Mixed hydrogenetic–hydrothermal genesis of Quaternary ferromanganese nodules in a tectonically active lacustrine basin, eastern Türkiye

This study presents a detailed investigation of the mineralogical, geochemical, and rare earth element (REE) characteristics of ferromanganese nodules found within Quaternary lacustrine sediments in the Ağrı region of Eastern Anatolia, Türkiye. These nodules, primarily composed of goethite and manganite with minor quartz, exhibit botryoidal and colloform textures typical of low-temperature diagenetic environments. Raman and XRD analyses identified significant iron and manganese mineral phases, while SEM-EDX revealed trace amounts of Co, Ni, and Zn associated with these phases. Chondrite-normalized REE patterns demonstrate light REE enrichment, a weak negative europium anomaly (Eu_N/Eu∗ = 0.69–0.84), and variable cerium anomalies (Ce_N/Ce∗ ranging from 0.74 to 1.95, corresponding to both weak negative and positive Ce anomalies), indicating a mixed hydrogenous and hydrothermal origin under fluctuating redox conditions. These geochemical signatures, together with sedimentological evidence, suggest formation within a restricted lacustrine environment characterized by episodic anoxia and limited detrital input. The results contribute to a broader understanding of Fe–Mn mineralization in continental settings and underscore the influence of volcanic and tectonic processes on nodule genesis.