Soutenance mémoire
The production of rare earth elements (REEs), a series of critical elements, has been virtually monopolized for the last few decades by China (Simandl 2014). Recently, the international market in REEs has been disturbed. In 2009, the Chinese government established a restrictive quota on exportation of REEs, and exploration for new sources of REEs rapidly increased around the world. The demand is expected to be strong because the market for green technologies, the main consumer of REEs, is expected to increase (Chakhmouradian and Wall 2012). Several geological processes or environments can produce economic REE concentrations (Simandl 2014). However, carbonatites are REEhosting rocks in which extractive metallurgy is known to work. According to Chakhmouradian and Zaitsev (2012), 20 of the 50 most advanced projects outside of China are in igneous carbonate rocks, whereas most actual or historical production has been from carbonatites or from their weathered components.
Although carbonatites are known to have high natural concentrations of REEs, in most cases that is not enough to be economical (Chakhmouradian and Wall 2012). A geological process to further concentrate the mineralization is needed. Three major processes are commonly accepted. 1) REE-bearing minerals, such bastnaesite [(REE)(CO3)F] or parisite [(REE)2Ca(CO3)3F2], precipitate from the primary carbonatite magma forming idiomorphic crystals of REE minerals in the late-stage carbonatite (Doroshkevich et al. 2008; Jones and Wyllie 1983; Mariano 1989). 2) REEs are remobilized and concentrated by hydrothermal solutions. Because minerals forming the late stage of an igneous carbonate complex are enriched in REE elements by fractional crystallization, a late hydrothermal solution can dissolve carbonates, apatite and sulphides, and become increasingly enriched in Ba, F, SO4 2−, Sr, REE and Th. In most carbonatites, hydrothermal REE minerals occur in veinlets or as interstitial fillings, and appear as finegrained polycrystalline clusters, commonly associated with barite, fluorite, hematite, quartz, strontianite and sulphides” (Mariano 1989). This second process accounts for most economical REE mineralizations in carbonatites around the world. 3) The third process is the result of chemical weathering of sub-surface carbonatite. The carbonate minerals are easily dissolved, and the Ca and Mg are removed, concentrating the less mobile elements, such as REEs (Mariano 1989).
However, despite the great abundance of REE concentrations by hydrothermal processes in carbonatite, the concentrations rarely form high-tonnage economic deposits (Mariano 1989). An early magmatic REE concentration would provide a more fertile ground to produce an economic deposit, and early concentration may represent the key factor for the formation of world-class REE deposits in carbonatites. To the best of our knowledge, few, if any, models combine magmatic and hydrothermal processes to generate economic REE deposits. The importance of prior magmatic input is not well documented because access to deeper portions of the carbonatite is not possible in most circumstances. Because most carbonatite minerals (carbonates, halides, etc.) are relatively soluble, the upper part is often weathered or leached and happens to be the most commonly studied. Furthermore, the upper part of the carbonatite is more likely to have been exposed to hydrothermal events. Consequently, access to the deeper parts of the carbonatites is fundamental to understanding the role of magmatic concentration in ore deposit formation.
Geological setting
The Saint-Honoré carbonatite complex is located in the Saguenay region. This region comprises three petrographic domains (Dimroth et al. 1981): a gneiss complex deformed during the Hudsonian Orogeny (1,735 Ma), anorthosite and charnockitemangerite batholiths dated to pre- to post-Grenvillian orogeny (935 Ma), and calc-alkaline intrusions related to the St. Lawrence River rift and related Saguenay Graben (Kumarapeli and Saul 1966). This extensional event provided anisotropies that channeled alkaline magmatism, including the 650 Ma Saint-Honoré carbonatite (Vallée and Dubuc 1970).
