EVIDENCE FOR A VOLCANOGENIC COMPONENT TO THE OBROCHISHTE MN ORE DEPOSIT, BULGARIA
The prevailing model for manganese ore formation of these deposits indicates a marine source for the sedimentary host rock. The model suggests that an Early Oligocene transgression mobilized high salinity, Mn-rich seawater from stagnant basins to shallow shelf environments, permitting the subsequent formation of Mn-carbonate. New data on mineralogy and trace element and rare earth element geochemistry of Obrochishte ores suggest a significant role of an additional volcanogenic source. Sediments deposited during the formation of Mn-rich strata record evidence of increased eruptive activity, most distinct in the Maikop and Menilitic basins of the Lesser Caucasus and Carpathians (Varentsov et al., 2003). Intense calc-alkaline volcanism in the Lesser Caucasus had ceased by the beginning of the Priabonian, to be resumed in the middle Rupelian, when the bentonitic clay intercalations were formed.
Bivariate discrimination plots of trace and rare earth elements (207Pb/204Pb, 87Sr/86Sr, Sr/Ca, Nb/Y) suggest a three component mixing system: a seawater, terrigenous, and volcanogenic component. The presence of a smectite-rich clay component further indicates the presence of a volcanic source that produced both a felsic air fall tephra component as well as well as the more andesitic tuffs described by previous authors.