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\section{Introduction}  There is abundant evidence for pore fluids and fracture processes in the upper crust to be physically and chemically coupled (\cite{Hubbert:1959ea}\cite{Hubbert and Rubey_1959}\cite{Garg_1973}\cite{Sibson_1981}{Frank:1965wi, (\cite{Hubbert:1959ea}\cite{Garg_1973}\cite{Sibson_1981}{Frank:1965wi,  Hubbert:1959he, Nur:1973ju, Sibson:1981kt}). Increases in pore fluid pressure, for example due to fluid injection, have been observed to lead to rupture and an increase in seismic activity. This is readily explained by the Navier-Coulomb criteria for brittle failure and the decrease in effective stress as a result of the elevated pore fluid pressure (Price NJ\cite{Price_1966}{Price:1966uh, Sibson:1981kt}). Conversely, the changes in groundwater levels and the surface effusions of warm water that sometimes occur along fault traces following earthquakes show that failure can also have a profound effect on fluid flow, heat and mass transport ({Nur:1974ht} Sibson et al., 1975, Sibson, 1981,). The flow is interpreted to result from either seismic pumping as a result of dilatancy diffusion type processes or a seismic valve mechanism in which fault rupture leads to leakage of an overpressured aquifer or reservoir of fluid (Nur, 1972, Nur et al., 1973, Sibson, 1981). Evidence for episodic and rapid flow of fluids associated with fracture and faulting in the geological record is more equivocal. The presence in exhumed fault systems of banded hydrothermal mineralisation of varying degrees of complexity is often taken as evidence of pulsed fluid flow driven by seismic activity. Rock failure may result in brecciation of earlier generations of veins and the opening of large dilation voids that are then cemented by mineral precipitation from upwelling hydrothermal fluids ({Wright:2009ej}). The extent, however, to which mineral precipitation in the veins is contemporaneous with, and directly coupled to failure is open to question. Observations of epithermal mineralisation associated with, for example dilation jogs between en-echelon fault segments suggests that fluid flow accompanied by rapid pressure fluctuations may trigger high level boiling, or effervescence and rapid degassing of hydrothermal fluids. This can result in precipitation of common gangue quartz and calcite as well as economic metalliferous deposits ({Sibson:1975cn, Sibson:1981kt, Sibson:1987dq, Henley:2000wk}).