Interaction Between Mafic Dike Rocks and Salt Deposits in the Rhine Graben, Southwest Germany

ABSTRACT Dikes of primitive olivine melilitites and monchiquites intruded into an Oligocene (Rupelian) potash salt deposit near Buggingen (SW Germany). Ocelli and amygdules reveal distinct mineral assemblages depending on whether the dike rocks are in direct contact with the potash layer or with bituminous shales (Fish Shale). Samples in contact with the potash salt layer show roundish textures that contain smectite ± talc ± chlorite, calcite, and in cases anhydrite and halite, while those close to the bituminous shale mainly comprise smectite, calcite, zeolite group minerals, and analcime. No textural or mineralogical evidence for high-temperature (magmatic) interaction between the dike rocks and the evaporites was observed. This is presumably related to (1) a very low magmatic water activity in the magma, which prevented exsolution of aqueous fluids and appreciable dissolution of the salt, and (2) fast cooling of the magmas, inhibiting melting of the salt deposits and potential liquid mingling and/or assimilation processes. Halite formation in the dike rocks is, rather, related to later, post-magmatic hydrothermal fluids that previously interacted with the salt-rich host rocks. Alteration of the initially glassy groundmass to smectites and zeolites caused an enrichment of Na in the residual fluid, but halite saturation was not attained, as indicated by the absence of groundmass halite. Only fluid–rock interaction in millimeter-sized vugs caused halite precipitation via desiccation by swelling of previously formed clay minerals. Locally, the boron silicate datolite formed in pseudomorphs after olivine. Its precipitation was controlled by the Si and B supply provided by the breakdown of serpentine and smectite.

Assessment of instability initiation in exposed salt rock roofs

Stability of rocks is the main requirement for the safe operation of mines. For this purpose, certain measures are applied for the protection and support of underground openings, including roof support design and roof arrangement in the most stable rocks. Stability assessment of underground excavations is largely related to their roof stability. Determination of possible instability conditions in mine roofs governs the choice of support system design and parameters of mine excavations. The Upper Kama Potash Salt Deposit represents a stratified layer of solid salt rocks. Roof instability develops as stratification and roof collapse. The Mohr–Coulomb criterion of coherent rocks is currently used to estimate parameters of a possible damage zone in the exposed roof. This criterion allows evaluating shear plane angles in roof rocks and, as a result, finding parameters of the possible collapse zone. The experience of mining operations in the Upper Kama deposit shows different failure conditions as against the Mohr–Coulomb criterion as the stress state is scarcely included in the criterion used. This study is an attempt to assess parameters of rock exposures by solving a Lame problem in terms of a single mine excavation. The analytical results were compared with the parameters obtained from the plane elasticity solutions by the finite element method. Based on the implemented studies, an engineering approach is developed for the assessment of anticipated instability parameters in exposed roofs in horizontal excavations driven in salt rocks.

Unit weight patterns of overlying rocks above commercial-value salt strata in the Upper Kama deposit

Stability of rocks is the main requirement for the safe operation of mines. For this purpose, certain measures are applied for the protection and support of underground openings, including roof support design and roof arrangement in the most stable rocks. Stability assessment of underground excavations is largely related to their roof stability. Determination of possible instability conditions in mine roofs governs the choice of support system design and parameters of mine excavations. The Upper Kama Potash Salt Deposit represents a stratified layer of solid salt rocks. Roof instability develops as stratification and roof collapse. The Mohr–Coulomb criterion of coherent rocks is currently used to estimate parameters of a possible damage zone in the exposed roof. This criterion allows evaluating shear plane angles in roof rocks and, as a result, finding parameters of the possible collapse zone. The experience of mining operations in the Upper Kama deposit shows different failure conditions as against the Mohr–Coulomb criterion as the stress state is scarcely included in the criterion used. This study is an attempt to assess parameters of rock exposures by solving a Lame problem in terms of a single mine excavation. The analytical results were compared with the parameters obtained from the plane elasticity solutions by the finite element method. Based on the implemented studies, an engineering approach is developed for the assessment of anticipated instability parameters in exposed roofs in horizontal excavations driven in salt rocks.

