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Ore forming fluids in the Trgovska gora and Petrova gora deposits, Croatia

The Banovina-Kordun (Petrova Gora) and Sana-Una (Trgovska Gora) Palaeozoic units represent a northwestern part of the Inner Dinarides, related to Apulian and African passive continental margin. They comprise Upper Paleozoic (Carboniferous– Lower Permian) synorogenic flysch sequences, composed of dark-gray silty shales, siltites interlaying with fine- to coarse-grained sandstones, primarily greywackes, interbedded with subordinate conglomerates. Mineralisation in Petrova and Trgovska gora Paleozoic units can be subdivided into two major paragenetic types: 1) Mesothermal siderite-quartz-polysulfide veins and metasomatic ankerites and iron-rich limestones and 2) Epithermal barite veins. 1) Petrova gora mesothermal siderite– polysulfide-quartz veins were precipitated from dominantly NaCl– CaCl2– H2O± CO2 solutions with total salinities between 0.35 and 26 wt% NaCl equ. Heterogeneous trapping with Th between 100-200 °C is recognized by different degree of filling. 2) Trgovska gora mesothermal siderite– polysulfide-quartz veins were formed from fluids containing dominantly NaCl-H2O, with salinity between 1– 13 wt% NaCl equ. and homogenization temperatures Th, at 90– 250 °C (Palinkaš, 1988 ; Jurković and Palinkaš, 2002). Fluid inclusions leachate chemistry confirmed presence of Na+, Ca2+, Mg2+, K+ and Cl-, which is in agreement with observed low eutectic temperatures ( Te, -51, and -33 °C). Observed ratios of Ca2+ and K+ follow the evolution trend of dolomitisation and calcium dissolution and do not keep on the trend of the basinal brines. It reflects extensive cation exchange with surrounding Upper Paleozoic sediments. Increased Br- concentrations found in fluid inclusions follow successively seawater evaporation line, suggesting evolved Permian sea water. The majority of  34S values in barite from both regions lies between +9 and +11‰ , supporting involvement of the Permian seawater in the ore forming processes, as well. Sulfides (galena, pyrite, chalcopyrite) yield lower values, between -3.2 and +2.70/00, caused by variable degree of the thermal reduction of the seawater in the closed system. Formation temperature calculated by the pyrite-chalcopyrite pair from Trgovska gora region gives the value of 324°C. The other calculations of sulfide geothermometres (gn-py ; gn-cpy ; py-cpy) show formation under disequilibrium conditions. Ore-forming fluids were within the pressures range between 1– 2 kbar, which is related to a deep circulation of the basinal brines derived primarily by Permian seawater, modified by evaporation and interaction with the Upper Paleozoic sedimentary sequence. Ar/Ar measurements were performed on a concentrates of white mica from the ore-bearing breccias taken for dating of mineralisation age in both ore deposits. The results show preferentially Variscan ages of the country rocks at 332.8– 353.8 ± 4.2 Ma and younger ages between 298.0– 265.6 Ma, which is considered to be the age of hydrothermal mineralisation (Borojević-Šoštarić et al., 2007). The deposits belong to the Early intracontinental rifting in the Tethyan domain.

fluid inclusions; sulfur isotopes ar age dating; siderite deposits; Dinarides; Trgovska and Petrova gora

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