THE AGE AND ORIGIN OF THE BARITE FLUORITE (PB-ZN) VEINS OF THE SIERRA-DEL-GUADARRAMA (SPANISH-CENTRAL-SYSTEM, SPAIN) - A RADIOGENIC (ND, SR) AND STABLE-ISOTOPE STUDY

Fluorite and barite-rich veins (+sulphides) of the Sierra del Guadarra ma (Spanish Central System) are usually hosted by Hercynian granitoids . Zoned fluorites from one of these veins have a Sm-Nd age of 145 +/- 18 Ma. This age is similar to the K-Ar ages obtained in other hydrothe rmal rocks of the Spanish Central System (156-152 Ma) and confirms the existence of an important hydrothermal event during the Late Jurassic , probably related to extensional fracturing accompanying the opening of the North Atlantic Ocean. The Sr-87/Sr-86 ratio found in fluorite a nd barite (0.7154-0.7207) is in most cases lower than that found in th e host granitoids (0.7194 and 0.7209), suggesting more than one source of strontium. This, in tum, supports previous models that involve mix ing of two fluids during mineralization, an ascending hot (> 300-degre es-C) and low-salinity fluid and a shallow, cooler (< 100-degrees-C) a nd more saline brine. The ascending fluid probably reacted to a variab le extent with the host granitoids, and the most likely source for the shallow fluid derived from a marine/evaporitic environment, with a lo w Sr-87/Sr-86 ratio (approximately 0.7069 in the Late Jurassic). The c alculated epsilon(Nd) composition of the fluorites (-8.8 to -7.6) as w ell as the Sm-147/Nd-144 variation found in this mineral (0.153-0.405) do not conflict with the mixing model although they can be largely ex plained by interaction of the ascending fluid with the host granitoids . Sulphur isotopic compositions of sulphides range between -3.4 and -0 .8 parts per thousand and those of barites from +15.5 to +15.8 parts p er thousand. The near 0 parts per thousand deltaS-34-value of the sulp hides suggests that their sulphur was derived from the hydrothermal le aching of the granitoids by the ascending fluid. The deltaS-34-value o f barite is consistent with Upper Jurassic marine sulphate as the sour ce of sulphate in the shallow fluid. The absence of equilibrium betwee n reduced and oxidized sulphur species can be easily interpreted as a consequence of the short residence time of the mixed fluid in the open fractures at temperatures between 250-degrees and 100-degrees-C. Alth ough no pre-Cretaceous sediments are found on this part of the Iberian Hercynian basement, the data presented here support the existence of a shallow platform joining central Spain with the Iberian or the Subbe tic realm in Late Jurassic times.