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

Julie Anne-Sophie Michaud 1, 2 Michel Pichavant 1, 2
2 Magma - UMR7327
ISTO - Institut des Sciences de la Terre d'Orléans - UMR7327
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.
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Julie Anne-Sophie Michaud, Michel Pichavant. The H/F ratio as an indicator of contrasted wolframite deposition mechanisms. Ore Geology Reviews, Elsevier, 2019, 104, pp.266-272. ⟨10.1016/j.oregeorev.2018.10.015⟩. ⟨insu-01918865⟩

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