Low-temperature hydrothermal alteration of intra-caldera tuffs, Miocene Tejeda caldera, Gran Canaria, Canary Islands

The Miocene Tejeda caldera on Gran Canaria erupted ~ 20 rhyolite-trachyte ignimbrites (Mogán Group 14-13.3 Ma), followed by ~ 20 phonolitic lava flows and ignimbrites (Fataga Group 13-8.5 Ma). Upper-Mogán tuffs have been severely altered immediately within the caldera margin, whereas extra-caldera Mogán ignimbrites, and overlying Fataga units, are apparently unaltered. The altered intra-caldera samples contain minerals characteristic of secondary fluid-rock interaction (clays, zeolites, adularia), and relics of the primary mineral assemblage identified in unaltered ignimbrites (K-feldspar, plagioclase, pyroxene, amphibole, and groundmass quartz). Major and trace-element data indicate that Si, Na, K, Pb, Sr, and Rb, were strongly mobilized during fluid-rock interaction, whereas Ti, Zr, and Nb behaved in a more refractory manner, experiencing only minor mobilization. The δ¹⁸O values of the altered intra-caldera tuffs are significantly higher than in unaltered extra-caldera ignimbrites, consistent with an overall low-temperature alteration environment. Unaltered extra-caldera ignimbrites have δD values between - 110‰ and - 173‰, which may reflect Rayleigh-type magma degassing and/or post-depositional vapour release. The δD values of the altered intra-caldera tuffs range from - 52‰ to - 131‰, with ambient meteoric water at the alteration site estimated at ca. - 15‰. Interaction and equilibration of the intra-caldera tuffs with ambient meteoric water at low temperature can only account for whole-rock δD values of around - 45‰, given that ∆D _clay-water is ca. - 30‰ at 100 °C, and decreases in magnitude at higher temperatures. All altered tuff samples have δD values that are substantially lower than - 45‰, indicating interaction with a meteoric water source with a δD value more negative than - 15‰, which may have been produced in low-temperature steam fumaroles. Supported by numerical modeling, our Gran Canaria data reflect the near-surface, epithermal part of a larger, fault-controlled hydrothermal system associated with the emplacement of the high-level Fataga magma chamber system. In this near-surface environment, fluid temperatures probably did not exceed 200-250 °C.