Summary:
The Ramu deposit is a typical laterite nickel and cobalt deposit formed by weathering and leaching of the original ultramafic intrusive rocks, mostly dunite with small amount of harzburgite and pyroxenite in a tropical climate with large amounts of rainwater, saturated with atmospheric carbon dioxide, and a local monsoonal rainfall pattern. When the ultramafic rocks are subject to strong weathering, olivine, augite and other ferromagnesian silicate minerals rich in nickel, cobalt and other elements will decompose; the released SiO2 is progressively removed by ground and/or surface water in the form of colloid or siliceous acid, and the ferrous iron is oxidised to ferric iron and converted into hydroxides and oxides, such as lepidocrocite, goethite and hydrohematite, and left in situ. Nickel, cobalt and other elements are absorbed by the clays in the saprolite, or directly precipitated from the colloidal solution, or enriched in secondary nickel silicate minerals, consequently forming a lateritic nickel-cobalt deposit within the weathering crust. At Ramu, nickel and cobalt have been enriched from a background of around 0.3% Ni and 0.01% Co in the ultramafic bedrock up to grades averaging 0.9% Ni and 0.1% Co in the laterite profile.
The Ramu laterite nickel and cobalt deposit occurs in the weathering crust of ultramafic intrusive rocks, mostly dunite. The deposit above the dunite bedrock is divided into six laterite layers, from top to bottom, the humic layer (Q), the red limonite (O), the yellow limonite (L), the saprolite (S), the upper rocky saprolite (R1) and the lower rocky saprolite (R2).
Nickel grade is less than 0.5% in the humic layer, and is generally less than 0.5%, but occasionally above 0.5%, in the red limonite. These two layers are generally considered as overburden for mining and are stripped off before mining the lower mineralised laterite layers, including yellow limonite, saprolite, upper rocky saprolite and lower rocky saprolite. The nickel grade in the lower mineralised layers averages approximately 1.0%, and the cobalt grade averages approximately 0.1%.
The distribution of each relevant element in the laterite profile shows different patterns. The nickel and magnesium grades generally increase from top to bottom; but at the bottom of the lower rock saprolite, nickel grades reduce but magnesium grades still increase. The aluminium grade reduces with the increasing depth as it is a residual component in the weathering and leaching process. Nickel is apparently enriched in both saprolite and rocky saprolite, but cobalt is only enriched in the saprolite.
The principal ore minerals identified in the Ramu deposit include goethite, asbolan and garnierite.
Goethite is found as ochre-coloured, porous, cryptocrystalline, needle-like matrix in the limonite and saprolite zones of the laterite. The highest concentrations of goethite occur in the yellow limonite. Goethite-silica, goethite-smectite and other goethite-clay mixtures dominate the matrix of these zones. The average nickel grade contained within the goethite structure has been measured by electron microprobe analyses at 1.6% Ni in the limonite zone and 2.9% Ni in the saprolite zone.
Asbolan occurs as bluish black dendrites and fracture coatings throughout the laterite profile. It has a range of compositions containing elemental mixtures of cobalt, nickel, manganese and aluminium. In the limonite zone, asbolan assays by electron microprobe analysis at 8.4% Co and 5.2% Ni, and in the saprolite zone, it assays 5.6% Co and 15.1% Ni.
Garnierite, or nickeliferous serpentine, is an apple green mineral found at deeper levels in the deposit in the alkaline weathering zone, generally at the base of the limonite horizon and in the saprolite and rock saprolite zones. The most common occurrence of this mineral is as one to ten centimetre wide veins, as fracture infillings and in the weathered rind of bedrock boulders. Garnierite may also occur as infill with serpentine in calcic magnesite breccia. Garnierite displays a range of compositions based on the proportions of serpentine, talc and lizardite minerals. From analysis of 14 samples, lizardite contains an average of 1.2% Ni.
The distribution of the laterite layers is generally controlled by topography. The laterite layers dip at angles generally between 10º to 35º, consistent with the topography dip angles. All laterite layers in the deposit vary significantly in thickness because of the topographic control and erosion.
Based on the MgO content, the weathering crust lateritic nickel and cobalt ore is divided into the iron ore type when MgO<10%, the ferromagnesian ore type when MgO content ranges from 10% to 20%, and magnesium ore type when MgO>20%. The average MgO content in the O, L and S layers is less than 10%, and therefore these zones belong to the iron ore type; the average MgO content in the R1 layer is approximately 18%, therefore it belongs to the ferromagnesian ore type; and the MgO content in the R2 layer is approximately 20.4%, and therefore it belongs to the magnesium ore type.