Flat vein formation in a transitional crustal setting by self-induced fluid pressure equilibrium - an example from the Geant Dormant gold mine, Canada

Gold-bearing veins grossly define a bimodal distribution within the Earth's crust as demonstrated by epithermal- (0 to 2 km) and mesothermal-type (sim ilar to 5 to 15 km) deposits. Vein formation in the epithermal and mesother mal environments is commonly attributed to the suction pump and the fault-v alve mechanisms, respectively. Characteristics of the Geant Dormant gold-be aring vein network are compatible with neither mechanism. In this paper, ve in morphology and geometry, alteration styles, and host rock characteristic s are used to constrain the following empirical parameters: tectonic regime , fluid flow vectors, crustal depth, host rock permeability, fluid pressure , mineral precipitating conditions and scale of filling processes (zoning). Based on these parameters, a three-stage model for vein formation is envis aged. The self-equilibrating mechanism involves the formation of mostly fla t veins stacked along cross-stratal dikes within an impermeable volcanic pi le. The dikes served as flow-restricted fluid feeders and as conduits for f luid discharge to the paleosurface. During the stable prefailure stage, the dike conduits acted as dampers in controlling the fluid discharge rate and in keeping fluid pressure at a constant level needed for the opening of pr eexisting fractures for vein formation (P-f approximate to sigma (1) + 0.3T ) at specific vertical intervals. The formation of veins corresponds to an equilibrating process that releases differential fluid pressure (DeltaP) bu ilt up vertically in the flow-restricted conduits. The DeltaP is induced by the decrease of the lithostatic pressure as longs as the hydrothermal flui ds move upward at a low velocity. At a critical state, when the deepest vei ns cannot physically absorb more fluid pressure accumulation, the excess fl uid pressure (DeltaP) is then transferred upwards along the QFP dikes, lead ing to the failure of the equilibrium process for vein formation at the net work-scale (failure stage). The postfailure stage involves draining to the paleosurface of the underlying pressurized hydrothermal reservoir. At an ad vanced state, hydrothermal self-sealing leads progressively to the restorat ion of the initial, prefailure, flow-restricted conditions of the dike cond uits. The proposed model involves a crustal depth of 2-5 km and a near-neut ral tectonic regime. These characteristics are intermediate to those involv ed for the suction pump and the fault-valve mechanisms and suggest that eac h tectonic regime has an optimal crustal depth for the formation of gold-be aring veins. (C) 2000 Elsevier Science B.V. All rights reserved.