THE LITTLE NAHANNI PEGMATITE GROUP, NORTHWEST TERRITORIES

The Little Nahanni Pegmatite Group (LNPG) is a rare-element bearing, lithium-caesium-tantalum (LCT)-type, sub-vertical granitic pegmatite dyke swarm covering an area of 5 x 11 km. The host rocks are Precambrian stratified rocks of the Selwyn Basin area, specifically the Fork Anticline, a northwest-trending upright fold formed prior to the intrusion of the pegmatites. The dikes of the LNPG are well-exposed over a vertical distance of up to 500 m, providing a rare opportunity to study the fluid/melt evolution of a granitic pegmatite system in three dimensions. Variations in mineralogy and crystallization textures of the dikes suggest that the outcrops present the transition from a litho- to hydrostatic pressure regime, which has seldom been studied in pegmatite systems.

At low elevations the mineralogy of the dikes is fairly complex (spodumene, lepidolite, quartz, feldspar etc) and classic pegmatitic textures occur (line-rock, comb layering). However, the textures are discontinuous, varying considerably over short distances (<1 m). It is suggested that the variations are pressure related, with the variability indicating that the system experienced frequent pressure fluctuations, potentially related to dike propagation. At higher elevations the mineralogy becomes simplified and increasingly quartz-rich, and the textures resemble those associated with porphyry systems, i.e. quartz stockwork and brecciation-induced cavities. These observations suggest that crystallization occurred in a dynamic physical environment at a relatively shallow depth.

The LNPG will be dated using the 40Ar-39Ar (mica) and U-Pb systems which will help determine its relationship with local S-type intrusions. Stable isotope (Li, O and H) and fluid inclusion studies, will take advantage of the good exposure of the LNPG system and determine whether the fluctuations occurring during crystallization can be quantified. As part of the stable isotope study, lithium isotope ratios will be measured in minerals from various elevations to determine the signature of the dikes and measure any potential fractionation occurring within the system. Li isotopes have not previously been used to study this type of setting in pegmatites, and the data collected here will make an important contribution to our knowledge of Li isotope systematics.