Geoscience and Exploration

Uranium-Lead Zircon Geochronology of Granitoids near Jolly Lake, Slave Craton, Northwest Territories

Soapbox Wednesday, November 21, 2018 - 13:28 to 13:34 Theatre 2


S.D. Stone (Presenting)
University of Alberta

B. Knox
NWT Geological Survey

T. Chacko
University of Alberta

As part of a Northwest Territories Geological Survey 1:50 000-scale mapping program, we have undertaken a study of major granitoid bodies in the Jolly Lake area of the Slave Craton (~ 220 km NE of Yellowknife). Earlier work in this area has focused on the volcanic belts surrounding the granitoid bodies and consequently, relatively little information is available about the age and nature of plutonic units. Whereas the granitoids themselves are of little economic interest, a better understanding of these rocks could lead to more meaningful exploration of proximal volcanic belts and associated metal deposits. In addition, the granitoids can serve as probes of the Slave Craton lower crust and underlying lithospheric mantle from which the granitoid magmas were derived. Given the proximity of the study area to the Lac de Gras kimberlite field (~ 50 km to the NE), additional information on the nature and evolution of the Slave lithosphere through time may be useful in further diamond exploration.

Previous workers have subdivided the granitoids of the Jolly Lake area into younger, older, and undivided suites. The older suite comprises primarily granitoid gneisses and migmatites. There is uncertainty surrounding whether some of the rocks of this suite have granitoid or supracrustal protoliths. The younger suite of granitoids consists of little deformed to undeformed syenogranites, monzogranites and granodiorites. The undivided granitoid suite is characterized by deformational characteristics and emplacement ages intermediate between the older and younger suites. Monzogranites near Winter Lake and syenogranites near Courageous Lake have been dated at 2796-2855 Ma and 2613 Ma respectively, while the granites surrounding Jolly Lake remain undated.

Observations made during the 2018 field season generally agreed with the interpretations of earlier geologists in the context of unit age relationships. However, the finer scale of the present mapping allowed for more detailed and comprehensive lithological descriptions. In particular, we subdivided the basement complex in this area into at least four distinct types of gneisses, classified on the basis of their mafic mineral content and metamorphic texture.

The granitoid samples collected in the fieldwork are currently being processed for uranium-lead zircon geochronology. The isotopic analyses of zircon will be done in-situ in standard petrographic thin section by laser ablation, multi-collector, inductively coupled plasma mass spectrometry (LA-MC-ICPMS). The goal of the geochronological study is to firmly establish a sequence of emplacement for the various granitoid suites and clarify the origin of the older gneisses and foliated granites.