BIF-hosted iron mineral system: A review

Secondary microplaty hematite (mplH) occurs in many different environments but importantly, where unmodified by metamorphism, it is the defining minor component of the high-grade BIF-hosted iron ores of the world that are dominated by martite (M, hematite pseudomorphs after magnetite). The term was introduced early during the CSIRO–AMIRA 1976–1994 program on the Hamersley Province iron ores to conveniently distinguish the then main export martite–microplaty hematite (M–mplH) ore type of Mt Whaleback, Mt Tom Price and Paraburdoo (ca2000 Ma) from the newly exploited, prolific supergene martite–goethite (M–G) ores (ca Cretaceous–Paleocene). The latter, with the channel iron ores, are now the major iron-ore exports of the Hamersley Province. The CSIRO-AMIRA model proposed that theM–mplH ores formed during regional metamorphism of Proterozoic M–G ores at low temperatures (∼80–100°C) by mplH growth in the supergene goethite. The conversion of goethite to hematite involves a ∼27% reduction in volume, with suggested mplH growth in the resulting microvoids by iron transfer through water resulting from the process itself, essentially an internal hydrothermal process requiring no introduced hypogene fluids. Alternative models have proposed oxidation of introduced siderite resulted in mplH + ankerite by reaction of heated meteoric fluids with hydrothermally metasomatised BIF 'protore' preserved in two locations in the Mt Tom Price deposits. This postulated 'protore' had earlier been described during the CSIRO–AMIRA program as local post-ore BIF metasomatic residuals. These new concepts have resulted in varying hydrothermal models that currently dominate the international literature of iron ore. A critical comparison of the two suggested mechanisms of mplH formation includes examples of mplH-related material from a range of milieus including BIF, GIF, iron ores and ferruginous sediments. Conversion of goethite to mplH appears the more likely process.

10.1080/08120099.2011.626453

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Supergene mineralisation of the Boyongan porphyry copper-gold deposit, Surigao del Norte, Philippines

AM Ignacio

Boyongan is a blind copper-gold porphyry deposit that was discovered by Anglo American Exploration (Philippines), Inc. in August 2000. It is located in Surigao del Norte, Philippines. Current inferred mineral resources for Boyongan are estimated at 219 million tonnes of combined oxide and sulfide material with an average grade of 0.51% copper and 0.74 grams of gold per tonne. Most of the high-grade mineral resource is within the oxide (supergene zone). Deep oxidation at Boyongan has developed a thick supergene enrichment profile (up to 600 meters) which has a complex supergene mineralogy, consisting of chalcocite, digenite, pseudo-covellite, native copper, cuprite, malachite, pseudo-malachite, azurite, chrysocolla, pseudo-chrysocolla, and pseudo-neotocite. Fine gold (<100μm) has been observed in goethite, chalcocite, chrysocolla, and malachite. Supergene mineralisation is associated with iron oxides (goethite with minor hematite) and clays (kaolinite, halloysite, illite and montmorillonite). Oxidation and the development of supergene minerals has been controlled mainly by fracturing and the availability of hypogene sulfides. The low pyrite content of hypogene mineralisation at Boyongan allowed supergene mineralisation to develop in-situ from near-neutral pH groundwaters. The initial stages of supergene mineralisation involved the replacement of hypogene sulfides such as chalcopyrite and bornite by chalcocite, digenite and pseudo-covellite. In some places, chalcocite replaced pyrite. Goethite formed during the weathering of pyrite, chalcopyrite, bornite and chalcocite. Copper that was released into solution precipitated as native copper, which has replaced chalcocite locally. Native copper was then oxidised to form cuprite, and also acicular and euhedral crystals of chalcotricite. Some cuprite may have precipitated directly from solution, and also where chalcocite reacted with oxygenated groundwaters. The final stages of supergene copper mineralisation at Boyongan produced copper carbonate (malachite, pseudo-malachite, azurite and pseudo-neotocite) and a copper silicate overprint (chrysocolla and pseudo-chrysocolla) onto earlier-formed copper oxides and sulfides. Copper generally has a more dispersed or erratic distribution than gold. Gold is restricted spatially to the early mineral intrusions. Copper grades in the cuprite-dominated zone in the west generally decrease with depth toward zones of patchy native copper. The copper carbonate (malachite-azurite)-dominated blanket above the cuprite zone contains both high grade copper and gold (>1% and >2 g/t, respectively). Chalcocite zones that have partially replaced hypogene copper sulfides have higher grades (>0.5% Cu and >1g/t Au) compared to zones of chalcocite replacing pyrite (<0.5% Cu and <0.5g/t Au). Chrysocolla and/or pseudo-chrysocolla is confined to zones that contain high copper and gold grades (>0.5% and >1 g/t, respectively). Isotopic compositions of malachite and azurite from Boyongan are consistent with deposition from ambient temperature (15°C to 20°C) meteoric water. These low temperatures are consistent with Boyongan being a low-sulfide porphyry system. Higher pyrite contents would probably have led to greater degrees of sulfide oxidation as well as higher groundwater temperatures. δ13C values of malachite are consistent with an organic carbon (soil?) source suggesting that malachite may have formed when Boyongan was uplifted and exposed. δ13C values of azurite are much higher, and could be derived from seawater, or by remobilisation of an inorganic carbon from carbonate wallrocks, or by sulfide oxidation by supergene-related bacteria above the water table.

