NICKEL
Last of the Giants
April 2019
The era of discoveries of large, high-grade nickel sulphide provinces appears to be over. It has been a century since the giant discoveries of Sudbury or Norilsk, each containing nickel approaching 20Mt. The last large, high-grade province discovered was Kambalda in Western Australia in the 1960s, with 5Mt of contained nickel. Recent nickel sulphide discoveries are either low-grade and large or high-grade, small, and very difficult to discover.

Are sulphide ores destined to be replaced by laterite ores? The following table summarises details of major nickel sulphide deposits around the world.

 

 

Nickel exploration depends a lot on the size, grade, and abundance of different styles of mineralisation. Nickel laterites are abundant, exposed near the surface and relatively easy to locate and delineate. Today, new nickel sulphide deposits are much harder to discover, as they are often buried, with high-grade deposits generally small in scale. Below, a brief review of nickel sulphide and laterite deposit types, as well as their key characteristics, is provided to illustrate the variation in potential exploration targets.

 

Nickel Sulphide Deposits

While nickel sulphide deposits are rare worldwide, they are highly productive. Sudbury in Canada, Norilsk–Talnakh in Russia and Mt Keith, Kambalda and Windarra in Western Australia are all prolific sulphide nickel producing regions. Nickel sulphide deposits are invariably associated with mafic or ultramafic igneous rocks (i.e. igneous rocks that are poor in silica, and primarily composed of magnesium- and iron-bearing silicates). These deposits usually carry copper, cobalt and sometimes gold, silver and recoverable PGMs.

Sulphide deposits have been classified into five major styles of deposit in the table above, based on their mode of formation.

 

Nickel Laterite Deposits

Nickel laterites form as a result of near-surface weathering of ultramafic rocks in generally warm tropical climates. Two main layers form within the laterite profile—an upper limonite layer (oxide) and a lower saprolite layer (silicate) sitting above unweathered bedrock.

The nickel laterites are formed by the progressive weathering of bedrock. Rain water, made slightly acidic through interaction with the atmosphere and decaying vegetation, leaches the rock as it passes downwards. More soluble components, such as magnesium, are leached from the profile, while the least soluble elements, like iron and aluminium, are preferentially concentrated towards the top of the profile. Elements with moderate solubility like nickel are initially leached, but then reprecipitated further down the profile as the acidity of the water changes as it interacts with less weathered, more alkaline rocks. The rate of weathering is controlled by the amount of rainfall and the permeability of the bedrock, which is in turn controlled by the degree of serpentinisation and fracturing.

An example of an idealised laterite profile is presented below. Passing down the profile, several zones are encountered, including:

  • Red Limonite – An upper most layer that is highly enriched in iron and aluminium, with minimal residual nickel content. This layer is essentially treated as waste, although at times it has been consumed as an iron feedstock.
  • Yellow Limonite – the major economic limonite layer, which exhibits low to moderate nickel grades increasing with depth from around 0.7–1.6%, and iron grades decreasing from over 45% to around 30% with depth. Nickel is contained within hydrated amorphous iron oxides. Very high cobalt grades are often developed at the base of this horizon above the saprolite horizon. The limonite horizon has traditionally been processed via HPAL or Caron processes, and more recently via blast furnaces to create a low-nickel grade, high-iron NPI product. HPAL processors gain the benefit of significant cobalt by-products, which blast furnace processors are generally unable to recover.
  • Saprolite – this zone is highly enriched in nickel, exhibiting grades of 1.5% to well over 1.8%, with minimal iron content and elevated magnesium and silica content. Nickel is generally contained in nickel silicate minerals such as garnierite. This saprolite horizon is processed via blast furnace or Rotary Kiln Electric Furnace (RKEF). Limited amounts of saprolite may also be processed along with limonite in an HPAL operation incorporating an atmospheric leach circuit, where the saprolite is used to neutralise residual acid in leach solution. HPAL processing of saprolite on a large scale is not feasible, as acid consumption rates are uneconomic.

Sulphide mines of the future are likely to look very different to those of today, as high-grade resources are exhausted, and as new discoveries are either smaller and at greater depth in higher grades, or in highly disseminated orebodies. As laterite ores are abundantly available and near the surface, the profile of these will alter less. The increase in metal production in recent years saw laterite ores overtake sulphide mines in terms of contained nickel in 2010, and their share of production will increase to about 74% by 2030.