Recurrent hydrothermal activity induced by successive extensional episodes: the case of the Berta F-(Pb-Zn) vein system (NE Spain)

A mineralogical and geochemical (fluid inclusion, stable and radiogenic isotopes) study of the Berta F-(Pb-Zn) vein system has identified the source and temperature of the fluid reservoirs involved and proved the existence of two separate hydrothermal events at the mine scale, which reflect distinct periods of regional fluid circulation. Main stage minerals (fluorite I, sulphides, calcite I and barite I) precipitated by mixing between a polysaline H2S bearing (delta(34)S = 11parts per thousand) brine (up to 23% NaCl eq salinity) and a more dilute fluid (delta(18)O from -3.2parts per thousand to 0parts per thousand), at temperatures between 80 and 150degreesC. The progressive increase in Sr-87/Sr-86 ratio from the early precipitated minerals (0.71242 in calcite 1) to the late ones (0.71894 in fluorite 11) is mainly (but not exclusively) due to a difference in age separating the two hydrothermal events. The assumed genetic model for the main stage fluorite (1) is based on a convective circulation of surficial waters leaching the crystalline basement rocks acquiring a high salinity, high Sr-87/Sr-86 ratios and a high temperature. These fluids then mixed with low salinity-low temperature waters, having a low Sr-87/Sr-86 ratio. An at least Jurassic age is suggested for the main period of vein filling, contemporaneous with the extensional regime during the Mesozoic, when fluid circulation was probably enhanced by crustal thinning. During the early Burdigalian (lower Miocene), a new period of important extension in this area took place. Hydrothermal activity related to this new and younger extensional regime is geochemically different and produced a distinctive mineralogical record, developing a set of veinlets filled with green octahedral fluorite (fluorite 11), calcite (11) and barite (11). The Sr isotope compositions of these late stage vein minerals are compatible with leaching the granodiorite host-rocks during recent times. The existence of successive hydrothermal events in the same area is not surprising as geothermal systems, like La Garriga-Samalus, are still active and currently precipitating fluorite. (C) 2002 Elsevier Science B.V. All rights reserved.