Iron ore genesis and post-ore metasomatism at Mount Tom Price

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About 90% of the pre-mining banded iron formation (BIF)-hosted iron ore resource of the Hamersley Province of Western Australia is of the Phanerozoic supergene martite-goethite type. The remaining 10% formed as supergene deposits at ~2000 ± 200 Ma, and were later modified by burial to form the Proterozoic martite-microplaty hematite ores. The supergenemetamorphic segment of this general unified model has been challenged by three recently published hypogenebased genetic models for the hematite ores, all requiring post-hypogene meteoric oxidation to produce the ore. The most recent and detailed, by Taylor et al.,22 is based on data from the small Southern Batter and North deposits of the Mount Tom Price mine. The authors proposed that during Stage 1 'overpressured bicarbonate-saturated basin brines from the dolomite aquifer of the Wittenoom Formation' at 150-250°C, dissolved free silica from BIF to produce a stratigraphically thinned, magnetite- carbonate-silicate-apatite rock. In Stage 2, meteoric oxidation of siderite to secondary microplaty hematite + ankerite, and magnetite to martite, at unspecified elevated temperatures and pressures, was followed by carbonate leaching (Stage 3). Finally, during Stage 4, weathering with removal of apatite, produced the typical low-phosphorus, porous martite-microplaty hematite ores of the Tom Price-Whaleback type. However, evidence of both microplaty hematite and martite reduced to magnetite, and of infill silica, apatite, and ferroan chlorite, with late-stage pyrite, in the pore space of martite-microplaty hematite ore, shows that the hydrothermal activity at Mount Tom Price is a post-ore event. Neither the pyrite nor ferroan chlorite now present could have survived the oxidation required to form hematite ore. Recrystallisation of microplaty hematite in these infilled zones to a typically coarser form than present in most of the normal ores is a further supporting factor. A more credible, post-ore alternative to the hypogene genetic model is suggested here. Re-exposure to leaching by ground water of the metamorphosed ore deposits in the Phanerozoic resulted in the partial removal of remnant goethite. Localised modifications in the porous BIF/ore contact zones were driven by exothermic oxygen/pyrite/carbon reactions in fractured areas of the footwall Mount McRae Shale. Cool descending oxygenated meteoric water in the permeable ore horizons, acted as one limb of a thermal convective cell. In the other limb, heated reducing solutions rose through the permeable contact zones, to produce the observed localised modifications of BIF and ore. Expelled silica from the carbonatised BIF invaded adjacent porous ore to form erratic silicified ore zones, and to produce the local quartz vein systems found in the associated overlying BIF.