Institute of Mineralogy and Petrography, University of Lausanne, Switzerland
The earliest Alpine event is recorded in a katophorite-paragonite schist hosted within the basement gneisses. The paragonites yield a total fusion 40Ar/39Ar age of 48 Ma. Mineral thermobarometry is consistent with formational temperatures in excess of 300oC, with minimum pressures of 1500 bars. A well defined pressure-temperature uplift path (Fig. 1) is recorded in a variety of overlapping Alpine events including stilpnomelane-epidote-quartz-calcite veins, quartz-chlorite veins, quartz-muscovite veins and a remobilization of the Variscan fluorite veins terminating with a gold mineralization event at 10 Ma (40Ar/39Ar, adularia, Fig. 2) at temperatures in the range 250 to 300oC with pressures from 500 to 1600 bars. The overall PTt path defines a geothermal gradient of 25oC/km, but portions of this PTt path are consistent with geothermal gradients as high as 50oC/km similar to those observed to the East along the Rhone-Simplon line. Fluid inclusion and stable isotope studies indicate a diverse source of Alpine fluids, ranging from metamorphic to meteoric with considerable fluid chemistry control exercised by the local granitic wallrocks. Fluids compositions range from highly saline brines with variable K:Na:Ca to the CO2 bearing fluids typically associated with higher grade metamorphic rocks and some mesothermal gold deposits.
Figure 1. Alpine pressure-temperature-time path. The derived pressure-temperature constraints for each locality are discussed in the text. The derived uplift rate of 0.44 mm/yr and a geothermal gradient of 50oC/km are based upon a maximum age of 10 Ma for the gold showing.
Figure 2. Temperature-time diagram showing the Alpine temperature-time constraints from Soom (1990) Seward and Mancktelow (1994) and the data from this study.