High-grade Hematite Ore Research Articles

Thermal magnetic analysis on iron ores and banded iron formations (BIFs) in the Hamersley Province: Implications of origins of magnetic minerals and iron ores

<abstract> <p>The genesis models of the iron-ores hosted in banded iron formations (BIFs) in the Hamersley Province of Western Australia have been debated since the iron-ore deposits were discovered in the 1960s. The existing models considered the few physicochemical conditions for the iron-ore enrichment from BIFs. This study incorporates the latest research outcomes in conversions among the major magnetic minerals under different physicochemical conditions with the thermal magnetic analysis for BIFs and iron-ores collected from the Hamersley Province to fill the gap in knowledge highlighted by existing studies of the iron ores and BIFs. The results indicate that the high-grade hematite ores might have been undergone a physicochemical process under hydrothermal conditions between 120 ℃ and 220 ℃ during the major stage of enrichment from the original BIFs in the Brockman Iron Formation. Such physicochemical conditions would require either that the BIF units were buried 4000–5000 m underground with tilted broad channels formed by large-scale deformation in the region that facilitates hydrothermal reactions and leaching by the fluids flowing down deep to 4000–5000 m, somehow similar to the deep-seated supergene model proposed in previous works, or that the BIF units were still buried but the hydrothermal fluids coming up from deeper sources spread widely over the broad channels to ensure the high-grade hematite ores are consistently uniform over the entire deposit. The large-scale martite-goethite deposits in the Marra Mamba Iron Formation might be derived from multiple supergene phases from hematite-martite ores below 100 ℃ in the natural process of oxidization near surface, somewhat similar to the existing model for the channel iron deposits. Magnetite contained within current BIFs and iron ores was least likely derived from primary hematite in BIFs.</p> </abstract>

Using In Situ Monazite and Xenotime U-Pb Geochronology to Resolve the Fate of the “Missing” Banded Iron Formation-Hosted High-Grade Hematite Ores of the North China Craton

AbstractHigh-grade hematite mineralization is widely developed in banded iron formations (BIFs) worldwide. However, in the North China craton where Neoarchean-Paleoproterozoic BIFs are abundant, economic high-grade hematite ores are scarce. High-grade hematite ores hosted in the Paleoproterozoic Yuanjiacun BIFs represent the largest occurrence of this type of ore in the North China craton. The orebodies are fault controlled and show sharp contacts with lower greenschist facies metamorphic BIFs. In situ U-Pb geochronology of monazite and xenotime intergrown with microplaty hematite and martite in high-grade ore established two episodes of metamorphic-hydrothermal monazite/xenotime growth after deposition of the BIFs. The earlier episode at ca. 1.94 Ga is interpreted as the timing of lower greenschist-facies metamorphism, and the later episode at 1.41 to 1.34 Ga represents the timing of high-grade hematite mineralization. Petrography and microthermometry of primary fluid inclusion assemblages indicate that the high-grade hematite ore formed from hot (313°–370°C), CO2-rich, and highly saline (~20 wt % NaCl equiv) hydrothermal fluids. These fluids channeled along faults, which concentrated iron through interaction with the BIFs—a process similar to typical hematite mineralization elsewhere. The deposition of hematite was probably related to tectonic extension in the North China craton related to the breakup of the Columbia/Nuna supercontinent. Our results challenge a previously proposed model ascribing the scarcity of high-grade hematite ores in the North China craton to the lack of prolonged weathering conditions. Rather, we argue that the high-grade ore formed in lower metamorphic-grade BIFs at shallower depths than magnetite mineralization and was largely eroded during later exhumation and uplift of the craton.

Ultrafast Iron-Making Method: Carbon Combustion Synthesis from Carbon-Infiltrated Goethite Ore.

