Abstract

Consideration of chloride complexes with the platinum-group elements (Fuchs and Rose, 1974; Westland, 1981) has shown that they can enter into solution at low temperatures under acid conditions with a high Eh. Complexes may also be formed with other ligand ions or soil colloids under similar conditions. Studies of lateritic soil profiles indicate that they provide the required conditions of acidity and high Eh and that, within these soils, gold has been taken into solution (Mann, 1984). Under these conditions, gold may be dissolved, transported, and redeposited to form large nuggets or small crystals with well-defined crystal faces. The conditions required for the same processes to act upon the platinum-group elements are shown to be less extreme than those required for gold, and it is concluded that solutions similar to those which can transport gold can equally act upon the platinum-group metals.Evidence that these criteria should be seriously examined is provided by the distribution of platinum-group minerals in relation to the geology of the Freetown layered gabbro. In particular, this is indicated by well-preserved crystals of laurite-erlichmanite [(Os,Ru)S 2 ], Os-Ir alloys, and large nuggets of platinum-iron alloy present in laterites but apparently not present in the underlying anorthosite. The crystals are notable for their very smooth crystal faces and sharp interfacial edges which have clearly not suffered abrasion. Other crystals of laurite-erlichmanite show the effects of resorption and examples of a platelike overgrowth on the crystal faces are illustrated. In two cases it is clear that there has been a late-stage overgrowth of (Os,Ru)S 2 approximately 5 mu m in thickness on the outside of a grain which had earlier suffered resorption. These features and the distribution of their occurrence indicate that it is more likely that these platinum-group minerals have grown in situ in the laterite than that they remain after weathering of the underlying basic rocks. The textures also show that under the varying conditions to be found within a laterite, partial resorption and regrowth can occur.This process is capable of concentrating the platinum-group elements from a large volume of rock and subsoil into a horizon where conditions favor deposition to form easily accessible concentrations of platinum-group minerals which could become targets with economic significance.

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