Nickeliferous Laterite Deposits

Abstract

Nickeliferous laterite deposits comprise in situ lateritic weathering products developed from peridotites. Nickel enrichment in the laterite profile is largely derived from olivine or serpentine. Four zones are noted in normal, uneroded profiles: (1) the upper zone—transported limonite and ferricrete, (2) in situ limonite, (3) intermediate zone—either nontronite or silica boxwork, and (4) lower zone—saprolite. Profiles are classed into four major types on the basis of the serpentine content of the host and the presence or absence of the intermediate zone. Profiles without an intermediate zone are characteristic of the humid equatorial zone or other locales with very high rainfall and minimal dry season. They may result from efficient leaching without reaching supersaturation conditions for smectite clays in the saprolite zone. They show marked mineralogical and structural variation from profiles developed over unser-pentinized peridotite (type Ia) through partly serpentinized hosts (type Ib) to serpentinite hosts (type Ic). Type Ia is characterized by a heterogeneous structure of partially leached joint blocks of saprolite with fresh rock cores and interblock veins of quartz and garnierite. Type Ic consists of a relatively homogeneous saprolite in which the principal mineral is parent rock serpentine which has undergone partial substitution of Mg by Ni and Fe+3. Type Ib, the commonest, has characteristics intermediate between Ia and Ic. Profiles with silica boxwork zones or nontronite zones typically occur in less humid tropical climates with a marked dry season. In general they result from inefficient or relatively slow leaching attaining supersaturation conditions for smectite clays within the saprolite zone. Elements may be strongly leached, supergene enriched, or residually concentrated as follows: leached: Mg, Si, Ca; supergene: Ni, Mn, Co, Zn, Y; and residual: Fe, Cr, Al, Ti, S, Sc, Cu. The leached elements show a loss which decreases approximately exponentially with depth. The supergene elements are concentrated in Mn, Co oxides; only nickel is enriched in supergene silicate phases. Nickel enrichment preferentially occurs in the saprolite zone, commonly on hills or moderate slopes where relatively strong fracturing or closely spaced jointing favors downward leaching. Differences in topographic control of laterite formation and nickel enrichment suggest three types of morphogenic development: (1) downwasting of an initially mountainous terrain to low foothills hosting nontronitic profiles possibly below a thick limonite; (2) downwasting of an initially mountainous terrain with cycles of renewed erosion, multiple terrace development, and deep Ni-enrichment root zones; and (3) erosion of a continental peneplain with development of a silica boxwork mesa and peripheral Ni-enrichment zones. Laterite ores develop from the decomposition of 20 to 100 m of peridotite rock. Many laterites are currently evolving at a rate such that a million years is adequate to develop most ore profiles; however, indirect age and geomorphic evidence suggest that development has, in most cases, taken place in stages over a time span of about mid-Tertiary to the present.