Geoscience and Exploration

Lithostratigraphy of the Sunset Rhyolite, Cameron-Beaulieu Volcanic Belts, Slave Province

Soapbox Wednesday, November 21, 2018 - 13:21 to 13:27 Theatre 2


S.B. Austin-Fafard (Presenting)
Mount Royal University

Y.M. DeWolfe
Mount Royal University

B. Knox
NWT Geological Survey

The Sunset Lake area contains Neoarchean volcanic rocks of the Beaulieu belt, which occurs at the northeast margin of the ca. 2.8 Ga Sleepy Dragon Complex. The exact age and the stratigraphy of these volcanic rocks is not fully understood. As a result, volcanic rocks within the Beaulieu belt cannot be easily correlated. The focus of this study is the Sunset Rhyolite, a large rhyolitic body at the south end of Sunset Lake that was originally mapped as a single, coherent rhyolite dome, and to correlate it with a rhyolite dome and associated rhyolite fragmental rocks that host the Sunrise volcanogenic massive sulfide (VMS) deposit some 6 km to the north.

The objective of this study is to document the litho- and chemo-stratigraphy of the Sunset Rhyolite and compare it to that of the Sunrise deposit. During 2014, the Sunset Lake area experienced forest fires resulting in great outcrop exposure. Detailed mapping completed this past summer (2018) indicates the Sunset Rhyolite is not a single, coherent rhyolite body, but instead represents a much more complicated rhyolite flow/dome complex. The south Sunset Lake area is composed of basalt, andesite and rhyolite lavas with varying compositions of volcaniclastic rocks. The lithologies from oldest to youngest are as follow: 1) massive to pillow flows that are basaltic to andesitic in composition (>200 m); 2) a dominantly coherent rhyolite that is weakly quartz-plagioclase porphyritic with local lobes and areas of in-situ breccia (50 – 100 m); 3) heterolithic volcaniclastic rocks (50 – 100 m) consisting of lapilli- to tuff-sized felsic (30 – 40%), mafic (10 – 15%), and andesite clasts (20 – 30%); 4) felsic volcaniclastic rocks ranging from tuff breccia to lapilli tuff with some tuff forming massive beds ~10-20 m thick with felsic volcaniclastic packages ~50-100 m thick. Felsic clasts share a similar composition to the coherent rhyolite.

The stratigraphy of the south Sunset Lake area is much more complicated than originally thought and does not represent a single coherent rhyolite lithofacies but instead is comprised of several different lithofacies. Detailed mapping of the area shows that volcaniclastic rocks make up 60-70% of the rhyolite in the area, the rest being massive. This has economic implications as the occurrence of a rhyolite dome indicates vent proximity and, along with the occurrence of volcaniclastic rocks, defining a topographic low that was filled with porous volcaniclastic debris, are key features needed for the formation of a VMS deposit. These lithofacies are also very similar to those observed at the Sunrise Deposit indicating they might represent the same stratigraphic horizon as that which hosts the VMS deposit.