Abstract

The atypical Cu-Ni mineralization in the central part of the Labrador trough is related to gabbro sills but is also slightly enriched in Au-(Co-Zn) and associated with intensely altered rocks along a thrust fault. Thus, it has features which suggest both magmatic and hydrothermal processes.The initial mineralization is of magmatic origin. Along a 13-km length of the Aulneau-Redcliff valley, which has a north-northwest trend parallel to the axis of the trough, are seven sulfide deposits included in seven synvolcanic gabbro sills of the Proterozoic Murdoch Formation. The mineralization is predominantly associated with a pyroxenitic facies of the gabbro.The host gabbro sills are found in a broad deformation zone, interpreted to contain the axial plane of an isoclinal anticline and a thrust fault, both striking north-northwest and dipping northeast. The sulfide deposits are elongate tabular bodies parallel to a well-developed stretching lineation that plunges 45 degrees NE. A part of the sulfide deposits has been remobilized by plastic flow, from the gabbro sills into the adjacent pyroclastic and sedimentary rocks.During the Hudsonian orogeny, hydrothermal Cu-Au mineralization associated with quartz-carbonate veinlets and intense chloritization was deposited in local shear zones cutting the gabbro. It was superimposed on the earlier formed Cu-Ni deposits. A gold-rich facies, associated with stilpnomelane-carbonate alteration, was deposited along contacts between the Cu-Ni sulfides and surrounding host rocks.The magmatic Cu-Ni deposits are interpreted to have been introduced with pyroxenitic gabbro sills, syn- to late volcanic in relation to the basalt of the Murdoch Formation. This mineralization is found at the base of the Murdoch Formation in the study area where it appears in the core of an anticlinal structure. The later hydrothermal Cu-Au mineralization is hosted by structurally competent rocks in the vicinity of the regional thrust and is in part superimposed on the earlier formed Cu-Ni mineralization.

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