Interplay of hydrothermal, seawater, and diagenetic fluids as principal driver of lithium and critical‑metal enrichment in the Cocos-Nazca spreading center ferromanganese crusts

Ferromanganese crusts (Fe-Mn crusts) are of significant importance due to their economic potential. However, hydrothermal crusts from the Eastern Equatorial Pacific have remained understudied as resource targets. This work evaluates seven Fe-Mn crust samples from an off-axis, fault-controlled hydrothermal system at the Cocos-Nazca Spreading Center using bulk geochemical and micro-textural mineralogical analyses techniques. The results indicate a clear hydrothermal-hydrogenetic formation continuum influenced by vent proximity for the Fe-Mn crusts. Proximal Fe-Mn crusts exhibit pronounced hydrothermal signatures: elevated Mn (up to 35 %); dominance of 7–10 Å manganates; fast growth rates (>878 mm/Ma); and high contents of hydrothermally derived metals (Li up to 607 µg/g, Ni ∼ 0.35 wt%, Cu and Zn ∼ 0.22 wt%). Distal crusts, conversely, exhibit attenuated hydrothermal features, increased hydrogenetic metal concentrations (497 µg/g Co, 620 µg/g ΣREY), and high-moderate Ni, Cu and Zn concentrations scavenged predominantly by Fe-oxyhydroxides and Fe-vernadite. Stratabound substrates reflect alternating hydrothermal-diagenetic influences, however only purely hydrothermal minerals are either metal-poor or Zn-rich, whereas diagenetically affected layers display notable Ni-Cu enrichments. Trace metal distribution patterns suggest that effective and selective metal scavenging is controlled by the types of fluids involved, but metal concentrations are regulated by environmental factors (e.g. metal solubility, precipitation of Fe or Mn oxides, temperature and distance to vent). Hydrothermal plume particles and colloids emitted by the vent effectively scavenge metals from both hydrothermal fluids and seawater for the Fe-Mn crust, while sediment‑derived metal remobilization via hydrothermal circulation drives Ni-Cu enrichment on the stratabound mineralization. Significantly enriched polymetallic hydrothermal crusts form primarily through interactions among hydrothermal, seawater, and diagenetic fluids rather than hydrothermal fluids alone. Similar Fe-Mn crusts as well as other mineral deposits in the Eastern Equatorial Pacific highlight the underexplored but significant resource potential of this region.