ORE PETROLOGY, WHOLE-ROCK CHEMISTRY AND ZONING OF THE KREMIKOVTSI CARBONATE-HOSTED SEDIMENTARY EXHALATIVE IRON(-BARITE-SULFIDE DEPOSIT, WESTERN BALKAN, BULGARIA(MN))

Siderite and hematite iron formations and barite as well as sulfide mi neralizations from primary orebodies recently exposed after continuous mining of the Kremikovtsi deposit have been studied. They were produc ed by superimposed sedimentary and hydrothermal Middle Triassic ore-fo rming processes involving precipitation and diagenesis, fissure- and c avity-filling, and replacement. Hydrothermal sedimentary-diagenetic ma gnesian-manganoan siderites, accompanied by a ferroan dolomite-ankerit e association, are fine-grained with spheroid relies of femicrite and predominantly massive, rarely fine-banded, texture. Epigenetic vein an d replacement siderites are coarser grained, predominantly idiomorphic and zonal. Analysis of the chemical composition of siderites from mor e than 25 deposits showed that those connected with hydrothermal activ ity (in veins, replacement and SEDEX orebodies) have Mn/Mg > 0.5, in c ontrast to sedimentary ones (Mn/Mg < 0.5). A replacement group with 1 > Mn/ Mg > 0.5 is distinctly outlined in the field of hydrothermal sid erites, which is probably the result of metasomatic exchange of elemen ts with the host rocks. Microquartz and relies of length-slow chalcedo ny cementing siderite grains are substantial evidence of a hydrotherma l SiO2 source and sedimentary origin of the Kremikovtsi iron formation s. Berthierine and illite are diagenetic products; chamosite replaces berthierine or is metagenetic. The high-grade iron oxide formation con sists of tabular hematite crystals with well-developed pinacoid faces. The barite is represented by lenses, nests and subvertical veins. Str ata-bound disseminated sulfide mineralization accompanied by barite is hosted in different iron facies and carbonate rocks. The close positi ve and negative chemical correlations established five defined geochem ical associations: carbonate, silicate, sulfide, sulfate and oxide. A combination of geochemical criteria has allowed proposing a hydrotherm al origin of the ore-forming matter. The high Mg and Br contents of si derite iron formation and dolomitic limestone imply evaporate depositi on. Primary zoning of the Kremikovtsi ore deposit extends upward and n orthwestward: pyrite mineralization --> siderite iron formation --> ba rite ore --> lead-bearing ferroan dolomite-ankerite rock --> hematite iron formation --> hematite-bearing Middle Triassic dolomitic limeston e. Geochemical zoning similar to the ore zoning has also been establis hed: Fe, Fe+Mn --> Ba --> Pb, Pb+Cu --> Fe and a decrease of the Cu/(C u+Pb+Zn) and Pb/Zn ratios. Close genetic and temporal-spatial relation s of iron formations and barite-sulfide ore are proposed on the basis of the following arguments: identical lead isotope compositions of gal enas from the lead-bearing orebodies, barite veins and disseminated su lfides of the siderite iron formation, absence of metal dispersion and extension of barite veins into the overlying host rocks; spatial coin cidence, similar mineralogy and geochemistry of the two ore types in t he main orebodies and underlying rocks. On the basis of the available data, a new sedimentary exhalative model of ore formation is proposed for Kremikovtsi, which is a zonal carbonate-hosted deposit of SEDEX-ty pe consisting of stratiform pyrite and barite ore (intermediate facies ), and MECS-IF-type iron (+/-Mn) formations with low-grade sulfide min eralization (distal facies), as well as respective stockwork and vein iron carbonate-barite-sulfide occurrences in the underlying rocks. The deposit was produced by Middle Triassic metallogenesis located in the marginal parts of a second-order graben-shaped structure adjacent to the West Balkan Early Paleozoic accreted block. The ore-forming proces ses took place in an incipient rift-related extension against the back ground of passive margin regional setting.