2004 thorne et al. north deposit

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A R T I C L E S

Warren Spencer Thorne Æ Steffen Gerd HagemannMark Barley

Petrographic and geochemical evidence for hydrothermal evolutionof the North Deposit, Mt Tom Price, Western Australia

Received: 13 June 2003/ Accepted: 2 September 2004/ Published online: 5 November 2004Ó Springer-Verlag 2004

Abstract High-grade iron mineralisation (>65%Fe) inthe North Deposit occurs as an E-W trending synclinalsheet within banded iron formation (BIF) of the EarlyProterozoic Dales Gorge Member and consists of mar-

tite-microplaty hematite ore. Three hypogene alterationzones between unmineralised BIF and high-grade ironore are observed: (1) distal magnetite-siderite-iron sili-cate, (2) intermediate hematite-ankerite-magnetite, and(3) proximal martite-microplaty hematite-apatite alter-ation zones. Fluid inclusions trapped in ankerite withinankerite-hematite veins in the hematite-ankerite-mag-netite alteration zone revealed mostly H2O–CaCl2pseudosecondary and secondary inclusions with salini-ties of 23.9±1.5 (1r, n=38) and 24.4±1.5 (1r, n=66)eq.wt.% CaCl2, respectively. Pseudosecondary inclu-sions homogenised at 253±59.9°C (1r, n=34) and sec-ondary inclusions at 117±10.0°C (1r, n=66). The

decrepitation of pseudosecondary inclusions above350°C suggests that their trapping temperatures arelikely to be higher (i.e. 400°C). Hypogene siderite andankerite from magnetite-siderite-iron silicate andhematite-ankerite-magnetite alteration zones have simi-lar oxygen isotope compositions, but increasingly en-riched carbon isotopes from magnetite-siderite-ironsilicate alteration (À8.8±0.7&, 1r, n=17) to hematite-ankerite-magnetite alteration zones (À4.9±2.2&, 1r,n=17) when compared to the dolomite in the Witte-noom Formation (0.9±0.7&, 1r, n=15) that underliesthe deposit. A two-stage hydrothermal-supergene modelis proposed for the formation of the North Deposit.

Early 1a hypogene alteration involved the upwardmovement of hydrothermal, CaCl2-rich brines (150– 250°C), likely from the carbonate-rich Wittenoom

Formation (d13C signature of 0.9±0.7&, 1r, n=15),within large-scale folds of the Dales Gorge Member.Fluid rock reactions transformed unmineralised BIF tomagnetite siderite-iron silicate BIF, with subsequent

desilicification of the chert bands. Stage 1b hypogenealteration is characterised by an increase in temperature(possibly to 400°C), depleted d

13C signature of À4.9±2.2& (1r, n=17), and the formation of hematite-ankerite-magnetite alteration and finally the crystallisa-tion of microplaty hematite. Late Stage 1c hypogenealteration involved the interaction of low temperature($120°C) basinal brines with the hematite-ankerite-magnetite hydrothermal assemblage leaving a porousmartite-microplaty hematite-apatite mineral assemblage.Stage 2 supergene enrichment in the Tertiary resulted inthe removal of residual ankerite and apatite and theweathering of the shale bands to clay.

Keywords Fluid inclusion Æ Hematite ore Æ

Iron mineralisation Æ Two-stage hydrothermal-supergene model

Introduction

Iron ore deposits in the Hamersley Province, defined bythe limit of outcrop of the Mt Bruce Supergroup(MacLeod 1966), can be subdivided into three geneticgroups: BIF-derived iron deposits (BID), channel iron

deposits (CID), and detrital iron deposits (DID).Stratigraphic relationships constrain CID and DIDdeposits to a period of Mesozoic–Cenozoic weathering.The BID deposits are of two main types: Mesozoic-Tertiary martite-goethite (M-G) ores and Precambrianmartite-microplaty hematite (Mr-mpH) ores. Mesozoic-Tertiary M-G ores are considered to be a result of supergene enrichment (Morris 1980, 1985, 1988;Harmsworth et al. 1990).

Historically, models for the formation of high-grademartite-microplaty hematite (>65 % Fe) iron ore

Editorial handling: B. Lehmann

W. S. Thorne (&) Æ S. G. Hagemann Æ M. BarleyCentre for Global Metallogeny,School of Earth and Geographical Sciences,University of Western Australia, Nedlands,WA, 6009, AustraliaE-mail: [email protected]

Mineralium Deposita (2004) 39: 766–783DOI 10.1007/s00126-004-0444-x



deposits hosted by banded iron formation (BIF)-irondeposits generally agreed on a process of de-silicificationwith or without the addition of iron (Park 1959; Dorr1965; MacLeod 1966, Beukes 1977, 1983; Morris 1980,1985, 1988; Harmsworth et al. 1990). Morris (1985) ar-gues against a hypogene origin based on the absence of evidence for fluid conduits or wall rock alteration belowthe hematite ore bodies, the presence of low-temperaturephases such as goethite in many deposits, and the ab-sence of clearly hypogene hydrothermal minerals(Morris 1985). Recent studies at the Mt Tom Price irondeposit by Barley and Pickard (1997), Barley et al.(1999), Hagemann et al. (1999), Ridley (1999), Tayloret al. (2001), and Thorne (2001) suggest that the super-gene enrichment model is not adequate to explain theincrease in ore grade from the bottom to the top, thegreat depth of some ore bodies (>400 m) with almostperfect textural preservation observed during BIF to oretransitions, and the removal of large amounts of silicafrom BIF to form pure iron oxide.

The lack of detailed studies of the hydrothermalalteration zonation and the fluid geochemistry to sup-

port hypogene transformation of BIF that contains$30 % Fe to high-grade iron ore that contains >65wt.% Fe hinder the widespread acceptance of the model.The preservation of hypogene alteration zones withinthe majority of deposits in the Hamersley Province, e.g.Channar, Paraburdoo, and Mt Tom Price (Fig. 1), arelimited due to widespread overprinting by deep Meso-zoic–Cenozoic weathering (Thorne 2001).

At the North Deposit at Mt Tom Price, the preser-vation of hypogene alteration zones below the limit of modern weathering (Barley and Pickard 1997; Barleyet al. 1999; Thorne 2001) provides an unique opportu-nity to expand the knowledge of the processes thatformed the high-grade (>65 % Fe) iron deposit. Thispaper provides a detailed description of the hypogenealteration zones that surround the high-grade iron-orebody at the North Deposit and their paragenetic se-quence within the mineralisation system. Fluid inclu-sions and carbon and oxygen isotope compositions of carbonates in the major alteration zones at the NorthDeposit were analysed to establish the pressure, tem-perature and composition of the hydrothermal fluids.An integrated hydrothermal-alteration model for thegenesis of the North Deposit is presented, and comparedto previous models for the Mt Tom Price deposit (Barleyet al. 1999; Hagemann et al. 1999; Taylor et al. 2001).

