Submarine hydrothermal activity and gold-rich mineralization at Brothers Volcano, Kermadec Arc, New Zealand.

Brothers volcano, of the Kermadec intraoceanic arc, is host to a hydrothermal system unique among seafloor hydrothermal systems known anywhere in the world. It has two distinct vent fields, known as the NW caldera and cone sites, whose geology, permeability, vent fluid compositions, mineralogy and ore forming conditions are in stark contrast to each other. The NW caldera site strikes for ~600 m in a SW-NE direction with chimneys occurring over a ~145 m depth interval, between ~1,690 and 1,545 m. At least 100 dead and active sulfide chimney spires occur in this field, as seen during manned submersible dives, and are typically 2-3 m in height, although locally reach 6-7 m. Their ages fall into three groups: 0-4 years (at time of sampling), 23 and 35 years old. The chimneys typically occur near the base of individual fault-controlled benches on the caldera wall, striking in lines orthogonal to the slopes. More rare are massive sulfide crusts up to 2-3 m thick. Two main types of chimney predominate: Cu-rich chimneys (with parts up to 28.5 wt.% Cu) and more commonly, Zn-rich chimneys (up to 43.8 wt.% Zn). Geochemical results indicate a "magmatic" suite of elements associated with the Cu mineralization, including up to 91 ppm Au, and an ‘epithermal’ suite of elements with the dominant Zn-rich chimneys. The cone site comprises the Upper cone site atop the summit of the recent (main) dacite cone, and the Lower cone site that straddles the summit of an older, smaller, more degraded dacite cone on the NE flank of the main cone. Huge volumes of diffuse venting are seen at the Lower cone site, in complete contrast to venting at both the Upper cone and NW caldera sites. Individual vents are marked by low relief (≤0.5 m) mounds comprised predominately of native sulfur with bacterial mats. Vent fluids of the NW caldera field are focused, hot (≤300°C), acidic (pH ≥ 2.8), metal-rich and relatively gas-poor. Calculated end-member fluids from various NW caldera vents indicate phase-separation has occurred with Cl values ranging from 93% to 137% seawater values. By contrast, vent fluids for the cone site are diffuse, noticeably cooler (≤122°C), very acidic (to pH = 1.86), relatively metal-poor and very gas-rich. Higher-than-seawater values of SO4 and Mg in the cone vent fluids show that these ions are being added to the hydrothermal fluid and are not being depleted via normal water/rock interactions. Three year old Fe-oxide crusts covering the main cone summit appear to have formed from Fe-rich brines. Evidence for magmatic contributions to the hydrothermal system at Brothers includes: high concentrations of dissolved CO2 (e.g., 206 mM/kg at the cone site), high CO2/³He values, negative δD and δ18OH2O values for vent fluids, negative δ34S values for sulfides (to -4.6‰), sulfur (to -10.2‰) and δ15N2 (to -3.5‰), vent fluid pH values to -1.9 and mineral assemblages of chalcopyrite, bornite, chalcocite, covelllite and euhedral grains of hematite. Changing physio-chemical conditions at the Brothers hydrothermal system, and especially the cone site, occur over months to hundreds of years, as shown by interlayed Cu/Au- and Zn-rich zones in chimneys, variable fluid and isotopic compositions between vents, similar shifts in 3He/4He values for both cone and NW caldera sites, and overprinting of "magmatic" mineral assemblages by water/rock dominated ones. Metals, especially Cu and possibly Au, may be entering the hydrothermal system via the dissolution of metal-rich glasses. They are then transported rapidly up into the system via magmatic volatiles utilizing vertical (~2.5 km long), narrow (~300 m diameter) "pipes", consistent with evidence of vent fluids forming at higher than hydrostatic pressures, at relatively shallow depths. The NW caldera and cone sites are considered to represent stages along a continuum between magmatic-hydrothermal and water/rock-dominated end-members.