Host Rock Type Research Articles

Subsurface microbial community structure shifts along the geological features of the Central American Volcanic Arc.

Subduction of the Cocos and Nazca oceanic plates beneath the Caribbean plate drives the upward movement of deep fluids enriched in carbon, nitrogen, sulfur, and iron along the Central American Volcanic Arc (CAVA). These compounds fuel diverse subsurface microbial communities that in turn alter the distribution, redox state, and isotopic composition of these compounds. Microbial community structure and functions vary according to deep fluid delivery across the arc, but less is known about how microbial communities differ along the axis of a convergent margin as geological features (e.g., extent of volcanism and subduction geometry) shift. Here, we investigate changes in bacterial 16S rRNA gene amplicons and geochemical analysis of deeply-sourced seeps along the southern CAVA, where subduction of the Cocos Ridge alters the geological setting. We find shifts in community composition along the convergent margin, with communities in similar geological settings clustering together independently of the proximity of sample sites. Microbial community composition correlates with geological variables such as host rock type, maturity of hydrothermal fluid and slab depth along different segments of the CAVA. This reveals tight coupling between deep Earth processes and subsurface microbial activity, controlling community distribution, structure and composition along a convergent margin.

The Geological Characteristics, Resource Potential, and Development Status of Manganese Deposits in Africa

Manganese is a critical metal extensively used in steel production, battery manufacturing, and the chemical industry. Manganese deposits are widely distributed globally, with African manganese deposits holding a particularly significant position. This study provides a comprehensive overview of the current status of manganese resources in Africa and analyzes the main types and genesis of the deposits. It also explores the significance of African manganese resources within the global manganese industry and examines the exploration, development, production, and trade status of African manganese ore, revealing dynamic changes and trends within the industry. Africa plays a key role in global manganese resources, with its reserves and resources accounting for 69% and 51% of the global totals, respectively. South Africa is the largest manganese producer, while Gabon and Ghana are also important countries in terms of manganese resources. Based on the host rock and genetic types, African manganese deposits primarily include banded iron formation (BIF)-hosted, black shale-hosted, karst-hosted, and volcanic sedimentary deposits. Additionally, manganese exploration projects in Africa are active, with significant production growth in South Africa and Gabon, and Africa’s standing in the global manganese market is steadily increasing. Trade data also indicate that South Africa dominates manganese exports in Africa and globally. Despite fluctuations in global export volumes, African manganese exports remain high. The prospects for Africa’s manganese industry are promising, and further strengthening of manganese exploration and development will help enhance Africa’s influence in the global manganese market, making a more significant contribution to global industrial and economic development.

The goodness-of-fit of models for fracture trace length distribution: Does the power law provide a good fit?

Fracture trace length distributions are often assumed to follow a power law, which implies that the distribution is scale-independent. The present study tests this assumption by evaluating the goodness-of-fit of three statistical models—the power law, piecewise power law, and lognormal distribution—upon a dataset of 57 trace maps that cover a range of fracture modes, host rock types, network scales, and topologies. The goodness-of-fit was assessed through the unbiased Kolmogorov-Smirnov (KS) test, which accounts for the fitting procedure and the degrees of freedom of each model. The results show that the power law provides a poor fit to trace length distributions, being rejected in 24 trace maps at a significance level of 0.05. In contrast, the piecewise power law and lognormal distribution demonstrated better fits across the fracture networks, with the piecewise power law performing the best overall. The poor fit of the power law can be attributed to mechanical and chemical controls on fracture growth, mainly fracture abutment, as well as stress shadowing and cementation, which affect growth rate at different length scales and result in scale-dependent trace length distributions. The consistent poor fit of the power law across various fracture networks suggests that these controls are prevalent in natural systems. While the power law remains a simple and effective model for trace length distribution, it should be recognized that it overlooks such controls that can influence the mechanical and hydraulic properties of fracture networks. Meanwhile, the fit of the piecewise power-law suggests the existence of a characteristic length where a transition in fracture growth occurs.

Rare-metal halos of lithium pegmatite at the Wolfsberg deposit, Austria, and their implications for exploration