Regional geological setting of the Hamersley Province

The Hamersley Group is a sequence of Late Archaean toPalaeoproterozoic marine sedimentary and volcanicrocks that overlie $40,000 km2 of the southern PilbaraCraton, Western Australia (Fig. 1; Harmsworth et al.1990). The Brockman Formation, host to the majoriron-ore deposits in the Hamersley Province (e.g. MtTom Price) is dominated by BIF with lesser fine tuff,mudrock, dolomite, and chert (Blake and Barley 1992).Deepening upward trends and bimodal volcanism indi-cate that deposition occurred in an extensional setting,most likely, back-arc setting between 2,490 and2,450 Ma (Barley et al. 1997).Fig. 1 Regional geological map of the Hamersley Province

showing location of Mt Tom Price and stratigraphic summary(after Taylor et al. 2001)

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The earliest structures (2,600–2,450 Ma) in the Ha-mersley Province are extensional and comprise NEtrending chert pods formed by listric normal rotation orrare, recumbent layer-parallel isoclinal F1 folds (Tylerand Thorne 1990). Pervasive F2 folds that trend E toESE and are inclined to the north, overprint F1 struc-tures (e.g. Turner Syncline). The F2 folding is related tonorth-directed compression during the Opthalmianorogeny (Powell and Horwitz 1994). Geological rela-tionships constrain the timing of deformation between2,400 and 2,050 Ma (Barley and Krapez 1997). Normalfaults (e.g. Southern Batter Fault) related to NE–SWextension crosscut F2 structures. Krapez (1997) suggeststhat normal faulting occurred during a period of diver-gent tectonics at 2,050–1,860 Ma. Open F3 folds over-print F2 folds and are localised within NW trending foldcorridors. In the Hamersley Province, the F3 folds areinformally known as the Panhandle folds. Theemplacement of WNW and NNW trending doleritedykes preceded iron mineralisation at Mt Tom Price(Taylor et al. 2001).

The southern region of the Hamersley Province has

undergone greenschist (epidote-actinolite) facies meta-morphism, attributed to burial metamorphism (Smithet al. 1982). Mineral assemblages and textures from BIF

at Mt Tom Price and Mt Whaleback were folded andmetamorphically recrystallised before mineralisation(Barley and Pickard 1997).

The Brockman Iron Formation of the HamersleyGroup hosts the major iron ore deposits of the Ha-mersley Province with resources in excess of 19,000 Mtof ore at >55% Fe. The Marra Mamba Iron Formationis host to iron-ore deposits with a resource of approxi-mately 9,000 Mt, at >55% Fe (Harmsworth et al.1990).

Mt Tom Price iron deposit

The Mt Tom Price deposit contains the second largestaccumulation of high-grade hematite (>65% Fe) ore inthe Hamersley Province of Western Australia with anoriginal resource of 900 Mt of almost pure hematite,averaging 63.9% Fe (Harmsworth et al. 1990). The de-posit has a current resource of 200 Mt with an annualproduction of approximately 15–20 Mt (Thorne 2001).

The Mt Tom Price orebody (Fig. 2) extends for sevenkilometres from the North Deposit in the NW to theSouth East Prong deposit in the SE, and is up to 1.6 kmwide (average 600 m), with a maximum depth of 250 mbelow the surface (Taylor et al. 2001). Surface outcropof the orebody occurs north of the WNW trendingSouthern Batter Fault (Fig. 2).

Fig. 2 Surface geology and major structural features of the MtTom Price iron deposit (after Taylor et al. 2001)

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The detailed stratigraphy of the mine area is pre-sented in Fig. 1. All the units are conformable andcomparable to other parts of the Hamersley Province.The lowest level of drilling reached at the South Eastand North East Prong deposits (Fig. 2) is the top of theMarra Mamba Iron Formation (Taylor et al. 2001). TheWittenoom Formation occurs on the northern side of Mt Tom Price where it crops out along the southernlimb of the Turner Syncline and in the North East andSouth East Prong open pits (Fig. 2). The ParaburdooMember (Fig. 1) of the Wittenoom Formation is eithernot present, or is significantly reduced in thickness at MtTom Price. The Bee Gorge Member (Fig. 1) has athickness of 110 m at Mt Tom Price compared to 200 mregionally (Harmsworth et al. 1990). The Bee GorgeMember passes upwards into the chert-rich Mt SylviaFormation. The overlying Mt McRae Shale is carbonrich in fresh deep mine exposures with total organiccarbon contents ranging from 1.8 to 6.3 wt.% (Tayloret al. 2001).

The upper Colonial Chert Member is locally minera-lised at Mt Tom Price and is referred to as the Footwall

Zone (FWZ). The Dales Gorge Member contains most of the Fe resource at Mt Tom Price and overlies the ColonialChert Member. The Dales Gorge Member consists of 17banded iron macrobands (Fig 1; DB0 to DB16) and 16shale macrobands (DS1 to DS16) that are persistentthroughout the orebody (Fig. 1; Trendall and Blockley1968).The Dales Gorge Memberhas been subdivided intothree units (Fig. 1) based on their shale content: the DG1(6% shale), DG2 (31% shale) and DG3 (7% shale). TheBIF macrobands comprise centimetre-scale bands,termed mesobands, of chert and iron-rich material in achert matrix. Mesobands commonly consist of millimetrealternations of chert, shale and iron-rich bands, termed

microbands (Trendall 1983). The Whaleback ShaleMember forms the hanging wall to Dales Gorge Memberiron mineralisation with BIF mesobands within WS1(Fig. 1) commonly mineralised. The Joffre Member ismineralised, where it is in faulted contact with DalesGorge Member (Fig. 2).

The orebody at Mt Tom Price is preserved as a fol-ded, E–W trending sheet, within and just north of themain closure of the Turner Syncline (Fig. 1; Taylor et al.2001). The F2 folds are well developed within the ore-body and are parasitic to the regional structure of theTurner Syncline (Fig. 2). On the northern side of syn-clinal closures, fold limbs are steeply dipping to over-

turned. The F2 folds are typically doubly plunging inareas affected by NW trending, noncylindrical F3 folds(Ridley 1999). The Southern Batter Fault (Fig. 2) is aNW striking, SW dipping normal fault with a throw of 300 m that is folded by F3 folds. The Box Cut Fault(Fig. 2) is a major normal fault that juxtaposes high-grade iron-ore (>65% Fe) of the North East Prongagainst dolomite of the Wittenoom Formation. Both theSouthern Batter Fault, and the Box Cut Fault areprominent within the mine area but appear to die outrapidly both laterally and vertically. A third fault, the

South East Prong Fault forms the northern edge of theSouth East Prong open pit. The fault is a NW striking,steeply NE dipping reverse fault with a minimum throwof 120 m (Taylor et al. 2001).

Several suites of upright, NW to WNW trending dol-erite dykes that crosscut F2 and F3 folds are correlated toregionally distributed Paleoproterozoic dykes that in-truded prior to iron mineralisation at Mt Tom Price(Ridley 1999). The earliest generations are less than 10 mthick (average 2–4 m). Locally, within the mine area, talcmetasomatism has altered wallrock several metres on ei-ther side of the dykes (Taylor et al. 2001).

Low phosphorus, martite-microplaty hematite (Mr-mpH) iron mineralisation (Fe >65 %; P <0.05 %) isthe most abundant ore type at Mt Tom Price with BIFderived macrobands that grade over 68 wt.% iron(Harmsworth et al. 1990). The ore preserves the meso-banding and microbanding of unmineralised BIF withintergranular porosity estimated at 30% (Ridley 1999;Taylor et al. 2001). The ore consists of martite andrandomly orientated fine-grained (10–250 lm) micro-platy hematite and martite. Subhedral to euhedral

martite exhibit extensive overgrowths of microplatyhematite from their grain boundaries. The hard lump(6.3–31.5 mm) ores of the Mt Tom Price deposit aregenerally enriched in anhedral, fine mosaic hematite thatis derived from the compaction of the more porousmicroplaty hematite. The shale bands, typically kaolin-itic, form thin pink seams within the ore and constitutethe main impurities (Taylor et al. 2001).

