Chemistry and genetic implications of tourmaline and Li-F-Cs micas from the Valdeflores area (Caceres, Spain)

Abstract

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(-1) (schorl), []AlNa(-1)Mg(-1) (foitite), AlOMg(-1)(OH)(-1) (olenite), and CaMgNa(-1)Al(-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 μm). White mica ranges from lithian muscovite-phengite to lepidolite/zinnwaldite, containing up to 8.40 wt% F, 6.0 wt% Li2O, and 10.73 wt% FeO. Dark mica shows a variable color and has compositions characterized by relatively high contents of Cs2O (1.14-2.78 wt%) and F (1.94-8.08 wt%), with a deficit in K2O (5.75-9.04 wt%). Log (f(H(2)O)/f(HF)) of fluids in equilibrium with biotite in the tourmaline-rich rocks was 4.02-4.17 at T ~ 400 °C. Log (f(H(2)O)/f(HF)) values of fluids in equilibrium with topaz (X(F) ~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.