Structure,hydrothermal alteration and composition of the Rubiales Pb-Zn orebody (Lugo,Spain): Genetic model

The Rubiales Pb-Zn ore deposit, northwestern Spain, is situated in the Westasturian-Leonese zone, according to the division of the Hercynian Chain in the Iberian Peninsula (Julivert et al. 1972). The orebody is placed in a subvertical shear zone developed at the eastern limb of the Baralla syncline, within the middle and upper members of the lower Cambrian Transition Series. The deposit is a vertical lenticular mass with a N30°W direction. Its length is about 1200 m in a N-S direction by 600 m wide and has an average thickness of 30 m.Its mineralogy is simple: 99% of the sulphides consist of sphalerite and galena with a ratio of 7 to 1. The remaining 1% is mainly formed by pyrite and chalcopyrite with pyrrhotite traces. The deposit shows a large aureole of hydrothermal alteration which is the result of three consecutive processes: (1) sericitization of slates and ankeritization of limestones; (2) silicification of ankeritic rocks; and (3) chloritization of the lower part of the deposit.Since the deposit was discovered in 1967 there has been controversy concerning its origin. Two hypotheses have been considered so far: (1) a sedex model formation (Gilissen 1977; Vazquez 1987); and (2) a hydrothermal origin in a shear zone during the Hercynian Orogeny (Merayo et al. 1984; Arias 1988). The data herein presented support the second hypothesis.