Hydrogeological constraints on the formation of Palaeoproterozoic banded iron formations

Abstract

Banded iron formations are critical to track changes in Archaean to Palaeoproterozoic ocean chemistry, with deposition triggered by water column iron oxidation. Recently, however, it was suggested that reduced iron minerals were the primary precipitates, and these were subsequently oxidized by oxygen-bearing groundwater. If true, this would cast doubt on our understanding of how banded iron formations were deposited and their ability to record early ocean chemistry. Here we present a hydrogeological box model, based on the approximately 2.5 billion year old Hamersley Basin of Western Australia, developed to evaluate the plausibility of secondary iron oxidation. The box model calculates the time required for groundwater to flux enough oxygen through the basin to oxidize a given amount of ferrous iron. Less than 9% of nearly four million model iterations returned oxidation times less than the age of the basin. Successful simulations required simultaneously steep hydraulic gradients, high permeability and elevated oxygen concentrations. Our simulations show that the postdepositional oxidation of large banded iron formations is unlikely, except on a limited scale (that is, during secondary ore formation), and that oxidized iron phases were probably the precursor to large Palaeoproterozoic banded iron formations. Banded iron formations could not have formed by postdepositional oxidation, according to four million hydrogeological box model iterations that failed to reproduce secondary oxidation on reasonable timescales.