GIANT BANDED IRON FORMATION-HOSTED PALEOPROTEROZOIC HEMATITE DEPOSITS FROM AUSTRALIA, SOUTH AFRICA AND BRAZIL: TOWARDS A UNIFIED MODEL

Giant banded iron formation (BIF)-hosted iron-ore deposits are the world’s major source of iron. In the Hamersley Province they can be divided into two types: ores that contain abundant goethite and ores that are dominantly hematite. Because they contain hydrous iron oxides, it has generally been accepted that the goethite-rich deposits result from supergene oxidation of BIF, accompanied by leaching of silicate and carbonate minerals. The second group of deposits includes the martite-hematite ores at Mount Tom Price, Whaleback and Paraburdoo. Similar deposits occur in South Africa and Brazil. This group is characterised by locally well preserved, primary lamination, but with variable loss of internal texture and the growth of abundant secondary prismatic and microplaty hematite. The origin of these ores has been controversial, with a general view that they represent goethite-rich ore bodies that were metamorphosed and dehydrated during burial. The presence of pebbles of ore of this type in ~2.0 Ga conglomerates confirms their Paleoproterozoic age of mineralisation. There are sufficient similarities between Paleoproterozoic hematite deposits in Australia, South Africa and Brazil to suggest that they all formed by a similar process. Development of early high-temperature iron oxide-carbonate assemblages indicates that the initial stage of mineralisation involved a heated carbonate-rich fluid that removed silica from BIF. Subsequent mixing with meteoric water leached all other phases and crystallised platy hematite with ore grades being achieved by loss of other phases rather than introduction of a large amount of iron. If this model is generally applicable it is likely Sishen-type deposits represent the near surface expression of deeper-seated Mount Tom Price-type mineralisation and the goethite-hematite and friable hematite deposits in the Hamersley and Brazil weathered hydrothermal iron oxide-carbonate deposits.