The Austroalpine Unit Pegmatite Province (AUPP) in the eastern European Alps hosts abundant Permian-aged, variably rare-metal-enriched pegmatite bodies. However, only a small number containing spodumene [LiAlSi2O6] have been discovered. Here, we characterize three drill cores from the AUPP’s largest spodumene pegmatite resource, the Wolfsberg deposit (Austria), to determine the extent of rare-metal halo formation within different host rock types generated by pegmatite-derived fluids and their applicability to spodumene pegmatite exploration. The halos in both mica schist and amphibolite host rocks, as quantified via whole-rock mass-balance calculations, are characterized by enrichments in Li, Cs, Sn, Rb and Tl, amongst others, elements which were highly mobile during initial interactions between pegmatite-derived fluids and host rocks. The concentrations of these elements within the halos can be several times that present in unaltered host rock. They are most enriched in altered host rock interbedded or at the contacts with pegmatite, and decrease with distance from pegmatite to a distance of at least 4 m in mica schist and <3 m in amphibolite. Lithium, and in mica schist, Cs, have the most consistent and furthest extending halos, with respective concentrations of >590 ppm and >40 ppm in mica schist, and >390 ppm Li in amphibolite indicating proximity to spodumene pegmatite. An ID-TIMS U-Pb titanite age of ca. 103 Ma derived from a rare-metal-enriched amphibolite within the alteration halo reflects recrystallization of the Permian protolith during the Cretaceous Alpine orogeny and indicates preservation of the original pegmatite signature through eclogite-facies metamorphism. Recognition of rare-metal halos surrounding pegmatite at Wolfsberg serves as a potential geochemical exploration tool for spodumene pegmatite within the AUPP and may be applicable to other regions.

Lawsonite and Garnet Oxygen Isotope Record of Fluid‐Rock Interaction During Subduction

AbstractDuring the subduction of an oceanic plate, fluids are released from metabasaltic crust, metasediment, and serpentinite under high‐pressure/low‐temperature conditions. Although some fluids may eventually leave the slab, some participate in metamorphic reactions within the slab during subduction and exhumation. To identify fluid sources and other controls influencing mineral composition, we report the in situ‐measured δ18O of lawsonite and garnet in blueschist‐ to eclogite‐facies rocks from 10 subduction zones that represent various field settings, including mélanges, structurally coherent terranes, and an eclogite xenolith derived from a subducted plate. Lawsonite records distinct δ18O depending on the host rock type and other rock types that were fluid sources during lawsonite growth. In general, lawsonite in metabasalt (7.6 ± 0.2–14.8 ± 1.1‰) is isotopically lighter than in metasediment (20.6 ± 1.4–24.1 ± 1.3‰) but heavier than in metagabbro (4.0 ± 0.4–7.9 ± 0.3‰). The extent of δ18O fractionation was evaluated for lawsonite–fluid and lawsonite–garnet pairs as a function of temperature (T). Results demonstrate that variations of &gt;1.7‰ in lawsonite and &gt;0.9‰ in garnet are not related to changing T. More likely, the relative contributions of fluids derived from isotopically heavier lithologies (e.g., sediments) versus lighter lithologies (e.g., ultramafic rocks) are the major control. Monte Carlo simulations were performed to investigate the sources of metasomatic fluids and the water/rock ratio that formed lawsonite‐bearing metasomatite. Results indicate that δ18OLws and δ18OGrt record interactions with fluids sourced from diverse lithologies (sediment, serpentinite), further supporting that δ18OLws is a useful indicator of subduction fluid‐rock interactions.

Modelling geomechanical stability of a large deep borehole in shale for radioactive waste disposal

Large diameter boreholes drilled to a depth of 1000–2000 m are currently investigated in Australia as a potential solution for waste emplacement and long-term containment for long-lived intermediate-level radioactive waste (ILW). Different potentially suitable types of host rocks are considered, including granite, shale and salt. This study is aimed at assessing the risk of borehole instability for a large diameter deep borehole in shale. The investigation is conducted with numerical simulations using the CSIRO-developed modelling tools FRACOD and FRCAOD3D, and the commercial code Irazu. Previous laboratory experiments on boreholes in deep shale reported in literatures have been used as validation tests of the numerical models. Under hydrostatic stresses, the FRACOD model showed extensive shear-dominated “log-spiral” fractures around a borehole in shale which is similar to the laboratory testings reported in literatures. Subsequently, 2D and 3D numerical models representing a 0.7-m diameter vertical borehole drilled to the depths of 1000 m and 2000 m have been used. “Strong” and “weak” shales are simulated. The strong and weak shales represent the higher and lower ends of the shale strength reported in literature, respectively. Results showed that a “strong” shale exhibited only small borehole breakouts at the depth of 2000 m. The “weak” shale showed large breakouts (>1.5 m) at depths of 1000 m and 2000 m caused by extensive shear fracturing. In the scenario where a pre-existing 3D fracture crosscuts the borehole, the existing fracture may propagate deeper into the rock formation depending on its orientation and on the in situ stress magnitudes. Predictions based on analytical solutions indicate a 0.02-m and 0.04-m thick casing could prevent severe failure of rock mass at 1000 and 2000 m depths, respectively. Insights from model-based investigations of borehole breakouts are critical during the site selection of rocks with suitable geomechanical properties for waste disposal and for optimising the design of large-diameter boreholes for drilling, waste emplacement and long-term safety.

