Abstract
ABSTRACTHeavy mineral concentrates from rivers and river terraces near York, Freetown Peninsula, Sierra Leone have been examined for their platinum-group mineral (PGM) content. The alluvial PGM are 0.1 to 1.5 mm in size and include Cu-bearing isoferroplatinum (Pt3Fe) and disordered Pt3–xFe (x≤ 0.38), tulameenite (Pt2FeCu), hongshiite (PtCu), cooperite–vysotskite (PtS–PdS), laurite (RuS2), erlichmanite (OsS2), Os-Ir alloy, Os-Ru alloy and native copper.Are the alluvial nuggets primary or a neoformation? Comparison of the PGM mineralogy of fresh rocks, weathered rocks and the saprolite, with the alluvial suite shows strongly contrasting features highlighted by the mineral assemblage. Cooperite in the fresh rocks is rare in the alluvium whilst Pt-Fe alloys become more abundant. Oxidized PGM are a feature only of the weathering process and disordering of the Pt-Fe alloys develops during weathering. Palladium is much less abundant in the alluvial suite than in the primary minerals whereas Cu, present as Cu-sulfides in the fresh rocks, occurs in the alluvium as a minor component of the Pt-Fe alloys and as hongshiite alteration to the Pt-Fe alloys. The size difference is striking; the primary mineralogy is micrometre-sized whereas the alluvial PGM are three orders of magnitude larger. Delicate PGM with alteration textures are seen only in the weathered rocks whilst delicate dendritic PGM are reported only from the alluvial suite. An organic coating to the alluvial PGM may be indicative of an organic or bacterial involvement. Some alluvial PGM occur in a drainage basin devoid of outcrops of PGE-bearing horizons.Together these contrasting features of the primary and placer PGM support the proposal that the Freetown nuggets developed as a result of breakdown of the primary PGM during weathering, movement of the PGE in solution, and growth of new PGM in placers with a different mineral assemblage, mineralogy and mineral chemistry.
Highlights
ALLUVIAL platinum-group minerals (PGM) in the western hills of the Freetown Peninsula, Sierra Leone, West Africa were first found in 1926 (Junner, 1929, 1930; Pollet, 1931, 1951).This paper is part of a special issue entitled ‘Critical-metal mineralogy and ore genesis’
Experiments with nuggets from the present study have shown that the ‘brass colour’ of the PGM is a coating of Fe- and Al-oxides and organic matter that can be removed by washing in warm 6M HCl for 1 to 2 days and reacting with 30% H2O2 for about one month
Contrasting explanations have been offered for the occurrence of alluvial PGM nuggets: (1) Nuggets derived from the erosion of igneous rocks can travel with soil that is washed down slope and into rivers to be trapped in patches of gravel that form a natural riffle
Summary
ALLUVIAL platinum-group minerals (PGM) in the western hills of the Freetown Peninsula, Sierra Leone, West Africa were first found in 1926 (Junner, 1929, 1930; Pollet, 1931, 1951). This paper is part of a special issue entitled ‘Critical-metal mineralogy and ore genesis’. The Applied Mineralogy Group of the Mineralogical Society and the IMA Commission on Ore Mineralogy have contributed to the costs of Open Access publication for this paper.
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