The metamorphic evolution of rocks cropping out near Stoer, within the Assynt terrane of the central region of the mainland Lewisian complex of NW Scotland, is investigated using phase equilibria modelling in the NCKFMASHTO and MnNCKFMASHTO model systems. The focus is on the Cnoc an t’Sidhean suite, garnet‐bearing biotite‐rich rocks (brown gneiss) with rare layers of white mica gneiss, which have been interpreted as sedimentary in origin. The results show that these rocks are polymetamorphic and experienced granulite facies peak metamorphism (Badcallian) followed by retrograde fluid‐driven metamorphism (Inverian) under amphibolite facies conditions. The brown gneisses are inferred to have contained an essentially anhydrous granulite facies peak metamorphic assemblage of garnet, quartz, plagioclase and ilmenite (±rutile, K‐feldspar and pyroxene) with biotite, hornblende, muscovite, chlorite and/or epidote as hydrous retrograde minerals. P–T constraints imposed by phase equilibria modelling imply conditions of 13–16 kbar at >900 °C for the Badcallian granulite facies metamorphic peak, consistent with the field evidence for partial melting in most lithologies. The white mica gneiss comprises a muscovite‐dominated matrix containing porphyroblasts of staurolite, corundum, kyanite and rare garnet. Previous studies have suggested that staurolite, corundum, kyanite and muscovite all grew at the granulite facies peak, with partial melting and melt loss producing a highly aluminous residue. However, at the inferred peak P–T conditions, staurolite and muscovite are not predicted to be stable, suggesting they are retrograde phases that grew during amphibolite facies retrograde metamorphism. The large proportion of mica suggests extensive H2O‐rich fluid‐influx, consistent with the retrograde growth of hornblende, biotite, epidote and chlorite in the brown gneisses. P–T conditions of 5.0–6.5 kbar at 520–550 °C are derived for the Inverian event. In situ dating of zircon from samples of the white mica gneiss yield apparent ages that are difficult to interpret. However, the data are permissive of granulite facies (Badcallian) metamorphism having occurred at c. 2.7–2.8 Ga with subsequent fluid driven (Inverian) retrogression at c. 2.5–2.6 Ga, consistent with previous interpretations.

Leaching zones within potash seams generally represent a significant risk to subsurface mining operations and the construction of technical caverns in salt rocks, but their temporal and spatial formation has been investigated only rudimentarily to date. To the knowledge of the authors, current reactive transport simulation implementations are not capable to address hydraulic-chemical interactions within potash salt. For this reason, a reactive transport model has been developed and complemented by an innovative approach to calculate the interchange of minerals and solution at the water-rock interface. Using this model, a scenario analysis was carried out based on a carnallite-bearing potash seam. The results show that the evolution of leaching zones depends on the mineral composition and dissolution rate of the original salt rock, and that the formation can be classified by the dimensionless parameters of Péclet (Pe) and Damköhler (Da). For Pe > 2 and Da > 1, a funnel-shaped leaching zone is formed, otherwise the dissolution front is planar. Additionally, Da > 1 results in the formation of a sylvinitic zone and a flow barrier. Most scenarios represent hybrid forms of these cases. The simulated shapes and mineralogies are confirmed by literature data and can be used to assess the hazard potential.

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Abstract. Storage caverns are increasingly located in heterogeneous salt deposits and filled with various fluids. The knowledge of phase behaviour in heterogeneous systems of salt, liquid and gas and the requirements for reliable analytical techniques is, therefore, of growing interest. A method that allows for the continuous monitoring of mineral compositions at distinct humidity and gas content using XRD measurements is presented here. Various saliniferous mineral compositions have been investigated in pure CO2, N2 or CH4 atmospheres with varying humidity in a closed chamber. All mineral compositions experience dissolution and/or mineral conversion reaction accompanied by volume loss. Dissolution-recrystallization reactions of complex mineral assemblages involving halite, sylvite, kieserite, carnallite and kainite were observed using this method. For carnallite-rich mineral assemblages, the mineral conversion from carnallite to sylvite was observed when humidity exceeded 50 % RH. In the presence of CO2, acidification of the aqueous phase occurs which enhances the dissolution rate and reaction kinetics.

Abstract. In order to better understand both the fixation and migration of gases in evaporites, investigations were performed in five horizontal boreholes drilled in an underground potash seam. One of the five boreholes was pressurised with Ar and the pressure signal and chemical gas composition were then monitored in the other holes. A further gas sample from a separate borehole was characterised for the chemical composition and for noble gas and carbon isotopic compositions to conclude on the origin and evolution of the gas in the salt rocks. Additionally, in order to determine the total gas amount in the salt rocks, a potash-bearing salt sample was dissolved in water and from the mass of 1 kg salt sample, 9 cm(STP)3 gas was liberated. Due to the relatively large permeability of the disturbed salt rocks (4×10-17 to 4×10-18 m2), which is about 3–4 orders of magnitude higher than in undisturbed salt rocks, we assume that the migration of injected Ar most likely takes place along micro-cracks produced during the mining process. The geogenic gas concentrations found in the observation holes correlate directly to the Ar concentration, suggesting that they were stripped from the rocks in between the holes. According to the He-isotopes (0.092 Ra), a small contribution of mantle gas can be found in the geogenic salt gas. The δ13CCO2-isotopic composition (−7.8 ‰ to 6.7 ‰) indicates a magmatic source, whereas 13C∕12C of CH4 (−22.2 ‰ to −21.3 ‰) is typical for a thermogenic gas. We assume that CO2 and CH4 are related to volcanic activity, where they isotopically equilibrated at temperatures of 513 to 519 ∘C about 15–16 Ma ago.

