The Wallaby deposit and its continuation to the south, Just in Case, are located within the Laverton Tectonic Zone, some 11 km to the west of Granny Smith in the Yilgarn craton of Western Australia.
The Wallaby gold deposit is hosted by a 1200 in thick mafic conglomerate which is intruded by an apparently comagmatic alkaline dyke suite displaying increasing fractionation through mafic-monzonite, monzonite, syenite, syenite porphyry to late-stage carbonatite. Within the main deposit area, the conglomerate is overprinted by a pipe-shaped zone of actinolite-magnetite-epidote-calcite (AMEC) alteration. 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 2,664±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.
Three sub-horizontal to flat dipping and sub-parallel mineralised lodes have been delineated over an area of 400 m along strike and 600 m down dip by early 1999, although all were still open on two sides. The main lodes have numerous, associated, smaller, less continuous, sub-parallel and cross linking zones. Two main sets of granitic intrusions are found within the mineralised zone. These are also mineralised where cut by the lodes, although the mineralisation tends to be disseminated and of lower grade. The main host is a series of Archaean polymict conglomerates.
The orebodies are masked by 15 to 70 m of lake sediments. Intersections include 18 m @ 4.1 g/t Au, 10 m @ 19.1 g/t Au, 28 m @ 10.3 g/t Au and 38 m @ 2.3 g/t Au. The resource in mid 1999 was 41.7 Mt @ 2.8 g/t Au for 115 t Au (3.7 Moz) to a depth of 400 m. With Just in Case, the total resource was around 130 t Au (4.2 Moz). In 2003, the deposit had a declared ore reserve of ~62 t (@ Moz)Au, and a total resource of 220 t (7.1 Moz) Au (Salier et al., 2004).
The Wallaby discovery was the result of drilling in mid 1998. Wallaby at that stage was held by the Granny Smith Joint venture (40% Delta Gold NL, 60% Placer Dome Inc), while the smaller Just in Case was in a Homestake Mining-Abednego lease.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2004)