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The Main Sulphide Zone (MSZ) is a sulphide-enriched zone in the P1 pyroxenite layer of the Great Dyke, Zimbabwe, that contains platinum-group element (PGE), gold, nickel and copper mineralization. In the Zinca area of the Sebakwe subchamber the surface trace of the MSZ has been well established, topographic relief is subdued and weathering is typical of semi-arid, tropical areas. The Zinca area was, thus, chosen as a suitable location for an investigation of the weathering and secondary dispersion of PGE from the MSZ. Weathered rock, well-drained and poorly drained soils and stream sediments have been studied mineralogically and geochemically and compared with the primary mineralization in nearby drill-holes.

Weathered rock (1.5 m depth) retains the distinct metal distribution patterns of fresh rock. Levels of platinum in weathered rock are similar to those in fresh rock, although palladium shows weak evidence of variable mobility. In weathered rock the sulphides of the MSZ are pseudomorphically replaced by aggregates of goethite, limonite and chrysocolla, which faithfully retain the primary cuspate intercumulus textures that are observed in fresh rock. PGE-bearing minerals in weathered rock are dominantly platinum-iron alloys of variable composition, containing minor palladium, copper, bismuth and tellurium. Textures indicate that the primary assemblage of platinum-palladium sulphides and bismuth-tellurides has been altered in situ to platinum-iron and platinum-palladium alloys by leaching of sulphur, bismuth and tellurium during weathering.

Levels of nickel, copper, platinum, palladium and gold in soils are lower than in the underlying weathered MSZ, showing that physical soil processes have diluted and dispersed these metals. There is an indication that palladium is depleted relative to platinum, suggesting some hydromorphic dispersion of the former. Differences of oxidation state and pH between poorly drained black and well-drained red soils appear to make little difference to PGE levels and inter-element ratios.

Most of the metals of interest are enriched in the fine silt (-150 #) fraction of stream sediments, both absolutely and relative to background values. In stream sediments close to MSZ outcrop Pd/Pt ratios are significantly lower than in unweathered rock. Whereas platinum duplicate values are high and erratic in -150-# silt, those of palladium are lower and reproducible. The palladium dispersion train decays over a greater distance than that of platinum. This suggests strongly that platinum is physically dispersed as small discrete grains but that palladium is dispersed either hydromorphically or as very fine particles.

These results confirm earlier findings of the differential dispersion of platinum and palladium during secondary dispersion. They also indicate that platinum and, to some extent, palladium can remain stable while enclosed in rock in the form of metal alloys, becoming available for dissolution only after the breakdown of the rock during soil formation and dispersion.

The finer fractions of soil and stream sediments are the most suitable media for sampling and analysis of Pt and Pd. Analysis of heavy-mineral concentrates and of iron oxide granules does not indicate any significant increase in sensitivity for Pt, Pd or Au in this case.