THE IMPORTANCE OF VEIN SELVAGING IN CONTROLLING THE INTENSITY AND CHARACTER OF SUBSURFACE ALTERATION IN HYDROTHERMAL SYSTEMS

The identity and abundance of secondary minerals produced when a hydrothermal fluid, initially in equilibrium with its host formation, convects or is otherwise propelled down a temperature gradient toward the surface are calculated with and without boiling. The calculations show that the mass of secondary minerals produced by a unit throughput of hydrothermal water is surprisingly dependent on selvaging. Mineral precipitation is over 1,000 times greater when chemical contact is maintained with the host formation than when the upwelling fluid is chemically isolated from the host rock by selvages. This means, for example, that a selvaged vein with only one-tenth the hydrothermal mineral alteration of a nonselvaged vein formed in a similar environment may nevertheless have transmitted over 100 times more hydrothermal fluid and precipitated 100 times the mass of ore minerals. These conclusions are for end-member equilibrium models. The magnitude of the effect suggests, however, that selvage formation will remain an important consideration in cases between the two end-member extremes and for cases subject to kinetic control.