Variscan pegmatites as new potential mineral resources – VARPEG

VARPEG combines several types of approaches and methods. Field studies (structural relations, geochronology, thermobarometry) concentrate on the 4 VARPEG target sites (Limousin, Montagne Noire, Galicia, Catalonia). New quantitative spatial analysis methods are developed and applied to constrain the location of pegmatite bodies in relation with their geological environment. The segregation and ascent of pegmatite melts from their source region is modelled numerically. High pressure and temperature experiments are being performed to investigate the genesis of pegmatitic melts as low melt fraction products of anatexis of crustal protoliths. The geochemical characteristics of variscan pegmatites are determined by combining major and trace element data on whole-rocks and minerals, as well as the use of isotopic tracers including among others Li isotopes. Processes of crystallization of pegmatitic melts are investigated experimentally to bracket conditions responsible for the development of typical pegmatitic textures and constrain timescales of pegmatite consolidation. Finally, mineralogical studies (composition of key mineral carriers and distribution of rare elements and critical metals in pegmatitic minerals) are complemented by experimental measurements of mineral/melt partition coefficients.

a. structural contexts of pegmatite emplacement. Following previous results on Limousin and Galice, work has started on the Montagne Noire and Cap de Creus. Structural contexts of pegmatite emplacement and spatial relations with their hosts have been determined for the Montagne Noire and a similar work is now being carried out on the Cap de Creus.
b. geochronological data. Ages have been obtained both for the Montagne Noire and the Cap de Creus districts. For the Montagne Noire, the Rb-Sr data on lepidolite confirm previous results. For the Cap de Creus, the pegmatites were dated by U-Pb laser ablation ICP-MS on zircon and columbite minerals (Van Lichtervelde et al., 2017).
c. conditions of partial melting. They have been determined by using the Perplex software for two samples from la série de la Dronne which represent potential source rocks for the Limousin pegmatites. Results yield conditions close to the solidus. The same conclusion applies to micaschists which host pegmatites in Galicia, suggesting that, in both areas, the pegmatite parent melts are not formed in situ but are generated at deeper structural levels from protoliths metamorphosed at higher grades.
d. 7Li/6Li database for Variscan pegmatites. The new data on micas from the Montagne Noire show strong similarities with results on Limousin. There is no correlation between 7Li/6Li and the degree of fractionation of the pegmatite. Results suggest control of the Li isotopic compositions by the nature of the protolith. For the Limousin, reconstructed compositions of pegmatitic and granitic melts using micas trace element data are very similar, and so pegmatites are not products of granite crystallization.
e. Testing of the protolith model. A partial melting trace element model of crustal protoliths has been constructed. At the difference of “global” melting models, the new “modal” model distinguishes between residual phases and minerals newly formed during partial melting.

Focus
1. Initiation and development of an integrated research project (structural geology, mineralogy, petrology) on the Cap de Creus
2. Comprehensive structural, mineralogical, geochronological and geostatistical work on pegmatites from the Montagne Noire
3. Age determinations for the Cap de Creus pegmatites
4. Development of new methodologies for geostatistical modelling of the distribution of pegmatites and applications to natural cases
5. Demonstration from mica trace element chemistries that leucogranites and pegmatites crystallize from independent crustal magma batches
6. Experiments simulating the crystallization of pegmatitic melts stress the importance of very high degrees of undercooling to generate pegmatitic textures.
Perspectives
1. Initiation of numerical models for the segregation and ascent of pegmatitic melts from their source region
2. Influence of deformation on the spatial distribution and trace element composition of experimental partial melts
3. Systematic experimental determination of the influence of the degree of undercooling, cooling rate and final temperature on the development of pegmatitic textures
4. Measurement of mineral/melt partition coefficients for rare elements and critical metals

