Gold-oxysulphides in copper deposits of the Greywacke Zone, Austria: A mineral chemical and infrared fluid inclusion study

Abstract

Native gold and unusual gold-oxysulphides, both associated with tetrahedrite, occur in several abandoned copper deposits in the low grade Palaeozoic Greywacke Zone in the Eastern Alps. The ore mineralogy, the mineral chemistry and fluid inclusions in tetrahedrite and quartz were studied from historic Cu mines at Larzenbach/Hüttau, St. Veit im Pongau, and Mitterberg (all Salzburg Province, Austria). Fluid inclusions in tetrahedrite, which is non-transparent in visible light, were studied by infrared (IR) microthermometry; tetrahedrite containing up ~ 2.5 wt.% As is transparent for infrared light. Fluid inclusions within tetrahedrite occur as isolated inclusions, in clusters and along trails. At room temperatures these are aqueous two-phase liquid (L) + vapour (V) inclusions of low to moderate total salinity ( T m − 3 to − 14 °C). Homogenisation temperatures ( T h V→L) range between 85 and 200 °C. The composition of these tetrahedrite-hosted fluid inclusions corresponds to that one type of fluid inclusion in quartz showing a similar range in salinity ( T m − 3 to − 12 °C) and total homogenisation temperatures ( T h V→L, 71.5 to 250 °C). Another type of higher saline aqueous two-phase (L + V) inclusions in quartz is apparently unrelated to gold mineralisation. Fluid inclusions within tetrahedrite coexist with small solid inclusions of chalcopyrite, gold-oxysulphides, minor arsenopyrite and gold that is rich in gold-oxysulphide microinclusions and occur along re-healed microfractures within tetrahedrite. These microfractures also control the replacement of As-poor tetrahedrite by arseniferous (up to 3.16 wt.% As) tetrahedrite. It is suggested that in addition to cooling the exchange of As for Sb on tetrahedrite surfaces was one of the factors controlling precipitation of gold. Oxysulphides are associated with (a) native gold in unweathered primary ores and (b) remobilised Cu-rich gold in weathered ores. Gold thiosulphate and possibly thioarsenite complexes are the preferred species for the transport of gold in the low-temperature (< 250 °C) hydrothermal fluids and gold thiosulphate complexes were involved in the supergene remobilisation of gold.