beukes

The Geology and Genesis of High-Grade Hematite Iron OreDeposits

N J Beukes1, J Gutzmer1 and J Mukhopadhyay1,2

INTRODUCTION

Most world-class high-grade (60 - 67 wt per cent Fe) hematiteiron ore deposits are the product of enrichment of Precambrianiron-formations but processes responsible for enrichment are stillunclear. Different models, ranging from syngenetic anddiagenetic (King, 1989), deep-seated hydrothermal (Powell et al,1999; Barley et al, 1999; Taylor et al, 2001; Gutzmer et al, 2002)and ancient supergene (Morris, 1980; 1985; Van Schalkwyk andBeukes, 1986) to modern supergene (MacLeod, 1966) have beensuggested. The high-grade hematite deposits stand in contrast tomartite–goethite ores which are generally accepted to be theresult of supergene enrichment of iron-formation because theyare found along Cretaceous to Tertiary lateritic weatheringprofiles (Morris, 1980; Harmsworth et al, 1990).

Uncertainties about the origin of the high-grade hematite oresappear largely due to the monomineralic composition of the ores.They are almost exclusively composed of hematite (asmicrocrystalline hematite and martite), a mineral with widestability field and very simple chemical composition that revealslittle about its origin. Second, many of the deposits have beenoverprinted by later deformation and chemical weatheringobscuring primary ore characteristics and zonation (Taylor et al,2001). Third, surprisingly few detailed geological studies havebeen undertaken on the deposits with the aim of unraveling oregenesis. Most studies focus on the present-day structure oforebodies and their general physical-chemical composition formining and beneficiation purposes. A more thoroughunderstanding of the origin of high-grade hematite iron oredeposits may be gained by combining available mining-

orientated data with systematic petrographic-geochemicalanalyses of different ore generations and hematite phases.

This paper presents a first summary of results of an ongoingstudy, started some two years ago, of high-grade iron oredeposits in South Africa, India and Brazil, including acomparison with the rather well studied deposits of theHamersley Province in Australia.

CLASSIFICATION

Following field investigations and literature surveys ofhigh-grade hematite ore deposits in South Africa (Sishen-Beeshoek and Thabazimbi districts), Brazil (QuadriláteroFerrífero and Carajas districts), India (Noamundi andDalli-Rajhara districts) and Australia (Hamersley Province)(Figure 1), we recognise three general genetic types, namelyancient supergene, hydrothermal and supergene-modifiedhydrothermal deposits (Figure 2). Ancient supergene deposits,represented by Sishen-type deposits in South Africa, occurimmediately below a major erosional unconformity and gradedownwards into unmineralised banded iron-formation (Figure2A). The ores are typically overlain by red beds, which maycontain detrital ores derived from the underlying hard lateriticores (Figure 2A). In contrast, hydrothermal ores, of whichThabazimbi in South Africa and hard hematite ores of MountTom Price, Mount Whaleback, Paraburdoo and Newman inAustralia are type examples, are not associated with anyunconformity and typically grade from the bottom upwards intounmineralised iron-formation (Figure 2B). Supergene-modifiedhydrothermal ores, represented by deposits of the QuadriláteroFerrífero and Carajas districts in Brazil, and the Noamundi andDalli-Rajhara districts in India, are characterised by the presenceof large volumes of friable saprolitic ores that were derived fromsupergene enrichment of earlier hydrothermally alterediron-formation, next to hard high-grade hydrothermal hematiteorebodies (Figure 2C).

Iron Ore Conference Perth, WA, 9 - 11 September 2002 23

1. Department of Geology, Rand Afrikaans University, PO Box 524,Auckland Park 2006, South Africa.

2. Department of Geology Presidency College, Calcutta, Calcutta700073, India.

FIG1 - Locality map illustrating the distribution of important high-grade hematite ore deposits.

Although there are large variations in size (from small tosuperlarge, depending on geological setting and erosionalpreservation) in each of the deposit types, it is interesting to notethat the largest known reserves of up to 13 billion tons of in situore are developed in association with supergene-modifiedhydrothermal ores in the Carajas district. Ancient supergene oredeposits can also be very extensive and contain in situ reserves ofup to two billion tons as in the case of Sishen deposit in SouthAfrica (Van Schalkwyk and Beukes, 1986). Hydrothermal oredeposits tend to be somewhat smaller with largest knowndeposits containing 0.9 - 1.4 billion tons of ore (Taylor et al,2001).

