Petrography of the niobium-bearing units
The niobium mineralization is predominantly distributed within the dolomitic portion of the carbonatite although it may also be found in minor amounts within the calcitic facies. There is no significant Nb mineralization in the central ankeritic core of the carbonatite. This study focuses solely on the Nb-bearing units comprising the dolomitic facies and younger mineralized calcitic units enclosed within the dolomitic facies.
The carbonatite is strongly banded at a centimetre to metre scale . Mineral proportions are highly variable both between and within the bands. Niobium mineralization appears in the form of elongated subvertical lenses varying from a few millimetres to several metres in width and having a complex geometry. These lenses are also visible within the calcitic units enclosed within the broad dolomitic assemblage.
Other petrological units are also observed, including xenoliths of syenites, glimmerites and cumulates of magnetite.
For our purposes, fluorcalciopyrochlore will be referred to as pyrochlore (Pcl) given that other pyrochlore species are much less abundant and have not been as extensively studied. Fluorcalciopyrochlore (pyrochlore) has been the main mineral exploited for Nb at the Saint-Honoré deposit since 1971. Unaltered pyrochlore grains are usually euhedral, 0.01–2 mm in size, up to a centimetre in a few samples. They are usually light brown to grey with a greenish tint. A few grains are zoned and are usually inclusion-free, excepting a few apatite or rare pyrite inclusions. Most of the economic pyrochlore mineralization is associated with magmatic apatite (AP1) in bands, lenses or clusters within the dolomitic unit. Pyrochlore grains are also distributed randomly in the carbonatitic matrix, but at a much lower proportion. Coarser and zoned pyrochlores are also observed in the calcitic units. The geochemistry of unaltered pyrochlores show the expected major elements: Ca, Na, Ti and F . Fresh, weakly and moderately altered pyrochlores are distinguished by the proportion of pores, their colour (from brown to blackish) and their shapes, varying from octahedral to anhedral.
In hand samples, columbite-(Fe) can easily be misidentified as magnetite. It is black with varying shapes and sizes (ranges from 10 μm–1 mm). Under cathodoluminescence, inclusions of calcite and fluorite are easily distinguished by orange and blue colours, respectively . The inclusions are irregular in shape and may account for up to 50% of a columbite-(Fe) grain. Calcite and/or fluorite inclusions are a discriminating characteristic of columbite-(Fe). As with pyrochlore, columbite-(Fe) is observed to be disseminated within the dolomitic matrix, but is in a higher proportion within magmatic apatite (AP1). In general, there is an orange microcrystalline apatite (AP2) associated with columbite-(Fe) . AP2 is orange in hand samples, but is dark orange unless under strong light under the microscope. The rock is generally more altered (darkened dolomite grains and partly chloritized phlogopite) when columbite-(Fe) is present rather than pyrochlore. A few grains of columbite-(Fe) are observed in the calcitic rocks as detrital xenocrysts with no inclusions of calcite or fluorite. Nevertheless, unlike pyrochlore, columbite-(Fe) grains show no zoning and are generally observed in association with altered dolomite intersected by very fine-grained orange apatite (AP2).
Unlike pyrochlore, columbite-(Fe) does not have a significant content of Ca, Na and F. However, it does have a high amount of Fe and Mn.
Trace elements in Nb mineralization
LA-ICP-MS analysis was performed on five pyrochlore and five columbite-(Fe) samples to characterize the trace elements in both minerals . Elements including Al, Si, K, Zr, Ta and Hf do not show any significant difference between pyrochlore and columbite-(Fe). These elements, except for K that is undocumented in the pyrochlore crystal structure, are generally found in the B-site and are therefore immobile (Atencio et al., 2010). Pyrochlore has a very high Th content compared to columbite-(Fe). On the other hand, columbite-(Fe) is enriched in U and it does not follow the same trend as Th as normally expected (both are recognized as being held in the A-site). Transitional metals such as V are surprisingly high in columbite-(Fe), up to 100 times higher than in the pyrochlore samples .
