Progress in Understanding the Evolution of Nickel Laterites
J. Paul Golightly, 2010. "Progress in Understanding the Evolution of Nickel Laterites", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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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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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
VOLCANIC-ASSOCIATED and sedimentary-exhalative massive sulfide deposits on land account for more than one-half of the world's total past production and current reserves of zinc and lead, 7 percent of the copper, 18 percent of the silver, and a significant amount of gold and other by-product metals (Singer, 1995). A new source of these metals is now being considered for exploitation from deep-sea massive sulfide deposits. Because the oceans cover more than 70 percent of the Earth's surface, many expect the ocean floor to host a proportionately large number of these deposits. However, there have been few attempts to estimate the global mineral potential. Significant accumulations of metals from hydrothermal vents have been documented at some locations (e.g., 91.7 Mt of 2.06% Zn, 0.46% Cu, 58.5 g/t Co, 40.95 g/t Ag, and 0.51 g/t Au in the Atlantis II Deep of the Red Sea: Mustafa et al., 1984; Nawab, 1984; Guney et al., 1988). Even more metal is contained in deep-sea manganese nodules. Current estimates in the U.S. Geological Survey (USGS) mineral commodities summaries indicate a global resource of copper in deep-sea nodules of about 700 Mt. In the Pacific "high-grade" area, an estimated 34,000 Mt of nodules contain 7,500 Mt of Mn, 340 Mt of Ni, 265 Mt of Cu, and 78 Mt of Co (Morgan, 2000; Rona, 2003). A number of countries, including China, Japan, Korea, Russia, France, and Germany, are actively exploring this area.