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IntroductionKAMBALDA is a type locality for Archean komatiite-associatednickel sulfide deposits (class 1A deposits of Lesher, 1989) anda world-class mining district, with a premined reserve of ~35million metric tons (Mt) containing 3 percent nickel (Gre-sham, 1986; Stone and Masterman, 1998), for a total metal in-ventory of ~1.1 Mt nickel metal. Most studies at the Kam-balda dome have addressed deposit-scale depositional andstratigraphic relationships and primary igneous geochemistryin support of volcanic ore genesis models (Groves et al., 1986;Lesher and Groves, 1986). The volcanic models are based onthermal erosion of a sulfidic sediment substrate to turbulent

lava channels as the key process in ore genesis (Lesher, 1989;Williams et al., 1998). Consequently, exploration models em-phasize dynamic lava channels identified on the basis of deepfootwall embayments (trough structures), presence of thickhigh Mg komatiite flows in the hanging wall, absence of sed-imentary rocks, and systematic variations in tenor (nickelcontent in 100% sulfides; Lesher, 1989; Williams et al., 1998;Lesher et al., 2001). However, these models developed with-out a full understanding of deformation overprints, as evi-dent at the Kambalda dome (Barrett et al., 1977; Cowdenand Archibald, 1987; Stone and Archibald, 2004; Stone et al.,2004). Although most previous studies recognize postvol-canic deformation, few detailed structural studies have beenundertaken.

The volcanic ore genesis model is based on the premisethat primary volcanic and stratigraphic relationships can be

Structural Setting and Shape Analysis of Nickel Sulfide Shoots at the Kambalda Dome,Western Australia: Implications for Deformation and Remobilization

WILLIAM E. STONE,†,*Kambalda Nickel Operations, WMC Resources Ltd., Kambalda, Western Australia 6442, Australia

STEPHEN W. BERESFORD,School of Geoscience, Monash University, P.O. Box 28E, Clayton, Victoria 3800, Australia

AND NICHOLAS J. ARCHIBALD

Geoinformatics Explorations Ltd., 57 Havelock Street, West Perth, Western Australia 6872, Australia

AbstractIntegrated three-dimensional structural studies have led to new insights into the geologic controls on the dis-

tribution of magmatic nickel sulfide ore shoots associated with metakomatiites at the Archean Kambalda dome,Western Australia. Exploration models emphasize volcanic channels in deep embayments (trough structures)modified by thermal erosion at the base of turbulent lava flows. However, analysis of three-dimensional mod-els of the Kambalda exploration database reveals ore shoot-scale controls consistent with the regional defor-mation sequence D1 to D4. Ore shoots in D1 settings (e.g., Ken shoot) are transposed into the north-northwestD1 trend and have asymmetric trough structures with reentrant updip margins indicating a sense of movementopposite to that on the Kambalda dome. Ore shoots in D2 settings (e.g., Lunnon shoot) have asymmetric troughstructures bound by low-angle thrusts indicating a sense of movement typical of that on the dome, with thrustmovement of ore into the talc and carbonated hanging-wall rocks lacking primary igneous features (e.g.,McMahon shoot). Ore shoots in D3 settings are offset by normal faults which host gold mineralization at thesouth end of the Kambalda dome (e.g., Hunt shoot) or by major reverse faults at the north end of the dome(e.g., Otter-Juan shoots). Ore shoots in D4 settings (Fisher shoot) are transposed from the north-northwesttrend into the north to north-northeast D4 trend. In contrast, ore shoots associated with shallow, poorly definedtrough structures, with or without serpentinized hanging wall showing relict igneous features (e.g., Durkinshoot), indicate preserved volcanic control.

The spectrum of volcanic and structural controls indicates a continuum of Kambalda ore shoots from a vol-canic controlled end member (3 shoots in shallow trough structures with relict igneous features) through struc-turally modified (7 shoots in deep trough structures without relict features) to a structurally controlled endmember (1 shoot tectonically emplaced in the hanging wall with deformation features). Shape analysis of thethree-dimensional models indicates that the structurally modified and controlled ore shoots have prolateshapes and two of the volcanic controlled ore shoots have oblate shapes. Comparison of the prolate shapes withthe shapes of strain indicators reveals the importance of constrictional D1 deformation in elongation of the oreshoots, particularly at the north and south ends of the Kambalda dome. The oblate volcanic-controlled endmembers reflect low-strain settings adjacent to major fault zones. Comparisons to previous experimental sys-tems suggest that the ore shoots and metakomatiite deformed preferentially by ductile flow. The importanceof deformation controls on nickel sulfide distribution at the Kambalda dome means that exploration for Kam-balda style mineralization should take into consideration the deformation history of terranes that host ore.

† Corresponding author: e-mail, wstone@nevadastar.com*Present address: Nevada Star Resources Corporation, 355 Burrard

Street, Vancouver, BC, Canada V6C 2G8.

©2005 Society of Economic Geologists, Inc.Economic Geology, v. 100, pp. 1441–1455

reconstructed with confidence. However, the Kambaldadome is a major tectonic structure in which the geologic se-quence experienced polyphase tectonism and metamorphism(Cowden and Roberts, 1990). The trough structures are faultbounded, transgress ore, and represent hinge zones of para-sitic synclines that formed on the flanks of the Kambaldadome (Stone and Archibald, 2004). The massive nickel oresshow strong tectonic fabrics (Ostwald and Lusk, 1978) thatpreserve evidence of the regional deformation sequence(Cowden and Archibald, 1987), and variations in ore tenor re-flect modification during metamorphic alteration (Heath etal., 2001; Stone et al., 2004). The role of deformation in mod-ifying and controlling the nickel ore shoots must be under-stood before intricate models of the volcanic and stratigraphicrelationships can be evaluated.

This paper documents the results of three-dimensionalstructural studies of the nickel ore shoots at the Kambaldadome, Western Australia. The extensive exploration database

for the district has been digitally modeled to define thetrough structures and other major structures and show thedistribution of nickel ore shoots, metamorphic alteration, sed-imentary rocks, and gold mineralization in three dimensions.From this modeling and through application of the shapeanalysis techniques of Blenkinsop (2004), we demonstratehow the deformation modified and controlled the geologyand geometries of the ore shoots. These results permit classi-fication of the shoots in a structural continuum model, chal-lenge important aspects of volcanic genesis models, and havesignificance in global exploration.

