Metamorphism and metallogeny in the Eastern Alps

The two Alpine orogenic phases of the Eastern Alps, in the Cretaceous and i n the Tertiary, were both accompanied by the formation of mineral deposits. However, subduction-related magmatic belts as well as the typical "Andean" ore deposits are missing. Therefore, the role of metamorphism in East Alpi ne metallogeny was tentatively explored for more than 60 y, although for a long time without tangible results. Microthermometric, geochemical and isot opic investigations of fluid inclusions from selected Alpine mineral deposi ts presented allow a preliminary confirmation of the involvement of metamor phic fluids in their origin. Deposits which were formed immediately after t he first, Cretaceous orogeny, were produced at high pressures by fluids of very high salinity and high density, and with an isotopic composition of th e water falling into the metamorphic field. These fluids are best understoo d as products of metamorphic de-volatilization of rocks of the subducted So uth Pennine domain. In contrast to this, the deposits formed after the seco nd, Tertiary orogeny, originated at relatively low pressures from fluids wi th an appreciable content of CO2 and of low to moderate salinities. Isotopi c compositions of this carbon indicate a deep crustal or even mantle source for CO2, while the water is isotopically more heterogeneous and may have m ixed sources, both surficial and metamorphic. Tectonic control of these min eralizations is late-orogenic trans-tensional faulting, which exposed hot m etamorphic rocks to fluid convection along brittle structures. These deposi ts conform best to the model of metamorphogenic metallogenesis by retrograd e leaching, although ponded metamorphic fluids and mantle volatiles may als o have been involved.