Abstract
The Puy-les-Vignes W deposit, located in the northwestern French Massif Central (FMC), is a rare occurrence of a wolframite-mineralized hydrothermal breccia pipe hosted in high-grade metamorphic gneisses. We present an integrated study of this deposit aiming to characterize the ore-forming hydrothermal system in link with the Variscan late-orogenic evolution of the FMC. Based on a set of representative samples from the host rocks and mineralization, we describe a detailed paragenetic sequence and we provide the major and trace element geochemistry of the granitic rocks and W–Nb–Ta–Sn–Ti oxide minerals, in situ U/Pb and 40Ar/39Ar geochronology, and a fluid inclusion study of quartz and wolframite. We demonstrate that the formation of this W-mineralized breccia pipe results from a multistage development related to four major episodes during the late Carboniferous. The first episode corresponds to the emplacement of an unexposed peraluminous granite at ca. 324 Ma, which generated microgranite dykes exposed at the present-day surface. The second episode is the formation of the quartz-supported breccia pipe and wolframite mineralization at ca. 318 Ma at a paleodepth of 7 km. The mineralizing fluids have a H2O–NaCl–CO2–CH4–N2 composition, a moderate-salinity (< 9 wt.% NaCl eq) and were trapped at high-temperatures (> 400 °C) during lithostatic to hydrostatic pressure variations caused by hydrofracturing of the host rocks. Wolframite deposition is interpreted to result from a W-rich intermediate-density magmatic fluid that exsolved from an evolved leucogranite and interacted with volatile-rich metasedimentary country rocks and/or possibly mixed with low-salinity metamorphic fluids of deep origin. The third episode corresponds to magmatic-hydrothermal Nb–Ta mineralization overprinting the W-mineralized system interpreted to be related to the intrusion at ca. 311 Ma of a rare-metal granite, which is part of a regional peraluminous rare-metal magmatism during the 315–310 Ma period. Finally, the last episode corresponds to disseminated Bi ± Au–Ag mineralization emplaced at ca. 300 Ma, which shares similar mineralogical features with late Carboniferous orogenic gold deposits in the FMC. The Puy-les-Vignes W deposit records, therefore, a multistage and long-lived development that extends over a timespan of 25 million years in a regional setting dominated by protracted peraluminous magmatism and high-temperature and low-pressure metamorphism. Although the local environment of ore deposition is atypical, our results show that the mineral assemblages, alteration styles, and fluid characteristics of the Puy-les-Vignes breccia pipe are similar to those of other peri-granitic W deposits in the FMC.
Highlights
Tungsten and tin mineralization worldwide is spatially associated with granitoid intrusions forming a large variety of magmatic-hydrothermal ore deposits including greisens, veins, stockworks, skarns, pegmatites, porphyries, and breccia pipes (Černy et al, 2005; Romer and Kroner, 2016)
We present an integrated study of the Puy-lesVignes W deposit aiming to characterize the formation of this atypical breccia pipe in link with the Variscan late-orogenic evolution of the French Massif Central (FMC)
We showed that the formation of the Puy-les-Vignes W breccia pipe results from a multistage development during 25 million years related to four major episodes in the late Carboniferous
Summary
Tungsten and tin mineralization worldwide is spatially associated with granitoid intrusions forming a large variety of magmatic-hydrothermal ore deposits including greisens, veins, stockworks, skarns, pegmatites, porphyries, and breccia pipes (Černy et al, 2005; Romer and Kroner, 2016). In France, most W–Sn deposits are known in the French Massif Central (FMC; Marignac and Cuney, 1999), the Armorican Massif (Chauris and Marcoux, 1994), and the French Pyrenees (Poitrenaud et al, 2019) These deposits include wolframite-cassiterite-bearing quartz veins, greisen cupolas, and scheelite skarns spatially associated with Carboniferous peraluminous biotite-cordierite or two-mica granites (Stussi, 1989; Marignac and Cuney, 1999; Bouchot et al, 2005). The Puy-les-Vignes W deposit, located in the northwestern FMC, represents the only known wolframite-mineralized hydrothermal breccia pipe in France. We demonstrate that the formation of the Puy-les-Vignes breccia pipe results from a multistage development between ca. 325 and ca. 300 Ma in a regional setting dominated by protracted peraluminous granitic magmatism and high-temperature and low-pressure metamorphism
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