A review of the Late Cretaceous Little Nahanni Pegmatite Group and associated rare element mineralization in the Selwyn Basin area, Northwest Territories

Abstract

The Late Cretaceous Little Nahanni Pegmatite Group (LNPG) is composed of more than 200 subvertical, lithium-cesium-tantalum (LCT) pegmatites, occurring in an area of approximately 55 km2. The pegmatite group is hosted by Selwyn Basin offshelf facies of the Neoproterozoic to Lower Cambrian Hyland Group and is located between, and subparallel to, major regional northwest-trending structures, such as the March Fault and the Fork Anticline. Since their discovery in 1961, these coarse-grained, mostly quartz, feldspar, and spodumene pegmatites have been investigated for economic concentrations of rare elements. Whole-rock geochemical analyses indicate granitic compositions and strong peraluminous signatures suggest an S-type affiliation. The depletion in rare earth element (REE) abundance, enrichment of Li, Ta, Cs, Sn, Rb, and Nb, and emplacement into low-pressure metamorphic rocks is typical of LCT pegmatites. There are similarities between the LNPG dykes and the Greenbushes pegmatite, Australia, which is rich in Sn, Ta, Nb, and Li (Wengzynowski, 2002). However, even closer geochemical comparisons can be made with examples of European late-Variscan, post-orogenic mineralized plutons and pegmatites. One such example, the Beauvoir granite, is suggested as an analog for the plutonic source of the LNPG dykes. Comparison of the setting and ages obtained for the emplacement of the LNPG with this well studied Variscan granite suggests a period of post-orogenic collapse and extension during the late Cretaceous period in this region of the northern Cordillera.