Carbon-infiltrated iron ores were prepared from a coal-tar solution and selected calcined iron sources (i.e., goethite (FeOOH) ore, high-grade hematite ore, and Fe2O3 reagent grain). A several hundred micrometer thick carbon layer was deposited on the surface of all iron sources. Because the tar solution successfully penetrated into its nanopores, only goethite ore possessed a significant amount of carbon in its interior nanopores. The carbon-infiltrated ores were heated rapidly in an oxygen atmosphere in the combustion synthesis experiments. Carbon combustion occurred at the ore surface, with the ore temperature increasing suddenly during the experiments. Fast reduction to metallic iron was observed only in the carbon-infiltrated goethite ore, regardless of the oxygen atmosphere. Close contact between the goethite ore and the carbon in its nanoporous interior facilitated the fast reduction. The apparent reduction reaction of goethite ore is akin to a direct reduction reaction (i.e., FeOx + C → FeOx–1 + CO).

Морфологічні різновиди гематиту та їх перерозподіл по продуктах збагачення відсіву дробильно-сортувальних фабрик шахт Криворізького басейну

Наведено результати вивчення петрографічного та мінерального складу відсіву дробильно-сортувальних фабрик шахт Криворізького басейну. Головні петрографічні компоненти відсіву – рядові і збагачені гематитові кварцити, багаті гематитові руди; другорядні – малорудні та безрудні кварцити, різного складу сланці, граніти, амфіболіти, жильний кварц. За даними мінералогічних досліджень, відсів має практично бімінеральний (гематит + кварц) склад. Детально досліджено особливості головного рудотвірного мінералу відсіву – гематиту. Виявлено характер перерозподілу його різних морфологічних відмін по різних продуктах рудопідготовки та збагачення. Показано, що емульсійний гематит нерудних прошарків гематитових кварцитів і дисперсний гематит «краскових» руд і малорудних гематитових кварцитів потрапляють, головним чином, до відходів збагачення. Спекулярит і мартит практично повністю потрапляють до концентрату. Наголошується на необхідності врахування одержаних результатів для мінералогічного обґрунтування оптимальної технології повторного збагачення відсіву з метою одержання високоякісного залізорудного концентрату.

Multiple episodes of hematite mineralization indicated by U-Pb dating of iron-ore deposits, Marquette Range, Michigan, USA

Abstract High-grade hematite ore (so-called “hard ore”) was first mined in the Lake Superior region of North America in A.D. 1848. The development of ore deposit models from the region has strongly influenced research on the origin of this deposit type worldwide. The inability to date high-grade hematite ores has hindered the testing of competing genetic models. In the Marquette Range, northern Michigan (USA), high-grade hematite ore is hosted by the 1.9–1.85 Ga Negaunee Iron Formation and unconformably overlying Goodrich Quartzite. Field relations and new in situ U-Pb geochronology of monazite and xenotime intergrown with hematite from six mines across the Marquette Range, Michigan, indicate that the formation of high-grade hematite ore bodies involved up to three events. The presence of pebbles of hematite ore in basal conglomerate of the Goodrich Quartzite indicates that the first episode of iron-oxide mineralization occurred after deposition and deformation of the Negaunee Iron Formation, but before the ca. 1.85 Ga Sudbury impact. Monazite in iron-ore deposits within the hinges of Penokean-aged folds yields an age of ca. 1.8 Ga, which is interpreted to record a second phase of hematite enrichment. Monazite and xenotime in iron-ore deposits along the limbs of regional folds give ages of ca. 1.77 Ga and are interpreted to record a later episode of hematite upgrading. The ca. 1.8 Ga and ca. 1.77 Ga phosphate ages coincide with the timing of regional tectonic events, suggesting hematite mineralization was coeval with the Penokean orogeny and Yavapai-interval accretion. Our results support the proposal that the formation of high-grade hematite ore bodies is a multistage process linked to discrete orogenic events.

