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PERTHYork
Agnew
Hyden
Wiluna
Albany
Menzies
Leonora
Northam
Laverton
Merredin
Esperance
Pemberton
Katanning
Busselton
Perenjori
Geraldton
Murchison
Peak Hill
Paynes Find
Northampton
Meekatharra
Mount Magnet
Gascoyne Junction
Bunbury
Norseman
Leinster
Sandstone
Coolgardie
Ravensthorpe
WidgiemoolthaSouthern Cross
Kalgoorlie-Boulder
WESTERN AUSTRALIA126°
123°
123°
120°
120°
117°
117°
114°
114°
111°
24°
26°
26°
28°
28°
30°
30°
32°
32°
34°
34°
36°
INDIAN
OCEAN
Honeymoon Well
Mount KeithYakabindie
Perseverance
PERTHYork
Agnew
Hyden
Wiluna
Albany
Menzies
Leonora
Northam
Laverton
Merredin
Esperance
Pemberton
Katanning
Busselton
Perenjori
Geraldton
Murchison
Peak Hill
Paynes Find
Northampton
Meekatharra
Mount Magnet
Gascoyne Junction
Bunbury
Norseman
Leinster
Sandstone
Coolgardie
Ravensthorpe
WidgiemoolthaSouthern Cross
Kalgoorlie-Boulder
WESTERN AUSTRALIA126°
123°
123°
120°
120°
117°
117°
114°
114°
111°
24°
26°
26°
28°
28°
30°
30°
32°
32°
34°
34°
36°
INDIAN
OCEAN
Honeymoon Well
Mount Keith
YakabindiePerseverance
Kambalda
PERTH
Cue
York
Agnew
Hyden
Moora
Wiluna
Albany
Collie
Gingin
Newman
Denham
Carbine
Cascade
Naretha
Coonana
Menzies
MalcolmLeonora
Denmark
Augusta
Bunbury
Dowerin
Northam
Dongara
Mullewa
Norseman
Kambalda
Laverton
Leinster
Corrigin
Pinjarra
Westonia
BodallinMerredin
Kalbarri
Condingup
EsperanceMunglinup
Pemberton
Lake KingNewdegate
Katanning
Busselton
BullfinchBencubbin
Perenjori
Geraldton
Sandstone
Murchison
Peak Hill
Tom PriceLearmonth
Carnavon
Coolgardie
Bremer Bay
Boddington
Yellowdine
Dalwallinu
Paraburdoo
Monkey Mia
Marvel Loch
Broad Arrow
Paynes Find
Northampton
Meekatharra
Ravensthorpe
Mount Barker
Moorine Rock
Mount Magnet
Widgiemooltha
Koolyanobbing
Three Springs
Margaret River
Southern Cross
Jigalong Mission
Minilya Roadhouse
Gascoyne Junction
Kalgoorlie-Boulder
Cosmo NewberyMission
Mount Margaret Mission
Cundeelee Aboriginal Community
WESTERN AUSTRALIATrainor 1
Lancer 1
>1624.6Empress 1A
>709
>1501
3838
81
44
76
109
491
144
>141 >989
>613
>114
>368
>517
>146
>363
>635
>614
>524
>100
31.5
>130 >203
>138>150
>168>238
>225>130
>102
412.5
>1770
>3803
>2040
>2000
