Originally published as: Steding, S., Zirkler, A., Kühn, M. (2020): Geochemical reaction models quantify the composition of transition zones between brine occurrence and unaffected salt rock. - Chemical Geology, 532. DOI: http://doi.org/10.1016/j.chemgeo.2019.119349 Chemical Geology 532 (2020) 119349 Contents lists available at ScienceDirect Chemical Geology journal homepage: www.elsevier.com/locate/chemgeo Geochemical reaction models quantify the composition of transition zones T between brine occurrence and unaffected salt rock Svenja Stedinga,b,*, Axel Zirklerc, Michael Kühna,b a GFZ German Research Centre for Geosciences, Fluid Systems Modelling, Potsdam, Germany b University of Potsdam, Institute for Geosciences, Potsdam, Germany c K+S Aktiengesellschaft, Kassel, Germany ARTICLE INFO ABSTRACT Editor: Karen Johannesson Brine occurrences belong to the most significant risks in salt mining as they can lead to mine flooding andland Keywords: subsidence. Especially within highly soluble potash seams, which contain some of the economically most in- Potash salt teresting types of salt, the interactions between brine and salt rock result in cavernous structures surrounded by Water rock interaction moisture penetration zones with different mineralogical regions. In order to facilitate an early detection of Thermodynamic equilibrium cavernous structures, the brine and rock composition along these transition zones was modelled. The results PHREEQC show that the potash salt composition, or more precisely the ratio between kieserite and sylvite, determines the Mine field data dissolution behaviour and, therefore, which types of secondary minerals occur. According to the volume balance, Cavernous structure cavitation is only possible close to the centre of a cavernous structure, where the solid-fluid-ratio is low. Furthermore, an open system with frequent brine exchange is required. Along the transition zone, the solid-fluid- ratio can increase to several tens of kilograms before the water is fully consumed and the unaffected salt rock is reached. A comparison with measured data from a natural brine occurrence validates the model results and confirms a correlation between the brine composition and the distance from the centre of a cavernous structure. In conclusion, the models are suitable to determine the location of the central part of a cavernous structure based on rock and brine samples from the vicinity. However, in order to simulate the temporal and spatial development of geogenic cavities, a coupling of chemistry and hydraulics will be necessary. 1. Introduction are transferred to the mine system, whereby a natural brine occurrence in a potash mine serves as case study. Extensive investigation measures In Germany, salt deposits play an important role as industrial raw during the last decades provide comprehensive information on the material which is won by mining. One of the most significant risks in geological extent and the chemical composition of the brine occurrence mining is the access of groundwater due to geological fault zones which and the surrounding transition zone. The aim of the modelling is to may lead to the formation of moisture penetration zones and local quantify the dissolution processes within potash seams in order to fa- cavities within the salt body. Stability and tightness of the salt rock are cilitate an early detection of brine occurrences and minimise risks for significantly reduced in these areas and encountering them, in theworst mining industry. case, can lead to the flooding of a mine (Baumert, 1953; Wittrup and Kyser, 1990; Warren, 2016). Among the different salt layers, potash 2. Modelling approaches seams are particularly endangered as they have a significantly higher solubility than rock salt (Gimm and Meyer, 1968). Their dissolution Potash salts not only contain the main components Na+ und Cl− but 2+ + 2− 2+ behaviour is very complex and has only been investigated for a few also Mg ,K , SO4 , Ca and crystal water. Upon contact with individual cases (Herbert, 2000; Bach, 2010; Bohn, 2014). Therefore, groundwater all components enter into the solution and a hexary there is little academic literature on the composition of transition zones system forms. This system was modelled by using the program code between a brine occurrence and the unaffected salt rock. PHREEQC (Parkhurst and Appelo, 2013). The dataset ‘THERE- In this paper, geochemical modelling is the basis to systematically DA_PIT_PHRC_r01’ (Altmaier et al., 2011) serves as database that, investigate the dissolution behaviour of various potash salts. The results among other things, takes temperature dependency into account. Due ⁎ Corresponding author at: GFZ German Research Centre for Geosciences, Fluid Systems Modelling, Potsdam, Germany. E-mail address: [email protected] (S. Steding). https://doi.org/10.1016/j.chemgeo.2019.119349 Received 14 May 2019; Received in revised form 18 October 2019; Accepted 23 October 2019 Available online 24 October 2019 0009-2541/ © 2019 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/). S. Steding, et