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Origin of siderite mineralisation in Petrova and Trgovska Gora Mts., NW Dinarides

Petrova and Trgovska Gora Mts. (Gora=Mountain) are Variscan basement units incorporated within Northwestern Dinarides during Alpine orogeny. They host numerous siderite– quartz– polysulfide, siderite– chalcopyrite, siderite– galena and barite veins, as well as stratabound hydrothermal– replacement ankerite bodies within carbonates in non– metamorphosed, flysch– like Permo– Carboniferous sequences. The deposits were mined for Cu, Pb, Ag and Fe ores since Medieval times. Fluid inclusion study on quartz from siderite– polysulfide– quartz and barite veins of both regions showed the presence of primary aqueous NaCl– CaCl2± MgCl2– H2O± CO2 inclusions. Quartz-sulfide stage of both regions show variable salinities 2.7– 26.2 wt% NaCl eq. for the Trgovska Gora region and 3.4– 23.4 wt% NaCl eq. for the Petrova gora region, and similar homogenization temperatures i (100– 230 °C). Finally, barite precipitated from low salinity– low temperature solutions (3.7– 15.8 wt % NaCl equ. and 115– 145 °C). P– t conditions estimated via isohore con-struction yield formation temperatures between 180– 250 °C for the quartz– sulfide stage and 160– 180 º C for the barite stage, using maximum lithostatic pressure of 1kbar (cc. 3 km of overburden). Sulfur isotope composition of barite from both deposits points to involvement of Permian seawater in ore fluids. This is supported by the elevated bromium content of the fluid inclusion leachates (120– 660 ppm in quartz, 420– 960 ppm in barite) in respect to the seawater, pointing to evaporated seawater as the major por-tion of the ore forming fluids. Variable sulfur isotope compositions of galena, pyrite, and chalcopyrite, be-tween – 3.2 and +2.7‰ , is interpreted as a product of incomplete thermal reduction of the Permian marine sulphate mixed with organically– and pyrite– bound sulfur from the host sedimentary rocks. Ore– forming fluids are interpreted as deep– circulating fluids derived primarily from evaporated Permian seawater and later modified by interaction with the Variscan basement rocks. 40Ar/39Ar data of the detrital mica from the host rocks yielded Variscan age overprinted by an Early Permian tectonothermal event dated at 266-274 Ma. These ages are interpreted as those reflecting hydrothermal activity correlated with an incipient intraconti-nental rifting in the Tethyan domain. Nevertheless, 75Ma recorded at fine-grained sericite sample from al-teration zone is interpreted as a result of later resetting of white mica during Campanian opening/clousure of the Sava back arc in the neighbouring Sava suture zone (Ustaszewski et al, 2008).

siderite deposit; hydrothermal replacement ores; Permian; Variscan basement; rift; extension; hydrothermal fluid

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