Syn- to Post-emplacement Alteration Processes

We now consider the range of syn- to post-emplacement alteration processes and their effects on volcanic and associated sedimentary deposit types and successions. These processes include devitrification, palagonitisation (in mafic glass), hydrothermal alteration, diagenesis, metamorphism, and tectonic deformation, which may significantly modify and in some cases completely disguise original volcanic textures. The process that induces textural change in the first instance is devitrification of volcanic glass as it cools, which is facilitated by ion and gas diffusion through the glass. Secondly, permeation of magmatic hydrous fluids, surface waters, and meteoric water in the subsurface can begin to remobilise elements and initiate hydrothermal alteration of the volcanic glass and original minerals. In older volcanic rock successions, post-depositional processes of ongoing devitrification, hydrothermal alteration, and then metamorphism and deformation can all have the effect of overprinting original volcanic textures and mineralogy. Such processes not only have the ability to overprint original textures but also, in some instances, to obliterate them totally and produce new textures (even apparent or “false”, pseudo-clastic, including fiamme-like textures) that can lead to misconceptions of the original rock types and their origins. This can be particularly important in ore-forming volcanic environments subjected to prolonged polyphase hydrothermal alteration and associated hydraulic fracturing, and can lead to incorrect reconstructions of their paleoenvironmental setting, eruption styles, and ore deposit types.