MICA AS A PETROGENETIC INDICATOR OF PEGMATITIC EVOLUTION: THE LI-F-BE-B-P-BEARING PINILLA PEGMATITE (ZAMORA, SPAIN)

The highly fractionated, Li-F-Be-B-P-bearing Pinilla de Fermoselle (PF) pegmatite (Zamora, Spain) appears as a cupola over the PF leucogranite, and it displays a complete sequence from a barren pegmatitic facies near the granite, to a highly evolved one in the uppermost part of the body.

Micas of the PF pegmatite show a wide compositional and textural variation. Muscovite is the most abundant mica in all the pegmatite. Li-micas only appear in the most evolved zone of the body. The Fe-rich micas are also common. Biotite is common in the leucogranite and in the barren zone. In the intermediate zone zinnwaldite appears, and in the most evolved zone a Fe-rich lepidolite is found.

Micas from the PF pegmatite may be grouped into two categories: Al-rich and Fe-rich micas. Al-rich micas show continuous compositional trends from the leucogranite to the most evolved zone, with a decrease in the Al, Mg, and Ti contents, simultaneously with an increase in the F and Li contents. Thus, a continuous evolution from muscovitic to lepidolitic compositions is observed from leucogranite to the most evolved zone. The different correlations observed between F, Li, Al, and Si suggest that the incorporation of Li was mainly controlled by the Li₃Al_-1 vac_-2 and SiLi₂Al_-2vac_–1substitution mechanisms.

Chemical composition of Fe-rich micas ranges from Fe-biotite to an intermediate composition between zinnwaldite and trilithionite, that means that both Al-rich and Fe-rich mica series evolve inside the pegmatite towards the same composition. For these Fe-rich micas, the different correlations observed between Li, F, Al and Fe indicate that the substitution schemes that may have operated to incorporate Li could be Fe²⁺_-2LiAl, SiLi₂Al_-2vac_-1 and Al_-1vac_-1Fe²⁺Li.

Salient mineralogical and chemical characteristics, as well as field and textural relationships suggest that the PF pegmatite derived from a granitic precursor and evolved by fractionation processes. This model is supported by: (1) the gradual enrichment in Li, Rb, Cs and F, parallel to the decrease in other elements such as Mg and Ti; (2) the convergent evolutionary trends towards Fe-rich lepidolite in the most evolved zone 3 showed by the Al- and Fe-micas; and, (3) the decrease in the K/Rb ratio in micas from zone 1 to zone 3 in the pegmatite.