Yellowknife Geoscience Forum

The Sunset Lake area contains Neoarchean volcanic rocks of the Beaulieu volcanic belt, which occurs along the northeast margin of the ca. 2.8 Ga Sleepy Dragon Complex ~100 km northwest of Yellowknife. The absolute age and stratigraphy of the volcanic rocks in this belt are not known and thus the volcanic rocks within the Beaulieu belt cannot be easily correlated within the Slave craton. This study focuses on the volcanic rocks, and in particular the rhyolite dome, located at the south end of Sunset Lake and correlating these rocks with those that host the Sunrise volcanogenic massive sulfide (VMS) deposit ~6 km to the north.

The objective of this study is to document the litho- and chemo-stratigraphy of the volcanic strata to determine its petrogenesis and compare it to the volcanic strata that host the Sunrise VMS deposit. Detailed mapping (1:2000) was completed over two summers (2018-2019).  The south Sunset Lake area is composed of dominantly massive to pillow basalt and andesite, with lesser amounts of massive to in-situ brecciated, weakly quartz-plagioclase porphyritic rhyolite, heterolithic tuff to lapilli-tuff and felsic tuff to tuff breccia. Felsic clasts are similar in composition to the coherent rhyolite. All units have trace element geochemical signatures typical of modern calc-alkaline arc rocks.

Although originally mapped as a coherent rhyolite dome, the volcanic rocks of the south Sunset Lake area form a more complex stratigraphy consisting of multiple rhyolitic, andesitic, and basaltic lithofacies. Volumetrically, a significant component of the strata consists of volcaniclastic rocks, tuff to tuff breccia units, interpreted to have been deposited by debris flows and eruption-fed density currents. The rhyolite dome and associated volcaniclastic rocks in the south Sunset Lake area are similar in terms of their geochemical signatures and lithofacies to those associated with the Sunrise deposit. While the volcanic strata in the south contain only minor local semi-massive sulfide mineralization at surface, the similarity with host strata for the Sunrise deposit, including evidence of a vent proximal environment (e.g. rhyolite dome) and significant  porous volcanic debris on the seafloor, key for replacement of sulfides by circulation of hydrothermal fluids (e.g. thick sequence of volcaniclastic rocks), suggests there is potential for more significant mineralization in the south Sunset Lake area.