Geologic Setting of the Kambalda DomeThe Kambalda dome is a doubly plunging tectonic struc-

ture in the south-central part of the Archean Norseman-Wiluna greenstone belt, Yilgarn craton, Western Australia(Fig. 1; Stone and Masterman, 1998). The dome is part of theregional Kambalda anticline (Stone and Archibald, 2004),

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FIG. 1. Geologic map of the Kambalda dome (modified from Stone and Masterman, 1998). The ore shoots are shown inplan projection. LTW = Loreto thrust wedge. Age for the Lunnon Basalt from Chauvel et al. (1985).

cored by a felsic intrusion, and truncated to the east by the re-gional Lefroy tectonic zone (Fig. 1). In cross section (Fig. 2),the Kambalda dome is steeply dipping to locally overturnedon the east flank and moderately dipping on the west flank.

The nickel ore shoots in the area surrounding the Kam-balda dome (Fig. 1) occur on the upper contact of the Lun-non Basalt Formation (footwall of the ore shoots) and withinthe overlying Kambalda Komatiite Formation (host rock andhanging wall of the ore shoots; Fig. 2).

The volcanic sequence at the Kambalda dome underwentpolyphase tectonism, D1 to D4 (Weinberg et al., 2003; Stoneand Archibald, 2004), felsic intrusion (Cowden and Roberts,1990), and upper greenschist facies regional metamorphism(Barrett et al., 1977; Bavinton, 1981). Mineral assemblagesindicative of lower amphibolite facies conditions of metamor-phism occur in contact aureoles about the felsic intrusions(Archibald, 1985). The tectonometamorphic evolution of thekomatiitic rocks involved hydration to serpentine-dominated

STRUCTURE AND SHAPE OF Ni SULFIDE SHOOTS, KAMBALDA, WA 1443

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FIG. 2. Cross sections of the Kambalda dome (modified from Gresham and Loftus-Hills, 1981; Archibald, 1985; and Ba-nasik, 1996). (A). Cross section of the northwest flank of the dome at 550 850 N (mine grid) across the McMahon, Gellatly,and Otter-Juan ore shoots. West-dipping reverse faults have formed a series of wedges of the Lunnon Basalt footwall. (B).Cross section of the south part of the dome at 544 570 N (mine grid) across the Hunt and East Alpha ore shoots on oppos-ing flanks of the dome. The Alpha shoot is the Lunnon ore shoot offset on the east side of the Alpha Island fault. The thick-ness of the ore shoots, sedimentary units, and felsic intrusions is exaggerated for clarity.

assemblages (serpentine metamorphic alteration) and subse-quent talc and carbonate metamorphic alteration (Cowdenand Roberts, 1990). Volcanic textures and primary volcanicand stratigraphic relationships were preserved through ser-pentinization and probably reflect original sea-floor condi-tions. In contrast, the talc and carbonate metamorphic alter-ation involved porphyroblastic growth of dolomite andmagnesite, replacement by talc and fabric development dur-ing D1 to D4 (Barrett et al., 1977; Cowden and Roberts,1990), which destroyed igneous textures and obscured pri-mary relationships.

Methods and Techniques

Three-dimensional modeling

The key geologic features modeled in three dimensionswere the upper contact of the Lunnon Basalt (footwall con-tact of the ore shoots), major structures, metamorphic alter-ation, and nickel contents of sulfides and metasedimentaryrocks. These features were modeled either as geologic sur-faces or geologic volumes.

Surface (wireframe) modeling of the footwall contact in-volved digital capture of ~130 cross sections at several scales.Mine cross sections (~200-m spacing) were based on under-ground maps and drill log interpretations. In addition, 20- to40-m-spaced sections of the Lunnon shoot (Williams et al.,1993) were incorporated along with selected cross sections atirregular spacings from regional stratigraphic drilling (Gre-sham, 1978) for the wireframe construction. The basalt con-tact was modeled around the entire Kambalda dome.

Two versions of the surface model of the footwall contactwere created, a fact model and a more fully interpretedmodel. The parameters for the fact modeling were: (1) 50 msections with a projection window for each section of 10 m;(2) isolated drill lines projected 50 m on opposite sides of thesection; and (3) drill section lines extended to the surface butwith minimal extension downdip from the deepest drill inter-section. The fully interpreted model was created using vari-ograms and kriging to interpolate the contact position inthree-dimensional space. In addition to the model footwallcontact, major macroscopic structures were modeled, as iden-tified from faults defined in underground mapping (e.g., Lun-non main shear, A fault, and others, identified from Middle-ton, 1980, and Archibald, 1985) and faults interpreted fromdisplacements in the morphology of the footwall basalt (e.g.,Loreto thrust, Fisher Wall fault). It should be noted that be-cause they are mainly oblique views, the scale bars shown donot apply equally to all parts of the models presented in thefigures.

The lithotype and geochemical databases were merged toprovide a means of interpolating the data into three-dimen-sional lattices about the ore shoots. Volume interpolationusing three-dimensional lattices is a means of visualizing largeamounts of lithological and geochemical data. The volumemodels were subsequently created by interpolating the drillhole data into three-dimensional blocks, 50 × 50 × 10 m insize. This method records the presence or absence of an en-tity within a block and produces a three-dimensional contour(isosurface) around a block when a threshold value (e.g., av-erage Ni concentrations of ≥10,000 ppm in 10-m vertical

intervals of sulfide rock) is exceeded. This modeling methodis useful for rapid visualization of entities which may be highlyirregular and difficult to model otherwise.

Nickel contents of sulfides and sedimentary rocks and goldcontents of Lunnon basalt were modeled as isosurfaces of val-ues interpolated into the regularly gridded three-dimensionallattices. The value of 10,000 ppm Ni in sulfides was selectedto model the ore shoots, because it marks the lower cutoff forresource calculations (e.g., Bonwick and Sheppard, 2004).These models resolve the ore shoots (narrow, elongate arraysof orebodies) and constituent orebodies (geologically distinctand coherent zones of ≥3% Ni). Volume models also werecreated to represent the distribution of the felsic intrusionsand the serpentine metamorphic alteration and talc and car-bonate metamorphic alteration in the hanging wall. Thenickel ore shoots are hosted exclusively in the Silver LakeMember and not in the Tripod Hill Member (Fig. 2), but thisstratigraphic subdivision of the Kambalda Komatiite Forma-tion was not systematically recorded in the database, makingit impossible to model the two members separately.

