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Open File Report 85

Geochemical exploration for platinum group elements in weathered terrain: P252 Final Report

Butt, C.R.M., Williams, P.A., Gray, D.J., Robertson, I.D.M., Schorin, K.H., Churchward, H.M., McAndrew, J., Barnes, S.J. and Tenhaeff, M.F.J.

This report summarizes the results of over three years of research that had the objectives of (i) supplementing existing knowledge of the aqueous geochemistry of platinum group elements (PGE) in the weathering environment, (ii) obtaining information concerning the distribution of these elements in the lateritic regoliths and the potential for supergene enrichment, and (iii) establishing criteria for exploration in lateritic terrain. These objectives were met by conducting detailed laboratory experiments and field-based geochemical investigations.

Laboratory studies demonstrated that, in addition to chloride ion and organic compounds, thiosulphate ion and arsenious acid could mobilize the PGE under weathering conditions. Mobilization as thiosulphate complexes might occur in the vicinity of sulphides oxidizing under neutral to alkaline conditions, whereas arsenious acid could only mobilize Pd. and then only in acid environments. The potential for mobilization by these ligands has not previously been considered, but may have significance in appropriate environments. Studies were also made of the processes that cause immobilisation of the PGE in the regolith. The PGE tend to be sorbed by most regolith materials, thus restricting their potential mobility, although in more organic samples, some redissolution occurred after a few months, possibly due to soluble organic species produced by biological activity. The results demonstrate that despite the theoretical models postulating mobility under specific physico-chemical conditions, the substrate will strongly influence the actual behaviour.

Field investigations on the distribution of PGE were carried out on the Ora Banda Sill, Western Australia, and the Tout Intrusion at Fifield, New South Wales. Lateritic regoliths are well preserved on the pyroxenites of the Ora Banda Sill and the duricrusts locally contain 1-2 ppm Pt + Pd. There has been some relative loss of Pd in the duricrust and, particularly, the ferruginous lag derived from it, but otherwise there seems to have been little mobilization of the PGE. The PGE enrichment appears to be residual and occurs in similar horizons, and by a similar factor, as other relatively immobile elements such as Cr, Cu, V, Ti and Zr. No PGE minerals or alloys were found in the regolith and selective leaching experiments suggest that both Pt and Pd occur predominantly in the minus 2 µm fraction. Platinum is mainly associated with hematite throughout the profile, implying early release from its primary host; in comparison, Pd is present in goethite, but only high in the profile, and is presumed to be released from a primary phase (e.g., chromite) late during weathering. Chromite compositions can discriminate between peridotite and pyroxenite in the regolith and could possibly be useful for indicating sulphide-rich zones within the bedrock. There has been rather greater secondary mobility of PGE, especially Pt. in the regolith over the serpentinized dunites of the Tout Complex, but again no secondary PGE phase was located and the mechanism of remobilisation could not be established, although organic or chloride complexes are most probable. PGE enrichment in alluvium is probably mechanical.

The surface exploration procedures of soil and lag sampling are effective in exploration in lateritic environments. Routine sampling of the laterite itself by shallow drilling may be the best general procedure, especially in areas where much of the laterite is buried. High Pt and Pd concentrations are themselves insufficient indicators, so that it is probably necessary to analyse selected samples for the other PGE. Copper, Cr and Ni are not effective pathfinder elements for none is necessarily associated with primary or secondary mineralisation at either Ora Banda or Fifield. The restricted mobility of the PGE and lack of suitable pathfinder elements may pose problems to effective exploration of areas eroded to the saprolite or shallow fresh subcrop, since the target will be small and any enlargement is thus dependent on limited physical dispersion at the surface.

Last updated: Friday, July 21, 2000 04:24 PM


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