Marshall

PACROFI VI - Electronic Program

Alpine Pressure-Temperature-time path for the N-E Mont-Blanc Massif: Fluid Inclusion, Isotopic and Thermobarometric evidence.

Dan Marshall, David Kirschner, Francois Bussy, H-R Pfeifer and J.C. Hunziker.

Institute of Mineralogy and Petrography, University of Lausanne, Switzerland

The Mont Chemin region at the N-E extreme of the Mont-Blanc massif, Canton Valais, Switzerland is predominantly comprised of the granitic rocks of the Mont-Blanc intrusive rock suite and the Mont-Blanc basement gneisses. Hosted within the metamorphic rocks are a variety of sub-economic Au and Pb-Zn-F deposits and veins. Fluid inclusions, fluid-mineral equilibria, stable and radiogenic isotope studies have been used to derive pressure, temperature, age (PTt) and fluid composition constraints for a number Alpine events.

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 300^oC, 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 300^oC with pressures from 500 to 1600 bars. The overall PTt path defines a geothermal gradient of 25^oC/km, but portions of this PTt path are consistent with geothermal gradients as high as 50^oC/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 50^oC/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.

References:

Seward D. and Mancktelow N.S. (1994) Neogene kinematics of the central and western Alps: Evidence from fission track dating. Geology v. 22, p. 803-806.
Soom M.A. (1990) Abkühlungs- und Hebungsgeschichte der Externmassive und der pennischen Decken beidseits der Simplon-Rhone-Linie seit dem Oligozän: Spaltspurdatierungen an Apatit/Zirkon und K-Ar-Datierungen an Biotit/Muscowit (Westliche Zentralapen). PhD. Thesis, University of Bern, 145 p.