Disseminated and Massive Nickel Sulphide deposits, Honeymoon Well Complex, Western Australia

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Journal of the Virtual Explorer (2000), ISSN 1441-8126 Volume No. 1

Introduction

The Honeymoon Well nickel deposits, locatedapproximately 45 km south of Wiluna (26º40'S,120º25'E), are the most northerly of numerous knownnickel sulphide deposits within the 2700 Myr old Agnew- Wiluna greenstone belt (Fig. 1; Marston,1984; Hill etal., 1995; Gole et al., 1998). Both disseminated andmassive nickel sulphide deposits are present within thedeformed and metamorphosed Honeymoon We l lultramafic complex. Despite alteration to lowergreenschist facies serpentine-rich and minor talc-carbonate assemblages igneous rock types can begenerally recognised through a combination of preservedrelict igneous textures, metamorphic mineralogy andgeochemistry. Recognition of igneous protoliths andreconstruction of the igneous stratigraphy show that thedisseminated and massive sulphide deposits are hostedby distinctly different komatiitic rocks that in turn reflectformation in markedly different volcanic settings.

Bedrock in the Honeymoon Well area is covered by 50to 120 m of overburden and most geological data havebeen derived from 830 diamond drill holes and a similarnumber of percussion holes as well as detailedaeromagnetic data. Much of these data have beenobtained since the early 1990’s and thus wereunavailable for earlier studies of the Honeymoon Welldeposits (Donaldson and Bromley, 1981; Gole and Hill,1988). Drill hole core and rock chips have been loggedfor igneous and metamorphic textures, rock and veinmineralogy, and assayed for a range of sulphide- andlithology-related elements. Igneous rock names are usedwhere appropriate in this report despite completemetamorphic reconstitution of the igneous silicatemineralogy and partial to complete reconstitution ofigneous oxides and sulphides.

The current sulphide resource is 155 MT at 0.71% Ni(0.4% Ni cut off, <300 m depth) in one massive andmatrix sulphide deposit and three disseminated sulphidedeposits. This includes a massive sulphide resource of

Disseminated and Massive Nickel Sulphide deposits,Honeymoon Well Komatiite Complex, Western Australia

MARTIN GOLE1 AND MIKE WOODHOUSE2

1 Martin Gole and Associates,8 Landor Road, Gooseberry Hill

Western Australia, 6076.

2 Outokumpu Mining Australia,1/15 Joel Terrace, PerthWestern Australia, 6004.

Abstract: Both disseminated and massive Ni sulphide mineralisation is hosted in a variety of komatiiticrocks within the Honeymoon Well ultramafic complex, located in the northern part of the 2700 Myr oldAgnew-Wiluna greenstone belt, Western Australia. The ultramafic complex, extending over a 10 x 2 km area,is a structurally thickened part of the Mt Keith Ultramafic Sequence. This sequence hosts the nickel depositsat Perseverance, Yakabindie and Mt Keith. The Honeymoon Well sulphide resource is 155.5 MT at 0.71%Ni (includes 2.5 MT at 3.36% Ni) in one massive and three disseminated sulphide deposits. Despitegreenschist facies metamorphism and several episodes of deformation, igneous textures are widely preservedin the ultramafic rocks. The host rocks of the disseminated deposits are totally serpentinised olivineadcumulate, mesocumulate and minor orthocumulate. Sulphide assemblages are dominated by pentlanditeand heazlewoodite and occur in modified magmatic-textured lobate to blebby aggregates interstitial toformer olivine grains. The massive sulphide deposit consists of massive sulphide, massive sulphide brecciaand matrix sulphide hosted in a sequence of spinifex-textured thin komatiite flows. These sulphides havesignificantly lower Ni/S ratio than the disseminated sulphides reflecting an Fe-rich sulphide assemblage ofpyrrhotite - pentlandite - pyrite - chalcopyrite. All the Ni sulphide deposits are thought to have beenoriginally part of a single mineralised horizon with lateral variations in physical volcanology leading to theformation of komatiite sequences ranging from those dominated by olivine mesocumulate - adcumulate tothose dominated by spinifex-textured thin flows. Subsequent stratigraphic stacking within a D1 thrust duplexand D2 folding and strike slip shearing has significantly displaced two of the deposits from their originalstratigraphic position. Late As-bearing carbonate fluids have affected parts of the sulphide deposits.

Gole, M. & Woodhouse, M. 2000. Honeymoon Well Complex, Western Australia Journal of the Virtual Explorer - Volume 1

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2.5 MT at 3.36% Ni. Nickel grades in all thedisseminated deposits are slightly to significantly higherthan at Mt Keith (460 MT at 0.58% Ni; George, 1996)and at Yakabindie (520 MT at 0.47% Ni; North Limited,2000) which are hosted by similar komatiite sequences.

Regional geological setting

At Honeymoon Well the greenstone belt is 6-7 kmwide and composed of a regional west-youngingsequence of a lower basalt/gabbro unit, including alaterally persistent basaltic komatiite flow, felsic tointermediate volcanic and volcaniclastic rocks, aheterogeneous komatiite sequence and a westernfelsic/basalt sequence (Liu et al., 1995). Local east-younging is due to minor folds associated with faultzones. The ultramafic sequence, which within theHoneymoon Well complex is 1.5 to 3.0 km wide,consists of a diverse suite of metamorphosed komatiitelithologies including spinifex-textured rocks, olivineorthocumulate, mesocumulate and adcumulate (oOC,

oMC, oAC). Minor augite and augite-plagioclasecumulates define fractionated cyclic units within thepresumed upper parts of the ultramafic sequence. A l lthe ultramafic lithologies are interpreted to haveformed in a volcanic setting (see Hill et al., 1990,1 9 9 5 ) .

In the area around Wiluna and within the HoneymoonWell ultramafic complex the komatiite stratigraphy hasbeen duplicated by faulting. Duplication is interpreted tohave taken place during D1 thrust faulting. Folding anddeformation during D 2 resulted in the formation of talc-carbonate-rich NNW trending strike slip faults and shearzones that further duplicated parts of the sequence.

The stratigraphic stacking accounts, in part, for theanomalous thickness of the ultramafic rocks atHoneymoon Well compared to the remainder of the belt.Much of the ultramafic sequence, particularly atHoneymoon Well and probably to the immediate south,appears to be allochthonous.

Strike-slip faulting has removed a 5 km long section ofthe ultramafic sequence immediately north of the

Figure 1. Honeymoon Well solid geology interpretation at 100 m below surface.


Honeymoon Well ultramafic complex. Some of the D2faults and shears are probably reactivated D1 thrustsurfaces. The main metamorphic alteration in the area isassociated with D2 strike slip fault development( E i s e n l o h r, 1992). This structural and metamorphicevolution is similar to that proposed for the southern partof the Norseman-Wiluna greenstone belt (Swager, 1997).Metabasalt assemblages at Honeymoon Well indicatel o w e r-most greenschist facies metamorphic grade(Donaldson and Bromley, 1981). Komatiites are alteredto serpentine-dominant and local talc-carbonateassemblages.

