Micas of the muscovite-lepidolite series from the fregeneda pegmatites (Salamanca, Spain)

Abstract

In the Fregeneda area different types of pegmatites can be recognized by their mineralogy, morphology, internal structure and field relationships. The most common type corresponds to a simple pegmatite with homogeneous internal structure, but zoned Li-bearing pegmatites also are relatively widespread. Cassiterite-bearing pegmatites are subordinate. The pegmatites are spatially associated to the Lumbrales granite. This is a parautochthonous, fine- to medium-grained, two-mica granite, one of the syntectonic massifs which were deformed during the third phase of Hercynian deformation. Representative micas selected from the different groups of pegmatites were studied to determine wether the pegmatites can be related by a common fractionation path, and how different pegmatite types are related to the spatially associated Lumbrales granite. Compositional variations in the micas depend on the pegmatite type. Muscovite coexisting with Li-micas in the Li-bearing pegmatites is one of the richest in Al2O3 (35.4–37.7 wt%) and the poorest in FeO (0.2–1.5 wt%) and MgO (0–0.3 wt%), whereas muscovite of the simple discordant pegmatites shows the highest FeO (2.2–3.3 wt%) and that from the Lumbrales granite is the richest in MgO (0.5–0.7 wt%) and TiO2 (0.6–1.1 wt%). On the other hand, Sn (70–1168 ppm), Li (< 5–22253 ppm), F (880–21470 ppm), Cs (< 5–1696 ppm), Rb (800–9181 ppm) and other trace elements seem to increase with distance from the Lumbrales granite, and K/Rb decreases. According to this ratio, the exterior Li-bearing pegmatites are the more evolved, whereas the interior pegmatites are less evolved, and are richer in Cs, Li and Zn than other pegmatite types.