Metallogenic, tectonic and geochemical evolution of the Cape Ray fault zone with emphasis on electrum mineralization

Wilton, Derek Harold Clement (1983) Metallogenic, tectonic and geochemical evolution of the Cape Ray fault zone with emphasis on electrum mineralization. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

The Cape Ray Fault Zone is host to two electrum/base metal sulphide-bearing quartz vein deposits. The deposits occur within an alaskitic granite (the Windowglass Hill Granite) and an intermixed graphite/ chlorite/ sericite schist unit (part of the Windsor Point Group). This study was initiated to determine the origin of these two deposits, but the metallogenesis could not be ascertained until the tectonic and lithogeochemical framework of the entire region was understood. -- The Cape Ray Fault cuts through an area of tectonic complexity in southwestern Newfoundland. The fault occurs within the Windsor Point Group, a northeasterly-striking, intermixed sequence of felsic and mafic pyroclastic/volcaniclastic and associated sedimentary rocks, which is centrally located between a tonalitic terrane to the northwest (the Cape Ray Granite) and an amphibolite facies gneiss unit, with synkinematic granite, to the southeast (the Port aux Basques Complex). The tonalite contains isolated metagabbroic remnants of an ophiolite sequence (the Long Range Mafic/Ultramafic Complex). The Windsor Point Group contains tonalitic fragments in the ignimbritic rhyolite at its base, and is in tectonic contact with the gneiss unit, wherein the gneiss is extensively retrogressed. The Windowglass Hill Granite intrude the Windsor Point Group in the northeastern part of the zone. Both sides of the fault zone were intruded by post-tectonic granites (the Strawberry Granite on the northwestern side and Isle aux Morts Brook Granite on the southeast). -- Geochronological determinations indicate that the tonalitic terrane is Ordovician in age, the Windsor Point Group and Windowglass Hill Granite are late Devonian, and the Strawberry/Isle aux Morts Brook Granites are early Carboniferous. -- Isograds defined in the Windsor Point Group indicate that metamorphic grade increases from lower to upper greenschist facies towards the SE, and that pressures are rather low. Within the Port aux Basques Complex metamorphic isograds record amphibolite facies parageneses, and pressures are estimated to be >2 kb higher. -- Except for the post-tectonic granites, all units in the fault zone were subjected to a three-phase deformational event of Acadian age which overprinted earlier fabrics in the gneiss complex, and produced D1 schistosities axial planar to F1 isoclinal folds with axes parallel to L1 mineral elongation lineations. D2 effected crenulation cleavages and assymmetrical, open to tight, F2 folds. D3 resulted in conjugate kink folding. D1 also produced mylonites within the Windsor Point Group. -- Geochemical evidence suggests that a) the tonalite and associated phases were derived as partial melts of the ophiolitic material with some continental crust involvement, b) the felsic volcanism in the Windsor Point Group, and the Windowglass Hill, Strawberry and Isle aux Morts Brook Granites formed from partial melts of underlying continental crust, and c) the Port aux Basques Complex was derived from felsic volcanic rocks with a probable island arc affinity (though continental crust is also implicated in their origin). The entire Cape Ray Fault Zone region appears to be a completely allocthonous terrane, from tonalite/ophiolite in the northwest to a meta-felsic eruptive igneous complex to the southeast, which was emplaced on Grenvillian basement. -- Wall rocks to the electrum-bearing quartz veins experienced a potassic alteration with LREE-depletion and minor elevations in chalcophile element concentrations. Background Au and Ag abundances are typically low, but there are elevations in those units in the immediate vicinity of the Windowglass Hill Granite. -- Ore metals in the showings are Au, Ag, Cu, Pb and Zn, and distributions resemble those seen in auriferous massive sulphide deposits. The minerals present are galena, sphalerite, chalcopyrite, pyrite and electrum with rare arsenopyrite; all of which are extensively intergrown. Temperatures of ore formation (from arsenopyrite and equilibrium sulphide mineral intergrowths) appear to be around 300°C. -- Sulphur isotope ratios indicate sulphur in the showings is of magmatic origin. Pb isotope ratios in galenas from the showings and felsic volcanic members of the Windsor Point Group are similar and have an oceanic crust/mantle isotopic signature. -- The precious/base metal-bearing quartz veins were deposited by hydrothermal fluids evolved as a vapour phase from the Windowglass Hill Granite. The ultimate origin of the magma was from partial melting of underlying Grenvillian crust. This same crustal material is implicated in the origins of the other granites.

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