BHP Billiton’s Spence copper cathode project is located 1,700m above sea level in the Atacama Desert in northern Chile, close to the mining town of Sierra Gorda, 50 km south west of Calama and 150 km NE of Antofagasta. Feasibility studies were completed in 2002 with mine approval granted in 2004.
The mine reached design capacity in the third quarter of 2007, setting it on target to reach an annual production rate of 200,000t by 2010. The mine produced 172,685t in 2009. Spence was the first greenfield large-scale mining project to be developed in Chile in six years and the first large SX-EW (solvent extraction and electrowinning) project constructed globally this decade.
The supergene enriched and partially oxidised porphyry copper deposit is of Upper Paleocene age (about 57 million years). The deposit is 100% covered by gravels and depth to mineralisation ranges from 80m to 100m below the surface. Oxide and supergene sulphide mineralisation is amenable to heap leaching (atacamite and chalcocite).
Proven and probable reserves of oxide ore as of the end of 2007 were 74.8 million tons at a grading of 1.24%. Total reserves of sulphide ore were 238.3 million tons with a grading of 1.08%.
The operations at Spence comprise an open pit mine, sulphide leaching and oxide processes, and an SX-EW process. Ore is crushed, agglomerated, and transferred to leach pads at a rate of 50,000t a day. Sulphide and oxide ores are leached on separate dynamic (on / off) pads. Solvent extraction relies on four trains in a series-parallel configuration with extraction stages for both oxide and sulphide Pregnant Leach Solution.
Copper is electrowon to produce cathodes at a nominal rate of 200,000t a year. Cathode is transported to Mejillones for shipping to customers.
Annual production of copper cathode at Spence for the 12 months to June 2008 was approximately 142,700t. Mine life is currently estimated at 19 years.