>38.1
>74.8
>140.3>166.6
>390.8
>386.7
>42.97
>101.5
>85.34
>207.6
>248.5
>154.53
>265.48
>198.12
>205.75
>121.92
>4195
>140.9
TD4TD2 TD1
PC2 NRH1
NJD1
N4 1
N3 1
N1 1
OCCP 7
Brown 1
Lennis 1
Kanpa 1A
Hussar 1
7926 1/C
Yowalga 3
Jubilee 2Jubilee 1
Dragoon 1
Eyre No. 1
Minigwal 2A
Lungkarta 1
BMR Wanna 1
BMR Rason 3 BMR Rason 2
BMR Neale 3
BMR Neale 2
BMR Brown 1
92 THAC 001 90 RCLE 005
90 RCHE 00590 RCHE 00490 RCHE 001
BMR Warri 20
BMR Madley 1
BMR Yowalga 4
Gambanga No. 1
BMR Yowalga1-3
BMR Westwood 2
BMR Westwood 1
BMR Throssell 1
TD3
Yowalga 2Yowalga 1
BMR Rason 1
BMR Neale 1
90 RCHE 00389 RCWA 012
89 RCWA 011
89 RCWA 01089 RCWA 009
89 RCWA 008
Yowalga 3/12
INDIAN
OCEAN
Interpreted Distribution of Archean Mafic-Ultramafic RocksAUSTRALIAN ARCHEAN MAFIC-ULTRAMAFIC MAGMATIC EVENTS: YILGARN CRATON, WESTERN AUSTRALIA
Y I L G A R N
C R A T O N
PERTH
Cue
York
Agnew
Hyden
Moora
Wiluna
Albany
Collie
Gingin
Denham
Carbine
Cascade
Naretha
Coonana
Menzies
MalcolmLeonora
Denmark
Augusta
Bunbury
Dowerin
Northam
Dongara
Mullewa
Norseman
Kambalda
Laverton
Leinster
Corrigin
Pinjarra
Westonia
BodallinMerredin
Kalbarri
Condingup
EsperanceMunglinup
Pemberton
Lake KingNewdegate
Katanning
Busselton
BullfinchBencubbin
Perenjori
Geraldton
Sandstone
Murchison
Peak HillCarnavon
Coolgardie
Bremer Bay
Boddington
Yellowdine
Dalwallinu
Monkey Mia
Marvel Loch
Broad Arrow
Paynes Find
Northampton
Meekatharra
Ravensthorpe
Mount Barker
Moorine Rock
Mount Magnet
Widgiemooltha
Koolyanobbing
Three Springs
Margaret River
Southern Cross
Minilya Roadhouse
Gascoyne Junction
Kalgoorlie-Boulder
Cosmo NewberyMission
Mount Margaret Mission
Cundeelee Aboriginal Community
Komatiitic rocks in
the Sandstone region
Cooke
Nepean
Siberia
Redross
Mount KeithKingston
Yakabindie
Flying Fox
Ghost Rocks
Murrin Murrin
Marshall Pool
Marriott-Mount Clifford
LongGibbVictor
Scotia
McLeayMcEwen
Rat Bat
Widgie 3
Wannaway
Ringlock
Kambalda
Emu Lake
Mount Edwards
Cosmic Boy
Black Swan
Mount Windarra
Maggie Hays
Binti Binti
PerseveranceMiranda Well
Digger Rocks
North Ironcap
Mount Edwards 26N
Honeymoon Well
Widgie Townsite
Mount Edwards 132N
Antimony Nickel
WESTERN AUSTRALIA
?
??
??
?? ?
?
?
