The following is as summary from a paper by Randy Miller, Search Minerals' V.P. Exploration (Miller, 2015). Regionally, the geology is considered to be part of the Eastern Grenville Province (Fig. 1). Terranes within it are distinguished on the basis of rock types, structures, ages, and metamorphic signatures, and are separated by major fault zones (Gower et al., 1987, 1988; Hanmer and Scott, 1990; Gower, 2010, 2012). The Foxtrot deposit is in the Fox Harbour volcanic belt, which is part of the Fox Harbour domain. The Fox Harbour domain is bounded to the north by the Lake Melville terrane, to the west and southwest by the Mealy Mountains terrane, and to the south by the Pinware terrane (Fig. 1 above).
The Fox Harbour Volcanic belt is approximately 64 km-long and ranges in width from <50 m in the northwest to 3 km in the east. Units strike westerly to northwesterly, parallel to bounding faults, and dip steeply northward. The belt contains one (in the northwest) to three (in the east) sub-belts of bimodal rocks with mainly REE-bearing felsic peralkaline flows and ash-flow tuffs and mafic to ultramafic volcanic and related subvolcanic units. Feldspar augen gneisses and porphyritic units, including crystal tuffs in the eastern portion of the belt, predominantly occur between the three sub belts. Sedimentary supracrustal units, including quartzite and locally derived volcaniclastic rocks sourced by felsic (commonly peralkaline) and mafic units, are locally abundant.
The three bimodal sub-belts (Road belt, Magnetite belt and South belt) have been the focus of REE exploration. The Road belt, is on the northern boundary of the Fox Harbour Volcanic belt, and can be traced throughout its full length, but the Magnetite and South belts have only been observed in the eastern 30 km. The mineralized units within the subbelts, predominantly pantellerite (a peralkaline rhyolite with high Fe and low Al contents) and commendite (similar to pantellerite but with less Fe and more Al), outcrop poorly and commonly occur in bogs or water-filled topographic lows. These units exhibit relatively high radiometric (anomalous U and Th values) and relatively high magnetic (anomalous concentrations of magnetite) signatures that, when combined, are excellent indicators of REE mineralization.
Airborne and ground-based radiometric-magnetic surveys clearly outline the three mineralized belts (Srivastava et al., 2012, 2013). High-grade mineralization, characterized by Dy from 100–300 ppm, is predominantly hosted by fine-grained, layered to massive, pantellerite. Lower grade mineralization, characterized by Dy from 20–100ppm, is predominantly hosted by fine-grained, mostly massive commendite. Mineralized units are commonly interbedded with mafic volcanic units, quartzite, and locally derived volcanogenic sedimentary rocks. Most of the REE mineralization occurs in allanite and fergusonite; minor amounts of REE occur in chevkinite, monazite, bastnasite and zircon (Srivastava et al., 2012, 2013). Most of the light REE (i.e., La to Sm) in the mineralization occurs in allanite, whereas most of the heavy REE (i.e., Eu to Lu) and Y occurs in both fergusonite and allanite (Srivastava et al., 2012, 2013).