Peculiarities in the chemical composition of fine gold from rocks of the Ishlya graben (western slope of the Southern Urals)

The study material is terrigenous rocks of the Ishlya graben (western slope of the Southern Urals) represented by sericite-quartz siltstones, siltschists and carbonaceous shales, which have quartz-calcite veinlets with fine gold. Au forms isometrically irregular precipitates up to 5 µm in size, dispersed in a quartz matrix. By the analytical results, the composition of copper Au from quartz-calcite veinlets differs from analogs described for various sites on the western sloped of the Southern Urals (Tolparovo ore occurrence and metamorphosed gabbroids of the Kurmanai complex). These sites have been identified for a general trend, particularly the reducing amount of copper and the increasing amount of gold with small variations in silver content. This trend is not interrelated and genetically determined. The composition of palladium-bearing Au is fundamentally different from analogs described at the Ural sites. The closest values are characteristic of magmatic sites: palladium-bearing gold from the Marathon ore occurrence, Canada, and, to some extent, palladium-bearing gold from the Norilsk deposits. One possible mechanism for the redistribution and concentration of gold is the formation of chloroauric acid, followed by the reduction of [AuCl4]– to metallic gold in an acidic environment via a reaction with Fe2+.