Element mobility during the hydrothermal alteration of rhyolitic rocks of the Los Azufres geothermal field, Mexico

The effects of hydrothermal alteration on major, rare-earth, and other trace-element concentrations in rhyolitic rocks of the Los Azufres geothermal field, Mexico, were investigated by statistically comparing the chemical compositions of altered drill cuttings (taken above 450 m depth) with those of fresh rock outcrop samples. Altered rhyolitic rocks show predominantly vitreous and fluidal textures, with alteration products (mainly clay minerals, chlorites and, less commonly, sericite) comprising up to 40% of the rock mass; cryptocrystalline quartz, chlorite, and zeolites fill fractures and vesicles. In altered rocks the greater statistical variances of several elements (Y, Ce, Pr, Nd, Sm, Lu, and Pb) are probably due to alteration effects, whereas smaller variances for CaO, Sr, Rb/Sr, and Rb/Ba suggest that alteration processes have resulted in more uniform chemical compositions. Only MnO, P₂O₅, Ta, Zr, and Nb have significantly different concentrations in hydrothermal altered rocks as compared to fresh rocks. MnO, P₂O₅, Ta, Rb/Zr, and Rb/Nb decrease, whereas Zr, Nb, and Nb/Y increase in the altered rocks. The present study stresses that caution should be taken when using these chemical parameters for petrogenetic studies of old hydrothermally altered areas, particularly with rhyolitic rocks. Rare-earth element (REE) concentrations were not significantly different between fresh and altered rhyolitic rocks. This may indicate that these elements were relatively immobile during the hydrothermal alteration processes affecting the rhyolites at Los Azufres, or more likely that they were reincorporated into hydrothermal minerals after being mobilized from the primary phases.