Honeymoon Well Geology

Regolith

The ultramafic sequence occurs below a 50-120 mcover of cemented sands and grits (5-25 m), transportedclays and minor basal gravels (0-40 m) and a residualregolith (30-60 m). The residual regolith varies markedlyover the ultramafic rocks compared to adjacentlithologies. Over the ultramafic rocks the regolith profileis highly variable and has been variably stripped since itsformation (Mitchell, 1997). Where complete it consistsof an upper brown, strongly leached, highly porous andpartially silicified, goethite-rich upper saprolite, a green-grey clay-rich lower saprolite, and then saprockoverlying the bedrock. Within the saprolite over oACand oMC are hard, highly porous, laterally discontinuoussilica-minor goethite layers that range up to 30 m inthickness.

Bedrock Lithologies

Ultramafic Rocks: Where igneous textures arepreserved oAC display polygonal-textured olivinepseudomorphs and triple-point junctions with adjacentpseudomorphs and have negligible igneous porosity(see Donaldson and Bromley, 1981). Olivine grainsizes range up to 2.5 cm but are mostly 4-10 mm.Stichtite after chromite forms lobate, porousaggregates at some triple-point junctions. Within oMColivine pseudomorphs are mostly 3-8 mm across whilestichtite content (0.5- 5%) and aggregate size is highlyvariable. Olivine orthocumulates have a wide variationin igneous porosity and texture. Low porosityorthocumulates generally have coarser, more even-grained olivine pseudomorphs, whereas high porosityrocks show a wide range in olivine grain size,sometimes within the same rock, and with grain shapesranging from euhedral to hopper to harrisitic. T h e s elatter rocks are mostly associated with spinifex-textured rock sequences.

At a few localities, delicate igneous textures are verywell preserved within 20-40 m thick sequences of thinspinifex-textured flows. Mostly, however, deformationand metamorphic recrystallisation has destroyed spinifextextures, with former thin flows now represented byinterlayered tremolite-chlorite and serpentine-tremolite-chlorite schists or talc-carbonate altered equivalents.

Some spinifex-textured rocks and associated oOCshow evidence of partial melting and high-temperaturerecrystallisation. In such rocks textures and mineralcompositions differ markedly from those in typicalkomatiites (Gole et al., 1990). Spinifex olivine plates arebent and interstitial spaces are composed of polygonalaggregates of partially altered clinopyroxene,orthopyroxene, minor olivine, plagioclase and spinel.Two pyroxene thermometry yields temperatures of 1055to 1140ºC, just below the low pressure komatiite solidus( T h y, 1995). These features reflect metamorphism,partial melting and subsequent slow cooling within a 40-60 m thick sequence of spinifex-textured flows that arelocated between two thick stratigraphic sequences ofolivine-rich cumulates. Thermal modelling shows thatpost-magmatic conductive cooling of these two cumulatepiles, if they were formed within a period of about tenyears, would re-heat the intervening flows to theobserved metamorphic temperatures (Gole et al., 1990).These very distinctive rocks, which require a veryrestrictive set of circumstances to form, are part of anhorizon, termed the Recrystallised Marker Horizon(RMH; Fig. 1), that is critical in the stratigraphicreconstruction of the Honeymoon Well ultramaficcomplex.

No traceable stratigraphic layering has beenrecognised within the main mass of oAC but has beenrecognised in various states of preservation within themarginal ultramafic sequences (Fig. 1).

Strain within the ultramafic rocks has been markedlyheterogeneous with widespread preservation of igneoustextures in massive-textured rocks with foliated andshear fabrics restricted to relatively discrete zones.However most rocks have a well developed network ofanastomosing veins, many of which appear to be minorfaults. It is thus likely that the bulk strain of rock units,even those with well preserved igneous textures, will behigh.

Many of the talc-carbonate rocks are foliated reflectingpenecontemporaneous alteration and deformation withinfault/shear zones permeable to CO2-bearing fluids.Lizardite + graphite schists reflect alteration anddeformation within H2O-rich fluid regimes.

Rock Compositions: Typical compositions of themain ultramafic rock types are given in Table 1.

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Gole, M. & Wodhouse, M. 2000. Honeymoon Well Complex, Western Australia Journal of the Virtual Explorer - Volume 1

Variations in composition between oOC, oMC and oAClargely reflect differences in igneous porosity (ie. olivinepacking density). The MgO/(MgO+FeO) ratio of oMCand oAC varies widely from ~0.7 to 0.95, but the extentto which this reflects differences in original olivinecomposition or effects of metasomatism duringserpentinisation is difficult to determine on current data.The low CaO value in the oMC is due to loss duringserpentinisation as is common in such rocks (Donaldson,1983).

The compositions of rocks containing fine-grainedrandom olivine spinifex and flow-top breccia sampledfrom all the Honeymoon Well deposits are generallysimilar, with anhydrous MgO contents of 27-29% (Table1). Spinifex-textured rocks and spatially associated, highporosity oOC at Honeymoon Well commonly have verysignificant S values due to metasomatic addition.

Other Lithologies: Rocks in contact with, and inplaces included within the ultramafic complex consist ofa wide range of volcanic rocks that include tholeiiticbasalt and related gabbro, andesite, dacite and minorrhyolite as well as intermediate and minor felsicvolcaniclastic rocks (Harrison, 1995; Russell, 1995).Minor pyritic black shale and chert are also present.

Within the intermediate to felsic rock sequenceimmediately east of Harrier are discontinuous, thinlayers of Ni sulphide-bearing peridotitic komatiite. Thestratigraphic significance of these is unknown.

The basalts and gabbros are composed of actinolite,albite, chlorite, quartz, and leucoxene (Donaldson andBromley, 1981). Some gabbros contain metamorphicmagnetite formed by alteration of igneous ilmenite andare highly magnetic. The andesitic and dacitic rockscontain feldspar, quartz, chlorite, mica, actinolite andcarbonate in widely varying proportions. Metasomatisedrocks contain abundant carbonate, mica and minorarsenopyrite and tourmaline.

Lithological Distribution

Coarse-grained oAC forms the core and makes up thebulk of the komatiite complex, with oMC, oOC,spinifex-textured rocks and minor, medium-grained oACoccurring around the margins and as narrow horizonswithin the complex (Fig. 1). Dips of lithological unitsand structures are highly variable ranging from 30º tovertical.

