Growth mechanisms of metasomatic reaction veins in dolomite marbles from the Bergell Alps

Abstract

Tremolite and olivine reaction veins in dolomite marble inclusions in the Bergell granite formed by a crack-reaction-seal mechanism during the cooling history of the area. Brittle failure of stressed marble opened extension cracks that served as conduits for infiltrating silica-rich aqueous fluids. Reaction of the fluids at 450–550 °C with dolomite along the fracture walls resulted in partial replacement of dolomite by reaction products whose mineralogy was controlled mostly by temperature. Aqueous silica dissolved in the fluid in the central fracture was transported by diffusion from the fracture wall to the reaction front in the dolomite marble. The velocity of the replacement reaction'front in the marble itself was controlled by the slower of the two processes: the surface-reaction kinetics of the replacement reaction and the diffusion rate of silica to the reaction site. Reaction veins with very different reaction front morphologies occur. Tremolite veins always have straight reaction fronts parallel to the central fissure and formed at about 450°C. Olivine veins are typically bounded by highly irregular, wavy, undulating reaction fronts. O1 veins formed at about 550 °C. It is concluded that Trmolite-vein growth was controlled by surface-reaction kinetics, whereas diffusion kinetics controlled the growth of olivine veins. The difference in the morphology of the reaction front surface is probably a consequence of small scale texture variations in the deposited vein rock and associated porosity differences. Both types of veins formed in about 1000 years.