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 Å 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 Å 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 energy-dispersive X-ray spectroscopy (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.