The Kakanui hydrothermal system.
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Potential hydrothermal deposits were recognised within the late-Eocene to early-Oligocene volcanic tuff and overlying volcaniclastic grit at Kakanui South Head, Oamaru, New Zealand. The deposits included discrete pink carbonate pods within the volcanic tuff, discrete infilled cavities within the overlying volcaniclastic grit and a sub-horizontal carbonate precipitate found between the two layers. Assessment of these deposits and the surrounding stratigraphy was carried out through a combination of field work and rock analyses. Stratigraphic columns were constructed from field work depicting the relationship between the surtseyan volcanics and the local sedimentary lithofacies. The magma intruded through a bryozoan-rich limestone initiating consecutive submarine eruptions which formed interlayered crystalline nephelene tuffs. Petrographic assessment of the volcanic tuff shows multiple interstitial minerals precipitated within pore spaces during heated interactions with seawater, including silcrete and calcite rims, zeolite clusters, chalcedony and saddle dolomite. Discrete hydrothermal conduits crosscut the volcanic tuff, continue up through the unconsolidated volcaniclastic grit and are found in close proximity to local normal faults. Adjacent to the conduits, the tuff exhibits abundant dolomitisation and the grit displays the characteristic green colour of chloritisation; however, confirmation on chlorite alteration was not achieved due to SEM malfunction. Additionally, hydrothermal fluids flowing through the conduits diverted at the contact with the unconsolidated volcaniclastic grit and flowed sub-horizontally between the layers, which enabled precipitation of a sub-horizontal carbonate. Intense chloritisation appears to have occurred adjacent to this deposit. A fossil-rich, carbonate precipitate later infilled the conduits; the type of fossils correspond to a limestone directly overlying the volcaniclastic grit. This marks the cessation of hydrothermal activity; where hydrothermal fluid flow decreased, the system cooled, and calcite fossils fell into the cavities from above, which became embedded in CaCO₃ precipitate. Carbonate samples from discrete conduits, infilled fractures and the sub-horizontal precipitate underwent δ¹³13C and δ¹⁸O isotopic analyses in order to identify the source of the hydrothermal fluids. Analyses yielded no results for fluid sources at chloritisation temperatures greater than 150 °C within these samples. However, isotope results for the saddle dolomite show a source of approximately 95 °C magmatic fluids mixed with cool oceanic water, and the isotope results for the calcite show a source of less than 75 °C magmatic fluids mixed with cool oceanic water, representing two successively cooler stages of hydrothermal activity. The isotope results for an infilled fracture inside the crater rim show a precipitate formed from magmatic fluids at around 95 °C, later diagenetically altered by meteoric water with a biogenic component. These results confirm that wide-spread hydrothermal alteration of the volcanic tuff occurred after eruptive material interacted with seawater during surtseyan eruption events. Furthermore, the results show an active hydrothermal system operated within the Kakanui volcano, sourced by a mix of magmatic fluid and circulating seawater. Isotopic analyses of carbonates suggest infiltrating seawater was restricted to the flanks of the volcano and did not penetrate through to the core of the edifice. The local normal faults adjacent to the hydrothermal conduits are interpreted as the product of deflation of the volcanic edifice during cooling. The close proximity of the conduits to these faults suggests faulting and fracturing of a volcanic edifice may play a dominant role in the size and development of a hydrothermal system. Additionally, a key confining feature within the Kakanui hydrothermal system is that of the unconsolidated volcaniclastic grit. This layer diverted hydrothermal fluids to flow sub-horizontally between the tuff and grit layers, represented by abundant hydrothermal alteration and precipitation of hydrothermally sourced minerals.