The development of the early Paleozoic St-Lawrence River rift system was associated with extensive alkaline igneous activity (Kumarapeli and Saull, 1966), including the emplacement of the Saint-Honoré carbonatite (650 Ma; Vallée and Dubuc, 1970).
A carbonatite is an igneous rock comprising at least fifty percent carbonate minerals. Occurrences on a global scale are sparse and widely distributed. Dominant carbonate minerals, in order of decreasing abundance, include calcite, dolomite, ankerite, and rarely siderite and magnesite.
The St-Honoré alkaline complex is almost completely covered by Trenton limestone of Palaeozoic age and is elliptical in plan view, with a major axial length of approximately four kilometres. It consists of a series of crescentic lenses of carbonatite with compositions becoming younger progressively inwards from calcitite through dolomitite to ferrocarbonatite. Rocks immediately surrounding the complex are composed of magnetite diorite as well as hypersthene syenite.
Mineralized envelopes vary between 45 and 180 meters in width (north-south direction) while their length can reach almost 750 meters. The zones have a known vertical extent of approximately 750 meters, although they remain open at depth. The deepest completed drill holes are mineralized at grades equivalent to those of current ore production. Dips are generally vertical or steeply dipping (>70°) to the northwest or northeast.
Average grades of the large mineralized envelopes are between 0.44 and 0.51% Nb2O5. The majority of the mineral reserves and the measured and indicated resources are located in the four first mining blocks between 90 and 560 meters in depth. The bulk of the mineral resources classified as inferred are exclusively found in mining blocks 4, 5 and 6, between depths ranging from 560 to 730 meters. Mine operations are concentrated between depths of 100 to 440 meters, currently operating simultaneously on three mining blocks.