ANCIENT SUPERGENE DEPOSITS

The Sishen, Manganore, Beeshoek, Welgevonden and Rooinekkedeposits that developed below the pre-Gamagara unconformity inthe Griqualand West area of the Transvaal Supergroup, SouthAfrica (Figure 3), are considered type examples of ancientsupergene deposits. The unconformity, which is regionallydeveloped over the entire Transvaal basin, is consistently markedby the presence of a ferruginous lateritic weathering profile inthe rocks immediately below (Figure 3). Weathering took placearound 2.18 - 2.2 Ga, following a period of folding and uplift ofTransvaal strata (Beukes et al, 2002). Because of the foldednature of the strata below, the unconformity transects a widevariety of rocks, the compositions of which determine lateralvariations along the lateritised surface (Figure 3).

High-grade hematite iron ores are only developed in areaswhere the unconformity transects iron-formations. The oresderived from leaching of chert from the iron-formation duringweathering. Three iron-formation successions are involved,namely a) the Asbesheuwels succession, comprised of theKuruman and Griquatown Iron Formations that are overlyingCampbellrand carbonates, b) the Rooinekke Iron Formation ofthe Koegas Subgroup and c) the Hotazel Iron Formationoverlying basaltic andesites of the Ongeluk Formation (Figure 3).However, large to superlarge deposits of high-grade iron ore areonly developed in karstic laterite settings, where Asbesheuwelsiron-formation has slumped into carbonates of the underlyingCampbellrand Subgroup along the Maremane dome (Figure 3).In these karstic slump structures the iron ores are typicallyoverlain by reworked conglomeratic iron ores and highlyaluminous diaspore-rich shales and closely associated pisoliticlaterite profiles (Gutzmer and Beukes, 1998).

A siliceous chert breccia, known as the Wolhaarkop breccia,marks the dissolution surface between the dolomite andoverlying ore-bearing iron-formation (Figure 3). In areas outsideof karst slump structures only thin (one to 2 m thick) high-gradehematite ore beds are locally preserved below the unconformity.This is, for example, the case where the unconformity transectsthe Hotazel Formation in the Kalahari manganese field (Figure3). However, this upper lateritic iron ore capping has often beenremoved by erosion prior to deposition of the overlyingGamagara red beds. In such cases, only a thick oxidisedpaleo-weathering profile, without removal of chert, is preservedin the underlying iron-formation (Holland and Beukes, 1990).

Two major ore types are present in Sishen-type deposits,namely hard microcrystalline hematite ores derived fromsupergene enrichment of Asbesheuwels iron-formation below theGamagara unconformity, and conglomeratic (detrital) oresderived from erosion of underlying laminated ores andconcentration in the lower part of the overlying GamagaraFormation. The microcrystalline ores can be described as ancientsaprolites, because they preserve original textures and banding ofthe iron-formation precursor.

Due to their karstic setting, the Sishen-type deposits on theMaremane dome have very irregular floor and thicknessdistribution (Figure 4). Bedding in the iron-formation precursoris often highly contorted and/or brecciated due to slumping. Themicrocrystalline orebodies have a more regular outline becausethey crosscut bedding in the iron-formation and their topsconform to the pre-Gamagara unconformity (Figure 4).Conglomeratic orebodies in the lower part of the GamagaraFormation tend to thicken into karstic depressions, which alsocontain thick accumulations of Al-rich shales. This indicates thatthe karstic depressions were present at time of deposition of theGamagara Formation, an important observation because at leastsome of the karst structures were reactivated in more recenttimes.

The ores of the Sishen-Beeshoek area have been affected byvery localised hydrothermal alteration as manifested by thepresence of coarse specularite which fills secondary pores andveinlets in both laminated and conglomeratic ores. Thespekularite must have developed late in the history of thesuccession because it also occurs in veinlets in the overlyingGamagara shales and quartzites.