Structural Setting at the Scale of the Kambalda DomeThe upper contact of the Lunnon Basalt in the Kambalda

dome domain is shown in Figure 3. The model reflects thedistribution of drilling data and is thus highly irregular. Thesurface reveals the general complexity of the contact mor-phology, particularly in the areas of the nickel shoots and onthe west flank of the Kambalda dome (i.e., Loreto thrustwedge). Studies of the morphology of the footwall in the oreshoot areas led to insights into the nature of the structural andmetamorphic alteration trends.

Inspection of the model of the footwall contact (Fig. 3A)confirms the presence of the trough structures in many of theore shoot areas, the features of which are summarized inTable 1. The trough structures in general are linear featuresup to ~3 km in length and hundreds of meters wide. Mosttrend north-northwest, parallel to the regional structuraltrend and are particularly shallow (<10 m deep), poorly de-fined and discontinuous features.

The model of the distribution of ≥10,000 ppm Ni in sulfides(Fig. 3B) indicates that the ore shoots are linear (the Lunnon,Hunt and Gellatly-Wroth shoots) to planar (the Long andDurkin shoots) features up to thousands of meters in length,hundreds of meters wide, and tens of meters thick. Other oreshoots (Fisher, Otter-Juan, Ken) show a complex variety ofshapes. Generally, the trend of the nickel ore shoots coincideswith the trough structures. Indeed, the ore shoots in thenorth-northwest–trending trough structures are larger (mil-lions of tons) and more continuous (thousands of meters)than those in the other troughs (Table 1). However, some oreshoots trend slightly but significantly oblique to the generalnorth-northwest trend (e.g., Durkin shoot) and to the associ-ated trough structure (e.g., Lunnon shoot; Stone andArchibald, 2004).

The distribution of the serpentine metamorphic alterationin the Kambalda Komatiite (hanging wall of the ore shoots) isrepresented in Figure 3C. The serpentine metamorphic al-teration dominates the hanging wall at the Durkin and Victorshoots, where igneous textures and primary depositional andstratigraphic relationships are preserved (Cowden, 1988;

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Beresford et al., 2002). Elsewhere, except in parts of theFisher shoot, talc and carbonate metamorphic alterationdominates the hanging wall, and igneous textures and primarydepositional and stratigraphic relationships are destroyed.

Structural Setting at the Scale of the Ore shootsThe nickel ore shoots, the footwall contact with the Lunnon

Basalt, and the hanging-wall contact with the Kambalda Ko-matiite were originally products of volcanism but have sincebeen the loci of intense deformation resulting in complexstructure. The relict features of volcanic deposition and the ef-fects of each of the four phases of deformation on key selectedore shoots, as observed in the three-dimensional models, aredescribed below. The geologic characteristics of the footwall,ore shoots, and hanging wall are summarized in Table 1.

Predeformation

The Durkin, Victor, and Coronet ore shoots at the Kam-balda dome (Fig. 3B, Table 1) are associated with very shal-low trough structures. Well-preserved volcanic and deposi-tional relationships, as well as relict igneous textures, areevident in the wall rocks (Cowden, 1988). Primary deposi-tional features are best illustrated in the three-dimensionalmodels of the Durkin shoot, which indicate minimal struc-tural overprint and clear volcanic control.

Durkin shoot: Durkin shoot is the major ore shoot on thenorth flank of the Kambalda dome (Fig. 3B, Table 1). Thefootwall contact shows a small D2 trough structure, ≤10 mdeep and up to 50 m wide with poorly defined margins at theeast end of the Durkin shoot (Durkin trough, Fig. 4A; Cow-den, 1988). The relief on the contact decreases westward to-ward the Fisher Wall fault (D4). West of that structure, whichmarks the east boundary of the Otter-Juan ore shoots, the re-lief on the footwall contact increases markedly up to 100 m.

The Durkin ore shoot (Fig. 4B) is 1,000 m in length and300 m wide and only partly confined by the D2 trough struc-ture. West of the Fisher Wall fault, the ore shoot bifurcatesinto the updip (south) Durkin West and downdip (north)Durkin Deep trends of the Otter-Juan shoots, reflecting thewestward increase in structural complexity (Fig. 4A). Hang-ing-wall serpentine metamorphic alteration also decreases to-ward the west (Fig. 4C), but talc and carbonate alteration in-creases.

D1 thrusting

D1 involved south- to north-directed thrusting and distrib-uted deformation in incompetent lithotypes. Major over-thrusts and related folds of the Lunnon Basalt into theKambalda Komatiite, showing sinistral asymmetries in north-south and west-east cross sections of the Ken, Otter-Juan, and

STRUCTURE AND SHAPE OF Ni SULFIDE SHOOTS, KAMBALDA, WA 1445

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FIG. 3. Structural setting at the scale of the Kambalda dome. (A). Three-dimensional model of the upper contact of theLunnon Basalt, showing the locations of the trough structures as indicated by tens to hundreds of meters of relief on the foot-wall contact. Plan view. The model is ~12 km in length, 4 km wide, and a maximum of 1 km deep. Also shown are the sur-face extent of the Lunnon Basalt (medium gray) and interpreted major and minor axial traces of the Kambalda dome. Thedashed line for the Fisher Wall fault indicates interpreted continuation to the north. Felsic intrusions omitted for clarity.LTW = Loreto thrust wedge. (B). Three-dimensional model of the nickel sulfide ore shoots overlain on the three-dimen-sional model of the Lunnon Basalt upper contact. Plan view. The ore shoot positions are indicated by the model of the dis-tribution of ≥10,000 ppm Ni in sulfides (dark gray). (C). Three-dimensional model of serpentine metamorphic alteration ofthe Kambalda Komatiite hanging wall (dark gray). Plan view. The serpentine metamorphic alteration is dominant at the Vic-tor and Durkin ore shoots and in parts of the Fisher ore shoot. Talc and carbonate metamorphic alteration is dominant at theremainder of the ore shoots.

