Massive Sulfide Stockwork Zones of Brothers Submarine Arc Volcano: Evidence for Large-scale Transport of Metals

Brothers volcano of the southern Kermadec arc, New Zealand, is host to extensive hydrothermal activity, including two prominent hydrothermal vent fields: the NW Caldera and Upper Caldera sites. Where demarcated by a magnetic low, these sites have an hour-glass shape in plan view, with a bulbous, open, top section 1 km wide up near the upper-most caldera walls which extends 800 m to the SE where it narrows to 400 m at the NW Caldera rim, occupying an area 0.68 km² (Upper Caldera site). This alteration zone then widens again to 1 km and extends a further 700 m towards the SE, encompassing an area of 0.76 km² that includes all of the NW Caldera walls, and which extends onto the caldera floor (NW Caldera site). The Upper Caldera vent field is host to numerous black smoker chimneys up to 20 m tall expelling fluids up to 320 °C. Only rare, diffuse venting occurs across the caldera rim. Hundreds of both active and inactive chimneys are perched on benches that cut across the NW Caldera walls, reaching heights of 15 m and discharging fluids up to 311 °C. No present-day venting occurs on the caldera floor. The top of the Upper Caldera walls occur at 1300 mbsl and the caldera floor is at 1800 mbsl, resulting in 500 vertical meters of present-day hydrothermal activity in this sector of Brothers volcano. The NW Caldera walls are characterized by normal faults, which are associated with caldera collapse. Extensive stockwork veining is exposed in the faces of several of these faults, with three examples occurring at 1600, 1655 and 1685 mbsl, respectively. The faults strike between 037° and 057° and extend for up to 210 m, typically 150 m. The throw on the faults is commonly 15 m though can reach up to 20-30 m. An anastomosing network of sulfide veins cut highly altered wallrock with the veins ranging between mm to >10 cm wide. Pyrite is the most dominant sulfide seen macroscopically, although Cu is likely present given the green staining of the adjacent wallrock. Extensive stockwork mineralization exposed at the NW Caldera vent field is testament to mobilization of metals that occurred prior to caldera collapse.