Time-dependent changes in fluid-rock interaction: an investigation of syntectonic vein microchemistry

Veins in rocks have been studied for well over 150 years, initially driven by interest in how economically mineralised veins formed (e.g. Buckland, 1836; Hulin, 1929). Veins provide information on how fluids migrate through the crust, and how material is transferred by these fluids. Many studies have suggested that veins (including ‘crack-seal’ textured veins; Ramsay, 1980) form as a result of brittle failure, fracture dilation, fluid infiltration and subsequent mineral precipitation (e.g. Ramsay, 1980; Cox and Etheridge, 1983; Sibson, 1985; Cox, 1987; Urai et al., 1991). However, some recent studies have also suggested that vein formation can occur without fracture, with vein growth driven by diffusional transport of material from the immediately surrounding rock mass (Means and Li, 2001; Wiltschko and Morse, 2001). The mechanism by which veins form has been increasingly debated over recent years, with argument centered over the necessity for void formation prior to vein growth, and whether veins record advective or diffusive mass transfer in the crust. Different vein textures (e.g. fibrous, elongate blocky, crack-seal, stretched crystal, blocky) form as vein opening and crystal growth kinetics vary (Ramsay, 1980; Cox and Etheridge, 1983; Cox, 1987; Urai et al., 1991; Oliver and Bons, 2001; Hilgers et al., 2001). Some previous workers have suggested that certain vein textures (e.g. fibrous textures) are related to diffusional mass transfer, while other textures (e.g. blocky) are related to advective flow regimes (Bons, 2000; Oliver and Bons, 2001). Fibrous veins are of particular interest, as they provide a record of progressive displacement paths during vein opening (Durney and Ramsay, 1973). Regularly oriented and spaced inclusion bands across mineral fibres are interpreted to indicate that fibrous to elongate blocky vein textures develop via a crack-seal mechanism (Ramsay, 1980).