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Abstract

The Abitibi greenstone belt, the largest and best-preserved Archean granite-greenstone complex in the world, consists of five major lithologic assemblages which formed in four distinctive geotectonic environments. These comprise: (1) a tholeiitic and komatiitic assemblage formed in an oceanic extensional environment; (2) a calc-alkalic volcanic and volcanic-derived sedimentary rock assemblage formed in an island-arc environment; (3) an assemblage of craton-derived quartzose sedimentary rocks with interbedded komatiitic volcanic rocks formed on a passive continental margin; and (4) an assemblage of molasse-type sedimentary rocks, alkalic volcanic rocks, and felsic intrusions. The oceanic, arc, and continental margin assemblages were imbricated and tectonically stacked during the collision of a northward-moving continent and its attached marginal sediments with the arc, as a result of subduction of the intervening ocean crust. The molasse assemblage formed along the collisional suture zone. Interbedded alkalic volcanic rocks, syn- to postdeformational felsic intrusions, and the CO2-rich hydrothermal fluids responsible for widespread hydrothermal alteration and gold mineralization, all broadly part of this molasse assemblage, were generated by linked lower crust-mantle processes. The effect of the high density of the oceanic rocks on pressure-temperature conditions at the base of the tectonically thickened crust may have been instrumental in triggering the processes which resulted in auriferous fluids. Anatectic melts and gold ore-forming hydrothermal fluids were generated at depth as an end-stage manifestation of the collisional event. Rocks of the oceanic assemblage were the possible source of the gold. This model provides a rational and internally consistent explanation for the association of gold mineralization with greenstone belts, major faults, molasse-type sedimentary rock assemblages, felsic porphyry intrusions, and widespread hydrothermal alteration. Also, the model is consistent with the late timing of the mineralization in the geologic development of the Abitibi belt in particular, and both Precambrian and Phanerozoic greenstone belts in general.

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