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Abstract

Nickel laterites are thick weathering profiles derived by leaching of ultramafic rocks by meteoric water. Olivine or derived serpentine provides the nickel. Profiles with economically significant deposits derive their Ni from 40-m (15−100 m, 10th−90th percentile range) thicknesses of protolith grading 0.16 to 0.3 percent Ni and 5.5 to 10.5 percent Fe. The profiles may be preserved in situ or transported to form a sedimentary unit that may be buried, lithified, and metamorphosed.

From bottom upward, in situ nickel laterites may be comprised of silicate saprolite, a nontronite clay zone, high Co and Mn limonite or ferruginous saprolite, low Co and Mn limonite, and allocthonous cover. Any of these units may be absent due to erosion or nondeposition and, importantly, one or all may be siliceous, usually due to quartz precipitation in the saprolite zone. Nickel is leached downward from the limonite zone, added to the saprolite and nontronite zones, and left residually enriched in limonite. Strong supergene enrichment requires downward leaching into saprolite and fractured rock above a deep water table. Zones of strong passive jointing and pre- or synweathering fracture zones all may lead to an order of magnitude increase in the rate of advance of the weathering front.

The rate of advance of the weathering front in tropical rain forest covered highlands is about 50m/m.y., regardless of whether the bed rock is ultramafic, dioritic, or felsic. Weathering fronts advance at progressively slower rates in terranes with less relief. Nickel laterite deposits accumulate on terraces or plateau landforms in karstlike basins or under semiarid peneplains.

The topographic controls of in situ nickel laterite deposits can be understood in terms of structural controls and three long-term climatic and topographic scenarios. The scenarios include: (1) permanently wet rain- forest setting in tectonically active terrane with moderate relief, (2) a formerly wet peneplain that has evolved toward aridity, and (3) a formerly arid peneplain setting that has evolved into a permanently wet environment.

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