Magmatic fractionation and the magmatic-hydrothermal transition in rare metal granites: Evidence from Argemela (Central Portugal)

Abstract

Rare Metal Granites (RMGs) are highly evolved intrusions with specific geochemical signatures that are the result of combined magmatic and hydrothermal processes. In most examples, the early evolution is overprinted by late subsolidus transformations and the role of magmatic and magmatic-hydrothermal fractionation processes remains unclear. The origin of fluids involved in magmatic-hydrothermal systems is also an open question. The Argemela RMG (Portugal) is a small subvolcanic intrusion, which exposes in remarkable continuity a sequence of magmatic, magmatic-hydrothermal and early hydrothermal processes. The intrusion comprises a main granitic facies and a border unit made of a complex alternation of aplitic and pegmatitic facies plus three generations of intragranitic veins. The granitic mineral assemblage includes quartz, K-feldspar, albite, muscovite and accessory montebrasite, cassiterite and columbite-tantalite. The whole-rock chemistry is strongly peraluminous, with very high P2O5, Li and F enrichments, elevated rare metal (Sn, Nb, Ta, W) concentrations and strong depletions in Ca, Fe, Ti, Mg. Trace element zonation in minerals, mainly muscovite and quartz, record crystallization/fractionation processes along the magmatic-hydrothermal transition. Three main stages are recognized, magmatic (crystal fractionation and accessory/minor mineral saturation), magmatic-hydrothermal (fluid/melt element partitioning) and early hydrothermal, corresponding to the segregation and collection of magmatic fluids in the intragranitic vein system. Those are followed by limited metasomatic transformations and a late sulphide stage. Stable isotope (O, H) data demonstrate a magmatic origin for the early fluids and a granite-buffered signature for the vein-forming fluids. The fractionation of rare elements (Sn, Nb, Ta, Li, W, Mn, B, Rb, Cs) from the magmatic to the early hydrothermal stage is quantitatively modelled from the combination of three mechanisms, crystal fractionation, magmatic crystallization of accessory/minor phase (MCMA) and fluid/melt partitioning. Sn and W show a markedly contrasted behaviour with Sn precipitated as disseminated mineralization within the intrusion and W partitioned toward the magmatic fluid and extracted from the intrusion. Geochemical trends along the magmatic-hydrothermal transition do not require phases with unusual properties, such as immiscible hydrous melts to be explained and they do not necessarily result in an amplification of initial rare metal melt enrichments. The Argemela RMG stresses the importance of MCMA to generate magmatic rare element pre-concentrations, which later can be hydrothermally redistributed to form large economic deposits.