The area is underlain by northeast striking, moderately northwest dipping felsic to intermediate metavolcanics and minor interlayered metasediments considered to be of Devonian to Mississippian age (unit EBFt, Paper 1987-2), within the Lower Cambrian to Mississippian Eagle Bay assemblage. The rocks hosting the fluorite deposit consist of a deformed and metamorphosed pile of alkali feldspar porphyry, porphyry breccia, lithic tuff and breccia of trachytic composition, with occasional pyritic schist of rhyolitic composition. Rocks of this trachyte unit are light grey in colour and stained rusty brown or yellow due to widespread pyrite. They may be massive, brecciated, or markedly schistose and lineated.
Fractured and sheared crystals of potash feldspar and albitic plagioclase, and rock chips of trachytic composition, occur in a fine-grained groundmass of feldspar and sericite. The trachyte unit, which is 15 to 120 metres thick, is apparently a mixture of intrusive porphyry and its extrusive equivalent tuffs and tuff breccias. It is likely related to a volcanic centre or vent. The above unit is structurally underlain by quartz-sericite schist, chlorite schist and dacitic and andesitic volcanic breccia, with interlayers of grey phyllite, slate, chert and sericitic quartz- ite.
The prominent schistosity, which is parallel to the compositional layering and was probably produced during the first phase of deformation, is deformed by tight, recumbent, east-trending second-phase folds. These structures are refolded by upright third-phase, north to northeast trending structures. Subsequent late kinks, and prominent north-trending tension fractures, are commonly followed by post-tectonic felsic and mafic dykes of Cretaceous or later age. High-angle, north-trending faults sharply control the distribution of the trachyte unit.
The fluorite zone occurs as a concordant, tabular body, largely in lithic tuff and tuff breccia of the trachyte unit. The zone, which on average strikes 035 degrees and dips 40 degrees northwest, is about 300 metres long, up to 50 metres wide and extends over 350 metres down dip. Within the zone, the fluorite occurs as massive lenses up to 1 metre wide, small irregular streaks parallel to foliation, scattered hairline veinlets and disseminations with other minerals.
Minerals associated with the fluorite include celestite, pyrite, feldspar, mica and minor amounts of minerals containing rare earths. In the more massive lenses, the fluorite forms a dark-purple to blackish matrix surrounding the other minerals. In other lenses, evenly distributed fine-grained fluorite gives a purple colouration to the host rock. Celestite, at times as abundant as fluorite, occurs as small crystals around the fluorite grains and fractures within them. Bastnaesite is irregularly distributed with the fluorite and celestite and strontium, cesium, lanthanum, ytterbium and yttrium are associated with the fluorite. Radioactivity is weak to moderate in the fluorite zone. Uranium zones, such as the Rexspar (082M 021), occur nearby.
The geological setting and presence of pyrite-mica zones suggest that the mineralized zone was formed by deuteric, volatile-rich fluids during a late stage in the formation of the trachyte unit. The fluorite is likely syngenetic with the host rock and thus of volcanogenic origin.
Measured recoverable reserves in 1975 were 1.36 million tonnes grading 23.5 per cent fluorite, 5.2 per cent SrSO4, 4.1 grams per tonne silver, 0.05 per cent molybdenum, 0.01 per cent copper, 0.06 grams per tonne gold, 0.17 per cent lead and 0.08 per cent zinc (Property File - Wright Engineers Ltd., 1975).
In 1926 Smuggler Hill Development Company was formed to explore and develop silver and lead deposits (Smuggler, 082M 023 and Foghorn, 082M 029), which were originally staked in 1918 by A.G. McDonald. The results of this early exploration activity were reported by H.G. Nicol, 1926 and D.B. Starrett, 1930. A manganese occurrence was examined by W. Elliot and N.C. Stines in 1929 (Smuggler Manganese, 082M 158). Further geological examinations of fluorite occurrences were reported on by D.B. Starrett, R.P.D. Graham and M.R. Wilson in the early 1940's.
The presence of uranium mineralization became known in late 1949. Dr. F.R. Joubin studied and reported on the mineral occurrences during 1950 and 1951. Rexspar Uranium, later reorganized as Consolidated Rexspar Minerals and Chemicals Ltd., acquired the rights to mineral claims incorporating the uranium bearing zones and delineated three uranium deposits in the late 1950's. However, the deposits were not brought into production. Denison Mines Ltd. resampled and undertook an economic feasibility study in 1969. Exploration programs and geological reviews were conducted in 1969-1972, directed mainly at determining fluorite reserves. Additional diamond drilling of the uranium bearing zones was carried out in 1976 and the drill core was used in a metallurgical test program undertaken to establish process flowsheets.
The Fluorite deposit and the three uranium deposits have been outlined by fairly close spaced diamond drilling and by surface sampling. A total of 368 surface and underground holes have been drilled from 1943 to 1976, for a total of approximately 17,280 metres. Of these, 121 holes were on the "A" deposit, 81 on the "B" deposit, 125 on the "BD" deposit and most of the others on the fluorite deposit. Drifts, cross cuts and raises for a total of 664 metres were driven in the "A" and "BD" uranium zones. The property has been prospected several times over the years. Geological mapping, radiometric surveying, soil sampling and metallurgical testing have also been performed. Work conducted by Placer Development Ltd. during October, 1981, included ground magnetometer and VLF - EM surveys.
In 1987, Consolidated Rexspar changed its name to Conrex Corporation and sold the property in 1988 to Gold Ventures Limited. American Bullion Minerals Ltd. attempted to get a permit to do exploration on the main fluorite zone in the early 1990's.
In 2007, International Ranger Resources Inc. held the property as the Foghorn Polymetallic Project, and commissioned a technical report to summarize the geology, mineralization and exploration potential.