Non-destructive characterization of variously colored gypsum and aragonite/calcite speleothems from the Cigalère Cave (Ariège, France)

Le résumé fourni par la source

The Cigalère Cave is a 21 km-long karstic cave located in the Ariège Department, in the French Pyrenees, and underlies directly the Bentaillou Pb-Zn-Fe sulfide ores. The cave hosts abundant gypsum mineralizations, some of them exhibiting various colorations including blue, yellow, purple, orange and black. Due to strict preservation policies, these colored mineralizations have not been studied much. Here we propose a non-destructive characterization of five distinct gypsum or carbonate speleothem structures from the Cigalère, exhibiting different colorations. To comply with the preservation policies, no sample was taken from the cave, and all speleothems were analyzed in situ using portable X-Ray fluorescence spectrometry (pXRF), with two custom-made calibrations respectively adapted to carbonate and sulfate speleothems. The “Cascade Noire” is suggested to be colored black and orange by coatings composed respectively of hematite/goethite and jarosite. The “Chapelle de Donnea” speleothems are likely colored black by different Mn oxides (likely birnessite) covered by a thin gypsum or calcite layer. The cores of gypsum speleothems are colored blue by Cu2+ substitution, which may be influenced by the presence of blue Zn-bearing phases. Yellow carbonate speleothems from the “Gino affluent” may be colored by the presence of humic substances, which could not be detected by pXRF. Field observations and comparison of the speleothems’ composition with ores from the overlying Bentaillou mine suggest that all the metallic elements and sulfate ions originate from the weathering of sulfide ores by groundwater and subsequent transportation through joints and fractures. This work also show the capabilities and limitations of pXRF analysis and its potential application in other preserved caves.

Ce résumé expose les affirmations des auteurs. BNTIC ne l’interprète pas comme une validation indépendante des résultats.