Magmatic versus seawater-dominated hydrothermal alteration: evidence from deep-sea drilling at Brothers volcano, Kermadec arc, New Zealand

The Brothers volcano forms part of the Kermadec intraoceanic arc, located NE of New Zealand. Previous investigations of the Brothers volcano noted the occurrence of two distinct styles of hydrothermal venting within the Brothers caldera: a magmatic fluid-dominated, low-temperature, sulfur-rich system within a central resurgent dacitic cone; and a seawater-dominated high-temperature "black smoker" system perched on the NW caldera walls. We present data from subseafloor core samples collected during the International Ocean Discovery Program Expedition 376 'Brothers Arc Flux' (2018). For the first time, we can compare the subseafloor alteration mineralogy between magmatic- (Site U1528) and seawater-dominated (Sites U1527 and U1530) hydrothermal systems.

Despite being located only ~3 km apart, the Cone (Site U1528) and NW Caldera hydrothermal systems (Sites U1527 and U1530) exhibit distinctly different alteration assemblages. Utilizing over 350 X-Ray diffraction (XRD) analyses, petrographic characterization, and SEM element maps, variation in alteration mineral assemblage with depth and between different vent sites was examined. The Cone Site is dominated by an alteration mineral assemblage of natroalunite + pyrophyllite + sulfur + rutile, whilst at the NW caldera site, the assemblage consists of chlorite + quartz + zeolite (e.g., de Ronde et al., 2019).

We demonstrate that, despite their proximity, the style of alteration and, by inference, fluid chemistry are distinctly different between these two hydrothermal systems. Furthermore, we find considerable variation in alteration mineralogy with stratigraphic depth, most notably in the lower part of NW Caldera Site U1530 at a depth of 190 meters below seafloor (mbsf). Here, an alteration mineral assemblage consisting of zunyite, diaspore, pyrophyllite, and anhydrite is progressively overprinted by chlorite- and illite-rich facies. This study highlights the spatial and temporal diversity of alteration mineralogy in subseafloor hydrothermal systems and provides valuable insights into the evolution of a submarine volcanic-hosted hydrothermal system.

de Ronde, C.E.J., Humphris, S.E., Höfig, T.W., Reyes, A.G., 2019. Critical role of caldera collapse in the formation of seafloor mineralization: The case of Brothers volcano. Geology 47, 762-766.