Sources and composition of fluids associated with fluorite deposits of Asturias (N Spain)

Abstract

The fluorite deposits of Asturias (northern Iberian Peninsula) are hosted by rocks of Permo-Triassic and Palaeozoic age. Fluid inclusions in ore and gangue minerals show homogenization temperatures from 80 to 170C and the presence of two types of fluids: an H2O–NaCl low-salinity fluid (<8 eq. wt% NaCl) and an H2O–NaCl–CaCl2 fluid (7–13 wt% NaCl and 11–14 wt% CaCl2). The low salinity and the Cl ⁄ Br and Na ⁄ Br ratios (Cl ⁄ Brmolar 100–700 and Na ⁄ Brmolar 20–700) are consistent with an evaporated sea water origin of this fluid. The other end-member of the mixture was highly saline brine with high Cl ⁄ Br and Na ⁄ Br ratios (Cl ⁄ Brmolar 700–13 000 and Na ⁄ Brmolar 700–11 000) generated after dissolution of Triassic age evaporites. LA-ICP-MS analyses of fluid inclusions in fluorite reveal higher Zn, Pb and Ba contents in the high-salinity fluids (160–500, 90–170, 320–480 p.p.m. respectively) than in the low-salinity fluid (75–230, 25–150 and 100–300 p.p.m. respectively). The metal content of the fluids appears to decrease from E to W, from Berbes to La Collada and to Villabona. The source of F is probably related to leaching of volcanic rocks of Permian age. Brines circulated along faults into the Palaeozoic basement. Evaporated sea water was present in permeable rocks and faults along or above the unconformity between the Permo-Triassic sediments and the Palaeozoic basement. Mineralization formed when the deep brines mixed with the surficial fluids in carbonates, breccias and fractures resulting in the formation of veins and stratabound bodies of fluorite, barite, calcite, dolomite and quartz and minor amounts of sulphides. Fluid movement and mineralization occurred between Late Triassic and Late Jurassic times, probably associated with rifting events related to the opening of the Atlantic Ocean. This model is also consistent with the geodynamic setting of other fluorite-rich districts in Europe.