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 170°C and the presence of two types of fluids: an H20-NaCI low-salinity fluid «8 eq. wt% NaCI) and an H20-NaCI-CaCI2 fluid (7-13 wt% NaCI and 11-14 wt% CaCI2). The low salinity and the Cl/Br and NalBr ratios (CI/Brmolar 100-700and NalBr molar 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 NalBr ratios (CI/Brmolar 700-13 000 and NalBrmolar 700-11 000) generated after dissolution of Triassic age evaporites.

The present paper reports the first detailed petrographical and geochemical studies of hydrothermal dolomites related to MVT Zn-(Fe-Pb) deposits in the Riópar area (Mesozoic Prebetic Basin, SE Spain), constraining the nature, origin and evolution of dolomitizing and ore-forming fluids. Mapping and stratigraphic studies revealed two stratabound dolostone geobodies connected by other patchy bodies, which replace carbonate units of Upper Jurassic to Lower Cretaceous ages. These dolostones are associated to the WE trending San Jorge fault, indicating a main tectonic control for fluid flow. Seven different dolomite types were identified: i) matrix-replacive planar-s (ReD-I); ii) matrix-replacive planar-e (ReD-II); iii) planar-e sucrosic cement (SuD); iv) non-planar grey saddle dolomite cement (SaD-I) pre-dating Zn-(Fe-Pb) sulfides; v) nonplanar milky to pinkish saddle dolomite cement (SaD-II) postdating Zn-(Fe-Pb) ores; vi) ore-replacive planar-e porphyrotopic (PoD); and vii) planar-s cloudy cement (CeD). Meteoric calcite types were also recognized. The different dolomite types are 14 wt.% eq. NaCl), whereas the dolomitization related to ore precipitation (sphalerite and SaD-II) spreads to higher salinity values (5 to 25 wt.% eq. NaCl). These data may respond to a mixing between a low salinity fluid (fluid A, less than 5 wt.% eq. NaCl) and a more saline brine (fluid B, more than 25 wt.% eq. NaCl) at different fluid proportions.

Calculated 6180 of fluids ranges from 03 to + 7.4%0 during barite precipitation, from +0.8 to +4%0 during quartz formation and around + 3%0 during carbonate deposition. The 634S of barite (+ 17 to + 56%0), is explained by sulphate reduction processes (either thermochemically or bacterially mediated) in a system closed with respect to sulphate. The 634S of sulphide (+ 0.6 to -32%0) is compatible with these processes although bacterial processes must have dominated at Villabona. Organic matter was an important source ofC in the fluids especially in Villabona (613C=-14.8 to -2.5%0 in calcites and from -7.9 to -2.2%0 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.

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The origin and age of the hydrothennal fluids related to the precipitation of fluorite, barite and calcite in the Villabona, La