Characterization of rare- element Variscan pegmatite fields: multi- approaches study for new metallogenic guides
Résumé
Rare- element pegmatite fields mainly enriched in Li-Nb and Ta are subject to increasing mining exploration in the last few years. Indeed, Nb and Ta are more and more employed in design of high-tech applications whereas Li from pegmatite deposits is recognized to be less sensitive to supply disruptions than Li from brine deposits. Thus, new metallogenic guides need to be defined/ to meet growing demand for Li, Nb and Ta metals and to ensure supply of strategic metals at European scale. Two Variscan rare-element pegmatite fields have been selected to apply a multi-approaches study in order to understand their distribution and their genesis. Firstly, three methods have been developed on the Monts d'Ambazac pegmatite field (Haute-Vienne, French Massif Central):i) spatial statistical analysis to constrain the spatial distribution of these deposits, ii) Li-isotopic analysis in micas to constrain the genesis relationships between different pegmatite types and iii) U/Pb geochronological approach to discriminate temporal relationships between pegmatites and surrounding granites. Secondly, the Forcarei-Lalin pegmatite field (Galice, Spain) has been selected to constrain spatial and temporal relationships between pegmatites and hosting- rocks with a structural field study and U/Pb geochronology on Colombo-tantalite crystals. The results demonstrate the main role of tectonic context during pegmatite emplacement and secondly, the role of chemistry and rheology of hosting-rocks on pegmatite differentiation type and on their morphology based on field observations. On the other hand, these two pegmatite fields seem to result from direct crustal anatexis and not from a classical granitic differentiation. To refine this hypothesis, preliminary numerical models have been developed to first, constrain the ascent of pegmatitic melt from a deeper crustal source with respect to their physico-chemical properties and second, to investigate the role of tectonic constraints during their emplacement on the pegmatite field 2D- distribution.