Hydrothermal origin for the 2 billion year old Mount Tom Price giant iron ore deposit, Hamersley Province, Western Australia

Giant iron-ore deposits, such as those in the Hamersley Province of northwe stern Australia, may contain more than a billion tonnes of almost pure iron oxides and are the world's major source of iron. It is generally accepted that these deposits result from supergene oxidation of host banded iron for mation (BIF), accompanied by leaching of silicate and carbonate minerals. N ew textural evidence however, shows that formation of iron ore at one of th ose deposits, Mount Tom Price, involved initial high temperature crystallis ation of magnetite siderite-iron silicate assemblages. This was followed by development of hematite- and ferroan dolomite-bearing assemblages with sub sequent oxidation of magnetite, leaching of carbonates and silicates and cr ystallisation of further hematite. Preliminary fluid inclusion studies indi cate both low and high salinity aqueous fluids as well as complex salt-rich inclusions with the range of fluid types most likely reflecting interactio n of hydrothermal brines with descending meteoric fluids. Initial hematite crystallisation occurred at about 250 degrees C and high fluid pressures an d continued as temperatures decreased. Although the largely hydrothermal or igin for mineralisation at Mount Tom Price is in conflict with previously p roposed supergene models, it remains consistent with interpretations that t he biosphere contained significant oxygen at the time of mineralisation.