Trace metal and sulfur cycling in a hydrothermally active arc volcano: deep-sea drilling of the Brothers volcano, Kermadec arc, New Zealand

Abstract

Brothers volcano, located on the Kermadec arc north of New Zealand, hosts two geochemically distinct hydrothermal systems. The NW Caldera and Upper Cone hydrothermal fields exhibit distinct fluid compositions that are significantly influenced by seawater and magmatic volatiles, respectively. In this study, we present trace metal chemistry and sulfur isotope compositions of pyrite within hydrothermally-altered volcanic rocks recovered from drill cores at depths of up to 429 m below the seafloor collected during the International Ocean Discovery Program's Expedition 376. Magmatic volatile-influenced alteration resulting in pyrophyllite ± natroalunite assemblages occurs at the Upper Cone and at the NW Caldera below 189 m. At the NW Caldera, a later seawater-derived hydrothermal fluid overprints magmatic volatile alteration forming chlorite-rich alteration. Pyrite at the Upper Cone is fine-grained, euhedral and enriched in Cu, As, Sb, Pb and Pt and has an average δ 34 S composition of -5.5 ± 2.9‰ (1σ, n= 32). In contrast, pyrite associated with pyrophyllite-rich alteration at the older NW Caldera site is coarse-grained, subhedral and has higher Co, Se, Te and Bi contents but a comparable average δ 34 S value of -4.8 ± 5.5‰ (1σ, n= 26). The difference in trace metal content between pyrite from pyrophyllite ± natroalunite assemblages at the NW Caldera and Upper Cone site indicates a change in the trace metal enrichment signature of pyrite with the age of the hydrothermal system. Pyrite from chlorite-rich alteration (NW Caldera) is depleted in Cu, Te and Bi relative to all magmatic volatile-influenced pyrite but has a similar average δ 34 S composition of -4.6 ± 3.5‰ (1σ, n= 20). The similarity in trace metal enrichment signature and average δ 34 S composition of pyrite, regardless of associated alteration mineral assemblage shows that the initial magmatic volatile trace metal signature and sulfur isotope composition of pyrite is preserved during fluid overprinting. The lower content of Cu, Te and Bi in pyrite from chlorite-rich alteration confirms the importance of seawater-derived hydrothermal fluids in metal mobilization and consequent formation of hydrothermal precipitates at the seafloor.

Fluid Flow, Mineralization and Deformation in an Oceanic Detachment Fault: Microtextural, Geochemical and Isotopic Evidence From Pyrite at 13°30′N on the Mid‐Atlantic Ridge

“…The relationship between the two types of zonations is distinct; intricate zonations are overprinted by later irregular zonations. Concentric zonations, albeit of a less intricate nature, have been documented in several ore forming systems, for example, SMS deposits (Martin et al., 2023), epithermal Au deposits (Tanner et al., 2013, 2016) and porphyry Cu deposits (Reich et al., 2013). In these deposits, the concentric zonations are interpreted to be primary features that record changes in the physicochemical conditions (e.g., pH, temperature, salinity) of the hydrothermal fluid that influences the solubility of different metals, that are subsequently recorded as variations in sulfide geochemistry as the grain grows.…”

“…Numerous studies have documented the remobilization of trace metals in seafloor hydrothermal systems (Evans et al., 2020; Gallant & Von Damm, 2006; Keith, Häckel, et al., 2016; Martin et al., 2022b), however, what is generally assumed that complete dissolution of the mineral or part of the mineral occurs and is followed by either the precipitation of a secondary mineral phase or loss of a given metal from the system in the fluid phase (e.g., Haymon, 1983). The remobilization of trace metals can also occur due to galvanic interactions between different sulfide minerals during oxidation, where two minerals with different rest potentials in an electrolyte solution act as a cathode (e.g., pyrite) and anode (e.g., chalcopyrite) (Abraitis et al., 2004; Fallon et al., 2017).…”

“…Pyrite in seafloor massive sulfide (SMS) deposits and associated hydrothermal systems can act as a recorder for past hydrothermal fluid conditions, tracing temporal fluctuations in fluid pH, temperature, salinity and redox, that are preserved as micron‐scale geochemical and isotopic variations across individual mineral grains (e.g., Martin et al., 2023). Assessing the relationship between faulting, fluid flow and mineralization is especially important in areas that have long, protracted growth histories characterized by multiple episodes of faulting, fluid flow and mineralization, such as oceanic core complexes and associated detachment fault zones (e.g., Escartin et al., 2017).…”

Magmatic evolution of the Kolumbo submarine volcano and its implication to seafloor massive sulfide formation

“…In porphyry Cu systems, Cu sulfides precipitate upon fluid cooling over the 425 to 350 °C range, assisted by fluidrock interaction (Sillitoe 2010 and references therein). Similar processes at Kolumbo may lead to formation of Cu porphyry-style mineralization at depth, which later remobilization by magmatic-hydrothermal fluids could provide metals to the chimneys (Martin et al 2023). 4) Submarine magmatic-hydrothermal systems are complex systems hosting various fluids of diverse origins: i.e., seawater-eventually modified by fluid-rock interaction-connate brines, magmatic-derived vapor, and brines (Diehl et al 2020;Hannington et al 2005).…”

“…Tectonic or magmatic events, changes in permeability may affect the dynamics of the hydrothermal fluid regime, leading to mobilization of different fluids and ultimately impacting the metal budget (de Ronde et al 2019). Finally, eventual mineralization caused by the different processes discussed above may be overprinted by later hydrothermal fluid circulation, eventually remobilizing the metals toward the seafloor (Martin et al 2023).…”