Geology of the North Deposit

The North Deposit is located NW of the Southern

Ridge, Synclines and Centre deposits at Mt Tom Price(Fig. 2). The strata consists of the Dales Gorge Member,Whaleback Shale Member and the Joffre Member. TheNorth Deposit is concealed below colluvium, canga andlow-permeability, unmineralised shales that limit thedepth of weathering and preserve hypogene alterationzones both below and lateral to the deposit (Thorne2001).

Banded iron formations both lateral and below theNorth Deposit show extensive replacement by carbon-ate. The transition from low phosphorus, martite-mi-croplaty hematite (>65% Fe) ore to zones of carbonatealteration is abrupt and is defined by shale bands DS 9

and 11, which are typically green-black and preservetheir original sedimentary thickness and texture. Shalebands within high-grade ore zones are pink to yellow,ferruginous and thinned by as much as 70% (Thorne2001).

High-grade iron mineralisation (>65% Fe) occurs asan E–W trending synclinal sheet (F2) within DG3 andupper DG2 of the Dales Gorge Member. The orebodydips gently (5–10°) to the south, and plunges gently tothe NW (3–5°), parallel to the plunge of the dominant F2fold that hosts the orebody (Fig. 3). The superposition

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of F3 folds with F2 folds locally produces a basin-and-dome interference pattern. The deposit lacks overturnedor steeply dipping folds observed elsewhere in theMt Tom Price deposit.

Two zones of high-grade mineralisation are presentwithin the North Deposit: martite-microplaty hematite(Mr-mpH) ore (>65% Fe) with low P levels(<0.05 wt.%) and Mr-mpH ore with high phosphoruslevels (>0.05 wt.%). Supergene Mr-mpH (low P) occursabove the depth of weathering and extends to nearsurface on the northern limb of the syncline where in-tense weathering makes identification of hypogenealteration and structures impossible. To the south, theore zone pinches out with mineralisation becoming re-stricted to DB16 (Fig. 3). The Mr-mpH (high P) oreoccurs below the depth of weathering and is preservedwithin a restricted zone south of the dolerite dyke(Fig. 3). The distribution of supergene low P, Mr-mpHabove and hypogene high P, Mr-mpH mineralisationbelow the depth of weathering, is similar to the SouthernRidge (Taylor et al. 2001).

Petrography of hypogene alteration zones

The transition from unmineralised BIF (30–35 wt.% Fe)to high-grade (>65 wt.% Fe) iron ore at the NorthDeposit was investigated in 42 petrographic samplestaken from 13 diamond drill holes (Fig. 3). Selectedsamples were examined using the scanning electronmicroscope at the Centre for Microscopy and Micro-analysis (University of Western Australia) to determinecarbonate composition and document textures of oxide,

carbonate, silicate, and sulphide minerals. Samples ta-ken from below the depth of weathering (Fig. 3), whichdisplay hydrothermal alteration minerals such as sider-ite, iron silicates, ankerite and microplaty hematite, areinterpreted to have formed through hypogene processes(Barley et al. 1999; Taylor et al. 2001; Thorne 2001).Samples taken from within the weathering profile anddisplaying hydrous iron oxides such as goethite andlimonite are considered supergenic in nature (Morris1980, 1985; Barley et al. 1999).

Fig. 3 a Surface geological mapof the North Deposit, Mt TomPrice, with diamond drill holecollar locations shown. b Awestward-looking cross-sectionof the North Deposit along lineA–B, showing the projectedlocation of diamond drill holes,geology and alteration zonescompiled from diamond corelogging and field mapping

(Thorne 2001)

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Host rocks

Unmineralised BIF at the North Deposit is laterallycomparable to that of the Southern Ridge deposit(Thorne 2001). The shale bands are significantly moreradiogenic than the banded iron formation, and thisfeature is used to assist in correlations between diamonddrill holes (Taylor et al. 2001).

Unmineralised, unweathered BIF is characterised byalternating magnetite-chert mesobands and microbandswith subordinate carbonate, iron silicates, and pyrite(Figs. 4 and 6). Magnetite occurs as euhedral crystals(50–250 lm) with rare, anhedral hematite inclusions(5–15 lm). Chert displays variable degrees of recrystal-lisation with microcrystalline quartz (2–20 lm) pre-served in some thicker chert mesobands (Fig. 5a), andcoarser polygonal (50–100 lm) or fibrous quartz inter-grown with magnetite in magnetite-chert mesobands.Stilpnomelane is the predominant iron silicate withinunmineralised BIF and occurs as thin laminae of finely(20–70 lm) granular to bladed crystals and/or as anoverlapping groundmass of pale green to brown crystals

in which individual crystal boundaries are not distin-guishable (Fig. 5b). Rare fibrous riebeckite (20–100 lm)occurs as overgrowths on stilpnomelane. Carbonatesrange in composition from siderite to dolomite and oc-cur as granular mosaics (50–400 lm) within chert mes-

obands. Unweathered shale bands within the DalesGorge Member are green/black, laminated (mm to cm),contain iron-rich chlorite, stilpnomelane, massivedolomite, euhedral pyrite (50–300 lm) and local mag-netite-rich bands. The shale bands lack hydrothermalalteration minerals, and with minor variations, exhibit ahigh degree of lateral continuity within the HamersleyProvince (Morris 1985).

A zone of predominantly dolomite, calcite, and to alesser extent, siderite veins (V1) below the North Deposit(Table 1) crosscut BIF and shale macrobands (Thorne2001). The veins are typically thin (0.50–6.00 mm), withvertical and lateral extents of <1 m and are devoid of hydrothermal wallrock alteration. Although spatiallyrelated to iron mineralisation, the timing of V1 veins ispresently unknown making their relationship to hypo-gene alteration unequivocal.

Hypogene alteration zonation

Pervasive hypogene footwall alteration forms a later-

ally extensive zone below low P, Mr-mpH ore (>65 %Fe, P<0.05 %; Fig. 3). The alteration comprises threezones, the distal magnetite-siderite-iron silicate, theintermediate hematite-ankerite-magnetite, and theproximal martite-microplaty hematite-apatite zones(Fig. 4). Hypogene alteration is restricted to BIFbands with shale bands preserving their original min-eralogy and textures. Hypogene alteration is restrictedto the Dales Gorge Member, and strongly developedwithin DG3 and upper DG2 (Fig. 3). The outer distalalteration zone becomes restricted to the proximity of BIF/shale band contacts (Fig. 3). Regardless of the

Fig. 4 Core samples from the North Deposit. a Unmineralised BIFshowing chert (Qtz) and magnetite (Mt) banding. b Magnetite-siderite-iron silicate (Mt-Sid-FeSil ) alteration showing preservationof banding. c Hematite-ankerite-magnetite alteration with localisedbrecciated of magnetite bands. d Martite-microplaty hematite (Mr-mpHm) ore. Note preservation of banding and goethite (Goe) infill

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intensity of the alteration zone, the progressive changesin alteration mineralogy, described below, always forma consistent pattern.

Distal alteration zone

Distal alteration zones are about 30 m in width andcharacterised by the mineral assemblage magnetite-sid-erite-iron silicate (Fig. 3). The assemblage reflects thepartial replacement of chert bands by bladed magnetite(50–200 lm), siderite (800–1,500 lm) and iron silicates(20–110 lm; Fig. 5b, d). The shape of the magnetite isatypical suggesting that magnetite has pseudomorphedpreexisting minerals, such as dolomite or siderite. Wherepreserved, microcrystalline quartz is intergrown witheuhedral magnetite and as inclusions within sideriteblades. Pyrite crystals (50–300 lm) are finely dissemi-nated within shale bands. Apatite occurs as euhedralcrystals intergrown with anhedral chlorite (Fig. 6).

