Abstract

Mineralogical and chemical studies have been made of the weathering profile developed from serpentinized dunite containing disseminated Ni-Fe sulfides. Sulfide oxidation proceeds with the replacement of pyrrhotite by marcasite, followed by the replacement of pentlandite by violarite, then the replacement of violarite by millerite, and finally by the decomposition of the sulfides. This sequence is interpreted in terms of local electrochemical reactions and is contrasted with supergene sulfide alteration of massive or matrix sulfides. Nickel, Mn, Co, Zn, and Cu are enriched in this part of the profile, termed the supergene zone. The serpentine host rock is bleached and some Mg is lost.With increased weathering, the serpentine becomes progressively replaced by clay minerals, and substantial amounts of quartz and dolomite are deposited. Geochemically, this zone, termed the quartz-dolomite zone, is marked by sharp decreases in S and Mg and by generally increasing Si, Fe, Ni, Cu, Mn, Co, Zn, Al, Ti, V, As, and Ba. Overlying this zone is the ferruginous zone, characterized by the dominance of goethite, the common presence of clay minerals, and the absence of serpentine. Manganese oxide minerals containing Ni, Co, and Cu are precipitated in the lower part of this zone, creating a local enrichment of these elements; in the upper part of the ferruginous zone these elements are depleted. Aluminum, Cr, Ti, V, Zr, and As increase toward the top of this zone, apparently owing to the introduction of these components from outside the serpentinite body. Toward the top of this same zone, original rock textures are destroyed and concretionary pisolites develop. The overlying hardpan-clay zone is developed from transported detritus.The Mt. Keith weathering profile is broadly similar to profiles developed on other serpentinized dunites in Western Australia, and particular differences can be explained by differences in their primary mineralogy and geomorphological setting. There does not appear to be a simple relationship between the Ni contents of the primary zones and those of derived surficial materials.

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