|Harvey-Wishart, H. (2020). Characteristics and origin of hydrothermal pebble dykes and associated host rock alteration on the Thames coast, Coromandel Volcanic Zone (Thesis, Master of Science (Research) (MSc(Research))). The University of Waikato, Hamilton, New Zealand. Retrieved from https://hdl.handle.net/10289/13543
|During the Miocene to early Pliocene (c. 18 Ma to 2 Ma), subduction-related volcanism migrated down the Coromandel Peninsula forming the Coromandel Volcanic Zone. This period resulted in the development of extensive hydrothermal systems which interacted with the associated volcanic sequences, the Coromandel Group andesites and dacites. As well as introducing extensive alteration and epithermal Au-Ag deposits, the active environment resulted in the emplacement of co-hydrothermal pebble dyke units. Examples of these features have been identified on the Coromandel west coast, exposed as bodies of fractured rock material, hosting well-rounded clasts due to milling during transport forming “pebbles”. This material has been incorporated from a variety of lithologies from different stratigraphic depths, providing a ground surface expression of the subsurface. As the formation of these dykes is not well defined, whether they are a result of volcanic or hydrothermal activity, this study aims to investigate their origin and the influence they may have had on the character of proximal hydrothermal alteration.
A combination of field descriptions, optical petrology, SEM, XRD, XRF and ICP-MS geochemistry was all involved in investigating the study site along the Thames coastline, where two clusters of dykes are exposed within a ~ 600 m span of beach. The local hydrothermal character was determined from the mineralogy, reflective of the fluids and physical conditions involved in the mineralization of altered products. The extent of the site hosts multiple alteration zones, recognised as propylitic and phyllic alteration associated with periods of mineralisation, and supergene argillic overprinting. The gangue mineralogy also mirrors the character of the broader system, reflecting near-neutral fluids associated with the low-sulfidation epithermal environment at temperatures of ~ 240 – 260 °C.
To understand the role of the pebble dykes within this setting and the origin of their deposits, identifying the mechanisms involved in their formation is crucial. Characteristics of dyke morphologies, clast lithologies and morphologies, and alteration character suggest fluidisation played a large role, particularly so in the
transport of fragmented rock material from depth. The extent of their ascent is best reflected by the occurrences of greywacke argillite and sandstone components found in each of the exposures. These lithologies are sourced from the Manaia Hill group of the Waipapa Terrane, a deep basement unit recognised as the basement rock of New Zealand. Therefore, the presence of greywacke and argillite clasts suggest possible significant depths of material transport and the interaction with high volumes of hydrothermal solutions. However, due to the alteration style and morphologies of the clasts, it is also possible they were instead fragmented at similar depths to the shallower local volcanic stratigraphy. If pebble dykes were to act as permeable pathways for ore-bearing solutions within a hydrothermal system, it is possible they could play a significant role in the concentration of metals in economically viable quantities. Therefore, under the right circumstances; emplacement during or pre-mineralisation, orientated preferential to fluid flow direction and hosted within otherwise impermeable host rock, they could yet act as a vector towards potential mineralisation.