Mining
The Pinos Altos mining project includes a series of open pits and an underground mine to exploit deposits along the Santo Nino Fault. Most of the ore is treated in a conventional process plant, with the lower grade ore heap-leached. The Creston Mascota project 7 kilometres away is a satellite open-pit, heap-leach facility.
The main surface mining includes the Santo Nino and Oberon de Weber pits, and, in future, the El Apache pit. Ore is mined in 7-metre-high benches, and pit wall slopes are 45° to 50°. Haul trucks of 100-tonne capacity remove ore and waste from the pit, moving about 17 million tonnes of material per year.
The same conventional surface mining methods are used at the Creston Mascota open pit, which is expected to mine an average of 4,000 tonnes of ore per day, beginning in 2011.
The underground mine at Pinos Altos began operation in Q4 2010 and will be phased in over four years. It uses transverse and longitudinal open stoping to extract ore from the Santo Nino, Cerro Colorado, Oberon de Weber and San Eligio deposits. The stopes are 30 metres high and 15 metres wide, and are filled with cemented pastefill after mining. Ore is trucked to surface via a ramp system. At maximum production, the underground mine will be able to provide 3,000 tonnes of ore per day. An expansion to the underground production capacity is being considered by the Company in order to better match the mine capacity to the mill, which is currently operating at more than the design 4,000 tonne per day production rate.
Open Pit and Underground mines 3D view
Mineral Processing
Heap leaching is used to extract gold and silver from up to 2,000 tonnes of lower grade ore per day at Pinos Altos. The rest of the Pinos Altos ore is treated in a 4,000-tonne-per-day mill.
The mill includes crushing, grinding, gravity concentration and agitated leaching followed by counter-current decantation. Gold and silver are recovered using the Merrill-Crowe method, and a refinery produces gold/silver doré bars on site. Metals recovery in the plant is estimated at 94% for gold and 50% for silver over the life of the mine.
Residual cyanide is destroyed before the tailings are filtered, making them non-toxic. The filtered tailings are used underground for paste backfill or discharged to an impoundment where they are dry-stacked. Water used in processing is reclaimed for reuse in the plant.
The heap leach facility involves stacking crushed ore on a lined pad and spraying it with a weak sodium cyanide solution that leaches the precious metals. The metals-bearing solution is piped to the plant where the precious metals are recovered by the Merrill-Crowe circuit. The expected metals recovery for the heap leach operation is 68% for gold and 12% for silver.
The 4,000-tonne-per-day heap leach facility at Creston Mascota is similar to the Pinos Altos facility, but the precious metals are recovered differently. A small carbon column circuit at Creston Mascota recovers the precious metals from the leach solution onto carbon, which is transferred daily to the Pinos Altos plant. There, the gold and silver are stripped from the carbon and poured as doré bars. The expected gold recovery at Creston Mascota is 71% and the expected silver recovery is 16%.
