THE MOBILITY OF THE PLATINUM-GROUP ELEMENTS IN THE SOILS OF THE FREETOWN PENINSULA, SIERRA-LEONE

Suggestions that platinum-group minerals (PGM) may develop in supergen e environments have created controversy. The opposing view is that the y act solely as resistate phases during weathering. There is, however, significant evidence that weathering processes can dissolve the plati num-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 th e formation of eluvial and alluvial PGM deposits using the Freetown de posits (Sierra Leone) as the principal example. There is unambiguous p artitioning of the more soluble palladium and gold into ground and sur face waters, and preferential deposition of platinum in soils and eluv ial and alluvial deposits. There is no preference of the PGE in the so ils dominantly for the coarse fraction, as there is in the eluvial dep osits. 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 cont ain compounds that have a high affinity for the PGE. These organic com pounds could well provide a potential means of PGE transport in soluti on. These observations provide eloquent support far the importance of controls by organic species during supergene weathering for the moveme nt of gold and the PGE.