Sheet 2 of 2Interpreted Characterisation of Komatiites
INDIAN
OCEAN
Interpreted Characterisation of Komatiites (right)The geochronology data (most ages are summarised in Hoatson et al., 2006) of the Archean komatiitic sequencesshown on this map include: U-Pb zircon ages of pyroclastic and felsic units intercalated with komatiitic sequences(i.e., direct age of komatiites); felsic footwall units (maximum age); felsic hangingwall units (minimum age); and felsicdykes cutting the komatiitic sequences (minimum age). Most of the ages in the west Yilgarn Craton prefixed by a ?indicate that the stratigraphic relationship between the dated rock and associated komatiitic rock has not beendetermined. Those ages shown with an * may have questionable significance based on recent geochronology andfield mapping by the Geological Survey of Western Australia
The komatiitic rocks of the Yilgarn Craton can be divided into two broad compositional groups (Barnes et al., 2004):(1) Al-undepleted komatiites (AUDK) or ‘Munro-type’ komatiites–Al 2O3/TiO2 ratios generally vary between 15 and 25,and they are typically depleted in incompatible trace elements; and
(2) Al-depleted komatiites (ADK) or ‘Barberton-type’ komatiites–Al2O3/TiO2 ratios are <15, and they are enriched inincompatible trace elements
The two chemical groups of komatiites are inferred to result from different melting conditions in the mantle, with agarnet residue indicated by the Al-depleted lavas. Large-tonnage nickel sulphide deposits (Mount Keith) aregenerally associated with ~2.70 Ga AUDK, whereas smaller high-grade deposits (Flying Fox) are often associatedwith older ~3.00-2.90 Ga ADK
Nickel Sulphide Resources(past production and remaining resources) Crustal Neodymium Model Ages
Layered mafic-ultramafic intrusions and sills
Thick occurrence, interpreted from magnetics
Thick occurrence outcrop
Thick occurrence outcrop
Thick occurrence, interpreted from magnetics
Layered mafic-ultramafic intrusion outcrop
Layered mafic-ultramafic intrusion orsill extent, interpreted from magnetics
Thin occurrence outcrop
Thin occurrence, interpreted from magnetics
Thin occurrence outcrop
Thin occurrence, interpreted from magnetics
Boundary of layered mafic-ultramafic intrusion
Magnetite layering, interpreted from magnetics
Ultramafic rocks
Banded iron formation
CliffsJericho
Anomaly 1
Mount Windarra
Black Swan
Maggie Hays
Digger Rocks -Diggers South
Kambalda
Cliffs
JerichoAnomaly 1
Mount Windarra
Black Swan
Maggie Hays
Digger Rocks -Diggers South
Dated komatiitic or associated rock andage in millions of years
Aluminium-depleted and undepleted komatiitic rock
Nickel sulphide deposit
Aluminium-undepleted komatiitic rock(AUDK: Al2O3 /TiO2 = 15-25)Aluminium-depleted komatiitic rock(ADK: Al2O3 /TiO2 <15)
126°00'124°30'123°00'121°30'120°00'118°30'117°00'115°30'114°00'23°00'
24°00'
25°00'
26°00'
27°00'
28°00'
29°00'
30°00'
31°00'
32°00'
33°00'
34°00'
25°00'
26°00'
27°00'
28°00'
29°00'
30°00'
31°00'
32°00'
33°00'
34°00'
35°00'
36°00'
124°30'123°00'121°30'120°00'118°30'117°00'115°30'114°00'112°30'111°00'
126°00'124°30'123°00'121°30'120°00'118°30'117°00'115°30'114°00'23°00'
24°00'
25°00'
26°00'
27°00'
28°00'
29°00'
30°00'
31°00'
32°00'
33°00'
34°00'
124°30'123°00'121°30'120°00'118°30'117°00'115°30'114°00'112°30'111°00'
25°00'
26°00'
27°00'
28°00'
29°00'
30°00'
31°00'
32°00'
33°00'
34°00'
35°00'
36°00'
LAMBERT CONFORMAL CONIC PROJECTIONCentral Meridian: 134oE Standard Parallels: 18oS, 36oS
Geocentric Datum of Australia
0 60 120 180 240 300 KilometresScale 1: 3 000 000
Size of nickel sulphide deposits (left)Mineral deposits with up to 10 000 tonnes of nickelMineral deposits with 10 000 to 100 000 tonnes of nickel
Mineral deposits with more than 1 million tonnes of nickel
