Contributions to Mineralogy and Petrology (2018) 173:40 https://doi.org/10.1007/s00410-018-1470-5 ORIGINAL PAPER Constraints on the source of Cu in a submarine magmatic- hydrothermal system, Brothers volcano, Kermadec island arc Manuel Keith1 · Karsten M. Haase2 · Reiner Klemd2 · Daniel J. Smith1 · Ulrich Schwarz‑Schampera3 · Wolfgang Bach4 Received: 15 December 2017 / Accepted: 17 April 2018 / Published online: 21 April 2018 © The Author(s) 2018 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 phe- nocryst phases, such as clinopyroxene, plagioclase and Fe–Ti oxide suggest that the surrounding lavas that host the Broth- ers 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. Keywords Brothers volcano, Kermadec arc · Magmatic sulphides · Hydrothermal sulphides · Magmatic degassing · Melt inclusions · Epithermal–porphyry deposits · VHMS deposits Introduction Communicated by Timothy L. Grove. Magmatic-hydrothermal systems along convergent plate Electronic supplementary material The online version of this margins host some of the Earth’s most important porphyry article (https ://doi.org/10.1007/s0041 0-018-1470-5) contains Cu–Mo and epithermal Au–Ag–Te deposits (Seedorff et al. supplementary material, which is available to authorized users. 2005; Simmons 2005; Sillitoe 2010). Most of these met- * Manuel Keith als are considered to be derived from fluids of magmatic [email protected] origin that exsolved from mid- to upper crustal magma chambers (Richards 2011). Recent studies have shown that 1 School of Geography and Geology, University of Leicester, submarine hydrothermal systems associated with island arc Leicester LE1 7RH, UK volcanism can be affected by a magmatic volatile compo- 2 GeoZentrum Nordbayern, Universität Erlangen-Nürnberg, nent similar to their subaerial counterparts (de Ronde et al. 91054 Erlangen, Germany 2005, 2014). Brothers volcano in the Kermadec island arc 3 Bundesanstalt für Geowissenschaften und Rohstoffe, represents one of the best-studied examples of arc-related 30655 Hanover, Germany submarine magmatic-hydrothermal activity; and fluid and 4 Fachbereich Geowissenschaften der Universität Bremen, sulphide chemistry suggest a magmatic volatile contribution 28334 Bremen, Germany Vol.:(0123456789)1 3 40 Page 2 of 16 Contributions to Mineralogy and Petrology (2018) 173:40 (de Ronde et al. 2005, 2011; Keith et al. 2016a). In addition enriched in incompatible trace elements and volatiles (e.g., to a magmatic volatile signature, acid-sulphate (advanced S and Cl) compared to mid-ocean ridge magmas (Wallace argillic) alteration, sulphosalts (e.g., enargite, Cu 3AsS4) 2005; Richards 2011; Kelley and Cottrell 2012; Lee et al. typical for high-sulphidation conditions (Einaudi et al. 2012). This discrepancy results in higher S concentrations at 2003) and high contents of economically important met- sulphide saturation (Fortin et al. 2015), and hence sulphide als (e.g., Cu and Au) in some submarine arc and back-arc segregation typically occurs later during magmatic differen- hydrothermal systems imply that they may be comparable tiation, perhaps triggered by Fe–Ti oxide fractionation and to epithermal–porphyry deposits on land, such as the world- decreasing fO2 (Jenner et al. 2010; Keith et al. 2017; Patten class Far Southeast-Lepanto Cu–Au deposits (Philippines; et al. 2017). However, distinguishing these different pro- Hedenquist et al. 1998). This includes, for example, the mag- cesses and parameters is still challenging and often ambigu- matic-hydrothermal systems of Brothers volcano (Kermadec ous (Hedenquist and Lowenstern 1994) and our knowledge arc; de Ronde et al. 2005; Berkenbosch et al. 2012) and about the interaction between the magmatic and hydrother- Kolumbo volcano (Hellenic arc; Kilias et al. 2013, 2016), mal system and its control on the composition of associated as well as SuSu Knolls in the Manus back-arc basin (Crad- ore deposits is limited. dock and Bach 2010; Craddock et al. 2010; Yeats et al. 2014; This study combines the chemical and mineralogical Thal et al. 2016). Hence, arc-hosted hydrothermal systems composition of magmatic and hydrothermal sulphides with are characteristically distinct in