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

Abstract

Understanding wolframite deposition mechanisms is a key to develop reliable exploration guides for W. In quartz veins from the Variscan belt of Europe and elsewhere, wolframites have a wide range of compositions, from hübnerite-(MnWO 4 ) to ferberite-rich (FeWO 4 ). Deposition style, source of Mn and Fe, distance from the heat/fluid source and temperature have been proposed to govern the wolframite H/F (hübnerite/ferberite ratio) defined as 100 at. Mn / (Fe + Mn). The Argemela mineralized district, located near the world-class Panasqueira W mine in Portugal, exposes a quartz-wolframite vein system in close spatial and genetic association with a rare-metal granite.Wolframite is absent as a magmatic phase, but W-rich whole-rock chemical data suggest that the granite magma is the source of W. Wolframite occurs as large homogeneous hübnerites (H/F = 64-75%) coexisting with montebrasite, K-feldspar and cassiterite in the latest generation of intragranitic veins corresponding to magmatic fluids exsolved from the granite. Locally, early hübnerites evolve to late more Fe-rich compositions (H/F = 45-55%). In a country rock vein, an early generation of Fe-rich hübnerites (H/F = 50-63%) is followed by late ferberites (H/F = 6-23%). Most Argemela wolframites have H/F ratios lower than at Panasqueira and other Variscan quartz-vein deposits which dominantly host ferberites. In greisens or pegmatitic veins, wolframites generally have intermediate H/F ratios. In those deposits, fluid-rock interactions, either involving country rocks (quartz-veins) or granite (greisens) control W deposition. In contrast, at Argemela, wolframite from intragranitic veins was deposited from a magmatic fluid. Differentiation of highly evolved peraluminous crustal magmas led to high Mn/Fe in the fluid which promoted the deposition of hübnerite. Therefore, the H/F ratio can be used to distinguish between contrasted deposition environments in perigranitic W ore-forming systems. Hübnerite is a simple mineralogical indicator for a strong magmatic control on W deposition.

Contrasting fluid behavior during two styles of greisen alteration leading to distinct wolframite mineralizations: The Echassières district (Massif Central, France)

“…Given the relatively large temperature difference between wolframite c from stockwork and Mazet, it is remarkable to see a similar chemical composition. This 20 also indicates that the Fe/Mn ratio variations in wolframite c are apparently not a function of fluid temperature, as recently suggested as a general characteristic (Michaud and Pichavant, 2019).…”

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

“…Because of Fe depletion in peraluminous magma during fractional crystallization [52], and because hydrothermal fluids can become enriched in Fe from interacting with the host rock (e.g., [15]), in stockwork or porphyry settings the Fe/Mn ratio in wolframite can be a good indicator of the degree of differentiation of the magmatic source, as well as of the extent of orthomagmatic input on the fluid composition and, consequently, on the mineralization [53]. The lower the Fe/Mn ratio, the more evolved the magmatic source should be (rare metal granite or pegmatite), and/or the more wolframite mineralization is controlled by a magmatic fluid (proximal or intragranitic deposit; i.e., host-rock assimilation by the fluid is limited).…”

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

“…Occurring ~13 km east of the world class Panasqueira W mine and ~1.5 km west of the Fundão composite granite, the Argemela granite forms a small almost elliptical body (i.e., 250 x 125 m) in map view and is characterized by a pipe-like geometry in cross-section extending to a maximum depth of 1 km (Michaud, 2019).…”

“…Contacts of the granite with the surrounding low-grade Beira schists (Schist and Greywacke Complex (SGC) of Neo-Proterozoic to Cambrian age) are mostly intrusive except for the northeastern contact, which is affected by a dextral shear corridor (Michaud, 2019).…”

“…In some systems, hydrothermal fluids from various sources can superimpose (e.g., Sheppard, 1977;Vallance et al, 2001). This paper presents geochemical data (major and trace element data on whole-rocks and minerals, O and H stable isotope data) on the Argemela rare metal granite (Central Portugal), a small subvolcanic intrusion in the Portuguese part of the Central Iberian Zone (Charoy and Noronha, 1996;Michaud, 2019). The intrusion is mostly free of subsolidus processes, such as greisenization and kaolinization and it allows the early stages of fractionation to be documented.…”