Fluid-Rock intraction in the miocene Tejeda Caldera, Gran Canaria, Canary Islands

Abstract

Rhyolite-trachyte tuffs deposited within the Miocene Tejeda caldera (Mogán Group 14-13.3Ma) show evidence of severe fluid-rock interaction. The altered tuffs are restricted to a peripheral zone directly inside the caldera margin, and occur at four distinct horizons within the mid-upper Mogán ignimbrite succession. Upper-Mogán tuffs display pervasive intermediate argillic alteration (smectite+illite+zeolites+adularia) and silicification (microcrystalline quartz+amorphous silica), indicative of low-temperature (≤250°C) near-neutral pH conditions. Quartz+kaolinite+muscovite+chlorite+calcite alteration of mid-Mogán tuffs and breccias may reflect boiling of higher-temperature, acidic hydrothermal fluids at depth. Si+Na+K+Pb+Sr+Rb were highly mobile during fluid-rock interaction, whereas Ti+Zr+Nb were dominantly refractory. Altered intra-caldera tuffs (n=65) have higher δ18O values than equivalent unaltered extra-caldera ignimbrites, reflecting an overall low-temperature near-surface environment in which meteoric water (δD ca.-15‰, δ18O ca.-3‰) was the dominant fluid source. A decrease in δ18O from upper- to mid- Mogán altered tuffs is consistent with an increase in fluid temperature with depth. Unaltered ignimbrites have δD values of -110 to -168‰ (n=6) and ≤0.2wt% H2O, indicative of Rayleigh-type H2O-exsolution. Altered tuffs have δD values of -52 to -117‰ (n=75) and up to 4wt% H2O, reflecting interaction with steam (δD\ll-15‰) or an evolved low-δD fluid. Apparently unaltered ignimbrites between altered horizons (n=13), and shield basalts directly outside the caldera margin (n=6), have elevated δD and H2O values relative to equivalent unaltered rocks, indicative of minor alteration. Supported by numerical modelling, our Gran Canaria data reflect an intrusion-related, structurally controlled epithermal system, in which fluids and/or vapours migrated through intra-caldera tuffs via channelised, porous flow. This study may help to unravel the complex processes of fluid-rock interaction characteristic of both active and fossil caldera-hosted epithermal systems that are presently inaccessible or poorly exposed.