Geochemical characteristics and metallogenic environmental constraints of ferromanganese nodules in the central Philippine Sea

Understanding the geochemical characteristics, genetic types, and key controls on ferromanganese nodule (FMN) formation in the Central Philippine Sea is essential to assess the nodule resources across the Western Pacific marginal seas. We examined the microstructure and elemental geochemistry of FMNs collected from the southern Kyushu-Palau Ridge (KPR). Results indicate that the microstructure of FMN displays a distinct micro-laminae sequence transitioning from mixed to parallel or laminated structures, reflecting a progressive increase in bottom current energy. Diagnostic geochemical signatures—Mn/Fe ratios averaging 0.95, positive δCe anomalies (mean ～1.7), and high Co/(Ni+Cu) values (mean ～0.5)—confirmed collectively a predominantly hydrogenetic origin. The pronounced Ti enrichment (mean 0.93%) indicated significant aeolian input from Asian sources. Conversely, the relatively low combined content of Co, Ni, and Cu (0.74%) reflected existence of strong bottom current that restricted long-term metal accumulation. The spatial distribution of the FMN was controlled by the Antarctic Bottom Water (AABW) flow paths and seafloor topography. Abundant FMN occurred along the AABW-influenced KPR and Parece Vela Basin, while the nodule growth in the topographically enclosed southern West Philippine Basin was obviously very limited due to oxygen-deficient conditions. Therefore, hydrogenetic FMNs in the central Philippine Sea were formed in a strongly oxidizing and low sedimentation rate environment, and their resource potential was constrained by the synergy of deep-ocean circulation and tectonic geomorphology. This research improved our understanding of FMN genesis and provided valuable insights for resource exploration strategies in the Western Pacific marginal seas.