Constraints on the source of Cu in a submarine magmatic-hydrothermal system, Brothers volcano, Kermadec island arc

Abstract

Most magmatic-hydrothermal Cu deposits are genetically linked to arc magmas. However, most continental or oceanic arc magmas are barren, and hence new methods have to be developed to distinguish between barren and mineralised arc systems. Source composition, melting conditions, the timing of S saturation and an initial chalcophile element-enrichment represent important parameters that control the potential of a subduction setting to host an economically valuable deposit. Brothers volcano in the Kermadec island arc is one of the best-studied examples of arc-related submarine magmatic-hydrothermal activity. This study, for the first time, compares the chemical and mineralogical composition of the Brothers seafloor massive sulphides and the associated dacitic to rhyolitic lavas that host the hydrothermal system. Incompatible trace element ratios, such as La/Sm and Ce/Pb, indicate that the basaltic melts from L'Esperance volcano may represent a parental analogue to the more evolved Brothers lavas. Copper-rich magmatic sulphides (Cu > 2 wt%) identified in fresh volcanic glass and phenocryst phases, such as clinopyroxene, plagioclase and Fe-Ti oxide suggest that the surrounding lavas that host the Brothers hydrothermal system represent a potential Cu source for the sulphide ores at the seafloor. Thermodynamic calculations reveal that the Brothers melts reached volatile saturation during their evolution. Melt inclusion data and the occurrence of sulphides along vesicle margins indicate that an exsolving volatile phase extracted Cu from the silicate melt and probably contributed it to the overlying hydrothermal system. Hence, the formation of the Cu-rich seafloor massive sulphides (up to 35.6 wt%) is probably due to the contribution of Cu from a bimodal source including wall rock leaching and magmatic degassing, in a mineralisation style that is hybrid between Cyprus-type volcanic-hosted massive sulphide and subaerial epithermal-porphyry deposits.

Site U1531

“…Although orthopyroxene and clinopyroxene coexist in Site U1531 dacites, hornblende was observed in many samples volcano-wide, which indicates relatively low magmatic H 2 O contents (cf. 2-4 wt% H 2 O in Keith et al, 2018). , and more coarse-grained aggregates (~1-2 mm) of plagioclase, clinopyroxene, and Fe-Ti oxide indicate mixing of a more mafic with a more silicic magma to produce the Brothers volcano dacite, which is consistent with observations of dacite from other volcanic arc systems (e.g., Grove et al, 2005;Millet et al, 2014).…”

Transition of eruptive style: Pumice raft to dome-forming eruption at the Havre submarine volcano, southwest Pacific Ocean

“…Syneruptive inputs of magmatic volatiles, fluids and metals into the shallow (~500 m) subseafloor around conduits in deep submarine settings cannot be assigned into classical epithermal or porphyry-style mineralization models (e.g., Large, 1992;Sillitoe and Hedenquist, 2003). Hybrid-styles of epithermalvolcanic-hosted massive sulfide-porphyry deposition have been proposed for both active modern and ancient ore bodies, e.g., Mount Lyell (Yosemite, California; Huston and Kamprad, 2001) and Brothers volcano (South Pacific Ocean; Keith et al, 2018). Greater understanding of these hybrid mineral systems could be attained by geothermal, chemical and hydrological modeling constrained by quantitative information from Havre volcano.…”

Metal fluxes during magmatic degassing in the oceanic crust: sulfide mineralisation at ODP site 786B, Izu-Bonin forearc

“…Irrespective of the sources and pathways of the fluids transporting metals to the mineralised zones, the primary bulk metal budget available for mobilisation in the oceanic crust is controlled by magmatic processes (Moss et al 2001;Jowitt et al 2012;Patten et al 2017). Metal behaviour during the magmatic differentiation of supra-subduction zone oceanic crust is strikingly different from MOR settings (Sun et al 2004;Jenner et al 2010;Patten et al 2017;Keith et al 2018). In particular, the exsolution of magmatic fluids during differentiation of volatile-rich suprasubduction magmas can release significant quantities of fluid-mobile metals to the hydrothermal system (Moss et al 2001;Yang and Scott 2002;Sun et al 2004;Keith et al 2018).…”

“…Metal behaviour during the magmatic differentiation of supra-subduction zone oceanic crust is strikingly different from MOR settings (Sun et al 2004;Jenner et al 2010;Patten et al 2017;Keith et al 2018). In particular, the exsolution of magmatic fluids during differentiation of volatile-rich suprasubduction magmas can release significant quantities of fluid-mobile metals to the hydrothermal system (Moss et al 2001;Yang and Scott 2002;Sun et al 2004;Keith et al 2018). The high oxygen fugacity of supra-subduction oceanic crust inhibits sulphide saturation of the magma and strongly chalcophile elements including Au, Cu and Se behave as incompatible elements during the early stages of magmatic differentiation (Sun et al 2004;Jugo et al 2009;Jenner et al 2010Jenner et al , 2012Patten et al 2017;Keith et al 2018).…”

“…In particular, the exsolution of magmatic fluids during differentiation of volatile-rich suprasubduction magmas can release significant quantities of fluid-mobile metals to the hydrothermal system (Moss et al 2001;Yang and Scott 2002;Sun et al 2004;Keith et al 2018). The high oxygen fugacity of supra-subduction oceanic crust inhibits sulphide saturation of the magma and strongly chalcophile elements including Au, Cu and Se behave as incompatible elements during the early stages of magmatic differentiation (Sun et al 2004;Jugo et al 2009;Jenner et al 2010Jenner et al , 2012Patten et al 2017;Keith et al 2018). This process enables the build-up of strongly chalcophile elements in the melt, increasing its fertility.…”