Ore petrology, whole-rock chemistry and zoning of the Kremikovtsi carbonate-hosted sedimentary exhalative iron(+Mn)-barite-sulfide deposit, Western Balkan, Bulgaria
Damyanov, Z. K.
published: Jul 29, 1998
ArtNo. ESP154017401001, Price: 29.00 €
Siderite and hematite iron formations and barite as well as sulfide mineralizations from primary orebodies recently exposed after continuous mining of the Kremikovtsi deposit have been studied. They were produced by superimposed sedimentary and hydrothermal Middle Triassic ore-forming processes involving precipitation and diagenesis, fissure- and cavity-filling, and replacement. Hydrothermal sedimentary- diagenetic magnesian-manganoan siderites, accompanied by a ferroan dolomite-ankerite association, are fine-grained with spheroid relics of femicrite and predominantly massive, rarely fine-banded, texture. Epigenetic vein and replacement siderites are coarser grained, predominantly idiomorphic and zonal. Analysis of the chemical composition of siderites from more than 25 deposits showed that those connected with hydrothermal activity (in veins, replacement and SEDEX orebodies) have Mn/Mg > 0.5, in contrast to sedimentary ones (Mn/Mg Mn/Mg > 0.5 is distinctly outlined in the field of hydrothermal siderites, which is probably the result of metasomatic exchange of elements with the host rocks. Microquartz and relics of length-slow chalcedony cementing siderite grains are substantial evidence of a hydrothermal SiO2 source and sedimentary origin of the Kremikovtsi iron formations. Berthierine and illite are diagenetic products; chamosite replaces berthierine or is metagenetic. The high-grade iron oxide formation consists of tabular hematite crystals with well-developed pinacoid faces. The barite is represented by lenses, nests and subvertical veins. Strata-bound disseminated sulfide mineralization accompanied by barite is hosted in different iron facies and carbonate rocks. The close positive and negative chemical correlations established five defined geochemical associations: carbonate, silicate, sulfide, sulfate and oxide. A combination of geochemical criteria has allowed proposing a hydrothermal origin of the ore-forming matter. The high Mg and Br contents of siderite iron formation and dolomitic limestone imply evaporate deposition. Primary zoning of the Kremikovtsi ore deposit extends upward and northwestward: pyrite mineralization → siderite iron formation → barite ore → lead-bearing ferroan dolomite-ankerite rock → hematite iron formation → hematite-bearing Middle Triassic dolomitic limestone. Geochemical zoning similar to the ore zoning has also been established: Fe, Fe+Mn → Ba → Pb, Pb+Cu → Fe and a decrease of the Cu/(Cu+Pb+Zn) and Pb/Zn ratios. Close genetic and temporal-spatial relations of iron formations and barite-sulfide ore are proposed on the basis of the following arguments: identical lead isotope compositions of galenas from the lead-bearing orebodies, barite veins and disseminated sulfides of the siderite iron formation; absence of metal dispersion and extension of barite veins into the overlying host rocks; spatial coincidence, similar mineralogy and geochemistry of the two ore types in the 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-type consisting of stratiform pyrite and barite ore (intermediate facies), an. MECS-IF-type iron (±Mn) formations with low-grade sulfide mineralization (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 processes took place in an incipient rift-related extension against the background of passive margin regional setting.