Improvement of operational exploration and mining safety using the shear-wave reflection method of mine seismology

During the whole life of mines, the geological survey departments are faced with the critical objectives of operational exploration. These objectives are sometimes impossible to be met adequately without geophysics. Neutrality of geophysical studies is often below the desired level. The shear-wave reflection method of mine seismology, with separation of reflections, which is developed by the present authors, allows: investigating the shape, thickness and dip angles of geologic beds and ore bodies, detecting geological discontinuities such as faults, joints and cavities, as well as determining physical and mechanical properties of rocks. The capacity of the shear-wave reflection method with separation of reflections is described in a case-study of the Upper Kama Potash Salt Deposit. The seismic tests were carried out in a few underground excavations. The developed method in the test conditions is accurate to the first meters at a spacing more than 300 m. Accordingly, it is possible to study the whole thickness of the salt strata in the Upper Kama Deposit, from the salt table to the anhydrous clay roof, specifically, to assess the structure and composition of the waterproof strata, to identify mine-threatening geological discontinuities, and to build the model of physical and mechanical properties of rock mass. Introduction of the newly developed method can greatly enhance efficiency of actual operational exploration. The method has yielded the best results during or immediately after heading of permanent and development openings. Considering similarity of surface and underground mineral mining in terms of access to solid rocks, it is expected to have the same outcomes in open pit mines.

Technology and specificity of surface seismic in the Upper Kama Potash Salt Deposit

Exploration surveys at the Upper Kama Potash Salt Deposit widely use the surface seismic method by the common reflection point at depth. Based on the implemented research, a technology is developed for shallow seismic using an explosion source of elastic vibrations for the purposes of geological exploration. The research involved the comparative analysis of the main elastic wave sources used in the shallow seismic. It is highlighted that it is important to consider carefully the near-surface section structure and the surface relief. The accuracy of the velocity analysis procedure in the high-velocity section of salt strata is analyzed. The specificity of acquisition in the shallow seismic with an explosion source is discussed. The actual test data show a considerable increment in the energy of reflections from the roof and floor of the salt strata, which, in the absence of a priori geological information and geophysical logging data (acoustic logging and vertical seismic profiling), affects the velocity analysis precision and, as a consequence, the accuracy of reflection identification at depth. It is found that the explosion source has a much higher signal/noise ratio as against a pulse cartridge, which greatly improves neutrality of interpretation results. The use of a pulse cartridge in the surveys in the depth interval of 200–400 m is only justified when the surface conditions are perfect and the low velocity layer is not thick.

Advanced Technology for Improving Degraded Slopes in Mountainous and Foothill Zones

The authors presented the technology and mechanization means for the restoration of degraded mountain areas with slope steepness up to 15 degrees.(Research purpose) To determine the optimal distance between the springy racks of the chisel cultivator KCHG-2.4; to characterize the developed block-module of the multifunctional unit KCHG-2.4, which performed three operations after the slitting process: sowing grasses, applying mineral fertilizers and rolling the sown seeds and granular fertilizers into the soil; to determine the effectiveness of the developed agricultural practices that increased productivity and the environment-forming role of mountain phytocenoses.(Materials and methods). The tests were carried out in the mountainous zone of North Ossetia - Alania at an altitude of 1540 meters above sea level with slope steepness up to 15 degrees. The experimental area was divided into three replicates. Each replicate was 2.4 meters wide, 10 meters long, dividing stripes 1 meter, side stripes 2.4 meters. The total test area, located across the slope, was 184.2 square meters.(Results and discussion) The springy Väderstad strut of the KCG-2.4 unit was used to slit the mountain slopes. For overseeding, a three-component grass mixture of Phleum pratense L., Dactylis glomerata L. and Trifolium pratense L. was proposed at a seeding rate of 15 kilograms per hectare. When applying mineral fertilizers, the following composition was used (in the active ingredient): ammonium nitrate – 35 percent; superphosphate – 20; potash salt – 40 percent, in total 445 kilograms per hectare. An increase in dry aboveground mass of 3.02-4.19 tons per hectare and the accumulation of underground plant mass in the range of 1.84-12.62 tons per hectare were got, depending on the variant of the experiment.(Conclusions) The authors found out that, a higher yield was obtained with a distance between the slits of 1.5 meters for 3 years of observations. It was found that the use of the unit increased the collection of feed units from 0.81-1.68 to 4.06-4.98 thousand per hectare. Taking into account all costs, the cost of one feed unit was 8.76 rubles.

