GEOLOGY, GENESIS, AND METAMORPHIC HISTORY OF THE NAMEW LAKE NI-CU DEPOSIT, MANITOBA

The Namew Lake Fe-Ni-Cu sulfide deposit, 60 km south of Flin Flon, Man itoba, is hosted by an undifferentiated metapyroxenite sill enclosed w ithin Early Proterozoic orthogneisses and covered by Paleozoic sedimen tary rocks. The Namew Lake pyroxenite contains a relict igneous minera l assemblage of olivine + orthopyroxene + clinopyroxene +/- spinel +/- sulfides, suggesting derivation from a low Mg komatiitic or high Mg b asaltic magma. Sulfide ore in the Namew Lake pyroxenite occurs as 3- t o 10-mm blebs of disseminated Fe-Ni-Cu sulfides that accumulated durin g crystallization of olivine and pyroxene. A 0.5- to 5-m-thick layer o f massive sulfide at the banging-wall contact may represent a gravitat ionally segregated basal layer of a sill and suggests that the mine st ratigraphy it inverted. Although the massive sulfides were composition ally modified during metamorphism and supergene alteration, the dissem inated sulfides exhibit coherent geochemical trends and may presence t he average composition of the sulfide melt (recalculated to 100% sulfi de: ca. 36% S, 42% Fe, 16% Ni, 6.1% Cu, 0.23% Zn, 148 ppm Pb, 5,300 pp b Pd, and 88 ppb Ir). Assuming a silicate/sulfide mass ratio of ca. 56 0 calculated from the platinum-group element (PGE) contents, the Ni co ntent of a parental magma that would be in equilibrium with sulfides o f that composition is similar to that of typical komatiites, but the C u, Zn, and Pb concentrations are much higher. Thus, a komatiitic magma with ordinary Ni and PGE contents may have partially assimilated volc anogenic Cu-Zn-Pb sulfides, resulting in a high Mg basaltic magma from which the silicates and sulfides subsequently crystallized and equili brated. The Namew Lake pyroxenite and associated sulfides were affecte d 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 displacemen t of the sulfides updip. D-4 shearing locally increased the thickness of the pyroxenite. D-5 and subsequent deformations produced brittle fa ulting with minor displacement, open folds of the ore horizon, and lar ge-scale domain-bounding faults. The Namew Lake pyroxenite was metamor phosed at upper amphibolite facies (M-1), at lower amphibolite facies (M-3), and at greenschist facies (M-5). M-1 metamorphism of the pyroxe nite produced partial replacement of the host pyroxenite by pargasitic amphibole. M-3 produced successive partial replacement by talc + dolo mite, tremolite, and biotite, with greatest alteration att he edges of the pyroxenite and near crosscutting dikes. M-5 produced minor serpen tine and cummingtonite. Most or all elements, including rare earth ele ments (REE), were mobile to varying degrees juring M-1, M-3, and M-5, resulting in the observed compositional variability of the Namew Lake pyroxenite.