Understanding the Akaroa magmatic system : a multi-method approach to erupted plutonic lithics.

The plutonic-volcanic connection remains one of the main challenges to understanding volcanic eruptions. Past studies have usually approached this using geochemical analysis. Quantitative microstructural analysis is a proven and essential technique for providing context for geochemical data, especially in the formation of cumulates; a necessary process in segregating eruptible melt. This thesis aims to understand the plutonic-volcanic connection by using microstructural analyses in conjunction with geochemical data on erupted plutonic lithics at the Akaroa Volcanic Complex (AVC). There are no exposures of in situ plutonic material at the AVC (other than the late-stage Onawe syenite and gabbro), but plutonic lithics within lava flows provide a window in to the crustal magmatic system. Here, quantitative microstructural analysis (i.e. EBSD) identifies uniaxial compaction crystallographic preferred orientations (CPOs) of plagioclase in cumulates. Rotation axis analysis confirms compaction of the crystal mush both during the settling and organization of crystals in a melt-rich environment as well as near-solidus viscous deformation. The correlation of compaction CPOs of increasing strength with a decrease in glass (from petrographic analysis) and late-stage crystallization of the plagioclase lattice (color-CL) provides some of the first direct evidence of coupled compaction and melt extraction in natural samples, specifically in plutonic lithics which, by definition, have a known volcanic counterpart. Highly luminescent plagioclase grain boundaries are crystallographically the same as the adjacent plagioclase but are compositionally different. These bright CL regions have chemistries that correspond to the evolved lava flows of the AVC, further suggesting that the material represents a residual magmatic melt that was progressively extracted from the crystal framework, evolved, and was potentially able to erupt. Those bright CL compositions that do not match the AVC eruptives are high in FeO and MgO and are likely low-silica immiscible melts. Immiscible melts are used to explain magmatic accumulation and evolution in stacked sill complexes which the AVC crustal magmatic system resembles. The presence of symplectite-style reactive textures supports this interpretation. This thesis aims to understand the plutonic-volcanic connection at the Akaroa Volcanic Complex (AVC) using erupted plutonic lithics. First, with plutonic lithics from one location, Goat Rock Dome, and the magmatic processes responsible for cumulate formation and melt extraction. This provides important microstructural and geochemical evidence for compaction, melt extraction, and cumulate formation which, while frequently used to explain magmatic processes, is rarely proven. Finally, these findings are applied to all plutonic lithic-bearing locations at the AVC to better explain crustal magmatism and cone-building on Banks Peninsula

Quantifying the role of hydrothermal alteration in creating geothermal and epithermal mineral resources: The Ohakuri ignimbrite (Taupō Volcanic Zone, New Zealand)

International audienceHydrothermal fluids can alter the chemical and physical properties of the materials through which they pass and can therefore modify the efficiency of fluid circulation. The role of hydrothermal alteration in the development of geothermal and epithermal mineral resources, systems that require the efficient hydrothermal circulation provided by fracture networks, is investigated here from a petrophysical standpoint using samples collected from a well exposed and variably altered palaeo-hydrothermal system hosted in the Ohakuri ignimbrite deposit in the Taupō Volcanic Zone (New Zealand). Our new laboratory data show that, although quartz and adularia precipitation reduces matrix porosity and permeability, it increases the uniaxial compressive strength, Young’s modulus, and propensity for brittle behaviour. The fractures formed in highly altered rocks containing quartz and adularia are also more planar than those formed in their less altered counterparts. All of these factors combine to enhance the likelihood that a silicified rock-mass will host permeability-enhancing fractures. Indeed, the highly altered silicified rocks of the Ohakuri ignimbrite deposit are much more fractured than less altered outcrops. By contrast, smectite alteration at the margins of the hydrothermal system does not significantly increase strength or Young’s modulus, or significantly decrease permeability, and creates a relatively unfractured rock-mass. Using our new laboratory data, we provide permeability modelling that shows that the equivalent permeability of a silicified rock-mass will be higher than that of a less altered rock-mass or a rock-mass characterised by smectite alteration, the latter of which provides a low-permeability cap required for an economically viable hydrothermal resource. Our new data show, using a petrophysical approach, how hydrothermal alteration can produce rock-masses that are both suitable for geothermal energy exploitation (high-permeability reservoir and low-permeability cap) and more likely to host high-grade epithermal mineral veins, such as gold and silver (localised fluid flow)