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

Overview & Synopsis

The Echassières district of central France hosts diverse magmatic and magmatic-hydrothermal deposits of rare metals, mostly related to the well-known Beauvoir granite. Tungsten mineralization crops out at three distinct wolframite occurrences, the two most important of which are related to two distinct magmatic bodies, emplaced ca. 335 and 310 Ma (Monnier et al., 2019). The mineralization occurred at 335 Ma formed during a hydrothermal episode marked by precipitation of topaz replacing quartz in a stockwork system and as veinlets in the surrounding schist. Fluid inclusions in topaz and quartz display similar features, i.e., all have low salinity, contain significant amount of LiCl, display constant liquid/vapor ratios, and homogenized within a narrow temperature range (T h ≈ 380°C). No evidence for fluid pressure variations was observed, and temperature decrease is considered to be the main cause for wolframite deposition. The younger W mineralization is related to greisenizing fluids that altered the Beauvoir granite and generated several quartz (± topaz and apatite) veins. All greisen-related fluid inclusions display low salinity, however, T h are spread from ca. 190 to 400°C, and several populations exhibit heterogeneous liquid/vapor ratios while others consist of only vapor-rich fluid inclusions. Respectively, these populations are interpreted to have been trapped during boiling or flashing (vaporization) of the fluid. In contrast with the other regional veins, flashing was particularly intense in the Mazet veins, which host the bulk of the last wolframite generation. Consequently, it is proposed that flashing is the key factor that triggered W precipitation. This work highlights the role of two physical parameters, pressure and temperature, whose variations played a preponderant role on wolframite mineralization. It documents, in depth, an example of greisen fluid evolution, providing critical information on W behavior in orthomagmatic fluids, and on greisen-related rare-metal deposits.