Geochemical features of the deep-sea sediments from a high polymetallic nodule area in the eastern part of the Clarion-Clipperton Fractures Zone (CCZ), NE Pacific were studied. Box-corer samples from six stations in individual depth layers 0–3, 3–5, 5–10, 10–20 cm were collected from 4300–4500 m depth. The deep-sea sediments were classified as clayey-siliceous oozes according to the mineral and grain-size analyses. The chemical composition of the studied samples is compatible with that of pelagic sediments. Manganese content varies from 0.16% to 0.70% being the highest in the geochemically active layer (top 7–12 cm) and decreases with depth. The Mn/Fe ratio, Ba, Co, Ni and Cu have the highest values in the first (0–3) and second (3–5) layers and decrease with depth. REE range is 195.84–357.79 ppm showing low variation between the layers. PAAS-normalized REE patterns show significant enrichment of MREE and HREE with strong negative Ce anomaly and weak positive Eu and Y anomalies. The geochemical characteristics of the studied sediments infer polygenic origin. The redox sensitive oxides and barite presense, Eh values and negative Ce anomaly, indicate oxidizing environment of sediment formation.

Deep-sea Fe-Mn polymetallic nodules formed nowadays at the deep-sea ocean floor were evaluated as promising critical raw materials (CRMs). Here, we report results of polymetallic nodules from the H22_NE block of the Interoceanmetal (IOM) exploration area in the eastern part of the Clarion–Clipperton Zone (CCZ), NE Pacific Ocean. The polymetallic nodules were studied with X-ray Diffraction, Raman spectroscopy, SEM-EDS, and LA-ICP-MS (bulk nodules and in situ nodule layers). Additionally, we combine geochemical data of polymetallic nodules with the previously reported data of pore waters and sediments from six stations. Our study aims to define the mineral composition and determine the content of CRMs in the polymetallic nodules and to assess the main factors controlling metal deposition and nodule enrichment in some CRMs. Mn content and the Mn/Fe ratio of the nodules classify them mostly as mixed hydrogenetic–diagenetic type. They are also enriched in Ni, Cu, Co, Zn, Mo, W, Li, Tl, and REE. The in situ REE patterns exhibit MREE and HREE enrichment and a variable Ce anomaly that argues for a changing oxic/suboxic environment and periodically changing of diagenetic and hydrogenetic nodule growth. The results of the joint study of the bottom sediments, pore waters, and polymetallic nodules show a complexity of processes that influence the formation of these deposits. The changing oxic and anoxic conditions are well documented in the chemistry of the nodule layers. Probably the most important controlling factors are sedimentation rate, bioturbation, adsorption, desorption, and oxidation. In addition, growth rates, water depth variations, electro-chemical speciation, phosphatization, and the structures of the Fe-Mn adsorbents are also considered. The polymetallic nodule deposits in the IOM contract area are estimated for future mining for Ni, Cu, Co, and Mn resources. They, however, contain additional metals of economic importance, such as REE and other trace elements (referred to as CRMs) that are potential by-products for metal mining. They can significantly increase the economic importance of exploited polymetallic nodules.

Seasonal variations of drainage waters and ochreous products of their discharge from the closed abandoned old gallery at the Grantcharitsa scheelite deposit (Bulgaria) were studied by field and laboratory methods for the period 2019–2023. The drainage is generated under anoxic conditions and is inherently diluted (EC = 100–202 µS/cm) with S (6–12 mg/L), Si (6–22 mg/L), Na (6–10 mg/L), Fe (0.2–3.3 mg/L), and W (0.19–3.5 µg/L), at a pH 4.4–6.5 and temperature 7–11.5 °C, with dissolved oxygen DO (2.1–7.7 mg/L). The concentrations of Fe and W and the pH of the water are variable and reach their maximum values during the dry (autumn) season. It was found that such parameters as pH, Eh, DO, Fe and W content change dramatically at a distance of up to 3 m from the water outlet; the values of pH, DO and Eh are sharply increased with a simultaneous nearly 5–6-times reduction in iron and tungsten content. The decrease in the contents of these elements is associated with the precipitation of ochreous material consisting of nanoscale ferrihydrite with an intermediate structural ordering between 2-line and 6-line ferrihydrite (major phase), hematite, goethite, quartz, montmorillonite and magnetite. The formation of ferrihydrite occurs as a result of abiotic and biotic processes with the participation of iron-oxidizing bacteria. Besides Fe₂O₃ (55.5–64.0 wt.%), the ochreous sediment contains SiO₂ (12.0–16.4 wt.%), SO₃ (1.3–2.4 wt.%), Al₂O₃ (3.1–6.8 wt.%) and WO₃ (0.07–0.11 wt.%). It has been shown that drainage waters and ochreous sediments do not inherently have a negative impact on the environment. The environmental problem arises with intense snowmelt and heavy rainfall, as a result of which the accumulated sediments are washed away and carried in the form of suspensions into the water systems. It is suggested that by providing atmospheric oxygen access to the closed gallery (via local boreholes), it is possible to stop the generation of iron-enriched drainage.