnetite being rich in elements involved in marine biogenic cycles. The origin of mineralogically different (e.g. quartz-, grunerite- and magnetite-rich) bands of the studied BIFs is explained with a model of HFS rich precipitation during episodic hydothermalism, subsequent HFO rich precipitation by microbial oxidation, and silica rich precipitation during no hydrothermal activity or non-depositional period of iron minerals.