Reyes, A.G. 2003 Petrographic evaluation of dredged samples from the Brothers, Macauley and Giggenbach submarine volcanoes, Kermadec Arc . Lower Hutt: Institute of Geological & Nuclear Sciences. Institute of Geological & Nuclear Sciences science report 2003/29 41 p.
Abstract: The rocks dredged from the northwest caldera, southeast caldera and cone of the Brothers Volcano include volcanic rocks dominated by dacite and rhyodacite with minor andesite; volcanic sandstone; and fragments of sulphide-rich chimneys. Thirty-eight secondary minerals were identified in 55 samples and include silicates, silica polymorphs; Na, Ca, Ba and Ba-Sr sulphates; Fe, Cu, Zn, As and Pb sulphides; Fe and Mn oxide/oxyhydroxides and native sulphur. The variety and range of mineral compositions show that these formed, with time, in different parts of a submarine hydrothermal systems/s characterised by differences in temperature (ambient seawater to >300 degrees C), fluid compositions, degree of hydrothermal water-rock interaction and degree of seawater influence. The Brothers submarine hydrothermal system is divided into a zone of direct magmatic influx into the seafloor, initially forming as a crack-zone from the crystallising magma to the seafloor surface, and later manifesting itself as sulphide-rich chimneys. Away from the main upflow zone, where faults form conduits for hydrothermal fluids to ascend to the seafloor, sulphate-rich chimneys or abiogenically precipitated opal-A + pyrite are deposited. On the surface of the seafloor is a zone of seawater-rock (-plume) interaction and just below this is a region where hydrothermal waters, seawater and rock interact. Resurgent volcanism is followed by renewed hydrothermal activity at depth. The secondary mineralogy of dredged samples from the Brothers Volcano originate from (1) a partly exhumed waning hydrothermal system/s (northwest caldera); (2) the main upflow of a relatively well-established active magmatic-hydrothermal system on the seafloor where sulphide-rich chimneys are extant (southeast caldera) and (3) a nascent magmatic-hydrothermal system where crack zones localise upwelling acid waters from depth (cone). (auth)