Alluvial native gold, tetraauricupride and AuSn2 from western Switzerland

The chemical composition of gold, gold alloys and gold-bearing intermetalli c compounds extracted from sediments collected in 16 rivers on the Molassic Plateau and in the Jura Mountains of Western Switzerland was analysed with an electron microprobe. Among the 70 particles studied, 68 consisted of Au -Ag alloys, one contained the mineral tetraauricupride (AuCu), and one the unnamed intermetallic compound AuSn2, No correlation between the compositio n of the gold particles and the catchment basins could be recognised. Au-Hg amalgams were found sporadically as thin coatings around native gold parti cles, and formed by interaction between detrital gold and anthropogenic Hg. Metallic Hg was actually observed in some of the heavy mineral concentrate s. The chemical composition of a tetraauricupride grain recovered from the Rio de Misery near Chandon (canton Fribourg), as well as the association wi th cupriferous native gold, indicate a natural origin for this mineral, who se occurrence is generally restricted to basic and ultrabasic-hosted Au min eralisations. The presence of an AuSn2, intermetallic compound in the Boiro n river (canton Vaud) is enigmatic. This compound has not yet been reported in nature, but the composition of the grain, its association with agentife rous native gold, the presence of frequent cassiterite grains, and the asso ciation with sediments highly enriched in humic acids, suggest a natural or igin for this compound.