Origin of siderite veins in the Western Carpathians: I. P– T– X– δ13C– δ18O relations in ore-forming brines of the Rudňany deposits

Abstract

Siderite–barite veins of the Rudňany ore field have precipitated from NaCl–CaCl 2–H 2O±CO 2 solutions with CaCl 2 weight fractions between 0.05 and 0.38, total salinities between 17 and 35 wt.%, and up to 1.8 mol% CO 2. Homogeneous and heterogeneous trapping modes have been distinguished according to volumetric phase ratios of the fluid inclusions, which have been trapped at 175–205 °C in veiny siderite, at 200–230 °C in siderite and quartz from drusy cavities, and at 90–180 °C in the siderite–postdating quartz–sulfidic assemblage. A sequential re-equilibration technique has been developed to locate precisely vapour-saturated halite liquidus of individual brine inclusions, which normally homogenise by halite dissolution. Slopes of the vapour-unsaturated halite liquidi constrained by isochores of coexisting two-phase aqueous inclusions have been approximately three to five times steeper in natural polycomponent brines than those in the experimental NaCl–H 2O system. Fluid pressures between 1 and 2 kbar have been determined using this method in drusy quartz, associating with siderite crystals. Oxygen isotope variations (16.8–19.9‰ V-SMOW) in siderite result from a temperature increase from ∼175 to ∼230 °C, whereas nearly constant δ 13C values (−3.9‰ to −4.6‰ V-PDB) indicate absence of CO 2 devolatilisation. Carbon isotope composition of the parental fluid changed from −9.5‰ to −8.8‰ V-PDB, while the oxygen isotope ratios remained essentially fixed at 7.7±0.3‰ V-SMOV during formation of siderite. The calculated δ 18O fluid value is attributed to formation water enriched in 18O during isotopic exchange with crustal rocks at low fluid/rock ratios. The δ 13C fluid values probably correspond to a mixture of CO 2 from dissolved matrix carbonates and that liberated by thermal decarboxylation of organic acids (acetates), which normally occur in formation waters. Leachate chemistry data have corroborated high concentrations of CaCl 2 and MgCl 2 in the ore-forming brines. High Br concentrations are similar to halite-fractionated basinal brines. Ca 2+ and K + concentrations in excess of those in normal basinal brines reflect extensive cation exchange with surrounding low- to medium-grade metamorphites, coincidental with calcite and K-feldspar dissolution, dolomitisation, and albitisation of plagioclases. High pressures of the siderite-forming fluids (1–2 kbar) must be linked with a deep circulation (∼4–8 km) of the basinal brines derived primarily from marine water, but significantly modified by interaction with low- to medium-grade Paleozoic rocks at low fluid-to-rock ratios. An anti-clockwise crystallisation PT path inferred from fluid inclusion and stable isotope data is inconsistent with the siderite precipitation during extensional tectonic regime coincidental with Permian–Triassic rifting suggested in previous models. More likely, the progressively increasing fluid pressures and temperatures could be attributed to compression and crustal thickening triggered by Jurassic subduction in the Meliatic–Hallstatt oceanic suture. The superimposed quartz–ankerite–sulfidic ores have likely crystallised from brines expelled during compression associated with a continental collision during Early to Middle Cretaceous times.