GENESIS OF SECONDARY MN-OXIDE ORES IN THE URKUT DEPOSIT, HUNGARY - ANOXYGEN-ISOTOPE STUDY

The Urkut Basin of western Hungary is well known for its early Jurassi c Mn-carbonate deposit that is related genetically to Jurassic black s hale formation. This carbonate deposit has undergone multistage oxidat ion during the Cretaceous that produced secondary Mn-oxides. Among the geochemical characteristics that distinguish the host sedimentary roc ks and Mn-carbonates from the Mn-oxides is an extreme enrichment in Sr (up to 1%) without an observable independent Sr-phase. This enrichmen t indicates a special process of formation. The delta(18)O values of t hese Mn-oxide ores fall between 1.5 and 5.5 parts per thousand (VSMOW) showing a slight bimodal distribution. This isotopic variability is i nterpreted as a product of superposing effects of the interaction of s urficial and deep-seated magmatic fluids. The delta(18)O(MnO2) values (from 1.5 to 5.5 parts per thousand), the assumed delta(18)O(water) va lues and formation temperatures fit well with the MnO2-water fractiona tion curve based on the data of Yeh et al. (1985) and Hoefs et al. (19 87).