Webb, Adam Douglas (2003) From banded iron-formation to iron ore: geochemical and mineralogical constraints from across the Hamersley Province, Western Australia. PhD thesis, James Cook University.

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Abstract

Three giant iron ore deposits - Mt. Whaleback, Mt. Tom Price and Paraburdoo occur in deformed parts of the southern Hamersley Province, where banded iron-formation (BIF) of the Dales Gorge Member has been has been converted to martite and microplaty hematite. The genesis of these high-grade hematite ores has remained controversial, in part because no study has systematically documented variations on the chemistry and mineralogy of stratigraphically equivalent rocks from undeformed and unmineralized strata in the north, through deformed but unmineralized strata in the south, and into the ore deposits. This study examines the powder colour, chemistry, mineralogy and petrography of 270 samples of the Dales Gorge Member and surrounding shales from several locations across the southeastern Hamersley Province, including Mt. Whaleback.

On the basis of these analyses, samples of BIF and shale can each be categorized into four groups. Unaltered BIF consists of sub-equal portions of SiO₂ and Fe₂O₃ and moderate amounts of MgO, CaO and loss on ignition (LOI). Compared to these rocks, weathered BIF has significantly higher SiO₂, but lower MgO, CaO and LOI. Likewise, oxidized BIF from Mt. Whaleback has lower MgO, CaO and LOI, but interestingly it has higher Fe₂O₃ than unaltered BIF. In striking contrast to all BIF, iron are is relatively enriched in Fe₂O₃ and depleted in all other major elements particularly SiO₂.

As for chemistry, the mineralogy of BIF varies significantly across the Hamersley Province. Unaltered BIF is largely composed of fine-grained chert and euhedral magnetite, although fine-grained siderite, stilpnomelane, ankerite, dolomite, minnesotaite, and riebeckite are significant in some samples. Weathered BIF contains hematite instead of magnetite (martite) and goethite pseudomorphs of carbonates and silicates. Chert remains unaltered. Oxidized BIF from Mt. Whaleback similarly consists of martite and chert, however it does not contain any goethite. Instead these rocks contain abundant microplaty hematite nucleated on martite euhedra or within small voids left by the dissolution of gangue. In comparison to all BIF, iron ore lacks chert and is composed entirely of martite in a porous network of microplaty hematite.

Profound differences in chemistry also occur between shales. Unaltered black shale predominantly consists of SiO₂ and Fe₂O₃ with relatively high contents of Al₂O₃, MgO, CaO, K₂O and LOI. Compared to these rocks altered black shale at Mt. Whaleback has slightly lower SiO₂ and Fe₂O3 and significantly higher Al2O₃. Red shale from Mt. Whaleback contains less SiO₂, MgO, and CaO and higher Fe₂O₃ and Al2O₃ than all black shales. In striking contrast to these rocks two Mt. McRae Shale samples from core located deep within the Mt. Whaleback pit are significantly enriched in MgO and CaO and depleted in SiO₂.

Unaltered black shales have highly variable mineralogies, consisting of very finegrained stilpnomelane, K-feldspar and quartz with subordinate amounts of siderite, dolomite, ankerite, muscovite, biotite, pyrite and chlorite. Altered black shales contain abundant chlorite and muscovite instead of stilpnomelane, K-feldspar and carbonates. Red shales are entirely composed of very fine hematite plates and kaolinite. The two shale samples from deep within the Mt. Whaleback pit consist predominantly of fine- to medium-grained dolomite cut by numerous chlorite and ferroan-dolomite/ankerite veins. Spatially associated with many of these veins are large porphyroblasts of finegrained ferroan-dolomite and ankerite.

Profound chemical and mineralogical changes between unaltered rocks and their altered equivalents suggest that after early diagenesis, low-grade metamorphism converted clays in black shales to stilpnomelane and talc. Coincident with, or following these changes, reduced metamorphic fluids altered phyllosilicates and K-feldspar in these rocks to chlorite and muscovite around Mt. Whaleback. These metamorphic fluids did not significantly affect BIF. However, subsequent acidic and oxidizing fluids around Mt. Whaleback converted magnetite to martite and dissolved carbonates and silicates from BIF. In black shales, these fluids also dissolved quartz and converted chlorite and muscovite to hematite and kaolinite respectively. Late in the paragenetic sequence, oxidized BIF was converted to highly porous high-grade hematite are by the dissolution of silica. Mass and volume change calculations coupled with detailed thickness measurements indicates ore genesis was a process not requiring iron addition. However, the significant increase in mieroplaty hematite growth between oxidized BIF and iron ore suggests mm- to em-scale (or m-scale?) mobilization and precipitation of iron. The paragenetie sequence for mineralization is probably more complicated as carbonate-rich, silica-poor shales deep within Mt. Whaleback and previously described magnetite-siderite rich assemblages at Mt. Tom Price suggest a stage involving carbonate replacement of silica prior to oxidation. During or after ore genesis weathered BIF developed from the interaction of unaltered BIF with acidic, oxidizing fluids associated with typical supergene weathering.

An internally consistent and testable model that explains rocks at both deposits, as well as the limited geochemical and mineralogical data at Paraburdoo, can be proposed. Rocks at all three deposits experienced regional metamorphism, although the grade was less at Mt. Tom Price and Paraburdoo. Hydrothermal fluids followed by heated meteoric fluids then locally affected rocks at all three deposits, forming carbonate-rich "BIF and shale" and, ultimately, high-grade hematite ore. Substantial northward flow of acidic, oxygenated fluids sometime during or after mineralization converted BIF, including carbonate-rich "BIF" to altered martite-rich BIF. The distinct lack of abundant carbonate-rich silica-poor rocks at Mt. Whaleback compared to Mt. Tom Price and similarities between oxidized BIF from Mt. Whaleback to equivalent sequences outside the deposit suggest this oxidation was more pervasive in the southeast. It is possible, and likely that formation of these particular giant, martite-microplaty hematite rich deposits was dependent on both a phase of silica loss and a phase of oxidation during or soon after extensional deformation at ca. 2200 Ma. Initial access of deepseated silica undersaturated fluids to parts of extensional faults triggering silica loss, may have created initial chemical porosity that subsequently allowed focusing of oxidized, surface-derived fluids.

Item ID:	34808
Item Type:	Thesis (PhD)
Keywords:	banded iron-formation; BIF; Dales Gorge Member; geochemistry; Hamersley Province; hematite; iron ores; martite; mineralogy; Mount Whaleback; Moutn Tom Price; Mt. Tom Price; Mt. Whaleback; ore deposits; Paraburdoo; petrography; WA; Western Australia
Date Deposited:	21 Jul 2015 05:47
FoR Codes:	04 EARTH SCIENCES > 0402 Geochemistry > 040202 Inorganic Geochemistry @ 50% 04 EARTH SCIENCES > 0403 Geology > 040306 Mineralogy and Crystallography @ 50%
SEO Codes:	97 EXPANDING KNOWLEDGE > 970104 Expanding Knowledge in the Earth Sciences @ 100%
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