Fluid participation in deep fault zones: Evidence from geological, geochemical, and 18O/16O relations

Fluid incursion into fault zones and their deeper level counterparts, brittle‐ductile shear zones, is examined in a number of different crustal environments. At the Grenville front, translation was accommodated along two mylonite zones and an associated boundary fault. The high‐ (MZ II) and low‐temperature (MZ I) mylonite zones formed at 580 to 640°C and 430 to 490°C, respectively, in the presence of fluids of metamorphic orgin indigenous to the immediate rocks. A population of posttectonic quartz veins occupying brittle fractures were precipitated from fluids with extremely negative δ 18 O at 200 to 300°C. The water may have been derived from downwards penetration into fault zones of low‐ 18 O precipitation on a mountain range induced by continental collision with, uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults. At Lagoa Real, Brazil, Archaean gneisses overlie Proterozoic sediments along thrust surfaces and contain brittle‐ductile shear zones locally occupied by uranium deposits. Following deformation at 500 to 540°C, in the presence of metamorphic fluids and under conditions of low water to rock ratio, shear zones underwent local intense oxidation and desilicification. All minerals undergo a shift of −10‰, indicating discharge of meteoric water recharged formation brines in the underlying Proterozoic sediments up through the Archaean gneisses during overthrusting: about 1000 km 3 of solutions passed through these structures. At Yellow‐knife, a series of large‐scale shear zones developed by brittle‐ductile mechanisms, involving volume dilation with migration of ∼5 wt % volatiles into the shear zone from surrounding metabasalts. This early deformation involved no departures in redox state or whole rock δ 18 O from background states of Fe 2+ /ΣFe = 0.72 and 7 to 7.5‰, respectively, attesting to conditions of low water/rock. Shear zones subsequently acted as high‐permeability conduits for pulsed discharge of >9 km 3 of reduced metamorphic hydrothermal fluids at 360–450°C. The West Bay fault, a late major transcurrent structure, contains massive bull quartz that grew at 200–300°C from fluids of 2 to 6‰ (possibly formation brines). In general, flow regimes in these fault and shear zones follow a sequence from conditions of high temperature and pressure with locally derived fluids at low water to rock ratios (during initiation of the structures) to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs. Later in the tectonic evolution and at shallower crustal levels there was incursion of oxidizing fluids from near‐surface reservoirs into the faults.