Palaeoproterozoic banded iron formationhosted high-grade hematite iron ore deposits of the Transvaal Supergroup, South Africa

Banded iron formation-hosted high-grade (>60 weight percent (wt%) iron (Fe)) hematite ore deposits make up the bulk of the world's iron ore production and reserves. They developed mainly through supergene and/or hydrothermal leaching of silica from the iron formation host rock under oxidizing conditions. Early Palaeoproterozoic iron formations of the Transvaal Supergroup host several such high-grade hematite ore deposits. The largest ones are developed in the Asbesheuwels Subgroup iron formation on the Maremane Dome, between Sishen and Postmasburg in the Northern Cape Province of South Africa. These deposits currently produce virtually all of South Africa's annual supply of about 78 million tonnes (Mt) of high-grade hematite ore, to the local and export market from the Sishen, Khumani, Beeshoek and Kolomela Mines. These represent ancient supergene deposits that formed along the approximately 2.2 to 2.0 Ga unconformity at the base of the Gamagara/ Mapedi red bed succession of the Keis (formerly Kheis) Supergroup. Smaller ancient supergene deposits are also developed along the same unconformity such as where it intersects the Rooinekke Iron Formation of the Koegas Subgroup at the Rooinekke Iron Ore Mine to the south of the Maremane Dome and the intersection of the Hotazel Iron Formation of the Voelwater Subgroup of the Transvaal Supergroup, in the Kalahari Manganese Field to the north of the dome. The supergene ores occur in four types, namely laminated, massive, brecciated and conglomeratic ores, the latter forming part of the Gamagara/Mapedi red bed succession. With the exception of the ore at Rooinekke, which is manganiferous, the supergene ores are of high grade (>60 wt%) and they also have low silica and phosphorous contents. Normalized rare earth elements, in contrast to the banded iron formation protolith, show light rare earth element depletion and the oxygen isotopes are slightly depleted to slightly enriched in 18 O. Hydrothermally enriched deposits include: Thabazimbi, situated in the Penge Iron Formation of the Transvaal outcrop area of the Transvaal Supergroup in the metamorphic aureole of the Bushveld Complex; and the Bovenzeekoebaart and magmatic hydrothermal Nauga East deposit, both located in the Kuruman Iron Formation of the Asbesheuwels Subgroup, along the southern extremity of the Griqualand West outcrop area of the Transvaal Supergroup. These hydrothermal iron ores are all developed at the bottom contact between the host iron formation and underlying carbonaceous shale that marks the transition into the Malmani- Campbellrand carbonate platform succession below. In some instances this contact is faulted. The ore grades upwards into oxidized iron formation, with isolated lenses sometimes developed at higher stratigraphic levels. Fault systems intersect the iron formations and the mineralisation is concentrated between mafic sills, higher up in the iron formations and the bottom shale contact. At Nauga East, mineralisation is developed in contact with a steeply dipping zoned syenite-carbonatite dyke. The hydrothermal ores are of high grade (>60 wt%), but can have higher phosphorous contents of up to 0.25 wt%. Normalized rare earth elements are enriched but similar to that of the host iron formations and the oxygen isotopes are generally depleted in 18 O. The mining of iron ore in South Africa dates from between 800 and 1200 AD, with modern exploration and mining starting from 1916. The first production of high-grade banded iron formation-hosted ore commenced in 1931 from the Maremane Dome and Thabazimbi. The first truly world class iron ore mine in South Africa, Sishen, was opened in 1953 and still operates today. The deposits at Khumani and Welgevonden have gone into production as recently as 2008 and 2011 respectively. Current estimates place the life of mine of the larger deposits at between 15 and 40 years.

A Study of Rock Magnetism of High-Grade Hematite Ores

Rock magnetism is useful in various applications. Hematite is one of the two most important carriers of magnetism in the natural world and its magnetic features were mostly studied through laboratory experiments using synthetic hematite samples. A gap exists between the magnetic behaviors of hematite contained in the natural rocks and ores and those of synthetic hematite samples. This paper presents the results of a rock magnetism study on the natural hematite ores from the Whaleback mine in the Hamersley Province in the northwest of Western Australia. It was found that high-grade hematite ores carry a much higher remanent magnetization than induced magnetization. Hematite ores with less than 0.1% magnetite appear to have an exponential correlation between the bulk susceptibility and hematite content in weight percentage, different from the commonly accepted linear relationship between the bulk susceptibility and hematite content obtained from synthetic hematite samples. The new knowledge gained from this study contributes to a better understanding of magnetic behaviors of hematite, particularly natural hematite, and hence applications to other relevant disciplines.