General Properties of Commingled Materials for Mix Designs

Tailings volumes continue to collectively increase worldwide, leading to larger dams and tailings management facilities. With numerous high-profile dam failures in the past decade, the risks of these management practices are also growing. A potential shift to waste management practices at mineral mines is to commingle waste rock and dewatered tailings. This blended material should have superior physical strength properties provided by the waste rock together with improved chemical stability characteristics associated with the low permeability of the tailings. Ideally, commingled tailings and waste rock can be used to construct various mined earth landforms that are both physically and chemically stable, which will enhance operational performance and ultimately provide for the sustainable decommissioning and closure of the mining facility. To study these materials, the University of Alberta Geotechnical Centre is working with global industry partners to test commingled materials from several mine sites with varying ore and host rock types and climate regimes. The first stage of this study is described here and is focused on the optimum density, saturated hydraulic conductivity, and soil–water characteristic curves of various blend ratios, performed at laboratory scale.

Geochemical features of host rocks of the Porokhovskoe tungsten deposit (South Urals) and their role in concentration of ore matter

The article contains the results of statistical processing of geochemical data for rocks of the Porokhovskoe tungsten deposit (South Urals) obtained during systematic geological exploration. Statistical samplings were formed for various types of host rocks: andesites and their tuffs, shales, metasomatites (including sericite-quartz, epidote and chlorite-bearing) and skarns. It is established that all rocks of the Porokhovskoe deposit are characterized by increased contents of W and Mo and, to a lesser extent, Be, Cu, Li, and Zn and by stable groups of correlated elements, both associated with greisenization process and those inherited from the&#x0D; protolith. No zoning is revealed in the distribution of elements in relation to the Yugo-Konevsky leucogranitic pluton, which is considered the ore-generating for the deposit. No correlation between W and Mo was identifed in all types of host rocks. A number of statistical methods is proposed for the identifcation of promising areas with unclear W-Mo mineralization in rocks.

Graphite Cast Iron GGG40 for Disposal Containers: The Corrosion Mechanism in Opalinus Clay Water

The overpack of the Pollux® 10 container, taken as a reference for the storage of fuel elements in different types of host rocks, is manufactured from spheroidal graphite carbon steel (GGG 40 / 0.7040) [1]. The near field environment after emplacement is constituted by bentonite, which turns moisty, warm, and anaerobic after some decades because of the decay of the heat generation and the consumption of oxygen by reaction with the container wall, microbes, and minerals [2]. Thus, the corrosion mechanism of metal containers in these conditions is an essential issue for the design of repository concepts of radioactive waste.For a better understanding of the corrosion processes taking place at the interface of steel and saturated bentonite, electrochemical and surface chemical analysis, including SEM-EDX and local XPS were performed on steel in Opalinus Clay water under different temperatures, pH´s and pressures. Electrochemical measurements were carried out in a dedicated reactor cell for working at high temperature and high pressures (up to 150 bars). A Solartron electrochemical interface 1287 connected to a frequency analyzer 1260 was used.Polarization curves performed in deaerated solutions are characterized by a low cathodic constant current separating the onset of water reduction and the active dissolution. The presence of oxygen increases the current of the cathodic plateau shifting the corrosion potential towards more positive values. The metallographic structure of the steel give rise to local predominantly anodic and cathodic elements, i.e., ferritic, and pearlitic phases, on the one side and cementite and graphite on the other side, respectively.The dissolution of the active phases is controlled by the formation of an oxide film mainly constituted by magnetite and silicates containing Fe, Al, Mg and Cr formed by oxidation of the silicon contained in the alloy. SEM pictures obtained after 48 h of corrosion under anaerobic conditions and 5 bar of pressure show the deposition of silicates with different morphologies: as clusters of spheres and as hexagonal platellets (see fig.1). Under 100 bar of pressure, however, an rather etched aspect can be observed, revealing clearly the different metallographic phases.The impedance diagrams obtained at the corrosion potential shows an inductive loop at low frequency. Based on the shape of potentiostatic anodic current transients, the inductive loops can be ascribed to the dynamic of formation-dissolution of the interfacial oxide-silicate film. The effect of hydrostatic pressure is particularly interesting. The shiny optical aspect of the surface after corrosion under pressure can a priori be explained by the reducing atmosphere created by water reduction under these conditions.The significance of the formation of silicates is evident in the surface analysis of samples after long-term experiments in contact with Wyoming bentonite saturated with pore Opalinus Clay water. TEM pictures of a cross sectional lamella including part of graphite sphere shows the formation of a silicate film covering the corroding surface with an irregular adherence. The accumulation of silicates at the surface creates an additional barrier for the metal dissolution, which reduces the corrosion rate by a factor of thousand.