Rare earth elements also display large variations between pyrochlore and columbite-(Fe) samples. LREEs are nearly ten times higher in terms of proportion in pyrochlore compared to columbite-(Fe) . Alternatively, HREEs and Y are characteristically higher in columbite-(Fe). A comparison of the median content of REEs in pyrochlore from the Aley carbonatite (Chakhmouradian et al., 2015) and pyrochlore from the Saint-Honoré carbonatite show lower or similar amounts. Major elements (e.g. Na, Ca, F) are, however, found at higher amounts in pyrochlores from Saint-Honoré than in those found in the Aley carbonatite.
Petrology of the Nb-bearing units and mineralization
Within the dolomitic units, apatite and other accessory minerals (Phl, Mag, Py) are agglomerated in lenses. They may appear foliation-like and could be misinterpreted as representing deformation. However, considering the post-Grenvillian geological setting, it is more likely an igneous texture (flowbanding) induced by the low viscosity of the carbonatitic magma (Treiman, 1989).
Accessory minerals in the calcitic units are generally disseminated and lenses are less frequent. It suggests that the magma chamber was less eruptive in the late magmatic stage. These calcitic units are thought to be from a later event given their coarser grain size, the presence of well-developed cleavages and the absence of alteration. Pyrochlores in these calcitic units are euhedral and mostly unaltered. A few columbite-(Fe) grains are also visible, but they are highly fractured and without calcite and fluorite inclusions. These columbite-(Fe) grains are likely antecrysts from the dolomitic facies. Antecrysts refer to crystals that did not crystallize from the calcitic magma, but still have a relationship with the magma (as described in Charlier et al., 2005).
Mineralization is associated with accessory minerals and more specifically to apatite. Both columbite-(Fe) and fluorcalciopyrochlore are intimately associated with apatite, a common characteristic in carbonatites (Hogarth et al., 2000; Knudsen, 1989). The first type of apatite (AP1) is translucent, euhedral and zoned as described by Chakhmouradian et al. (2017). Primary textures suggest this apatite to be of magmatic origin. A few inclusions of AP1 were observed within pyrochlore grains suggesting it is syngenetic.
Origin of halite
The presence of ubiquitous halite in the Saint-Honoré carbonatite is intriguing. Sodium is certainly magmatic in the Saint-Honoré carbonatite as it is a major constituent of fluorcalciopyrochlore (up to 8% Na2O), one of the first minerals with apatite to crystallize in a carbonatitic magma (Hogarth et al., 2000; Knudsen, 1989). The strong relationship of pyrochlore to apatite and the textural evidence also argue for a magmatic origin.
The origin of chlorine is, however, enigmatic. Based on the spatial distribution of halite in and around magmatic minerals from the Saint-Honoré carbonatite, Kamenetsky et al. (2015) proposed halite, and specifically chlorine, to be mantle-derived. However, our study of the Nb-bearing minerals of the carbonatite offers a slightly different understanding of the petrogenesis of halite. Petrographic observations and geochemical analyses of pyrochlore and columbite-(Fe) demonstrate that Na was leached during alteration whereas Cl was related to hydrothermal fluid. Thus, the Na is considered as magmatic in origin and Cl as hydrothermal (Tremblay et al., in preparation). As such, these findings refine the idea of halite having a magmatic origin of Kamenetsky et al. (2015) and suggest a magmato-hydrothermal origin. The absence of Cl in other magmatic minerals (e.g. apatite and phlogopite) reinforces our hypothesis of a hybrid origin.
Table des matières
1.1 Geological setting
1.2 Mining overview
2.1 Sample collection and preparation
2.2 Sample analysis
3.1 Petrography of the niobium-bearing units
3.1.1 Dolomitic rocks
3.1.2 Enclosed calcitic units
3.2 Nb mineralization
3.5 Trace elements in Nb mineralization
3.6 Crystallization of halite
4.1 Petrology of the Nb-bearing units and mineralization
4.2 Alteration of pyrochlore
4.3 Origin of alteration
4.4 Origin of halite
CHAPITRE II :CONCLUSION GÉNÉRALE