Most contacts between major rock units, both withinand bounding the ultramafic complex, are faults. There

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Table 1Typical whole rock compositions, Honeymoon Well ultramafic complex,

recalculated to 100% on a volatile-free basis

Randomolivine spinifex oOC oMC* oAC*

93HWD227 93HWD227 74HWD003 74HWD008309.6 m 304.5 m 228.0 m 324.0 m

Si02 45.9 44.8 44.1 42.6

Ti02 0.32 0.24 0.04 0.01

AI203 6.76 4.90 0.41 0.12

Cr203 0.44 0.36 0.25 0.24

FeOt 11.1 10.9 6.44 4.23

MnO 0.19 0.19 0.05 0.06

MgO 28.9 34.6 48.1 51.9

NiO 0.21 0.26 0.42 0.38

CaO 6.10 3.60 0.01 0.10

Na2O 0.08 0.14 0.00 0.15

K2O 0.04 0.05 0.02 0.01

P205 0.02 0.01 0.01 0.00

S 0.77 0.44 0.14 0.18

LOI 7.7 10.1 15.7 18.3

Ni 1660 2030 3330 3010

Cr 2990 2450 1680 1630

* Data from Gole & Hill (1988)


are two very broad types: a) discrete faults defined by 5- 20 cm wide mylonite zones with well preservedigneous-textured rocks on either side and with littleassociated carbonate alteration. These are interpreted tobe relicts of D1 thrust faults: b) foliated and brecciatedzones, highly variable in width and definition, thatappear to be contemporaneous with metamorphicalteration. Some of these zones, particularly those alongthe margins of the ultramafic complex, are associatedwith intense carbonate alteration. Some appear to bestrike slip faults and some may be reactivated D1 thrustfaults. Within ultramafic rocks these faults may changealong strike or down dip from relatively well-definedshear zones into a wide, poorly-defined anastomosingnetwork of veins and small shears.

The Harrier and Corella sulphide deposits areinterpreted to be hosted by equivalent stratigraphicsequences. This correlation is based on several features.In terms of host lithology, mineralisation style andsulphide compositions (Table 2) the deposits are verysimilar. Secondary they are linked by patchy zones ofbedrock disseminated Ni sulphides. Further, to the westand stratigraphically overlying these deposits, is thedistinctive RMH, a discontinuous horizon not onlycontaining recrystallised spinifex-textured flows but also

intermediate to mafic volcanic rocks and minorsedimentary units (Fig. 1).

The two main ultramafic horizons in the south of theprospect are thought to be stratigraphic equivalentunits that have been duplicated by a strongly foliated,talc-carbonate altered D2 strike slip fault located alongthe east side of the Harrier deposit and extendingnorthwards along the eastern contact of the komatiitecomplex (Fig. 1).

The host sequences of the Hannibals - Harakka andWedgetail deposits and other weakly mineralisedspinifex-textured sequences occur as thin, fault-boundedunits along the western ultramafic contact (Fig. 1). Theseare interpreted to be D1 thrust slices. Based on thepresence of Ni sulphide mineralisation, the similarity insulphide composition between the Hannibals andHarrier-Corella deposits (Table 2) and particularly thepresence within the Hannibals deposit of recrystallisedspinifex rocks (see below), it is thought that these faultslices were derived from lateral equivalents to theHarrier-Corella mineralised horizon.

Augite and augite-plagioclase cumulates, tremolite-chlorite rocks (after spinifex-textured rocks) and highp o r o s i t y, fine-grained olivine orthocumulates locatedalong the western contact of the south east ultramafic

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Table 2

Composition of sulphide-bearing lithologies, Honeymoon Well

1 2 3 4 5 6 7Harrier Harrier Corella Hannibals Wedgetail Wedgetail HarrierLow Cu HighCu High Cu Po-rich sulphides

osMC osMC osAC/MC osMC Massive osOC osMCsulphide

n 17 27 6 14 7 42 20ppmNi 10190 9573 13078 9735 62440 6620 2200δCu 8 885 414 892 2980 370 227Co 224 180 230 247 1580 120 185S 7880 10790 14070 13830 282700 77800 13320ppb

Pt 100 68 197 50 12Pd 200 162 500 108 14Ir 14 13 115 9

Ni/S 0.29 0.89 0.93 0.70 0.22 0.56 0.17Ni/Cu 104 11 32 11 21 18 10Ni/Pd 65 60 125 61 157Pt/(Pt+pd) 0.33 0.30 0.28 0.32 0.54Pd/Ir 14 13 4 12


sequence appear to define the stratigraphic top of themain Honeymoon Well ultramafic unit. Similarultramafic and mafic rocks together with felsic rocks thatform a structurally disrupted block within the northernpart of the ultramafic complex are also probablyremnants of this upper stratigraphic contact. This top unitis, however, not preserved elsewhere within theultramafic complex.

The thickness of the oAC in the central and northernpart of the prospect is thought to be a result of repetitionby D1 thrust faults of an originally extensive and thickoAC body. The abrupt northern termination of theultramafic complex, where the width changes from 3 kmto 0 m over 1.5 km of strike, occurs at the intersection ofthese thrust faults with a D2 strike slip fault that definesmuch of the eastern ultramafic contact (Fig. 1). Noultramafic rocks are present along strike for at least 5 kmto the north of the Honeymoon Well complex.

Ultramafic Rock Mineralogy

Olivine adcumulate and oMC do not retain igneousminerals except for minor to trace amounts of partlyaltered chromite that may be present within stichtiteaggregates (see also Donaldson and Bromley, 1981). Thedominant metamorphic assemblages are lizardite -pyroaurite - stichtite - magnetite + brucite + chromite andlizardite - brucite - pyroaurite - stichtite (i.e. magnetite-absent).

This latter assemblage, which occurs in zones 20 mwide up to areas 0.5 x 1.5 km in extent, is highly unusualin serpentinised olivine-rich cumulates. Pyroauriteincludes other sjogrenite group minerals, particularlyiowaite (Donaldson and Bromley, 1981). Antigorite -carbonate - magnetite + stichtite and particularly talc -carbonate + magnetite assemblages, although they maydominate in large volumes of rock, are mostly associatedwith faults and shear zones particularly where theseoccur along ultramafic/country rock contacts.

Olivine orthocumulate and most spinifex-texturedrocks consist of variable proportions of serpentine(mostly antigorite), tremolite, chlorite, carbonate,magnetite and relict igneous chromite.

Chromite generally occurs as equant grains, rimmedand veined by magnetite although partly skeletalchromites are present within osOC at Wedgetail andCorella. In places oOC contain minor igneous kaersuititeand rarely clinopyroxene.

Recrystallised spinifex-textured rocks and associatedoOC generally retain coarse-grained pyroxenes and,except in some rocks from drill hole 74HWD003 (Goleet al., 1990), only rarely retain other minerals from theirhigh temperature metamorphic assemblage, theseminerals being altered during regional metamorphism.

Augite and augite-plagioclase cumulates may retainigneous clinopyroxene but mostly consist of tremolite-chlorite and actinolite-albite-epidote-chlorite-leucoxeneassemblages respectively.

In the ultramafic rocks the presence of pseudomorphiclizardite, antigorite and talc-carbonate assemblagesindicates that these formed directly from the igneousmineralogy (Wicks and Whittaker, 1977). Carbonate-richfluids appear to have been spatially related to relativelydiscrete permeable zones. Non-pseudomorphic (i.e.igneous texture destroying) antigorite-carbonate iscommonly superimposed on pseudomorphic lizarditeassemblages and occurs in selvedges to carbonate-bearing veins, shears and faults. Talc-carbonate rocks aremostly non-pseudomorphic suggesting that many ofthese assemblages also developed in already alteredrocks. This assemblage is, however, commonly spatiallyassociated with faults and shears, movement of whichwould cause recrystallisation and thus also result information of non-pseudomorphic textures. T h e s erelationships and the spatial distribution of assemblagessuggest that, over time, carbonate fluids expanded awayfrom initially relatively restricted CO2 fluid pathways. Inrestricted areas, for example at Wedgetail and the centralparts of Harrier, late arsenic - carbonate + gold-bearingfluids have further altered the rocks.

