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. They are commonly associated with geological units with higher content in accessory minerals such as magnetite, biotite and apatite (facies C3b, C3c and C3a).

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.

Niobec has been using the open stope mining method.

To break the ore, vertical holes, with a diameter of 17 centimetres (6.5 inches) are drilled off filled with explosives and detonated. The broken-up ore is collected at the base of the open room in a draw point with a loader and transported by truck to the ore pass. After, the ore is crushed underground, and hoisted to the surface to be crushed again prior to being transferred to the concentrator.

Niobec’s concentration plant uses the one of the most complex ore recovery processes in the world. In order to produce a pyrochlore concentrate (58% niobium pentoxide, Nb2O5), the following steps must occur during the transformation process, which takes place at the plant: milling, sieving, desliming, magnetic separation, three types of flotation, leaching, two-step filtration and concentrate drying. Each ton of ore (1,000 kg) produces in average 2.4 kg of niobium oxide. About 15 different chemicals are used during the concentration process. However, the concentration plant’s waste is pH neutral. In other words, it does not generate acid and is essentially carbonate-bearing.

The crushed ore is deslimed in two stages of cycloning, and the underflow is sent for conditioning prior to carbonate flotation. The carbonate concentrate is sent to the tails. The carbonate flotation rougher and cleaner tails are cycloned in two stages to change the process water and then sent to the pyrochlore rougher flotation. The rougher concentrate is sent to five stages of cleaner cells. This is followed by pyrite flotation to remove sulfur, hydrochloric acid leaching to remove phosphorus and then dried to produce a concentrate with less than 0.2% moisture before being converted into ferroniobium.

The conversion of the niobium concentrate into ferroniobium (FeNb) is performed on site by way of thermite reaction. This process is divided into 5 components: niobium oxide concentrate (Nb2O5), aluminum powder and pellets, metallic iron, sodium nitrate, fluxing agents and fine recycled matter. This combination produces a powerful exothermic reaction that generates enough heat to bring the temperature up to 2,250ºC, thus melting the entire load in less than 10 minutes. A typical load of 6,400 kg generates 2,400 kg of ferroniobium. Following a cooling period, the ferroniobium is crushed to sizes specified by various steel plants, packaged in 10 to 1,500 kg containers, and shipped worldwide.