Radiometric Signatures of Gold Mineralization Zone in Pongkor, West Java, Indonesia: A Baseline for Radiometric Mapping Application on Low-Sulfidation Epithermal Deposit

Radiometric mapping could play a prominent role in locating the host rock or alteration that leads to gold mineralization. Nevertheless, in low-sulfidation epithermal gold deposits, the radiometric signatures have to be priorly characterized due to their geometry. It is comprised of a small ore vein system within the large alteration zones. The Pongkor gold mine is a low-sulfidation epithermal deposit and was selected for this purpose. The method started with the surface identification of radiometric signatures on altered and unaltered rocks near Pongkor using portable spectrometers. They are followed by the characterization of the underground mining front, which is comprised of different types of veins and host rocks. The results show that the altered rocks were characterized by a high K% and a low eTh/K ratio. Vice versa, the mineralized veins show low radioelement concentrations. Following the characterization of the geometry of alteration zones and mineralized veins, a study of the relationship between radioelements detected by radiometric mapping and gold pathfinder elements was conducted. Gold pathfinders of Mn, Fe, Zn, As, and Pb were selected for correlation studies with the radioelement. The pathfinders and radioelements were more significantly correlated in veins compared to the host rock. Based on this study, radiometric mapping has the potential and benefit of being applied in the exploration of low-sulfidation epithermal gold deposits. An alteration zone could be delineated by K or eTh/K as an anomaly indicator, and the vein bodies could also be delineated using low K or eTh as an anomaly indicator.

Corrosion processes at the GGG40 steel–bentonite interface

Abstract. Spheroidal graphite cast iron (GGG40/0.7040) is used as an overpack material for the reference Pollux 10 container, a prototype for the storage of fuel elements in different types of host rocks (Hassel et al., 2019). Corrosion processes are expected to be triggered after some decades, as the contacting bentonite, which is used as fill material, becomes soggy because of the decay of the fission heat generation and the intrusion of pore water as the temperature decreases under the condensation point (King and Padovani, 2011). The distinct electrochemical reactivities of the graphite, ferrite, pearlite, and cementite phases exposed by this material introduce local galvanic elements which influence the topographic evolution of degradation (Spence, 2005). The aim of this work is to elucidate the corrosion dynamics of active and passive areas by following the chemical evolution at the interface of cast iron–bentonite during the first stages after its saturation with geological pore water. Polarization experiments and electrochemical impedance spectroscopy were applied to monitor the corrosion process in a bentonite cell, where GGG40 steel is put into contact with a light compacted Wyoming bentonite slurry 1 : 10 w/w of bentonite to Opalinus Clay pore water (Fig. 1a). Experiments were performed at 30 and 50 ∘C for longer than 3 months. The surface chemistry and morphological changes were investigated by local XPS, SEM-EDX, and TEM (X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray, and transmission electron microscopy, respectively). These experiments were complemented with corrosion studies performed in pore water under different temperatures, hydrostatic pressures, and pH and with and without dissolved oxygen. The polarization curves indicate a constant corrosion rate of GGG40 steel in saturated bentonite after 30 d. SEM micrographs reveal a preferential dissolution of the ferrite phase around the graphite spheres and the ferrite lamellae contained in the pearlitic eutectic. A strong localized dissolution of ferrite along the graphite boundary can be also observed; this is a typical case of crevice corrosion caused by the unhindered access of oxygen to the graphitic cathodic areas (Wang et al., 2022). The surface chemical studies indicate the accumulation of iron oxides, which can be attributed to a hydrated magnetite and the formation of iron silicates (Zhang et al., 2021). TEM pictures of a cross-sectional lamella, including part of graphite sphere, show the formation of a silicate film covering the corroding surface with an irregular adherence (Fig. 1b). The initial relatively high corrosion rate of GGG40 steel can be attributed to the dissolution of the more active ferrite with the formation of poorly passivating iron oxides and silicates. The system is driven towards the dissolution of pearlite at more positive electrode potentials. The bentonite slurry limits the access of oxygen to the graphitic cathodic areas, reducing the corrosion rate by 1 order of magnitude in comparison with that in aerated pore water. A surface enrichment of cementite with superior passive properties and the neutralization of the local elements by approaching the corrosion potential of graphite (Kadowaki et al., 2019) is also expected. Thus, the consumption of oxygen and the transport limitation of the cathodic reaction by bentonite forecast a considerable reduction in the degradation of the container after a sacrificial corrosion phase.

Time to change the geoscientific perspective!?

