Gold‐quartz veins in metamorphic terranes and their bearing on the role of fluids in faulting

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Abstract
Gold‐quartz vein fields in metamorphic terranes such as greenstone belts provide evidence for the involvement of large volumes of fluids during faulting and may be products of seismic processes near the base of the seismogenic regime. In the Val d'Or district of the Abitibi greenstone belt, Canada, quartz‐tourmaline‐carbonate veins form a vein field (30 × 15 km) in the hanging wall of a crustal‐scale fault zone, which was the main channelway for upward migration of the deeply generated fluids. The veins occur in small high‐angle reverse faults and in adjacent horizontal extensional fractures extending up to 75 m in intact rocks. They have formed incrementally during active reverse faulting in response to crustal shortening, at depths corresponding to those at the base of the seismogenic zone in actively deforming crust. Detailed structural and fluid inclusion studies provide evidence for generally lithostatic but fluctuating fluid pressures (ΔPƒ of the order of 200 MPa) and for cyclic stress reversals during vein formation and provide good support for the fault valve model. A comparison of vein characteristics with “standard” earthquake rupture parameters suggests that each slip increment along veins in reverse faults was accompanied by a small earthquake (4 > M > 3 or less). The large vein field thus represents both the extent of fluid dispersion in the hanging wall of a crustal‐scale channelway and the distribution of small earthquakes integrated over the lifetime of the hydrothermal system. It is proposed that such small earthquakes along veins in reverse faults are related to large earthquakes (M > 6) nucleating near the base of the seismogenic regime along the nearby crustal‐scale fault, either as aftershocks or as a precursory smarm.