Clay alteration of volcaniclastic material in a submarine geothermal system, Bay of Plenty, New Zealand

• Published in: Journal of Volcanology and Geothermal Research
• Main Authors: Hocking, Michael W.A., Hannington, Mark D., Percival, Jeanne B., Stoffers, Peter, Schwarz-Schampera, Ulrich, de Ronde, C. E. J.
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2010
• Subjects: Bay of Plenty; New Zealand; White Island
• Online Access: https://oceanrep.geomar.de/id/eprint/23680/; https://oceanrep.geomar.de/id/eprint/23680/1/Hocking.pdf; https://doi.org/10.1016/j.jvolgeores.2010.01.018

The Calypso Hydrothermal Vent Field (CHVF) is located along an offshore extension of the Taupo Volcanic Zone (TVZ), an area of abundant volcanism and geothermal activity on the North Island of New Zealand. The field occurs within a northeast-trending submarine depression on the continental shelf approximately 10–15 km southwest of the White Island volcano in the Bay of Plenty. The graben has been partially filled by tephra from regional subaerial volcanic eruptions, and active hydrothermal venting occurs at several locations along its length. The vents occur at water depths of 160 to 190 m and have temperatures up to 201 °C. Recovered samples from the vent field include variably cemented and veined volcaniclastic sediments containing an assemblage of clay minerals, amorphous silica, barite, As–Sb–Hg sulfides, and abundant native sulfur. The volcanic glass has been altered primarily to montmorillonite and mixed-layer illite–montmorillonite; illite, and possibly minor talc and mixed-layer chlorite–smectite or chlorite–vermiculite are also present. A hydrothermal versus diagenetic origin for the smectite is indicated by the presence of both illite and mixed-layer clays and by the correlation between the abundance of clay minerals and the abundance of native sulfur in the samples. The mineralization and alteration of the volcanic host rocks are similar to that observed in near-neutral pH geothermal systems on land in the TVZ (e.g., Broadlands–Ohaaki). However, the clay minerals in the CHVF have a higher concentration of Mg in the dioctahedral layer and a higher interlayer Na content than clay minerals from Broadlands–Ohaaki, reflecting the higher concentrations of Mg and Na in seawater compared to meteoric water. Minerals formed at very low pH (e.g., kaolinite and alunite), typical of steam-heated acid-sulfate type alteration in the TVZ geothermal environment, were not found. Mixing with seawater likely prevented the formation of such low-pH mineral assemblages. The occurrence of illite and mixed-layer ...