Dissolution-precipitation processes induced by hot water in a fractured granite Part 1: Wall-rock alteration and vein deposition processes

Mass transfer and fluid-rock interactions have been investigated in zoned alteration halos around hydrothermal veins which crosscut the La Peyratte granite (Armorican Massif, France). Temperature conditions have reached 340°C in the veins, according to a previous fluid-inclusion study. The width of the vein varies from 300 μm to 20 μm and the mineral composition of the deposit changes : phengite (300 to 200 μm); adularia (50 μm); quartz and feldspar overgrowths ( 50 μm wide) and plengite around the larger ones (300 μm wide). Both chlorite and albite determine sharp fronts inside the altered rock. The width of zone 1 is 4 mm for the plengite vein and 1 mm for the adularia one. Zone 2 extends from the chlorite front the unaltered rock; only dissolution voids are observed in oligoclase crystals. The width of zone 2 is 12 mm for the phengite vein and 10 mm for the adularia one. Illite/smectite mixed-layer clays have precipitated in some of these voids. Considering zones 1 and 2, two observations have been made : 1, the smectite percentage increases from 40% to 70u toward the vein; 2, the volume of oligoclase replaced by white mica or dissolved decreases exponentially with increasing distance from the vein. The widths of the chloritized-biotite zone (W CH ) and of the zone with white mica in oligoclase (W WM ) increase with the width of the vein (W V ). W CH and W WM are linearly correlated indicating that the formation of chlorite and white mica are interdependent processes. Combining average chemical compositions and quantities of primary and secondary minerals, we have calculated the chemical mass balance in the two altered zones for a reference volume of rock (40 dm 3 ). Results obtained by this way are consistent with the mass balance calculated with Gresens' method. In spite of the strongly altered aspect of the rock, the bulk chemical composition is little changed, with addition of water and small amounts of sulphur, and loss of Si and Ca. The major part of the elements leached out of the primary minerals is consumed by the crystallization of secondary minerals in the altered zone and inside the fracture, through a sequence of interdependent mineral reactions. The end-product of this sequence is phengite or adularia (or overgrowths of primary minerals in narrow veinlets) which seal progressively the open fracture. After the sealing of the fracture, the temperature decreased and illite/smectite mixed-layer minerals precipitated in the oligoclase dissolution voids (<100°C)