The regional setting, primary mineralogy, and economic geology of the Nemeiben Lake ultramafic pluton
Macfarlane, Neil Daniel
The Nemeiben Lake ultramafic pluton, in the Churchill Province of the Canadian Shield in 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 is present in the northwestern reaches of the pluton. Disseminated primary magmatic pyrrhotite, pentlandite, chalcopyrite, magnetite, and chromite occur throughout the body and reach proportions of interest to prospectors in the northern outcrops. A secondary assemblage of fine-grained oxides, sulphides, and native metals occurs in serpentinized and uralitized peridotites and pyroxenites. Primary opaque minerals are largely altered to marcasite, violarite, and haematite. The pluton intruded a supracrustal succession in the Early Aphebian before or in the early stages of the Hudsonian orogenic events. The metamorphic peak during Hudsonian time reached middle to upper amphibolite facies, granitized the country rocks, and contributed to the amphibolization of the ultramafic pluton. The ultramafic rocks in the "La Range-Rottenstone" domain are largely coeval with, and related to, the La Ronge-Group volcanics. The Nemeiben Lake pluton may have been part of the feeder system for Early Aphebian volcanics. It is proposed the La Ronge and Rottenstone domains (Lewry and Sibbald 1977), are parts of the same crustal zone which is progressively exposed to deeper levels as one approaches the Needle Falls shear zone, here interpreted as the westernmost fault zone due to compressional and uplift forces caused by the closure of an ocean previously formed between fragments of Archean crust. The Ni-Cu prospect of the Nemeiben Lake pluton is not now economic though standard technology for recovery of sulphide minerals in pyroxenites would probably be adequate. Most base-metal minerals in the altered peridotites belong to the fine-grained secondary assemblage of opaques due to the formation of retrograde hydrous silicates and are probably too fine-grained for standard recovery methods.