Mineral deposits with 100 000 to 1 million tonnes of nickel
Neodymium two-stage model ages, in billion years (Ga) (right)
2.48-2.10 Ga
2.66-2.48 Ga
2.80-2.66 Ga
2.87-2.80 Ga
2.91-2.87 Ga
2.96-2.91 Ga
3.02-2.96 Ga
3.10-3.02 Ga
3.15-3.10 Ga
3.21-3.15 Ga3.27-3.21 Ga
3.35-3.27 Ga
3.67-3.35 Ga
Up to 0.50 million tonnes of nickel
0.50 to 1.00 million tonnes of nickel
1.00 to 12.00 million tonnes of nickel
Nickel sulphide resources (left)
LAMBERT CONFORMAL CONIC PROJECTIONCentral Meridian: 134oE Standard Parallels: 18oS, 36oS
Geocentric Datum of Australia
0 60 120 180 240 300 KilometresScale 1: 3 000 000
Interpreted dominant age (in billion years: Ga) andcompositional groups of komatiitic rocks
~2.72-2.69 Ga; AUDK + ADK (minor)
~?2.93-2.70 Ga
~3.02-2.73 Ga; ADK + AUDK (minor)
~2.87-2.70 Ga; AUDK + ADK (minor)
~?3.00-2.80 Ga
~?3.01-2.74 Ga
~3.00-2.90 Ga; ADK + AUDK (minor)
7
6
3
5
4
2
3
7
6
3
5
2
6
Archean granite orgranitic gneiss
Archean greenstone
Proterozoic rocksPermian rocks
Drillhole with depth (m) to basement
Bottom-hole rock type
Minigwal 2A412.5
Distribution and Characterisation Maps (left and right) Characterisation Map (right)
MAP LOCALITY
NORTHERNTERRITORY
YILGARNCRATON
QUEENSLAND
NEW SOUTH WALES
VICTORIA
TASMANIA
ACT
SOUTH AUSTRALIAGAWLERCRATON
PILBARACRATON
Yilgarn Craton boundary
Possible banded iron formation, interpreted from magnetics
Proterozoic rocks, undivided outcrop/subcrop
Archean greenstone sequences, undivided outcrop/subcrop
Possible banded iron formation or Archeanultramafic rock, interpreted from magnetics
Phanerozoic rocks, undivided outcrop/subcrop
Archean granite or granitic gneiss, undivided outcrop/subcrop
LAMBERT CONFORMAL CONIC PROJECTIONCentral Meridian: 134oE Standard Parallels: 18oS, 36oS
Geocentric Datum of Australia
Scale 1: 6 000 0000 50 100 150 200 250 Kilometres 0 50 100 150 200 250 Kilometres
LAMBERT CONFORMAL CONIC PROJECTIONCentral Meridian: 134oE Standard Parallels: 18oS, 36oS
Geocentric Datum of Australia
Scale 1: 6 000 000
No resource data
Kurnalpi Terrane
Southern Cross Domain
Kalgoorlie Terrane
Nickel sulphide resourcesin terranes and domain
Limit of isotope data
Limit o
f isoto
pe da
ta
Model-based 'average' age of the crust as indicated by granitic rocks (using a two-stage evolution of Sm/Nd andassuming a depleted mantle, Champion and Cassidy, 2008). Highest nickel sulphide endowment (KalgoorlieTerrane) is associated with intermediate age crust (neodymium two-stage model ages of 3.02 to 2.87 Ga)Model age surface (cell size 1 km) calculated using interpolating grid routine in ArcGIS. Interpolating method:Inverse Distance Weighted with a fixed search radius (number of neighbours is 12)Sample locations for dating are not uniformly distributed across the Yilgarn Craton
Crustal Neodymium Model Ages (right)
1
No age or compositional data
WESTERN AUSTRALIA
HAMERSLEYBASIN
2
2
1
1
1
7
7
4
4
4
Sample location
Compiled by D.M. Hoatson, S. Jaireth, A.J. Whitaker, D.C. Champion, andJ.C. Claoué-LongCartography by G.A. Young
Acknowledgements:The authors acknowledge the information, comments, and suggestions on this mapprovided by colleagues in Geoscience Australia, and by various State and NorthernTerritory geological surveys. In particular, Charlotte Hall, Martin Van Kranendonk,Paul Morris, Michael Wingate, Stephen Wyche (all Geological Survey of WesternAustralia: GSWA); Wayne Cowley, Martin Fairclough, Anthony Reid (PrimaryIndustries and Resources, South Australia: PIRSA); and Julie Hollis and IanScrimgeour (Northern Territory Geological Survey) are acknowledged for theirvaluable contributions. The two map sheets were reviewed by Michael Huleatt,Lynton Jaques, Yanis Miezitis, Alastair Stewart (all Geoscience Australia) and bycolleagues at GSWA and PIRSA
