GEOLOGY AND MINERALOGY OF THE LITTLE NAHANNI RARE-ELEMENT GRANITIC PEGMATITES, NORTHWEST TERRITORIES

Abstract

The Little Nahanni rare-element granitic pegmatite group (LNPG; ca.82 Ma) in the western part of the Northwest Territories, occurs as subparallel dike swarms over an area of ~11 ϫ 5 km in the walls of a series of cirques dominated by schists of the Upper Proterozoic Hyland Group.These pegmatite bodies, up to a few meter wide, are divisible into spodumene-bearing and spodumenefree varieties that occur close together; some dikes split into these two variants.The two types are mineralogically similar.Both contain K-feldspar, plagioclase, quartz, mica (muscovite to lepidolite), columbite-group minerals, cassiterite, tourmaline, beryl, lithiophilite and garnet; they differ in the abundance of mica and accessory phases.The spodumene-free pegmatites have more mica, particularly lepidolite, than the spodumene-bearing variety.Apatite and montebrasite occur primarily in the former, whereas most lithiophilite is present in the latter.Sparse galena, titanian rutile, fluorite and helvite are also found in both variants.Subsolidus phases are zeolite-group minerals, microlite, and secondary phosphate minerals such as triploidite.Internal zoning in both pegmatites varies from complex to symmetrical.Where well developed, the zoning is typical from border through wall and intermediate to core zone.Quartz and the feldspars occur throughout the dikes.Micas and columbite-group minerals initially increase from the border to the wall zone and subsequently decrease toward the core, whereas tourmaline becomes less abundant toward the core.Spodumene and cassiterite first appear in the wall zone and become more abundant toward the core zone, whereas the reverse is true for beryl.Phosphate minerals exist mainly in the intermediate zone of the lepidolite pegmatites, and