Origin of a carbonate-hosted Fe-Mn-(Ba-As-Pb-Sb-W) deposit of Langban-typein central Sweden

The Sjogruvan deposit is one of the Langban-type Fe-Mn oxide deposits hoste d by marble interbeds within Svecofennian metavolcanic rocks in the Bergsla gen region, central Sweden. Mineralogical and geochemical studies have been carried out to clarify the premetamorphic origin of this type of deposit, which is set apart from most other Mn mineralizations by a significant enri chment in Ba, As, Sb, Pb, W and Be contained by various oxyminerals. The pr incipal ore types at Sjogruvan are (1) hematite + quartz +/- magnetite, (2) hausmannite + calcite + tephroite and (3) braunite + celsian + phlogopite. The Mn ores are compositionally akin to modern Mn deposits formed by subma rine hydrothermal processes (with a high Mn/Fe ratio and low contents of Co , Ni, Th, U and REE) and likely owe their existence to similar mechanisms o f formation. Pb isotope data indicate that the metal source and timing of d eposition is similar to the major stratabound basemetal and iron deposits i n Bergslagen. All the key elements have been leached from the local felsic volcanic units and were deposited on the sea floor; the excellent Mn-Fe sep aration occurred in an Eh-pH gradient that essentially corresponded to the mixing zone of hydrothermal solutions and seawater. The braunite ore is che mically distinct from the hausmannite ore, with a high concentration of ref ractory elements (Al, Ti, Zr) and a positive Ce anomaly, which indicate a d etrital/hydrogenetic contribution to its protolith. Carbon isotope (delta C -13) values around 0 parts per thousand (relative PDB) suggest that carbona tes in the deposit formed directly from seawater.