Discovery of hydrothermal sulfide mineralisation from the Kermadec - Havre arc - back-arc system (SW Pacific)

We report the discovery of hydrothermal sulfide mineralization within the summit caldera of two southern Kermadec frontal arc volcanoes (Brothers and Rumble II West) of the ∼1200 km long, Kermadec–Havre arc–back-arc system. The Brothers and Rumble II (West) calderas, both with re-surgent domes and comprising mostly dacite and basalt–andesite host rocks, respectively, rise to water depths of ≤1500 m. These calderas comprise mostly effusive lavas and volcaniclastic deposits, including talus breccias along their inner caldera walls. These two southern Kermadec hydrothermal sites have similar geological settings and mineralogy to other arc front vent sites. Two principle ore assemblages are identified, comprising: (i) chalcopyrite–pyrite–barite, and (ii) sphalerite–marcasite–barite ± pyrite. The most prevalent ore texture consists of massive chalcopyrite + pyrite ± sphalerite, commonly overprinting barite. Early marcasite cores within large, euhedral pyrites, and trails of pyrite inclusions within chalcopyrite are indicative of recrystallization. The ore mineralogy is consistent with postulated venting temperatures of ≥300°C. Sulfide geochemistry is consistent with the ore mineralogy with concentrations of Cu, Fe, and Zn up to 15.3, 19.1, and 18.8 wt%, respectively. The sulfide geochemistry is distinct, however, from comparable western Pacific vent sites, and may reflect source heterogeneity due to proximity to continental New Zealand and sediment recycling along the southern Kermadec trench–arc system. The recovery of two partial caridean vent shrimps suggests that present-day hydrothermal venting is occurring within the Brothers caldera.