The dominant veins (V2) in the distal alteration zoneare filled with siderite and iron-silicates (Table 1) thatcrosscut and parallel BIF bands. The wallrock adjacentto V2 veins is locally brecciated with clasts (5–60 mm) of magnetite-rich mesobands in a matrix of bladed mag-netite, siderite and iron silicates (Fig. 5c).

Intermediate alteration zone

Intermediate alteration zones are about 15 m in width(Fig. 3), characterised by the mineral assemblagehematite-ankerite-magnetite, as a result of ankerite and

microplaty hematite replacing quartz, siderite, and iron-silicates (Fig. 6). Microplaty hematite (10–60 lm) hascrystallised as both individual blades and dense clustersthat form overgrowths on magnetite (Fig. 5f), and asindividual plates within ankerite crystals. Euhedral andbladed magnetite show minor replacement by martitealong crystal boundaries. Anhedral and microplatyhematite have replaced iron-silicates (Fig. 5d). Ankeritecrystals (0.20–4 mm) form variably recrystallised mosa-ics with irregular grain boundaries. Apatite is present asinclusions within magnetite and microplaty hematiteand as anhedral crystals within ankerite crystals.

There are two vein sets in the intermediate alterationzone: ankerite-hematite (V3, Table 1) and pyrite veins(V4, Table 1). The V3 veins are abundant and crosscutBIF and shale bands, whereas pyrite veins are rare andoccur within the fracture zones that postdate hematite-ankerite-magnetite alteration. Breccias are matrix-sup-ported and consist of angular and rotated clasts(5–40 mm) of altered wallrock within an ankerite-mi-croplaty hematite matrix. The presence of microplatyhematite in the matrix of a hydrothermal breccia and

within ankerite-hematite (V3) veins indicates that itcrystallised at elevated fluid pressures.

Proximal alteration zones

Proximal alteration zones are about 15 m in width andcharacterised by the mineral assemblage martite-micro-platy hematite-apatite (Fig. 3). Martite and anhedralhematite replace magnetite and iron silicates, respec-tively (Figs. 5f and 6). Other characteristic features arethe complete absence of ankerite and pyrite. Apatiteoccurs predominantly within the intergranular space of

martite mesobands. Intergranular porosity increasessignificantly to about 15%. Collapse breccias are clastsupported and consist of angular and rotated clasts (2– 40 mm) of altered martite mesobands within a martite-microplaty hematite matrix.

Petrography of supergene (Mr-mpH, low P)alteration zone

Areas of supergene alteration are about 55 m in width(Fig. 3) and characterised by the mineral assemblage

Fig. 5 Microphotographs showing hypogene alteration mineral-ogy. a Unmineralised magnetite (Mt) and chert (Qtz) microbands.Note microcrystalline dolomite (Dol ) within chert microband. bIntergrown platy siderite (Sid ) and iron silicates (FeSi ) pseudo-morphing chert mesoband. c Brecciated magnetite microband withmatrix of siderite (Sid ) and iron silicates (FeSi ). d Radial andindividual bladed magnetite (Mt), partially oxidised to microplatyhematite (mpHm) within siderite (Sid ) matrix. e Hematite-ankerite-magnetite alteration with ankerite (Ank) replacing siderite andmicroplaty hematite (mpHm) replacing iron silicates. Euhedralmagnetite (Mt) remains unoxidised. f Ankerite (Ank) veins, V3,crosscutting magnetite (Mt) mesobands with wallrock crystallisa-tion of microplaty hematite (mpHm) on magnetite. g Martite (Mr)and microplaty hematite (mpHm) within ankerite (Ank). H Skeletalmartite (Mr) crystals within goethite (Goe) matrix. Note minorinterstitial quartz (Qtz)

b

Table 1 Summary of vein types at the North Deposit

Vein type Thickness (mm) Vein mineralogy Association

V1 0.50–6.00 Dolomite, calcite, siderite Within unaltered BIF below and lateralthe North Deposit. Common

V2 0.20–10 Siderite, iron silicates Associated with magnetite-siderite-ironsilicate alteration. Common

V3 0.10–60 Ankerite-microplaty hematite Associated with hematite-carbonate-magnetitealteration zone. Common. Contain numerousfluid inclusions

V4 1.00–5.00 Pyrite Associated with brittle fracture zones that post-datehematite-ankerite-magnetite alteration. Rare

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martite-microplaty hematite±goethite (Fig. 6). Thisassemblage reflects the replacement of remnant magne-tite to goethite and the removal of most of the apatite.With increased proximity to the topographic surface, theabundance of goethite and skeletal and cellular martiteincreases (Fig. 5g). Fibrous quartz and colloform goe-thite (Fig. 5h) are locally developed within the fracturezones. Intergranular porosity is estimated to be about

30%. Shale bands are reduced in volume by up to 60%by the removal of carbonates, the oxidation of pyrite to

limonite, and the replacement of shales by pink kaolin-itic clays.

Fluid chemistry of hypogene alteration zones

Analytical methods

Fluid inclusion microthermometric measurements wereconducted using the fully automated Linkam THMSG600 heating and freezing stage. The precision of mea-surements was ±0.1°C with an accuracy of ±0.1°C for

CO2 melting (À56.6°C). For C and O isotope analyses,the carbonates were analysed using an MM602E mass

Fig. 6 The paragenetic alteration sequence in BIF at the NorthDeposit. Proximity to mineralisation increases to the right. Zonewidths shown here bear no resemblance to actual widths observedin the field

Fig. 7 Carbonate isotope andfluid inclusion sample locationsfrom the North Deposit

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spectrometer at the University of Queensland, Depart-ment of Earth Sciences. All isotopic data are reported inper mil relative to PDB (d13C) or V-SMOW (d18O).Analytical accuracy is better than ±0.1 (1r) for carbonand carbonate oxygen isotope determinations.

Fluid inclusion microthermometry

Sample locations

A total of ten 100–200 lm thick double polished car-bonate sections were examined petrographically and sixselected for microthermometric analysis. Samples weretaken from drill-holes ND2/79, ND77/79, ND83/79,ND93/80 (2) and ND94/80 (Fig. 7). The samples werecollected from altered BIF and ankerite-hematite veins(V3, Table 1) from hypogene hematite-ankerite-magne-

tite alteration in the BIF (Fig. 8a, b).

Carbonate vein texture and timing and abundanceof fluid inclusions

Ankerite-hematite veins consist of variably recrystal-lised ankerite crystals and microplaty hematite thatformed contemporaneously with hypogene mineralisa-tion (Fig. 8b, c). The variably recrystallised ankeritecrystals suggest that the crystallisation of the ankeriteoccurred during the ongoing deformation. Detailed

petrography revealed pseudosecondary and secondaryinclusions in ankerite. Pseudosecondary fluid inclusionsin wallrock- and vein-carbonates occur in orientatedclusters throughout individual crystals (Fig. 8d). Dis-continuous trails, with variable orientation, are also

present within individual carbonate crystals. Secondaryfluid inclusions occur within trails that crosscut crystalboundaries (Fig 8e). Secondary trails contain a highabundance of secondary fluid inclusions with thegreatest abundance of pseudosecondary and secondaryinclusions present at vein/wallrock contacts. The fluidinclusions observed did not exhibit stretching or leak-ing.