Magnetic Mineralogical Characteristics of Hamersley Iron Ores in Western Australia

This paper presents the research outcomes of a magnetic mineralogical study on Hamersley iron ores. Thermal magnetic analysis shows that typical high-grade martite-microplaty hematite or M- (mpl H) and martite-microplaty hematite-goethite or M-(mpl H)-g ores contain a small amount of original magnetite. A small amount of magnetite/maghemite and pyrite/pyrrhotite/siderite may exist in typical martite-goethite (M-G) and martite-ochreous goethite (M-oG) ores. In “the hardcap zone”, M-(H)-g ores contain a small amount of magnetite and maghemite. Compared with XRD, thermal magnetic analysis is not only more sensitive in identifying trace of magnetite contained in high-grade hematite ores, but also more diagnostic in identifying other unstable magnetic minerals like maghemite and pyrite/pyrrhotite/siderite co-existed in the ores.

Towards Zero CO2 Continuous Steelmaking Directly from Ore

In-line continuous processing of high-grade hematite ore (crushed ore or fines) with a pure hydrogen reductant is assessed. An appraisal is made of the rate controlling mechanisms involved in the reduction of a pure layer of molten wustite being transported by floating on a molten carrier iron carbon-free medium at temperatures just in excess of the iron melting point. Published research clearly indicates that under these conditions the kinetics are principally controlled by molecular gaseous diffusion. Thus, the rate is essentially not influenced by total gas pressure above 1 atmosphere. Accordingly, on safety grounds it is recommended that high pressure should not be used for hydrogen steelmaking in the future, but the operation should be conducted close to atmospheric pressure with low pressure steam encapsulation of the plant items involved. Using hydrogen as the reductant means that sub-surface nucleation of CO bubbles cannot disrupt continuous processing. The operation is then no different to processing a normal liquid phase. The off-gases from the reduction zone of a melt circulation loop are super-clean and only contaminated with iron vapor. Accordingly, the best available technology becomes available for energy conservation without risk of non-fusible solids deposition. The net result is that the energy requirements are expected to be superior to other potential processes.

MICROSTRUCTURE AND CRYSTALLOGRAPHIC PREFERRED ORIENTATION (CPO) OF HEMATITE ORES FROM THE SISHEN MINE, NORTHERN CAPE PROVINCE, SOUTH AFRICA

Banded iron formations (BIFs) are described as banded chemical sedimentary rocks composed of major amounts of magnetite, hematite and quartz, with variable amounts of iron bearing minerals like carbonates (e.g. siderite, ankerite) and silicates (e.g. greenalite, grunerite). The BIFs are transformed to high grade iron ores by removal of quartz and transformation of almost all iron bearing minerals to hematite. On macroscopic and microscopic scale the high grade hematite ores generally reveal a more or less visible lamination caused by alternating layers of different grain sizes. In addition to the microstructure (grain size, grain shape etc.), the crystallographic preferred orientation is a powerful tool to characterize rocks and ores. The overall crystallographic preferred orientation (CPO, texture) of volumes can be determined by neutron diffraction and the local texture on small areas of polished sections by electron backscattering diffraction (EBSD). The CPO of volumes of all hematite ore types (including a conglomerate pebble) of the Sishen Mine is characterized by a weak, usually circular c-axis maximum aligned to the normal of the lamination of the ores, and seems to be typical for this essentially unmetamorphosed and undeformed ores. The crystallographic orientation of grains of the hematite ores determined locally by EBSD shows in general the same preferred orientation. But, it is remarkable that partially the CPO is quite different and inherited from grain orientations of preceding primary minerals (e.g. magnetite, goethite, siderite/calcite, ankerite/dolomite) of the Banded Iron Formation, without any microscopic visible indication.

