Hydrothermal fluids associated with seafloor mineralization at two southern Kermadec arc volcanoes, offshore New Zealand

Abstract

Massive sulfide samples have been recovered from Brothers and Rumble II West volcanoes of the southern Kermadec frontal arc, northeast of New Zealand. Both are caldera-forming volcanoes with Brothers largely dacitic and Rumble II West basaltic to basaltic-andesite in composition. The sulfide samples are considered to be pieces of chimneys from near the base of the caldera walls and are dominated by Cu–Fe–Ba ± Pb (chalcopyrite–pyrite–barite ± galena) and to a lesser extent, Zn–Fe–Ba ± Pb (sphalerite–pyrite–barite ± galena) mineralization. Gold is present in the samples up to 6.1 ppm. Paragenetic relationships suggest that the two hydrothermal systems have heated up and later cooled down. Oxidation of the sulfides is common and suggests that parts of the deposits are relatively old. Fluid inclusion microthermometric data show salinities fall predominantly in the range 3.0 to 3.4 wt% NaCl equivalent, similar to seawater values (3.2 wt%). About 15% of the Brothers salinity data range to both lower (2.2 wt%) and higher (3.9 wt%) than seawater values. Homogenization temperatures for type I inclusions in barite range between 175 and 322 °C for the Brothers samples, and between 205 and 268 °C for one Rumble II West sample. A dramatic decrease in temperature and/or change in the oxidation state of the hydrothermal fluid are considered the main mechanisms responsible for gold deposition at the Kermadec vent sites. This was achieved by mixing the hydrothermal fluid with ambient seawater within what are inferred to have been <300 °C "white smoker" chimneys, consistent with S isotope results. Fluid inclusion volatiles are dominated by H2O (99.81–99.98 mol%) followed by CO2 (0.03–0.17) and lesser amounts of N2 (0.004–0.023), CH4 (0.002–0.026) and trace COS and C2–C3 hydrocarbons. Fluid inclusions hosted by barite contain noticeably lesser amounts of gas than those within sulfides, consistent with mixing. The source of the relatively high concentrations of CH4 is enigmatic, although may be related to the thermal degradation of organic-rich sediments intercalated within the volcanic pile. Addition of a pressure correction and the effect of dissolved gases show that the hydrothermal fluids did not boil at the chimney depths. However, the range of measured salinities in the Brothers field combined with slightly positive trends in fluid inclusion CO2/CH4 versus CO2/N2 plots suggest sub-seafloor phase separation may have occurred in the hydrothermal system prior to venting.