Fluid-rock interaction in the Tejeda Caldera, Gran Canaria, and the Las Canadas Caldera, Tenerife (Canary Islands)

Abstract

This PhD thesis investigates the processes, products, and timing of fluid-rock interaction in the fossil hydrothermal systems of the Tejeda caldera, Gran Canaria and the Las Canadas caldera, Tenerife (Canary Islands), using field observations, and a combination of mineralogical (thin section petrography, XRD, SEM), geochemical (XRP, O- and H-isotopes, K-Ar dating), and numerical modelling techniques. Three case studies are presented. Chapters 2-4 investigate the mineralogical and geochemical changes brought about by fluid-rock interaction in a sequence of altered ignimbrites (ash- flow tuffs) exposed along the margin of the Tejeda caldera on Gran Canaria. Chapter 5 examines the hydrothermal processes occurring in the Tejeda Intrusive Complex on Gran Canaria, while Chapter 6 explores the nature and timing of hydrothermal alteration in the Las Canadas caldera on Tenerife. The major findings of each case study are summarised below. Rhyolite-trachyte ignimbrites (‘Los Azulejos’) deposited within the Miocene Tejeda caldera on Gran Canaria show evidence in the field of severe hydrothermal alteration (e.g. colour changes, veining, and alteration haloes around lithic clasts). The altered ignimbrites are restricted to a narrow zone directly inside the caldera margin, and occur at four distinct horizons within the mid-upper Mogan ignimbrite succession (14-13.3 Ma; Ar-Ar/feldspar; van den Bogaard & Schmincke, 1998). The intra- caldera ignimbrites are affected by intermediate-argillic, zeolitic, silicic, and Fe-Ti-Mn oxide alteration, reflecting the development of a low-temperature (<150°C), shallow-level (<200 m) epithermal system in the periphery of the Tejeda caldera. Field relationships suggest that alteration took place during a single epithermal event between ~13.2 and 12.5 Ma. related to the high-level emplacement (2-3 km depth) of the Fataga magma chamber. The altered ignimbrites are characterised by Ca. Rb, Sr, and Pb enrichment relative to strati graphically equivalent, unaltered extra-caldera ignimbrites, reflecting the mobilization of these elements during fluid-rock interaction, and their incorporation in secondary Ca-zeolites and clay minerals. In contrast, Na, K, and Ba are severely depleted, indicative of base-cation leaching from primary mineral phases (e.g. feldspar). The almost complete absence of major-oxide and trace-element correlations in the altered ignimbrites on Harker plots indicates that virtually all elements (apart from Al) were mobilised to some degree during fluid-rock interaction, even those traditionally regarded as ‘fluid-immobile’ (Ti, Zr, Nb, Y). The δ18O range for the altered intra-caldera ignimbrites extends to much higher values than that for the unaltered extra-caldera ignimbrites, consistent with a low- temperature environment, in which local meteoric water was the dominant fluid source. The δD values and H2O concentrations of the altered ignimbrites were also raised by exchange with meteoric water. Reworked volcaniclastic deposits in the stratigraphically deepest sample locality display evidence for a late phase of acid alteration (quartz-kaolinite-fluorite veins), and have the lowest (on average) δ18O values, perhaps related to localised, late-stage boiling of relatively high temperature (>300°C), low pH (<3), mixed meteoric-magmatic hydrothermal fluids at depth. Supported by numerical modelling, it is concluded that the ‘Los Azulejos’ deposits on Gran Canaria reflect the remnants of an intrusion-related, structurally-controlled epithermal system, in which water and/or steam migrated along the caldera margin and through the intra-caldera ignimbrites via channelised, porous flow.