Twin Hills is located in the Drummond Basin, a large intracratonic basin that developed in central Queensland between the Late Devonian and the early Carboniferous. Sedimentation in the Drummond Basin has been sub-divided into three tectono-stratigraphic cycles separated by minor orogenic events (disconformities and unconformities) Cycle 1 contains coarse intermediate to felsic volcanic and intrusive rocks possibly mixed with coarse clastic, mostly terrestrial, sediments. The volcanics occur in separate sub-basins around probable eruptive centres. Andesitic intrusions and volcanoclastic rocks are common. All the known economic epithermal gold deposits in the Drummond Basin are hosted by Cycle 1 sediments. Cycle 1 formations in the Twin Hills area include the Bimurra Volcanics and the Silver Hills Volcanics. A combination of Tertiary and Quaternary cover masks the underlying geology and makes exploration quite difficult.
Duval Mining (later the Battle Mountain Gold Company - “Battle Mountain”) discovered the Pajingo low-sulphidation epithermal system in the northern Drummond Basin in 1983, and identified the Scott Lode in November 1984. This find triggered a stampede of interest in the potential for near-surface epithermal gold mineralisation in this previously neglected area. Subsequently, the Wirralie and Yandan low-sulphidation epithermal gold deposits were discovered and mined, along with the Koala and Glen Eva mines located near Mount Coolon and the Belyando and Lucky Break gold deposits located south-east of the Twin Hills project area.
In 1987 Metana Minerals NL discovered the 309 mineralisation at Twin Hills. Drilling of the 309 deposit using RC and diamond coring commenced early in 1988 and by the end of that year a small inferred resource had been outlined at 309 and a large low-grade body of mineralisation outlined at Lone Sister.
In 1998 the first resource to JORC standards was published by Homestake. A combined total of 8.3 Mt @ 2.1 g/t Au had been outlined at that time at the 309 and Lone Sister deposits. Drilling definition has improved since the Homestake estimate in 1998, with more holes having been drilled at much closer spacing. As is often the case, this has resulted in a significant reduction in the tonnage estimate (from 8.26Mt in 1998 to 1.80Mt in 2007 for the 309 and Lone Star deposits combined) but a substantial improvement in the overall grade estimate from 2.1 g/t Au in 1998 to 4.7 g/t Au in 2007.
Mining commenced at 309 in 2006 with the ore being hauled 280 km to the Rishton Mill, located 35km south-east of Charters Towers. The fixed costs of operating the Twin Hills Mine and Rishton Mill were high and required ore to grade over 15g/t gold to be profitable. Unfortunately this grade could not be achieved consistently. BMA decided to close the Twin Hills Mine and sell all the surface equipment to assist with the funding of a new exploration programme designed to significantly increase the resources at Twin Hills.
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