The preliminary economic assessment on Nevada Copper’s Pumpkin Hollow open-pit project has given the company food for thought, just as it starts construction on the underground mine.Golder Associates and Sedgman Canada produced the PEA, which showed a 37,000 t/d (short) open-pit mine producing 80,286 t/y of payable copper at a C1 cash cost of $1.67/Ib (net of by-product credits) could be built for $592 million.This plan is a departure from the 70,000 t/d (short) open-pit project outlined in 2013, which would have required $926 million of upfront capital and produced 85,900 t/y of copper over a 23-year mine life.The latest PEA envisages an expansion to 70,000 t/d (short) in year eight of the mine, costing an additional $447 million, but would see higher-grade ore mined in the first five years of operation to increase the potential returns and decrease the amount of pre-stripping required.Matt Gili, President and CEO of Nevada Copper, said: “The phased development approach…aligns with our strategy of pursuing optionality through low-capital intensity and staged production growth to generate shareholder returns. We have used this same margin-over-tonnes philosophy with the Pumpkin Hollow underground mine, which will be in production in 2019, and we are looking forward to the next steps in advancing the open-pit project.”The Nevada Copper board signed off on the construction of the underground mine late last month.Pumpkin Hollow Underground is expected to process 5,000 t/d and produce some 50 MIb/y (22,680 t/y) of copper over a 13.5-year life at all-in sustaining costs of $1.96/Ib.The open-pit project, meanwhile, is envisaged as a conventional truck-and-shovel operation planned to use a combination of hydraulic and electric cable shovels and haul trucks.The mining fleet includes 240 t class trucks, loaded by a 45 yd3 (34 m3) diesel-hydraulic and electric shovel and 22 yd3 (17 m3) wheel loader. Drill and blast will be undertaken with track mounted drill rigs drilling 10 ¾ inch (273 mm) holes.The treatment technology proposed for the project is conventional flotation concentration. The processing plant will consist of crushing and grinding circuits, followed by a flotation process to recover and upgrade copper and silver from the feed material.Crushed material with the approximate particle grind size P80 of 6 inch (152 mm), will be fed to the grinding circuit via SAG mill feed conveyor. Oversized material from the SAG mill trommel screen will be conveyed to the pebble crusher. The pebble crusher will discharge to the SAG mill.The product from the SAG mill will be fed into the grinding cyclone feed pump-box, from where it will be pumped to the primary cyclopack. The cyclone underflow product will report via chute to the ball mill for further grinding. The cyclone overflow product, with the approximate particle grind size P80 of 150 microns, is the final ground product and will report to the rougher flotation conditioning tank.Flotation will consist of one rougher and two stages of cleaner flotation, with the single tower/Vertimill being used for the fine grinding of the rougher concentrate.The copper concentrate will be thickened using a hi-rate thickener and the underflow pumped to the agitated stock tank prior to filtration, and the thickener overflow will be collected in the process water tank.The tailings will be disposed of by dry stacking of filtered tailings. The tailings will be thickened prior to moving onto the tailings filtration plant.