The pilot knob magnetite deposit in the Proterozoic St. Francois Mountains Terrane, southeast Missouri, USA: A magmatic and hydrothermal replacement iron deposit

The Pilot Knob magnetite deposit is located in southeast Missouri within the 1380–1480MaSt. Francois Mountains terrane rhyolitic/trachytic volcanic rocks. The deposit is tabular, dips about 45°, and is sill-like in nature, being approximately parallel to the bedding in the host tuffs. The deposit was uncovered by erosion and exposed to weathering in the late Proterozoic, and is overlain in angular unconformity by the Cambrian Lamotte Sandstone. This Proterozoic weathering cycle apparently had little effect on the deposit with only the updip edge being converted to hematite. Textural and mineralogical features of the deposit suggest a combined magmatic and hydrothermal replacement origin. The magnetite-rich ores that make up the bulk of the deposit are interpreted as having crystallized from an iron-rich magma, and a surrounding envelope of lower- to moderate-grade ores where magnetite has clearly replaced the tuffaceous host rocks are interpreted as hydrothermal in origin. After the development of the higher-grade magnetite ores and the envelope of lower- to moderate-grade ores, late hydrothermal minerals were deposited as cross-cutting veins and breccia fill. The two most abundant minerals in the higher-grade portions of the deposit are magnetite and albitic plagioclase, and petrologically the higher-grade ores could be described as a magnetite sodic syenite. The most abundant gangue mineral within the lower-grade impregnated envelope of ores around the higher-grade ores is K-feldspar, apparently relict from the rhyolites/trachytes. Thin lenses within the higher-grade ores contain calcite as a matrix mineral to the magnetite and are considered to indicate carbonatitic affinity. Rare earth elements are elevated in one of five whole rock chemical analyses of the ores and the REE-bearing mineral ferriallanite has been identified. Minor portions of the deposit, below the Proterozoic weathering cap, consist of high-grade hematite ores having equilibrium textures. At depth, the deposit is intruded by the Shepherd Mountain gabbro, a 120m thick, near-horizontal dike, which resulted in minor contact metamorphism of the ore.

Effect of 25% Goethite on the Hydrophobicity and Oleate Flotation of Hematite

Flotation tests conducted on the two Iron Duke samples using oleate showed that the presence of goethite adversely affected the flotation response of the hematite but contact angle measurements showed a higher hydrophobicity for the sample containing 25% goethite. Correcting the oleate addition for the changes in surface area still showed that the goethite reduced the flotability of the hematite. Aspects of processing iron ores containing goethite will need to be addressed as the high grade hematite ores are depleted, especially if flotation is employed in the beneficiation process.

Genesis of High-Grade Iron Ores of the Archean Iron Ore Group around Noamundi, India

High-grade hematite ores of the Iron Ore Group in the Noamundi area, Jharkhand state, India, are hosted by a laterally extensive, 220-m-thick banded iron formation (BIF) in a folded greenstone belt succession of Paleoarchean age. Single orebodies, which are up to 3 km long along strike and several hundred meters wide, depending on dip of the beds, are strata bound and composed of two major ore types—namely, hard hematite ore that is of ancient geologic origin and ores related to recent weathering along a lateritized Cretaceous-Cenozoic land surface. The ancient hard hematite orebodies comprise laminated hematite ores in which microplaty hematite is dominant, and massive ores composed almost entirely of martite. Supergene ores, in contrast, are comprised of goethite-rich duricrust and soft saprolitic hematite ores representing the leached zone of the Cretaceous-Cenozoic laterite profile. Hard hematite-martite ores formed by hydrothermal replacement of BIF protolith, not only through leaching of silica but possibly also through introduction of iron by hydrothermal fluids of meteoric origin. Hematite iron ore pebbles in a conglomerate at the base of the Meso- to Neoproterozoic Kolhan Group near Noamundi attest to the antiquity of the hard hematite orebodies. Soft saprolitic iron ores formed as a product of lateritic weathering processes in Cretaceous-Cenozoic times. Supergene alteration took place under the influence of reducing and acidic meteoric water, which is typical for lateritic soil profiles developed under humid tropical climatic conditions with lush plant cover. Goethitic duri-crusts capping the laterite profile suggest alternating wet and dry seasons in this environment.

