Cyclic fluid infiltration in structurally controlled Ag–Pb–Cu occurrences (Schladming, Eastern Alps)

Polysulphide occurrences in the Schladming Crystalline Complex, Eastern Alps, are associated with the intersection of moderately N-dipping shear zones (Type-I structures) and NW–SE and NE–SW trending steep faults (Type-II structures). Geochemical and mineralogical profiling across the Type-I structures shows progressive mineral alteration (feldspar breakdown, silicification, sericitisation) from the margin to the center. These observations are best interpreted by an external acidic fluid infiltrating these structures. Such alteration halos in deep-seated structures are excellent leaching sites and transport channels for available metals. However, precipitation of ore took place at the contact with carbonates where metal-bearing complexes became unstable as a result of fluid neutralisation. The origin of this carbonate is linked with Type-II structures showing distinct carbonatisation but no wall rock alteration. It is therefore suggested that the ore occurrences in Type-I and Type-II structures were formed during the cyclic infiltration of an acidic and a CO2-bearing fluid. Fluid inclusions from quartz–carbonate–ore veins are consistent with this interpretation. PT estimates and the orientation of shear zones and faults are consistent with the geodynamic setting of Cretaceous cooling and exhumation of the Austroalpine realm east of the Tauern window. The exhumation of the Schladming Crystalline Complex is therefore interpreted as the setting for a cyclic process of increased fluid pressure, vein opening, fluid infiltration, ore precipitation and vein closure.