Abstract

Lateritic weathering of massive and disseminated sulfides, consisting largely of pyrrhotite and pentlandite, has produced a supergene pyrite-violarite assemblage that has been replaced by goethite, carbonates, and quartz toward the surface. In the zone of supergene sulfides, iron released by the conversion of pyrrhotite to pyrite is fixed either as siderite or as magnetite, depending on the local HCO 3 (super -) activity. The depth of supergene alteration is controlled by electronic conductivity in the massive sulfides and by ground-water permeability in the disseminated sulfides. Supergene nickel enrichment of the sulfides is not economically significant.The Oxide Zone is dominated by goethite, with carbonates toward the base and heavy silicification toward the surface. Sulfide relict textures are well preserved. The depth of the Oxide Zone was controlled by Cenozoic water table fluctuations, permeable fault zones, and the presence of sulfides. Major and minor elements in the Oxide Zone have been redistributed into a pattern consistent with laterite weathering profiles in ultramafic rock. Nickel enrichment at about 23 meters from surface is due to the presence of takovite, a green hydrocarbonate of nickel and aluminum.The development of the supergene profile is attributed dominantly to an episode of lateritic weathering between the Eocene and middle Miocene epochs. Some subsequent modifications have taken place, including silicification and erosion. It is unlikely that significant supergene alteration is occurring at present.

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