High‐density nitrogen inclusions in barite from a giant siderite vein: implications for Alpine evolution of the Variscan basement of Western Carpathians, Slovakia

Abstract

Contrasting compositions and densities of fluid inclusions were revealed in siderite-barite intergrowths of the Drozˇdiak polymetallic vein hosted in Variscan basement of the Gemeric unit (Central European Carpathians). Primary two-phase aqueous inclusions in siderite homogenized between 101 and 165°C, total salinity ranged between 18 and 27 wt%, and CaCl 2 ⁄ (NaCl + CaCl 2 ) weight ratios were fixed at 0.1-0.3. By contrast, mono-and two-phase aqueous inclusions in barite exhibited total salinities between 2 and 22 wt%, and the CaCl 2 ⁄ NaCl ratios ranged from NaCl-to CaCl 2 -dominated compositions. The aqueous inclusions in barite were closely associated with very high-density (0.55-0.745 g cm )3 ) nitrogen inclusions, in some cases containing up to 16 mol.% CO 2 . Crystallization P-T conditions of siderite (175-210°C, 1.2-1.7 kbar) constrained by the vertical oxygen isotope gradient along the studied vein, isochores of fluid inclusions and the K ⁄ Na exchange thermometer corresponded to minimal palaeodepths between 4.3 and 6.3 km, assuming lithostatic load and average crust density of 2.75 g cm )3 . Maximum fluid pressure during barite crystallization attained 3.6-4.4 kbar at 200-300°C, and the most dense nitrogen inclusions maintained without decrepitation the residual internal pressure of 2.2 kbar at 25°C. Contrasting fluid compositions, increasing depths of burial ($4-14 km) and decreasing thermal gradients ($40-15°C km )1 ) during initial mineralization stages of the Drozˇdiak vein reflect Alpine orogenic processes, rather than an incipient Permian rifting suggested in previous metallogenetic models. Siderite crystallized at rising P-T in a closed, rock-buffered hydrothermal system developed in the Variscan basement during the north-vergent Cretaceous thrusting and thickening of the Gemeric crustal wedge. Variable salinities of the barite-hosted inclusions reflect a fluid mixing in open hydrothermal system, and re-equilibration textures (lengths of decrepitation cracks proportional to fluid inclusion sizes) correspond to retrograde crystallization trajectory coincidental with transpression or unroofing. Maximum recorded fluid pressures indicate $12-km-thick pile of imbricated nappe units accumulated over the Gemeric basement during the Cretaceous collision.

U-Pb-Th geochronology of monazite and zircon in albitite metasomatites of the Rožňava-Nadabula ore field (Western Carpathians, Slovakia): implications for the origin of hydrothermal polymetallic siderite veins

“…The foliationparallel orientation of veins was interpreted as reflecting an arcuate deformation front during the Early-Cretaceous thrusting (Lexa et al 2003;Hurai et al 2008a). This model is supported by superdense CO 2 inclusions in quartz-stibnite veins (Urban et al 2006) and very high-density N 2 inclusions in barite from siderite veins (Hurai et al 2008b), which indicated a 14-15 km burial under imbricate stack of nappes thrusted over the Gemeric basement (Hurai et al 2010).…”

Modification of fluid inclusions in quartz by deviatoric stress I: experimentally induced changes in inclusion shapes and microstructures

“…Experimental studies over recent years have advanced understanding of the microstructural changes induced by variations in hydrostatic confining pressure (Gratier and Jenatton 1984;Pêcher and Boullier 1984;Boullier et al 1989;Sterner and Bodnar 1989;Vityk and Bodnar 1995;Vasyukova and Fonarev 2006). Recognition of the characteristic microstructures in natural samples has permitted reconstruction of the history of pressure evolution in the host rocks (Vityk et al 1994(Vityk et al , 1996Ayllon et al 2003;Hurai et al 2008;Mukherjee and Sachan 2009;Stöckhert et al 2009). …”

Mobilization of ore fluids during Alpine metamorphism: evidence from hydrothermal veins in the Variscan basement of Western Carpathians, Slovakia

“…All the siderite‐hosted inclusions contain aqueous liquid with a 5–15 vol.% of vapour phase (Varček 1968; Borisenko et al. 1984; Hurai et al. 1998, 2002, 2008).…”

“…Hurai et al. (2002, 2008) argued that high fluid pressures indicated by oxygen isotopes and fluid inclusions in siderite and barite from the Droždiak vein in the northern part of the Gemeric unit were much greater than a lithostatic load of Permo‐Triassic autochthonous sedimentary cover.…”