The lizardite-dominant rocks in particular have a highvein density. At Honeymoon Well vein densities,measured as veins (>1mm wide) per metre, are onaverage 10 - 12 whereas at Mt Keith and Yakabindiecasual observations suggest values of 1 - 4. Vein mineralsconsist of pyroaurite, iowaite, brucite, carbonate,magnetite, serpentine, trace amounts of gypsum and,within or near the sulphide deposits, minor Fe, Cu andZn sulphides. The vein assemblages are markedly Mg -Fe-rich and Si-poor relative to their host rocks andsuggest that, in places, extensive metasomatism hasaccompanied serpentinisation. Pyroaurite - iowaite -brucite-rich and carbonate-rich veins tend to be mutuallyexclusive and occur in separate zones althoughindividual veins are commonly laminated with differentassemblages. In antigorite and particularly talc -carbonate rocks the vein mineralogy is less complexbeing dominated by carbonate and magnetite.

Sulphide Textures

Disseminated Sulphides: In olivine sulphidemesocumulate (osMC) and olivine sulphide adcumulate(osAC) sulphide aggregates were moulded betweenclosely-packed olivine pseudomorphs and hence have ageneral lobate shape (see Donaldson and Bromley, 1981,for detailed descriptions). Sulphide aggregates in olivinesulphide orthocumulate (osOC) tend to be more blebby

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and may be relatively coarse-grained (up to 5 mm) due tothe greater space between former olivine grains.

Metamorphic reconstitution of the rocks has, in manycases, greatly modified the shape and the mineralogy ofthese aggregates. In rocks that have been stronglyrecrystallised, and particularly where non-pseudomorphic antigorite - carbonate or talc - carbonatehas replaced a large proportion of lizardite, all evidenceof former igneous sulphide aggregates has beendestroyed and sulphides have been redistributed intoirregular, scattered and smaller aggregates.

In rocks with relatively low Ni/S ratios sulphideaggregates are commonly veined and partly surroundedby magnetite, whereas in rocks with higher Ni/S ratiosmagnetite is a minor component or is entirely absent. Inthese latter rocks heazlewoodite - pentlandite andheazlewoodite-only assemblages occur and sulphideaggregates are commonly not located along formerolivine grain boundaries, have ragged to irregularoutlines and do not retain igneous lobate shapes. In theserocks igneous textures are rarely preserved due toextensive recrystallisation of serpentine.

Sulphide aggregates are also commonly intergrownwith antigorite and carbonate. Even in lizardite-dominated lithologies (i.e. most osMC and osAC)antigorite needles and carbonate occur along formerolivine grain boundaries and in interstitial areas andhence are commonly intergrown with sulphides. Inplaces, antigorite needles and, particularly in osOC,chlorite and talc are strongly intergrown with sulphidessuch that the sulphides fill angular, wedge-shaped areasbetween silicate grains.

Massive sulphides: In both massive sulphide andmassive sulphide breccia, which are present in theWedgetail deposit, interlocking anhedral to blockygrains of pyrrhotite, pentlandite, pyrite and minor totrace gersdorffite generally form a massive fabric, andthere is only minor development of weakly foliatedmassive sulphide. Minor chalcopyrite forms smalli n t e rg r a n u l a r, irregular shaped grains. Gersdorffite isrelated to arsenic - gold - carbonate alteration thatprobably significantly postdates the main regionalmetamorphism (Hagemann et al., 1995) suggesting thatthe massive sulphide ores acquired their microtexturesvery late in the structural/metamorphic evolution of thea r e a .

Supergene A l t e r a t i o n : S u p e rgene alteration ofprimary sulphide assemblages, similar to that seen inother Ni deposits in the Yilgarn Craton (e.g. Butt andNickel, 1981) occurs in the upper, very weathered partsof the deposits (Mitchell, 1997) and in patches deepwithin the deposits. Violarite, pyrite and magnetite arethe common alteration products in rocks with a Ni/S ratio

below about 1.3. In rocks with Ni/S ratio >1.3 milleritemay occur as alteration along grain boundaries andfractures in pentlandite and heazlewoodite or be finelyintergrown with violarite.

Deposit Geology

Nickel sulphides occur in two deposit types:disseminated sulphides (trace to 5 modal percent) inolivine-rich cumulates (osMC, osAC, and osOC in theHarrier, Corella, Hannibals, and Harakka deposits) andsulphide-rich rocks (massive sulphide, sulphide brecciaand osOC in the Wedgetail deposit) hosted by spinifex-textured flows. The Harakka deposit is relatively smalland will not be described.

The main deposits are described below in order of theirstratigraphic preservation with the Harrier deposit beingthe best preserved and the Hannibals and Wedgetaildeposits occurring in allochthonous fault blocks.

Harrier Disseminated Sulphide Deposit

The Harrier deposit occurs in the south of theHoneymoon Well prospect along the eastern contact ofthe main oAC-bearing ultramafic unit (Fig. 2). Thedeposit contains 43.0 MT at 0.64% Ni (0.4% Ni cut off,<300 m depth). The nickel sulphides at Harrier arehosted by osMC with minor osOC. The deposit, which isoverlain by between 55 - 120 m of overburden, extendsalong strike for at least 1700 m and varies in width from30 to 140 m.

To the east, the ultramafic sequence is in fault contactwith metamorphosed andesitic to dacitic lavas andvolcaniclastic rocks, and thin, discontinuous olivinespinifex-textured komatiite units, in places containingdisseminated and massive nickel sulphides. In plan viewthis faulted contact is sub-parallel to the stratigraphy(Fig. 1) although in cross section it cuts the stratigraphyat highly variable angles (Figs. 3 and 4).

The lower part of the ultramafic stratigraphy has beenremoved by the eastern boundary fault. The stratigraphyto the west of the fault, however, appears to be relativelyintact, and indeed the Harrier deposit appears to be theleast structurally disrupted of the Honeymoon We l ldeposits. The nickel sulphide-bearing sequence gradesup-sequence into barren oMC and then, in places, intooAC. North of 11000N the olivine cumulate pilebifurcates around the RMH (Fig. 2), here composed ofspinifex-textured flows, andesitic and minor basalticvolcanic rocks and minor pyritic and cherty sedimentaryrocks. This unit is up to 100 m thick and dips 60-75o tothe east (Fig. 4), although younging directions derivedfrom the flows are westerly indicating that the sequenceis overturned. Between 10500N and 11000N, where the


oAC is relatively thick and is in direct contact with themineralised sequence, the RMH is absent (Fig. 3). TheRMH re-appears south of 10500N as a 50 m thick unit ofoOC, minor recrystallised spinifex-textured rocks andbladed metamorphic olivine - pyroxene rocks, the lattersimilar to those in areas of amphibolite metamorphicfacies in the south of the Agnew - Wiluna belt. Theserelationships suggest that the RMH has been thermallyeroded within a 500 m wide channel-like feature bykomatiite lava prior to the formation of the oAC. Northof 11000N the oAC on the western side of the RMHthickens into the oAC body that forms the core of theHoneymoon Well ultramafic complex (Fig. 1)

The nickel-sulphide mineralisation, north of 10700N,is constrained to one osMC unit with minor layers ofosOC in the north. This unit extends to the south where

other sulphide-bearing units are also present. These unitsare separate igneous layers with different bulkcompositions. The western-most unit is composed ofosAC and is lens-shaped in plan view. It is located in thearea where the RMH is absent (Fig. 2), at or near the baseof a possible oAC-filled channel. Other mineralised unitsin this area appear to be igneous layers rather thanstructural duplicates of the extensive osMC horizon. Thetrue width of the sulphide-bearing horizons is highlyvariable ranging from 10 m to 140 m.