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TAB

LE

1. S

umm

ary

of N

iS O

re S

hoot

Cha

ract

eris

tics

at th

e K

amba

lda

Dom

e

Ore

sho

otK

enL

unno

nM

cMah

onH

unt

Ott

er-J

uan

Fis

her

Dur

kin

Vic

tor

Lon

gC

oron

etG

ella

tly-W

roth

Foo

twal

lTr

ough

str

uctu

reYe

sYe

sN

oYe

sYe

sYe

sPa

rtia

lPa

rtia

lYe

sPa

rtia

lYe

sTr

ough

tren

d in

N

NW

-NN

NW

Non

eN

WN

NW

NW

and

NW

NW

NN

WN

NW

NW

NN

W, N

plan

vie

wTr

ough

leng

th (

m)

2500

2500

Non

e15

0030

0010

0010

0010

0020

0060

013

00Tr

ough

wid

th (

m)

100

100

Non

e40

-80

250-

400

100-

400

300

50-2

00up

to 3

0010

050

-100

Trou

gh d

epth

(m

)5-

5050

Non

e10

10-3

010

-200

<5-2

0<5

-20

up to

75

<550

Plun

ge/d

irec

tion

10°–

15°/

15°/

SSE

NW

10°–

20°/

SW20

°–45

°/Va

riab

leN

one

10°/

SSE

0°–2

0°/S

SE45

°NW

20°/

NN

WN

W-N

NW

NN

WTr

ough

ree

ntra

nt

Upd

ipD

ownd

ipN

one

Dow

ndip

Dow

ndip

Dow

ndip

Dow

ndip

Dow

ndip

Dow

ndip

Dow

ndip

Upd

ipm

argi

nL

ocal

ly u

pdip

Oth

er m

ajor

str

uctu

res

D1,

D2

D2,

D4

D2

D2,

D3

D2,

D3

D2,

D4

D2

D2

D2

D2,

D3

D1,

D2

Oth

er m

inor

str

uctu

res

D4

D4

D4

D1

D4

D4

D4

D4

Ore

Trou

gh c

onfin

emen

tPa

rtia

lPa

rtia

lN

oYe

sYe

sYe

sPa

rtia

lPa

rtia

lYe

sYe

sYe

sM

ajor

ore

pos

ition

Con

tact

Con

tact

Han

ging

wal

lC

onta

ctC

onta

ctC

onta

ctC

onta

ctC

onta

ctC

onta

ctC

onta

ctC

onta

ctM

inor

ore

pos

ition

Non

eH

angi

ng w

all

Con

tact

Han

ging

wal

lN

one

Han

ging

wal

lN

one

Non

eN

one

Han

ging

wal

lH

angi

ng w

all

Mai

n or

e ty

peM

assi

veM

assi

veB

recc

iaM

x di

ssM

assi

veM

assi

veM

assi

ve m

xM

x di

ssM

x di

ssM

x di

ssM

x di

ssSi

ze (

Mt)

0.6

4.5

0.3

1.3

~10

1.7

3.5

1.0

~60.

60.

1N

i gra

de (

wt%

)3.

32.

72.

32.

73.

52.

33.

74.

03.

73.

52.

5O

re d

ip/d

irec

tion

Vari

able

/NW

45°/

E45

°/N

W45

°/SW

Hig

hly

vari

able

Vari

able

/SW

20°/

N45

°/E

45°–

80°/

E45

°SW

20°/

NN

WL

ocal

ly o

vert

urne

dN

oN

oN

oYe

sYe

sYe

sN

oN

oYe

sN

oYe

s

Han

ging

wal

lD

omin

ant a

ltera

tion

Talc

+ c

arb

Talc

+ c

arb

Talc

+ c

arb

Talc

+ c

arb

Talc

+ c

arb

Serp

+ ta

lc +

car

bSe

rpen

tine

Serp

entin

eTa

lc +

car

b±se

rpTa

lc +

car

bTa

lc +

car

bSe

dim

enta

ry r

ocks

Pres

ent

Abs

ent

Pres

ent

Abs

ent

Abs

ent

Abs

ent

Abs

ent

Abs

ent

Abs

ent

Abs

ent

Pres

ent

Not

es: D

iss

= di

ssem

inat

ed, m

x =

mat

rix,

ser

p =

serp

entin

e, ta

lc +

car

b =

talc

and

car

bona

te a

ltera

tion

Gellatly-Worth ore shoots, are attributed to D1 faulting andfolding. The effects of D1 are best represented in the three-dimensional models of the Ken ore shoot.

Ken shoot: The Ken shoot is the largest of the three oreshoots on the Loreto thrust wedge (Fig. 3B, Table 1). Thethree-dimensional model of the footwall contact of the Kenshoot (Fig. 5A) shows a well defined trough structure (Ken

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FIG. 4. Depositional features at the Durkin ore shoot. (A). Three-dimen-sional model of the upper contact of the Lunnon Basalt (light gray), showinga shallow, poorly defined trough structure in the east part of Durkin shoot.View looking to the south along the horizontal plane. (B). The model of thedistribution of ≥10,000 ppm Ni in sulfides (dark gray) shows the ore shootoverlying an essentially planar contact. The ore shoot is bound to the west bythe Fisher Wall fault (D4). (C). The model of the distribution of serpentinemetamorphic alteration in the Kambalda Komatiite hanging wall (dark gray).

FIG. 5. D1 at the Ken ore shoot. (A). Three-dimensional model of theLunnon Basalt upper contact, showing the locations of the Ken, McMahon,and Gellatly-Worth ore shoots on the Loreto thrust wedge, northwest flankof the Kambalda dome. Note that the main trough structures at Ken, Gel-latly, and Wroth have a reentrant updip east margin, characteristic of D1.KLT = Ken lower trough. Plan view. (B). The model of the distribution of≥10,000 ppm Ni in sulfides (light gray). Approaching the Ken lower troughstructure from the east, the ore shoot is transposed from the 310o northwesttrend of the Ken main flank and Ken lower flank orebodies into the 330o

north-northwest D1 trend of the Ken lower trough orebodies.

Lower Trough), 3 km in length and up to 100 m wide and 50m deep, which makes it the longest trough structure at Kam-balda. The updip east margin of the trough structure is flatrelative to rock unit contacts (reentrant), such that Lunnonbasalt locally overlies Kambalda komatiite and coincides witha major D1 mylonite zone (Archibald, 1985). In contrast, thedowndip west margin is steep relative to rock unit contacts(upright offsets), such that Lunnon Basalt is adjacent to Kam-balda Komatiite. This asymmetry of the lateral trough mar-gins suggests updip-over-downdip movement, opposite to thetypical sense of movement on the Kambalda dome (Stone andArchibald, 2004) and indicates D1 fault-fold couplets.

