Geology and genesis of uranium mineralization in subaerial felsic volcanic rocks of the Byers Brook formation and the comagatic [sic] Hart Lake Granite, Wentworth area, Cobequid Highlands, Nova Scotia

Gower, David Patrick (1988) Geology and genesis of uranium mineralization in subaerial felsic volcanic rocks of the Byers Brook formation and the comagatic [sic] Hart Lake Granite, Wentworth area, Cobequid Highlands, Nova Scotia. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Byers Brook Formation and the Hart Lake Granite are situated in the Cobequid Highlands of northern Nova Scotia. The anorogenic granite and bimodal volcanic rocks are comagmatic and are suggested to be products of extensional tectonism. The granite has an Rb/Sr whole-rock age of 339 ± 4 Ma and the rhyolites 343 ± 5 Ma. -- The Byers Brook Formation is a generally east-west (110-120 az.) striking, steeply dipping (70ﾟN) sequence of rhyolitic pyroclastic, flow and volcaniclastic rocks intercalated with a relatively minor volume of basalt flows. All units are cut by an abundance of diabase and composite dikes with the highest density of dikes associated with rhyolite dome/flow complexes. Rhyolite dikes are restricted to these complexes. -- Two periods of bimodal volcanism were separated by a brief hiatus represented by a 50-100 m thick conglomerate unit which grades eastward into laminated to thickly bedded lacustrine? siltstones. Both stages of volcanism are characterized by early pyroclastic eruptions which grade upwards into rhyolite and basalt flows. The base of the early sequence is intruded by the Hart Lake Granite. The later sequence is approximately 2200 m thick and grades into the dominantly basaltic Diamond Brook Formation. -- Geochemically the granite and rhyolites are subalkaline, metaluminous, except for a discrete suite of peralkaline rhyolite and composite dikes. All rocks are generally enriched in Fluorine. Strong alkali metasomatism is pervasive throughout the extrusive rocks, however, late subvolcanic rhyolite and composite dikes are unaltered. -- The largest and highest grade concentrations of uranium occur in breccia zones in and adjacent to rhyolite dome/flow complexes. It is hypothesized that these complexes were areas of high thermal gradient caused by the injection of late composite and rhyolite dikes, and thus the locus for the very large hydrothermal system which caused the pervasive alkali metasomatism. This large hydrothermal system leached uranium liberated during devitrification of the volcanic pile and concentrated it into permeable structures, such as lithophysae-rich zones, faults and breccia zones. -- Other associated mineralization includes; cassiterite in amygdules of basalt flows, massive pyrite beds with up to 276 grams per ton silver in the lacustrine siltstones, and fluorite-zircon-sphene-calcite-allanite veins with highly anamalous rare earth element concentrations hosted by the granophyric margin of the Hart Lake Granite. -- Although the Byers Brook Formation in the Wentworth area has been subject to extensive exploration in the past, the data presented here suggest that perhaps the most prospective parts of the Formation are the dome/flow complexes, and these have not been adequately tested. In light of this the Byers Brook Formation and the Fountain Lake Group in general should be considered good prospects to host lithophile element mineralization.

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