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

Abstract

Fluorite and barite-rich veins ( + sulphides) of the Sierra del Guadarrama (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 hydrothermal 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 87Sr/86Sr ratio found in fluorite and barite (0.7154-0.7207) is in most cases lower than that found in the host granitoids (0.7194 and 0.7209), suggesting more than one source of strontium. This, in turn, supports previous models that involve mixing of two fluids during mineralization, an ascending hot ( > 300 ° C) and low-salinity fluid and a shallow, cooler ( < 100 ° C) and more saline brine. The ascending fluid probably reacted to a variable extent with the host granitoids, and the most likely source for the shallow fluid derived from a marine/evaporitic environment, with a low 87Sr/86Sr ratio (0.7069 in the Late Jurassic) . The calculated ENd composition of the fluorites (-8.8 to -7.6) as well as the 147Sm/144Nd variation found in this mineral (0.153-0.405) do not conflict with the mixing model although they can be largely explained by interaction of the ascending fluid with the host granitoids. Sulphur isotopic compositions of sulphides range between - 3.4 and - 0.8 ‰ and those of barites from + 15.5 to + 15.8‰. The near δ34S-value of the sulphides suggests that their sulphur was derived from the hydrothermal leaching of the granitoids by the ascending fluid. The δ34S-value of barite is consistent with Upper Jurassic marine sulphate as the source of sulphate in the shallow fluid. The absence of equilibrium between 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° and lOO°C. Although 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 Subbetic realm in Late Jurassic times.