Experimental evidence for non-redox transformations between magnetite and hematite under H 2-rich hydrothermal conditions

Transformations of magnetite (Fe IIFe 2 IIIO 4) to hematite (Fe 2 IIIO 3) (and vice versa) have been thought by many scientists and engineers to require molecular O 2 and/or H 2. Thus, the presence of magnetite and/or hematite in rocks has been linked to a specific oxidation environment. However, the availability of reductants or oxidants in many geologic and industrial environments appears to have been too low to account for the transformations of iron oxides through redox reactions. Here, we report the results of hydrothermal experiments in mildly acidic and H 2-rich aqueous solutions at 150 °C, which demonstrate that transformations of magnetite to hematite, and hematite to magnetite, occur rapidly without involving molecular O 2 or H 2: Fe 3O 4(Mt) + 2H (aq) + ↔ Fe 2O 3(Hm) + Fe (aq) 2+ + H 2O The transformation products are chemically and structurally homogeneous, and typically occur as euhedral single crystals much larger than the precursor minerals. This suggests that, in addition to the expected release of aqueous ferrous species to solution, the transformations involve release of aqueous ferric species from the precursor oxides to the solution, which reprecipitate without being reduced by H 2. These redox-independent transformations may have been responsible for the formation of some iron oxides in natural systems, such as high-grade hematite ores that developed from Banded Iron Formations (BIFs), hematite-rich deposits formed on Mars, corrosion products in power plants and other industrial systems.

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

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, martite–goethite 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.

Specularitic iron ores and shear zones in the Quadrilátero Ferrífero District

In the eastern high-strain domain of the Quadrilátero Ferrífero, Brazil, hard and soft high-grade hematite ore may occur as schistose bodies in shear zones mainly related to the Cambotas-Fundão thrust system. These orebodies are almost entirely constituted of oriented specularite platelets that develop both as a product of crystal–plastic deformation of hematite and solution-precipitation processes. Both mechanisms interact in the partitioning of the strain, while solution-precipitation is responsible for the precipitation of hematite and localised leaching of silica resulting in the formation of syntectonic ores. The analyses of fluid inclusions from specularite crystals of the Conceição and Timbopeba deposits, located in the northern and southernmost areas of the eastern high-strain domain of the Quadrilátero Ferrífero, were studied using infrared microthermometry and synchrotron X-ray fluorescence. The results indicate the participation of highly saline aqueous H2O–NaCl fluids containing other additional cations (Ca, Fe, K and probably Ti) in solution and homogenisation temperature Th averaging between 130 and 190°C. However, the partial fusion of metastable saturation phases indicates crystallisation temperatures of ∼400°C. The mineralisation process produces high-grade orebodies as well as Fe-rich specularitic itabirites that are explored in that area. The shear zones are related to thrust faults mapped and interpreted as related to the Pan-African/Brasiliano Orogeny that affected the Quadrilátero Ferrífero, thrusting the Paleoproterozoic platform sequence of the Minas Supergroup over Mesoproterozoic quartzites of the Espinhaço Supergroup. They intersect and even overprint an early metamorphic and tectonic fabric of Paleoproterozoic age (Transamazonian Orogeny). The geological and fabric relationships lead to the conclusion that the luids involved in the mineralisation were most probably of external origin, of still unknown source (possibly igneous), during the closure of the São Francisco Craton.

