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Wallaby, Just In Case
Western Australia, WA, Australia
Main commodities: Au

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The Wallaby deposit and its continuation to the north, Just in Case, are located on the western margin of the Laverton Tectonic Zone, some 11 km WSW of Granny Smith and 30 km SW of Laverton in the Yilgarn Craton of Western Australia.

The Wallaby deposit is located on the edge of the ephemeral Lake Carey salt flat and is covered by 25 to 125 m of alluvial and lacustrine sediments, including running sands and saturated clays which severely hampered drilling. Gold was known at the historic Just in Case mine, 3.5 km to the north. The deposit was detected by air core drilling of a large concealed ovoid magnetic anomaly in 1977 and intersecting anomalous gold. The magnetic anomaly was targeted because it was believed to be similar to the expression of the Jupiter syenite hosted gold deposit previously discovered a few km to the west. Other explorers had previously attempted to test his magnetic feature since the early 1990s, then known as the Neptune Anomaly, but had been unable to penetrate the cover sequence. The successful drilling was conducted by Plutonic Resources, later to be acquired by Homestake Gold of Australia, but was in the northern extension of the main deposit, immediately to the north of their tenement boundary with that of the Granny Smith Exploration Joint Venture (GSEJV) which included what was to become the main deposit. The release of Plutonic data to the ASX by their joint venture partner alerted the GSEJV who then commenced drilling to outline the Wallaby deposit. Resource drilling was completed by December 1999 with a pre-mining reserve and resource of 60 and 220 t of gold respectively. Open pit production commenced in 2001 and was completed in 2006. Underground mining began in December 2005 and, to 2018, 15.6 Mt @ 5.71 g/t Au for 89 t of gold has been produced.

The Wallaby gold deposit is hosted by a 1500 m thick matrix supported mafic conglomerate, the Wallaby Conglomerate, which on the basis of rare graded beds appears to dip moderately to the SE. Clasts are composed of basalt, komatiite, dacite, andesite, felsic porphyry and banded iron formation. The conglomerate is interpreted to have been deposited as a result of uplift of the Mount Margaret metamorphic core complex to the NW (see the regional setting in the Granny Smith record).

The Wallaby Copnglomerate is intruded by a highly fractionated, 2664±3 Ma alkaline dyke suite, displaying increasing fractionation through mafic-monzonite, monzonite, syenite, syenite porphyry to late-stage carbonatite. Syenite is dominant. These dykes strike at 90° and dip at 50°S at surface, shallowing to 45° at depth. This intrusive persists to depths of at least 2 km and is interpreted to have been intruded along the intersection of the north-south Chatterbox Fault and another NE-SW structure. Early biotite lamprophyres predate the intrusive complex, whilst late, steep, fine grained lamprophyres with chilled margins cut both the intrusive complex and gold mineralisation (Tully et al., 2017).

The Wallaby Conglomerate is limited to the north by the south-dipping, 250 m wide, ductile, Thet's Shear Zone which separates it from underlying meta-volcanic and meta-sedimentary rocks. Although this shear predates the gold mineralisation, it is apparently not mineralised. A generally parallel, 1 to 10 m thick zone of brittle fracturing and faulting, the Wedge Fault, marks the northern margin of the intrusive suite. These two structures are interpreted to represent syn-sedimentary normal growth faults that were subsequently inverted (Tully et al., 2017).

Wallaby is developed close to the hinge of the NW-SE elongated Mount Margaret Dome, and along the southern projection of the steeply dipping, north-south Chatterbox Fault.

The Wallaby Conglomerate has been metamorphosed to greenschist facies which has produced a pervasive chlorite-calcite assemblage. Within the main deposit area, the conglomerate and this metamorphic assemblage is overprinted by a pipe-shaped 800 x 600 m zone of actinolite-magnetite-epidote-calcite (AMEC) alteration that plunges at ~50° towards an azimuth of 180°. This alteration suite parallels the plunge of the intrusive suite and has been observed to overprint the more mafic intrusive dykes, However, sulphur isotopes suggest the alteration predates the syenite ans syenite porphyry dykes. This pipe of brittle, iron-enriched alteration is interpreted to have both increased fracture permeability and geochemical conditions favourable to the precipitation of gold (Tully et al., 2017).

