Fe-oxyhydroxide deposits at Semenov hydrothermal field (13°30′N), Mid-Atlantic ridge: insights into formation, modification and resource potential

Secondary Fe-oxyhydroxide (FeOOH) forms at seafloor massive sulphide (SMS) deposits through the oxidation of sulphide minerals by oxygenated seawater. Secondary FeOOH deposits are enriched in economically important metals, such as Cu and Zn, potentially representing an additional resource. However, how the metal content of secondary FeOOH evolves through post-formational modification at the seafloor, is poorly understood. Moreover, FeOOH can form as metal-poor primary precipitates, but our knowledge is insufficient to effectively discriminate between primary and secondary FeOOH at SMS deposits. At the Mid-Atlantic Ridge hosted Semenov hydrothermal field, primary FeOOH deposits are typically metal-depleted (< 0.4 wt% Cu + Ni + Zn, n = 6) and form layered chimney structures with alternating Mn-oxide and FeOOH bands, with the fluid conduit lined with green smectite. In contrast, secondary FeOOH deposits are enriched in Cu, averaging 2.40 wt% (n = 31), and exhibit diverse textural morphologies including chimney, brecciated, layered, ochre, ocherous and massive, inherited from the sulphide protolith. Secondary FeOOH behaves similarly to terrestrial gossans, acting as a metal trap through sorption or precipitation of metals released during underlying sulphide oxidation. A correlation between the Cu content of secondary FeOOH and that of sulphide suggests its potential use as a geochemical vector in exploration. Finally, the Cu content in the secondary FeOOH remains stable even under prolonged exposure to seawater at the seafloor, indicating that older, off-axis, buried and oxidised SMS deposits may still contain secondary FeOOH with appreciable amounts of Cu. Overall this study demonstrates the potential of secondary FeOOH as a potential metal resource and a tool to guide exploration at SMS deposits.