FLUID INCLUSION, STABLE-ISOTOPE AND AR-AR EVIDENCE FOR THE AGE AND ORIGIN OF GOLD-BEARING QUARTZ VEINS AT MONT-CHEMIN, SWITZERLAND

A new Swiss gold occurrence at Mont Chemin, comprising gold-bearing qu artz veins, displays many characteristics that are typical of mesother mal gold deposits within the Alps and globally. The most notable of th ese features are: i) the presence of NaCl-H2O-CO2-bearing fluid with a n XCO2 of approximately 0.016 and NaCl equivalents in the range 4.6 to 10.6 weight percent, ii) greenschist formational temperatures and pre ssures in the range 265-285 degrees C and 700-1400 bars; and iii) the proximity of the occurrence to the Rhone-Simplon Line, a deep crustal structure in the Swiss Alps. Corrected Ar-Ar data for hydrothermal adu laria, considered to be comtemporaneous with mineral deposition from t he gold-bearing fluid, yields an age of 9.9 +/-1.0 Ma. Geothermal grad ients and uplift rates derived from the Ar-Ar age data and the geother mometry are in agreement with existing data for this region, and indic ate that the hydrothermal activity at the Mont Chemin gold occurrence records one of the last Alpine metamorphic events in the northeastern Mont Blanc massif. Temperature estimates from fluid-muscovite-quartz-f eldspar equilibrium and oxygen isotope thermometry of coexisting adula ria and quartz are combined with the fluid inclusion isochores to deri ve depositional pressures. These data yield geothermal gradients on th e order of 50 degrees C/km and uplift rates of 0.44 mm/a for the NE po rtion of the Mont Blanc massif.