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Ljubija siderite-barite-polysulfide ore field, NW Bosnia: Stable isotope and REE evidence for hydrothermal-metasomatic origin and Permian time of formation

The Ljubija ore field, NW Bosnia, is a siderite-polysulfide-barite type of mineralization, hosted by Middle/Upper Carboniferous carbonates and shales. It is placed at the margin of the Mesozoic carbonate platform. The recent geochemical data suggest hydrothermal-metasomatic origin and Permian time of formation during the stage of Early intracontinental rifting. The following types of siderite ore are distinguished: 1) Black, microcrystalline siderite, containing organic matter ; 2) "Zebra" siderite with cavities infilled with quartz and sulphides and 3) Veins containing siderite, quartz and sulphides. The carbon and oxygen isotopic composition of mineralized and barren carbonates were determined in order to constrain the source and nature of Fe-rich mineralizing fluid. Limestones vary in interval from -0.52 to 3.77o/oo 13C (VPDB) and 20.00 to 27.28o/oo 18O (VSMOW), ankerite from -0.90 to 0.34o/oo 13C and 20.65 to 22.37o/oo 18O and siderite from -2.25 to 0.03o/oo 13C and 20.67 to 22.78o/oo 18C, respectively. The average 13C value of the limestones corresponds well to the world-wide Phanerozoic marine carbonates values. The limestones within contact with Fe-mineralization are slightly depleted in 13C. Based on 13C values of ankerite and siderite, -0.05 +/- 0.27o/oo and -1.84 +/- 0.22o/oo, respectively, can be concluded that Fe-rich fluid reacted with the limestone, replacing it with ankerite. Further supply of Fe-rich fluid replaced ankerite with siderite, and even caused recrystallization within siderite (yellowish coarse-grain siderite veins within dark fine-grain siderite matrix, 13C -2.06 +/- 0.14o/oo). The 34S values of barite samples (+9.2 +/- 0.2o/oo V-CDT) fall within the range of Permian marine evaporites. The 34S values of the ore sulfides increase in the order galena (-2.3 to +3.0o/oo) < chalcopyrite (-0.8 to +2.3o/oo) < sphalerite (+0.4 to +4.4o/oo) < pyrite (+5.4 to +8.5o/oo), reflecting isotopic equilibrium and high-temperature formation. Isotopic fractionation temperature of sphalerite-galena pairs are around 240°C. All samples analyzed for REE show a negative Ce anomaly, typical for marine carbonates, and the negative Eu anomalies in limestone represent the typical Eu depletion of seawater. The positive Eu anomalies in ankerite and siderite suggest that Eu and Fe were metasomatically added to a sedimentary precursor.

Ljubija; siderite; stable isotopes; REE; Early intracontinental rifting

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