generated. The point of the circle that first touches the envelope represents the plane along which a fracture forms. A newly formed fracture leads to changes in the stress field and tensile strength of the fractured rock and causes a drop in stress magnitude. If a stress increases again, a new fracture will most likely be generated along the same fracture plane. This process is known as the crack-seal mechanism Crack-seal veins are thought to form quite quickly during deformation by precipitation of minerals within incipient fractures. This happens swiftly by geologic standards, because pressures and deformation mean that large open spaces cannot be maintained; generally the space is in the order of millimeters or micrometers. Veins grow in thickness by reopening of the vein fracture and progressive deposition of minerals on the growth surface. Tectonic implications Veins generally need either hydraulic pressure in excess of hydrostatic pressure (to form hydraulic fractures or hydrofracture breccias) or they need open spaces or fractures, which requires a plane of extension within the rock mass. In all cases except brecciation, therefore, a vein measures the plane of extension within the rock mass, give or take a sizeable bit of error. Measurement of enough veins will statistically form a plane of principal extension. In ductilely deforming compressional regimes, this can in turn give information on the stresses active at the time of vein formation. In extensionally deforming regimes, the veins occur roughly normal to the axis of extension. Mineralization and veining Veins are common features in rocks and are evidence of fluid flow in fracture systems. Veins provide information on stress, strain, pressure, temperature, fluid origin and fluid composition during their formation. Typical examples include gold lodes, as well as skarn mineralisation. Hydrofracture breccias are classic targets for ore exploration as there is plenty of fluid flow and open space to deposit ore minerals. Ores related to hydrothermal mineralisation, which are associated with vein material, may be composed of vein material and/or the rock in which the vein is hosted. Gold-bearing veins In many gold mines exploited during the gold rushes of the 19th century, vein material alone was typically sought as ore material. In most of today's mines, ore material is primarily composed of the veins and some component of the wall rocks which surrounds the veins. The difference between 19th-century and 21st-century mining techniques and the type of ore sought is based