Linking of regional and local hydrothermal systems in the mid-crust by shearing and faulting

The concentration of mass during regional metamorphism and metallogenesis r equires interactions between regional and local hydrothermal systems that a re scale- and time-dependent. During regional low- to medium-temperature me tamorphism in which fluid flow is driven by broad thermal or mechanical gra dients, fluid flow rates are (on average) < 10(-8) m/s, and are limited by restricted permeabilities and gentle gradients in hydraulic head. In contra st, coseismic fluid flow rates in major fault zones may reach greater than or equal to 10(-2) m/s. As a consequence, there may be a limited capacity f or individual faults to substantially control fluid flow in the region beca use of the large contrast in flow rates and permeabilities between faults a nd low permeability rocks. To produce a major epigenetic ore deposit in ove rpressured crust, fluid flow systems with such diverse flow rates must inte ract in such a way that metal dissolution and transport from regional fluid flow systems culminate in focused fluid flow with metal deposition. This i s best achieved through two steps; one in which km-scale hydrothermal syste ms channel into 100 m-scale ductile shear zones largely by reaction- and de formation-enhanced permeability in the shear zone; and a second one in whic h rapid fluid pressure changes accompanying brittle faulting (or intense sh earing) within the broader shear zones causes metal deposition and ore gene sis. (C) 2001 Elsevier Science B.V. All rights reserved.