Geology, hydrothermal activity, and sea-floor massive sulfide mineralization at the Rumble II West mafic caldera

Abstract Sea-floor imagery, volcanic rock, massive sulfide, and hydrothermal plume samples (delta (super 3) He, pH, dissolved Fe and Mn, and particulate chemistry) have been collected from the Rumble II West volcano, southern Kermadec arc, New Zealand. Rumble II West is a caldera volcano with an approximately 3-km-diameter summit depression bounded by ring faults with a resurgent central cone. Rocks recovered to date are predominantly mafic in composition (i.e., basalt to basaltic andesite) with volumetrically lesser intermediate rocks (i.e., andesite). On the basis of its size, geometry, volcanic products, and composition, Rumble II West can be classified as a mafic caldera volcano. Rumble II West has a weak hydrothermal plume signature characterized by a small but detectable delta (super 3) He anomaly (25%). Time-series light scattering data though, obtained from vertical casts and tow-yos, do show that hydrothermal activity has increased in intensity between 1999 and 2011. Massive sulfides recovered from the eastern caldera wall and eastern flank of the central cone are primarily comprised of barite and chalcopyrite, with lesser sphalerite, pyrite, and traces of galena. The weak hydrothermal plume signal indicates that the volcano is in a volcanic-hydrothermal quiescent stage compared to other volcanoes along the southern Kermadec arc, although the preponderance of barite with massive sulfide mineralization indicates higher temperature venting in the past. Of the volcanoes along the Kermadec-Tonga arc known to host massive sulfides (i.e., Clark, Rumble II West, Brothers, Monowai, Volcano 19, and Volcano 1), the majority (five out of six) are dominantly mafic in composition and all but one of these mafic volcanoes form moderate-size to large calderas. To date, mafic calderas have been largely ignored as hosts to sea-floor massive sulfide deposits. That 75% of the presently known massive sulfide-bearing calderas along the arc are mafic in composition (the dacitic Brothers volcano is the exception) has important implications for sea-floor massive sulfide mineral exploration in the modern oceans and ancient rock record on land.