Stable isotope and fluid-inclusion evidence of hydrothermal speleogenesis (Kryštálová cave, Western Carpathians)

The stable isotopic composition of wall rock and cave minerals is a sensitive tool to recognize the hypogene component of speleogenesis (Spötl et al., 2021; Temovski et al., 2022), to elucidate the cave evolution (Dublyansky, 1995, 2013), and to characterize the paleo-fluids (Dublyansky & Spötl, 2009). Here, we report evidence of water-rock interaction associated with hydrothermal speleogenesis at a site that shows the prevalence of brittle deformation in cave formation.

Kryštálová cave located in the Krivánska Malá Fatra (Western Carpathians) is a 57 m-long cave hosted in Triassic limestones, with walls covered by phreatic calcite spar. We conducted laser scanning of the cave and the immediate surroundings, as well as small-scale geological mapping. High-resolution stable isotope profiling was applied to the calcite spar and two wall rock cores overgrown by calcite. Calcite crystals were also studied by fluid inclusion microthermometry.

Field evidence, cave mapping, and laser scanning showed that the cave follows an extension fracture which begins and terminates in larger rooms. Several small-diameter, short, blind branches are present in the far end of the cave. In the cave and the surrounding area, we observed jointing, conjugate fracture sets, as well as some fracture planes displaying linear features, which we interpret as slickensides. Solutional forms suggest hypogene dissolution; there are no signs of epigene karst overprint. Part of the cave is filled with brown detrital sediment, presumably an insoluble bedrock residue, largely removed during exploration of the cave. The sediment (in places) and the cave walls (entirely) are lined by a layer of sparry calcite. The wall rock immediately underneath the crystals and along the fractures is stained brown.

Temperatures of calcite-depositing waters (based on fluid inclusion homogenization temperatures) were 53 ± 5°C (n=129). Assuming equilibrium precipitation and combined with the calcite δ¹⁸O values of -14.2 to -16.5 ‰ (VPDB) suggests paleo-water δ¹⁸O values of -8.4 to -10.9 ‰ (VSMOW), similar to modern meteoric water in the area (Holko, 2012). Unaltered limestone bedrock shows δ¹⁸O values of -3.4 to -6.5 ‰ (VPDB) and δ¹³C values of 2.7 to 2.1 ‰ (VPDB). Isotope wall rock alteration was detected in both cores (C1, C2), showing a strong depletion in δ¹⁸O near the cave wall (-7.6 to -14 ‰, and -7.3 to -13.2 ‰ for C1 and C2, respectively). Depletion in δ¹³C was minor: 1.8 to -0.8 ‰ (C1), and 0.1 to 2.1 ‰ (C2). Covariation between δ¹⁸O and δ¹³C in the cores is significant (R² = 0.68). 

Kryštálová cave formed as an extensional fissure, subsequently enlarged by hypogene dissolution. Heated meteoric groundwater interacted with the wall rock in the cave and along fractures, producing an isotopic alteration “halo” and eventually precipitated calcite crystals.

“Crystal caves” such as Kryštálová cave are widespread in the central Western Carpathians, sharing the same host rock lithology and similarities in their cave deposits (detrital sediments and calcite crystal linings). Calcite spar can – in principle – be radiometrically dated and represents a largely untapped archive of the local and regional paleohydrogelogy and related processes of cave formation.