Magmatic fluids in seafloor hydrothermal systems : an in-depth mineralogical, trace element and isotopic study of sulfide chimneys from Brothers volcano, Kermadec arc

Brothers submarine caldera volcano is one of 30 large volcanic centres that comprise the Kermadec arc, which stretches for ~1300 km NE of New Zealand. The NW Caldera vent field at Brothers straddles the caldera wall and hosts numerous, active, high-temperature (up to 302¬¨‚àûC) black smoker chimneys and a greater number of inactive, sulfide-rich spires. The addition of magmatic fluids to the hydrothermal system is indicated by high \\(^3He\\), `CO_(2(g))`, and `H_2S_((g))` concentrations, low pH, and negative `˜í¬•^(15)N` and `˜í¬•D_(H2O)` values for the vent fluids, in concert with local advanced argillic alteration assemblages in the host rocks. This study examines the mineralogy, trace element composition and Cu isotopes of the sulphide chimneys to test the hypothesis that magmatic fluids significantly affect the composition of mineralization at Brothers NW Caldera vent field. Petrographic analysis was undertaken to describe chimney mineralogy and formation. Chimney types were identified based on the composition and relative proportion of mineralogical layers. Two are Zn-rich, i.e., sphalerite-chalcopyrite and sphalerite-barite chimneys, and two are Cu-rich, i.e., chalcopyrite-sulfate and chalcopyrite-bornite chimneys. Discovery of small Bi-Au telluride inclusions explains previously enigmatic whole rock Au contents up to 91 ppm. Enriched Bi contents are commensurate with large amounts of sediment being subducted at the Kermadec trench, whereas the Bi-Au association suggests liquid Bi scavenged Au. Both findings are consistent with magmatic contributions to the NW Caldera vent site. Synchrotron radiation X-ray fluorescence microscopy (XFM) was used to produce high-resolution trace element maps (2 ˜í¬¿m beam, covering `84 ‚Äö- 136 mm^2`) of Fe, Cu, Zn, As, Se, Sr, Pb ¬¨¬± Ga, Au, Bi and U distribution across the inner chimney walls. In addition, lower resolution (47 ˜í¬¿m beam) maps generated by laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) imaged those same elements, plus Ca, Co, Ni, Mo, Ag, Cd, In, Sn, Sb, Ba, Tl ¬¨¬± V and Te. Elemental zonation and textural features of sphalerite in the two Znrich chimneys show a progression of sphalerite replacement by chalcopyrite. The two Cu-rich chimneys show contrasting formation styles based on their massive chalcopyrite linings. The first displays elongate chalcopyrite grains radiating into and infilling the conduit, which merge together some millimetres from the centre. The second style shows deposition of successive laminations (0.25 - 1 mm) of chalcopyrite inside the conduit that progressively narrowed the orifice. Additionally, fine (15 - 40 ˜í¬¿m) rings of concentrated trace elements occur within, and between, the laminations of Co, Ni, Zn, As, Se, Mo, Ag, Cd, Sn, Te, Au, Tl, Pb, Bi and U. The presence of U specifically indicates repeated, brief incursions of seawater into the chimney interior, during which perturbation of the resultant chemical gradients induced abrupt precipitation of these elements. Thus, the rings are a proxy for secular variations in vent fluid composition. Calculated enrichment factors, used to differentiate magmatically-derived elements, are generally consistent between the chimney trace element rings, fumarole condensates from subaerial volcanoes, and a 'pond' of molten (condensed) sulfur atop a submarine volcano. These indicate that Au, Te, Bi, Se, Ag and Cu in Brothers chimneys have a magmatic source. Isotopic analysis of primary chalcopyrite was utilized to investigate high-temperature hydrothermal Cu isotope fractionation at Brothers. The majority of the samples range between `˜í¬•^(65)Cu` = ~0.00 and 0.50‚ÄövÑ‚àû, which is representative of a mantle source for the Cu. A few higher `˜í¬•^(65)Cu` values (>0.90‚ÄövÑ‚àû) occur randomly distributed through chalcopyrite of the same age (<1 yr) in two chimneys. This suggests the higher `˜í¬•^(65)Cu` values are not related to seawater oxidation, which would decrease `˜í¬•^(65)Cu values in residual chalcopyrite, but rather could indicate isotopic variation within the vent fluids. Theoretical studies show significant isotopic fractionation can occur between aqueous and vaporous complexing species. Thus, given the evidence for magmatic volatiles at Brothers, vapour transport of Cu could account for the observed isotopic fractionation, again consistent with a magmatic origin. In summary, the application of techniques ranging from petrography to element mapping to Cu isotopes, shows that Au, Te, Bi, Se, Ag and Cu in this high-temperature, seafloor hydrothermal system are derived by magmatic fluids, where Bimelts concentrated Au effectively and Cu may be transported by vapour.