Paleomagnetic data shows that ore formation and deposition ofoverlying Gamagara red beds took place in a near equatorialsetting (Evans et al, in press), supporting a lateritic supergeneenrichment origin for the Sishen-Beeshoek deposits. A positiveconglomerate test on hematite pebbles in the lower GamagaraFormation indicates that the ores formed prior to transport anddeposition of the pebbles.

24 Perth, WA, 9 - 11 September 2002 Iron Ore Conference

N J BEUKES, J GUTZMER and J MUKHOPADHYAY

FIG 2 - Classification of major types of high-grade hematite oredeposits.

A

B

C

Hematite in the ores display oxygen isotope compositions thatvary between -3‰ and +3‰ (relative to SMOW) indicatingprecipitation from normal meteoric water at low temperature.REE distributions (normalised to PAAS) in Sishen-type oredisplay positive Ce anomalies, in contrast to parentiron-formations. Some samples also display strong enrichment inHREE, a feature characteristic of lateritic weathering profiles.The presence of positive Ce anomalies in Sishen-type oresuggests leaching of REE from the system during supergenealteration but with retention of Ce due to highly oxygenatedground water conditions.

HYDROTHERMAL DEPOSITS

Type examples of hard high-grade hydrothermal hematite oresfrom the Brockman Iron Formation of the Hamersley Group inWestern Australia and the Penge Iron Formation of the TransvaalSupergroup at Thabazimbi in South Africa (Figure 1) share manyfeatures. The parent iron-formations are of correlative age (in theorder of 2460 - 2480 m.y.) and overly black shales cappingcarbonates. In both areas the main orebodies are located near thebase of the iron-formation succession in contact with underlyingblack shale. Mineralisation also invariably appears to beassociated with early normal faults (Taylor et al, 2001;Netshiozwi, 2002; Netshiozwi et al, 2002). At Thabazimbi, lossof stratigraphy along the contact between the Penge Iron


THE GEOLOGY AND GENESIS OF HIGH-GRADE HEMATITE IRON ORE DEPOSITS

FIG 3 - (A). Illustration of a truncation model of folded Transvaal strata by the lateritised erosion surface on subcontinental scale below thepre-Gamagara unconformity. (B). Lateral variations along laterite profiles. All laterite profiles illustrated were intersected in drill core depths

of >100 m and are free of modern weathering. The Wolhaarkop profile (note depths below red bed cover rocks) display liesegang andcorestone weathering patterns in saprolite derived from massive lava and diamictite bedrock. Karstic laterites on dolomite bedrock contain

supergene manganese ores at Glosam and iron ore crusts derived from supergene leaching of iron-formation at Sishen, in addition toAl-rich claystones. Supergene iron ore beds are also sporadic preserved below the unconformity at the base of the overlying Mapedi andGamagara red beds, in areas outside of the karstic environment, such as in the vicinity of Hotazel. The red beds contain hematite pebble

conglomerates derived from the iron ore beds and pisolitic laterites. In the Hekpoort profiles the saprolite zone preserves original textures ofthe parent basalt but the pallid and upper ferruginous zones have a massive reconstituted fabric.

FIG 4 - Schematic diagram illustrating the setting of ancientsupergene hematite ores in karstic solution collapse structures

at the Sishen deposit, South Africa.

Formation and underlying Malmani dolomite indicates thatmineralisation took place along early low-angle normal lystricfaults that were later rotated to steep dip by thrust deformation(Figure 5). Similarly, at both Mount Tom Price and Paraburdoo,high-grade iron ores formed along steep normal faults (Taylor etal, 2001).

Ore formation can be ascribed to leaching of silica (chert)from iron-formations and recrystallisation of other iron mineralphases to specularite and martite. There may have been somemobility of iron in the system, as indicated by the replacement ofchert-bands by hematite without significant volume loss.However, for the most part hematite enrichment can apparentlybe accounted for by leaching of silica, compaction and anincrease in porosity (Taylor et al, 2001).

