TOWARD A UNIFYING MODEL FOR GIANT MARTITE-MICROPLATY HEMATITE ORE DEPOSITS OF THE HAMERSLEY PROVINCE, WESTERN AUSTRALIA

Proterozoic sedimentary rocks of southern Hamersley Province, Western Australia (WA), host several giant martite-microplaty hematite orebodies whose genesis remains controversial. Most ore exists within Dales Gorge Member (DGM). In flat-lying sequences of northern Hamersley Province, DGM comprises m-scale horizons of banded iron formation (BIF) and black shale surrounded by black shale of Mt. McRae Shale and Whaleback Shale. BIF is mostly made of mm- to cm-scale layers of magnetite and chert but with some carbonate. By contrast, this strata was deformed in southern Hamersley Province ca. 2.3 Ga . Within ore deposits, one or more processes converted magnetite and chert of deforment BIF to martite and microplaty hematite (+/- porosity), and some black shale to red shale. Controversy arises over the timing and mechanism of this conversion. One view has BIF altered to goethite during supergene weathering, and subsequently metamorphosed to martite-microplaty hematite; an alternate view has chert replaced by carbonate when basinal fluids migrate along normal faults, and oxidizing, and carbonate removed and hematite produced by later acidic, oxidizing fluids.

Here we present the chemistry, mineralogy and petrography of hundreds of rocks from DGM, Mt. McRae Shale and Whaleback Shale across Hamersley Province, including at and beneath ore at Mt. Whaleback. Two crucial findings are: (1) silica within Mt. McRae Shale beneath Mt. Whaleback ore and along a major normal fault has been replaced by carbonate immediately after deformation, and (2) BIF surrounding Mt. Whaleback (and all across the southwest Hamersley Province) is comprised of hematite and chert without carbonate, indicating oxidation without silica removal. Combined with other recent results (Taylor et al., Econ. Geol., 2001), an internally consistent and testable basic model for martite-microplaty hematite orebodies of WA (and perhaps elsewhere) can be forwarded. Rocks of DGM were deposited during special ocean conditions, folded ca. 2.3 Ga, and subsequently intruded by carbonate-rich fluids along major normal faults. Pervasive oxidizing and acidic fluids converted magnetite to hematite and removed carbonate across large regions of the southern Hamersley Province. The orebodies lie precisely where sedimentary rocks of unusual chemical composition were locally altered and then overprinted by regional fluid flow.