Génesis del yacimiento de magnesita de Rubián (Lugo): nueva interpretación basada en datos petrológicos y geoquímicos

Abstract

The Rubian sparry magnesite (Galicia, NW Spain) is hosted in Lower Cambrian carbonate rocks (Caliza de Cándana Fm.) included in a strongly deformed and faulted thick succession of siliciclastic and carbonate rocks. The magnesite shows massive and banded fabrics. δ18 O SMOW values range from 13,32‰ to 17,22‰ whilst δ13C PDB values range from -2,32‰ to 2,37‰. Radiogenic strontium isotopes from magnesite exceed 0.711435, considerably higher than those commonly reported for Cambrian marine carbonates. Similarly, REE patterns do not show any sort of anomaly indicative of marine environment. Fluid inclusions can be interpreted as a result of NaCI-CaCI2(±MgCI2) mineralising brines, probably by mixing of two CaCI2 -rich fluids of contrasted salinity, and allow to conclude that magnesite formed at a minimum temperature of 170±15QC. Accordingly, a hydrothermal/metasomatic replacement of preexisting dolostone is proposed as a more reliable process to explain the origin of the sparry magnesite of Rubian, which is in contrast with previous interpretations that supported a syn-diagenetic model for the formation of the deposit.