From magmatic to hydrothermal Sn-Li-(Nb-Ta-W) mineralization: The Argemela area (central Portugal)

Abstract

The Argemela mineralized area (AMA), central Portugal, exhibits a nearly continuous magmatic to hydrothermal ore-forming sequence typical of Variscan granite-related rare metal deposits. Disseminated and vein-type mineralization are distributed in two systems, the Argemela Mine (AM) and the Cabeço da Argemela (CA). Disseminated mineralization includes montebrasite, cassiterite and columbite-tantalite dispersed in rare metal granites. Vein-type mineralization occurs both in granites and country rocks. The CA system exposes three generations of intragranitic veins with montebrasite in the two earliest and montebrasite, wolframite, cassiterite and columbite-tantalite in the latest. Country rock veins include a swarm of cassiterite and montebrasite veins (AM) and rare isolated wolframite veins (CA). Disseminated cassiterites are enriched in Ta 2 O 5 and Nb 2 O 5 compared to vein cassiterites. Disseminated columbite-tantalites evolve toward Mn-and Ta-rich chemistries that contrast with the more Fe-and Nb-rich compositions in intragranitic veins. In the CA system, both intragranitic and country rock veins crystallize early Mn-rich wolframites followed by late more Fe-rich compositions associated with Nb-, Ta-poor cassiterite, the later replaced by stannite. Field relations, structural, mineralogical and geochemical data suggest that the disseminated and vein-type mineralization are expressions of a continuous metallogenic evolution initiated at the magmatic stage, pursued during the magmatic-hydrothermal transition and ended with hydrothermal circulations in country rock. The two mineralized systems share similarities, but they also show major differences. Structural analysis demonstrates that the intragranitic veins in the CA and the country rock veins in the AM were emplaced under the same tectonic regime. Therefore, the two systems are variants of the same local metallogenic evolution. Sn and W show markedly contrasted behaviors. Both have a magmatic source but only Sn reaches concentrations leading to saturation of the melt with cassiterite. W is deposited later and preferentially to Sn in intragranitic veins. At the hydrothermal stage, cassiterite and wolframite deposition are disconnected and, overall, wolframite occurs only in minor amounts. The metallogenic model proposed for the AMA emphasizes magmatic rare metal concentration processes, exsolution of magmatic fluids and selective deposition of metals during the magmatic-hydrothermal and later hydrothermal stages. The Sn-Li-(Nb-Ta)-dominated, and W-poor signature of the AMA makes it similar to LCT pegmatites of the Central Iberian Zone but is distinctive from the world class W-(Sn-Cu) Panasqueira deposit nearby.

The H/F ratio as an indicator of contrasted wolframite deposition mechanisms

“…Although this Mn/Fe ratio is unknown, Variscan RMG (e.g., Beauvoir, Raimbault et al, 1995) and pegmatites (Chèdeville, Raimbault, 1998), and equivalent volcanic products (Macusani, Pichavant et al, 1987b), demonstrate a general increase of Mn/Fe during magmatic differentiation. The whole-rock data for the Argemela granite show such an increase (Charoy and Noronha, 1996), also marked in the enrichment of disseminated columbo-tantalites in MnTa 2 O 6 (Michaud et al, 2018, see also Raimbault, 1998;Linnen and Cuney, 2005). Therefore, the magmatic evolution of the Argemela RMG led to the exsolution of relatively high Mn/Fe igneous fluids.…”

“…Three generations of intragranitic veins occur within the granite. Field evidence (confinement of veins inside the granite), structural (granite and veins are emplaced in the same structural context), mineralogical (veins lack hydrothermal alteration aureoles and the earliest have a "pegmatitic" texture and a granitic mineral assemblage) and stable isotope data indicate that the intragranitic veins are the expression of fluids exsolved from the granite at the magmatic-hydrothermal transition (Michaud et al, 2018). Wolframite occurs in variable amounts in the third and latest generation of intragranitic veins which are numerous but of restricted size, never exceeding a 10 cm width.…”

“…At Argemela, the W-bearing intragranitic veins show no hydrothermal interaction aureole at their margins. The earliest veins exhibit "pegmatitic" textures and crystallize a granitic mineral assemblage (Michaud et al, 2018). Quartz in the granite and veins both have high δO 18 (respectively 13.7-14.5 and 14.5-15.9 ‰).…”

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

“…1). Geometrical relations, structural and mineralogical data enable three generations of veins to be distinguished (see details in Michaud et al, 2020). The first and earliest generation (Type I, Fig.…”

Incipient Wolframite Deposition at Panasqueira (Portugal): W-Rich Rutile and Tourmaline Compositions as Proxies for the Early Fluid Composition

“…Rb-Sr (Priem and Den Tex, 1984) methods. Close to the deposit, the Fundão granodiorite (Garcia, 2004) and the small Argemela RMG intrusion (Michaud et al, 2020) are associated with a quartz-160 cassiterite vein system (Inverno et al, 2009). 161…”