THE GEOLOGY OF THE GORO PLATEAU

• Geological profile
• Laterites
• Goro laterites
• Seven geological horizons

The deposit on the Goro Plateau in New Caledonia is a classic deposit in its composition, that is, it comprises iron oxides on the surface (laterites) and magnesium silicates beneath (saprolite). The nickel and cobalt contained therein result from chemical concentrations within this residual formation. Of the world’s laterite deposits, this has one of the highest nickel contents.

Geological profile

The specificity of the geological profile of the Goro plateau comes, in part, from its size and its geometric and mineralogic continuity which define deposits exceptionally high in nickel and cobalt; and additionally from its location in a particularly humid environment, fractured and drained by underground water, which has contributed to the development of morphology in basins, thus explaining its size and concentrations of residual metals.

Of the world’s laterite deposits, this has one of the highest nickel contents. Laterites may be red, orange, ochre or yellow in colour. Unlike sulphide ore reserves, these lend themselves particularly well to opencast methods of exploitation.

There are two types of lateritic nickel ore at Goro: laterites and saprolite. These comprise a leached layer between the surface and the parent rock.

Laterites

Laterite deposits are formed in tropical and subtropical climates by the prolonged erosion of ultrabasic parent rock, peridotite, which is produced with the passing of more or less marked periods of droughts and rain over millions of years. Rain and underground water pass through the fractured parent rock, gradually leaching mobile elements such as magnesium and silica, and leaving the (less mobile) nickel and cobalt in residual concentrations. Over a very long period of time, mineral and chemical stratification takes place within the developing lateritic profile

In addition to New Caledonia, the biggest laterite deposits are in Cuba, Australia, Indonesia and the Philippines. The dry laterites of Australia contain between 20% and 30% humidity, while wet laterites, such as those at Goro, may contain up to 50% free water.

Goro laterites

The total thickness of the Goro Plateau profile may reach as much as 60 metres, while, on average, it is more like 40 metres. The wet laterites of the Goro Plateau are richer in nickel and cobalt than the typical dry deposits in Australia such as those at Murrin Murrin and Cawse.

They are, however, lower in content than the Caledonian saprolite resources currently being exploited (known locally as ‘garnierites’). This is why they have remained unexploited until now. It is the hydrometallurgical process developed by Inco, allowing them to be exploited profitably, which gives them value.

Inco geologists have been aware of the economic potential of the laterite resources of the southern tip of Grande Terre since the beginning of the 1960s. Vale Inco Nouvelle-Calédonie could become the world’s biggest producer of low-cost nickel. The lifespan of the Goro Plateau mine is estimated to be at least 30 years.

Seven geological horizons

Seven principal geological horizons define the lateritic profile of the Goro plateau:

• A layer of iron duricrust

• A layer of granular iron

• A layer of red laterite whose nickel and cobalt concentrations are not considered economic within the context of Vale Inco Nouvelle-Calédonie’s processes.

• A layer of yellow laterite (limonite) containing from 0.8 to 1.7% nickel and from 0.1% to 0.5% cobalt. Just one area of yellow laterite (over 1.2% nickel) is considered to be economic.

These are much lower content levels than the saprolites which are currently being exploited in New Caledonia, but the Goro Plateau resources are much more abundant. It is the mainform of ore in terms of volume and chemical composition (extremely low magnesium content).

• A thin transition zone containing from 1.2 to 2.0% nickel and from 0.1 to 0.5% cobalt

• An irregular layer of saprolite, silicate ore whose nickel content varies from 0.9 to 2.5%, including boulders. By sorting the fine grains, rich in nickel and low in magnesium, an exploitable ore is obtained, containing around 2% nickel.

This ore would not be exploitable using a conventional pyrometallurgical process.

The Vale Inco Nouvelle-Calédonie saprolite will, therefore, have the lowest content of all New       Caledonia deposits where this type of ore is exploited. 

• The peridotitic parent rock, from 0.3 to 0.5 % nickel content.