Environmental and economic perspectives on Fe-oxyhydroxide and atacamite at seafloor massive sulphide deposits

Seafloor Massive Sulphide (SMS) deposits have received global interest because of their high average grades of copper and zinc. However, the weathering processes of SMS deposits and the fate of metals during sulphide oxidation remain poorly understood, posing a potential ecological risk within the context of seafloor mining operations. Atacamite and Fe-oxyhydroxide (FeOOH) are key weathering products at SMS deposits, formed during the oxidation of sulphide minerals. These weathering products are hypothesized to trap base metals (i.e., copper and zinc), limiting their release into seawater. Here we study the processes that control the formation of sulphide weathering products and metal retention to provide new insights in metal release during seafloor mining and the valorisation of sulphide weathering products at these systems.

For this study, 31 samples of FeOOH with and without atacamite veins were obtained from the Semenov hydrothermal field on the Mid-Atlantic Ridge. Bulk geochemical analysis reveal copper grades of 2.40 wt.%, comparable to those in volcanogenic massive sulphide deposits, highlighting the potential of FeOOH and atacamite as a secondary resource at SMS deposits. Leaching experiments on nine samples of FeOOH with different amounts of atacamite show that the majority, i.e., 63.5% of copper within sulphide weathering products is found in atacamite veins hosted within FeOOH, while the FeOOH itself holds 32.5% copper. Additionally, our leaching experiments show that the occurrence of atacamite veins within FeOOH correlates with increased copper enrichment in FeOOH. While this may be due to copper adsorbing onto FeOOH during atacamite dissolution, this supports the natural capacity of FeOOH to adsorb copper in conditions where atacamite is unstable, akin to what could happen in natural SMS systems. For example, during chalcopyrite oxidation, copper is mobilised in acidic pore fluids, which may adsorb onto FeOOH at a low pH where atacamite is unable to precipitate. The mobilised fluid will then precipitate as atacamite veins when mixing with sufficient amounts of seawater.

To assess the ecological impact of sulphide weathering products, we conducted oxidative dissolution experiments on mixed sulphides under simulated seafloor conditions (~2.1°C, pH 8.2) over a two-week period in synthetic seawater. Results show that dissolved copper and zinc concentrations increased by approximately an order of magnitude above background synthetic seawater levels, i.e 35 vs 4.1 μg/L and for copper and 274 vs 33 μg/L for zinc. Extrapolation of constant oxidation rates from our experiments suggest that stockpiling 1,000 tonnes of sulphide ore at the seafloor could release up to 2,730 kg of zinc and 280 kg of copper annually. However, FeOOH precipitation occurring during our experiments, adsorbs dissolved metals from seawater, with a maximum of 23.1% of the total dissolved copper captured, thereby reducing metal release into seawater during the oxidative dissolution of sulphides.

Together, these findings demonstrate the potential of sulphide weathering products as secondary resources and as mitigators of ecological risk. The observation of atacamite could serve as an indicator for copper-rich FeOOH, while FeOOH precipitation provides a natural mechanism to adsorb metals, reducing their release during potential SMS mining operations.