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In brief...
Introduction
It is largely accepted that the development of Archaean ore deposits is related to the mobilisation of fluids through the crust, their channelling along shear zones and concentration in domains bounded by zones of low permeability. We present here a model of plumbing system that derives from 2D thermal modelling, 3D physical modelling, and field studies in the Pilbara and the Murchison Province of Western Australia.
Archaean Regional Finite Strain Field: Example from the Pilbara Craton.
In many Archaean cratons a number of modern structural studies argue that the regional structure, together with the characteristics of metamorphism and plutonism are compatible with the gravity-driven exhumation of granitic domes and foundering of greenstone basins. Archaean domes and basins correspond to the development of gravitational instabilities that follows the emplacement of greenstone covers. Mapping of regional finite strain fields in the Pilbara confirms that strain, in Archaean domes and basins, displays a large but coherent spatial variability. Planar fabric develops in domal position, planar-linear fabric develops at the contact between two granitic domes, and intense linear and vertical fabric develops at foliation triple junction where three or more granitic domes interact. Together these structures define a connected 3D network of permeable zones throughout the Archaean crust. This network is the backbone element of the Archaean plumbing system that has channelled and concentrated ore bearing fluids.
Foliation Triple Junctions and Crustal Scale Fluid Flow
Foliation triple junctions are of particular interest for fluid flow in Archaean cratons. The Warawoonna syncline in the Pilbara is a spectacular examples of triple junction. This region shows spectacular vertical stretching lineations in the greenstones. The Warawoonna syncline is pervasively intruded by quartz veins oriented at high angle to the direction of maximum finite stretching, whereas the greenstone is pervasively retrogressed into chlorite and talc schists. In the core of the triple junction, dykes of hydraulic breccias testify that pore pressure was at least transiently above confining pressure. In both the Warawoonna syncline and the Bagda batholith, structural and petrological features strongly suggest that foliation triple junctions represent vertical drains, a few hundred meters to a few kilometres in diameter, extending deep into the crust.
Thermal and Mechanical Modelling
Thermal and mechanical numerical modeling (Mareschal and West, 1979; West and Mareschal, 1980) as well as analogue modeling (Dixon and Summer, 1983) offer a firm physical basis for the development of crustal gravitational instability in the Archaean. In addition thermal modelings show that the emplacement of mantle plume as little effect of crustal geotherm unless it is emplaced at or near the base of the crust, a situation which can be ruled out because of the diachronism that commonly exists between the emplacement of the greenstones and the development of granitic domes.
2D thermal modelings show that triple junctions may have acted as heat-driven pumps during the emplacement and the cooling of granitic domes. As fluids flow down temperature, foliation triple junctions acted as sinks for fluid at an early stage of granites emplacement as heat flows away from the granite toward the triple junctions. At a later stage, as the temperature in the triple junctions rose above that of the surrounding granites, triple junctions released fluids collected early. The fluids flow upward using the vertical fabric of the triple junctions as pathway to the surface.
References
[1] West, G., and J.C., Mareschal, 1979. A model for Archaean tectonicm. Part 1. The thermal conditions. Can. J. Earth Sci., 16, 1942-1950.
[2] Mareschal, J.C., and G. West, 1980. A model for Archaean tectonicm. Part 2. Numerical models of vertical tectonism in greenstone belts. Can. J. Earth Sci., 17, 60-72.
[3] Dixon, J.M., and J.M., Summers. 1983. Patterns of total and incremental strain in subsiding troughs: experimental centrifuged models of inter-diapir synclines. Can. J. Earth Sci., 20, 1843-1861.
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