Other maps in this series:This map is part of a Geoscience Australia series showing the geographic extentand time-space relationships of Archean and Proterozoic mafic-ultramaficmagmatism and associated mineral deposits across the Australian continent
Part 1: Western Australia was published in October 2006 and documents 15Proterozoic mafic-ultramafic magmatic eventsPart 2: Northern Territory–South Australia was published in July 2007 anddocuments 19 Proterozoic mafic-ultramafic magmatic eventsThe Australian Proterozoic map which comprises two sheets, was published inAugust 2008 and documents 30 Proterozoic mafic-ultramafic magmatic eventsacross the continent
The geological and geochronological information accompanying the Proterozoicmap series is summarised in Geoscience Australia Record 2008/15 (GeoCatNumber: 66624) Guide to using the 1:5 000 000 map of Australian Proterozoicmafic-ultramafic magmatic events by Hoatson, Claoué-Long, and Jaireth. AGeoscience Australia Record summarising the Archean mafic-ultramafic magmaticevents is planned for release in 2009. Two map sheets by Claoué-Long andHoatson (GeoCat Number: 69213) showing the distribution of Proterozoic LargeIgneous Provinces in Australia are also to be released in 2009Copies of the maps (in pdf and jpg formats) and Geoscience Australia Record2008/15 are available free online in a resource package at:http://www.ga.gov.au/map/index.jsp#mum
References for digital geological and geophysical datasets:● Geological base maps and solid-geology rock polygons (Sheet 1)Western AustraliaDistribution of Precambrian mafic and ultramafic rocks in Western Australia:1:500 000 Interpreted Bedrock Geology Map of Western Australia, GeologicalSurvey of Western Australia (2008)
South AustraliaCowley, W.M. (Compiler), 2006. Solid geology of South Australia. Department ofPrimary Industries and Resources, South Australia. Mineral Exploration DataPackage 15 (version 1.1.)
● Geological and tectonic province boundaries (Sheets 1 and 2)Yilgarn CratonCassidy, K.F., Champion, D.C., Krapez, B., Barley, M.E., Brown, S.J.A., Blewett, R.S.,Groenewald, P.B. and Tyler, I.M., 2006. A revised geological framework for the YilgarnCraton, Western Australia. Geological Survey of Western Australia, Record 2006/8, 8 pp
Pilbara Craton1:2 500 000 Tectonic Units of Western Australia, June 2001, Geological Survey ofWestern Australia
Gawler CratonFairclough, M.C., Schwarz, M.P. and Ferris, G.M., 2003. Interpreted crystallinebasement geology of the Gawler Craton, South Australia, Geological Survey, Specialmap, 1:1 000 000 scale
● Distribution of Archean dolerite dykes and sills in Western Australia (Sheet 1)Thorne, A.M. and Trendall, A.F., 2001. Geology of the Fortescue Group, Pilbara Craton,Western Australia. Geological Survey of Western Australia, Bulletin 144, 249 pp
● Geological and geophysical datasets (Sheets 1 and 2)Interpreted distribution of Archean mafic and ultramafic rocksWhitaker, A.J. and Bastrakova, I.V., 2002. Yilgarn Craton aeromagnetic interpretation(1:1 500 000 scale map), Geoscience Australia
Geochronology of Archean mafic and ultramafic rocksHoatson, D.M., Jaireth, S. and Jaques, A.L., 2006. Nickel sulphide deposits in Australia:Characteristics, resources, and potential. Ore Geology Reviews, 29, 177–241Van Kranendonk, M.J. and Ivanic, T.J., 2009. A new lithostratigraphic scheme for thenortheastern Murchison Domain, Yilgarn Craton. Geological Survey of Western AustraliaAnnual Review 2007–08, 35–53Van Kranendonk, M.J., Hickman, A.H., Smithies, R.H., Williams, I.R., Bagas, L. andFarrell, T.R., 2006. Revised lithostratigraphy of Archean supracrustal and intrusive rocksin the northern Pilbara Craton, Western Australia. Geological Survey of WesternAustralia, Record 2006/15, 57 ppWilde, S.A., 2001. Jimperding and Chittering Metamorphic Belts, southwestern YilgarnCraton, Western Australia–a field guide. Geological Survey of Western Australia, Record2001/12, 24 pp
