The Nonacho Lake area of the South Rae craton, Northwest Territories, consists of ca. 1.91-1.82 Ga conglomerate, sandstone and mudstone of the Nonacho Group unconformably overlying Archean to Paleoproterozoic granitic and gneissic basement rocks. The Nonacho Group is interpreted as a mostly fluvial sequence deposited in a sinistral strike-slip basin.
There is a substantial metamorphic break between the 2.05-1.92 Ga greenschist-facies metamorphism in the Porter domain 45 km east of Nonacho Lake and the 2.08-1.95 granulite-facies metamorphism in the southwestern Taltson Magmatic Zone west of Nonacho Lake. This metamorphic discontinuity suggests the presence of a significant structure between the Taltson basement complex (Tbc) and the Porter domain. Moreover, the ca. 2.4 Ga (Hbl) and 2.3 Ga (Bt) 40Ar/39Ar cooling ages recorded in the Porter domain contrast with the younger (ca. 1.85 Ga) cooling ages recorded in the Tbc, and necessitate the presence of a major structure that accommodated the ca. 1.85 Ga exhumation of the 2.08-1.95 Ga high-grade metamorphic rocks of the Tbc.
The goal of this project is to investigate the deformation history of the western flank of the South Rae craton, in the Nonacho Lake area, by constraining the geometry, kinematics and timing of major domain-bounding structures, documenting the exhumation history of the crustal blocks, and examining potential temporal and spatial links between these major structures and structures that host polymetallic (Cu-Au-Ag) and U+/-REE mineralization. Detailed mapping during the 2019 field season involved ground truthing of structures delineated from aeromagnetic data. This revealed three major SW-striking ductile shear zones, informally termed the Udder Island shear zone, Magrum Lake shear zone, and Gray Lake shear zone. Located along the western flank of the Nonacho group, the Udder Island shear zone is 1-2 km wide with a strike length of at least 70 km and likely extends as far as the Athabasca basin in Saskatchewan. The shear strain is most notable within siltstone of the Chief Nataway formation and some basement units. The zone dips steeply to the NW and displays a strong sub-horizontal stretching lineation and ubiquitous sinistral shear sense indicators. The 2-3 km wide Magrum Lake shear zone bounds the eastern margin of the Nonacho Group and has been traced for at least 30 km. Sandstone and basement units have a steeply NW-dipping shear fabric and stretching lineations plunging shallowly to the SSW, that indicate oblique normal and sinistral movement. The Gray Lake shear zone consists of K-feldspar porphyritic granite and tonalite-amphibolite gneiss with a steeply NW-dipping shear fabric, and appears to separate the higher-grade Archean rocks of the Porter domain from the lower-grade rocks to the west.
Future work will include: 1) 40Ar/39Ar thermochronology to constrain timing of basement uplifts and delineate crustal-scale boundaries; 2) U-Pb geochronology of syn-to-post tectonic granitic dykes to constrain timing of deformation and shear zone development.