terms of their chemical and silicate glass and melt inclusion data for the first time to mineralogical composition compared to those along mid- constrain the source characteristics and main enrichment ocean ridges, which were interpreted to be the classic mod- processes of Cu in an island arc-hosted magmatic-hydro- ern analogues of volcanic-hosted massive sulphide (VHMS) thermal system. deposits mined on land (Hannington et al. 1998, 2011; de Ronde et al. 2014; Keith et al. 2016b). High bulk ore contents of certain metals and semi-metals, Geological setting and sample localities such as Cu (up to 27 wt%), Au (up to 57 ppm), Ag (up to 2010 ppm) and Te (up to 53 ppm) at PACMANUS and SuSu Island arc systems with a submarine expression have a Knolls (Manus Basin; Moss and Scott 2001) have attracted combined length of about 21,700 km (93% in the Pacific attention for potential future submarine mining operations Ocean; de Ronde et al. 2014). Brothers volcano is part of the (Gena 2013). Consequently, it is important to improve our ~ 1200 km long Kermadec island arc, northeast of New Zea- understanding of the ore-forming processes in subduction land (Fig. 1a), which post-dates the earliest phase of Havre zone-related magmatic-hydrothermal systems and to identify Trough back-arc rifting (Wright et al. 1996, 1998). The chemical and mineralogical parameters for exploration and elongated volcanic edifice of Brothers is 13 km long, 8 km their economic evaluation. wide and strikes NW–SE at a water depth of 2200 mbsl (de A key process for the formation of a Cu-rich magmatic- Ronde et al. 2005). The caldera floor (1850 mbsl) has a basal hydrothermal ore deposit is the loss of magmatic volatiles dimension of 3 to 3.5 km and is surrounded by a 350–450 m prior to the formation of immiscible sulphide liquids. In high caldera wall (Fig. 1b; Wright et al. 1998). The caldera contrast, early sulphide segregation, i.e., prior to degassing, hosts a resurgent dome rising to a water depth of 1300 mbsl would extract most chalcophile metals including Cu and at its summit (Fig. 1b; Wright and Gamble 1999; de Ronde Au from the melt leading to a barren arc system (Sun et al. et al. 2001). Hence, at least two stages of magmatic activ- 2004; Jenner et al. 2010; Richards 2011; Park et al. 2015; ity can be distinguished at Brothers volcano including (1) Fontboté et al. 2017). This idea appears to be supported by an effusive incremental caldera-forming eruption (Wright previous studies, which suggest that aqueous S- and Cl-bear- and Gamble 1999) and (2) the subsequent formation of two ing magmatic volatiles are the main source and transport younger volcanic cones (Embley et al. 2012). The Brothers medium for Cu in submarine hydrothermal arc systems, such volcanic rocks range in composition from dacite to rhyolite as Brothers volcano (de Ronde et al. 2005; Berkenbosch (Haase et al. 2006). In addition, one basaltic andesite sample et al. 2012; Gruen et al. 2014). Therefore, economic miner- was recovered from Brothers Ridge, a young SW-NE strik- alisation within an island arc can be controlled by sulphide ing mafic dyke beneath the Brothers caldera (Wysoczanski liquid immiscibility during silicate melt evolution, which is et al. 2012). highly sensitive to changes in pressure, temperature, oxygen Several hydrothermal fields were distinguished includ- fugacity (fO2), the degree of fractional crystallisation and ing hydrothermal venting of evolved seawater derived fluids the initial Fe and S contents of the melt (Mavrogenes and with limited magmatic contribution at the NW caldera wall O’Neill 1999; Jenner et al. 2010; Yang et al. 2014; Keith and magmatic volatile-dominated systems at the Brothers et al. 2017). Arc magmas characteristically show higher fO2 cone complex (Fig. 1b; de Ronde et al. 2005, 2011; Baker (up to FMQ + 2) and H 2O contents (up to 6–8 wt%) and are et al. 2012; Caratori Tontini et al. 2012). Hydrodynamic 1 3 Contributions to Mineralogy and Petrology (2018) 173:40 Page 3 of 16 40 Fig. 1 a Map of the Kermadec island arc showing the location of and upper cone complex. Map modified after Baker et al. (2012). LC Brothers volcano, modified after de Ronde et al.