Greenalite and its role in the genesis of early Precambrian iron formations – A review

Abstract

Iron formations are marine sedimentary rocks comprising Fe-rich and Si-rich bands that were most widely deposited before ~2.45 Ga, prior to the Great Oxidation Event (GOE). They host most of the world's iron resources but their scientific importance lies in their potential for tracking the evolution of ancient seawater chemistry. Most models of their deposition (see reviews by Bekker et al., 2010, 2014; Konhauser et al., 2017) assume that iron oxides in iron formations were derived from original Fe(III)-oxides/hydroxides that were precipitated in the photic zone from Fe(II)-rich seawater via biologically mediated oxidation, the Fe(II) ultimately being derived from hydrothermal vent plumes. In this review, we present an alternative model for the origin of iron formations centred on petrographic observations that indicate that the depositional (original) iron phase was an Fe(II)-silicate (greenalite) that precipitated in vent plumes as nanometer-sized particles and silt-sized flocs. The greenalite was deposited throughout the oceans and, in times of increased mafic-ultramafic volcanism, on continental shelves. Paragenetic studies indicate that the bulk of the iron oxides in iron formations formed by replacement of Fe(II)-rich precursors during metamorphism, deformation and exhumation. The origin of early Precambrian iron formations by precipitation of greenalite from vent plumes implies that iron deposition was largely an abiotic chemical process that did not require biologically driven oxidation, consistent with the lack of microfossils and indigenous organic matter.