Criteria and problems in estimating global lateritic nickel resources

Abstract

Although two-thirds of the present nickel production is derived from nickel sulfides, lateritic nickel reserves are estimated to be around three times as large as those of sulfides. This discrepancy is largely caused by the mineralogical complexity of lateritic nickel which precludes low-cost beneficiation by conventional physical methods. Nevertheless, lateritic nickel reserves are bound to become increasingly important because of expected metallurgical advances. Lateritic nickel deposits were formed from peridotitic rocks on peneplaned surfaces during a tropical or subtropical climate. The most important deposits were formed during the Tertiary and the lateritization process still continues in deposits situated in the present tropical or subtropical belts. Other deposits, like those in East Europe and Greece, were formed during much older periods (Palaeogene-Cretaceous). The various types of lateritic nickel deposits, covered or uncovered,in situ or transported, are listed and their characteristics briefly mentioned. Large differences exist in depth, grade, specific gravity, and mineralogical composition—the latter especially in the nickel silicate section as opposed to the overlying limonitic section. These large differences exist even within one individual mining area. Because of the mineralogical complexities and highly irregular boundaries of lateritic nickel deposits (especially the upper and lower boundaries of the nickel silicate section), thorough studies are required to determine the mineralogical composition and ratios of the components. Only then may it be possible to specify with confidence the most suitable metallurgical process which has a strong bearing on the cost and consequently the viability of the deposit. This condition makes the exploitability rating in the McKelvey classification diagram relatively more difficult. Because of the quickly changing geological attributes of deposits even within limited areas, it seems necessary to develop various deposit models for many, if not most, individual metallogenic regions. It does not seens justified or practical to use estimation methodologies based on straight extrapolation. This is especially the case when dealing with the young uncovered type of deposit which constitutes the bulk of global resources. Consequently methodologies no. I (areal value estimation. Cargill, Meyer, Picklyk, Urquidi, 1977) and no. 2 (volumetric estimation) are not considered suitable. It is thought that methodology no. 3 (abundance estimation) in conjunction with no. 5 (Delphi estimation) would work satisfactorily but, as could be expected, the most meaningful results should come from a combination of methodologies nos. 4 (deposit modeling), 5 (Delphi estimation), and 6 (integrated synthesis). A classification of leteritic nickel deposit types is proposed.