Fluid transport in lineaments

Abstract
Fluid infiltration into fault zones and their deeper level counterparts, brittle-ductile shear zones, is examined in five different tectonic environments. In the 2.7 Ga Abitibi Greenstone Belt major tectonic discontinuities have lateral extents of hundreds of kilometres. These structures, initiated as listric normal faults accommodating rift extension of the greenstone belt, acted as sites for the extrusion of komatiitic magmas, and formed submarine scarps which delimit linear belts of clastic and chemical sediments. During reverse motion on the structures, accommodating shortening of the belt, these transcrustal faults were used as a conduit for the ascent of trondhjemitic magmas from the base of the crust, alkaline magmas from the asthenosphere, and for discharge of hundreds of cubic kilometres of hydrothermal fluids. Such fluids were characterized by δ18O = 6 ± 2, δD = —50 ± 20, δ13C = —4 ± 3, and temperatures of 270-450 °C, probably derived from devolatilization of crustal rocks undergoing prograde metamorphism. Hydrothermal fluids were more radiogenic (87Sr/86Sr = 0.7010-0.7040) and possessed higher values of μ than contemporaneous mantle, komatiites or tholeiites, and thus carried a contribution from older sialic basement. Mineralized faults possess enrichments of l.i.l. elements, including K, Rb, Li, Cs, B and C02, as well as rare elements such as Au, Ag, As, Sb, Se, Te, Bi, W. Fluids were characterized by XCO2≈ 0.1, neutral to slightly acidic pH, low salinity (less than 3% by mass), and K /N a ≈ 0.1, carried minor CH4, CO and N2, and underwent transient effervescence of CO2during decompression. At Yellowknife, a series of large-scale shear zones developed by brittle-ductile mechanisms, involving volume dilation with the migration ofca.5% (by mass) volatiles into the shear zone from surrounding metabasalts. This early deformation involved no departures in redox state or whole-rock δ18O from background states of Fe2/eFe = 0.7 and δ18O of 7-7.5 ‰ respectively, attesting to conditions of low water/rock ratios. Shear zones subsequently acted as high-permeability conduits for pulsed discharge of more than 9 km3of reduced metamorphic hydrothermal fluids at 360-450 °C. The West Bay Fault, a late major transcurrent structure, contains massive vein quartz that grew at 200-300 °C from fluids of 2- 6 % salinity (possibly formation brines). At the Grenville Front, translation was accommodated along two mylonite zones and an intervening boundary fault. The high-temperature (MZ II) and lowtemperature (MZ I) mylonite zones formed at 580-640 °C and 430-490 °C, respectively, in the presence of fluids of metamorphic origin, indigenous to the immediate rocks. A population of post-tectonic quartz veins occupying brittle fractures were precipitated from fluids with extremely negative δ18O at 200-300 °C. The water may have been derived from downward penetration into fault zones of low18O 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 500-540 °C, in the presence of metamorphic fluids and under conditions of low water/rock ratios, shear zones underwent local intense oxidation and desilication. All minerals undergo a shift of — 10‰ δ18O, indicating discharge up through the Archaean gneisses of formation brines recharged by meteoric water in the underlying Proterozoic sediments during overthrusting: about 1000 km3of solution passed through these structures. The shear zones and Proterozoic sediments are less radiogenic (87Sr/86Sr = 0.720) than contemporaneous Archaean gneisses (87Sr/86Sr = 0.900), corroborating transport of fluids and solutes through the structure from a large external reservoir. Major crustal detachment faults of Tertiary age in the Picacho Cordilleran metamorphic core complex of Arizona show an upward transition from undeformed granitic basement, through mylonitic to brecciated and hydrothermally altered counterparts. The highest tectonic levels are allochthonous, oxidatively altered Miocene volcanics, with hydrothermal sediments in listric normal fault basins. This transition is accompanied by a 12‰ increase in δ18O from 7 to 19, and a decrease of temperature of 400 °C, because of expulsion of large volumes of metamorphic fluids during detachment. In the Miocene allochthon, mixing occurred between cool downward-penetrating meteoric thermal waters and hot, deeper aqueous reservoirs. 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/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.
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