Shape

Pseudosecondary fluid inclusions in ankerite are char-acterised by their irregular shape and large size (15–

40 lm, Fig. 8f). Rare rectangular fluid inclusions, con-trolled by carbonate cleavage directions, are present.Liquid/vapour ratios range from 0.05 to 0.80 (n=38).Secondary inclusions are characterised by their ellipticalto subrounded shape and their relative small size (5– 30 lm, Fig. 8g). Liquid/vapour ratios range from 0.05 to0.30 (n=66).

Compositional types of inclusions and fluid inclusionassemblages

Petrological observations and heating and freezingmeasurements define two main inclusion types: type 1a

‘‘H2O–CaCl2-NaCl’’ (L-V) and type 1b ‘‘H2O–CaCl2 – NaCl-solid’’ (L-V-S, Fig. 8h). The exact species of thesolids is presently unclear. The solids are colourless andrectangular in shape and are likely to be either calcite,dolomite or halite daughter crystals. Liquid/vapour ra-tios in type 1a inclusions range from 0.05 to 0.80(n=100) and 0.05 (n=4) for type 1b of inclusions. Bothtype 1a and 1b inclusions are observed in ankerite ininternal trails and clusters as pseudosecondary inclu-sions and in cross-cutting trails as secondary inclusions,i.e. forming a fluid inclusion assemblage.

Fig. 8 Carbonate and fluid inclusion textures from hypogenehematite-ankerite-magnetite alteration. a Diamond drill coresample showing interbedded magnetite-microplaty hematite (Mt-mpHm) and ankerite (Ank) mesobands. b Ankerite-microplatyhematite (V3) veins crosscutting magnetite-microplaty hematite(Mt-mpHm) mesobands. c Crystallisation of microplaty hematite

(mpHm) with ankerite (Ank) within and along vein margins. bPseudosecondary fluid inclusion cluster within ankerite (Ank)crystal. e Secondary fluid inclusion trail crosscutting ankerite(Ank) crystal boundaries. f Irregular shaped two-phase (L+V)pseudosecondary fluid inclusions within ankerite crystal. G Ovoidshaped two-phase (L+V) secondary inclusions within ankerite(Ank) crystal. H Sub-rounded three phase (L+V+S) inclusionwithin ankerite crystal

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Microthermometric results

Microthermometric data were collected from bothpseudosecondary and secondary fluid inclusions. Initialand final ice melting (Te and Tmice, respectively), totalhomogenisation temperature (Thtot(L)), temperature of daughter crystal dissolution (Tdiss) and temperature of decrepitation (Tdecrep) are presented in Table 2. Themajority of pseudosecondary inclusions homogenised

into a liquid at about 250°C (one inclusion homogenisedat 449°C) with several inclusions decrepitating at about350°C (Fig. 9a). Secondary inclusions homogenised intoa liquid at about 120°C.

Salinity, bulk composition and density

The salinity (eq.wt.% CaCl2), bulk composition, anddensity of aqueous inclusions were calculated using theMacFlincor program (Brown and Hagemann 1996) andthe Zhang and Frantz (1987) equation of state for theH2O–CaCl2±CO2 system. Pseudosecondary type 1aand 1b inclusions show initial ice melting temperaturesbetween À64.4 and À52.0°C (n=38) and À56.1°C(n=1), respectively. Secondary type 1a and b inclusionsshow the initial ice melting temperatures lie betweenÀ64.7 and À44.0°C (n=66) and À55.9 and À54.5°C(n=3), respectively. These temperatures approximatethe eutectic melting temperatures for the system H2O– NaCl–CaCl2 (À55°C, Borisenko 1977). Lower freezingtemperatures may indicate the presence of other cationssuch as iron (Baldassaro and Bodnar 1998).

Final melting temperatures of ice for pseudosecond-ary 1a inclusions were between À32.4 and À21.1°C(À25.7±2.9°C, 1r, n=37) indicating salinities of 23.9±1.5 (1r, n=38) eq.wt.% CaCl2 (Fig. 9b). The final

melting temperature of ice for one pseudosecondary 1binclusion was À26.2°C (n=1) indicating a salinity of 24.6 eq.wt.% CaCl2. Final melting temperatures of icefor secondary 1a inclusions were À32.8 and À16.2°C(À26.1±3.6°C, 1r, n=63) indicating salinities of

24.4±1.5 (1r, n=66) eq.wt.% CaCl2 (Fig. 9b). Finalmelting temperatures of ice for secondary 1a inclusionswere between À27.1 and À19.1°C (À23.7±4.1°C, 1r,n=3) indicating salinities of 23.4±2.0 (1r, n=3)eq.wt.% CaCl2 (Fig. 9b). Note that these salinities arelikely minimum salinities due to: (a) the occurrence of low eutectica in some samples (<< À55°C), which in-dicates additional cations besides Ca2+, and (b) the lackof an appropriate equation of state that allows the cal-culation of salinities from fluid inclusions that containdaughter crystals, which dissolve upon heating. Theseinclusions likely display salinities substantially higherthan the maximum salinity calculated based on the finalice melting temperatures (i.e., >30 eq.wt.% NaCl).

Corresponding aqueous densities for pseudosecond-ary type 1a and 1b inclusions range from 0.94 to 1.17 g/cc (1.04±0.06 g/cc, 1r; n=37) and 1.12 g/cc (n=1),respectively. Densities for secondary type 1a inclusionsrange from 1.12 to 1.28 g/cc (1.14±0.02 g/cc, 1r,n=63). Densities for secondary type 1b inclusions rangefrom 1.14 to 1.18 g/cc (1.16±0.02 g/cc, 1r, n=3).

Interpretation of fluid inclusion data

There is no petrographic or microthermometric evidencefor fluid immiscibility (unmixing) of an aqueous fluid,thus the homogenisation temperatures do not representthe true trapping temperatures (Pichavant et al. 1982;Roedder 1984). Therefore, a pressure (temperature)correction has to be applied to the homogenisationtemperatures in order to determine the true trappingtemperatures of the fluid inclusions (Roedder 1984).Using the pressure correction table of Potter (1977) for25 eq.wt.% NaCl in the system H2O–NaCl and assum-ing a pressure of 500 bar (based on an overburden of about 2 km at the time of mineralisation), a pressurecorrection of 50°C has to be applied to the measuredhomogenisation temperatures. Consequently, it is pro-posed that the pseudosecondary and secondary fluidinclusions were trapped during cooling of an aqueous

Table 2 Microthermometric results for pseudosecondary and secondary fluid inclusions

Pseudosecondary inclusions Secondary inclusions

2-phase(L+V)

2-phase (L+V)decrepitated

3-phase(L+V+S)

2-phase(L+V)

3-phase(L+V+S)

N 34 4 1 66 3TmiceRange À32.4 to À21.1°C À31.5 to À22.8°C À32.8 to À16.2°C À27.1 to À19.1°CAverage ± 1r À25.7±2.9°C À26.7±3.4°C À26.2°C À25.8±3.6°C À23.7±4.1°C

Thtot(L)Range 153 to 449°C 103 to 157°C 103 to 128°CAverage ± 1r 253±59.9°C 182°C 117±10°C 118±13°C

TdecrepRange 298 to 345°CAverage ± 1r 329±21°C

ThdissRange 240 to 280°CAverage ± 1r 239°C 307±21°C

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hydrothermal fluid between 300–350°C and 170°C,respectively.

