Arsenian Pyrite and Cinnabar from Active Submarine Nearshore Vents, Paleochori Bay, Milos Island, Greece

Abstract

Active, shallow-water (2–10 m below sea level) and low temperature (up to 115 °C) hydrothermal venting at Paleochori Bay, nearshore Milos Island, Greece, discharges CO2 and H2S rich vapors (e.g., low-Cl fluid) and high-salinity liquids, which leads to a diverse assemblage of sulfide and alteration phases in an area of approximately 1 km2. Volcaniclastic detritus recovered from the seafloor is cemented by hydrothermal pyrite and marcasite, while semi-massive to massive pyrite-marcasite constitute mounds and chimney-like edifices. Paragenetic relationships indicate deposition of two distinct mineralogical assemblages related to the venting of high-Cl and low-Cl fluids, respectively: (1) colloform As- and Hg-bearing pyrite (Py I), associated with marcasite, calcite, and apatite, as well as (2) porous and/or massive As-rich pyrite (Py II), associated with barite, alunite/jarosite, and late-stage hydrous ferric oxides. Mercury, in the form of cinnabar, occurs within the As-rich pyrite (Py I) layers, usually forming distinct cinnabar-enriched micro-layers. Arsenic in colloform pyrite I shows a negative correlation with S indicating that As1− dominates in the pyrite structure suggesting formation from a relatively reducing As-rich fluid at conditions similar to low-sulfidation epithermal systems. On the contrary, As3+ dominates in the structure of porous to massive pyrite II suggesting deposition from a sulfate-dominated fluid with lower pH and higher fO2. Bulk sulfide data of pyrite-bearing hydrothermal precipitates also show elevated As (up to 2587 ppm) together with various epithermal-type elements, such as Sb (up to 274 ppm), Tl (up to 513 ppm), and Hg (up to 34 ppm) suggesting an epithermal nature for the hydrothermal activity at Paleochori Bay. Textural relationships indicate a contemporaneous deposition of As and Hg, which is suggested to be the result of venting from both high-salinity, liquid-dominated, as well as CO2- and H2S-rich vapor-dominated fluids that formed during fluid boiling. The CO2- and H2S-rich vapor that physically separated during fluid boiling from the high-salinity liquid led to calcite formation upon condensation in seawater together with the precipitation of As- and Hg-bearing pyrite I. This also led to the formation of sulfuric acid, thereby causing leaching and dissolution of primary iron-rich minerals in the volcaniclastic sediments, finally resulting in pyrite II precipitation in association with alunite/jarosite. The Paleochori vents contain the first documented occurrence of cinnabar on the seafloor in the Mediterranean area and provide an important link between offshore hydrothermal activity and the onshore mercury and arsenic mineralizing system on Milos Island. The results of this study therefore demonstrate that metal and metalloid precipitation in shallow-water continental arc environments is controlled by epithermal processes known from their subaerial analogues.

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

“…At decreasing wave action, the seawater-fluid interface moves upward towards the seafloor leading to the formation of colloform pyrite-marcasite upon mixing of the discharging fluid with ambient seawater (Figure 3). Cinnabar (HgS) has been described at Paleochori Bay (Voudouris et al, 2021) and a similar Hg-rich mineralization has also been observed at Calypso SE (Figure 3C; Stoffers et al, 1999a), which is associated with charcoal fragments hosted by the volcaniclastic sediments (Supplementary Figure S1B). This may imply that the Hg precipitation is related to organic matter, as suggested by Prol-Ledesma et al, (2002) for the Punta Mita vent system, Mexico.…”

“…This combined leaching and replacement process of detrital magnetite was also described at other shallow marine hydrothermal vents, such as Lihir, Papua New Guinea (Pichler et al, 1999) andPunta Mita, Mexico (Prol-Ledesma et al, 2002). The presence of alunite-jarosite at Paleochori Bay suggests the occurrence of oxidised low pH (<3) fluids likely reflecting a boilingrelated vapor-dominated origin (Valsami-Jones et al, 2005;Price et al, 2013;Voudouris et al, 2021;Wu et al, 2016). Marcasite is only stable at low pH (<5) conditions (Murowchick and Barnes, 1986), and hence we propose that the pyrite-marcasite alternations, as observed at Paleochori Bay (Figure 3) may be the result of the periodic discharge of lower pH vapor-dominated fluids (cf.…”

“…Milos Island hosts several epithermal deposits associated with geothermal activity (Kilias et al, 2001;Naden et al, 2005;Smith et al, 2018). Shallow submarine hydrothermal venting is concentrated along the southern coastline, where Paleochori Bay is located with its intense hydrothermal activity and As-rich mineralization (Price et al, 2013;Voudouris et al, 2021). Hydrothermal venting is structurally controlled by an active rift, which extends offshore from the subaerial Fyriplaka Volcano (Figure 2B; Fytikas et al, 1986;Varnavas and Cronan 2005;Price et al, 2013).…”

“…Different types of fluids were described, including low Cl vapor-rich fluids with a focused discharge through volcaniclastic sediments, brine-rich seeps, and low-pH fluids discharging through rock fissures (Table 1). Intense discharge also raises sandy/pebbly plumes of volcaniclastic sediments that are cemented by pyrite and marcasite to sulphide mounds, which are associated with gas bubbles and the occasional discharge of saline liquids (Valsami-Jones et al, 2005;Voudouris et al, 2021).…”

“…However, the trace element composition of hydrothermal pyrite in shallow marine hydrothermal systems (<200 mbsl) has not been investigated to date, and hence little is known about the enrichment processes of trace metals and metalloids in these environments (Valsami-Jones et al, 2005;Wu et al, 2016;Houghton et al, 2019). In a recent study, the mineralogical and major (and minor) element composition of hydrothermal pyrite and marcasite from Paleochori Bay (Milos, Greece) has been described and related to the venting of high-Cl liquid-and low-Cl vapordominated fluids (Voudouris et al, 2021). We significantly extend this approach by a micro-analytical trace element study combined with S isotope analyses on samples from Paleochori Bay and Calypso Vents (Bay of Plenty, New Zealand).…”