Exeter Resource Corp. has reported the results of a positive prefeasibility study (PFS) for combined development of the oxide and sulphide ores at its Caspiche gold-copper-silver project in northern Chile.

The study follows an earlier, positive, stand-alone, oxides-only PFS that the company completed in June 2011 (E&MJ, July-August 2011, p. 16). The new study predicts an average annual production of 696,000 oz of gold, 244 million lb of copper, and 844,000 oz of silver over a 19-year mine life. The study is based on proven and probable gold reserves of 19.3 million oz and proven and probable copper reserves of 4.6 billion lb.

Capital expenditures to develop the project are estimated at $4.8 billion.

The oxides-sulphides PFS evaluated three mining and processing options for the Caspiche deposit. All options included an open-pit to mine the near-surface, heap-leachable ore. The preferred development option is an open-pit operation processing 150,000 mt/d of sulphide ore and a heap-leach operation at an initial design production rate of 72,000 mt/d. The PFS envisages the heap leach operation will start before the main concentrator, with the leachable ore being removed as part of the pre-stripping operation.

A key component of the PFS is the inclusion of high-tonnage, in-pit crushing and conveying (IPCC) systems for the movement of waste rock. The cost savings per ton of total material moved using the IPCC system instead of a large truck fleet is estimated at about $0.25/mt, or $0.80/mt of ore mined. In addition to the operational savings, Exeter estimates an initial and sustainable capital saving of approximately $1 billion by building the tailings dam wall largely from a conveyor-stacker system rather than using conventional truck haulage for material movement. The waste dump would form the backbone of an engineered tailings dam face and provide a significant margin of safety in the event of an earthquake or other event.

The oxide ore and some low-copper sulphide ore would be treated in a conventional valley-fill heap leach operation. The sulphide orebody would be processed through a conventional concentrator. The copper concentrate produced would be treated in an offsite roaster to ensure that its arsenic content is below 0.2%. Additional gold recovery would be achieved by leaching the flotation scavenger tailings.