| Abstrakt: |
Relatively fresh volcanic rocks have been sampled by a remotely operated vehicle in situ from the NE caldera wall of Brothers submarine volcano, associated with Seafloor Massive Sulfide‐SMS deposits. Here, we present the first complete stratigraphic column of the NE caldera wall, comprising at least 12 massive dacitic lava flows, up to 80 m‐thick intercalated with multiple volcaniclastic layers associated with tuffaceous sediment layers. Detailed petrographic and geochemical analyses from hand specimen to crystal to silicate melt scale show chemical variability with depth, correlating partly with an increase in pervasive alteration due to volatile degassing. Moreover, while sulfide saturation occurred prior to volatile exsolution—which sequestrated most chalcophile elements as confirmed by the low metal contents of melt inclusions (e.g., Cu ≤ 1.3 μg/g and Au ≤ 7.0 μg/g)—silicate glass records a Cu enrichment and Au loss with differentiation, with interstitial glass accounting for Cu = 4.2 μg/g and Au = 6.6 μg/g and matrix glass for Cu = 6.0 μg/g and Au = 2.8 μg/g, respectively. Our findings suggest multiple sources for metals compensating for the low initial metal contents: (a) from hydrothermal fluids and volatile percolation ensuing interaction with the host rock and thus also replacement and/or dissolution of pre‐existing magmatic sulfides, (b) directly from the magma, consistent with metal release during magma degassing of metal‐ and Cl‐, and S‐ rich volatiles, and (c) from fluid circulation within unusually metal‐rich andesitic volcaniclastic layers (Cu = 40 μg/g, Au = 1.5 ng/g, and Pt = 0.99 ng/g). Our results elucidate the capacity of such hybrid mineralizing submarine volcanic systems to effectively scavenge, transport, and concentrate metals. Plain Language Summary: Based on a set of samples collected by a remotely operated vehicle, we composed the first stratigraphic column of the NE Caldera wall of the Brothers active submarine volcano. In order to understand the source and transport mechanism of metals in the volcanic system, we performed petrographic observations and chemical analyses of glass inclusions (representing the source of the magma), minerals and their host rocks. Our findings show a chemical variability with depth, down the caldera wall, correlating with alteration caused by degassing. Moreover, our results indicate that although the magma at depth was not unusually high in metals, likely because of early sulfide saturation (sequestrating most metals at depth), multiple other processes compensated for the metal loss and contributed to the metal enrichment of the system, including: (a) dissolution of pre‐existing magmatic sulfides, (b) magma degassing of metal‐ and Cl‐, and S‐rich volatiles, and (c) from fluid circulation within metal‐rich lithologies. Key Points: Underwater high‐resolution imagery and in situ rock sampling allow the reconstruction of a 375 m stratigraphic column of the caldera wallPetrography and geochemistry from hand specimen to crystal to silicate melt scale indicate weak to moderate chemical variability depending on depth and/or lithologyMultiple metal inputs from the magma, from magmatic sulfide dissolution, and from fluid circulation within metal‐rich volcaniclastics [ABSTRACT FROM AUTHOR] |
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