The ore deposit takes the form of a layered blanket on a 2500 x 1700 m oval shaped hill that rises some 200 m above the surrounding countryside. The thickness of the ore varies from a few up to more than 100 m. The lateritic profile is as folllows, from the surface to the bedrock:

1) Canga - up to more than 20 m thick, comprising a hard ferruginous crust of lateritic soil cemented by hydrated iron oxide covering a large aprt of the deposit, and having a low Ni content;
2) Laterite - comprising an amorphous, reddish-brown, soft soil made up of hydrated iron and aluminium oxides, with generally <1.5% Ni;
3) Saprolite - partially weathered clays that retain the texture and appearance of the peridotite bedrock. Two types of saprolite are recognised, namely a). a brown coloured variety, and b). a green variety which has a higher Ni content and ferrous:ferric iron ratio, found below the canga capping. Overall the saprolite averages >3% Ni;
4) Saprolitised peridotite - composed of fractured bedrock with soft, enriched weathered margins to the fractures and joint planes, sometimes containing garnierite;
5) Silica boxwork - occuring a veins of chalcedony and quartz on fractures within the peridotite bedrock - essentially barren at surface, but reaching ore grade in the lower sections of the deposit.

The original peridotite had a high magnesia and silica content, with the balance being predominantly iron and 0.2 to 0.3% nickel. Magnesia, silica and nickelwere dissolved during weathering and re-deposited lower in the weathering profile, while iron and aluminium remained near the surface as relatively insoluble hydrated oxides. Nickel maxima in the profile correspondes to a zone of 15 to 25% MgO. The ore comprises a zone of saprolite and saprolitised peridotite. The footwall is un-weathered peridotite.

Due to the lateral extent of the orebody and the trend of Ni grade lowering with depth, the Cerro Matoso pit has generally been mined in a “top-down” method – which is appropriate for this style of orebody. The mining method is traditional truck-and shovel utilising shovel, excavator and front end loaders (FELs) as required for both primary digging and stockpile rehandle. The excavator in backhoe configuration is the primary loading unit in ore, while shovels and FELs are used in bulkier materials or stockpile reclaim.

Mining activities include removal of any growth medium (topsoil), free-digging, drilling, blasting, loading, hauling and mining support activities. Approximately 10% of the material mined is classed as free-dig (not requiring blasting). Approximately 12% of the ore can be sent directly to the crusher. The remaining ore is sent to the ROM fingers or the off-specification stockpiles near the waste dump, and later rehandled to the crusher as required to achieve the required blend.

Ore mined is blended with ore from stockpiles before it is passed through a two-stage crushing sequence. The two stages of crushing reduce the size from a nominal minus 600 mm to minus 50 mm.

Cerro Matoso is a major producer of nickel contained in ferronickel which is used to make stainless steel. Ore mined is blended with ore from stockpiles, which is then dried in rotary kilns and smelted in two electric arc furnaces where ferronickel is produced.

Description
After crushing, the ore is mixed and homogenised before it is partially dried in two rotary driers. An upgrading process is then undertaken whereby lower nickel grade is removed. After the upgrading process, the dried ore is then blended with a reductant agent (locally bought coal) and calcined in two rotary kilns.

Calcined ore is then transferred to two 75 MW electric furnaces where ferronickel is tapped at around 1,460° C. The crude metal is transported to the refinery where impurities are removed. The refined metal is granulated and packed.

Cerro Matoso has a water management system which allows it to recycle 95 per cent of the water used in operations. The remaining 5 per c ........