ALBERT

Description: Highlights • Regional investigation of precious and base metal-rich deposits in the Cyclades • Pb-isotope and trace element analyses in galena by laser ablation ICP-MS • Galena inherits the Pb-isotopic signature of the underlying basement. • Distinct Pb-isotope signatures in the west and north-central Cyclades, Greece. • 206Pb/204Pb line follows the trace of known tectonostratigraphic units. Polymetallic ore deposits of the Aegean Sea, Greece, are temporally and spatially associated with the emplacement of Miocene granitoids and Pliocene to Quaternary volcanic units along the active volcanic arc and occur adjacent to the crustal-scale detachment systems. A variety of mineral deposits (e.g., carbonate-replacement, skarn, porphyry, vein-type, and low- to high-sulfidation epithermal deposits) formed during different stages of back-arc evolution and can be found in the footwall and hanging wall of the major detachment systems and all tectono-stratigraphic units. New galena Pb-isotope data collected by laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analysis exhibit a range of isotopic ratios: 206Pb/204Pb: 18.68–18.91, 207Pb/204Pb: 15.67–15.75 and 208Pb/204Pb: 38.83–39.18, with a resolvable geographic pattern. Together with compiled Pb-isotope data of Cycladic galena, large-scale regional similarities in the Pb-isotope signatures are recognized, suggesting distinct sources of lead in the underlying basement. Base metal-rich deposits in the west Cyclades have a common lead source with a more radiogenic signature, whereas lead in the deposits in the north-central Cyclades was derived from a less radiogenic source. Similarities between the Pb-isotopic signature of galena and regional host rocks suggest, that the pre-Alpine Cycladic Basement was the main source of lead for the deposits in the north and central, and the metamorphosed volcano-sedimentary Lower Cycladic Blueschist Nappe was the source of lead for the deposits in the west. Moreover, there is a close spatial relationship of the 206Pb/204Pb 18.84 line and trace of the proposed Trans-Cycladic Thrust. Regional trends were also observed in the new LA-ICP-MS trace element analysis in galena. Galena in carbonate-replacement and skarn deposits in the west is enriched in base metals and silver, whereas galena in vein-type deposits in the north and low- to high-sulfidation epithermal deposits along the active volcanic arc is enriched in gold, tellurides and selenium. Combined trace element and isotopic data of galena indicate distinct metal sources in the Cyclades. Heterogenous rock assemblages of mixed provenance and age account for the differences in lead isotopes. Whereas the base metals and silver are ultimately sourced from the basement, the precious metals might be derived from the metasomatized lithospheric mantle.

Description: Polymetallic veins and breccias and carbonate-replacement ore deposits in the Cyclades continental back arc, Greece, formed from a range of fluid and metal sources strongly influenced by the dynamics of the late Mesozoic-Cenozoic Hellenic subduction system. These complexities are recorded in the isotopic signatures of hydrothermal barite. We investigated 17 mineral occurrences on four Cycladic islands and from Lavrion on the mainland. Here, barite occurs in almost all deposit types of Miocene to Quaternary age. We used a multiple isotope and geochemical approach to characterize the barite in each deposit, including mineral separate analysis of δ34S and δ18O and laser ablation-inductively coupled plasma-mass spectrometry of 87Sr/86Sr and δ34S. Barite from carbonate-hosted vein and breccia Pb-Zn-Ag mineralization on Lavrion has a wide range of δ34S (2–20‰) and δ18O (10–15‰) values, reflecting a mix of magmatic and surface-derived fluids that have exchanged with isotopically heavy oxygen in the carbonate host rock. Sulfur (δ34S = 10–13‰) and oxygen (δ18O = 9–13‰) values of barite from the carbonate-hosted vein iron and barite mineralization on Serifos are permissive of a magmatic sulfate component. Barite from epithermal base and/or precious metal deposits on Milos has δ34S (17–28‰) and δ18O (9–11‰) values that are similar to modern seawater. In contrast, barite from vein-type deposits on Antiparos and Mykonos has a wide range of δ34S (16–37‰) and δ18O (4–12‰) values, indicating a seawater sulfate source modified by mixing or equilibration of the hydrothermal fluids with the host rocks. Strontium isotope ratios of barite vary regionally, with 87Sr/86Sr ≥ 0.711 in the central Cyclades and 87Sr/86Sr ≤ 0.711 in the west Cyclades, confirming the strong influence of upper crustal rocks on the sources of fluids, Sr, and Ba in the formation of ore.

Description: Introduction: Active hydrothermal vents of volcanic origin provide a remarkable manifestation of life on Earth under extreme conditions, which may have consequences for our understanding of habitability on other terrestrial bodies as well. Methods: Here, we performed for the first time Illumina sequencing of bacterial and archaeal communities on sub-seafloor samples collected from the Santorini-Kolumbo volcanic field. A total of 19 (3-m long) gravity corers were collected and processed for microbial community analysis. Results: From a total of 6,46,671 produced V4 sequences for all samples, a total of 10,496 different Operational Taxonomic Units (OTUs) were identified that were assigned to 40 bacterial and 9 archaeal phyla and 14 candidate divisions. On average, the most abundant phyla in all samples were Chloroflexi (Chloroflexota) (24.62%), followed by Proteobacteria (Pseudomonadota) (11.29%), Firmicutes (Bacillota) (10.73%), Crenarchaeota (Thermoproteota) (8.55%), and Acidobacteria (Acidobacteriota) (8.07%). At the genus level, a total of 286 known genera and candidate genera were mostly dominated by members of Bacillus, Thermoflexus, Desulfatiglans, Pseudoalteromonas, and Pseudomonas. Discussion: In most of the stations, the Chao1 values at the deeper layers were comparable to the surface sediment samples denoting the high diversity in the subsurface of these ecosystems. Heatmap analysis based on the 100 most abundant OTUs, grouped the sampling stations according to their geographical location, placing together the two hottest stations (up to 99°C). This result indicates that this specific area within the active Kolumbo crater create a distinct niche, where microorganisms with adaptation strategies to withstand heat stresses can thrive, such as the endospore-forming Firmicutes.