Interprétation de l'anomalie en europium des fluorines hydrothermales — Données de la résonance paramagnétique électronique: Application au gite de fluorine de Montroc (Tarn, France)

Abstract

Rare-earth impurities have been analyzed for twelve samples of fluorites taken from a cross-section of the Montroc vein; two types of measurements have been used: electron paramagnetic resonance (EPR) and neutron activation analysis (NAA). Results obtained show that the sign and the amplitude of the Eu anomaly in the chondrite-normalized lanthanide spectra may be correlated with the majority valence state of Eu in the crystal: a marked positive anomaly relates to a deficit of paramagnetic divalent Eu and, inversely, if divalent Eu is the majority valence state, the Eu anomaly appears to be negative.The results obtained for the Montroc vein are consistent with a model involving discontinuous injections of hydrothermal solutions. They may be related to variations of oxygen fugacity arising from cooling of these solutions and from precipitation of sulfides during fluorite precipitation.