Mining
The mine design, mineral reserve estimate, mining costs and mining fleet requirements for the Project were prepared by NCL Brasil LTDA. Conventional open pit mining will utilise owner operated truck and shovel combinations to maximise production. The mine is designed as a two pit operation using three phases per pit mined over ten years (including ramp up and ramp down), plus a year of pre-stripping. The mine has a planned throughput of 7.0 million tonnes of ore per year.
The average waste to ore strip ratio for the life of the mine is estimated to be 6.88:1. Bench heights of 10 meters will be mined using 15m shovels loading 180 tonne haul trucks. Waste rock will be hauled to dedicated waste management facilities adjacent to the open pits. Saprolite ore will be stockpiled and up to 5% will be fed to the plant annually.
Metallurgy
Previous metallurgical test work included gold recovery tests using medium and high grade composites, with varying gold head grades from approximately 1 g/t to 4 g/t from each of the Grota Seca and Ouro Verde ore bodies.
The following tests made up the gold recovery section of the metallurgical test work program:
-- Chemical characterization, size distribution and diffractive x-ray analysis of the head samples.-- Preg-robbing tests.-- Gravity separation tests using centrifugal concentrators.-- Leaching tests under a variety of conditions including grind size, slurry density, cyanide concentration and dissolved oxygen concentration.
The highlights of the gold recovery tests included:
-- Between 25% to 50% of gold can be recovered in a gravity concentration step prior to leaching.-- The optimal leaching conditions for the Ouro Verde and Grota Seca composites were a grind size of 80% passing 75 um, slurry density of 50% by weight, 0.5 g/L NaCN concentration, 10 ppm dissolved oxygen concentration at a pH of 10.5.-- The Ouro Verde ore, with a gold head grade varying between 1.17 g/t to 3.80 g/t, has an average gold recovery of 94%.-- The Grota Seca ore, with a gold head grade varying between 1.06 g/t to 3.90 g/t, has an average gold recovery of 94%.
Metallurgical testwork has defined the most effective gold recovery process for Volta Grande as follows:
-- Primary crushing with a gyratory crusher,-- Grinding using a SAG/Ball Mill combination,-- Gravity recovery,-- Leaching in a carbon in leach circuit,-- Cyanide destruct prior to discharge to the tailings management facility,-- Carbon elution and regeneration,-- Electro-winning and smelting to produce a gold dore.
Overall gold recovery using this process has been estimated at 94%. The plant will be operated continuously with a planned throughput of 7.0 million tonnes per year assuming an overall 90% availability. Annual gold production is projected to average approximately 313,100 ounces. Dore produced at the site will be sold for further refining.
Operating Costs
Life of mine operating cost based on an annual production of 7.0 million tonnes of ore are estimated at US$17.36 per tonne of ore mined or US$2.26 per tonne of material mined, excluding production royalties. This estimated cost equates to US$681.90 per ounce of gold produced, which includes: mining, process, general and administrative costs. The total operating costs are broken down as follows:
Operating Cost Breakdown
---------------------------------------------------------------------------- Cost per tonne Cost per Ounce Ore in Produced inCategory US$ US$----------------------------------------------------------------------------Mining Cost per Tonne material moved 2.26Mining 17.36 389.60----------------------------------------------------------------------------Processing 10.50 235.70----------------------------------------------------------------------------General and Administrative 2.52 56.60----------------------------------------------------------------------------Total US$30.38 US$681.90----------------------------------------------------------------------------Strip Ratio 6.88:1
