Mineralogy, geochemistry, and geochronology of the Little Nahanni Pegmatite Group, Logan Mountains, Southwestern Northwest Territories
Creators: name: Mauthner, Mark H. F.
Publisher: name: University of British Columbia
Year: 2009

description: While pegmatites of the Canadian shield have been extensively studied, Canadian Cordilleran pegmatites have not. A cooperative study of the regional geology of pegmatites in the Canadian Cordillera was begun in 1992, by workers from the University of British Columbia and the Canadian Museum of Nature. Attention in this thesis has been focussed on the Little Nahanni Pegmatite Group, located in the Logan Mountains, about 47 km northwest of Tungsten, Northwest Territories. Fieldwork and laboratory studies were conducted in an attempt to classify, outline and map the extent of the pegmatite group, to describe the setting, mineralogy, geochemistry and internal evolution of the pegmatite dikes, to examine the interaction between the pegmatitic fluid and the host rocks, and to date the pegmatite intrusive event. The group is hosted by deformed, upper greenschist to lower amphibolite facies metasedimentary rocks of the Upper Proterozoic Hyland Group. The dikes that make up the group are characteristic of the albite-spodumene type of rare-element class of granitic pegmatite. The dikes are up to only several metres wide (most are less than 1 m), yet can extend up to several kilometres along strike. Mineralogical and geochemical data show the group to belong to the LCT family as classified by Cerny (1991a). Internal zonation consists of a fine-grained albite(Ab)-quartz(Qtz) (± tourmaline(Tur), ± apatite(Ap)) border zone, a mediumto coarse-grained Ab/Kfs-Qtz-muscovite(Ms) wall zone, a coarse-grained Ab and/or Kspspodumene( Spd) intermediate zone, and rarely, a coarse-grained quartz core. Late albitic units (Ab-Qz±Ms±lepidolite(Lpd)) comprise bands and irregular pods cutting all previously crystallized zones, and, in some cases, comprise entire dikes. Two notable expressions of rhythmic banding in the dikes are: 1) the alternation of pegmatitic, Spd-bearing bands and aplitic Fsp-Qtz bands, and 2), banded aplitic and pegmatitic Lpd-Ab-Qtz. No one mechanism seems to be responsible for both styles of banding. Accessory minerals include: columbite group minerals, cassiterite, apatite (commonly Mn-bearing), beryl, lithiophilite, triploidite, montebrasite, rutile, uraninite, uranmicrolite, helvite, galena, sphalerite and zeolites. The dikes are strongly enriched in Li and F, moderately enriched in P, and weakly enriched in B, Mn, Be, Nb and Ta. Several phases show enrichment in Mn with respect to Fe, and in Ta with respect to Nb. Beryl crystals with Cs-rich rims are associated with late-stage features. The virtual absence of magmatic Fe-Mg allowed B to remain, for the most part, partitioned in a vapour phase throughout the crystallization of the pegmatites, at the end of which B left the system to form tourmaline exocontacts in the host rock. Geochronological and Pb isotopic studies were carried out to constrain the petrogenesis, emplacement age(s) and relationships of the Little Nahanni pegmatite group to other granitoids in the region. A U-Pb age of 81.6 + 0.5 Ma for columbite was obtained. K-Ar ages for micas were determined to be 65.4 ± 4.0 Ma for muscovite, and 65.8 ± 3.4 Ma and 65.4 ± 3.6 Ma for lepidolite. The U-Pb age is interpreted as the emplacement age for the pegmatites and the KAr ages are interpreted to be the timing of a thermal event responsible for resetting the K-Ar mica system. The U-Pb columbite age is 8 to 13 Ma younger than the average age of the plutons in the Selwyn plutonic suite and indicate that these pegmatites represent a magmatic event that has not been previously recognized. Likewise, the K-Ar ages point to a later thermal event not previously recognized in this area. Fractionation trends in the pegmatite, Pb isotopic compositions of the feldspars, mineralogy and the general peraluminous nature of the pegmatites suggest that they are the product of moderate to high levels of fractionation of an upper-crustally derived granitic melt. This melt is likely to be the result of partial melting associated with tectonic movement during the Late Cretaceous formation of the Mackenzie fold and thrust belt.