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A basaltic tephra layer consisting of brownish‐olive glass shards. and about 0.2 mm thick. was found in cores from four lakes in northwest Germany. According to pollen analysis it was deposited during the early Boreal period (corresponding to about 8700 BP). The petrographic properties. the geochemical composition and the age agree with those of the Saksunarvatn tephra. which was first found on the Faroe Islands. The position of the tephra layer in the pollen stratigraphy and in the absolute time‐scale is discussed. Procedures for locating the tephra in other cores are suggested.

The present work focuses on the description of the assemblage of detrital platinum-group minerals (PGM) found in rivers draining the Great Dyke. This PGM assemblage distinctly contrasts with the suite of PGM in the pristine, sulfide-bearing Main Sulfide Zone (MSZ) of the Great Dyke, the assumed source of the detrital PGM. Specifically, PGE-bismuthotellurides and-sulfarsenides, common in the MSZ ores, and PGE-oxides or-hydroxides present in the oxidized MSZ, are missing in the assemblage of detrital PGM in the fluvial environment. Instead, conspicuously high proportions of grains of Pt-Fe alloy are common in the sediments, followed by sperrylite, cooperite, braggite, laurite, rare Pd-Sb-As compounds, and Os-Ir-Ru alloys. Possibly, some of the Pt-Fe alloy grains originate from the MSZ, but the majority appear to represent true neo-formations that formed in the course of weathering of the MSZ ores and concomitant supergene redistribution of the ore elements. Sperrylite, cooperite/braggite, and laurite appear to be direct descendants from the primary MSZ. Rare Pd-dominated minerals (Pd-Hg ± As and Pd-Sb ± As) are considered to be neo-formations that probably formed from dispersed elements during supergene processes. Os-Ir-Ru alloy grains, present in the samples from the Umtebekwe River in the Shurugwi area, possibly originate from the Archean chromitite deposits of the Shurugwi greenstone belt and not from the Great Dyke. Geochemically, the Pt/Pd proportions increase from pristine via oxidized MSZ ores to the fluviatile environment, corroborating earlier findings that Pd is more mobile than Pt and is dispersed in the supergene environment. Detrital PGM can be expected to be present in rivers draining PGE-bearing layered intrusions, and economic placers may form under particular sedimentological conditions. Therefore, the work also highlights the fact that simple field methods have their value in mineral exploration, especially if they are combined with modern micro-analytical methods. Furthermore, it is established that the PGE-bismu thotel lurides, PGE-sulfarsenides, and PGE-oxides, dominating the PGM assemblages in the pristine and oxidized MSZ, are components that are unstable during weathering and mechanical transport. Including the genetically somewhat disputed Pt-Fe alloys, the order of decreasing stability in the supergene environment is as follows: (1) Pt-Fe and Os-Ir-Ru alloys (very stable) → (2) sperrylite (stable) → (3) cooperite/braggite (variably stable/"metastable") → (4) PGE-bismuthotellurides, PGE-sulfarsenides, and PGE-oxides (unstable).

T h e r m o d y n a m i c analysis of the stability conditions for gold-silver solid solutions reveals that the silver c o m p o n e n t is unstable with respect to chlorargyrite and silver sulfate in most weathering environments. As a consequence, supergene (secondary) gold and gold found in modern river placers (detrital and authigenic) tends to be pure. Impure, detrital placer gold is subject to natural purification processes as evidenced by silver depleted, spongey grain margins. In contrast, under hydrothermal conditions the silver content of primary gold can be controlled by equilibrium reactions with argentite.

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