The structural/stratigraphic development of the Sishen South (Welgevonden) iron ore deposit, South Africa, as deduced from ground gravity data modelling

The Palaeoproterozoic Transvaal Supergroup in the northern Cape Province of South Africa hosts high grade (>60%Fe) hematitic and specularitic iron and manganese mineralisation. The mineralisation is primarily exposed along the western margin or in the central part of a major interference fold structure known as the Maremane dome. The bulk of South Africa's iron ore production is currently provided by two active mines, Kumba Resources megaton Sishen iron ore mine on the northern end of the dome, and the smaller Assmang Beeshoek mine on the southern closure. The undeveloped Sishen South deposit, which contains ∼400 Mt of high-grade hematite ore, is located ∼9 km towards the southwest of Postmasburg. It forms a linear extension of the Beeshoek mine and as such contains comparable laminated, breccia and conglomeratic iron mineralisation. The Transvaal Supergroup sediments were deposited in two distinct fault-controlled basins on the Archean Kaapvaal craton. Basement rocks in the western Griqualand West basin are formed by a carbonate platform sequence (Campbellrand Subgroup), which is overlain by a thick sequence of banded iron formation of the Asbesheuwels Subgroup. A major unconformity separates these rocks from the red-bed Gamagara Formation, which is correlated with the base of the Olifantshoek Supergroup. Diamictites and lavas of the Postmasburg Group have been thrust over these units from the west. The mineralisation process is related to regional scale supergene iron enrichment of Kuruman banded iron formations along the unconformity. During regional tectonism associated with the Kheis orogeny (2400–1700 Ma) the Maremane dome was uplifted, and the basal dolomite units thus exposed to erosion and karstification. The overlying banded iron formations collapsed into karst sinkhole structures where leaching of silica and enrichment of hematite occurred. Although the exact mechanism of the mineralising process remains unclear, karstification preserved components of the original concordant laminated and massive ores. These ores were subsequently eroded and deposited as conglomeratic and gritty ores down-dip of the Maremane dome in alluvial fan systems. A thin veneer of recent calcrete or Tertiary Kalahari clay beds masks the subsurface geology and most of the orebodies, thus ground geophysical gravity surveys proved excellent exploration tools to penetrate the surficial cover. These surveys were used extensively for delineation drilling and to deduce the subsurface geology and structural development of the area. Finally Sishen South orebodies were modelled with ModelVision software in an attempt to obtain an exploration template for the surrounding area.

The Genesis of the Hope Downs Iron Ore Deposit, Hamersley Province, Western Australia

The banded iron formation (BIF)-hosted Hope Downs high-grade hematite ore deposits are situated within the Marra Mamba Iron Formation with subsidiary deposits in the Brockman Iron Formation of the Archean to Proterozoic Hamersley Group of Western Australia. The main orebody extends to 260 m below the surface and is unusually rich in martite (pseudomorphous hematite after magnetite) and poor in limonite and goethite compared to other ore deposits of the Marra Mamba Iron Formation. The high-grade hematite ore is mainly within the Newman Member but also occurs in parts of the Nammuldi Member together with low-grade limonitic ore that becomes high grade after calcining. Karst erosion of the overlying Wittenoom Formation has produced steep-sided buried valleys adjacent to the in situ orebodies that contain thick deposits (<160 m) of goethitic and sideritic sediments, including remnants of Robe Pisolite Formation, bedded siderite, hematite gravels, red ochreous detrital material, and enriched scree deposits that are additional sources of ore. The ore consists of low phosphorous martite-limonite-goethite derived from chert-free BIF by supergene weathering. No evidence of the complete carbonate replacement of chert has been found at Hope Downs nor were any traces of preexisting chert bands seen in the ore, despite the abundance of chert bands in BIF elsewhere. A variety of textures and composition shown by cherty BIF adjacent to the orebodies is described from which the origin of the chert-free BIF is inferred, including sedimentary structures consistent with density-current deposition. A model is presented for the origin of the host iron formation and the ore deposits, in which density currents transported reworked iron silicates and hydroxides in colloidal suspension onto an unstable sea floor. The amorphous silica produced during diagenesis of Al-poor iron silicates formed the characteristic chert bands of BIF but some of the hydrous amorphous silica was lost prior to lithification to form chert-free BIF. Weathering of the chert-free BIF produced the high-grade hematite ore that is exposed today.