Just to the west of the southern-most part of the RMH,and effectively on the southern margin of the oAC-filledchannel way, is a 20 - >100 m thick body composed ofosOC (Fig. 2). The sulphides consist of pyrrhotite andminor pyrite and occur intergrown with magnetite inmodified magmatic-textured lobate aggregatesintergranular to former olivine. These rocks are stronglydepleted in nickel such that despite sulphur contents ofup to 1.7%, Ni contents are mostly below 1700 ppm(Table 2).

The mineralogy of the Ni sulphide host lithologies isdominated by lizardite-rich assemblages. A significantproportion of the host sequence has, however, beenaltered by carbonate-bearing fluids resulting in a zonedpattern with lizardite assemblages giving way toantigorite-carbonate assemblages which, in places,envelope talc-carbonate rocks. Most talc-carbonate rocksare strongly recrystallised, massive rocks or are foliated,although rare pseudomorphed igneous textures arepresent in low strain zones.

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Figure 2. Geology plan of the Harrier deposit at 100 m below surface.

Figure 3. Cross section along 10600N, Harrier deposit.


Nickel Mineralisation: The depth to theoxide/supergene sulphide boundary ranges from 70 to100 m being deepest in the north. The width of thesupergene zone is variable being almost non-existent onsome sections but up to 65 m thick on 11600N. Thetransition zone is generally 20 - 30 m thick and again isthicker in the north.

In the primary zone the sulphide content of themineralised ultramafic rocks typically varies between 1to 5 modal percent, being highest in osOC which maycontain >3% Ni. In lizardite-rich osMC sulphidestypically form lobate aggregates between former olivinegrains whereas in osOC sulphides in the intercumulusspace are blebby. In antigorite-carbonate and particularlytalc-carbonate rocks strong recrystallisation of gangueminerals has destroyed much of the magmatic texture ofsulphide aggregates, and sulphides are commonlyintergrown with antigorite blades, talc aggregates orrelatively coarse-grained carbonate grains. A r s e n i c -bearing minerals are patchily associated with these latterrocks.

Within the primary sulphide zone pentlandite + tracechalcopyrite, pentlandite - heazlewoodite and minorheazlewoodite-only are the dominant sulphideassemblages in lizardite and most antigorite-rich rocks.In lizardite-rich assemblages chalcopyrite mostly occursin veins with pyroaurite, brucite, magnetite and pyrite,and is only rarely present in relict magmatic sulphideaggregates. In talc and some antigorite-rich rocks the

sulphides consist of pentlandite, millerite, tracechalcopyrite, pyrrhotite and patchy gersdorffite, niccoliteand maucherite.

As in the other disseminated sulphide deposits themineralisation can be divided into two types based onNi/S ratio and Cu contents (Table 2): 1.High-Cu mineralisation, containing 40 - 2000 ppm Cu,

having relatively low Ni/S ratios (range 0.5 - 1.3) andwith the sulphide mineralogy dominated bypentlandite with trace tochilinite, chalcopyrite and inplaces pyrrhotite. Alteration to violarite and minorpyrite is relatively abundant even deep within thedeposit. This mineralisation type is generally similar tothat in the Mt Keith and Yakabindie disseminated Nisulphide deposits further south in the Agnew - Wilunabelt (Burt and Sheppy, 1975; Hill et al., 1990).

2.Low-Cu mineralisation containing 0 - 40 ppm Cu,having high Ni/S ratios (range 1.2 - 3.0) and with thesulphides being pentlandite and heazlewoodite withminor alteration to millerite.In rocks with high Ni/S ratio magnetite is absent from

the mineral assemblage. Most samples of this typecontain no detectable Cu (detection limit of 5 ppm)despite Ni grades of up to 2% and elevated Co and PGE.This ore type forms about 40% of the Harrier resource.

These mineralised types form large coherent blockswithin the deposit (Fig. 5). There are slight differences inthe gangue mineralogy between these two ore types. Thehigh-Cu ore is commonly hosted in dark green lizardite-

17

Figure 4. Cross section 11200N, Harrier deposit. The central spinifex-textured flows and andesitic volcanic

rocks form the Recrystallised Marker Horizon.


rich assemblages and some antigorite - carbonateassemblages with veins containing a relatively highproportion of carbonate. The low-Cu ore is mostly hostedby dark to very light green lizardite-rich rocks containinga low proportion of antigorite-assemblages and the veinmineralogy is dominated by brucite andpyroaurite/iowaite. Based on Cu, the contact between oretypes is sharp (over 2 - 6 m) whereas other geochemicaldifferences (S, Fe) are gradational (over 5 - 50 m). Thesegeochemical gradients are reflected in a zoned pattern inprimary sulphide + magnetite assemblages of Ni-poorpentlandite - chalcopyrite + pyrrhotite-magnetite (Ni/Sratio <0.8), pentlandite + chalcopyrite + magnetite, (0.8 - 1.3), Ni-rich pentlandite - heazlewoodite + tracemagnetite (1.3 - 2.0) and heazlewoodite + rare magnetite(2.0 - ~3.0).

Corella Disseminated Sulphide Deposit

The Corella deposit is located along the north eastcontact of the Honeymoon Well ultramafic complex. The

deposit contains 53.5 MT at 0.62% Ni (0.4% Ni cutoff,<300 m depth) and extends for 1400 m with widths of 20to 100 m (Fig. 6). The Corella sequence is composed ofa wide variety of lithologies including oAC, osAC, oMC,osMC, high porosity osOC and oOC and minor spinifex-textured flows. These flows have been recrystallisedunder high temperature metamorphic conditions (Gole etal., 1990) and together with minor andesitic and basalticvolcanic rocks form the RMH.

Spinifex-textured flows within this horizon show bothwesterly and easterly younging directions. The latter arethought to be due to local folds associated with shearzones.

The ultramafic sequence is strongly deformed andsheared with many lithological units, includingmineralised lithologies, occurring as fault-boundedboudins on various scales. Lithologies interdigitate as aresult of both original stratigraphic relationships andstructural dislocation. Because of the shearing thedensity and thickness of veins are significantly higher atCorella than at the other deposits.

18

Figure 5 Distribution of high and low Cu mineralisation and Ni/S ratio, Section 10650N, Harrier deposit.


