Ta and Sn concentration by muscovite fractionation and degassing in a lens-like granite body: The case study of the Penouta rare-metal albite granite (NW Spain)

The Penouta peraluminous low-phosphorous granite is the most important low-grade, high-tonnage Sn-Ta-Nb-bearing albite granite from the Iberian Massif. A sheet or laccolith shape, instead of a stock, is inferred for the Penouta granite, maybe in relation with the low viscosity and high mobility of a fluorine-bearing melt. Subhorizontal lateral extension of the magma is also inferred via vertical and horizontal geochemical variations. The absence of compositional gaps in variation diagrams, coupled with continuous evolutionary trends of compatible and incompatible elements with height, discard a multi-pulse intrusion and point to a single magma pulse. Mineral chemistry, trace element and least-squares mass balance modelling support a differentiation process from bottom to top in the emplacement place. The absence of switch from incompatible to compatible behaviour (bell-shaped trends) in Sn, Nb and Ta variation diagrams, coupled to experimental constraints on tantalite and cassiterite saturation, suggest that Nb-Ta oxides and probably cassiterite were not fractionated mineral phases, their crystallisation being related to concentration gradients within a trapped intercumulus melt. Major and trace element modelling support that the concentration upwards of Ta and the Ta/Nb ratio could be a consequence of mineral fractionation, with a key role of muscovite (mainly primary) for the Ta/Nb ratio, as this mineral has a higher partition coefficient for Nb than Ta. Our results suggest that fluorine and peraluminosity had a limited effect in the Ta/Nb ratio variations. Hence, Ta enrichment is mainly controlled by fractional crystallisation processes. In most cases, Sn enrichment was also concomitant with Ta, indicating that crystal-melt fractionation processes also played an important role in Sn concentration. Nevertheless, the strongest Sn enrichment in the granite (e.g., central part of the granite body) does not correspond to a significant Ta enrichment. The high affinity of Sn for fluids and the high partitioning of Ta for melt could explain this decoupling. Nevertheless, the magmatic signature of cassiterites in these strongly Sn-enriched zones (central part of the granite body) rules out a hydrothermal subsolidus origin for this fluid. By analogy with models carried out in sill-like bodies it seems likely that the Sn enrichment in the central part of the granite body is related to fluid saturation/degassing occurred in the lower margin, as a consequence of cooling and crystallisation of mostly anhydrous minerals (i.e. second boiling). The vapour exsolved migrated into the hotter melt up to the central part, where it probably was reabsorbed, yielding cassiterite with a magmatic signature. Moreover, we suggest that heat loss in the upper margin of the granite body might also contribute to the formation of a second fluid-saturated zone. As a result, pegmo-aplites and greisen were developed.