Geochemical footprints of peraluminous rare-metal granites and pegmatites in the northern French Massif Central and implications for exploration targeting
Résumé
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 km 2 ), 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.
Origine | Fichiers éditeurs autorisés sur une archive ouverte |
---|---|
Licence |