The model of the distribution of ≥10,000 ppm Ni in sulfidesshows that two of the three orebodies at the Ken shoot arepresent updip rather than within the D1 trough structure(Fig. 5B). The bulk of the ore was in the Ken main and Kenmiddle orebodies, confined to irregular spoon-shaped de-pressions defined by north trending D4 faults. In contrast,only minor amounts of ore were in the Ken lower trough ore-body. The Ken shoot is the best example of a Kambalda oreshoot that is not confined to a trough structure. Together, thethree orebodies show transposition of the ore shoot from a310o trend into the 330o trend of the D1 lower Ken troughstructure.

D2 inclined thrust-related folding

D2 was the peak deformation event regionally and involvedsouth-southwest– to north-northeast–directed thrust-folding,

pervasive deformation, formation of the Kambalda anticlineand development of the Lefroy tectonic zone (Fig. 1). Thrustsand high-angle reverse faults and related folds, which cut theD1 thrusts and show typical asymmetry for the Kambaldadome, are attributed to D2 faulting and folding. At the oreshoot scale, the D2 event formed the majority of the troughstructures as fold-thrust couplets with downdip-over-updipsense of movement, consistent with movement updip towardthe crest of the Kambalda anticline or dome (Stone andArchibald, 2004). Such movement resulted in the thrust em-placement of some contact ore zones internally within thehanging wall (i.e., the hanging-wall ore at the McMahon,Lunnon, and Hunt shoots). These effects of D2 are best rep-resented in the three-dimensional models of the Lunnon andMcMahon shoots.

Lunnon shoot: Lunnon is the major ore shoot at the southend of the Kambalda dome (Fig. 3B, Table 1). It was the dis-covery nickel shoot at Kambalda (Woodall and Travis, 1969)and is the best understood geologically (Ross and Hopkins,1975; Gresham and Loftus-Hills, 1981; Findlay, 1998).

The Lunnon trough structure is 2.5 km in length, 100 mwide, and 50 m deep, with an irregular floor (Fig. 6A). Thedowndip east margin of the trough structure is defined by aseries of en echelon east-dipping D2 reverse faults termed theEast thrust zone (Fig. 6B; Middleton, 1980; Cowden, 1986),with an updip sense of movement characteristic of D2. In con-trast, the updip west margin of the trough structure is definedby an upright offset along a D3 fault zone (Main Shear) which

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FIG. 6. D2 at the Lunnon ore shoot. (A). Three-dimensional model of the Lunnon Basalt upper contact, showing a long,narrow, and deep trough with well-defined margins. The reentrant downdip east margin defines downdip-over-updip asym-metry, which indicates upward movement toward the Kambalda dome. The irregular relief on the floor and at the marginsof the trough structure (up to 10 m) mark crosscutting D4 faults. View looking to the south-southeast from slightly above thehorizontal plane. Image rotated 45o to the west. (B). Wireframe models of the East thrust (D2), which defines the downdipeast margin and Main shear (D3), which defines the updip margin of the Lunnon trough structure and truncates the Eastthrust. (C). The model of the distribution of ≥10,000 ppm Ni in sulfides showing partial confinement of ore to the troughstructure. The blocky nature of the ore models reflects data distribution and not the physical nature of the ore shoots. Viewlooking to the north along the horizontal plane.

truncates the D2 East thrust zone. Partial confinement of theore shoot to the trough structure (Fig. 6C) is consistent withprevious reports (Stone and Archibald, 2004) of slight but sig-nificant discordance in the trends of the trough structure andthe volcanic channel.

McMahon shoot: The McMahon shoot is a small but geo-logically significant ore shoot on the northwest flank of theKambalda dome (Table 1), 1 km north of the Ken shoot (Fig.3). The controlling structure of the McMahon ore shoot is adiscrete D2 fault (McMahon thrust) within the hanging wall,25 to 30 m upsection of the footwall contact (Fig. 7), ratherthan a trough structure in the footwall contact. Thrust move-ment on this low-angle fault explains the displacement of theMcMahon hanging wall from its original downdip position(Nevill, 1980; Hayward, 1986).

D3 doming and faulting

D3 coincided with the main phase of felsic intrusion (Fig.1), peak of metamorphism, and formation of the Kambaldadome, with related normal faults in some places and reversefaults elsewhere which cut the D2 structures. The effects ofD3 at the ore shoot scale are best represented in the three-di-mensional models of the Hunt shoot and the Otter-Juanshoots.

Hunt shoot: The Hunt shoot is a small ore shoot (Table 1)at the south end of the Kambalda dome (Thomson, 1985;Cowden and Roberts, 1990), 1 km to the southwest of theLunnon shoot (Fig. 3B). The structural geology of the Huntshoot is well understood, due to the presence of gold miner-alization superimposed on the footwall contact of the nickelsulfide orebodies (Roberts and Elias, 1990). The footwallcontact (Fig. 8A) has a shallow, poorly defined trough struc-ture (D2) that is cut by the upright D3 A fault and the West-ern Flanks fault. Kinematic indicators indicate sinistraloblique movement along the A fault (Archibald, 1985). To-gether with indications of dextral oblique movement alongthe D3 Main Shear at the Lunnon shoot, on the opposite side

of the Kambalda dome (Fig. 3B), this is consistent with dom-ing during D3. At the Hunt shoot, displacement along the Afault formed the offset A zone orebody (Fig. 8B). The A faultalso influenced the siting of later gold mineralization at thesouth end of the Kambalda dome (Fig. 8C). The A fault ishighly mineralized in gold where it cuts the Lunnon Basaltimmediately below its contact with the D-zone Deeps andoffset A zone nickel sulfide orebodies (Roberts and Elias,1990), consistent with movement and hydrothermal activityduring D3 faulting.