Shales, dykes and sills in the iron-formations at bothThabazimbi and in the Hamersley Province appear to exceed animportant control in the localisation of the orebodies (Taylor etal, 2001; Netshiozwi, 2002). Ores are often only developed alongone side of dolerite dykes that intruded prior to ore formation.Similarly, impermeable shale beds and dolerite sills appear tohave acted as hydrological seals that restricted ore formation.This is illustrated by the observations that high-grade iron oresare only rarely developed above the Whaleback shale in theHamersley District, or above a prominent pre-mineralisationdiabase sill in the Penge Iron Formation at Thabazimbi. It is alsoimportant to note that the orebodies are always surrounded by ahalo of oxidised (hematite-rich) iron-formation. This is very wellillustrated at Thabazimbi, where earlier carbonate and silicatefacies iron-formations have been transformed into hematite-richiron-formation in vicinity of the orebodies (Netshiozwi, 2002).

The age of ore formation in the Hamersley Province and atThabazimbi also appears rather similar. At Thabazimbi, hematiteore replaces contact metamorphic grunerite in the iron-formation

related to intrusion of the Bushveld Complex at ~2.058 Ga.However, mineralisation took place prior to deposition of circa2.0 Ga conglomerates of the unconformably overlying WaterbergGroup that contain pebbles of iron ore. Mineralisation thusappears to have taken place shortly before 2.0 Ga, very similar tothe age of ore formation in the Hamersley Province recentlyproposed by Taylor et al (2001). Following ore formation,the ores at Thabazimbi have been duplicated by a series ofnorth-verging post-Waterberg thrust faults. At Paraburdoo, theores were tilted in southern direction by the post-WylooCapricorn orogeny at 1.7 - 1.8 Ga (Taylor et al, 2001).

The only significant difference between the two areas appearsto be the depth of post-Gondwana late Mesozoic to Tertiaryweathering. At Thabazimbi, significant weathering to depths of50 m to 100 m along the Late Cretaceous – Early TertiaryAfrican Land Surface was associated with karstification of theunderlying dolomites and solution collapse of ore into the karstcavities. The present day weathering surface appears to have verylittle effect on the ores (Figure 5). In contrast, the depth ofweathering in the Hamersley Province is commonly between100 m and 200 m deep (Taylor et al, 2001).

With reference to the mineralogical composition of the ores itis important to note that an early phase of magnetite and hematitemineralisation, intimately associated with metasomatic carbonateformation, has been identified at Mount Tom Price (Taylor et al,2001). Similar carbonate-bearing ores are developed atThabazimbi (Figure 5) (Netshiozwi, 2002). REE patterns inhydrothermal ores from Thabazimbi and the Hamersley Provincediffer from those in ancient supergene Sishen-type deposits inthat they display slight negative to no Ce anomalies. Overall, thepatterns remain very similar to that in parent iron-formations,strongly suggesting that no significant preferential concentrationor introduction of REE took place during ore formation.



FIG 5 - Schematic cross-section of the Donkerpoort West high-grade hematite orebody at Thabazimbi Mine, South Africa.

Carbon and oxygen isotope signatures of hydrothermaldolomite associated with iron ores at Thabazimbi in comparisonwith diagenetic carbonates in the iron-formation protore, indicaterock-buffered conditions for δC13 and δO18. Sparitic calcite, onthe other hand, appears to have formed at significantly greaterfluid-rock ratios with highly depleted δO18 values relative to thatof diagenetic carbonates in parent iron-formations (Gutzmeret al, 2002).

Oxygen isotopic compositions of hematite in the Thabazimbiand Hamersley ores are marked by δO18

SMOW values as low as-7‰. Fluid inclusion studies on carbonates and quartz associatedwith hematite ores at Thabazimbi indicate the presence of twodistinct hydrothermal fluids, namely a high-salinity fluid thatresulted in deposition of early dolomite at temperatures around150 - 160°C, and a low salinity fluid that led to precipitation ofquartz at temperatures of between 120 and 140°C (Netshiozwi,2002). Fluid inclusions in calcite have compositions ranging incomposition between the two end members in both salinity andtemperature. It is thus suggested that the calcite was depositedduring mixing of the two fluid end members. These results are inexcellent agreement with homogenisation temperatures andsalinities of fluid inclusions from quartz veins associated withhematite mineralisation at the Mount Tom Price deposit (Tayloret al, 2001). At 160°C the oxygen isotopic composition of thehydrothermal fluid from which hematite and calcite precipitatedat Thabazimbi can be estimated at -2‰ (relative to SMOW).Fluids of such light oxygen composition can only be meteoricwater, ie water-rich fluids of shallow crustal origin that have notinteracted with silicate rocks (Netshiozwi, 2002).

