An AMF-PGC International Study Tour
Developed & Managed by Porter GeoConsultancy
OzGold '97
Australian Gold Deposits
26 November to 4 December 1997 - In Two Separate Modules
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Bronzewing pits, Yilgarn, WA, Australia
Image: Bronzewing open pits, WA.   
DEPOSIT DESCRIPTIONS - MODULE 1

This module of the tour and the selected gold deposits included on the itinerary comprised:

Module 1 - Archaean,
Wed 26 to Sat 29 November 1997


Perth Seminar

The tour commenced with a seminar in Perth, with presentations on:
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  • "The Geological and Tectonic Framework of Western Australia, with Particular Emphasis on the Archaean Cratons" by Dr John Myers, Chief Geoscientist of the Geological Survey of WA.
  • "The Distribution, Styles and Controls of the Archaean Gold Deposits of Western Australia" by Professor David Groves of the Key Centre for Strategic Mineral Deposits, University of WA:
  • "The Distribution and Behaviour of Gold in the Weathering Profile" Dr Charles Butt, Program Leader, CRC-LEME, CSIRO Exploration & Mining, Perth.

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Boddington

The principal gold mineralisation originally exploited was present as very fine, generally <5 µm, gold grains spread through a 35 m thick lateritic weathering profile.  The deposit is developed within and over rocks of the Archaean Saddleback greenstone belt in the Western Gneiss Terrane of the Yilgarn Craton, and falls within the Darling Ranges bauxite province to the east of Perth where the same profile is mined on a large scale for bauxite.  Some 30% of the gold forms a semi-continuous blanket within the same vertical zones as the bauxite, namely the upper lateritic gravels, the hardcap laterite and the B-zone laterite.  The remaining 70% is found in the underlying clay and saprolite zones.  In addition to the lateritic ore, there is also a variety of other ore types present in the greenstone belt bedrock, including rich lode deposits and large low grade bulk mineable disseminated ores.  In these the gold is associated with copper and molybdenum in the ratio of 1:10:1000 respectively.  The original laterite resource was 60 Mt @ 1.6 g/t Au in 1987, covering an area of 4.5 x 1 km.  The current resource is 253 Mt @ 1.05 g/t Au, including more than 130 Mt as low grade primary leach ore in the bedrock.  Production in 1996 totalled 8.66 Mt @ 1.26 g/t Au, from both open cut and underground operations.

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Kalgoorlie Super Pit

The KCGM Super Pit exploits the Golden Mile string of deposits which have yielded around 1200 t Au to the present.  To 1987 this had come from 103.7 Mt of ore averaging 11.4 g/t Au.  The December 1997 measured/indicated/inferred resource was 262 Mt @ 2.5 g/t Au for 656 t of contained Au.  Production in 1995 was 8.42 Mt @ 2.52 g/t Au, for 18.7 t Au.  The Kalgoorlie gold field is developed within the Archaean Norseman-Wiluna greenstone belt of the Yilgarn Craton.  The ore is controlled by a complex series of shears and is hosted within the Golden Mile Dolerite and to a lesser extent the Paringa Basalt.  The ores are mineralogically refractory and complex, containing free gold and a significant proportion of gold- and silver-bearing telluride minerals, often intimately associated with pyrite.  The mineralisation is localised within lodes that occur as alteration zones of ankerite+quartz+sericite+pyrite ±telluride.  The lodes are subdivided into the Eastern and Western Lode System on the flanks of the Kalgoorlie Syncline.  Individual lodes occupy 20 to 50% of a lode channel and may be 30 to 1800 m long, 0.1 to 10 m thick and extend 30 to 1160 metres down dip.

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Kanowna Belle

Kanowna Bell is 18 km from Kalgoorlie, within the Archaean Norseman-Wiluna greenstone belt.  It lies within an Archaean volcano-sedimentary sequence which overlies the anticlinal Scotia-Kanowna granitic body.  This sequence has a lower mafic-ultramafic sequence and an upper succession of felsic volcanics, polymictic conglomerate, felsic conglomerate and coarse felsic grits and arenites, with common porphyry intrusives.  The principal hosts are sediments within the felsic sequence and the Kanowna Belle feldspar porphyry.  The alteration and mineralisation is predominantly associated with the latter.  The highest grade ore is associated with brecciation, silicification, pyritisation and crackle veining within the inner two alteration zones which comprise quartz-albite-pyrite and a more extensive sericite-ankerite-dolomite-pyrite-fuchsite.  These are progressively flanked by chlorite-calcite and the outermost hematite zone.  The ore occurs as a series of plunging shoots within the Fitzroy Fault, a major ENE trending reverse shear system, and is associated with splays from this structure.  The deposit originally contained around 124 t Au.  In 1997 measured/indicated/inferred resources totalled 25.8 Mt @ 5 g/t Au.  Production in 1994/95 totalled 1.3 Mt @ 3.97 g/t Au.