Between the shears and faults within the ultramaficrocks, igneous textures are generally well preserved. Themetamorphic mineralogy of the osOC and oOC consistsdominantly of pseudomorphic antigorite - carbonate withminor chromite, tremolite, chlorite and rarely kaersuititicamphibole. The originally more olivine-rich rocks aremostly altered to lizardite - brucite - stichtite-bearingassemblages. In places these assemblages are altered tonon-pseudomorphic antigorite - carbonate and talc -carbonate assemblages. Talc - carbonate alteration occursmostly along the eastern ultramafic fault contact and isgenerally outside the limits of the Ni sulphide resource.

The ultramafic sequence is sub-vertical over the N-Sextent of the deposit although in the south, the deeperparts of the sequence dip to the west (Figs. 7 and 9). To

the east the ultramafic rocks are in fault contact withbasalts and minor gabbros in the northern part, and withandesitic rocks to the south (Fig. 1 and 6). In plan thisfault is sub-parallel to the mineralised sequence. In crosssection however the fault has variable attitudes. In thenorth it dips to the west and truncates the mineralisedsequence at depth (Fig. 8), and in the south it dips to theeast away from the mineralisation (Fig. 7).

Due to this fault the lowermost part of the ultramaficstratigraphy is missing. Within the preserved part of theultramafic sequence the interpreted stratigraphy is, fromeast to west, barren oAC, the mineralised sequence ofosAC, then osMC with overlying osOC and oOC. Thissequence was originally capped by spinifex-texturedflows and mafic to intermediate volcanic rocks that formthe RMH. Up-sequence and to the west of the RMH isminor oMC, then medium-grained oAC grading into thevery coarse-grained oAC that forms the central core ofthe Honeymoon Well ultramafic complex. T h i sstratigraphy is very similar to that at the Harrier deposit.

The patchy distribution of the RMH along the easternside of the Honeymoon Well complex (Figs. 1 and 6) isa function of probable extensive thermal erosion bykomatiite magma from which the overlying oAC unitformed and also later structural dislocation.

Nickel Mineralisation: The mineralised sequencevaries along strike being dominated by osOC and osMCin the north to osAC and minor osOC in the south (Figs.6, 8 and 9). To the north the sequence thins and gradesinto barren oAC whereas to the south the mineralisedsequence is truncated by a NW trending fault.

19

Figure 6. Geology plan of the Corella deposit at 100 m below surface.

Figure 7. Cross section 16 100N, Corella deposit.


The depth to the oxide/supergene boundary is 55 to 75m, and is the shallowest of the deposits because thetransported clay layer is absent over most of the deposit.The thickness of the supergene zone is mostly 10 m orless. In the central and northern parts of the deposit,transition zone sulphide assemblages occur in patchesand zones throughout the deposit, even at the deepestlevels. In the south the transition zone occurs only as anarrow horizontal layer below the supergene zone and atdepth the sulphides are almost exclusively primarysulphides.

The thicker mineralised parts of the deposit in the areaof 16100-16200N and 16700-17000N appear to resultfrom stratigraphic repetition by faulting, presumablyrelated to early thrusting. Although the thicker parts ofthe mineralisation appear to form coherent blocks, indetail a combination of original stratigraphy anddeformation has resulted in numerous small,discontinuous lenses of mineralisation occurring inotherwise barren rocks. This is particularly the case onthe sections south of about 16400N.

Nickel grades vary from 0.35 to >2.5% being highestin the osOC. As at Harrier both high-Cu and low-Cumineralisation types are present although the proportionof the low-Cu type is relatively low.

Hannibals Disseminated Sulphide Deposit

The Hannibals deposit is located along the westerncontact of the Honeymoon Well ultramafic complexwithin a fault-bounded, west-younging sequence of oMC

and minor oAC, oOC and spinifex-textured rocks thatcan be traced for about 2.4 km along strike (Fig. 1). Thissame sequence hosts the Harakka deposit. The depositcontains 36.1 MT at 0.70% Ni (0.4% Ni cut off, <300 mdepth). It is divided into western and eastern overlappingfault blocks (Fig. 10), which are stratigraphic equivalentsrepeated along a central fault zone (probably areactivated D1 thrust fault). The eastern boundary of theeastern fault block and much of the western block areagainst the central oAC along a major, discrete, planarfault with mylonite fabrics preserved in places.

This fault transgresses the mineralised sequence at alow angle to the stratigraphy and is interpreted as arelatively well preserved D1 thrust fault.

To the west the mineralised sequence is in fault contactwith a sequence of tholeiitic basalt flows, some of whichhave doleritic to gabbroic centres, and minor interflowcarbonaceous and pyritic shale. The ultramafic rocksalong this contact are mostly schists derived fromspinifex-textured flows. In places spinifex textures arepreserved and indicate both easterly and westerlyyounging directions. Flows further from the contact areconsistently west-younging and the easterly directionsare due to folding along the faulted contact.

In the eastern block the stratigraphy of the sulphide-bearing sequence is relatively intact and igneous layeringis sub-vertical (Fig. 11). A high Ni-grade core within themineralisation plunges in a general SE direction. ThinoOC and spinifex-textured rock units occur lateral to the

20

Figure 8. Cross section 16900N, Corella deposit.

Figure 9. Cross section 16300N, Corella deposit.


high grade core suggesting that the sulphide shoot fills apossible 30-80 m deep, 150 m wide channel.

In the western fault block the lithological distributionappears to be more complicated due to possible layer-parallel faulting. The block extends southward forseveral hundred metres as a narrow (20-60 m wide)sequence between the central fault and metabasalts to thewest. A high Ni-grade core appears to be sub-horizontalwithin the mineralised sequence which thinsdramatically to the north and south. The mineralisedsequence bottoms at depth against the folded, westernmetabasalt/ultramafic fault contact and is truncated onthe east by a fault contact with oAC (Fig. 11 ) .Recrystallised spinifex-textured rocks and associatedfine-grained oOC occur in fault slices (Fig. 10) that areinterpreted to be derived from the RMH.

Nickel Mineralisation: Mineralisation consists ofdisseminated sulphides containing trace to ~3 modalpercent sulphide with Ni grades ranging from 0.35 % to2.4%. The sulphide-bearing units are variously preservedigneous horizons with gradational to sharp contacts withbarren or weakly mineralised oMC. Many of the sharpmineralisation, contacts are on minor faults. Down dipand along strike from the central portions of themineralisation the sulphide layers thin and generallybecome lower grade and interdigitate with barren rocks.

The depth to the oxide/supergene boundary rangesfrom 60 to 104 m and the supergene zone averages 15 mthick.

Most sulphides typically occur as scattered aggregatesenclosed by oxide, carbonate and silicate gangueminerals. Sulphide aggregates typically have modifiedlobate shapes which reflects the predominance ofmesocumulate-textured host rocks and general lack ofstrong recrystallisation. Chalcopyrite mostly occurs inveins with pyroaurite, brucite, magnetite and pyrite andis only rarely present in relict magmatic sulphideaggregates.