SUPERGENE-MODIFIED HYDROTHERMAL ORES

Supergene-modified hydrothermal hematite ores occur in theQuadrilátero Ferrífero in Brazil and are hosted in the Caué IronFormation of the 2.1 - 2.6 Ga Minas Supergroup, a sedimentaryunit which unconformably overlies greenstones of the 2.7 -2.8 Ga Nova Lima Supergroup. Supergene-modified iron ores ofCarajas and India are developed in iron-formations of Archeangreenstone belts.

Apart from bring characterised by the abundance of friable(soft) hematite ores in very deep (>100 - 500 m) lateriticweathering profiles (Figures 6, 7 and 8), the supergene-modifiediron ores of Brazil and India display many features that aresimilar to those of less altered hydrothermal iron ore deposits inWestern Australia and at Thabazimbi, South Africa. The friableto powdery saprolitic hematite ores, usually composed ofmicroplaty hematite, specularite and martite, are invariablyassociated with bodies of hard hematite ore. These hard oresoften occur as large tabular bodies in the lower part of the



FIG 6 - Schematic cross-section illustrating the general setting ofsupergene modified hematite ores at the N4E deposit, Carajas,

Brazil.

FIG 7 - Schematic cross-section illustrating the general setting ofsupergene modified hematite ore at the Noamundi deposit, India.

FIG 8 - Schematic cross-section illustrating the general setting ofAguas Claras deposit, Quadrilátero Ferrífero, Brazil.

iron-formation successions with smaller, lenticular bodies higherup in the stratigraphy. In certain cases, tabular orebodies are alsodeveloped below impermeable roof rocks such as dolerite sills,lava and/or shale. The hard orebodies are typically surrounded byhighly porous friable ores and/or friable hematite-rich bandediron-formation (itabirite) (Figures 6, 7 and 8). The friableitabirite (best described as iron-formation saprolite) is widelyutilised as low-grade ore in the Quadrilátero Ferrífero becausehematite can easily be concentrated from the closely associatedquartz by dense media separation. Near surface, the orebodiesare typically capped by a veneer of hydrated goethitic ore belowa cover of laterite and canga that has been incised by streams ingeologically recent times.

The origin of the highly porous soft ores in the deposits is amatter of contention at present. They have always beenconsidered the result of leaching of silica from bandediron-formation under deep lateritic weathering conditions(Dardenne and Shobbenhaus, 2000). However, in recent yearsthrough deep drilling and mining in Brazil, it has becomeobvious that banded carbonate-hematite rocks are preserved atdepth below friable hematite ores at Carajas and the AguasClaras mine in the Quadrilátero Ferrífero (Figures 6 and 7).These rocks obviously developed from the replacement of chertbands in iron-formation by carbonate. Aguas Claras is the bestexample, where original chert bands in the lower part of the CauéIron Formation have been completely replaced by ferroandolomite. In the deepest levels of this mine there are excellentexposures illustrating how leaching of dolomite from the bandeddolomite-hematite rock gives rise to saprolitic friable hematiteore (Figure 7). How much of this applies to other deposits in theQuadrilátero Ferrífero is unknown at present, but it is certainlyapplicable to the Carajas district, where rather similarcarbonate-replaced hematite-rich iron-formation has beenintersected in drill core at depth. Another argument in favor ofleaching of carbonate rather than silica to form friable ores is thefact that large volumes of friable itabirite are preserved close tosurface in many of the mines in the Quadrilátero Ferrífero and atCarajas. At Aguas Claras lenticular bodies of itabirite arepreserved in soft ore close to surface (Figure 7). Rather thanrepresenting iron-formation corestones in the ore from whichsilica has not been leached, they may well represent pockets ofiron-formation that were not affected by carbonatemetasomatism.

