iron–nickel deposits

iron–nickel deposits Nickel and iron are important elements used in a variety of everyday materials. Both are important constituents of stainless steel, where nickel, for example, gives the steel strength and anti-corrosion properties.

Two main types of iron–nickel deposits exist: ores related to laterites and those associated with mafic and ultramafic rock types in a wide range of tectonic settings. Each type of deposit is discussed below.

Lateritic ores

Laterites are formed in subtropical climates where silicate minerals are broken down leaving behind soils composed of insoluble oxides such as aluminium oxide (Al₂O₃) and iron oxy(hydr)oxides (FeOOH, Fe₂O₃). Dissolved silica and other alkali metals such as potassium, sodium, and calcium are leached from the rock into the underlying water-table, where they may subsequently react to form secondary minerals.

Where laterites form above ultramafic igneous rocks the nickel and iron contained within the minerals that comprise the rocks may be concentrated to form economic deposits. The climate and topography of the region in which laterite formation occurs will control the grade of ore produced. In tropical climates weathering of the silicate minerals is too rapid for new silicate minerals to be formed at depth, and in temperate climates weathering is too slow to generate an economic deposit. Subtropical regions therefore yield the richest lateritic nickel deposits. In areas where topography is steep, weathered material may be removed before nickel has been released from the underlying formations and had an opportunity to form new nickel-rich minerals. Important lateritic nickel deposits are found in New Caledonia, Indonesia, and Cuba, all of which are associated with island arc volcanism and plate margins. The typical structure of a nickel-rich laterite is shown in Fig. 1.

Nickel sulphide ores

Nickel sulphide ores can be subdivided into two main types: those associated with dunites/peridotites in Archaean Greenstone belts (e.g. Kambalda and Agnew deposits, Australia) and those associated with layered igneous intrusions (e.g. the Bushveld complex, South Africa). The most productive nickel ore deposit in the world is in Sudbury, Ontario (Canada). The geological interpretation of this deposit has been a source of controversy, but it is now generally agreed that it was formed as a result of an asteroid impact that triggered magmatic activity.

Sulphide ores associated with peridotites occur within flows of ultramafic lavas classified as komatiites, which usually contain elevated magnesium concentrations. The nickel sulphide ores are found at the base of the sequence of lava flows. The main ore mineral is pentlandite ((Fe,Ni)₉S₈); other common minerals include pyrrhotite (Fe_1–xS), chalcopyrite (CuFeS₂), pyrite (FeS₂), magnetite (Fe₃O₄), and iron-rich chromite (FeCr₂O₄). The ores are thought to have formed by the segregation of an immiscible sulphide-rich liquid from an ultrabasic magma, but the precise mechanism of the process is not fully understood.

Ores associated with intrusive dunites tend to occur in lens-shaped bodies in which the primary mineral is olivine or its breakdown products. One theory proposes that these deposits are the feeders for nickel sulphide deposits in komatiitic formations. Sulphide mineralization in these deposits tends to be disseminated throughout the body and is dominated by pyrrhotite and pentlandite phases. The origin of the deposits is considered to be similar to that of the volcanic–peridotitic ores, where segregation from an ultrabasic magma appears the most likely scenario. The disseminated nature of the deposits indicates, however, that segregation probably took place at a later stage than for the komatiitic-associated ores.

Elizabeth H. Bailey