The mineralisation can be divided into low and highCu types with the low-Cu ore type forming about 60% ofthe volume of the deposit. The eastern and western faultblocks have different mixes of these two mineralisationtypes with the eastern block having a higher proportionof the low-Cu type, hence having a lower overall Cu

21

Figure 10. Hannibals geology plan at 100 m below surface.

Figure 11. Cross section 13800N, Hannibals deposit.


content and significantly higher Ni/S ratio together withhigher lizardite, pyroaurite/brucite and lower carbonatecontents compared to the western block.

Wedgetail Massive Sulphide Deposit

The Wedgetail deposit is located along the northwestern margin of the oAC complex. The deposit has astrike length of 1700 m and varies in width from 10 to 80 m (Fig. 12). The deposit contains an estimated 22.9MT at 1.08% Ni (0.5% Ni cut off, <300 m depth). Of this2.5 MT at 3.36% Ni represents a high grade zone which

includes massive sulphide and massive sulphide brecciamineralisation.

The Wedgetail deposit comprises disseminated andmassive sulphides hosted by a north-striking, west-younging, moderate to steep easterly dipping sequenceof oOC and spinifex-textured rocks.

In the south the mineralised sequence is in fault contactwith the central oAC (Fig. 13). At depth and to the norththe sequence is separated from the oAC by a fault wedgeof felsic volcanic rocks up to 80m thick (Fig. 14). To thewest the mineralised sequence is in fault contact with

22

Figure 12. Plan of the Wedgetail massive sulphide deposit at 100 m below surface.


variably deformed felsic to mafic metavolcanic andmetasedimentary rocks that typically have a strong sub-vertical cleavage. North of the inflection in the oACcontact around section 18800N, the mineralisedsequence pinches out at depth where the faults that markthe eastern and western contacts join. On some sectionsa thin layer of breccia massive sulphide extends downdip along the combined fault plane (Fig. 14). South of18800N the sequence is open below the depth of drilling(300-500 m vertical depth).

Spinifex-textured rocks within the mineralisedsequence usually occur as small patches and zonesgenerally close to the western contact. Complete flowprofiles are mostly not preserved, although rarely there issufficient textural preservation in A-zones to allow ayounging direction to be determined. However at depthin the southern part of the deposit a 30 m thick sequenceof very well preserved spinifex-textured flows is presentthat indicate a younging direction to the west, i.e. thesequence is overturned (Fig. 13). Mineralised rocksoccur above and below the flows which grade laterallyinto osOC, probably reflecting a change fromcrystallisation within a channel way (osOC) to atemporary overbank environment (rapidly cooled thinflows). Similar lateral changes occur withindisseminated mineralisation at the Perseverance nickel

deposit (Barnes et al., 1988). It must be born in mind,however, that Wedgetail occurs in a fault block, and assuch is only a slice of the original stratigraphic sequence.

Olivine orthocumulates and osOC mostly contain non-pseudomorphic antigorite assemblages with only minorpreservation of lizardite assemblages. Talc-carbonateassemblages are generally confined to contacts althoughmuch of the southernmost part of the mineralisedsequence is altered to assemblages containing talc,carbonate, and chlorite.

Nickel Mineralisation: Nickel grades within themineralised sequence generally increase from east towest, with massive sulphide present in places along orclose to the western faulted contact.

Sulphide breccia, which comprises 30-90% sulphideand contains 1-20 cm sized, angular clasts of ultramaficand foliated country rock, also occurs along this contact.The massive sulphide/sulphide breccia varies inthickness from a few centimetres to several metres(maximum 12 m). No massive sulphide is recognised asbeing in its original stratigraphic position, all beingthought to have been remobilised along faults and shears.

The base of the oxide zone ranges from 70 to 90 mwith the underlying supergene zone extending to 117 to204 m depth. The transition zone ranges from 178 to 265 m depth with some deeper patches along fracturesand faults.

Primary zone sulphides consist of pyrrhotite,pentlandite, pyrite, minor chalcopyrite and tracegersdorffite. Massive sulphide consists of interlockingpolygonal shaped, relative coarse grains (up to 1.5 mm)of pyrrhotite and pentlandite, with pyrite in somesamples. Chalcopyrite forms small interg r a n u l a r,irregular shaped grains. A preferred alignment of grainsor compositional layering is only rarely observed in themassive sulphide. In osOC the sulphides are stronglyintergrown with metamorphic gangue minerals and formirregular aggregates that mostly do not show typicalmagmatic shapes.

Very minor low-Cu ore, comprised of pentlandite andheazlewoodite, is restricted to low grade zones along theeastern contact of the mineralised sequence.Nickel/sulphur ratios within Wedgetail mineralisationare significantly lower than those of the otherHoneymoon Well deposits, reflecting the Fe-S-richnature of the sulphide assemblages (Table 2). Massivesulphide exhibits Ni/S ratios of 0.17-0.4, whereas ratiosfor osOC range from 0.2-0.9, with only very minor areasof higher values.

Other Sulphide Occurrences

Minor to trace sulphides occur as layers and patches 1-20 m thick in places within the main mass of oAC. S o m e

23

Figure 13. Cross section 18300N, Wedgetail deposit


of these disseminated sulphides form lobate aggregatesbetween former olivine grains and appear to bemodified igneous sulphides. Others, however, occur asragged grains within olivine pseudomorphs and,although they may be associated with elevated wholerock Ni values (mostly 0.3-0.45%, rarely higher), donot have elevated Cu, Pt or Pd. Such sulphides appearto be of metasomatic origin, forming duringserpentinisation from Ni and Co released from alteredolivine and from S introduced with the metamorphicfluids. In places rocks containing such sulphides haveNi grades above that of barren rocks suggesting someloss of Mg and other major element components duringserpentinisation to allow residual enrichment of barrenNi values to these slightly elevated values.

Within and around the margins of the Ni sulphidedeposits are veins containing trace to 90% sulphide. Veinthickness ranges from 0.5 mm to rarely >1 m. The veinsmostly occur in lizardite-rich rocks and apart fromsulphide contain pyroaurite, brucite, magnetite, lizardite,chrysotile and rarely antigorite and carbonate. The veinmineralogy suggests that they are part of the greenschistfacies regional metamorphic and metasomatic alterationof the ultramafic rocks.

In lizardite-dominant mineralisation a high proportionof chalcopyrite occurs within such veins. Around the

margins of deposits the primary zone vein sulphides arepyrrhotite, pyrite, tochilinite, chalcopyrite, tracepentlandite and sphalerite. The distribution of thesulphide-bearing veins around Ni sulphide depositsappears to be highly erratic.

Model for evolution of Honeymoon WellComplex

An interpreted stratigraphic reconstruction of theHoneymoon Well ultramafic complex is shown inFig.15. The lowermost komatiite unit contains all theknown Honeymoon Well Ni sulphide deposits. Thiscorrelation is made on the basis of; (a) physical linkingof similar stratigraphic packages between Corella andHarrier; ( b) the western sulphide deposits are all hostedin fault blocks with some contacts being mylonites; (c)the presence of significant Ni sulphide mineralisationand the similarity in mineralisation style andcomposition between the western and easterndisseminated deposits; and (d) the presence ofrecrystallised spinifex-textured thin flows along thelength of the RMH as well as within Hannibals.

