Pervasive metasomatism that involved the formation of tourmaline-rich rocks and influx of Li, F, and Cs into Ordovician psammo-pelitic metasediments occurred in the Valdeflores area (Caceres, Spain). Numerous Li- and Sn-bearing, mineralized, greisen-type veins also can be observed here, in the vicinity of geochemically specialized granites. Tourmaline-rich rocks appear as: (1) massive, fine-grained, dark green to black rocks; and (2) fine-scale tourmaline-rich laminae, which alternate with quartz-rich layers parallel to the bedding. Electron microprobe analyses indicate that the tourmaline lies mostly within the space defined by the exchange vectors from dravite: FeMg (sub -1) (schorl), AlNa (sub -1) Mg (sub -1) (foitite), AlOMg (sub -1) (OH) (sub -1) (olenite), and CaMgNa (sub -1) Al (sub -1) (uvite). The Fe/(Fe+Mg) ratio ranges mainly from 0.87 to 0.54 and increases with Al in the Y-site. Analytical results and substitutional relations show an insignificant elbaite component. Mica in the tourmalinized rocks is very fine-grained (mostly <50 mu m). White mica ranges from lithian muscovite-phengite to lepidolite/zinnwaldite, containing up to 8.40 wt% F, 6.0 wt% Li 2 O, and 10.73 wt% FeO. Dark mica shows a variable color and has compositions characterized by relatively high contents of Cs 2 O (1.14-2.78 wt%) and F (1.94-8.08 wt%), with a deficit in K 2 O (5.75-9.04 wt%). Log (f H2O /f HF ) of fluids in equilibrium with biotite in the tourmaline-rich rocks was 4.02-4.17 at T nearly equal 400 degrees C. Log (f H2O /f HF ) values of fluids in equilibrium with topaz (X F nearly equal 0.8) in country rock adjacent to contacts with veins, and in equilibrium with amblygonite-montebrasite (X amb = 0.2) in the veins were about 4.30-4.60 and 6.4-6.7, respectively. These variations denote the existence of gradients in relative a HF more than differences of temperature during metasomatism. The lack of tourmaline in the veins is interpreted to reflect the alkalinity and low Fe-Mg contents in the fluids, which precluded the formation of tourmaline. Consequently, most of the boron was expelled into metasediments where tourmaline was produced as a result.
Chemistry and genetic implications of tourmaline and Li-F-Cs micas from the Valdeflores area (Caceres, Spain)
Alfonso Pesquera, Jose Torres-Ruiz, Pedro P. Gil-Crespo, Nicolas Velilla; Chemistry and genetic implications of tourmaline and Li-F-Cs micas from the Valdeflores area (Caceres, Spain). American Mineralogist ; 84 (1-2): 55–69. doi: https://doi.org/10.2138/am-1999-1-206
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