Invisible and Microscopic Gold in Pyrite: New Data for Volcanogenic Massive Sulfide Ores of the Urals

The study of modes of occurrence of gold in sulfides (including "invisible gold") covers two giant slightly metamorphosed volcanogenic massive sulfide (VMS) deposits: Uchaly (intensively deformed) and Uzelginsk (altered by late hydrothermal processes) from the Magnitogorsk megazone (South Urals) as well as the small Valentorsk and Galkinsk deposits (nondeformed) from the Tagil megazone (North Urals). We used scanning and transmission electron microscopy, microprobe analysis and mass spectrometric analysis with laser ablation (LA-ICP-MS), thermochemical methods to understand the nature and form of gold in sulfides. For weakly metamorphosed VMS deposits relatively high contents of Au and other trace elements (Co, Se, Te, As, Sb, Hg) occur in chalcopyrite (Au 0.01-10.6 ppm, usually <5 ppm, INAA and LA-ICP-MS), sphalerite (Au 0.01-18.6 ppm, usually <1 ppm) and esspecially in pyrite (Au 0.01-29.4 ppm, usually <5 ppm). Minerals of these elements are more usual and size of the grains are coarser for deposits affected by late hydrothermal processes (including metamorphic ones).