Trace Element Signatures in Pyrite and Marcasite From Shallow Marine Island Arc-Related Hydrothermal Vents, Calypso Vents, New Zealand, and Paleochori Bay, Greece

Abstract

Fluid conditions of shallow marine hydrothermal vent sites (<200 mbsl) in island arcs resemble those of subaerial epithermal systems. This leads to a distinct mineralization-style compared to deeper arc/back-arc (>200 mbsl) and mid-ocean ridge-related environments (>2000 mbsl). At Calypso Vents in the Bay of Plenty and Paleochori Bay at the coast of Milos Island, fluids with temperatures <200°C are emitted through volcaniclastic sediments in water depths <200 mbsl. The hydrothermal mineralization from these fluids is dominated by pyrite and marcasite showing diverse textures, including colloform alternations, semi-massive occurrences surrounding detrital grains, vein-type pyrite, and disseminated fine-grained assemblages. Pyrite and marcasite from Calypso SE show elevated concentrations of volatile elements (e.g., As, Sb, Tl, Hg) implying a vapor-rich fluid phase. By contrast, elements like Zn, Ag, and Pb are enriched in hydrothermal pyrite and marcasite from Calypso SW, indicating a high-Cl liquid-dominated fluid discharge. Hence, vapor-liquid element fractionation induced by fluid boiling is preserved in the seafloor mineralization at Calypso Vents. Hydrothermal mineralization at very shallow vent sites (<10 mbsl), like Paleochori Bay, are affected by wave action causing a seasonal migration of the seawater-fluid interface in the sediment cover. The δ34S composition of native S crusts and crystalline S (0.7–6.7‰) is indicative for host rock leaching and thermochemical reduction of seawater sulphate. By contrast, the highly negative δ34S signature of native S globules in sediments (−7.6 to −9.1‰) is related to microbial sulphate reduction or a subordinate magmatic fluid influx. Alunite-jarosite alteration (Paleochori Bay) and a mineral assemblage consisting of orpiment, realgar, and native S (Calypso Vents) may also suggest a contribution by an oxidised (sulphate-rich) low pH fluid of potential magmatic origin. However, fluid boiling is pervasive at Calypso Vents and Paleochori Bay, and the condensation of vapor-rich fluids in a steam-heated environment may produce a similar alteration and mineralization assemblage without a significant magmatic fluid influx, as known from some subaerial epithermal systems.

Trace Element and Isotope Systematics in Vent Fluids and Sulphides From Maka Volcano, North Eastern Lau Spreading Centre: Insights Into Three-Component Fluid Mixing

“…The Fe/Zn ratio of Cu-poor (<2 wt%) sphalerite I can be used to estimate the precipitation temperatures during the early stage (Keith et al, 2014), which are suggested to be in the range of 230-260 °C for the Maka hydrothermal system (Figure 9). These temperature estimations also agree with the precipitation conditions of marcasite that typically forms at temperatures <240 °C (Murowchick and Barnes, 1986;Nestmeyer et al, 2021). This early sulphide precipitation stage insulates the hot ascending fluids in the central conduit from the surrounding seawater leading to higher precipitation temperatures at the onset of the intermediate-stage (Figure 9).…”

“…Section 5.4). However, similar conditions may be achieved by the condensation of boiling-induced vapour into hydrothermal fluids or seawater (Naden et al, 2005;Nestmeyer et al, 2021;Schaarschmidt et al, 2021;Voudouris et al, 2021), as suggested for Maka South (cf. Sections 5.2, 5.3).…”

“…This agrees with the late stage occurrence of bornite, covellite and chalcocite-digenite (Table 3 and Figure 9) either indicating a decreasing fluid discharge resulting in seawater ingress caused by a gradient in fluid and seawater pressure in favour of the latter (Tivey, 1995) or a supergene occurrence by chalcopyrite replacement (Berkenbosch et al, 2012;Keith et al, 2016a;. However, marcasite is absent in the bornite, chalcocite-digenite and covellite bearing samples from Maka South (Table 2), for which reason we propose that marcasite was not stable under the given fluid conditions probably due to elevated fluid temperatures (>240 °C, Murowchick and Barnes, 1986;Nestmeyer et al, 2021). Instead, decreasing a (H 2 ) and a (H 2 S) conditions in high temperature fluids (>300 °C) could result in a similar sequence of chalcopyrite followed by bornite and eventually chalcocite/covellite as the activities of H 2 and H 2 S decrease towards the outer chimney wall (Figure 10), as observed at Maka South.…”

“…Section 4.3.1). The formation of pyrite III overlaps with sphalerite II (intermediate-stage) and late-stage marcasite II (Figures 2G, 9); however, the precipitation of marcasite II continues beyond the pyrite III deposition implying a final decrease in fluid temperature (<240 °C, Murowchick and Barnes, 1986;Nestmeyer et al, 2021). This agrees with the late stage occurrence of bornite, covellite and chalcocite-digenite (Table 3 and Figure 9) either indicating a decreasing fluid discharge resulting in seawater ingress caused by a gradient in fluid and seawater pressure in favour of the latter (Tivey, 1995) or a supergene occurrence by chalcopyrite replacement (Berkenbosch et al, 2012;Keith et al, 2016a;.…”

“…Depending on their volatility the dissolved elements either partition into the vapour (e.g., As, Sb, Te) or liquid phase (e.g., Fe, Co, Ni) and/or precipitate (e.g., Au) as a consequence of boiling-induced changes in fluid pH, redox, and ligand availability (e.g., Cl, H 2 S; Drummond and Ohmoto, 1985;Grundler et al, 2013;Monecke et al, 2014;Pokrovski et al, 2018). Although, liquid-vapour partitioning and precipitation affect fluid chemistry in predictable ways (e.g., Von Damm et al, 1997;Stoffers et al, 2006;Schmidt et al, 2017) it is still unclear how these processes are recorded in the geological archive by sulphide chemistry (Tardani et al, 2017;Román et al, 2019;Keith et al, 2020;Falkenberg et al, 2021;Nestmeyer et al, 2021;Schaarschmidt et al, 2021).…”