Abstract. The Repository Site Selection Act (StandAG) explicitly emphasises that targeting the disposal of high-level radioactive waste is a so-called learning process. For example, progress in scientific knowledge, in the methodological approach or in new data should be accounted for. We believe that now is a good time to reflect on what we have learned so far and to put the methodological approach to the test. Therefore, we need to question the perspective of the problem: is the bottom-up strategy (data-based), which reduces the remaining search area more and more, objectively feasible enough, or is there a complementary methodological approach? We are of the opinion that the procedure and the available data should be combined with geoscientific knowledge in a top-down (knowledge-based) manner to support the identification of siting regions. The current bottom-up strategy of continually narrowing down the areas can be purposeful, but it postpones a fundamental problem to a later point in time: how do we compare host rock types? So far, this question has only been addressed qualitatively (BGR, 2007). If this approach were to be developed intellectually and quantitatively, then the following fundamental question could be addressed: does one type of host rock in Germany generally always perform better or worse than another? If these questions could be answered on the basis of data and knowledge and if a clear result were to emerge, then numerous sub-areas could possibly be excluded at this stage. In order to pursue such a complementary approach, the following points need to be examined: a. quantitative ranking of properties for different host rock types is required for assessments;b. geoscientific knowledge to be used, not only pure data, but also basic findings and expert knowledge;c. synthetic, idealised sites must be defined with the best possible parameters according to the state of science. We propose this workshop based on the questions outlined above and invite all experts who have dealt with the search for a repository site from a geoscientific perspective. The aim of the workshop is to start the exchange of previous experiences and what has been learned, to discuss alternative or complementary approaches scientifically, and thus to further develop the investigation of the remaining sub-areas.

Basis for a research project on marking strategies for a deep geological high-level radioactive waste (HLW) repository in Germany

Abstract. Marking has been considered as an approach for the preservation of information and awareness after the closure of a deep geological repository (DGR) for high-level radioactive waste (HLW). The discussion to use markers to warn people off from – in oversimplified terms – digging into a DGR dates back to the 1980s. Based on the recent recommendations by the Nuclear Energy Agency of the Organization for Economic Co-operation and Development (OECD NEA) regarding the preservation of records, knowledge and memory (RK&amp;amp;M) of DGRs in a systemic approach (NEA, 2019), the Federal Office for the Safety of Nuclear Waste Management (BASE) has identified the need for research regarding relevant aspects of potential future marking strategies in Germany. The first elaborate and sophisticated concept has been developed for the Waste Isolation Pilot Plant (WIPP) in the USA (DOE, 2004). Far from being “one marker”, it is a monumental multi-component system. Internationally, the WIPP concept provided abundant food for thought and inspired a new wave of discussions – not as a design to be copied to other DGRs (the WIPP concept has been developed for this specific DGR at its unique desert location), but as a model for site-specific ambitions to design a marker system that aims in earnest to preserve information and awareness on-site by non-mediated means. In NEA (2019) marking is described as one approach – out of nine – for the preservation of RK&amp;amp;M of DGRs. Marking was found to be focused, in particular, on preserving awareness. However, for a robust system, individual preservation mechanisms should not be planned in isolation, but rather “the components of a systemic RK&amp;amp;M preservation strategy should work together, complement and strengthen each other, act as indexes to each other and thus provide diversity and redundancy with a view to maximising information accessibility, understandability and survivability over the various timescales considered” (NEA, 2019, p. 95). In Germany, there are no regulatory requirements with regards to marking. The Site Selection Act (StandAG, 2017/2023), which sets the regulatory framework for the targeted DGR for HLW, does not mention marking. It mentions, however, two points with specific relevance to RK&amp;amp;M preservation: (1) important data and documents must be permanently stored, and (2) the DGR concept must ensure recoverability of the waste for 500 years after closure, thereby setting a post-closure time frame of specific interest. Against this background, a research project is planned to explore the relevant aspects of potential future marking strategies in Germany. Different types of markers may play different roles. And these roles may again change over time: what are the specifics imposed by the 500-year recoverability requirement? How could surface markers be combined with, e.g., concepts for the re-use of the site or possible added-value projects for the local community? Could off-site markers/monuments play a role? Who would be the stakeholders in a participatory process? With respect to deep geological markers, different questions arise, like the following: could markers within the DGR potentially play a role in recoverability concepts without negatively impacting long-term safety? What dependencies exist on the type of host rock yet to be selected? Could the same markers help warn an inadvertent intruder in the long term? The potential role of markers in a systemic RK&amp;amp;M preservation strategy needs to be investigated by taking other preservation approaches into consideration, too. Which mechanisms could strengthen the efficacy of markers? Which could be strengthened by markers? This presentation will deliver insight into the preliminary studies preceding the planned research project.

AREHS: effects of changing boundary conditions on the development of hydrogeological systems: numerical long-term modelling considering thermal–hydraulic–mechanical (–chemical) coupled effects

