New South Wales has a history of exploration going back over 150 years. The announcement of a discovery near Bathurst in 1851 sparked a gold rush, and many other discoveries were made. None of these, however, was on a grand scale. The Lachlan Fold Belt (LFB) was consequently somewhat overlooked by mineral explorers until the discovery of a large base metal deposit at Woodlawn in 1972, followed by the rich gold-copper deposits at Cadia in 1992.
The Lachlan Fold Belt forms a small portion of the Tasman Fold Belt, stretching from Queensland to Tasmania. In the eastern part of the belt, Ordovician-Silurian volcanoes and intrusions were formed during the Palaeozoic Era in a north-south trending island arc setting, known as the Macquarie Arc. The Ordovician and Silurian rocks which outcrop in the LFB study area consist of deep water sediments interspersed with intermediate to mafic volcanics, and intruded by shoshonitic intrusions. The best gold and copper mineralization, for example at Cadia, appears to be closely associated with Ordovician monzonites.
The gold endowment of the study area, which covers some 120,000 square kilometres, is close to 50 million ounces. However, within the greater Lachlan Fold Belt, which runs into Victoria, the total endowment is over 150 million ounces. The data mining study was carried out on our own initiative, but has subsequently been licensed to Rimfire Minerals Corporation.
The exploration data sets used in the LFB study consisted of geology and structure, gravity, magnetics, radiometrics, geochemistry, Landsat and terrain. Footprints of as many known gold deposits as possible were established using the nationwide OZMIN database, compiled and maintained by Geoscience Australia, as well as by reviewing publications and company reports.
Data Mining Results
The process worked well in the LFB, where there is a first-class collection of modern data sets and a fine selection of known deposits. A strong, coherent statistical signal was obtained from the exploration data as a whole.
Like the Walker Lane, the Lachlan Fold Belt is an active exploration area, so we can’t show the overall target map that resulted from the data mining process. However, we can show a small portion of the map. The accompanying figures show a pair of typical neural network targets, marked as Target A and Target B in the figure above. Target A in the north is actually a small known deposit, which was used as part of the training set. The white squares are on a 100m grid, and represent the footprint of the known gold mineralization. The mine shaft symbols represent old workings which produced minor amounts of gold.
The larger Target B in the south is sharply defined and covers an area of roughly two square kilometres. A target like this would normally be quickly staked and followed up on the ground. Unfortunately, as can be seen in the satellite image below, this target lies directly under the small country town of Forbes, which happens to be where gold was first discovered by the prospector “German Harry” Stephens in June 1861.