Origin of the atypical Puy-les-Vignes W breccia pipe (Massif Central, France) constrained by trace element and boron isotopic composition of tourmaline

Abstract

The Puy-les-Vignes W deposit is an atypical wolframite-bearing hydrothermal breccia pipe hosted in migmatitic biotite-sillimanite gneisses in the northwestern French Massif Central. The deposit is characterized by volumetrically important tourmaline alteration allowing to investigate the fluid evolution of the ore-forming hydrothermal system. Four generations of hydrothermal tourmaline (Tur 1–Tur 4) formed during pre-, syn-, and post-mineralization stages were identified based on detailed petrographic observations and were analyzed in situ for their chemical and boron isotopic compositions. At the grain scale, tourmaline commonly shows oscillatory zoning and dissolution textures resulting from a multi-stage crystallization in a fluid-dominated system. The different generations of hydrothermal tourmaline have dravite-schorl compositions and show similar major and trace element contents falling into the field of metamorphic rocks. High concentrations of V, Cr, Sr and low concentrations of Li, Sn in tourmaline suggest a metamorphic-dominated origin of these elements. The boron isotopic compositions of tourmaline range between −13.3‰ and −7.8‰ and cannot unambiguously distinguish between a magmatic and a metamorphic fluid origin. Our data indicate that the chemical and boron isotopic composition of tourmaline was dominantly controlled by high-temperature fluid-rock interactions between the metamorphic basement and boron-rich, reduced, and low-salinity hydrothermal fluids. Based on these results, we propose a fluid evolution scenario for the Puy-les-Vignes ore-forming hydrothermal system. Release of magmatic-hydrothermal fluids from an unexposed peraluminous leucogranite at ca. 324 Ma is proposed as the main mechanism responsible for early greisenization and formation of disseminated Tur 1. This episode was followed by massive tourmalinization and hydraulic brecciation of the overlying gneisses producing a tourmaline-rich (Tur 2) crackle breccia by interactions with boron-rich hydrothermal fluids. Fluid-assisted reopening and collapse of the former tourmalinite at ca. 318 Ma yielded a matrix-supported quartz-tourmaline breccia pipe and formation of wolframite-bearing quartz veins accompanied by deposition of Tur 3. Finally, post-ore hydraulic fracturing at ca. 300 Ma led to formation of tourmaline-rich microbreccias (Tur 4) possibly from metamorphic fluids.