Potash mining mountain waste and its contribution to river water salinisation: the case of the Llobregat River, Catalonia, Spain

<p>It is well known that mining activities have negative effects on fluvial ecosystems. Such activities alter the water quality by introducing heavy metals and associated pollutants and alter the sediment regime by creating a point source sediment that may affect the entire basin. </p><p>In the Llobregat River, a medium Mediterranean river basin (ca. 5000 km<sup>2</sup>), potash salt mining activities have been undertaken for several decades. Salinisation of surface river water has become an environmental issue of great concern for the water administrators given that the water of this river supplies half of the population of the metropolitan area of Barcelona (ca. 2,500,000 inhabitants) and it is also used for irrigation in the lowermost part of the river and its delta.</p><p>This study aims to describe the magnitude of the dissolved solids inputs that are detected in the river surface water after rainfall events by means of continuous electrical conductivity monitoring. Electrical conductivity records (EC) were obtained from an automatic water quality monitoring station set by the Water Catalan Authorities and located 3 km downstream from the potash mountain waste.  The study also tries to predict the EC peak according to different hydrometeorological parameters selected from the episodes recorded.</p><p>Data analysed was continuously recorded at 15-minute interval between January 1st, 2018 and September 30th, 2020 and a total of 74 EC episodes were considered. Mean EC of the episodes recorded was 3,488 µS/cm, with a standard deviation of 3,638 µS/cm, and a coefficient of variation of 104.3%. The median was 2,390 µS/cm. Data obtained show that after rainfall events a peak of electrical conductivity in the river is detected. However, it exhibits a high variability in its magnitude, ranging from 939 µS/cm up to 26,900 µS/cm. Despite this, the coefficients of determination of the regression lines between the meteorological variables, such as rainfall intensity or total rainfall amount, and the peak EC exhibit poor correlations (R<sup>2</sup>=0.355 and R<sup>2</sup>=0.229, respectively), although they are significant.</p><p>Results indicate that washload processes in the salt mountain waste take place and reach the river producing extremely high EC peak values during a short period of time. Such values can have harmful effects on the river ecosystem and affect the lowerland river area, where water is diverted for potabilization and irrigation purposes. However, the low correlation between rainfall and EC peak indicates that additional variables intervene in the rainfall-runoff processes and further research is required to fully understand the connectivity and transmission of the salt moutain waste into the river. Understanding such processes and analyasing the consequences on the fluvial system, will probably be the way to tackle the restoration of this enormous impact on this river ecosystem.</p>

Gas-dynamic roof fall during the potash deposits development

In the development of practically all potash salt deposits, the study of gas-dynamic phenomena (GDP) is one of the most difficult tasks to ensure mining safety. Sudden salt and gas outbursts, dynamic breakdown, which are accompanied by intense gas release and possible broken rock carry-over into the mine workings, are associated with GDP. Geological preconditions for the GDP development are often the layered structure of the salt rock mass, the presence of interlayers and layers of salt clays. For the conditions of the Usolsky potash plant mine, complex studies of factors that characterize the possibility of gas-dynamic roof fall of the stoping rooms were carried out. In mine studies, free gases pressure and the initial velocity of gas release in the rocks of the roof workings were determined. The obtained experimental estimations were used as a parametric basis for mathematical modeling of geomechanical processes under conditions of a near-contact accumulation of free gas. The deformation of a layered salt mass produced by a room development system was described by the model of an ideal elastic-plastic medium with internal friction. The parabolic envelope of Mohr circles was used as a plasticity criterion in the compression area. In the numerical implementation, the deformation of clay contacts was modeled by Goodman contact elements. Based on the results of multivariate numerical calculations, it is established that the main factors determining the possibility of implementing GDP are the additional gas pressure at the contact, the width of the workingspan, and the distance from the roof to the first gas-containing contact. With multi-level lamination of roof rocks, there is a danger of large sources of GDP formation and the mechanism of successive fall of layers in an instant mode is implemented.

Spatial and Temporal Evolution of Leaching Zones within Potash Seams Reproduced by Reactive Transport Simulations

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.