Preliminary Results of Reconnaissance-Scale Bedrock Mapping and U-Pb Geochronology in the Nonacho Lake Area (NTS 75F), South Rae Craton, Northwest TerritoriesWednesday, November 21, 2018 - 10:40 to 10:59 Theatre 1
In the Nonacho Lake area, the ca. 1.91-1.82 Ga Nonacho Group (NG) consists of breccia, conglomerate, sandstone, mudstone and rare volcanic rocks deposited in a complex system of braided steam, lacustrine, and alluvial fan environments. These rocks are polydeformed and unconformably overlie Archean to Paleoproterozoic gneisses and granitic rocks of the Rae craton. The basal unconformity of the NG has been the target of significant uranium exploration. In addition to U and REE, previous exploration discovered several polymetallic (Cu-Au-Ag) showings hosted in both basement and NG rocks. The stratigraphy and structural geology of the NG has not been investigated in detail since the original mapping of Aspler (1985). Questions remain regarding the depositional setting and regional correlation of this Paleoproterozoic basin. The basement rocks remain un-mapped, although detrital zircon U-Pb dates from the NG suggest that the basement records a protracted Eoarchean to Paleoproterozoic history. The Nonacho Lake region currently lacks the geological framework necessary to facilitate mineral potential assessment and scientific work. Reconnaissance investigations conducted in 2018 will be followed by detailed studies in 2019 and 2020, with the goal to resolve the tectonometamorphic history and assess the economic potential of this underexplored region.
Basement rocks in the Sparrow Bay-Stewart River area comprise foliated granodiorite, dioritic to granitic injection gneiss with abundant ultramafic inclusions, local biotite-sillimanite paragneiss, and cross-cutting weakly foliated to undeformed granites. Preliminary U-Pb zircon geochronology reveals a spread of ages typical of the Rae craton, Queen Maud block, and Arrowsmith orogeny. Two foliated granodiorites yielded ages of ca. 2.60 and 2.50 Ga, a dioritic component of the injection gneiss is ca. 2.43 Ga, and the cross-cutting granites are ca. 2.35 to 2.28 Ga. Mapping at Hjalmar Lake revealed a unique body of carbonate-bearing hornblendite (carbonatite?) and NW-trending feldspar-phyric, calcite-amygdule-bearing dykes. The latter are undated but intrude the NG and were previously considered coeval with the 1.83 Ga Sparrow Dyke swarm. They bear a resemblance to ultrapotassic rocks of the ca. 1.83-1.75 Ga Baker Lake Group.
Evidence of two regionally extensive and likely distinct mineralizing systems has been observed: (1) a unconformity-related uranium mineralizing event with showings in basement and NG rocks; and (2) polymetallic sulphide mineralization associated with carbonate-fluorite veins at numerous showings in basement and NG rocks. The interpretation that most of the polymetallic showings are genetically related is based on similarities in metal tenor, alteration and emplacement characteristics.
Future work will include: 1) U-Pb geochronology to understand the major episodes of magmatism and metamorphism in the basement; 2) Lu-Hf tracing of magmatic rocks to constrain the age and isotopic character of their source, and to test for their interaction with the ancient crust that contributed ca. 3.9-3.0 Ga detrital zircons to the NG; 3) Ar-Ar thermochronology to delineate crustal-scale boundaries and constrain the timing of basement uplifts; 4) investigation into the source of mineralizing fluids in polymetallic sulphide-carbonate-fluorite bearing veins; 5) geochemistry, geochronology and economic significance of the carbonate-bearing hornblendite; and 6) detailed sequence stratigraphy and detrital zircon geochronology of the NG.