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Oxygen and carbon isotopic study on the genesis of the Ljubija ore field

The Ljubija Fe carbonates-sulfide-barite ore field (44.55° N, 16.3° E) is located at the margin of Mesozoic carbonate platform, within Sana-Una river Paleozoic terrain, Inner Dinarides, 180 km NW from Sarajevo, Bosnia and Herzegovina. Fe carbonates occur as stratabound siderite and ankerite replacement in limestones and siderite-sulphide veins placed within shales. Mineralization is hosted by Middle Carboniferous sedimentary formation represented mainly by clastics (subgraywackes, graywackes, sandstones and organic-rich shales) and carbonate blocks placed within shales. Barite veins with fluorite are occurred within upper parts of ore deposit (Palinkaš, 1988). The main types of iron ore are (1) microcrystalline dark siderite and ankerite ; (2) sparry siderite veins within shales, contain sulfides and quartz ; (3) "zebra" siderite contains cavities infilled with sparry ankerite, quartz, sulfides and alteration phyllosilicates as tossudite and cookeite. Carbon and oxygen isotopic composition of mineralized and barren carbonates were determined in order to constrain the origin and nature of Fe-mineralization. The data have been obtained on the 54 carbonates from 32 hand samples. Based on textural and geochemical characteristics several generations of carbonates can be distinguished. The average d13C value of organic rich fine-grained limestone non-affected by mineralizing fluid of +1.6 +/- 0.4 ‰ V-PDB corresponds well to the world-wide Phanerozoic marine carbonates values. Affected limestones are slightly depleted on 13C. Light calcite veins which cut dark limestones affected by mineralizing fluid show δ 13C value of 0.2 +/- 0.16‰ . Microcrystalline dark ankerite with the average d13C and d18O values of -0.1 +/- 0.3 ‰ and +19.3 +/- 0.2 ‰ , respectively, has been identified to belong to the first Fe-carbonate generation. Microcrystalline dark siderite and dark siderite from zebra ore have the same average d13C. The next generation of Fe-carbonates is presented by light siderite from zebra ore with average d13C and d18O values of -2.3 +/- 0.8 ‰ and +19.7 +/- 0.6 ‰ , respectively. Ankerite sample isolated from zebra ore shows slightly depletion on 13C with average d13C value of -2.4 ‰ and d18O value of +15.9 ‰ . Sparry vein siderite is distinguished as the last generation of Fe-carbonates with average d13C and d18O values of -2.5 +/- 1.1 ‰ and +20.4 +/- 2.0 ‰ , respectively. Variations in d13C values of the host limestones and Fe carbonates point to progressive metasomatic replacement of calcite by siderite and can not be explain as a product of the synsedimentary event. The replacement of limestones by Fe-carbonates is a volume-for-volume process and result of two separate but coeval reactions - dissolution of the host mineral and precipitation of the replacing mineral (Garrels and Dreyer, 1952). Zebra ore was formed at the place where hot, acid fluid affected limestones. Volumetric rate of calcite dissolution was higher then volumetric rate of siderite precipitation. Dark fine grained (metasomatic) siderite was precipitated when fluid of lower temperature and acidity reacted with the host limestones. Volumetric rate of calcite dissolution was equal to volumetric rate of siderite precipitation i.e. primary texture was retained. Bicarbonate ions have been leached from limestones in reaction with mineralizing fluids. Fluids enriched on HCO3- deposited vein siderite within shales as the latter generation of Fe carbonates. Crystallization temperature of dark siderite was calculated from oxygen isotope fractionation between siderite and recrystallized calcite. Calcite-water and siderite-water equations by Zhang (1999) gave an apparent isotope equilibration temperature of 224°C. Fluid in isotopic equilibrium with both siderite and recrystallized calcite at the calculated temperature should have a d18O of 11.5‰ V-SMOW.

siderite; ankerite; replacement; Dinarides; stable isotopes; REE

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