The Indian deposits that we have investigated thus far(Mukhopadhyay et al, 2002) have only been explored and minedto depths of 50 m - 100 m so and there is virtually no informationavailable on the precursor rock for the friable ores. However, inmines around Noamundi, bodies of hard unaltered chert-richiron-formation are commonly preserved in friable saprolitichematite ore. Contacts between the friable ores andiron-formation bodies are sharp and difficult to explain bysupergene leaching of silica. Rather, it would appear that theiron-formation was replaced by hematite and perhaps carbonateprior to lateritic weathering. Similarly, in the Dalli-Rajhara area,there are abundant examples of unreplaced hard and friable(saprolitic) siliceous iron-formation bodies preserved in softhematite ore.

Massive and laminated hard orebodies are present in both theBrazilian and Indian deposits. The massive ores are typicallyvery dense and often magnetic. They are rather reminiscent ofmagnetite-rich ores described by Taylor et al (2001) at depth atMount Tom Price in Australia. The massive martite-rich ores areless abundant than surrounding laminated hematite ores that tendto form the bulk of hard ore reserves.

In all of these deposits hematite mineralisation appears to havedeveloped late in the geologic history of the successions afterearlier folding, faulting and/or intrusion of dolerite sills. At thePico de Itabirito Mine in the Quadrilátero Ferrífero it was

observed that ore formation postdated intrusion and folding ofdolerite sills. These sills could be related to volcanic activity thattook place around 2.12 Ga (Alkmim and Marshak, 1998) duringdeposition of the Sabara Group. Dykes that crosscut the ore havebeen dated at 1.75 Ga (Alkmim and Marshak, 1998) and iron oreformation may thus have taken place during or shortly after aperiod of extensional tectonics described by Alkmim andMarshak (1998). In the Dalli-Rajhara deposits of India excellentexposures of crosscutting relationships between orebodies andbedding in intensely folded iron-formation can be observed.Unfortunately the age of folding is not known at present.

GENETIC MODEL FOR HYDROTHERMAL ORESAND THEIR SUPERGENE MODIFICATION

Information available at present strongly suggests that all majorhigh-grade hematite iron ore deposits of Western Australia,Thabazimbi in South Africa, the Quadrilátero Ferrífero andCarajas in Brazil, and eastern and central India are very similarin character and may thus share a very similar mode of origin.Differences between the ores are largely the result of varyingintensities of supergene modification in late Cretaceous toTertiary times, after the breakup of Gondwana.

All indications are that primary orebodies owe their origin tothe interaction of iron-formation with a hydrothermal fluid ofmeteoric origin at temperatures of not more than 160°C to200°C. Ore formation was essentially a process of silicadissolution and transformation of ferrous iron minerals, includingsiderite, ankerite and iron silicates into microplaty hematite andmagnetite, the latter of which was later transformed into martite.An early phase of magnetite + carbonate mineralisation ispresent in some deposits and could have formed at elevatedtemperatures and very low oxygen fugacity under the influenceof an alkaline hydrothermal fluid that carried Ca++ and Mg++, anenvironment also most suitable for the leaching of silica. Coolingof this hydrothermal fluid would have favored the formation ofhematite, a process that does not necessarily require higheroxygen fugacity than magnetite formation.

A simple system of meteoric fluid circulation alongextensional fault-controlled aquifers is envisaged for formationof the iron ore deposits. Some faults acted as aquifers formeteoric water recharge and others for discharge. Deep seatediron ore deposits formed in the vicinity of normal faults thatacted as channels for fluid escape. Meteoric water flow was mostprobably topographically driven with recharge taking place inareas of uplift related to either rise of basement domes, like inthe Quadrilátero Ferrífero, or areas of extensional tectonics. It isquite possible that lateritic supergene ores formed in areas ofrecharge but because of uplift they would have had a smallpreservation potential.

Following breakup of Gondwanaland the ore deposits weresubjected to different degrees of supergene alteration, determinedby climatic evolution on the various continents, and timing ofexposure of deposits. At present, the deposits are mainlypreserved at high elevations below old lateritised land surfaces.These old land surfaces are preserved at high elevations becausefluvial erosion takes place by pediplenation (back-cutting) ratherthan peneplenation (down-cutting).

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