Abstract. The objective of the AREHS project, funded by BASE (FKZ 4719F10402), is to model the effects of changing external boundary conditions on the hydrogeologically relevant parameters and effects (e.g. hydraulic permeability, porosity, migration pathways, fluid availability, hydraulic gradients) of a generic geological repository in Germany in all three potential host rocks (clay, salt, and crystalline rocks) in its hydrogeological setting. Special attention is paid to the cyclic mechanical loading due to glaciation events. This results in stress changes (M – mechanical processes) as well as induced temperature effects (T – thermal processes) due to permafrost and warm periods. Since the thermal, hydraulic (H – hydraulic processes), and mechanical processes are strongly coupled, they have to be covered by state-of-the-art coupled thermal–hydraulic–mechanical (THM) (C) modelling. The presentation consists of a (a) presentation of important findings for all three host rock types and a (b) presentation of the overall workflow. Complex, time-varying boundary conditions have been formulated for modelling glacial cycles. Investigating the effects of these boundary conditions with THM simulation reveals a variety of coupling effects. To conduct the complex modelling workflow for claystone and salt rock effectively, an automated workflow was developed and tested. It handles the transformation from a simulator-independent geological model to a numerical model specific to the simulator. This contains a suitable finite-element mesh and a parameterization of the fully coupled or optionally isolated thermal–hydraulic–mechanical processes, which are implemented and executed in the OpenGeoSys simulator. Simulation results are presented for selected physical quantities at characteristic local and temporal points with respect to the position of a migrating glacier. The investigation results are reproducible through full automation, container deployment, and a continuous integration process running on a GitLab instance (see Fig. 1a). To simulate the response of a fractured crystalline rock mass to THM impacts, the explicit integration of the fracture network in numerical models is necessary. The workflow couples the distinct element method (DEM) software “3DEC” and the discrete fracture network (DFN) software “DFN.Lab” automatically (Fig. 1b).

Acceleration potential of a Swiss-inspired approach to the site selection procedure

Abstract. The Federal Company for Radioactive Waste Disposal (BGE), the implementer of Germany's site selection procedure to identify a site suitable to safely dispose of its high-level nuclear waste, recently proposed a timeline suggesting the period between 2046 and 2068 as potential time slot for the final site decision (BGE, 2022). Thus, the decision is expected to be made substantially later than envisaged by the Site Selection Act, which states that one should aim for 2031 (StandAG, 2017). The BGE document not only details the timeline resulting in a site decision by mid-century but also outlines an alternative exploration approach inspired by the Swiss site selection procedure, which – if pursued – would enable substantial time gains. The core of this approach is a paradigm shift from exploration programs designed to acquire all data legally required for the safety analyses to data acquisition explicitly aiming at a site comparison during the respective exploration phase. Consequently, full-scale exploration would only be performed for those siting regions having passed the comparison. This contribution aims to detail and discuss this approach more thoroughly, based on the BGR's experience regarding the exploration of the subsurface. It focuses on the geoscientific aspects of this approach, while neglecting others such as the legal and administrative challenges. Owing to the basic idea of this alternative exploration approach, first a set of parameters suitable to differentiate potentially “better-suited” siting regions from “worse-suited” ones has to be identified. There are two main requirements for this set of parameters, namely the (i) general acceptance by the scientific community and (ii) capability to account for the specific characteristics of the individual host rock types while (still) ensuring the comparability of different rock types.

Geology, geochemistry, fluid inclusion microthermometry and depositional environment of the Early Cretaceous Meymeh sub-seafloor replacement sideritic-ankeritic iron deposit, Malayer-Esfahan Metallogenic Belt, Iran

Meymeh sideritic-ankeritic iron deposit in the eastern part of Malayer-Esfahan Metallogenic Belt (MEMB) is formed in the sedimentary-volcanic rocks of the Early Cretaceous sequence. Ore mineralization in the study area based on stratigraphic location and type of host rocks divided in two ore horizons. Lower ore horizon located in the dolomitic-sandy limestone (Kc3) unit, which upper ore horizon is formed in thin-bedded limestone (Km) rocks. Petrographic studies indicates that mineralization comprises three ore facies: stockwork, bedded and massive ore facies. The most important primary minerals are siderite, ankerite, ferroan-dolomite, pyrite, pyrolusite, barite and minor chalcopyrite. The most frequent textures in the ore zones include laminae, replacement, vein-veinlet, massive and banded. Dolomitization and silicification are the main wall rock alteration styles; alteration intensity increases towards the ore zones. Based on relationships between ore minerals and rock forming minerals, ore mineralization (hypogene and supergene) in the Meymeh deposit formed during three main stages: fine-grained Fe-carbonate bands are intricately interlayered with dolomite beds. Sideritic-ankeritic bands exhibit classic sedimentary textures, such as laminations and bedding, indicative of a syn-sedimentary to early diagenesis origin. Coarser-grained stage two siderites and ankerites show breccia and vein-veinlet textures, and are considered to have formed by replacement during burial diagenetic sub-seafloor fluid flow. In stage three, siderite and ankerite were converted to secondary iron oxides such as oxide/hydroxide Fe minerals during meteoric water flow through the inverted normal and thrust faults and uplift. The primary two-phase fluid inclusions in the quartz-2 and ankerite-2 minerals that have been investigated from the ore mineralization section of the Meymeh deposit are homogenized at temperatures between 110.3 to 226.9 °C. Salinities of the primary fluid inclusions range from 3.39 wt.% to 14.77 % NaCl eq. This finding suggests that hydrothermal brine fluid mixing with seawater could be the primary mechanism that prompted ore formation. The similarity of REE patterns between siderite and ankerite in different ore facies and host rock carbonates indicates their derivation from the same ore fluids. The Meymeh deposit is considered a typical case of sedimentary hydrothermal diagenetic sideritic-ankeritic mineralization, in which minerals deposited when hydrothermal fluid was released from anoxic to suboxic water columns.

