Origin of a Toarcian manganese carbonate/silicate deposit from the Krížna unit, Tatra Mountains, Poland

A Mn-bearing sequence of the Krížna unit in the Western Tatra Mountains occurs locally between shallow-water crinoidal tempestites (Lower Toarcian) and pelagic red nodular limestones (Middle Toarcian–Aalenian). The Mn-bearing sequence forms a lens-shaped body up to 2 m thick extending for a few hundred meters. It consists of several distinctive facies: Fe-rich layer (∼15 cm thick), jasperite (up to 0.5 m thick), X-ray amorphous Mn-oxide layer (10 cm thick), shale (∼510 cm thick), and Mn-rich bed (up to 1.1 m thick). Minerals in the Mn-rich bed, such as braunite, caryopilite, Mn-bearing calcite, and rhodochrosite, are distributed in two distinctive zones, carbonate- and silicate-dominated. The most important geochemical characteristics of these deposits are high Mn contents (up to 62.8 wt.% MnO), low concentrations of transitional elements (Co + Ni + Cu < 0.01 wt.%) and high Ba (up to 4500 ppm). Mineralogical and geochemical features coupled with the occurrence of specific microbial structures and fauna assemblage suggest an origin related mainly to exhalative shallow submarine vents. Manganese mineralization was controlled by synsedimentary tectonics that produced extensional faults providing channelways for geofluid migration. The internal facies variation of the Mn-bearing sequence may be explained by changes in bottom-water chemistry and geofluid temperature. Present-day texture and mineral composition of the Mn-bearing sequence is largely the result of diagenetic processes.