Invasion of a karst aquifer by hydrothermal fluids: evidence from stable isotopic compositions of cave mineralization

Abstract

Mineral deposits in the Cupp‐Coutunn/Promeszutochnaya cave system (Turkmenia, central Asia) record a phase of hydrothermal activity within a pre‐existing karstic groundwater conduit system. Hydrothermal fluids entered the caves through fault zones and deposited sulphate, sulphide and carbonate minerals under phreatic conditions. Locally, intense alteration of limestone wall rocks also occurred at this stage. Elsewhere in the region, similar faults contain economic quantities of galena and elemental sulphur mineralization. Comparisons between the Pb and S isotope compositions of minerals found in cave and ore deposits confirm the link between economic mineralization and hydrothermal activity at Cupp‐Coutunn. The predominance of sulphate mineralization in Cupp‐Coutunn implies that the fluids were more oxidized in the higher permeability zone associated with the karst aquifer. A slight increase in the δ34S of sulphate minerals and a corresponding δ34S decrease in sulphides suggest that partial isotopic equilibration occurred during oxidation. Carbonate minerals indicate that the hydrothermal fluid was enriched in 18O (δ18OSMOW ∼ + 10‰) relative to meteoric groundwater and seawater. Estimated values for δ13CDIC (δ13CPDB ∼ − 13‰) are consistent with compositions expected for dissolved inorganic carbon (DIC) derived from the products of thermal decomposition of organic matter and dissolution of marine carbonate. Values derived for δ13CDIC and δ18Owater indicate that the hydrothermal fluid was of basinal brine origin, generated by extensive water–rock interaction. Following the hydrothermal phase, speleothemic minerals were precipitated under vadose conditions. Speleothemic sulphates show a bimodal sulphur isotope distribution. One group has compositions similar to the hydrothermal sulphates, whilst the second group is characterized by higher δ34S values. This latter group may either record the effects of microbial sulphate reduction, or reflect the introduction of sulphate‐rich groundwater generated by the dissolution of overlying evaporites. Oxygen isotope compositions show that calcite speleothems were precipitated from nonthermal groundwater of meteoric origin. Carbonate speleothems are relatively enriched in 13C compared to most cave deposits, but can be explained by normal speleothem‐forming processes under thin, arid‐zone soils dominated by C4 vegetation. However, the presence of sulphate speleothems, with isotopic compositions indicative of the oxidation of hydrothermal sulphide, implies that CO2 derived by reaction of limestone with sulphuric acid (‘condensation corrosion’) contributed to the formation of 13C‐enriched speleothem deposits.

Identifying low-temperature hydrothermal karst and palaeowaters using stable isotopes: a case study from an alpine cave, Entrische Kirche, Austria

“…The purpose of this paper is to demonstrate that stable isotope data are a valuable geochemical tool to identify and quantify palaeowater-rock interactions in low-temperature hydrothermal karst systems (e.g. Bakalowicz et al 1987;Bottrell et al 2001;Plan et al 2006).…”

Petrography and geochemistry of the magnesites and dolostones of the Ediacaran Ibor Group (635 to 542 Ma), Western Spain: Evidences of their hydrothermal origin

“…It could also be modified in response to water-rock interactions or by contri butions of oxidized carbon with meteoric waters (Souza et al, 1995). The magnesites of the Ibor Group also show isotope values similar to those found in rocks which have been altered by hydrothermal wa ters such as those found in mineralizations of cave deposits formed by hydrothermal fluid flows (Bottrell et al, 2001). The studied rocks have undergone several diagenetic events and, thus, isotope values may not correspond to the original isotope values, but rather to those resulting from interactions with later post-depositional fluids.…”

Sulfate and Phosphate Speleothems at Jenolan Caves, New South Wales, Australia

“…3) and will be controlled by local influences such as the original d 34 S of fluids, water-rock interactions and potential mixing processes. Bottrell et al, (2001) found d 34 S ranges of +9.8 to +13.3‰ in gypsum floor crusts from Cupp-Coutun/Promeszutochnaya cave system. While these values are similar to those identified in this work, the geological setting is totally different with no upwelling of basinal brines identified at Jenolan.…”

“…3). These results contrast with more depleted isotopic signatures of bat guano from Cambodia with average d 34 S of +6.8‰ (Hosono et al 2006 The influence of hydrothermal fluids or sulfidic groundwater in gypsum precipitating karst systems has also been described (Bottrell et al, 2001;Galdenzi andMaruoka, 2003, Onac et al, 2009). However, the potential range of sulphate-d…”