Geology, genesis, and metamorphic history of the Namew Lake Ni-Cu deposit, Manitoba

Abstract

The Namew Lake Fe-Ni-Cu sulfide deposit, 60 km south of Flin Flon, Manitoba, is hosted by an undifferentiated metapyroxenite sill enclosed within Early Proterozoic orthogneisses and covered by Paleozoic sedimentary rocks. The Namew Lake pyroxenite contains a relict igneous mineral assemblage of olivine + orthopyroxene + clinopyroxene + or - spinel + or - sulfides, suggesting derivation from a low Mg komatiitic or high Mg basaltic magma.Sulfide ore in the Namew Lake pyroxenite occurs as 3- to 10-mm blebs of disseminated Fe-Ni-Cu sulfides that accumulated during crystallization of olivine and pyroxene. A 0.5- to 5-m-thick layer of massive sulfide at the hanging-wall contact may represent a gravitationally segregated basal layer of a sill and suggests that the mine stratigraphy is inverted. Although the massive sulfides were compositionally modified during metamorphism and supergene alteration, the disseminated sulfides exhibit coherent geochemical trends and may preserve the average composition of the sulfide melt (recalculated to 100% sulfide: ca. 36% S, 42% Fe, 16% Ni, 6.1% Cu, 0.23% Zn, 148 ppm Pb, 5,300 ppb Pd, and 88 ppb Ir). Assuming a silicate/sulfide mass ratio of ca. 560 calculated from the platinum-group element (PGE) contents, the Ni content of a parental magma that would be in equilibrium with sulfides of that composition is similar to that of typical komatiites, but the Cu, Zn, and Pb concentrations are much higher. Thus, a komatiitic magma with ordinary Ni and PGE contents may have partially assimilated volcanogenic Cu-Zn-Pb sulfides, resulting in a high Mg basaltic magma from which the silicates and sulfides subsequently crystallized and equilibrated.The Namew Lake pyroxenite and associated sulfides were affected by five phases of deformation and three phases of metamorphism. The first deformational event in the area, D 1 , involved migmatization and isoclinal folding at upper amphibolite facies conditions. In addition, sheeted tonalites and diorites intruded prior to or during D 1 . D 2 folded the gneisses into domes, synforms, and antiforms. D 3 involved minor boudinage of the pyroxenite, intrusion of dikes, and displacement of the sulfides updip. D 4 shearing locally increased the thickness of the pyroxenite. D 5 and subsequent deformations produced brittle faulting with minor displacement, open folds of the ore horizon, and large-scale domain-bounding faults.The Namew Lake pyroxenite was metamorphosed at upper amphibolite facies (M 1 ), at lower amphibolite facies (M 3 ), and at greenschist facies (M 5 ). M 1 metamorphism of the pyroxenite produced partial replacement of the host pyroxenite by pargasitic amphibole. M 3 produced successive partial replacement by talc + dolomite, tremolite, and biotite, with greatest alteration at the edges of the pyroxenite and near crosscutting dikes. M 5 produced minor serpentine and cummingtonite. Most or all elements, including rare earth elements (REE), were mobile to varying degrees during M 1 , M 3 , and M 5 , resulting in the observed compositional variability of the Namew Lake pyroxenite.