The Mount Gibson Banded Iron Formation-Hosted Magnetite Deposit: Two Distinct Processes for the Origin of High-Grade Iron Ore

The Mount Gibson banded iron formation lies within the Windanning Formation of the Luke Creek Group, which is found in almost all greenstone belts throughout the Murchison province. The banded iron formation (BIF) typically consists of alternating bands of magnetite and microcrystalline quartz (chert) with rare carbonaceous and iron silicate-rich shale partings and layers and rare, thin fine-grained tuff bands. Owing to repetition by isoclinal folding and attenuation by faulting, the true thickness of the Mount Gibson BIF is unknown but appears to be on the order of 100 m. Although the Mount Gibson BIF is typical of many Algoma-type iron formations, situated within a greenstone belt on an Archean craton, it is also similar to Hamersley-type BIF in petrology, areal extent, and ore genesis. High-grade hematite deposits formed within BIF were thought to have formed by the supergene leaching of chert from typical cherty BIF. Recent evidence suggests that at least some of these deposits are formed by hypogene replacement of chert by carbonates with subsequent supergene leaching of the carbonate and accessory minerals and oxidation of magnetite to hematite. Magnetite-carbonate BIF, in which there is clear evidence of hydrothermal replacement of chert by carbonate, forms distinctive magnetite-goethite ore with magnetite locally persisting to the surface. Mount Gibson shows clear evidence of the formation of high-grade ore by this process but also contains high-grade hematite ore and chert-free BIF that show no evidence of the hypogene replacement of chert. High-grade hematite occurrences, up to 1 km in strike length, are found within the weathered zone overlying the magnetite BIF at Mount Gibson and continue into unweathered chert-free BIF at depth that show no evidence of hydrothermal carbonate or supergene enrichment. The cherty BIF shows sharp contacts against chert-free BIF and high-grade ore, even when strongly weathered. This suggests that deep saprolitic in situ high-grade ore may be produced by different processes, including hydrothermal replacement of chert mesobands by carbonates with subsequent supergene leaching of the carbonate and by the oxidation of chert-free BIF, in which chert bands either never developed or were apparently removed during diagenesis. Neither model requires supergene selective leaching of quartz (chert) during deep weathering.

Giant iron-ore deposits of the Hamersley province related to the breakup of Paleoproterozoic Australia: New insights from in situ SHRIMP dating of baddeleyite from mafic intrusions

Banded iron formations of the ca. 2770–2405 Ma Hamersley province of Western Australia were locally upgraded to high-grade hematite ores during the Early Paleoproterozoic by a combination of hypogene and supergene processes after the initial rise of atmospheric oxygen. Ore genesis was associated with the stratigraphic break between the Lower and Upper Wyloo Groups of the Ashburton province, and has been variously linked to the Ophthalmian orogeny, late-orogenic extensional collapse, and anorogenic continental extension. Small-spot in situ Pb/Pb dating of baddeleyite by sensitive high-resolution ion microprobe (SHRIMP) has resolved the ages of two key suites of mafic intrusions, constraining for the first time the tectonic evolution of the Ashburton province and the age and setting of iron-ore formation. Mafic sills dated as ca. 2208 Ma were folded during the Ophthalmian orogeny and then cut by the unconformity at the base of the Lower Wyloo Group. A mafic dike swarm that intrudes the Lower Wyloo Group and has a close genetic relationship to iron ore is ca. 2008 Ma, slightly younger than a new syneruptive 2031 ± 6 Ma zircon age for the Lower Wyloo Group. These new ages constrain the Ophthalmian orogeny to the period between ca. 2208 and 2031 Ma, before Lower Wyloo Group extension, sedimentation, and flood-basalt volcanism. The ca. 2008 Ma dikes pre s ent a new maximum age for iron-ore genesis and deposition of the Upper Wyloo Group, thereby linking ore genesis to a ca. 2050–2000 Ma period of continental extension similarly recorded by Paleoproterozoic terrains worldwide well after the initial oxidation of the atmosphere by ca. 2320 Ma.