Composition of komatiitic rocksBarley, M.E., Blewett, R.S., Cassidy, K.F., Champion, D.C., Czarnota, K., Doyle, M.G.,Krapez, B., Kositcin, N., Pickard, A.L. and Weinberg, R.F., 2006. Tectonostratigraphicand structural architecture of the eastern Yilgarn Craton. Final AMIRA ReportP763/pmd*CRC Project Y1, December 2006Barnes, S.J., Hill, R.E.T., Perring, C.S. and Dowling, S.E., 2004. Lithogeochemicalexploration for komatiite-associated Ni-sulfide deposits: strategies and limitations.Mineralogy and Petrology, 82, 259–293Lesher, C.M. and Keays, R.R., 2002. Komatiite-associated Ni-Cu-PGE deposits:geology, mineralogy, geochemistry, and genesis. In: Cabri, L.J. (editor), The Geology,Geochemistry, Mineralogy and Mineral Beneficiation of Platinum-Group Elements.Canadian Institute of Mining, Metallurgy and Petroleum, Special Volume 54, 579–617
Nickel sulphide resources for deposits and regional nickel endowmentOZMIN–Geoscience Australia’s national database of mineral deposits and resources
Neodymium model agesChampion, D.C. and Cassidy, K.F., 2008. Geodynamics: Using geochemistry andisotopic signatures of granites to aid mineral systems studies: an example from theYilgarn Craton. In: Korsch, R.J. and Barnicoat, A.C. (editors), New Perspectives:The Foundations and Future of Australian Exploration. Abstracts for the 11–12thJune 2008 pmd*CRC Conference. Geoscience Australia, Record 2008/09, 7–16
For more information:Dean Hoatson: phone (02) 6249 9593; email [email protected] Claoué-Long: phone (02) 6249 9418; email [email protected]
© Commonwealth of Australia, 2009
This work is copyright. Apart from any fair dealings for the purposes of study,research, criticism or review, as permitted under the Copyright Act, no part may bereproduced by any process without written permission. Inquiries should be directedto the Communications Unit, Geoscience Australia, GPO Box 378, Canberra, ACT,2601, Australia
Geoscience Australia has tried to make the information in this product as accurateas possible. However, it does not guarantee that the information is totally accurateor complete. THEREFORE YOU SHOULD NOT RELY SOLELY ON THISINFORMATION WHEN MAKING A COMMERCIAL DECISION
Published by Geoscience Australia, Department of Resources, Energy andTourism, Canberra, Australia. Issued under the authority of the Minister forResources, Energy and Tourism
Copies of this map can be obtained from:xxxxxSales Centre, Geoscience AustraliaxxxxxGPO Box 378, Canberra, ACT, 2601, AustraliaxxxxxPh (02) 6249 9966, Fax (02) 6249 9960xxxxxEmail: [email protected]
It is recommended that this map sheet be referred to as:Hoatson, D.M., Jaireth, S., Whitaker, A.J., Champion, D.C. and Claoué-Long, J.C.,2009. Australian Archean Mafic-Ultramafic Magmatic Events: Yilgarn Craton,Western Australia, Sheet 2 of 2 (1:3 000 000 and 1:6 000 000 scale maps),Geoscience Australia, Canberra
GeoCat Number: 69347 ISBN: 978-1-921672-09-5
AUSTRALIANARCHEAN MAFIC-ULTRAMAFIC
MAGMATIC EVENTS:Sheet 2 of 2
2009
YILGARN CRATON, WESTERN AUSTRALIA
Interpreted
boundary
of
Yilgarn
Craton
Interpreted
boundary
of
Yilgarn
Craton
Interpreted
boundary
of
Yilgarn
Craton
Archean greenstone sequences, undivided and under basin cover
Interpreted
boundary
of
Yilgarn
Craton