The Peninsular Ranges orogenic gold belt: Supporting evidence from the San Pedro Mártir mining area (Baja California, Mexico)

The gold deposits in the northern Sierra San Pedro Mártir region of the Peninsular Ranges batholith (PRB) occur within a middle-Cretaceous suture zone that juxtaposes the Alisitos island arc against the continental margin of North America. Mineralization shares affinities with orogenic-type gold deposits and typically consists of shear zone-related gold-bearing quartz-carbonate veins, with minor amounts of sulfides, white mica and native gold. Host rock types encompass varied synorogenic plutonic rocks and greenschist to amphibolite facies metamorphic rocks. In this paper, we present four 40Ar/39Ar ages of hydrothermal white micas for representative gold deposits: 106.2 ± 0.2 Ma (Valladares), 100.5 ± 0.2 Ma (La Fortuna-San José), 94.2 ± 0.2 Ma (El Socorro), and 92.2 ± 0.2 Ma (Corazón de Oro de Jesús). The first two ages correspond to highly accurate determinations of the timing of the orogenic gold event, whereas we attribute the younger ages to thermal resetting caused by the emplacement of a nearby large La Posta-type pluton (∼97‒90 Ma). Additionally, in order to establish the maximum age of mineralization for the granitoid-hosted deposits, three new U–Pb zircon ages were obtained for plutonic rocks (136.7 ± 0.9 Ma, Corazón de Oro de Jesús quartz diorite orthogneiss; 107.0 ± 0.5 Ma, Valladares tonalite pluton; 105.0 ± 2.0 Ma, Santa Cruz diorite pluton) Furthermore, fluid inclusions were analyzed from six gold deposits. Four different types of ore fluids were distinguished, with likely temperatures of trapping between ∼300° and >600 °C, and salinities between ∼1 and 17 wt.% NaCl equiv. Gold mineralization formed subsequent to the initial collisional phase, occurring simultaneously with regional metamorphism and pluton emplacement. At least some synorogenic granitoid-hosted deposits were formed under a transient dextral transpressive regime, intercalated between the predominant regional orthogonal compression. A connection is established between these gold deposits and others found in historical mining districts along the exhumed axis of the PRB (e.g., El Álamo), and we propose that all form part of the ‘Peninsular Ranges orogenic gold belt’.

A paleoplacer component to the gold hosted in meta-conglomeratic units of the Neoarchaean moto greenstone belt, DRC

The giant Karagba-Chauffeur-Durba (KCD) gold deposit and its peripheral gold satellite deposits are located within the Kibali gold district in the Neoarchaean Moto Greenstone Belt in the northeast of the Congo Craton, Democratic Republic of Congo. The gold mineralisation within the KCD area occurs within altered chemical sedimentary rocks such as banded iron formation (BIF) and chert, as well as in meta-carbonaceous argillite, meta-sandstones and poorly-sorted pebble meta-conglomerates. To elucidate and characterise the controls on gold mineralisation in the siliciclastic rocks of this sedimentary sequence dating from approximately 2630 to 2625 Ma, the present study critically considers an extensive data set comprising borehole logs, petrographic observations, gold assay data, and mineral chemistry. Detailed high magnification petrographic analyses of polished sections enabled evaluation of 3824 gold grains from meta-conglomerate and siliciclastic sedimentary host rock specimens. These were compared to an additional 985 gold grains from deformed quartz-carbonate (ACSA) altered rocks and BIF host rock types, representing end-members for a purely hydrothermal style of gold mineralisation. These analyses revealed that most of the gold grains in the meta-conglomerates occur within the gangue mineral assemblage in the form of free gold generally distal to any sulphide minerals, with a second population associated with the sulphides, mainly pyrite, followed by arsenopyrite and lesser pyrrhotite. The gold grains in the meta-conglomerates show larger average sizes (433 µm2) than in the grains in the ACSA (311 µm2) and BIF (180 µm2) host rock types. A large dataset of assayed gold results of 9,632,470 data from 3953 drill holes highlights that the ACSA-altered rocks and the chemical sedimentary rocks (chert and BIF) are the main hosts for gold, showing average grades of 3.9, 2.5 and 2.4 g/t when a 0.5 g/t cut-off is applied, respectively. Conversely, meta-conglomerates have a relatively low Au content (1.84 g/t), yet this dataset has an extensive set of outlying values, with 86 data points reflecting grades over 100 g/t. Scanning Electron Microscope (SEM-EDS/WDS) performed on 397 gold grains from meta-conglomerates and deformed quartz-carbonate (ACSA) altered rocks shows mainly Au-Ag alloys, dominated by native gold with averages of 85.46 wt% of Au and 9.76 wt% of Ag in the meta-conglomerates and 86.56 wt% of Au and 12.84 wt% of Ag within the ACSA. Minor amounts of Au-Ag alloys electrum and Au-Hg-Ag alloys amalgams are present predominantly at Gorumbwa and KCD deposits. Our analysis of textural features, mineral chemistry, morphology, assay results, and mineral associations suggests that there are significant differences between the purely hydrothermal mineralisation and the mineralisation hosted by the meta-conglomeratic units. We suggest that the pre-enrichment of these meta-conglomerates by placer processes has played a role in enhancing the fertility for gold mineralisation in the KCD area. This has far-reaching implications for the existing ore-genesis models and exploration paradigms employed in the KCD area. This work presents new opportunities for exploration by highlighting the significance of understanding the stratigraphy and identifying sub-basins that may have undergone early placer enrichments. To achieve this, the early sedimentary environment should be thoroughly investigated.

