Alteration products of seafloor massive sulphides: a source of critical metals?

Abstract

The global shift toward renewable energy technologies has driven demand for critical and strategic metals (e.g., Cu and Zn). As terrestrial deposits become more expensive and difficult to explore, seafloor massive sulphide (SMS) deposits emerge as promising alternative metal resources. When exposed at the seafloor, sulphides oxidise, forming secondary minerals including Fe-oxyhydroxide (FeOOH) and atacamite, which trap metals otherwise lost to seawater. Despite their potential significance, key unknowns remain regarding their variety, metal content, approximate tonnage, and geochemical evolution at the seafloor, leaving their resource potential and long-term fate uncertain. This study investigates secondary FeOOH deposits at the Semenov hydrothermal field (13°30′N, Mid-Atlantic Ridge) and aims to understand their formation, evolution, and resource potential. Here, I distinguish two types of FeOOH: i) metal-poor (<0.4 wt.% Cu+Zn) primary precipitates forming hydrothermal chimneys with Mn-oxide banding and green smectite; and ii) metal-rich (average 2.55 wt.% Cu+Zn) secondary FeOOH deposits occurring as fine-grained ochres, layered and brecciated deposits, remnant hydrothermal chimneys, and massive deposits. Approximately 50,000 - 880,000 t of secondary FeOOH material at Semenov is estimated that can sequester 1,300-22,400 t Cu and Zn. These figures suggest that secondary FeOOH could serve as viable supplementary resources. However, Semenov sits close to the MAR and does not represent an off-axis deposit. Therefore, understanding how the geochemical composition of FeOOH evolves at Semenov is crucial to determining whether sulphide weathering products in older, off-axis SMS deposits can retain their economic value over extended timescales. Initially, these deposits inherit metals directly from their sulphide protolith. Seawater interaction then modifies their composition, imparting a REE fingerprint with negative Ce anomaly while sulphur is depleted. Crucially, Cu and Zn mobilised during sulphide oxidation (pH 4.5–8.2) can become sequestered onto FeOOH through adsorption. With sufficient seawater mixing with these Cu-rich pore waters, atacamite veins can precipitate, further concentrating Cu within secondary FeOOH deposits. Laboratory studies suggest that, with time, the initial FeOOH mineralogy (which is dominated by ferrihydrite) transforms to goethite with the potential release of up to 90% Cu and Zn up to 57%. Despite this, goethite crystallisation from ferrihydrite may not be extensive under seafloor conditions at SMS deposits, and thus secondary FeOOH deposits can continue to function as long-term metal traps. In contrast, Au and Ag are not incorporated into secondary FeOOH or atacamite, but concentrate at the shrinking sulphide phase as it progressively oxidises. On completion of sulphide oxidation, Au and Ag depart from the secondary FeOOH deposits. It is speculated that these precious metals are re-mobilised as thiosulphate complexes that precipitate upon seawater contact or at the FeOOH-sulphide boundary deeper within the SMS deposit, potentially creating hitherto unrecognised enrichment zones. The fate of sulphide weathering products depends on the interplay between oxidation and reduction. Sulphide oxidation typically follows non-linear kinetics, with secondary FeOOH forming protective barriers that preserve sulphide cores and weathering products under oxic conditions. Under reducing conditions, however, FeOOH and atacamite may dissolve, potentially stripping metals from the system. Beyond resource potential, secondary FeOOH and atacamite can serve as exploration vectors. Atacamite highlights Cu-enriched areas and potential drilling targets for underlying Cu-rich sulphide ores. Seafloor samples can be rapidly screened by portable XRF to map Cu anomalies in real-time during surveys, guiding drilling efforts toward concealed ore bodies. This advances our understanding of sulphide weathering and metal cycling on the seafloor while providing a framework for evaluating weathering products as both resources and exploration guides for critical metals.

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Identifiers

ORCID for Christian Star Bishop: orcid.org/0000-0003-0652-3008

ORCID for Stephen Roberts: orcid.org/0000-0003-4755-6703

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