Pseudosecondary and secondary aqueous fluidinclusions have similar salinities but vary significantly intheir homogenisation temperatures (Fig. 9b). The dif-ference of about 100–200°C in trapping temperaturesbetween the two fluid inclusion types is compatible withtwo distinct fluid pulses: a hot (>300–350°C) saline(24 eq.wt.% CaCl2) brine that circulated during the

formation of the hematite-ankerite-magnetite alteration,and a later stage (170°C) saline (24 eq.wt.% CaCl2)brine. The latter fluid could represent either the finalcooling stage during hematite-ankerite-magnetite alter-ation or a separate fluid phase.

Isotope geochemistry

Eight siderite samples from hypogene magnetite-siderite-iron silicate alteration (Population 1, S18-25) and 17

ankerite samples from hypogene hematite-ankerite-magnetite alteration (Population 2, A1-17) were col-lected by microdrilling in order to determine the carbonand oxygen composition of carbonates (Fig. 6). Hypo-gene samples were compared with diagenetic carbonatesidentified from unmineralised BIF at the North Deposit(Rivers 1998) and from the marine carbonates of theWittenoom Formation (Rivers 1998). All data, includingcarbonate isotope data on the magnetite-siderite-iron

silicate alteration in the North Deposit obtained byBarley and Pickard (1997) are presented in Table 3 andFig. 10.

Results

Population 1 samples (Barley and Pickard 1997; Thorne2001) fall largely within or near the domain of the un-mineralised, diagenetic North Deposit carbonates withtwo samples having similar isotopic composition toPopulation 2 samples (Fig. 10). Population 2 samples

0

50

100

150

200

250

300

350

400

450

500

-35 -30 -25 -20 -15 -10

A

H o m o g e n i s a

t i o n T e m p e r a t u r e ( o C )

Final Ice Melting (oC)

0

50

100

150

200

250

300

350

400

450

15 20 25 30 35 40

B

H o m o g e n i s a t i o n T e m p e r a t u r e ( o C )

eq. wt%CaCl2

xPseudosecondary Inclusions (Tdecrep) Pseudosecondary Inclusions (Tdiss)

Secondary InclusionsPseudosecondary Inclusions

Fig. 9 a Temperature of finalice melting, and b equivalentweight percent CaCl2 versushomogenisation temperature(L) data for fluid inclusionstrapped in ankerite withinankerite-hematite veins in thehypogene hematite-ankerite-magnetite alteration, with datadistinguished by inclusion type

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have similar d18 O values to carbonate and siderite

samples from the North Deposit but d13C values fall

between Population 1 and Wittenoom Formation sam-ples (Fig. 10). Samples from the Wittenoom Dolomiteform a distinct population with a small range in d

13C buta large range in d

18O.

Interpretation of isotopic data

Depletedd13

C values of ankerite (d

13C; À4.9±2.2&,1r, n=15) from the hematite-ankerite-magnetite alter-ation zone indicate that the bulk of the carbon withinthe alteration zone is not derived from the BIF sequence.Similar oxygen isotope compositions, but increasinglyheavy carbon isotopes from magnetite-siderite-iron sili-cate alteration to hematite-ankerite-magnetite alterationzones, suggest the progressive exchange (mixing) with anexternal fluid with a heavy carbon isotope signature. It islikely that an ascending, saline fluid mixed with theWittenoom Formation (d13C; 0.9±0.7, 1r, n=15)provided such a fluid source. Evidence from deep

drilling at the Mt Tom Price deposit (Taylor et al. 2001)suggests that the Wittenoom Formation is stratigraphi-cally thinned below the Mt Tom Price deposit and isstructurally linked via the Southern Batter Fault. Thissuggests that the Wittenoom Dolomite may haveacted as a possible source for carbonates with carbonisotopic signature heavier than that in the unmineralisedBIF.

Structural and hydrothermal model for the North Deposit,Mt Tom Price

Hypogene alteration at the North Deposit is constrainedby the NW trending dykes that crosscut the CheelaSprings Basalt dated at 2,209 Ma (Fig. 11; Martin et al.1998). At Paraburdoo and Channar (Fig. 12) abundanthigh-grade hematite clasts occur within the basal MtMcGrath Formation indicating that the main period of iron mineralisation within the southern HamersleyProvince occurred before 1,843 Ma (Taylor et al. 2001).

Table 3 The C and O isotopic compositions of hypogene and unmineralised carbonate samples from Mt Tom Price

Sample d13CPDB d

18OV-SMOW

Diagenetic carbonates from unmineralised BIF,North Deposit (Rivers 1998)

(À10.4 to À7.7) À8.8±1.0 (n=12) (15.5 to 24.6) 18.6±2.7 (n=12)

Magnetite-siderite-iron silicate alteration, North Deposit(Barley and Pickard 1997; Thorne 2001)

(À10.6 to À6.3) À8.8±0.7 (n=17) (13.2 to 18.7) 16.2±1.5 (n=17)

Hematite-ankerite-magnetite alteration,North Deposit (Thorne 2001)

(À7.5 to À0.9) À4.9±2.2 (n=17) (14.3 to 17.0) 14.3±1.0 (n=17)

Wittenoom Formation, Mt TomPrice deposit (Rivers 1998)

(À3.2 to 0.0) À0.9±0.7 (n=15) (16.6 to 25.4) 20.0±2.5 (n=15)

5

10

15

20

25

30

-12 -10 -8 -6 -4 -2 0 2

Thorne (2001): Hem-Ank-Mag alteration

(North deposit)

Thorne (2001): Mg-Sid-FeSi alteration

(North deposit)

Rivers (1998): Dolomite from the Wittenoom

Formation (Mt Tom Price deposit)

Rivers (1998): Diagenetic carbonates form

unmineralised North deposit BIF

Barley and Pickard (1997): North Deposit Mg-Sid-FeSi

alteration (North deposit)

δ13 (PDB)

δ 1 8

( V - S M O W )

Diagenetic carbonates from

unmineralised BIF (North deposit)

Population 1

Magnetite-siderite-iron silicate


Population 2Hematite-ankerite-magnetite


Wittenoom Formation

(Mt Tom Price deposit)

Fig. 10 d18O-d13C diagram

showing the isotopiccomposition of the variouscarbonate populations in theNorth and Mt Tom Pricedeposits

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Similar stratigraphic evidence is not present at Mt TomPrice. However, the main period in which the HamersleyProvince is likely to have experienced a higher thannormal geothermal gradient was during deposition of the Wyloo Supersequence in a rift basin associated withlithospheric extension (Krapez 1999). This tectonicframework places the timing of hypogene mineralisationbetween 2,209 and 1,843 Ma (Barley et al. 1999; Thorne2001). At Mt Tom Price, stratigraphic relationshipsconstrain supergene alteration to a period of Mesozoic– Cenozoic weathering (Morris 1980, 1985; Harmsworthet al. 1990; Taylor et al. 2001; Thorne 2001).

Model

The timing relationships outlined above, in combinationwith the distinction of hypogene and supergene alter-ation zones and related geochemical constraints on theore fluids, provide constraints on the preliminarystructural-hydrothermal model for the North Deposit.In this model, a clear distinction is made between pro-gressive stages of early and late hypogene alteration(stages 1a–c; Fig. 11) that can be distinguished in thetransition from BIF (30–35% Fe) to high-grade iron ore(>65% Fe), and supergene alteration (stage 2; Fig. 11).

Stage 1a Early magnetite-siderite-ironsilicate hypogene alteration

Initial hypogene alteration (Fig. 1a) at the North De-posit occurred within the dominantly magnetite-chertlayers of the Dales Gorge Member. Unmineralised,magnetite-chert BIF wallrocks are transformed laterallyand vertically into magnetite-siderite-iron silicate BIF

with subsequent desilicification of the chert bands. Theassemblage reflects the partial replacement of chertbands by bladed magnetite, siderite and iron silicates.