The Saint-Honoré alkaline complex is a vertical intrusion composed of a carbonatite core surrounded by alkaline silicate rocks. Fenitization is present in host rocks and is characterized by the metasomatism of pyroxenes into sodic amphibole and carbonates, sericitization of plagioclase and formation of many carbonatitic red to green veins (Belanger 1977). The alkaline silicate units are mainly composed of nepheline syenite and younger alkaline syenite (all miaskitic, Na2O+K2O/Al2O3 < 1) (Belanger 1977). Some xenoliths of alkaline syenite are observed in the carbonatitic part, suggesting that the carbonatite is younger than the syenites. The carbonatitic units have an elliptical shape oriented northeast-southwest (Fig. 2). From the periphery to the center of the carbonatitic complex, a concentric pattern is formed by Ca-carbonatite to Mg-carbonatite to Fecarbonatite in the center. The main carbonatite crystallized at high temperatures (1000- 1150°C, apatite-phlogopite geothermometer; Fournier 1993), clearly suggesting a magmatic origin. According to crosscutting relationships and common knowledge of carbonatite complexes, a progression exists from early calcitic rocks to late Fe-carbonatite. This progression is in agreement with geochemical data: Mn, which reflects the evolution of a carbonatite complex (Heinrich 1980), increases from the calcitic rocks to the Fecarbonatite. Table 1 shows partial geochemical analyses of the different units from Belanger (1977). La, Ce and metals increase from older to younger units. This pattern is interpreted as the result of magmatic fractionation of these less compatible elements during the crystallization of carbonates (Fournier 1993). Furthermore, the REE deposit is hosted in the youngest and most evolved unit, the Fe-carbonatite. Yttrium, which behaves similarly to heavy rare earth elements (HREEs), decreases with fractionation, suggesting a prior precipitation of HREEs or a low HREE concentration in the starting liquid. Only the LREEs are concentrated by fractional crystallization to form the REE zone (Belanger 1977; Fournier 1993; Gauthier 1979). This fractionation between HREEs and LREEs is common in carbonatite and can be explained by the higher compatibility of HREEs (Green et al. 1992). Compared to the carbonatitic units, the alkaline silicate units are significantly poorer in REEs but richer in Zr. The alkaline complex is covered by Ordovician Trenton limestone and the uppermost 60-120 meters of the Fe-carbonatite were heavily weathered to an orange or red color prior to deposition of the Trenton limestone.
According to previous studies (Fournier 1993; Gauthier 1979), the principal economic REE minerals are late-stage fluoro-carbonates (bastnaesite, parisite and synchisite) that occur as needles a few μm in diameter and up to 20 μm in length. Although both previous studies (Fournier 1993; Gauthier 1979) suggested that mineralization involved a late-stage orthomagmatic hydrothermal origin, they were limited to the first few hundred meters below ground level of the Fe-carbonatite.
Petrographic studies
The macroscopic observations were performed by logging 19 deep drill cores located almost entirely in the REE zone. Microscopic observations were made from 100 polished thin sections taken from these drill cores, selected to cover a representative vertical and lateral distribution. The thin sections were prepared by SGS Canada Inc. with oil-based lubricants and ethylene glycol to preserve the highly soluble minerals, such as halite, present in these rocks.
Scanning Electron Microscope (SEM)
A Zeiss EVOMA15 HD scanning electron microscope coupled with an EDS-SDD from Oxford Instruments (Model X-Max 150) from IOS Services Geoscientifiques Inc., Chicoutimi, Canada, was used. Back-scattered electron images, chemical mapping and phase identification were obtained with the following parameters: 40 Pa, 20 kV with a working distance of 11,5 μm.
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Table des matières
INTRODUCTION GÉNÉRALE
LA ZONE À TERRE RARE DE LA CARBONATITE DE SAINT-HONORÉ, QUÉBEC,
CANADA : UN MODÈLE COMBINÉ MAGMATIQUE ET HYDROTHERMAL
Abstract
1.0 Introduction
2.0 Geological setting
3.0 Methods
3.1 Petrographic studies
3.2 Scanning Electron Microscope (SEM)
3.3 Whole rocks analysis
3.4 Trace element analysis of minerals
4.0 Petrography of the Fe-carbonatite
4.1 REE-poor Fe-carbonatite (PFeC)
4.2 REE-rich Fe-carbonatite (RFeC)
5.0 Geochemistry of the Fe-carbonatite
6.0 Mineral chemistry
7.0 Discussion
8.0 Conclusion
Acknowledgments
9.0 References
Figures for the manuscrit.
Tables for the manuscrit.
CONCLUSION GÉNÉRALE
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