Abstract

The Eastmain River deposit is located within the Archean Upper Eastmain River greenstone belt, approximately 320 km north of Chibougamau. The regional stratigraphy is dominated by ultramafic to felsic volcano-sedimentary units and aluminous sedimentary strata that have undergone amphibolite-grade regional metamorphism.The deposit is hosted by a narrow brittle–ductile shear zone striking northwest and dipping 45 °to the northeast. This shear zone affects a lithologically varied unit composed of MgO-rich rocks and mafic and felsic volcanics. Kinematic indicators within this deformation zone indicate thrust-sense displacement.Within this shear zone, surface drilling has outlined three ore zones characterized by deformed quartz–sulphide veins that form elongated lenses whose long axes parallel the steeply pitching stretching lineation, which plunges to the northeast.Hydrothermal alteration is restricted to within a few metres of the ore veins. It consists of two different mineral assemblages that are both associated with ore veins: a prograde garnet-bearing assemblage, and an epidote-bearing assemblage that locally retrogrades the garnet-bearing assemblage. The main pulse of Au mineralization appears to be relatively late, as suggested by grains of native gold occurring in fractures within quartz and sulphide.The general absence of carbonate alteration around the lodes in rocks generally susceptible to such a process contrasts strongly with Archean lode-gold deposits developed in greenschist-facies metamorphic terranes. Furthermore, the presence of garnet within ore veins raises a number of questions regarding the timing of mineralization of this deposit. On the other hand, the main pulse of Au mineralization appears relatively late, as is the case for the majority of Archean lode-gold deposits.Structural relations and alteration assemblages suggest a deeper structural level of emplacement, at least for the quartz-sulphide veins and the hydrothermal alteration. It is suggested that the Eastmain River deposit formed from a syntectonic hydrothermal system during peak metamorphic conditions.

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