Trace elements and sulfur isotopic compositions of sulfides in the giant Dahongshan Fe-Cu-(Au-Co) deposit, SW China: Implications for fluid evolution and Co enrichment in IOCG systems

Iron-oxide-copper–gold (IOCG) deposits are characterized by Cu-Au polymetallic mineralization and may contain large amounts of critical metals (e.g. Co, REE, and U) as by-products. Sources of ore-forming fluids and metals of the IOCG deposits remain controversial, either a magmatic or basinal source has been advocated. The ∼1660 Ma Dahongshan Fe-Cu-(Au-Co) deposit in the Kangdian region in southwestern China has spatially separated Fe and Fe-Cu orebodies, in which Co-rich pyrite has been identified in different types of ores and host rocks. Here, we integrate in-situ sulfur isotopic compositions and trace element geochemistry of sulfides to characterize the nature of ore fluids and the Co enrichment processes.Paragenetic sequences of sulfides in the Dahongshan deposit include: Pyrite (Py I) associated with magnetite mineralization in Fe ores, which has rims (Py II-1) intergrown with hematite-chalcopyrite assemblage; Pyrite and chalcopyrite (Sulfide II) in the Fe-Cu ores with protolith of arenaceous dolostone (Sulfide II-2) and interbedded argillaceous siltstone and dolostone (Sulfide II-3); and Sulfide III in post-ore reworking veins formed during regional metamorphism. Py I shows a magmatic sulfur isotopic signature (−2.2 to 1.2‰) and relatively high Au-Cu-Zn contents, indicating a predominance of magmatic-hydrothermal component during magnetite formation. By contrast, the combined sulfur isotopes and trace element compositions of Sulfides II in Fe-Cu ores indicate the mixing of two distinct fluid endmembers: Sulfides precipitated from one endmember show magmatic affinity with low δ34S values (as low as −2.1‰), high Se/S ratios (∼11.8 × 10-4), high Co contents (∼10600 ppm) and Co/Ni ratios (∼110), likely from a magmatic-hydrothermal source of mafic magmas affinity; whereas sulfides from the other endmember have overall higher δ34S values (up to 15.8‰), relatively lower Se/S ratios (∼0.09 × 10-4), lower Co contents (∼900 ppm) and Co/Ni ratios (∼13). Together with the published B-O isotopic data, the high-δ34S fluid is attributed to externally-derived basinal brine at Dahongshan. The clear negative correlations between δ34S values and Se/S ratios, Co contents, as well as Co/Ni ratios of Py II highlight that the incursion of exchanged basinal brines may be vital for triggering economic sulfide mineralization in Dahongshan. Furthermore, when assuming a similar predominance of magmatic fluids, Py I associated with magnetite ores is less enriched in Co (with lower Co/Ni ratios) than Py II from Fe-Cu ores, which can be tentatively ascribed to the decreasing Co solubility limits and formation of cattierite (CoS2) solid solution in pyrite as the fluid evolved. Finally, sulfide III in reworking veins show comparable δ34S values and Se/S ratios, but higher Co contents (up to 3.1%) than sulfide II, implying that Co can be further enriched during later remobilization events. This study exemplifies the utility of sulfide as a robust monitor of ore-forming process in IOCG system.

Naleziště sekundárních puklinových fosfátů v Českém masivu a jejich členění a porovnání v závislosti na typu hostitelské horniny

In the Bohemian Massif, there are many localities with the occurrence of secondary phosphates bound to the fissures of the host rocks. The type of these rocks partly determines the character of individual phosphate associations. As part of this comprehensive work, based on the author’s PhD thesis, the main host rock types have been classified and selected mineral associations have been studied in detail. This paper is a summary not only of the author’s research, but also of the historical knowledge in the field of secondary fissure phosphates in the Bohemian Massif. The paper concludes with a brief outline of the likely conditions for the formation of fissure phosphates.