Origin of the Atrevida Vein (Catalonian coastal ranges, Spain); mineralogic, fluid inclusion, and stable isotope study

Abstract

Atrevida is a Ba-F base metal vein located in the Catalonian coastal ranges of Spain about 100 km southwest of Barcelona. The vein cuts Paleozoic metasediments and granites of the Hercynian basement and Triassic red beds at the base of an overlying transgressive sequence. The Atrevida vein can be divided into three parts on the basis of mineralogy. The lower level contains sugarlike quartz + fluorite + minor barite. The middle level contains a breccia of banded barite + basement rocks cemented by a complex sulfide-arsenide assemblage with galena, Ni sulfide, Ag sulfosalt, and native silver veinlets. The upper level contains symmetrical bands of white barite. Late calcite and marcasite are found in geodes. The only alteration observed is quartz + sericite + chlorite after K feldspar and biotite where the vein cuts granite host rocks.Fluid inclusions in fluorite homogenize at 110 degrees C and have salinities of 21 wt percent NaCl equiv. Inclusions in sphalerite homogenize at 90 degrees C and have salinities of 22 wt percent NaCl equiv. Eutectic results in both minerals are as low as -59 degrees C, indicating that the trapped fluids are complex CaCl 2 -bearing polysaline brines. Fluid inclusions in late calcite homogenize at -0.3 degrees C, indicating that low-salinity waters invaded the vein system during calcite deposition.Sulfur and oxygen isotope analyses reveal that vein barites have delta 34 S values of 14 to 17.9 per mil (CDT) and delta 18 O values of 9.5 to 12.2 per mil (SMOW). Barite also occurs as cement in the Triassic red beds with delta 34 S = 15.3 per mil and delta 18 O = 10.6 per mil. Both the sulfur and the oxygen isotope compositions are within the ranges reported for Lower Triassic- to Lower Jurassic-age evaporites.Banded galena from the middle level of the vein has delta 34 S values between -6.3 and -8.9 per mil, and rimming anglesite has delta 34 S values of -21 to -30.9 per mil. Galena from the central breccia has delta 34 S values of -14.9 to -16.1 per mil and coexisting sphalerite is about -10 per mil. Quartz in the lower level of the vein has delta 18 O in the range 20.7 to 22.7 per mil.We infer that sulfate in the vein was derived directly from seawater or from a solution which dissolved evaporites as it descended through the Triassic section along the fault zone. Barite and sulfides were precipitated where the sulfate-bearing solution mixed with an ascending, Ba-rich brine. The reduced sulfur found in the vein may have been carried by the ascending brine (banded galena) and may also have been produced by bacterial reduction of sulfate from a surficial source (galena in breccia). Oxidizing, nonmarine waters were introduced into the hydrothermal system giving rise to anglesite rims on galena.Mineralization took place in the Late Triassic during an extensional tectonic regime characterized by a high-geothermal gradient. Atrevida, and similar barite veins found throughout the Catalonian coastal ranges, formed along basin margin faults near a paleosurface formed on Hercynian basement. On a broader scale, low-temperature veins are known to occur throughout western Europe and northern Africa. If Atrevida is representative of the group, then the formation of these veins may be linked to Mesozoic extensional tectonics.