Abstract: Fluid flow through a fractured granite core and leaching experiments on granite powder (at constant temperatures of 50 and 100°C and during cooling from 100 to 50°C) indicate that the dissolved silica content of the fluids originates from the dissolution of feldspars and phyllosilicates. The dissolution of quartz is not ruled out but it should be a minor phenomenon. The precipitation of quartz may control the dissolved Si content during constant temperature leaching. During cooling from 100 to 50°C chalcedony, alumino-silicates, and chlorite are all capable of precipitation, possibly leading to some clogging or sealing of fissures.