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Mesozoic fluid mixing and Alpine remobilization of Mississippi Valley-type (MVT) F–Ba–Pb–Zn–(Ag) veins in the Grand Châtelard district (Western Alps)
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DOI：10.1007/s00126-026-01475-y.png)
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Abstract
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Ore districts in orogenic belts can record polyphased geological histories integrating both pre‑ and syn‑orogenic processes. The F–Ba–Pb–Zn–(Ag) mineralization of the Grand‑Châtelard district, located within the external crystalline massifs of the Western Alps, exemplifies such complexity and results from two distinct superimposed hydrothermal events. The first, dated to the Mesozoic, is linked to the circulation of moderately saline brines during the rifting of the Western Tethyan margin. These fluids, trapped at [140–175 °C] and low pressures [0.35–0.85 kbar], precipitated fluorite–barite–galena–sphalerite veins filling normal faults affecting both the Variscan basement and its sedimentary cover. The mineralizing system involved basinal brines [22–25 eq. wt% NaCl] migrating along the basement unconformity and normal faults, which locally mixed with hot ascending dilute fluids (< 5 eq. wt% NaCl), producing fluids with intermediate salinities ranging from [1 to 14 eq. wt% NaCl]. This mixing process with fluid–rock interactions was the main driver of metal precipitation, consistent with Mississippi Valley-type deposits elsewhere in Europe. A second Alpine hydrothermal event (ca. 20 Ma; Rb–Sr dating on adularia) overprinted the Mesozoic mineralization during external crystalline massifs exhumation. Circulation of dilute hot fluids ([285–305 °C]; [1.8–2.2 kbar]) induced local remobilization of earlier sulfide and gangue minerals, along with partial chloritization and adularization of the granite host rocks. This fluid‑assisted recrystallization led to a depletion in Ga and Ge in sphalerite, together with local Bi–Ag and Sn–In enrichments in galena and chalcopyrite. These patterns indicate dissolution–reprecipitation-driven redistribution of trace metals and show that Alpine orogeny significantly remobilized critical elements at the local scale, even in the well-preserved Grand Châtelard Mississippi Valley–type district.
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