Fig. 11 The stages of hypogene and supergene alteration at theNorth and Southern Ridge deposits. Both deposits are shown juxtaposed to one another only to illustrate relationships of fluidflow and alteration. a Stage 1a early hypogene magnetite-siderite-iron silicate formed by ascending 150–250°C basinal brines. b Stage1b early hypogene hematite-ankerite-magnetite alteration formedby ascending 300–400°C basinal brines. c Stage 1c late martite-microplaty hematite-apatite alteration formed by ascending

$120°C basinal brines. d Stage 2 supergene martite-microplatyhematite-goethite alteration formed by descending shallow mete-oric waters (<100°C)

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Siderite-iron silicate veins (V2) crosscut and parallel BIFbands. The wallrock adjacent to V2 veins is locallybrecciated with clasts of magnetite-rich mesobands in amatrix of bladed magnetite, siderite and iron silicates.

A trend from heavy carbon isotope compositions of dolomites from the Wittenoom Formation, whichunderlies the Mt Tom Price deposit, to progressivelyheavier carbon isotope compositions of siderite in thestage 1a hypogene alteration zone (Fig. 9) suggeststhat hydrothermal fluids (brines) were released fromthe underlying Wittenoom Formation and directedupward along normal faults, probably during thedeposition of the Wyloo supersequence and associatedlithospheric extension (Krapez 1999). These fluids werefocussed within the silica-rich rocks of the DalesGorge Member by the shales of the underlying MtMcRae Shale Member and overlying Whaleback For-mation, which acted as an aquitard. Within the DalesGorge Member, hydrothermal fluids migrated laterallywithin large-scale folds with permeability controlled byshale bands and the NW trending dolerite dyke sets(Fig. 11a).

Stage 1b Hematite-ankerite-magnetite hypogenealteration

Continuing reactions between the ascending hydrother-mal fluids and magnetite-siderite-iron silicate alterationproduced the hypogene hematite-ankerite-magnetitealteration, leaving only the remnants of the former(Fig. 11b). The replacement of iron silicates by hematiteand the replacement of siderite by ankerite via thereaction

2FeCO3 þ Ca2þxMg2þ ! Ca xMgð1 À xÞFe½ CO3ð Þ2

þ xFe2þ

accompanied continued desilicification of the chertbands. Microplaty hematite has crystallised as bothindividual blades and dense clusters that form over-growths on magnetite and as individual plates withinankerite crystals. Apatite is present as inclusions withinmagnetite and microplaty hematite and as anhedralcrystals within ankerite crystals. Abundant ankerite-hematite (V3) crosscut BIF and shale bands and withinthe hematite-ankerite-magnetite alteration zones cross-cut BIF and shale. Wallrock breccias are matrix-sup-

ported and consist of angular and rotated clasts of altered wallrock within an ankerite-microplaty hematitematrix. Rare pyrite veins (V4) that crosscut all other veintypes and the crystallisation of microplaty hematitesuggests locally a late-stage influx of sulfide-bearingfluid. The increase in heavy carbon isotope values fromstage 1a magnetite-siderite-iron silicate alteration tostage 1b hematite-ankerite-magnetite alteration (Fig. 10)suggests the ongoing progressive isotopic exchange viathe influx of hydrothermal brines, likely sourced fromthe underlying Wittenoom Formation. Fluid inclusion

evidence suggests that the hematite-ankerite-magnetitealteration involved a hot (>300–400°C) saline(24 eq.wt.% CaCl2) brine, which increased the solubilityof carbonates (Rimstidt 1997) resulting in the wide-spread dissolution and stratigraphic thinning of theWittenoom formation below the Mt Tom Price deposit:

CaMgðCO3Þ2 ! Ca2þ + Mg2þ + 2CO2þ3

Stage 1 Late martite-microplaty hematite-apatitehypogene alteration

The final stage of hypogene alteration involved thetransformation of magnetite and iron silicates tohematite, and the dissolution of the ankerite from theprecursor stage 1b hematite-ankerite-magnetite rock(Fig. 11c). The preservation of low-temperature, salinesecondary fluid inclusions ($120°C and 24 eq.wt.%CaCl2) preserved in relict ankerite, suggests that ankeritedissolution occurred late in the hydrothermal evolutionof the North Deposit. The high salinity of the fluids

precludes meteoric water as a fluid source and suggeststhat these brines likely relate also to the release of fluidsfrom the underlying Wittenoom Formation.

Stage 2 Martite-microplaty hematite-goethitesupergene alteration

The second stage (Fig. 11d) resulted in the removal of most of the phosphorus from the BIF bands with goe-thite and anhedral hematite replacing martite. Shalebands were weathered to clay with a considerablereduction in volume. The high intergranular porosity of the ore (30%), the stratigraphic thinning of shale bands,and subsequent compaction results in a ‘‘mechanical’’enrichment factor. The distribution of phosphorus andpresence of goethite within the alteration zone suggestthat this stage involved cool (<100°C), presumablyshallow meteoric fluids.

Discussion

Taylor et al. (2001) have proposed the deep circulationof oxidised, low salinity meteoric fluids (150–250°C) forthe formation of microplaty hematite and hematite-

martite-apatite-ankerite mineralisation at the SouthernRidge deposit, Mt Tom Price. Evidence from hypogenealteration at the North Deposit establishes that crystal-lisation of microplaty hematite and ankerite occurred atsignificantly higher temperatures (253±60°C, 1r, n=34)and elevated salinities ($24 eq.wt.% CaCl2). Heavyd13C isotopic signatures of ankerite, texturally in equi-

librium with microplaty hematite, indicate that ascend-ing hydrothermal fluids that interacted with carbonatesfrom the Wittenoom Formation were largely responsiblefor hypogene alteration. At the Southern Ridge deposit,

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mineralisation invariably appears to be related withearly normal faults (Southern Batter Fault; Hagemannet al. 1999; Ridley 1999; Taylor et al. 2001) and is likelythe conduit for ascending hydrothermal brines that werereleased into the Dales Gorge Member.

The coexistence of microplaty hematite and pyriteconfirms that the transformation of magnetite tohematite could not have occurred by oxidation reactions(Kamei and Ohmoto 2000). The mineralogy of thehydrothermal alteration at the North Deposit supports

an argument, where the ascending hydrothermal brineshave caused an in situ enrichment of BIF by mobilisingFe from magnetite, iron silicates and carbonates andprecipitating it as microplaty hematite (Hagemann et al.1999) with simultaneous removal of silica. Studies of thetransformation of hematite to magnetite by Ohmoto(2003) show that a non-redox reaction can transformmagnetite to hematite (and vice versa), especially inhydrothermal environments. Ohmoto (2003, Fig. 3)demonstrates that reactions between BIF and meteoricfluids, which typically have Fe2+ contents less than$10À5 m, should transform magnetite to hematite whileincreasing the Fe2+ content of fluids. The precipitation

of microplaty hematite would be favoured in this envi-ronment. Such fluid interactions are supported by Tay-lor et al. (2001), who argue on the basis of mass balancecalculations that there is sufficient iron in the precursorbanded iron formation to account for all the iron in theorebody, and that no external source is required toconvert BIF to high-grade ore.

