Rutile from Panasqueira (Central Portugal): An Excellent Pathfinder for Wolframite Deposition

Abstract

Abundant W-rich rutile in the tourmalinized wall-rocks from the Panasqueira W-deposit appears to be a marker of the onset of the main wolframite depositing event. Rutile displays spectacular zoning, both sector (SZ) and oscillatory (OZ). An extensive set of compositional data obtained on crystals, beforehand studied using back-scattered electron images and X-ray maps, was used to address (i) the effects of SZ on differential trapping of minor elements, and (ii) the significance of the OZ in deciphering fluid sources and fluid circulation dynamics. Particular attention was paid to Sn, W (Nb, Ta) concentrations in rutile as pathfinders of the W deposition. Concerning the sector zoning, W is more incorporated than (Nb, Ta) onto more efficient faces, whereas Sn contents are nearly not impacted. The net effect of the sector zoning is thus a progressive increase of the relative weight of Sn from pyramid to prism faces, in combination with a less significant increase in the relative weight of Nb + Ta. The oscillatory zoning concerns most minor elements: W, Nb (Ta), Fe, V, Cr and Sn. In the frequent doublets, the clear bands are in general enriched in W relatively to the dark ones, whereas the inverse is true for Nb and Ta. The doublets may be viewed as the result of the successive influx of (i) a W-rich, Nb + Ta poor fluid, abruptly replaced by (or mixed to) (ii) a Nb + Ta-rich and W-poor fluid. The Nb + Ta-rich fluid could be in turn related to a rare-metal granite layer observed atop of the Panasqueira granite.

Multistage development of a hydrothermal W deposit during the Variscan late-orogenic evolution: the Puy-les-Vignes breccia pipe (Massif Central, France)

“…Such primary textures are similar to those described for Nb-rich ferberite from the Echassières W deposit in the northern FMC (Aïssa et al, 1987;Harlaux et al, 2018a,b). Nb-Ta-rich rutile is found as disseminations within ferberite-1 and forms corrosive overgrowths on Fe-W-rutile, the latter being interpreted as coeval with wolframite deposition similarly to the findings of Carocci et al (2019) in the Panasqueira W deposit (Portugal). Several studies showed that the association of Ta-Nb-ferberite, W-Nb-Ta-rutile, wolframoixiolite, and columbite-tantalite minerals is a marker of the magmatic-hydrothermal transition in RMG-RMP (Breiter et al, 2007(Breiter et al, , 2017Alekseev et al, 2011;René and Škoda, 2011;Zhu et al, 2015Zhu et al, , 2019Michaud et al, 2020).…”

Multiple Generations of Wolframite Mineralization in the Echassieres District (Massif Central, France)

“…These can occur in a variety of geodynamic settings related to plates convergence, i.e., anorogenic magmatism, cordillera-type arc environments (porphyry setting), and mostly collisional setting either during onset or collapsing stages [4][5][6][7]. Although it is recognised that wolframite mineralization in stockwork is commonly associated to greisen alteration ("an assemblage of quartz plus muscovite, accompanied by varying amounts of other distinctive minerals such as fluorite, topaz and tourmaline" [8]) in underlying granites, the genetic link between wolframite mineralization and greisen fluids is still being discussed by some authors [9][10][11][12][13], in particular because wolframite is only rarely observed within the greisen itself (e.g., in the Erzgebirge [14]). Indeed, Lecumberri-Sanchez et al [15] highlight that Fe and Mn, necessary for effective wolframite crystallization, mostly originate from the schist host rock, which would explain the fact that wolframite is commonly restricted to quartz veins located in the host rock.…”

High pressure and temperatures during the early stages of tungsten deposition at Panasqueira revealed by fluid inclusions in topaz

“…-Stage I: after pervasive tourmalinisation of the wall rocks (Stage 0), wolframite (Wfm) crystallised as euhedral prisms at the fracture edges (Carocci et al, 2018(Carocci et al, , 2019(Carocci et al, , 2020. It was followed by massive quartz (Qtz I) precipitation following alpine fissure type opening, with development of euhedral Wfm and then a nearly full vein infilling by Qtz I (Fig.…”