Geology of the Nemeiben Lake ultramafic complex, north-central Saskatchewan

CIM Bulletin, Vol. 75, No. 837, 1982

N.D. MACFARLANE and D.J. MOSSMAN, University of Saskatchewan, Department of Geological Sciences, Saskatoon, Saskatchewan

The Nemeiben Lake ultramafic complex, in the Churchill Province of the Canadian Shield in north-central Saskatchewan, is a small 1.6-km circular plug largely composed of high-alumina pyroxene cumulates forming a series of grossly concentric layers of clinopyroxenite, websterite and wehrlite. Minor dunite and gabbro are present in the northwestern part ofthepluton.Disseminated primary magmatic pyrrhotite, pentlandite, chalcopyrite, magnetite and chromite occur throughout the intrusion. Sulphides occur in proportions of economic interest in the northern outcrops. A secondary assemblage of finegrained oxides, sulphides and native metals occurs in serpen-tinized and uralitized peridotites and pyroxenites. Primary opaque minerals are largely altered to marcasite, violarite and hematite. The ores represent a primary magmatic assemblage extensively modified by metamorphism and, to a lesser extent, by supergene alteration.The complex intruded an Early Aphebian supracrustral succession during the waning stages of the Hudsonian orogeny. The metamorphicpeak during Hudsonian time reached middle to upper amphibolite fades, granitized the country rocks and contributed to the partial amphibolitization of the ultramafic pluton.The Nemeiben complex has a number of features in common with the zoned intrusions of southeastern Alaska, including: the diopsidic composition of clinopyroxene, highly magnesian olivines, ubiquitous primary magnetite, relatively high alumina content of clinopyroxenes and the unlikelihood that associated gabbroic rocks are derived from a magma common to the ultramafic rocks. However, at Nemeiben, or-thopyroxene occurs as a widespread postcumulus phase and, to a lesser extent, as a cumulus mineral. This fact, together with geochemical data on whole-rock and mineral compositions, indicates a tholeiitic parentage.The nickel-copper deposits of the Nemeiben Lake ultra-mafic complex are not currently economic, although standard technology for recovery of sulphide minerals in the clinopyroxenite (and websterite) would probably be adequate. Most ore minerals in the altered peridotites belong, to the finegrained secondary assemblage of opaques due to the formation of retrograde hydrous silicates and are probably too fine grained for standard recovery methods.

Keywords: Geology, Nemeiben Lake complex, Ultramafic complexes, Churchill Province, Nickel, Copper, Petrogenesis, La Ronge domain, Rottenstone domain.