Summary:
Carbonatite-associated deposits can be subdivided into magmatic types. Magmatic deposits are formed through processes associated with the crystallsation of carbonatites, whereas metasomatic deposits form by the reaction of fluids releases during crystallisation with preexisting or country rocks. Spatially distinct niobium and rare-earth element (REE) mineralisation at the Niobec mine, Québec, are examples of magmatic and metasomatic mineralisation.
The main minerals in the Niobec deposit are carbonates (50%), oxides (magnetite, hematite) (10%), silicates (biotite, chlorite) (10%), apatite (10%), sulfides (1%) as well as fluorite, barite and zircon (1%, collectively).
Niobium minerals in a bipyramidal shape are scattered throughout the carbonatite. These minerals are fine-grained (0.2 mm to 0.8 mm in diameter) and not easily visible to the naked eye. Commonly, geological units with higher content in accessory minerals such as magnetite, biotite, and apatite (facies C3b, C3c, and C3a) are associated with this.
The ore is analysed in terms of percentage of Nb2O5. The mineralised lenses located on the northern edge of the deposit are characterised with the presence of hematisation, which seems to decrease in intensity with depth. Lenses farther south of the deposit are contained in a more calcitic carbonatite with syenite xenoliths.
The mineralisation mainly includes ferric-pyrochlores, sodic-pyrochlores and columbite as Nb bearing minerals. The mineralogical characteristics, the alteration and the mineralisation type greatly influence metallurgical recovery, which can vary between 30% and 70%.
The mineralised envelopes vary from 45 m to 185 m in width (north to south), and can reach close to 760 m in length. The areas have a known vertical extension of about 750 m. The deepest completed drillholes (down to block 12, close to a depth of 1,280 metres) show mineralisation at equivalent grade to those of the current mine production. The deposit has a sub-vertical dip.
The average grades of major mineralised envelopes are 0.44% to 0.51% Nb2O5. Considering the block caving method, the definition of high grade mineralised envelopes is no longer necessary because all carbonatite will be mined. Potential mineralised zone remains open at depth.