Variability in fluid sources in the fluorite deposits from Asturias (N Spain): Further evidences from REE, radiogenic (Sr, Sm, Nd) and stable (S, C, O) isotope data

Abstract

Fluorite deposits in Asturias (Iberian Peninsula, N Spain) are hosted in Permo-Triassic and Paleozoic rocks. The three main districts of Berbes, La Collada and Villabona preferentially occur along the margins of a Mesozoic basin and comprise veins and stratabound mineralization composed of fluorite, barite, calcite, dolomite, quartz and sulphides. Although the geological framework is similar and fluorite dominates in all deposits, variability in sources and processes has led to each area having its own distinctive characteristics. Sr isotope data of fluorite, calcite and barite (87Sr/86Sr=0.7080 to 0.7105) are compatible with a mixing between seawater and an evolved groundwater that interacted with basement rocks. Sm/Nd ratios in fluorites from Villabona district provide an isochron age of 185±28 Ma (Late Triassic–Late Jurassic), consistent with other hydrothermal events in the Iberian Peninsula and Europe. The total REE content of fluorite increases from Berbes to Villabona by an order of magnitude (0.4 to 9.3). The La/Lu ratio in fluorites decreases from a mean value of 0.36 in Berbes, 0.17 in La Collada to 0.09 in Villabona indicating a strong fractionation between LREE and HREE. Calculated δ18O of fluids ranges from 0.3 to +7.4‰ during barite precipitation, from +0.8 to +4‰ during quartz formation and around +3‰ during carbonate deposition. The δ34S of barite (+17 to +56‰), is explained by sulphate reduction processes (either thermochemically or bacterially mediated) in a system closed with respect to sulphate. The δ34S of sulphide (+0.6 to −32‰) is compatible with these processes although bacterial processes must have dominated at Villabona. Organic matter was an important source of C in the fluids especially in Villabona (δ13C=−14.8 to −2.5‰ in calcites and from −7.9 to −2.2‰ in dolomites). Differences in host rock and position within the basin, and the lithology of the basement, seem to have exerted a strong control on the chemistry of mineralizing fluids providing each district with distinctive characteristics.