Geochemical footprints of peraluminous rare-metal granites and pegmatites in the northern French Massif Central and implications for exploration targeting

Abstract

Peraluminous rare-metal granites and pegmatites (RMGP) formed in late-orogenic settings represent an important source of elements essential to the energy transition and the electronics industry, such as Li, Ta, Nb and Sn. However, exploration for RMGP in crystalline basement is hampered by their typically small size, lack of distinctive petrophysical characteristics, and proximity with earlier, larger barren composite granitic plutons that may mask their presence. This paper reviews the main geochemical features of late-Variscan, 315-to-310 Ma RMGP in the northern French Massif Central (FMC) to better define useful geochemical proxies relevant to exploration targeting. The northern FMC defines a ca. 150 km-long metallogenic province that hosts three types of RMGP: (i) the Beauvoir and Montebras leucogranites, (ii) the Chédeville pegmatite field, and (iii) the Richemont rhyolite. Based on a compilation of whole-rock geochemical data for these RMGP (n = 151) and other late-Variscan peraluminous granites in the FMC (n = 1953), we examine the enrichment of rare elements (Li, F, Be, Nb, Ta, Sn, W) as well as indices of magmatic fractionation and hydrothermal alteration in these settings. The RMGP are variably enriched in Li (372–11,200 ppm; avg = 3500 ppm), F (1475–40,000 ppm; avg = 15,010 ppm), Be (3–506 ppm; avg = 107 ppm), Nb (25–200 ppm; avg = 79 ppm), Ta (23–447 ppm; avg = 116 ppm), Sn (19–13,311 ppm; avg = 1069 ppm) and W (3–312 ppm; avg = 38 ppm), of which the Beauvoir leucogranite is the most enriched. Compared to the peraluminous cordierite-biotite and two-mica granites in the FMC, the RMGP are strongly fractionated with very low Σ(Fe + Mg + Ti), low Nb/Ta (<2), Zr/Hf (<20) and Th/U (<1) ratios, and high Rb/Sr ratios (>10). In addition to these striking features, the RMGP show a systematic spatial association with evolved two-mica granites containing elevated contents of Li (avg = 627 ppm), F (avg = 4462 ppm), Be (avg = 28 ppm), Nb (avg = 32 ppm), Ta (avg = 14 ppm), Sn (avg = 122 ppm) and W (avg = 19 ppm), which are 2–4 times higher relative to other peraluminous two-mica granites in the FMC. The emplacement of these evolved granites was synchronous with magmatism at ca. 330–315 Ma, preceding and partly overlapping the formation of RMGP. Interpolated element distribution maps from the studied areas show large geochemical halos for Li, Be, F and Sn (±W) around the granites spatially associated to RMGP, with concentrations exceeding ten times upper continental crust values, and extending several km from the plutons. Due to their highly-evolved character and large geochemical footprints (>10 s km²), the granitic plutons associated with RMGP represent pre-enriched, specialized precursors for rare-metal mineralization. Given the regional extent of evolved peraluminous leucogranites in the northern FMC, we conclude that there is a significant mineral potential for rare metals (especially Li) and that future geochemical surveys should considerer areas near specialized leucogranites as high-priority targets to search for new concealed RMGP occurrences.