ABSTRACT
Veins are mineral deposits left in fractures by flowing fluids. At low pressures, rocks are strong enough to sustain open fractures. Hot water, especially at the higher temperatures achievable at the higher pressures at depth, can dissolve considerable rock material. Open fracture networks permit deeply penetrating surface waters to flow into and out of hot regions by density-driven convection, and other hydrostatic forces. As the fluids migrate, the local conditions of pressure and temperature will change, resulting in the fluid cooling, boiling, mixing with other fluids, and so on. If the fluids become supersaturated with one or more solid phases, minerals will precipitate on the fracture walls. The minerals may grow freely into fluid-filled space, and can fill the space completely. If vein filling is incomplete, the resulting minerals can have fine crystal forms. If a single vein could be followed for a long distance, the precipitated mineral assemblage would probably change along its length, the result of changing conditions and fluid composition as it moved along the fracture.