Gold mineralisation is associated with dolomite-albite-quartz-pyrite alteration, and is hosted in a series of sub-horizontal, structurally controlled zones that are largely confined within the magnetite-rich pipe. Monzogranite and carbonatite dykes of the Wallaby syenite intruded at 2664±3 Ma, at least 5 m.y. and probably 14 m.y. before gold mineralisation at 2650±6 Ma, implying the gold mineralisation and causitive fluids were not derived from magmas associated with the Wallaby syenite (Salier et al., 2004).

Two main phases of gold mineralisation have been recognised: i). a low grade stage associated with an early oxidised Fe-mineral phase of hematite-albite-dolomite-sericite-pyrite alteration; and ii). a subsequent high grade stage associated with a reduced hydrothermal fluid that produced and alteration assemblage of dolomite-albite-sericite-quartz-pyrite±fuchsite with visible gold (Tully et al., 2017).

Gold mineralisation is principally present as series of sub-horizontal, vertically stacked lodes that are each 5 to 20 m thick, dipping at between 0 and 40°NE (Tully et al., 2017). The uppermost of these have lateral dimensions of 400 m along strike and 600 m down dip (Nielsen and Currie, 1999). These thicker lodes are separated by by thinner, less continuous, more steeply dipping, sub-parallel and cross linking structures, generally dipping at between 40 and 70°N. Gold typically occurs in microfractures within disseminated pyrite, or rarely as coarse visible gold in hydrothermal brecciated quartz veins. Mineralisation is highest grade within the conglomerate where it is adjacent to the margin of the actinolite-magnetite alteration halo, or proximal to the intrusive complex. Whilst mineralisation is found within the intrusive complex, it is generally of substantially lower tenor compared to the conglomerate (Tully et al., 2017).

Mineral Resources at Wallaby in mid 1999 were - 41.7 Mt @ 2.8 g/t Au for 115 t of gold to a depth of 400 m.
  including Just in Case, the total Mineral Resource was - ~130 t Au (Nielsen and Currie, 1999).
In early 2000, the total Mineral Resource at Wallaby and Just in Case was - 58.4 Mt @ 2.65 g/t Au for 152 t of gold.

Mineral Resources and Ore Reserves at 31 December, 2018 were (Gold Fields 2018 Ore Reserves and Mineral Resources Report)
  Measured + Indicated + Inferred Mineral Resources - 40.22 Mt @ 5.64 g/t Au for 227 t of gold;
  Proved + Probable Ore Reserves - 12.571 Mt @ 5.54 g/t Au for 69.64 t of gold.

The most recent source geological information used to prepare this decription was dated: 2017.     Record last updated: 6/9/2019
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Nielsen K I, Currie D A  1999 - The Discovery of the Just in Case / Wallaby Gold Deposit, Laverton District, Western Australia: in   New Generation Gold Mines 99, Case Histories of Discovery, Conf. Proc., Perth, Nov, 1999 AMF, Adelaide    pp 1-13
Salier B P, Groves D I, McNaughton N J and Fletcher I R  2004 - The world-class Wallaby gold deposit, Laverton, Western Australia: An orogenic-style overprint on a magmatic-hydrothermal magnetite-calcite alteration pipe?: in    Mineralium Deposita   v39 pp 473-494
Tully, R.B., Platt, H.K., Chirnside, A.R. and Murray, A.L.,  2017 - Gold deposits of the Granny Smith - Wallaby District: in Phillips, G.N., (Ed.), 2017 Australian Ore Deposits, The AusIMM, Melbourne,   Mono 32, pp. 255-259.
Zhang, Y., Schaubs, P.M., Sheldon, H.A., Poulet, T. and Karrech, A.,  2103 - Modelling fault reactivation and fluid flow around a fault restraining step-over structure in the Laverton gold region, Yilgarn Craton, Western Australia: in    Geofluids   v.13, pp. 127-139.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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