These latter two points are particularly powerfularguments for the correlation. It would be expected thatmagmatic sulphides from temporally different igneoussequences or even perhaps from widely separatedlocalities within the same sequence would haveundergone different magmatic histories and thus havedissimilar compositions (eg. Naldrett and Barnes, 1986;Barnes et al., 1997). The very similar compositions,particularly the similar Ni/Pd, Pt/(Pt + Pd) and Pd/Irratios (Table 2), of the disseminated deposits thusstrongly argues for these sharing the same magmatich i s t o r y. The recrystallised spinifex-textured rocksrequire such restricted and unusual circumstances toform (Gole et al., 1990) that these are unlikely to beduplicated within two different sequences in the samegeneral locality. Honeymoon Well is the only knownlocality for these rocks in the world.

Lateral variations in stratigraphic profiles along thisunit reflect crystallisation in different volcanic facies(see Hill et al., 1995, 1996). These range fromcrystallisation within 5-8 km wide, long-lived turbulentflow channels sastained by very high lava supply ratesthat form thick, layered olivine-cumulate dominatedsequences to periodically emplaced, thin, rapidly cooledlava flows that results in sequences of spinifex-texturedflows.

Although Wedgetail is within a fault slice and thusnot the complete stratigraphic sequence, and also thatthe massive sulphide is interpreted to be physicallyremobilised along a fault, the presence of spinifex-

24

Figure 14. Cross section 18900N, Wedgetail deposit.


textured flows and of matrix and massive sulphidesuggests that in volcanic setting it is generally similar tothat of a Kambalda style deposit (Type 1 of Barnes et al.,1994) and is likely to have formed near the flow front ofan advancing komatiite flow field or marginal to anyregional-scale lava channel way (Hill et al., 1996).Reconstruction of the stratigraphy of the disseminateddeposits shows significant differences between thedeposits with spinifex-textured flows occurring withinthe Hannibals mineralised sequence, and relatively thick,laterally persistent oOC and osOC units occurring withinthe Corella sequence, whereas at Harrier olivine-richcumulates dominate the mineralised sequence. Thesedifferences may reflect more marginal (at Hannibals) tomore central positions (at Corella) relative to a majorflow channel. The variable but relatively high modalabundance of sulphide within the Honeymoon Welldisseminated deposits suggests that much or all of themagmatic sulphide was physically introduced into thedeposits within turbulently flowing silicate lava ratherthan being the result of insitu olivine-sulphide cotecticcrystallisation as proposed at the slightly lower Ni gradedeposits at Mt Keith and Yakabindie (Hill et al. , 1996).The similarity of the sulphide compositions betweenHoneymoon Well disseminated deposits suggests that themagma from which the deposits were formed came froma very large homogeneous source.

The mineralised unit is capped by spinifex-texturedrocks which are in turn overlain by basalt and andesiticvolcanic and minor sedimentary rocks. The base of theunit is not preserved but originally the unit was probably,in places, >250 m in thickness.

The upper komatiite unit formed within a few tens ofyears after the lower unit (Gole et al., 1990). Its charactervaries along strike and in the Honeymoon Well areacontains a coarse-grained oAC body, originally 500 -1000 m thick by >6 km long in apparent cross section,formed in a massive komatiite lava channel (Hill et al. ,1990, 1995). To the south the oAC body thins into anoOC-dominated sequence whereas to the north proximallateral equivalents are not preserved. The unit is capped

by spinifex-textured flow rocks and in placesfractionated sequences of pyroxenite and gabbro.Thermal erosion occurred along the basal contact of thisvery broad lava flow channel, removing in places theunderlying intermediate and mafic lavas andvolcanoclastic rocks and part of the top section of themineralised komatiite sequence. During igneous coolingremnants of this top section were partially melted andmetamorphosed to high temperatures (Gole et al., 1990).

Some of the scattered disseminated Ni sulphides withNi grades of 0.35-0.6% Ni within the central oAC bodyappear to have formed by local magmatic S-saturationand cotectic crystallisation of olivine and sulphide.

The pyrrhotite-rich sulphides present in osOC andosMC east of and up-sequence of the RMH at Harrieroccur in relatively high modal proportions and must havebeen physically introduced to their present position ashighly fractionated sulphide liquid within stronglychalcophile-element depleted silicate lava.

The ultramafic stratigraphy outlined above is generallysimilar to that proposed by Barnes et al. (1988) for thePerseverance Ni deposit in the southern part of theAgnew-Wiluna belt.

Early deformation of the Honeymoon Well sequenceoccurred within a D1 thrust duplex with an apparentsense of movement of north block to the south. Thegeometry of this duplex was probably greatly influencedby the size and competency of the unmetamorphosedoAC body. The oAC unit itself was stacked and thinthrust slices of the lower mineralised ultramafic horizonwere emplaced along the top (now western) margin ofthe duplex.

Folding of the greenstone belt sequence and strike-slipfaulting during D2 was accompanied by lowergreenschist facies metamorphism. Strike-slip faultingresulted in further duplication of the stratigraphy in thesouthern part of the Honeymoon Well prospect, againwith an apparent sense of movement of north block to thesouth, as well as removing the northern proximalequivalents of the Honeymoon Well ultramafich o r i z o n s .

25

Figure 15. Stratigraphic reconstruction of the Honeymoon Well ultramafic complex.

At Wedgetail the bulk composition of massivesulphide dominantly reflects magmatic processes, andapart from relatively minor redistribution of Cu and theaddition of As to parts of the deposit there appears tohave been relatively little adjustment of the sulphidebulk composition during metamorphism.

The Ni-rich bulk sulphide compositions of thedisseminated sulphide deposits are, however, afunction of interaction with metamorphic fluids. T h esulphide deposits are located adjacent to major faultsthat have acted as pathways for migration of larg evolumes of fluid. Highly reduced fluids that may formduring progressive alteration of ultramafic rocks(Frost, 1985) have caused complete leaching of Cu andpartial loss of Fe, S and Zn from parts of the sulphidedeposits and of Fe from large volumes of barrenultramafic rocks. In the latter, magnetite is absent fromlizardite - brucite - iowaite dominated alterationassemblages resulting in low magnetic susceptibilityfor such serpentinites present, for example, south eastof Wedgetail and east of Hannibals - Harakka (see Fig.5, Bourne, 1996). In the disseminated sulphidedeposits the metasomatism has formed a zoned patternin Cu content, whole rock Ni/S ratio and primarysulphide assemblages.

The sulphide zones are defined by pentlandite-minorpyrrhotite-chalcopyrite, pentlandite-only, pentlandite-heazlewoodite and heazlewoodite-only assemblages.Zones containing pyrrhotite and chalcopyrite representthe least altered sulphide assemblages whereasheazlewoodite-only assemblages occur in the moststrongly leached rocks.

Acknowledgments

We warmly thank the many geologists from Rio TintoExploration and Outokumpu Mining Australia who haveworked on the project since 1969 for their geologicalcontributions upon which this paper is largely based.

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Disseminated and Massive Nickel Sulphide deposits, Honeymoon Well Complex, Western Australia

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