Concreciones de ferromanganeso en el mar Bálticocaracterización y mecanismos de formación

Resum

Ferromanganese concretions are polymetallic ore deposits present in all the oceans of the planet. Economically they are attractive because of their composition, rich in strategic and critic metals, and also are interesting for the paleo- ceanographic record study. Concretions from 15 different locations within the Baltic Sea have been analyzed. The obtained results reveal three types of concretions. Type 1: subspherical concretions with diameters of 1–2 cm containing mainly iron oxides (goethite) and lesser extent manganese oxides (birnessite); Mn/Fe=0.40, trace metals abundances 0.29%. Type 2: tabular concretions (length range between 1-7 cm) with irregular surfaces and formed mainly by silicates; Mn/Fe=0.06, trace metal content 0.65%. Type 3: rough discoidal concretions with diameter between 2–3 cm composed primarily of silica- tes and iron oxides (goethite); Mn/Fe=0.18 and small trace metals content (0.13%). The three groups content small amounts of rare earths (REE) (<0.04%). Electron probe micro-analyzer reveals a complex mineral structure. All the characteristics of concretions suggest a high correlation between the formation of the ferromanganese concretions and the diagenetic proces- ses, which, in turn, are controlled by stagnation and renewal periods in the Baltic sea.