al. Chemical Geology 532 (2020) 119349 Table 1 Invariant points and their adjacent stability spaces in the hexary system Na-K- Mg-Cl-SO4-Ca-H2O at 22 °C. Point Minerals the solution is in equilibrium with (besides halite) M glaserite, schoenite, sylvite, polyhalite N leonite, schoenite, sylvite, polyhalite P kainite, leonite, sylvite, polyhalite Q carnallite, kainite, sylvite, polyhalite R carnallite, kainite, kieserite, polyhalite T glaserite, schoenite, bloedite, polyhalite U leonite, schoenite, bloedite, polyhalite V epsomite, leonite, bloedite, polyhalite W epsomite, kainite, leonite, polyhalite X hexahydrite, kainite, epsomite, polyhalite Y hexahydrite, kainite, kieserite, polyhalite Table 2 Salt minerals taken into account. Mineral Molecular formula Mineral Molecular formula Anhydrite CaSO4 Kainite K4Mg4Cl4[SO4]4 * 11 H2O Fig. 1. Jänecke projection for an NaCl saturated, quinary system Na-K-Mg-Cl- Bloedite Na2Mg[SO4]2 * 4 H2O Kieserite MgSO4 *H2O SO4-H2O at 22 °C; at points S, T, U … the solution is in equilibrium with all Carnallite KMgCl3 * 6 H2O Langbeinite K2Mg2[SO4]3 minerals of the adjacent stability fields. Epsomite MgSO4 * 7 H2O Leonite K2Mg[SO4]2 * 4 H2O Glaserite K6Na2[SO4]4 Polyhalite K2MgCa2[SO4]4 * 2 H2O Glauberite Na2Ca[SO4]2 Schoenite K2Mg[SO4]2 * 6 H2O to the high salinity also the Pitzer approach (Pitzer, 1973) is used. Gypsum CaSO4 * 2 H2O Sylvite KCl It can be assumed that within a cavernous structure in salt rock the Halite NaCl Syngenite K2Ca[SO4]2 *H2O system is always saturated with NaCl (Herbert and Schwandt, 2007; Hexahydrite MgSO4 * 6 H2O Thenardite Na2[SO4] Stadler et al., 2012). Depending on the relationship between the other ion concentrations, the solution is in thermodynamic equilibrium with NaCl. At every step, 1 mol of potash salt is added to the solution. different minerals. This can be seen, for example at the Jänecke pro- Assuming that thermodynamic equilibrium is archieved, the pro- jection (Jänecke, 1912) for a Ca-free, NaCl-saturated system (Fig. 1). gressive evolution of the solution composition can be quantified. The Points such as P, Q or R denote concentration ratios where the solution temperature is set at 22 °C which is in accordance with the natural is in equilibrium with three other minerals besides halite. The location conditions in the mine in the area of brine occurrence. The aim is to of these points is temperature-dependent. In addition, it should be determine which precipitates are formed upon the contact of potash salt noted that in this representation not the absolute concentrations are and NaCl solution, which amount of rock is affected and under which taken into account but only their relationship to each other. Thus, a conditions the dissolution process stops. The composition of the solu- solution may have the same concentration ratio as, for example, at tion and the surrounding rock are assumed to be spatially homo- point P, but still be undersaturated with all adjacent mineral phases. geneous. Therefore, this titration model applies only to closed systems Therefore, it is important to always consider the absolute concentra- where a limited amount of NaCl solution is in contact with potash salt tions and saturations of the solution as well. Nevertheless, the Jänecke over a longer period of time. projection is useful to graphically show the reaction path of a solution. However, especially in the vicinity of geological fault zones, open When considering a hexary system, the solution composition does systems with regular brine exchange may occur. This is particularly the not move across the surface shown in Fig. 1 but through a tetrahedon case in layers located close to the edges of salt deposits. The 1D model whose fourth vertex is formed by the calcium concentration (Herbert used in this study shows which interactions occur if a NaCl saturated and Schwandt, 2007). The stability fields become stability spaces and solution flows through a potash seam. In particular, the changing so- additional minerals such as anhydrite, polyhalite or syngenite occur lution composition as well as precipitations along the flow path are within the tetrahedron. The starting point of a reaction path always considered. The 1D model consists of several successively arranged cells corresponds to the salt point of the dissolved rock which results from which initially contain the same amount of NaCl solution (Fig. 2). Based the ratio of the ion concentrations in it. For example, the salt point of on the titration model, 1 mol of potash salt is added to each cell and the kieserite (MgSO4 *H2O) is located at the left border of the Jänecke dissolutions and precipitations are calculated assuming thermodynamic projection midway between Mg and SO4 vertex (Fig. 1). For kieseritic equilibrium. Subsequently, the solution is moved one cell to the right, potash salt, the salt point is usually located within the stability field of while from the left a 6-molar NaCl solution flows into the first cell. Solid glaserite or more specifically, within the stability space behind it.