Evidence from the North Deposit indicates that theinteraction of hydrothermal saline brines with the hostBIF involves predominantly desilification and thetransformation of iron-bearing minerals to other forms

such as microplaty hematite. It is likely that theprogressive transformation of unmineralised BIF tosiderite-magnetite-iron silicate to ankerite-magnetite-hematite and then martite-hematite is controlled by thephysiochemical (P-T-X) properties of the ascending ba-sinal brines, rather than the transport of Fe-rich basinalbrines. Barley et al. (1999) proposed that the initial stageof mineralisation at the Mt Tom Price deposit involvedcrystallisation of magnetite (hematite)-siderite (iron sil-icate) assemblages at temperatures above 150°C (andpossibly above 250°C) and high pressures (i.e. hydro-thermal conditions). Petrographic textures and fluidinclusion studies from the North Deposit show that the

interaction of magnetite-siderite-iron silicate alterationwith higher temperature (300–400°C) and highly saline($24 eq.wt.% CaCl2) fluids produced the hematite-ankerite-magnetite alteration. Although speculative,given the tentative connection of the data sets, it isproposed that the hydrothermal alteration at the NorthDeposit formed from ascending hydrothermal brine thatincreased in temperature, from approximately 150– 400°C. The increasing temperature of the saline brinesincreased the solubility of carbonates (Rimstidt 1997)resulting in the brines becoming increasingly Ca-rich.

Temperatures of hydrothermal alteration at theNorth Deposit are higher when compared to other high-

grade iron ore deposits. At the Mount Whaleback mi-croplaty hematite deposit (Fig. 1), vein formation tem-peratures from 203 to 231°C have been obtained fromprimary fluid inclusions within quartz veins. The quartzveins may have formed from feeder or effluent fluidsfrom microplaty hematite ore formation (Brown andOliver 2002). Similarly, fluid inclusion data on carbon-ates and quartz associated with hematite ores at theThabazimbi iron deposit (South Africa) indicate thepresence of two distinct hydrothermal fluids, a high-salinity fluid that resulted in deposition of early dolomite

Fig. 12 Schematic diagram showing the relative and absolutetiming of events at the North Deposit

781



at temperatures around 150–160°C and a low-salinityfluid that led to precipitation of quartz at 120–140°C(Netshiozwi 2002). It is unclear if the higher tempera-tures of hydrothermal alteration at the North Depositare anomalous because the data sets form an incompletecontinuum dependent on tectonic and regional settings.The most striking similarities of the hydrothermalalteration at the North Deposit with the alteration at theCarajas mines (State of Para) and the Aquas Clarasmine in the Quadrilatero Ferrifero, in Brazil (Beukeset al. 2002) are the carbonate-hematite rocks, developedfrom the replacement of chert by carbonate, that typi-cally form an aureole around hard hematite ores(Geudes et al. 2002)

The hydrothermal model proposed for the NorthDeposit is analogous to the basinal brine model forMississippi Valley-type deposits (MVT), where gravity-driven fluid flow is the preferred driving force for fluidmigration, although tectonic pumping, shale compaction,and episodic dewatering have been all suggested asalternative driving forces (Kendrick et al. 2002). TheMVT deposits commonly occur where mineralising

brines are focussed by the upward movement of hydro-thermal saline (NaCl-CaCl2) brines along inclined faultsand associated fracture networks and laterally along thebedding planes of permeable lithologies (Sicree andBarnes 1996). However, the higher temperatures of hydrothermal fluids (i.e. 300–400°C) responsible forhypogene alteration at the North Deposit differ signifi-cantly from those associated with MVT deposits (75– 200°C; Shelton et al. 1992). Barley and Pickard (1997)have suggested that enhanced hydrothermal fluid flowmay have formed in response to elevated geothermalgradients during the uplift of the basement domes duringlithospheric extension and deposition of the Wyloo Su-

persequence. The emplacement of the Cheela SpringsBasalt and associated dyke swarms also reflect this ther-mal pulse. Evidence from sedimentary facies indicatesthat basement domes were being uplifted and eroded astectonically active highlands during deposition of theBeasley River Quartzite (Thorne and Seymour 1991).

Conclusions

Detailed petrographic analyses of diamond drill coretaken from the North Deposit identified three hypogenealteration zones between unmineralised BIF and high-

grade (>65% Fe) iron ore. They are the: (1) distalmagnetite-siderite-iron silicate, (2) intermediate hema-tite-ankerite-magnetite, and (3) proximal hematite-apa-tite alteration zones.

Based on petrological and geochemical data, a two-stage hydrothermal-supergene model is proposed for theformation of the North Deposit. Early stage 1a and 1bhypogene alteration involved the upward movement of hydrothermal, chlorine-rich brines via the SouthernBatter Fault and laterally within large-scale folds of theDales Gorge Member. The saline brines are interpreted

to originate from the interaction of hydrothermal fluidswith the carbonate-rich Wittenoom Formation. Reac-tion of the hot, 150–250°C hydrothermal saline brines,transformed unmineralised BIF to magnetite-siderite-iron silicate BIF with subsequent desilicification of thechert bands. As the temperature of hydrothermal fluidsincreased to about 300–400°C continued interactionbetween the ascending hydrothermal fluids and magne-tite-siderite-iron silicate alteration produced hematite-ankerite-magnetite alteration and the crystallisation of microplaty hematite. Late stage 1 hypogene alterationinvolved the interaction of lower temperature, $170°C,meteoric fluids with the precursor hematite-ankerite-magnetite hydrothermal alteration leaving a poroushematite-apatite mineral assemblage. Stage 2 involvedsupergene enrichment in the Cretaceous/Tertiary andresulted in the removal of most of the phosphorus fromthe BIF bands and the weathering of shale bands to clay.

Although the exact timing and tectonic frameworkare speculative, it is clear that petrographic and geo-chemical evidence from the North Deposit at Mt TomPrice provides new insight into the processes associated

with the formation of high-grade (>65% Fe) iron-oredeposits of the Hamersley Province. Further investiga-tions on the detailed structural control and alterationzonation of other Tom Price iron orebodies, combinedwith microthermometric investigations on hypogenealteration zones, are necessary to further develop therole of hydrothermal fluids on the formation of high-grade iron deposits and establish a robust genetic modelfor the Mt Tom Price deposit.

Acknowledgements We would like to thank Prof. Carlos Rosie ` re formany insightful discussions and comments on several parts of thepaper. He was also of great assistance in elucidating the complexiron ore textures and microstructures. Warren Thorne would par-ticularly like to acknowledge Shankar Madan who organised thisresearch project. The Hamersley Iron Pty Ltd geologists Tim An-drews, Raul Bitencourt, Naomi Wall and Rod McKenzie arethanked for their invaluable assistance during fieldwork and alsofor willingly sharing their detailed knowledge of the Mt Tom Pricedeposit. We would also like to thank the detailed reviews andinsightful comments by J. Gutzmer and A. R. Cabral.

References

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Barley ME, Krapez B (1997) Placing the Late Archaean to Pa-leoproterozoic Hamersley Province BIF and iron ore in theirglobal geological context. In: Advances in understanding theHamersley Province. Key Centre for Strategic Mineral Depos-its, M.Sc. course notes

Barley ME, Pickard AL (1997) The importance of variations inBIF composition and regional tectono-stratigraphic evolutionto the development of high-grade iron ore. Hamersley Iron Pty.Ltd Report, p 25

Barley ME, Pickard AL, Sylvester PJ (1997) Emplacement of alarge igneous province as a possible cause of banded iron for-mation 2.45 billion years ago. Nature 385:55–58

Barley ME, Pickard AL, Hagemann SG, Folkert SL (1999)Hydrothermal origin for the 2 billion year old giant iron ore

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