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 × 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 spodumene-free 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 lithiophilite in the spodumene pegmatites. Garnet remains a trace mineral in all zones, with the exception of the core zone. where it is absent. Sporadic small cavities containing quartz, K-feldspar, apatite, Mg-rich beryl, elbaite and calcite are present in some dikes. The composition of the rock-forming minerals in both types of pegmatite is similar. An example of the chemical evolution of the minerals is shown by tourmaline, which becomes richer in Al and Li, but poorer in Mg, and shows decreasing Na/(Na + vacancy) and increasing Al/(Al + Fe) values from the schist to the inner (wall + intermediate) zone of the pegmatite. These trends, together with the changes in the abundances and paragenetic sequence of the major minerals, are interpreted to reflect an increase in the Rb, Cs, Li, F, P, Be, and Ta contents, and a decrease in the contents of Mg, Fe, and B contents, in the felsic liquids during crystallization.