Otter-Juan shoots: The Otter-Juan shoots at the north endof the Kambalda dome, 2 km northeast of the McMahonshoot (Fig. 3B), constitute the largest nickel sulfide deposit atKambalda (Table 1) and are essentially continuous with theDurkin ore shoot to the east. The footwall contact (Fig. 9A)shows extreme relief as a result of major offsets along D3 (andD1) reverse faults. The major D3 fault domain consists of a se-ries of closely spaced, steep west-dipping wedges of LunnonBasalt (Fig. 9A), which have been structurally emplaced intothe hanging wall. One of the largest of these structures is theJuan Deeps F fault (Fig. 9B-C), along which a wedge of Lun-non Basalt has been emplaced hundreds of meters into theKambalda Komatiite (Fig. 2A), disrupting the ore shoots andobscuring stratigraphic relationships.

Overall, the two Otter-Juan ore shoots (arrows, Fig. 9B)have an along-plunge extent of 3 km to a depth of at least1,200 m below surface, and are up to 400 m wide and containnumerous orebodies (Marston and Kay, 1980; Heath et al.,2001). The ore shoots were disrupted and offset dextrallyalong the Juan Deeps F fault (Fig. 9C) during D3 faulting.Unlike the Hunt shoot, the D3 reverse faults at the Otter-Juanshoot lack gold mineralization.

D4 oblique strike-slip

D4 involved dextral shearing on steep, north to northeasttrending faults and fault zones, which offset the D3 structuresand felsic intrusions. The effects of D4 are evident at the Ken,Lunnon, Hunt, Long, and Victor shoots (Table 1) but aremost obvious in the three-dimensional models of the Fisherore shoot.

Fisher shoots: The Fisher shoots constitute the largest oredeposit on the west flank of the Kambalda dome (Fig. 3B,Table 1). The footwall contact has very high relief (Fig. 10A)and multiple ore trends (Fig. 10B). A well-defined north-trending trough structure (main trough) is defined by majorstructures at its margins. The east margin of the main troughis defined by the upright offsets along the major D4 FisherWall fault (Hancock, 1978; Gresham and Loftus-Hills, 1981;Smith, 1983). The west margin is defined by the major west-dipping Loreto thrust (D2). East of the Fisher Wall fault, twoore shoots are partly confined to minor northwest trendingtrough structures (arrows, Fig. 10B). At the intersection withthe main trough structure, the south ore shoot is disruptedand transposed into parallelism with the D4 north trend of theFisher Wall fault. The north ore shoot appears to be trun-cated by that fault.

Summary

Interpretation of the three-dimensional models indicatesthat the nickel sulfide ore shoots at the Kambalda dome have

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FIG. 7. D2 at the McMahon ore shoot. The model of the distribution of≥10,000 ppm Ni in sulfides (dark gray) overlain on a wireframe model of theMcMahon thrust (D2). View looking to the north and from below the hori-zontal plane. The McMahon shoot is located on the McMahon thrust withinthe Kambalda Komatiite, 25 to 30 m upsection of the upper contact of theLunnon Basalt. MU = McMahon upper orebody, ML = McMahon lower ore-body, and KLT = Ken lower trough.

been strongly affected by polyphase deformation. The majorcontrols on the ore shoots are structural and metamorphic,and the ores occur in the most deformed rocks at Kambalda.Despite the deformation, prevailing genetic and explorationmodels (e.g., Williams et al., 1998; Lesher et al., 2001) em-phasize primary depositional relationships. Most mines arenow closed, but the three-dimensional models clearly indi-cate the importance of structure and provide valuable guidesfor exploration.

Three groups of deposits can be recognized, based on thedegree of deformation and metamorphic overprinting. TheDurkin, Victor, and Coronet shoots are the least deformed,are at least partly controlled by primary volcanic features (i.e.,shallow troughs), and are associated with mainly serpen-tinization of the hanging-wall rocks. The majority of the oreshoots (i.e., Lunnon, Hunt, Long, Otter-Juan, Fisher, Ken,McMahon, Gellatly-Worth) occur in structurally controlledtroughs and the host rocks are intensely altered to talc andcarbonate. Exploration models based on primary volcanolog-ical controls must consider the deformation and metamor-phism in these shoots. The McMahon ore shoot occupies amajor thrust and is intensely deformed. Exploration modelsbased on primary volcanological controls are of limited ap-plicability in this setting.

Three-Dimensional Shape AnalysisFollowing Blenkinsop (2004), the shape of orebodies can

be described by three orthogonal axes U ≥ V ≥ W (maximum,

intermediate, and minimum). The ratio values of these axescan be expressed as parameter j = (U/V – 1)/(V/W – 1) andrepresented in a shape plot of U/V versus V/W, in a mannersimilar to strain ellipsoids. Values of j vary from >1 to ∞ fortubular (prolate) shapes to <1 to 0 for tabular (oblate) shapes.In shape plots, a line representing j = 1 divides the plot intoprolate and oblate fields.

The shape analysis technique was developed by Blenkinsop(2004) for studying the origin of hydrothermal gold depositsbut has been adopted here for studying the deformation ofthe magmatic nickel deposits. Measurements of the threeaxes of the Kambalda ore shoots and orebodies were made onthe models of the distribution of ≥10,000 ppm Ni in sulfides.Following Blenkinsop (2004), closed ore shoots have all theirboundaries fully defined, whereas open ore shoots remain un-defined at depth or are truncated by the surface or by felsicintrusions. Most of the Kambalda ore shoots are closed.

The three-dimensional shape analyses were carried out onall 11 of the major nickel ore shoots on the Kambalda dome(Table 2). In cases where the ore shoots were too complex inthree-dimensional shape to be treated as an ellipsoid, individ-ual orebodies were analyzed and the results presented asmean values. Generally, the U axes are oriented downplungeand the V and W axes are oriented along and across the strike(thickness), respectively. The majority of ore shoots and ore-bodies show strong prolate shapes (Table 2, Fig. 11). Thestructurally controlled end member, the McMahon shoot,and the structurally modified Long shoot (Fig. 3B) plot in the

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FIG. 8. D3 at the Hunt ore shoot. (A). Three-dimensional model of the Lunnon Basalt upper contact showing a relativelyshallow trough structure and the intersection traces of the A fault and Western flanks fault (D3). View looking to the south-east from above the horizontal plane. (B). The model of the distribution of ≥10,000 ppm Ni in sulfides, showing the positionsof the main orebodies. The A zone orebody was offset along the A fault from the main D zone Deeps orebody. Orebodies:DZD = D zone deeps and HWZ = hanging-wall zones. (C). Model of the distribution of >100 ppb Au in the Lunnon Basalt(50% translucent) in the immediate footwall of the contact ore shoot. Comparison of (A) and (C) highlights the strong struc-tural control of the D3 faults, particularly the A zone fault, on gold mineralization.

prolate area but near the j = 1 line. The least deformed vol-canic end member Durkin and Coronet ore shoots have moreoblate shapes.

