engleski

Rude, Samobor, siderite-hematite, -polysulfide-barite mineral deposit within the Zagorje-Mid-Transdanubian Zone, NW Croatia

Introduction Permo-Triassic siderite-barite-polysulfide (SBD) mineral deposits placed at the margin of the Mesozoic carbonate platform and along Inner Dinarides demarcate opening of the Tethys. There is a long queue of deposits from NW Monte Negro, Central Bosnia, Ljubija, Trgovska gora and Petrova gora following the general SE-NW Dinaridic lineaments. The deposits of similar genetic style, Rude-Samobor, Bistra-Medvednica, Rudabanya (Hungary) accommodate along the Zagorje-Mid-Transdanubian zone (ZMT), a northwestern boundary of the Internal Dinarides, stretching SW-NE, and terminate within Gemerides with Rudnyani, Rožnava, and other deposits in Slovakia. Beside geological and paragenetic similarity, there is an ever increasing record on fluid inclusion P,T,X characteristics and isotope data which unambiguously confirm their unique geotectonic and genetic heritage. One of the striking fact is a uniform δ34S value of paragenetic barite within all of the deposits, around 10 0/00 , suggesting participation of Permian sea water in the hydrothermal ore-forming process, plain or modified by evaporation and halite fractionation or Permian evaporates themselves. Results Rude-Samobor ore deposit is situated 25 km, west of Zagreb, in Samoborsko gorje Mts., within MZT zone. This is a small-scale siderite-hematite, submarine exhalative-sedimentary type (SEDEX) hosted within fine to medium grained clastic sediments of Upper Paleozoic age. The hanging wall of the sedimentary ore is a manganiferous gypsum-anhydrite layer, at places rich in siliceous component, with δ34S values between 9 and 12 0/00. Mesothermal quartz-siderite veins with chalcopyrite, sphalerite, galena, and accessory tetrahedrite, and epithermal barite-galena veins sideways, clearly epigenetic in character, are developed underneath the SEDEX type deposit, as a feeder zone (Šinkovec, 1971) . Sulfphur isotopes. According to the δ34S values of gypsum and anhydrite, they are a product of the Permian sea water evaporation. The range of values between 3 to 7 0/00 shows influence of ascending hydrothermal fluids, and higher values close to 13.50/00, deposition of gypsum within partly restricted sedimentary pool. Vein galena and coexisting barite with 3.0 and 11.6 0/00 values respectively, were not precipitated under equilibrium conditions, but show influence of Permian sea water within hydrothermal system convincingly, as do in the other SBP deposits (Šiftar, 1989). The chemistry of fluids in fluid inclusions (FI) was deteremined by a modified Bottrel et al. (1988) technique, mainly in siderite and quartz. The results of ionic chromatography on microquantity of Na-Cl-Br in leaches are presented as the molar ratio of Na/Br and Cl/Br. The molar compositions of the fluids plot well along the "evaporation trend" as the fluids from the other SBP deposits of Dinarides and Eastern Alps (Erzberg, Austria, Prochaska,1997). It confirms supposition of engagement of the Permian sea water, modified by evaporation and fractionation of halite in the ore forming process. It has also important inference on the age of the mineralization. Microthermometric measurements were performed on quartz wafers from quartz-siderite veins, the only available measurable material from the deposits at present. There were recognized three fluid inclusion (FI) populations: 1. Primary, L+V, highly irregular in shape but with uniform degree of fill (F=0.9 to 0.98). Tfm=-65 to -45 0C, indicates presence of CaCl2 - NaCl - H2O fluids, Tmhyd=-23.5 0C, Tmice=-15.5 to -4.4 0C, salinity 6.7-18.5 wt.% NaCl equ., Homogenisation L+V---L, Th=78 to 286 0C. 2.Rare V+L, primary FI, Tfm= -49 to -44.5 0C, Tmice=-15 0C, salinity around 18 wt.% NaCl equ., Th=352 0C, V+L--V. 3.Secondary, L, Tfm=-65 to 55 0C, indicates CaCl2 - NaCl - H2O system, Tmice=-14.0 to 4.8 0C. Wide span of salinity between 6.7 to 18 wt.%NaCl equ., and Th between 78 to 352 0C, and presence of CaCl2 in sodium chloride-aqueous fluids, is a phenomenon already observed on FI-s from Ljubija, Trgovska gora and Petrova gora (Palinkaš, 1990). It suggests mixing of surface sea water, modified by evaporation and highly evolved hydrothermal fluids, affected probably even by boiling (low density fluids). It should be mentioned, however, that fluids from hydrothermal quartz within pillow lavas, present above the deposit within Triassic beds, contain fluids with very similar chemistry. Pb-isotopes in galena using Doe-Stacey, Stacey-Kramer and Doe-Zartman models of lead growth curve give Permo-Triassic age (Palinkaš, 1985). Source of metals including lead were reworked Paleozoic orogenic sediments, leached by hydrothermal evaporitic brines modified by evaporation and halite fractionation and deep hydrothermal circulation, what can be recognized from the appropriate μ value on the growth curves. Conclusion. Rude-Samobor, ore deposit is a SEDEX type with a well developed feeder zone underneath. It is formed from hydrothermal water formed by mixing of the Permian sea water, modified by evaporation and halite fractionation within restricted pool, and highly evolved NaCl-CaCl2-H2O hot fluids, with high salinity, probably affected by subsurface boiling. The deposit bears all characteristics already observed at the other SBP deposits from Inner Dinarides and Eastern Alps, confirming unique geotectonic affiliation of Zagorje-Mid-Transdanubian zone to the Inner Dinarides, as stated by Pamić (1998). The hydrothermal activity commenced with incipient magmatism caused by early continental rifting of Tethys.

siderite-hematite-polysulfide ore deposit; fluid inclusion chemistry; Pb- isotopes; S-isotopes; early continental rifting; Zagorje-Mid-Transdanubian zone; Inner Dinarides

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