Regional and Local Geology

The Wallbrook Project occurs within the Norseman – Wiluna Archaean Greenstone belt in the Eastern Goldfields province of the Yilgarn Craton. The Project is located within the Edjudina Region in the Laverton Tectonic Zone, centrally between Kalgoorlie and Laverton, and 35km north of Northern Star Limited’s Carosue Dam Gold Mining Operation.

The granite-greenstone belt is approximately 600 kilometres in length and is characterised by thick, possibly rift-controlled accumulations of ultramafic, mafic, felsic volcanic, intrusives and sedimentary rocks. Greenstone successions of the southern Eastern Goldfields have been segregated into elongate structural terranes bounded by regional NNW-trending faults (Swager, 1995). These terranes include the Kalgoorlie Terrane, Gindalbie Terrane, Kurnalpi Terrane and the Edjudina Terrane. These terranes contain distinct similarities, including timing of the deposition of volcano-sedimentary sequences (2720-2675 Ma) and regional deformation and plutonism (2675-2620 Ma). The terranes differ only in lithostratigraphic development and early tectonic history (Swager, 1995).

The Wallbrook Project area is located between two major converging tectonic features, the Laverton and Keith-Kilkenny tectonic zones. The Laverton Tectonic Zone (LTZ) forms the central portion of the Laverton Greenstone Belt, running north-south in the eastern parts of the Wallbrook Project. The LTZ is recognized as a world class gold province, with a mineral endowment (production + resources) of over 20 Moz of gold. Major deposits include Sunrise Dam (8.0 Moz), Wallaby (8.0 Moz) and Granny Smith (3.6 Moz). The Keith-Kilkenny Tectonic Zone (KKTZ) has a northwest-southeast orientation and is an important vector to mineralisation in the region between Leonora and Leinster. The southern extension of the KKTZ intersects the Carosue Dam Operation (4.275 Moz).

The lithologies at Wallbrook are dominated by intermediate (andesitic) volcanics, intrusive felsic porphyries and granite. The dominant feature in the project area is the Wallbrook Monzonite. North of the monzonite are relatively smaller granitic intrusions and related narrow felsic porphyry dykes/sills which run predominantly parallel to the regional trend.

The project area covers the convergence of two major trends wrapping around the northern end of the tear-shaped Wallbrook Monzonite. There are several phases of alteration observed, including:

• chlorite + magnetite (associated with regional deformation);
• hematite + silica + sulphides (+ associated felsic intrusives); and
• sericite + silica + carbonate + pyrite + gold (late tectonic + mineralising event).

As with many of the gold deposits within the Eastern Goldfields, gold mineralisation occurred relatively late in the deformational history of the area. Within the felsic lithologies there is a relationship between the hematite/silica alteration and gold mineralisation. Arnold (1999) suggests gold mineralisation is related to hematite bearing oxidized alteration assemblages, with deposition occurring where gold bearing fluids have come into contact with earlier magnetite-hematite assemblages.