The considerable variation in the shapes of the ore shootsand orebodies reflects heterogeneous strain throughout theKambalda dome. According to Archibald (1985) and Cowdenand Archibald (1987), strain related to D1 was dominantly

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FIG. 9. D3 at the Otter-Juan ore shoots. (A). Three-dimensional fully in-terpreted model of the Lunnon Basalt upper contact (light gray), showing thedomain of closely spaced D3 reverse faults offsetting the footwall contact byup to hundreds of meters in the west part of the Otter-Juan shoots. Plan view.(B). The model of the distribution of ≥10,000 ppm Ni in sulfides (light gray),showing two subparallel ore shoots (indicated by arrows), which are dis-rupted along the trace of the Juan Deeps F fault. Plan view. (C). The modelof distribution of ≥10,000 ppm Ni in sulfide (dark gray) overlain on the three-dimensional model of the Lunnon Basalt upper contact (50% translucent).The ore shoots are offset in a dextral sense along the D3 Juan Deeps F fault,as indicated by the arrows (parallel to plunge). View looking down and ro-tated 45o west.

FIG. 10. D4 at the Fisher ore shoot. (A). Three-dimensional model of theLunnon Basalt upper contact (light gray), highlighting the structural com-plexity at the Fisher shoot. The major trough structure trends north and isdefined at the margins by the Fisher Wall fault (D4) to the east and theLoreto thrust (D2) to the west, with vertical relief of up to hundreds of me-ters. Plan view. (B). The orebodies of the north ore shoot (NOS) and thesouth ore shoot (SOS) are defined in the model of the distribution of ≥10,000ppm Ni in sulfides (dark gray). Plan view. The northwest-trending SOS istransposed along the north trending Fisher Wall fault, whereas the NOS istruncated.

constrictional and produced prolate bodies outside the thrustzones and oblate bodies inside the zones. Strain related to D2

was dominantly flattening and produced oblate shapes, andstrain related to D3 and D4 involved strong flattening andwould have produced strongly oblate shapes. The Fisher andOtter-Juan shoots plot in the D1 field, consistent with domi-nantly constrictional deformation. The Durkin and theMcMahon shoots plot in the D2 field, consistent with domi-nantly flattening deformation. None of the ore shoots plot inthe D3-D4 field. Overall, the dominance of strong prolateshapes is most consistent with the constrictional strain char-acteristic of D1 outside the major thrust zones.

In contrast to the Long shoot, the strongly prolate shape ofthe Victor shoot, 1 km to the south (Fig. 3B), indicates a verydifferent deformation history. The Victor shoot was deformedduring D1 but occupied a low-strain zone adjacent to theLefroy tectonic zone during D2 flattening.

The ore shoots and orebodies with U/V ratios >3 are struc-turally modified and located at the north and south ends ofthe Kambalda dome. The ore shoots with lower U/V ratios,except the volcanic end-member Durkin shoot, are located on

the east and west flanks of the dome. This broad correspon-dence of elongation and position about the Kambalda domesuggests a structural control on the shapes of the orebodies atthe scale of the dome. This may also be indicated by the in-crease in thickness of the Silver Lake Member of the Kam-balda Komatiite at the north and south ends of the Kambaldadome (Figs. 1, 3). Such heterogeneous strain partitioning is afeature of structural domes (Dalstra et al., 1998). The excep-tional oblate shape of the Durkin shoot at the north end of theKambalda dome could reflect a strain shadow setting relativeto D1 thrusting.

Some of the shape characteristics of the ore shoots can beaccounted for by the stress-strain behavior of massive ore(>80% sulfides), matrix ore (40–80% sulfides), hanging-walltalc and carbonate-altered serpentinite, and footwallmetabasalt. Experimental curves for stress versus strain inores and host rocks of the Lunnon ore shoot (Fig. 12) indi-cate that the massive and matrix ore are significantly weakerthan the wall rocks. The ore had higher ductility and lowerstrength and deformed plastically by ductile flow at lowerstresses compared to the hanging-wall metakomatiite and

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TABLE 2. Shape Dimension Data and j Values for the Nickel Sulfide Ore Shoots at the Kambalda Dome

Main structural setting Continuum class U (m) V (m) W (m) U/V V/W J Shape Definition

Ore shootsLunnon D2 Structurally modified 2345 538 373 4.4 1.4 7.6 Prolate Closed

Durkin Other Volcanic end member 961 580 224 1.7 2.6 0.4 Oblate Open

Long D2 Structurally modified 1900 622 216 3.1 2.9 1.1 Prolate Open

Mean orebodiesKen D1 Structurally modified 713 223 146 3.2 1.5 4.9 Prolate Closed

s.d. 458 49 13 3.3n 3

McMahon D2 Structural end member 324 158 85 2.0 1.9 1.2 Prolate Closed

Hunt D3 Structurally modified 845 254 131 3.3 1.9 2.5 Prolate Closeds.d. 520 102 44 1.5n 3

Otter-Juan D3 Structurally modified 703 264 167 2.7 1.6 3.5 Prolate Opens.d. 340 63 54 2.4n 8

Fisher D4 Structurally modified 493 226 170 2.2 1.3 14.8 Prolate Closeds.d 93 95 56 21.8n 6

Gellatly-Wroth D1 Structurally modified 1132 203 154 5.6 1.3 21.6 Prolate Open

Victor Other Volcanic end member 469 148 134 3.2 1.1 49.7 Prolate Open

Coronet Other Volcanic end member 415 164 60 2.5 2.7 0.8 Oblate Open

Key orebodiesKen lower trough D1 Structurally modified 1241 259 160 4.8 1.6 6.2 Prolate Closed

Hunt D zone deeps D3 Structurally modified 1433 352 156 4.1 2.3 1.8 Prolate Closed

Juan Deeps west D3 Structurally modified 1472 329 200 4.5 1.6 5.4 Prolate Open

Notes: n = number of measurements, s.d. = standard deviation

particularly the footwall metabasalt. In contrast, the footwallmetabasalt deformed by brittle fracture. The behaviour ofthe metakomatiite was intermediate between the ore andmetabasalt, deforming initially by brittle fracture and thenplastically by ductile flow, although at higher stresses thanthe ore. Field observations confirm that ductile deformationof the sulfides occurred at conditions under which themetabasalt was brittle (Stone and Archibald, 2004) and sug-gest that the prolate shape of the ore shoots was due to duc-tile flow of sulfide. Ductile flow of the metakomatiite duringD1 to D2 deformation could provide an explanation for thethickened hanging wall at the ore shoots (cf. Gresham andLoftus-Hills, 1981). Remobilization of matrix sulfides alsocould account for the formation of massive sulfide oreshoots.

