Basement-hosted quartz-barite sulfide veins in the French Alps; a record of alpine tectonic fluid expulsion in the external crystalline massifs; structural, fluid inclusion, and isotope (S and Sr) evidences

Abstract

Quartz-barite sulfide veins hosted by a late Variscan monzonite in the polymetamorphic basement of the Aiguilles Rouges massif, western Alps, France, have been the subject of a combined structural, microthermometric fluid inclusion, and sulfur and strontium isotope study. The mineralized veins have a maximum age of 40 Ma and were emplaced along vertical tension fractures formed, or possibly reopened, during the Mesoalpine thrust phase. Two mineralization stages have been recognized: a major stage of quartz-barite veins with minor and irregularly distributed sulfides and sulfosalts, and a minor crosscutting stage of quartz-stibnite veins. Fluid inclusion microthermometry, homogeneous 87 Sr/ 86 Sr ratios of the barite, and the texture and mineralogical composition of the polymetallic quartz-barite veins indicate that they were formed mainly due to the cooling of a single aqueous fluid with a salinity around 9 wt percent NaCl equiv. Contemporaneous pressure variations cannot be excluded. Sulfur isotope thermometry yields formation temperatures between 255 degrees and 270 degrees C, and combined with the fluid inclusion isochores they indicate pressures of 0.45 to 1.55 kbars, corresponding to a depth of 3.6 (super +2.3) (sub -1.9) km assuming lithostatic conditions. Sulfur in the fluid was dominated by H 2 S which was oxidized at the ore site due to cooling of the fluid. The vein barites are enriched in 87 Sr with respect to the host rocks and indicate that the strontium and the barium were leached from deeper lithologies. After the emplacement of the quartz-barite veins, the hydrothermal system was opened toward shallower structural levels, allowing the ingress of meteoric water. The late quartz-stibnite veins are associated with a distinct and warmer CO 2 -bearing fluid. We conclude that the H 2 O-NaCl ore-bearing fluid originated at a depth of about 15 km by dewatering of Mesozoic sedimentary rocks during the collisional-thrust Mesoalpine phase. Subhorizontal thrust planes between the basement crystalline rocks and overlying sedimentary units inhibited mixing or regeneration of the system with superficial fluids. Subvertical fluid migration was restricted to faults in the polymetamorphic basement. Late Alpine extension tectonics allowed the upward migration of deep CO 2 -bearing fluids and also favored the influx of surface fluids that had remained isolated from the H 2 O-NaCl hydrothermal system.