Fluorite and barite
Fluorite is abundant in the Massif Central and occurs in veins in the Hercynian basement and sedimentary formations overlying the Hercynian basement (Triassic sandstones and conglomerates, Infralias).
All these mineralisations are formed close to the basement/sedimentary basin unconformity, generally by the upwelling of fluids that have percolated through the basement and extracted the metals, the deposition being favoured by mixing with basin fluids.
A growing number of direct dates (Sm/Nd on fluorite) and indirect dates (K-Ar on surrounding clays) have enabled us to link these hydrothermal manifestations to major geotectonic events, such as the oceanic rifting stages between 150 and 110 Ma (opening of the Atlantic in the west, then of the Bay of Biscay, and Neothetys in the southeast). This model applies to basement and overburden fluorites, whose modes of formation are virtually identical.
Several major districts mineralised with fluorite (F)-barite (Ba) or lead-zinc (Pb-Zn) are distinguished:
- north-west Massif Central and Poitou Sill (Melle (Pb), Chaillac (fluorite-barite), La Charbonnière),
- Morvan (fluorite (Ba)) Pierre Pertuis, Maine, Antully...), which is the main district with significant exploitable reserves,
- Albigeois (Montroc, Le Burc, Peyrebrune (Zn-F), Trébas, Le Moulinal),
- Cévennes (Les Malines, Zn (Pb-Ba), ...)
Some of these deposits have earned a worldwide reputation among mineralogists thanks to fluorite's exceptional colour or barite's particular habitus (Maine in Morvan).
Sky-blue fluorites are the best-known, with the Burc (Tarn) and Beix deposits in Auvergne and the yellow fluorite of Valzergues (Aveyron).
Barite from Malines (St Laurent le Minier) forms balls of dense, immaculately white crystals on spandrels covered with reddish sphalerite.