Chemical variation and significance of micas from the Fregeneda-Almendra pegmatitic field (Central-Iberian Zone, Spain and Portugal)

Abstract

Field, textural, paragenetic, and chemical data for micas from pegmatites of the Fregeneda-Almendra pegmatitic field (Central-Iberian Zone) are used to characterize and evaluate their petrogenesis. These pegmatites show a zonal distribution from barren to evolved, with an increase in degree of evolution with increasing distance from the Mêda-Penedono-Lumbrales leucogranite. Five types of evolved pegmatites have been recognized: (1) petalite-rich, (2) spodumene-rich, (3) Li-mica + spodumenerich, (4) Li-mica-rich, and (5) cassiterite-rich pegmatites, plus six types of barren and intermediate pegmatites. Representative micas from the different pegmatite types and from the leucogranite were analyzed for major and trace elements. All micas belong to the muscovite-lepidolite series. Lithium is incorporated into Li-micas via the Li 3 Al -1 o -2 and Si 2 LiAl -3 substitutions, where o represents vacancies. The Al 4 Si -3 o -1 and Al 2 o 1 R 2+ -3 substitutions, where R 2+ = (Fe + Mg + Mn), account for the compositional variability of micas from the Li-mica-free pegmatites. The Li, Rb, Cs, Be, Ta, and Nb contents of micas increase in the order: leucogranites and barren pegmatites < intermediate pegmatites < spodumene-bearing and petalite-bearing dikes < Li-mica-bearing pegmatites. The Ba content decreases in the same order, and Sn and Zn are relatively abundant in the intermediate pegmatites.These variations are consistent with rare-element enrichment via fractionation processes combined with partitioning of rare elements from the pegmatite melt into the minerals and volatile phases. However, some pegmatite types occurring in this area, such as the cassiterite-rich dikes, do not seem to form part of the same evolutionary trend.

Evolved granitic systems as a source of rare-element deposits: The Ponte Segade case (Galicia, NW Spain)

“…Albite-rich leucogranites II are highly enriched in Al 2 O 3 , P 2 O 5 , Rb, Cs, Li and F. As was established by Shaw (1968), Cerny et al (1985), London (2008), Vieira et al (2011), Rb and Cs are highly incompatible elements that are accumulated, as are Hf and Li, in the most evolved magmatic residual melt. Both Rb and Cs substitute K in minerals such as muscovite and K-feldspar.…”

Geology and mineralogy of Li mineralization in the Central Iberian Zone (Spain and Portugal)

“…In some cases these pegmatites form a pegmatite field around a granitic body. This is the case for the Li-Sn-rich Fregeneda-Almendra pegmatite field (Salamanca-Guarda) (Roda et al, 1999;Vieira et al, 2011) and the Barroso-Alvão field (Northern Portugal) (Lima, 2000;Martins et al, 2012), among others. In other cases, Li enrichment is observed in the marginal and/or apical parts of leucogranites, as in the Pinilla de Fermoselle pegmatite (Zamora) (Roda-Robles et al, 2012b).…”

“…In both cases the dominant mechanism of Li incorporation appears to be the exchange vector Li 3 Al -1 Vac -2 , and to a lesser extent Si 2 LiAl -3 (e.g. Roda-Robles et al, 2006;Vieira et al, 2011, Martins, et al 2012). In the beryl-phosphate pegmatites the mica composition follows the muscovite-biotite trend, with no Li (nor F) enrichment (Fig.…”

“…The Li aluminophosphates and Li-Fe-Mn phosphates were analysed at the Université Paul Sabatier (Toulouse, The Li content of micas was calculated using different equations based on the positive correlation between Li and F (e.g. Tischendorf et al, 2004Vieira et al, 2011;RodaRobles et al, 2006). For other minerals, Li was calculated assuming the ideal content in the structural formulae (for more detail, see Tables 3 to 6).…”

Micas, feldspars and columbite-tantalite minerals from the zoned granitic lepidolite-subtype pegmatite at Namivo, Alto Ligonha, Mozambique

“…Micas and feldspars are good petrogenetic indicators of pegmatite evolution, and consequently their geochemistry has been used to track the consolidation history of pegmatite bodies (e.g., Jahns, 1982;Jolliff et al, 1987;Monier et al, 1987;Č erný, 1994;Č erný et al, 1995;Kile & Foord, 1998;Alfonso et al, 2003;Roda Robles et al, 2006Van Lichtervelde et al, 2008;Neiva & Ramos, 2010;Vieira et al, 2011). However, chemical zoning in micas from pegmatites has rarely been studied (Neiva et al, 2008(Neiva et al, , 2012Van Lichtervelde et al, 2008).…”