Ore-forming timing of polymetallic-fluorite low temperature veins from Central Pyrenees: A Pb, Nd and Sr isotope perspective

Abstract

Vein fluorite deposits (Tebarray, Lanuza, Bielsa-Parzán, Bizielle and Yenefrito) as well as one MVT-style fluorite mineralization (Portalet) in the Central Pyrenees are the focus in this contribution. These deposits are made up of fluorite, barite, base metal sulphides, calcite, and quartz and are hosted in sedimentary rocks and granites of Palaeozoic age. Generally, these mineral occurrences, typically associated with Late Palaeozoic steeply dipping faults are similar with respect to geologic setting, mineralogy and geochemical trends to other fluorite and base metal veins located in the Central Pyrenees. Veins occurring along such faults most likely represent channelways used by mineralizing solutions that were expelled from the basement. Previous work argued for genetic processes involving circulation of mineralising fluids during the Triassic–Lower Cretaceous period, which is often considered to represent a period of heat, fluid, and mass transfers related to rifting events in the western European basins, which is related to the opening of the Atlantic.A major goal of this study was to decipher the timing of fluid flow and ore formation on the basis of Nd–Sm dating of fluorite sampled from a number of deposits sharing a similar geological framework. No precise age(s) could be obtained due to a scatter in data, but results from the Portalet MVT-style deposit point to a mid-Triassic age (around 220Ma) for this mineralization. The model that best explains the diagenetic stratabound mineralization at Portalet is gravity-driven fluid flow involving basinal brines during a rifting stage. Indeed, the formation of horst and graben structures during Early Alpine extensional tectonics favoured the infiltration of meteoric water into uplifted blocks, followed by fluid migration through the deeper parts of the basins whereby heat and dissolved components were acquired. This model also explains diagenetic changes recorded in the host limestone at Portalet. Also, overall Pb, Sr and Nd isotopic ratios measured in galena and fluorite suggest that differences in host rock and in the lithology of the basement seem to have exerted control on the chemistry of mineralizing fluids providing each deposit with distinctive characteristics.