Abstract

Deposits of ferruginous nickeliferous lateritic soils formed by weathering in place of ultramafic rocks occur at several places in northwestern California and southwestern Oregon, mostly in the Klamath Mountains province. Most of the deposits have been derived from relatively fresh peridotite, although at least one deposit was formed on serpentinite. The accumulations of lateritic soil are on flat-lying to gently sloping surfaces in a terrain that has been extensively dissected and deeply entrenched.The thickness of the deposits ranges from a few feet to more than 50 feet, and may vary widely in any given deposit, for the bedrock surface is commonly highly irregular. The deposit at Nickel Mountain, Oregon, is unique, because it is the only one in which the nickel silicate, garnierite, is plentiful. At Nickel Mountain, garnierite, accompanied by abundant microcrystalline quartz, occurs in veinlike bodies and boxworks that lie at the base of the soil zone and persist in depth along joints and fractures into the underlying slightly weathered or fresh peridotite.Mineralogical studies and chemical analyses show that weathering of the ultramafic rocks destroyed olivine and much of the orthopyroxene and serpentine minerals. Secondary minerals formed are predominantly hydrated ferric oxides (goethite); minor amounts of a montmorillonite mineral, chlorite, and talc were also identified. The kaolin group and bauxite minerals were not found. Except for garnierite at the Nickel Mountain deposit, no nickel-bearing mineral was identified. Nickel may occur, however, in montmorillonite, chlorite, and talc where it substitutes for Mg to a limited extent; in serpentine minerals that were not destroyed by weathering; and possibly in combination with ferric hydroxides. Cobalt may also occur in the clay minerals and probably is associated with trace amounts of manganese oxide or hydroxides.The lateritic soils in northwestern California and southwestern Oregon are similar to soils formed by lateritic weathering of ultramafic rocks in Cuba, the Philippines, New Caledonia, and other tropical regions of the world. They have, however, a higher content of SiO 2 and MgO, and a lower content of Fe 2 O 3 than deposits formed in tropical climates. The California and Oregon deposits are considerably smaller and lower in average metal content which, because they are widely scattered in a rugged, relatively isolated terrain, makes them unpromising as likely reserves for commercial exploitation in the foreseeable future.Most of the deposits are regarded as having been formed by chemical weathering in a climate having alternating wet and dry seasons, probably similar to that prevailing today. Their age is uncertain but may range from post-Miocene to Pleistocene.

First Page Preview

First page PDF preview
You do not currently have access to this article.