Timing and genesis of Hamersley iron-ore deposits

Abstract

Two types of genetic model exist for the in situ enriched ores in the Hamersley Basin: hypogene; and supergene. Although the supergene model has been adopted by a large number of geologists, conflicting geological/geophysical evidence suggests that the other model cannot be ruled out. This paper reports the results of a pilot palaeomagnetic study in and around the Tom Price mine. A negative fold test on results from the Wittenoom Dolomite demonstrates that the Tom Price region underwent an overprint event after folding in the Palaeoproterozoic. The direction of this overprint remanence is similar to that of a previously reported syn-folding remanence found in the Mount Jope Volcanics of the underlying Fortescue Group, and most of the remanence directions reported from the micropiaty hematite ore bodies at Tom Price and Paraburdoo. When these results are combined with geological observations, we are led to a tentative interpretation that the in situ enriched ores, particularly the microplaty hematite ores, could have been formed during Palaeoproterozoic extensional deformation, when the tectono/thermal fluids carried the iron to favourable structural positions, and/or leached components other than iron from those structural positions. This tentative model differs from the supergene model in that it requires orogenic/thermal fluids from the deeper sedimentary sequence rather than meteoric fluids percolating down from the weathering surface.