The Rae craton is part of the Canadian Shield and is separated from the Slave craton and the Buffalo Head Terrane by the Thelon Tectonic Zone (TTZ) and Taltson Magmatic Zone (TMZ). The least understood region of the Rae craton is its western margin, exposed in southeastern Northwest Territories (NWT). At least two periods of collision along the western margin of the craton have been identified; the ~2.5-2.3 Ga Arrowsmith orogeny and the 1.9-1.93 Ga TTZ/TMZ orogeny. However, many uncertainties remain regarding the tectonic evolution of the area.
As part of a concerted effort to update existing knowledge of the South Rae craton, the NWT Geological Survey is leading a thematic geoscience program, now in the second of three years, in the Nonacho Lake area. This area includes the western flank of the South Rae craton between the Porter domain in the core of the Rae craton, to the east, and the TMZ, to the north and west. In the Nonacho Lake area, Archean and Proterozoic gneissic basement is unconformably overlain by clastic rocks of the ~1.9 Ga Nonacho Group. Preliminary results are reported on herein for U-Pb geochronology, metallogeny, sedimentology, and structural-evolution studies undertaken during the 2019 field season.
Reconnaissance U-Pb geochronology of basement orthogneisses and granitoids documents ca. 2.60 Ga magmatism typical of the Rae craton, and 2.5-2.3 Ga magmatism and metamorphism overlapping in age with the Arrowsmith orogeny and ca. 2.5 Ga igneous rocks of the Queen Maud block (QMb). Considered with field mapping data, these results suggest that ca. 2.5 Ga rocks of the QMb and/or Taltson basement complex extend into the Nonacho area, well inboard of the TMZ.
Preliminary work on the sedimentology of the Nonacho Group reveals evidence for significant intervals of marine deposition in the lower, sandstone-dominated units of the Nonacho Group, previously thought to be entirely fluvial-alluvial-lacustrine. Marine influence is inferred from the presence of thick successions that have abundant hummocky cross-stratification and wave ripples topping most beds, local herring-bone cross-stratification, and tidal bundles.
Preliminary results of structural evolution studies show that at least three major shear zones are present between the dominantly Neoarchean Porter domain and dominantly Paleoproterozoic TMZ. Two of these shear zones record greenschist grade sinistral strike-slip or oblique normal-sinistral movement which occurred after the deposition of the <1.9 Ga sediments of the Nonacho Group and post-date at least one generation of regional folding and hydrothermal activity.
At least four mineralogically and temporally distinct mineral assemblages record a widespread and long-lived post-Nonacho hydrothermal system: i) pink to reddish feldspar-quartz±magnetite, ii) epidote ± hematite, iii) black chlorite ± U, Au, Ag, Cu-sulphides, specular to black hematite, and muscovite, and iv) quartz ± Cu, Zn, Pb, sulfides, Ag, barite, carbonate, and fluorite. While it is too early to classify the numerous mineral occurrences of the Nonacho area, it is noted that the various occurrences share similarities with IOCG, skarn, Manto-type, unconformity-type U, and vein-hosted base metal deposits.