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<p>Salt deposits host an important industrial raw material and provide storage capacities for energy and nuclear waste. However, leaching zones can seriously endanger the development and utilisation of salt deposits for these purposes, especially if these occur in potash seams. Their increased solubility enables even NaCl-saturated solutions, if present, to deeply penetrate these seams. The resulting salt dissolution processes generate fluid flow paths and affect the mechanical rock integrity. To model the timely evolution of leaching zones and to assess their hazard potential, a reactive transport model has been developed, taking into account not only the complex dissolution and precipitation behaviour of potash salts, but also the resulting porosity and permeability changes as well as density-driven chemical species transport. Additionally, the model makes use of an approach to describe transport and chemical reactions at the interface between impermeable (dry) salt rocks and permeated leaching zones (Steding et al., 2021). In the present study, we focus on the effect of heterogeneity of the mineral distribution within potash seams and on the influence of mineral- and saturation-dependent dissolution rates.</p><p>The applied reactive transport model is based on a coupling of the geochemical module PHREEQC (Parkhurst & Appelo, 2013) with the TRANSport Simulation Environment (Kempka, 2020) as well as the newly developed extension of an interchange approach (Steding et al., 2021). A numerical model has been developed and applied to simulate the leaching process of a carnallite-bearing potash seam due to natural density-driven convection. The results show that both, the mineral composition and dissolution rate of the original salt rock, strongly influence the shape and evolution of the leaching zone (Steding et al., 2021).</p><p>In nature, strong variations of the mineralogy occur within potash seams with random or stratified distributions. Furthermore, dissolution rates depend on the mineral itself as well as on its saturation state. Both may considerably influence the growth rate of a leaching zone. Therefore, the reactive transport model has been extended by mineral- and saturation-dependent dissolution rates. A scenario analysis has been undertaken to compare the impact of homogeneous and heterogeneous rock compositions. For that purpose, the carnallite content in the potash seam was varied from 5 to 25 wt. % including different stratifications and random distributions. The simulations were classified by means of the Péclet and Damköhler numbers, and the long-term behaviour as well as hazard potential are discussed.</p><p> </p><p>References:</p><p>Parkhurst, D.L.; Appelo, C.A.J. (2013). Description of Input and Examples for PHREEQC Version 3 - a Computer Program for Speciation, Batch-reaction, One-dimensional Transport, and Inverse Geochemical Calculations. In Techniques and Methods; Publisher: U.S. Geological Survey; Book 6, 497 pp</p><p>Kempka, T. (2020). Verification of a Python-based TRANsport Simulation Environment for density-driven fluid flow and coupled transport of heat and chemical species. Adv. Geosci. 54, 67–77. </p><p>Steding, S.; Kempka, T.; Zirkler, A.; Kühn, M. (2021). Spatial and temporal evolution of leaching zones within potash seams reproduced by reactive transport simulations. Water 13, 168. </p>

<p>Fluid inclusions are voids enclosed in the rock matrix and contain, depending on their origin and development, various amounts of gaseous, liquid or solid phases. Depending on their occurrence within the crystalline structure or in healed micro-fractures, primary and secondary inclusions can be distinguished. Their characteristics are utilized in various geological settings to reconstruct rock history and fluid involvement. Fluid inclusions could also be considered to be small equivalents to large cavities. As salt is regarded a favorable host rock for the storage of natural gas and other materials in artificial caverns, knowledge on gas migration and retention is crucial.</p><p>Here, we present results of a fluid inclusion study in various salt rocks using Raman spectroscopy in addition to conventional microscopic characterization and gas analysis on whole rock samples. This approach allows for a better understanding of fluid generation and migration in different salt lithologies over geological times.</p><p>Various salt minerals (halite, sylvite, kieserite and carnallite) from an area of potential overprint of CO<sub>2</sub>-dominated gas migration were investigated. Numerous fluid inclusions exhibit chevron structure and are small sized. Large single- or two-phased inclusions are observed with irregular shapes, often indicative for leakage or necking down. Interestingly, although the CO<sub>2</sub> concentrations in whole rock samples were high, fluid inclusions were dominated by an aqueous phase and often contain numerous daughter minerals. This suggests that CO<sub>2</sub>-rich gas is stored along distinct fractures or grain boundaries within an otherwise intact rock.</p>