Deep-seated iron ores from banded-iron formation

Abstract

Very large iron ore deposits have formed in many parts of the world, evidently by the supergene alteration of Precambrian banded-iron formation (BIF). Some of these deposits extend to great depths, ranging to 2,400 m at Krivoyrog1, beyond the likely reach of oxygenated water. We propose here, on the basis of studies in the Hamersley Ranges, Western Australia, a mechanism for deep-seated ore formation in which electronic conduction through the magnetite layers in BIF connects a cathodic region near the surface, where oxygen is reduced, to an anode at depth, where iron (II) from magnetite, carbonates and silicates is oxidized and precipitated as iron (III) hydroxyoxides. The electrical circuit is completed by ionic conduction through groundwaters. The model is based broadly on a mechanism suggested by Sato and Mooney2 to explain self-potentials associted with sulphide ore bodies. It is based more specifically on a model demonstrated by Thornber3 for the weathering of massive nickel–iron sulphide deposits at Kambalda, Western Australia, but with substantial differecnes in the physical situation and the reactions at depth.