Chalcocite

Bornite

Supergene (geology)

Covellite

Hypogene

Pyrolusite

Alunite

Cuprite

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Genesis modelling for the Hamersley BIF-hosted iron ores of Western Australia: a critical review

Australian Journal of Earth Sciences (2011)

Richard Morris M. Kneeshaw

Enrichment iron ore of the Hamersley Province, currently estimated at a resource of over 40 billion tonnes (Gt), mainly consists of BIF (banded iron-formation)-hosted bedded iron deposits (BID) and channel iron deposits (CID), with only minor detrital iron deposits (DID). The Hamersley BID comprises two major ore types: the dominant supergene martite–goethite (M-G) ores (Mesozoic–Paleocene) and the premium martite–microplaty hematite ores (M-mplH; ca 2.0 Ga) with their various subtypes. The supergene M-G ores are not common outside Australia, whereas the M-mplH ores are the principal worldwide resource. There are two current dominant genetic models for the Hamersley BID. In the earlier 1980–1985 model, supergene M-G ores formed in the Paleoproterozoic well below normal atmospheric access, driven by seasonal oxidising electrochemical reactions in the vadose zone of the parent BIF (cathode) linked through conducting magnetite horizons to the deep reacting zone (anode). Proterozoic regional metamorphism/diagenesis at ∼80–100°C of these M-G ores formed mplH from the matrix goethite in the local hydrothermal environment of its own exhaled water to produce M-mplH ores with residual goethite. Following general exposure by erosion in the Cretaceous–Paleocene when a major second phase of M-G ores formed, ground water leaching of residual goethite from the metamorphosed Proterozoic ores resulted in the mainly goethite-free M-mplH ores of Mt Whaleback and Mt Tom Price. Residual goethite is common in the Paraburdoo M-mplH-goethite ores where erratic remnants of Paleoproterozoic cover indicate more recent exposure. Deep unweathered BIF alteration residuals in two small areas of the Mt Tom Price M-mplH deposits have been used since 1999 for new hypogene–supergene modelling of the M-mplH ores. These models involve a major Paleoproterozoic hydrothermal stage in which alkaline solutions from the underlying Wittenoom Formation dolomite traversed the Southern Batter Fault to leach matrix silica from the BIF, adding siderite and apatite to produce a magnetite–siderite–apatite 'protore.' A later heated meteoric solution stage oxidised siderite to mplH + ankerite and magnetite to martite. Weathering finally removed residual carbonates and apatite leaving the high-grade porous M-mplH ore. Further concepts for the Mt Tom Price North and the Southern Ridge Deposits involving acid solutions followed, but these have been modified to return essentially to the earlier hypogene–supergene model. Textural data from erratic 'metasomatic BIF' zones associated with the above deposits are unlike those of the typical martite–microplaty hematite ore bodies. The destiny of the massive volumes of dissolved silica gangue and the absence of massive silica aureoles has not been explained. Petrographic and other evidence indicate the Mt Tom Price metasomatism is a localised post-ore phenomenon. Exothermic oxidation reactions in the associated pyrite-rich black shales during post-ore removal by groundwater of remnant goethite in the ores may have resulted in this very localised and erratic hydrothermal alteration of BIF and its immediately associated pre-existing ore.

Banded iron formation

Iron ore

Ore genesis

10.1080/08120099.2011.566937

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Citations (51)

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

Applied Earth Science Transactions of the Institutions of Mining and Metallurgy Section B (2007)

D. F. Lascelles

The Koolyanobbing South Range in the Yilgarn Province, WA, Australia, hosts a variety of high-grade 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.

Supergene (geology)

Yilgarn Craton

Hypogene

Saprolite

Iron ore

Banded iron formation

10.1179/174327507x167055

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Leaching of silica bands and concentration of magnetite in Archean BIF by hypogene fluids: Beebyn Fe ore deposit, Yilgarn Craton, Western Australia

Mineralium Deposita (2012)

Paul Duuring Steffen Hagemann