Abstract

Lateritization of the Perseverance nickel-sulfide deposit, near Agnew, Western Australia, has produced mineralogical and chemical changes in the sulfides and serpentinized dunite host rocks culminating in a thick goethite-quartz-magnesite gossan. The trace and minor elements Au, Ag, Pd, Pt, Ir, As, Sb, Tl, Se, Co, and Cr were analyzed in 30 samples to assess their mobility under conditions of extreme chemical weathering. Gold, Pd, Pt, Se, As, Sb, and Tl are retained or enriched in samples from the oxidized zone, while Ag is lost. The additional metal must have been derived from the overlying gossan, since removed by erosion, because the metal abundances in the surrounding barren host rocks are too low to act as a potential source.The different sampling techniques employed in the study allow an assessment to be made of geochemical sampling procedures in weathered terrain. Grab samples generally yield the same trends as do channel samples; however, the grab samples display greater variability, reflecting small-scale heterogeneity in the fresh parent rocks.The observed absolute enrichments and depletions of the trace elements investigated in this study in the oxide zone at Perseverance indicate that their retention or loss is a complex process, not simply controlled by the Eh and pH fluctuations during lateritization.The enrichment of As and Sb in all the strongly oxidized weathered material and the retention of Tl in the lower parts of the oxide zone indicates that primary dispersion halos of these elements should persist in intensely weathered terrain. The use of Ir and Pd (and Pt) to discriminate barren gossans from those developed above nickel sulfides has been confirmed.

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