This map depicts outcropping Archean ultramafic-mafic igneous rocks, layered mafic-ultramafic intrusions, andinterpreted subsurface extensions of equivalent rocks under younger cover throughout the Yilgarn Craton. Outcroplocations, rock types and regional groupings, e.g., distribution of greenstone belts, were taken from the 1:250 000Geological Map Series (compiled by the Geological Survey of Western Australia and Geoscience Australia). Thedistribution of subsurface equivalent rocks within the craton was largely derived from an interpretation ofaeromagnetic data contained in the National Airborne Geophysical Database (Geoscience Australia; as at 2007).Stratigraphic continuity was used to assign rock type information from exposed outcrop to adjacent aeromagneticanomalies over concealed areas. Some anomalies remain unassignedThe Yilgarn Craton has an area of at least 660 000 km2. Granite and greenstone rock sequences throughout thecraton mostly range in age from ~3.0 Ga to ~2.6 Ga, although some mafic igneous rocks as old as ~3.7 Ga occur inthe westGreenstone belts, which are composed largely of mafic volcanic rocks with lesser felsic volcanic, sedimentary, andultramafic rocks, make up approximately 20% of the craton. Granite and granitic gneiss (shown in pale pink)constitute the other 80%. Ultramafic rocks (purple), which host significant nickel mineralisation, occur withinregionally extensive north-northwest-trending linear greenstone belts (pale green), and are also depicted on the mapStrata-coincident aeromagnetic anomalies of at least moderate magnetisation generally correlate with banded ironformation (BIF: blue), or olivine-rich ultramafic rocks (komatiite; serpentinisation alteration produces magnetite as aco-product: purple), or magnetite-rich layers (black) in layered mafic and mafic-ultramafic intrusions (dark green).Units with extreme magnetisation (+ 1500 nT above average Yilgarn rock magnetisation) are almost exclusivelyassociated with BIF. Generally, however, the amplitude of magnetic anomalies, from very high to moderately lowlevels, is not unique to any of the three rock associations, and reflects, amongst other things, magnetite content,susceptibility, and remanence, in combination with unit thickness, attitude (dip), and depth extent. Thus thedistribution of outcropping and interpreted subsurface BIF has also been included both for magnetic interpretationcontext and to allow for possibly incorrect rock type assignment in areas of coverThis interpretation does not address all subsurface ultramafic rocks. Ultramafic rock types such as pyroxenites andhigh-magnesium basalts are generally poorly magnetised and are not readily mapped by aeromagnetic dataThe Yilgarn Craton, as portrayed by Whitaker and Bastrakova (2002), extends under laterally extensivePaleoproterozoic (brown) basin cover in the north and Phanerozoic (beige) cover in the east. The subsurfaceboundaries of the craton in these areas were located by gradients in aeromagnetic and gravity data which truncatethe widespread internal north to north-northwest lithological/anomaly trends. Across these boundaries the crust isinferred to be different in composition and to have undergone different deformational histories. The interpreted extentof subsurface greenstone belts underneath basin cover (indicated by paler green with stipple) was also defined usinga combination of aeromagnetic and gravity dataGreenstone belts under the basin cover may also be prospective for komatiite-related nickel deposits as inferred fromthe presence of ultramafic rocks in regionally adjacent sub-cropping belts. The thicker cover precludes geophysicalmapping of all but extremely magnetised BIF units in these belts. Selected drill hole data have been added in thearea of cover to provide information on depths to basement
Interpreted Distribution of Archean Mafic-Ultramafic Rocks (left)
Spotted Quoll Spotted Quoll
Flying Fox Flying Fox