A common misconception is that only high-grade gold orebodies contain a significant component of gravity recoverable gold (GRG). As a result, the impact of gravity recovery on project profitability for low-grade deposits is often not considered. The fact though, as shown in this case study of the Karlawinda Gold Project, is that low-grade orebodies can contain significant GRG. The impact of GRG on the projects cashflow was shown to be in the order of A$44.1 million dollars over the life-of-mine. The Karlawinda Gold Project is located in the Pilbara region of Western Australia and is 100%-owned by Capricorn Metals Ltd. Its Bibra orebody is a typical low-grade, high tonnage deposit with a current reserve of 27.5 million tonnes at 1.0 g/t gold containing 0.89 million ounces. The feasibility study for the project confirmed up to 69.6%, or 0.696 g/t, of the gold in the deposit is amenable to recovery by gravity. Detailed test work undertaken for the project characterised the GRG in terms of its particle size, and its impact on cyanidation and gold recoveries. It identified that a significant proportion of the GRG in the Bibra deposit is moderate to coarse in size. On average 50% of the GRG is greater than 80 m in size, with 20% greater than 150 m. This moderate to coarse gold component was found to be slow leaching, impacting on leach residue grades. Removal of the GRG component prior to cyanide leaching was demonstrated to significantly reduce sodium cyanide consumption. Modelling of the GRG data, using the AMIRA P420 Gravity Model, was used to assist in the design of the gravity circuit to ensure optimum gravity effort was applied for maximising GRG recovery and minimising leach residue grades. By applying a high gravity effort, treating 50% of the bleed stream to gravity, it is estimated that leach residue grades will be reduced by 0.02 to 0.03 g/t. This alone equates to approximately A$26 million dollars in additional revenue, and savings of approximately A$10 million dollars in sodium cyanide consumption over the life-of-mine.
CITATION:Helm, M M, Hunter, I, Staunton, W P and Martin, M, 2019. Does gravity gold recovery make a difference in the profitability of a low-grade orebody?, in Proceedings World Gold 2019, pp 188195 (The Australasian Institute of Mining and Metallurgy: Melbourne).