Ni-Cu-Co-rich hydrothermal manganese mineralization in the Wallis and Futuna back-arc environment (SW Pacific)

The Wallis and Futuna back-arc system is a complex area composed of at least two active oceanic spreading centers (the Futuna and Alofi spreading centers) and young volcanic zones characterized by diffuse magmatism locally affected by the Samoan hotspot. This geological setting is favorable to the establishment of hydrothermal systems, in the form of either high-temperature (HT) hydrothermal venting or low-temperature (LT) diffuse flow. During the 2010 Futuna cruise aboard the R/V L'Atalante, three remarkable inactive LT Fe-Si-Mn deposits were discovered (Utu Uli, Anakele and Utu Sega). Some of the Mn-rich precipitates discovered exhibit the highest base metals concentrations so far recorded in ferromanganese rocks, including in the well-documented hydrogenetic crusts and polymetallic nodules. The deposits lie on top of volcanoes and formed in close association with the volcanic facies. The manganese mineralization occurs in the form of massive layered crusts and Mn-rich cements within strongly altered basaltic pyroclastic rocks, brecciated lavas and, more rarely, in sediments. Field observations and mineralogical and chemical studies support a hydrothermal origin for the mineralization and show that nickel, cobalt and copper enrichments are controlled by the precipitation of 7 Å and 10 Å manganates. The conventional geochemical classifications (e.g. Bonatti et al., 1972) used to decipher the origin of Mn mineralization cannot be used for this new type of deposit and new robust discrimination diagrams need to be established. We suggest that the unusual enrichment of metals recorded in our samples is due to (i) a lack of precipitation of high-temperature massive sulfides at depths that would have retained metals (e.g. Cu, Ni, Co); (ii) isolation of the hydrothermal system, thereby avoiding Ni, Co and Cu losses in the water column; and (iii) the ability of birnessite and buserite/todorokite to scavenge Co, Ni, and Cu from aqueous fluids. The Utu Uli and Anakele deposits share certain characteristics with the active hydrothermal system at Loihi seamount (e.g. the depth of mineralization, relationships with pyroclastic volcanoes, and the influence of a mantle plume source) and thus might represent late-stage products of this specific type of hydrothermal activity. Elsewhere, the Co-rich mineralization of the Calatrava volcanic field (CVF) in Spain may be a potential analog of the Utu Sega deposit. The Mn-(Co) deposits of the CVF formed in close proximity to Pliocene volcanic rocks. Metals were transported by epithermal hydrothermal solutions with high fO₂ and cobalt was scavenged by Mn oxides. Together with the well-documented stratabound Mn deposits (González et al., 2016; Hein et al., 2008; Hein et al., 1996), the Mn deposits discovered in the Wallis and Futuna back-arc provide crucial insights into LT hydrothermal activity in the deep ocean. The metal-rich character of this LT hydrothermal activity may be of major importance for future research on the net flux of hydrothermally derived metals (e.g. Ni, Co, Cu) to the open ocean.