The timing and extent of the sulfide remobilization remainsdifficult to quantify. Offsets of syn-D1 dikes along the top ofthe footwall metabasalt at the Otter-Juan ore shoots suggestmovement on the order of tens to hundreds of meters (Hay-den, 1976; Cowden, 1986). This is consistent with the pres-ence of inclusions of footwall metabasalt, quartz-carbonateveins and intermediate dikes in ore and in the hanging wall,and the lack of expression of some high-angle faults in thehanging wall (Stone and Archibald, 2004). Remobilization at

this scale also could explain the three-dimensional prolateshapes of the orebodies and some of the ore shoots.

Summary and Significance in ExplorationFollowing magmatic emplacement, the nickel ore shoots

surrounding the Kambalda dome were structurally deformed,and the hanging-wall rocks were altered. Strain localizationwithin and about mechanically weak sulfide ore and metako-matiite resulted in sulfide remobilization and structural mod-ification of the original ore-hosting features. The dominanceof structurally modified ore shoots at Kambalda means thatdeformation controls, in addition to volcano-stratigraphiccontrols, must be considered in exploration. The structuralcontinuum model also presents new exploration targets.Three-dimensional analysis indicates the possibility of dis-placement, transposition, and offset of the ore shoots alongtrough structures and thereby helps to identify new explo-ration targets. The recognition of the structurally controlledMcMahon shoot suggests that other deposits of this typemight be present at Kambalda.

The geometry of the trough structures reflects the regionaltectonostratigraphic and metamorphic setting. Althoughreentrant and elongate at the Kambalda dome, trough struc-tures elsewhere in the Kambalda nickel field (Stone et al.,2004) are broader, shallower features that are wider at the topthan at the bottom (Stone and Archibald, 2004). In othercomplex fold-thrust belts, cuspate-lobate fold patterns at thecontacts of metakomatiite units (e.g., Tramways belt, WesternAustralia; Stone and Archibald, 2004) and trough structures

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FIG. 11. Shape plot for the nickel sulfide ore shoots at the Kambaldadome (adapted from Blenkinsop, 2004). The diagonal line with the positiveslope (i.e., j = 1) separates the fields of prolate and oblate shapes. The fieldslabeled D1, D2, and D3-D4 are defined by strain indicators for the regionaldeformation events, based on axial ratios of deformed sulfide blebs in ser-pentinite, pillows in the Lunnon Basalt, and clasts in conglomerate(Archibald, 1985). The fields represent change from constrictional strain dur-ing D1 to flattening during D2, and to more well-defined flattening during D3

and D4. Ore shoots (solid squares); D = Durkin, Lg = Long, Ln = Lunnon.Mean orebodies (solid circles): C = Coronet, F = Fisher, GW = Gellatly-Wroth, H = Hunt, K = Ken, M = McMahon, OJ = Otter-Juan, V = Victor. In-dividual orebodies (solid triangles): HDZD = Hunt D zone deeps, JWD =Juan west deeps, and KLT = Ken lower trough.

FIG. 12. Stress vs. strain curves for experimentally deformed massivenickel sulfide ore (>80% sulfides), matrix nickel sulfide ore (40–80% sul-fides), footwall metabasalt, and (hanging-wall) talc and carbonate-altered ser-pentinite (metakomatiite) at 500oC and for massive nickel sulfide ore andpyrrhotite at 400oC, at 3-kbars confining pressure and 10–5 sec–1 strain rate(adapted from Cowden, 1986, and McQueen, 1987). All materials from theLunnon shoot, Kambalda.

in hanging-wall units and thrust wedges (e.g., theWidgiemooltha dome, Western Australia) may reflect thepresence of mineralized troughs on a buried footwall contact.

Many Kambalda-style nickel deposits occur in structuralanticlines and domes. In addition to the Kambalda dome, theWidgiemooltha and Kanowna-Scotia domes of Western Aus-tralia host significant nickel sulfide deposits (Barrett et al.,1977; McQueen, 1981; Stolz and Nesbitt, 1981; Hicks andBalfe, 1998). Anticlinal structures to the south of the Kam-balda dome at Foster-Jan (St Ives) and Tramways belt alsohost large deposits (Cowden and Roberts, 1990; Stone andArchibald, 2004). Elsewhere, the Shaw, Bartlett, Halliday,and La Motte domes in the Abitibi greenstone belt, Ontarioand Quebec, host significant nickel sulfide deposits (Naldrettand Cabri, 1976; Green and Naldrett, 1981). Thus the impor-tant structural controls on nickel sulfide orebodies in theKambalda dome may have general applicability to other set-tings that host Kambalda-type mineralization.

AcknowledgmentsThis paper is a result of collaborative research of WMC Re-

sources Ltd. Kambalda Nickel Operations (KNO) and FractalGraphics Pty. Ltd. This research would not have been possiblewithout the generous support of Chris Banasik, Jim Reeve, JonRutter, Peter Bewick, and Rob Behets and Peter Bewick ofKNO, and access to the internal reports made by numerousgeologists over 35 years at Kambalda. Valuable contributionswere also made by Peter Ketelaar, Darren Holden, and SteveNichols of Fractal Graphics Pty. Ltd. The paper has benefitedfrom thorough and insightful reviews by Kevin Frost and Mar-tin Prendergast, lengthy discussions with Steve Cox and SteveBarnes, and editorial comments from Kevin Cassidy and espe-cially Mark Hannington. We are grateful to GeoinformaticsExplorations Ltd. and Nevada Star Resource Corp. for sup-porting the final stages of this research.

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STRUCTURE AND SHAPE OF Ni SULFIDE SHOOTS, KAMBALDA, WA 1455

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