Alluvial native gold, tetraauricupride and AuSn2 from Western Switzerland

Meisser N., & Brugger J.

Alluvial native gold, tetraauricupride and AuSn2 from Western Switzerland.

2000, Schweiz. Mineral. Petrogr. Mitt., 80, 291-298

Abstract

The chemical composition of gold, gold alloys and gold-bearing intermetallic 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 tetra-auricupride (AuCu),and one the unnamed intermetallic compound AuSn2 .No correlation between the composition of the gold particles and the catchment basins could be recognised.Au -Hg amalgams were found sporadically as thin coatings around native gold particles,and formed by interaction between detrital gold and anthropogenic Hg.Metallic Hg was actually observed in some of the heavy mineral concentrates.The chemical composition of a tetraauricupride grain recovered from the Rio de Misery near Chandon (canton Fribourg),as well as the association with cupriferous native gold, indicate a natural origin for this mineral, whose occurrence is generally restricted to basic and ultrabasic-hosted Au mineralisations. The presence of an AuSn2 intermetallic compound in the Boiron river (canton Vaud)is enigmatic.This compound has not yet been reported in nature, but the composition of the grain, its association with argentiferous native gold, the presence of frequent cassiterite grains,all embedded in sediments highly enriched in humic acids, suggest a natural origin for this compound.

Keywords: alluvial minerals, native gold, tetrauricupride, AuSn2, anthropogenic mercury, Switzerland.