Fe-Mn nodule morphotypes from the NE Clarion-Clipperton Fracture Zone, Pacific Ocean: comparison of mineralogy, geochemistry and genesis

Abstract Ferromanganese nodules are polymetallic ore deposits considered to be archives of paleoceanographic proxies and targets of deep-sea mining. In the present paper, nodules and underlying sediments from a claim area (UK-1) within the eastern Clarion-Clipperton Fracture Zone (CCFZ) and from Area of Particular Environmental Interest (APEI-6) were analyzed. In each area, nodules are characterized by contrasting morphology, mineralogy, geochemistry and abundance. The obtained results reveal three types of Fe-Mn nodules. Type 1 : smooth irregular nodules with sizes of 1-1.5 cm containing mainly 10 A phyllomanganates and phillipsite; Mn/Fe = 2.50, high trace metal abundances Co+Ni+Cu = 2.10%, high rare earth element and yttrium (REY) concentrations (1515 ppm), positive Ce anomaly; hydrogenetic origin. Type 2 : smooth discoidal nodules with sizes of 2-4 cm composed of 10 A phyllomanganates and buserite I; Mn/Fe = 3.65, high trace metal enrichment Co+Ni+Cu = 2.35%, positive Ce anomaly; mainly formed by hydrogenous precipitation. Type 3: nodules (3-15 cm) with botryoidal surfaces and well-defined concentric zonality; Mn/Fe = 5.1, trace metal content Co+Ni+Cu = 2.42%. The textural and chemical characteristics of nodules suggest that they acquire metals as a result of both hydrogenetic and diagenetic processes during nodule growth in alternating oxic/suboxic conditions. Scanning electron microscopy (SEM), in conjunction with X-ray spectral analysis (EDS), reveals abundant pseudomorphs, which is considered possible evidence for biomineralization within the nodules. The morphology, mineralogy and geochemistry of the studied nodules are strongly affected by post-accretional diagenetic processes, which are closely linked to biomineralization.