Leopold F., Gumiaux C., Gloaguen E., Guillou-Frottier L., Druguet E. (2017). Pegmatites as partial melting products of host migmatites, as demonstrated by field data and spatial statistical analysis. Submitted to Lithos.
Villaros A., Pichavant M., Deveaud S. (2017). Mica-melt trace elements partitioning and the granite-pegmatite connection: The St-Sylvestre complex (Western French Massif Central). Submitted to Geochimica and Cosmochimica Acta.
Van Lichtervelde, M., Grand’Homme, A., de Saint-Blanquat, M., Olivier, P., Gerdes, A., Paquette, J.-L., Melgarejo, J. C., Druguet, E., Alfonso, P. (2017). U-Pb geochronology on zircon and columbite-group minerals of the Cap de Creus pegmatites, NE Spain. Mineral. Petrol., 111:1–21.
Devineau K., Champallier R., Pichavant M. (2016). Searching for the formation of specific textures of granitic pegmatites. 15th International Conference on High Temperature Materials Chemistry, 29 mars-1er avril, Orléans, France.
Gumiaux, C., Gloaguen, E., Deveaud, S., Silva, D., Michaud, J., Leopold, F., Branquet, Y., Lima, A., Pichavant, M. (2016). Analyse spatiale statistique appliquée aux champs de pegmatites minéralisées. 25ème RST, Caen, p. 153.
Pichavant, M., Villaros, A., Deveaud, S., Scaillet, B. and Lahlafi, M. (2016). The influence of redox state on mica crystallization in leucogranitic and pegmatitic liquids. Can. Mineral., 54, 1-24.

The supply of raw materials to European countries is becoming under increasing pressure. France makes no exception to this situation which stimulates national initiatives in the field of mineral exploration. The mineral potential of granitic rocks is now being re-evaluated, and new types of granite-related mineralizations, such as granitic pegmatites, are being considered as exploration targets. Pegmatites form a significant proportion of Variscan terranes. They consist of complex internally zoned small to medium size (< 0.1-1 km3) bodies with marked variations in texture as well as in grain size, and are characterized by the occurrence of unusually large crystals. Granitic pegmatites are among the most fractionated igneous rocks on Earth. They concentrate a wide range of critical elements (Be, Cs, Li, Nb, Sb, Sn, Ta, W). Besides being potential sources of these elements, granitic pegmatites also produce quartz, feldspars and mica phases of interest for the mineral and ceramic industries.
Since the renewal of mineral exploration must be accompanied by an update of the scientific knowledge, an integrated research programme coordinated at the French national scale is proposed on Variscan granitic pegmatites. The program can be viewed as a comprehensive test of the anatectic origin of pegmatites. It provides a re-evaluation of the classical model of granite derivation and constitutes a possibly new paradigm for these potential mineral resources. It comprises three main parts and includes fundamental conceptual aspects and others more practical allowing exploration operations to be optimized and pegmatite bodies to be targeted.
(A) Classical models of pegmatite formation assume their derivation from less fractionated magmas represented in the field by granitic plutons. However, many examples of pegmatites show neither spatial nor genetic relations with granites, leading to the alternative model of pegmatitic melts being generated by crustal melting. In this new model, pegmatite emplacement would be determined by the ascension of pegmatitic batches from their source. It is proposed to test this new model on selected examples of pegmatite fields from Limousin, Galicia, Catalogna and Montagne Noire. In parallel, spatial analysis techniques will be applied on the same pegmatite fields to quantify the distribution of the pegmatite bodies. Finally, a numerical approach will be implemented to better constrain the physical processes associated with the ascent of pegmatite melts.
(B) The genesis of pegmatites by low degree partial melting of crustal sources will be tested experimentally. Granitic to pegmatitic melts will be generated under variable P-T conditions, for different protoliths (including enriched lithologies) and under different fluid regimes. The resulting glasses and residual phases will be analyzed for major and trace elements (Li and critical elements). In addition, an isotopic characterization approach will be implemented to constrain the origin of pegmatites from the above districts. It will be based on the coupled use of 18O/16O and 7Li/6Li which allows the influence of the source and of magmatic differentiation to be deciphered. Finally, studies of crystallization kinetics will be carried out to constrain timescales of pegmatite consolidation.
(C) the differentiation and internal evolution of rare-element pegmatites involves poorly known mechanisms such as melt immiscibility and boundary layer processes. These aspects will be studied experimentally in presence of HFSE metals (Nb-Ta-Ti) to assess their partitioning between immiscible melts and their concentration in boundary layer melts. In parallel, crystallization experiments from pegmatitic melts doped with rare metals will be performed to measure the partition coefficients of the various metals between HFSE minerals and to calibrate thermobarometers.