Geoscience and Exploration

Petrographic Analysis of Syn- To Post-Tectonic Granitic Dykes in the Yellowknife Greenstone Belt, NWT: Evidence for Late-Stage H2O-Rich Fluids

Soapbox Wednesday, November 21, 2018 - 13:35 to 13:41 Theatre 2


S.C. Speight (Presenting)
University of New Brunswick

D.R. Lentz
University of New Brunswick

C.R.M. McFarlane
University of New Brunswick

The Yellowknife greenstone belt (YGB) is located in the Archean Slave Province, in the Northwest Territories, Canada. It is host to the historic Con and Giant mines which have produced a total of 14.2 Moz gold at an average grade of 16 g/t. Historic and recent field work has documented narrow granitic dykes that were previously not the subject of an academic study. To date these dykes have only been observed proximal to the Western Plutonic Suite in the amphibolite grade Chan Formation of the Kam Group, and not in the overlying greenschist grade Crestaurum, Townsite, or Yellowknife Bay formations. The granitic dykes are typically = 30cm wide and up to 1m, and often have a dark pink-red, earthy hematized colouring. The dykes have sharp contacts with no observable chill margins and an overall aplitic texture with occasional fine- to medium-grained white feldspars. The granitic dykes often have cm-scale quartz cores that are typically continuous along the length of the dyke, or occasionally occurring as discontinuous pods. The granitic dykes were found to cross-cut all lithologies in the map area, including feldspar-quartz porphyries, tonalites, granodiorites, and mafic flows, making them the youngest dykes based on relative timing relationships.

Petrographic analysis compliments field data by allowing for a more detailed study of microstructures. Microtextural analysis of granitic rocks has shown that there is a correlation between microstructures and metamorphic grade. Therefore, the examination of the various quartz and feldspar textures may lead to a better understanding of the relative timing of these syn- to post-tectonic dykes as they developed overprinting prograde to retrograde metamorphic and deformation textures. These relative timing relationships will be combined with U-Pb geochronology to establish the age of discrete deformation events.