Magnetism — key to the Wallaby gold deposit

The Wallaby deposit in Western Australia, which consists of several stacked gold lodes, is now recognised as being hosted by an actinolite-magnetite alteration pipe. Identification of the pipe geometry resulted from a combined interpretation of the aeromagnetic anomaly and drill core magnetic susceptibility measurements.

Gold was discovered in 1997 in the northern part of the deposit, called Just in Case. By September 1999 the complete Just in Case - Wallaby zone had been drilled to 500 m depth, outlining gold mineralization within magnetic basaltic conglomerate. The challenge then was to develop targets to 1000 m.

Magnetic susceptibility measurements made on all core proved invaluable. A susceptibility block model was studied in conjunction with models derived from the surface magnetic anomaly. Although the gold zones tended to be flat-lying, susceptibilities displayed an annular pattern in plan, and a southerly dip in a central north-south section. The magnetic zone was interpreted as being a plunging pipe, with a plunge of 50° toward 190° determined from modelling the aeromagnetic anomaly. Complementary to recognition of the pipe was evidence that gold was effectively restricted to the magnetic zone.

The pipe model provided a well-defined target, and drilling to 1000 m confirmed the structure as well as discovering four more gold lodes.