Banded iron formation hosted high-grade hematite deposits, a coherent group?

Iron formation-hosted iron ore deposits account for the majority of current world iron ore production and consist of three classes: unenriched primary iron formation with typically 25 to 45 wt-%Fe; martite-goethite ore formed by supergene processes, with abundant hydrous iron oxides containing 60 to 63 wt-%Fe; high-grade hematite ores thought to be of hypogene or metamorphic origin overprinted by subsequent supergene enrichment with 60 to 68 wt-%Fe. Individual iron ore deposits range from a few millions of tonnes to over two billion tonnes at .64 wt-%Fe, although most are within the range of 200 to 500 Mt. In the Hamersley province of Western Australia, martitegoethite ores are largely developed in the Marra Mamba iron formation, although high (.0 . 08 %P) phosphorous mineralisation is also well developed in the stratigraphically higher Brockman iron formation. While the vast majority of high-grade microplaty hematite ore is best developed in the Brockman iron formation, the present paper provides the first textural evidence of locally significant microplaty hematite mineralisation in the Nammuldi member of the Marra Mamba iron formation, in the Chichester Ranges at the Christmas Creek, Cloud Break and Mount Nicholas prospects. Petrographic studies have identified a high-grade Fe texture composed of nanometre scale plates of hematite in mineralised sections of primary microplaty hematite deposits in the Pilbara and elsewhere, well below the normal depth of weathering or dehydration. The population of nanometre to micrometre scale hematite plates is interpreted to represent various stages of nucleation, crystallisation and progressive growth of hematite from the primary ore-forming fluid in areas that were once iron-rich carbonates or silicates in the banded iron formation.

Section: High-grade Hematitementioning

confidence: 99%

Iron formation-hosted iron ores in the Hamersley Province of Western Australia

Clout¹

2006

“…17), no such zone has previously been identified at the biggest martite-microplaty hematite deposit, Mount Whaleback. 7,16 Given the location of carbonate alteration at Giles Mini, 3,4 it is difficult to imagine how a similar zone could be missing at Mount Whaleback if basic aspects of the Taylor et al 12 model are correct. Indeed, unless affected by post-ore fluids, 16 the model predicts the occurrence of carbonates below ore on the hanging wall of the Mount Whaleback fault.…”

Section: Introductionmentioning

confidence: 99%

Carbonate alteration of the Upper Mount McRae Shale at Mount Whaleback, Western Australia – implications for iron ore genesis

Webb¹,

Dickens²,

Oliver³

2006

The genesis of giant hematite ore deposits in northwest Australia remains a contentious topic in part because key evidence supporting a unifying genetic model has not been found at all deposits. In particular, several papers have proposed that carbonate replacement of silica layers in banded iron formation (BIF) by basinal brines is a requisite first step toward ore formation. This initial hypogene stage is inferred primarily from the presence of silica-poor, carbonate-rich rocks found below ore and along normal faults at the Mount Tom Price and Giles Mini deposits. However, until recently such rocks have not been identified at the largest martite-microplaty hematite deposit, Mount Whaleback. In the present study, samples of the upper Mount McRae Shale are examined for their chemistry, mineralogy and petrography. These samples were collected from several key locations, including within drill core that penetrates an area immediately beneath ore along the Mount Whaleback fault at Mount Whaleback. Compared with unaltered black shale from Wittenoom Gorge in the north and altered black and red shale at Mount Whaleback, reddish-green shale below the Mount Whaleback deposit is significantly enriched in MgO and CaO and depleted in SiO 2 . Samples of this shale consist predominantly of fine-to medium-grained dolomite and ankerite cut by chlorite and carbonate veins, contrasting with unaltered equivalents that contain mostly stilpnomelane, K-feldspar and relatively minor carbonates. This alteration is distinct from assemblages developed during regional metamorphism, and possibly represents hydrothermal alteration after metamorphism. Although several questions remain unanswered, these results support models that invoke an early hypogene stage during the formation of the martite-microplaty hematite deposits in the Hamersley Province.

“…The absence of chert bands in the chert-free BIF appears to predate the metamorphic and tectonic events that produced the prevailing textures and structure of the BIF, which are indistinguishable between high-grade ore and cherty BIF. However, the magnetite-siderite protore is directly comparable to the hydrothermal replacement of chert bands by siderite at Mount Gibson and Mount Tom Price, 4,5,16,17,21 although no direct evidence of these hydrothermal veins was seen at Koolyanobbing. Supergene processes upgraded the protore by leaching the carbonate and silicate minerals to produce highgrade ore without affecting chert or quartz in adjacent cherty BIF.…”

Section: Discussionmentioning

confidence: 89%

Genesis of the Koolyanobbing iron ore deposits, Yilgarn Province, WA, Australia

Lascelles¹

2007

The Koolyanobbing South Range in the Yilgarn Province, WA, Australia, hosts a variety of highgrade iron ore deposits illustrating processes of formation ranging from diagenetic loss of chert to hydrothermal replacement of chert bands by carbonate followed by supergene leaching of the carbonate. Quartz is leached from iron-rich superficial screes to form high-grade lump ore deposits, but there is no evidence of supergene or hypogene selective solution of quartz in the saprolite ore. Hydrothermal fluids have produced dramatic recrystallisation of the haematite in weathered ore and cherty banded iron formation (BIF), but have not affected the bulk chemistry of the rocks.