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St Ives Complex

The St Ives Complex of orebodies is also within the Archaean Norseman-Wiluna greenstone belt, and is 75 km SSE of Kalgoorlie.  The host sequence reflects a progression from tholeiitic basalts to komatiites and komatiitic basalts, to high Mg basalts, which are overlain by clastic sediments, felsic volcanic-volcaniclastic sequences.  Gold mineralisation is best developed in the mafic dominated part of the Archaean sequence, above the Kambalda Komatiite which hosts the nearby major nickel orebodies of the Kambalda complex of mines.  Hosts range from basalts to dolerites to slates.  Mineralisation in all of the gold complex deposits is contained within minor structures which include quartz-vein arrays, breccia zones and central, quartz rich and mylonitic parts of shear zones.  These structures are associated with post-peak metamorphic faults and thrusts which post date felsic dykes and are splays from the nearby regionally extensive  Boulder-Lefroy Fault.  The alteration zonation has a core development of albite-ankerite/dolomite-quartz-pyrite, with gold being associated with the pyrite; this is enclosed by a biotite zone containing albite-biotite-quartz-calcite/dolomite-magnetite-muscovite; passing in turn into a chloritic zone with an assemblage of chlorite-albite-quartz-calcite-magnetite±biotite/muscovite/epidote, and then into the regional greenschist metamorphic assemblage.  This zonation is not always present, or fully developed.  These deposits were known to contain in excess of 93 t Au at the start of mining.  The measured/indicated/inferred resource in 1997 was 59.9 Mt @ 3.54 g/t Au, while the total reserves were 31.3 Mt @ 4.0 g/t Au.  Production in 1995 was 2.9 Mt @ 3.97 g/t Au, for 11.6 t Au.

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Mt Hunt Traverse

The Norseman-Wiluna Greenstone Belt extends from Norseman in the south, through the St Ives-Kambalda district and Kalgoorlie, to Wiluna in the north.  It lies within the Eastern Goldfields Province of the Yilgarn Craton and is the most productive greenstone belt in Western Australia.  The greenstones within it are younger than those in the other provinces of the craton, with ages of between 2730 and 2650 Ma.  The belt contains tholeiitic and komatiitic volcanics and intrusives, volcaniclastic sediments and epiclastic sediments, and is also characterised by the absence of banded iron formation, with the interflow sediments being carbonaceous, sulphidic shales and cherts.

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Bronzewing

Bronzewing is located within the Yandal belt of deposits, 400 km north of Kalgoorlie.  This belt, which lies on the eastern margin of the Archaean Norseman-Wiluna greenstone belt, contains five significant gold deposits, all found within the 1990's.  The sequence in the belt varies along strike, containing interlayered tholeiitic and high Mg basalts, Fe rich shales, ultramafic rocks and internal granites, with felsic rocks to the south and ironstones to the north.  The structural trend in the mine area is dominated by NNW shearing, with NE-SW & E-W conjugate structures.  The main structure, which comprises anastomosing highly strained rocks enclosing lenses of lower strain material, is 100 to 500 m wide at Bronzewing.  Alteration associated with gold mineralisation includes calcite and ankerite in mafic rocks, and talc with dolomite-magnesite in ultramafic lithologies.  Gold is hosted in quartz veins and altered basalt wallrock, with the better grade in the former.  Mineralised veins include 1 to 2 m thick laminated quartz veins and stockworks.  Pyrite is the predominant sulphide.  There are three main orebodies.  The original resource at Bronzewing totalled 20 Mt @ 4.7 g/t Au, for 95 t Au.  In 1997 the measured/indicated/inferred resource was 38.3 @ 2.9 g/t Au, while the reserves totalled 12.05 Mt @ 4.6 g/t Au.

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For more information contact:   T M (Mike) Porter, of Porter GeoConsultancy   (mike.porter@portergeo.com.au)

This tour was designed, developed, organised, managed and escorted by
T M (Mike) Porter of Porter GeoConsultancy Pty Ltd.

Porter GeoConsultancy Pty Ltd
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LINDEN PARK, 5065
South Australia
Telephone: +61 8 8379 7397
Mobile: +61 422 791 776


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