Abstract

Suggestions that platinum-group minerals (PGM) may develop in supergene environments have created controversy. The opposing view is that they act solely as resistate phases during weathering. There is, however, significant evidence that weathering processes can dissolve the platinum-group elements (PGE) and permit their transport and deposition of PGM in eluvial deposits, in which the PGM are significantly different in grain size and morphology from those derived from an igneous source. We present here three complementary lines of evidence relating to the formation of eluvial and alluvial PGM deposits using the Freetown deposits (Sierra Leone) as the principal example. There is unambiguous partitioning of the more soluble palladium and gold into ground and surface waters, and preferential deposition of platinum in soils and eluvial and alluvial deposits. There is no preference of the PGE in the soils dominantly for the coarse fraction, as there is in the eluvial deposits. Simple mechanical concentration from the soil cannot, therefore, account for eluvial or alluvial deposits downslope. Polymeric acids (humic acids) have been extracted from the soils and are shown to contain compounds that have a high affinity for the PGE. These organic compounds could well provide a potential means of PGE transport in solution. These observations provide eloquent support for the importance of controls by organic species during supergene weathering for the movement of gold and the PGE.

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