Dolomite Formation Research Articles

An Ecological Perspective on Dolomite Formation in Great Salt Lake, Utah

Protodolomite was detected in sediments from Great Salt Lake (GSL), Utah, U.S.A. that have no history of elevated temperature or pressure, conditions that are thought to promote dolomitization of sedimentary carbonates. Protodolomite was abundant in a non-oolitic sediment core from the South Arm (SA) of GSL at 26% salinity. Protodolomite was also abundant in a non-oolitic NA sediment hand sample yet was absent in a nearby oolitic sediment hand sample in locations that likely receive allochthonous nutrients. Protodolomite was not detected in benthic photosynthetic mats from the SA. However, the mats comprised aragonite with halite and minimal calcite; benthic photosynthetic mats do not form in the NA. To begin to identify potential controls on the formation of protodolomite in the SA and NA of GSL, the composition and abundance of 16S rRNA gene transcripts, carbon cycling activities, and porewater geochemistry of the sediment cores were characterized. Transcripts affiliated with a dominant halophilic, heterotrophic sulfate-reducing bacterium were detected in the uppermost sections of the SA core and their abundance was positively correlated with rates of acetate oxidation/assimilation and concentrations of sulfide. Differences in the quality of organic matter between the SA and NA cores, as indicated by carbon to nitrogen ratios, indicate fresh deposition of photosynthetic biomass at the SA sediment core site but not in the NA sediment core site. Sediment grains from the SA core exhibit micrometer-sized euhedral protodolomite crystals that were not detected in the NA core. Collectively, these observations suggest that deposition of photosynthetic biomass drives the development of a sharp, anoxic lens of heterotrophic sulfate reduction. Sulfide, in turn, may promote dehydration of Mg2+-water complexes and primary protodolomite nucleation and growth. The scarcity of dolomite in the NA sediment core may result from constraints imposed by a combination of extreme hypersalinity and depositional environment on phototrophs and sulfate reducers, their activities, and the thermodynamics of protodolomite formation.

Phase transformation involved in the reduction process of magnesium oxide in calcined dolomite by ferrosilicon with additive of aluminum

AbstractMetal magnesium is mainly produced from the calcined dolomite by the silicothermic production. However, in this process, the reduction temperature is higher while the reaction speed is slow, which results in higher energy consumption and serious environmental problems. In this paper, adding aluminum into the ferrosilicon reducing agent is expected to lower the reaction temperature so as to solve the problems above. The phase transition involved in the whole reduction process including with and without aluminum addition were investigated in details by theoretical calculation and experimental research. The influence of aluminum on the magnesium oxide reduction path was analysis to clarify the internal mechanism. The results show that aluminum added into the ferrosilicon would first react with magnesium oxide to form magnesium vapor and alumina under vacuum pressure of 10 Pa when the temperature rises to 720°C. Then, calcium aluminate would be formed by the reaction of aluminum oxide and calcium oxide. Once the temperature reaches 1150°C, silicon begins to reduce the magnesium oxide to create the silicon oxide that will finally react with calcium oxide to form calcium silicate. When the temperature rises above 1150°C, both the aluminum and silicon will participate in the reduction of magnesium oxide. In the process of heating up, the mixture of aluminum, ferrosilicon and calcined dolomite forms Mg2Al4Si5O18 and Ca3Al2(OH)12 phase with the components in calcined dolomite. Mg2Al4Si5O18 and Ca3Al2(OH)12 phase finally form Ca12Al14O33 phase. The interaction between aluminum and ferrosilicon in the mixture is less; the mixture of aluminum and ferrosilicon first forms Al3FeSi2 phase, and finally has the trend of forming Al4.5FeSi phase. There is a great difference between the phase transformation of aluminum in the mixture of aluminum, ferrosilicon and calcined dolomite and that of aluminum in the mixture of aluminum and ferrosilicon.

Organomineralization of proto-dolomite by a phototrophic microbial mat extracellular polymeric substances: Control of crystal size and its implication for carbonate depositional systems

Many have postulated that a specific microbial metabolism or the presence of microbes or/and their extracellular polymeric substances (EPS) can lead to the formation of dolomite. Although now there is the consensus that dolomite can be formed in the presence of microorganisms, the exact nature of the involvement of microbes in the dolomite nucleation remains a matter of debate. The focus is now in understanding how microbial mats determine the mineralogy of dolomite. Here we report the effect of the EPS extracted from phototrophic microbial mat isolated from a sabkha in Qatar dominated by cyanobacteria (Lyngbya aestuarii) in the formation of dolomite precursors at 25 °C and 40 °C. Both the temperature and the presence of EPS impact the size and morphology of minerals, promoting spherulitic and dumbbell growth in sulfate free solutions. The formation of proto-dolomite was enhanced by the abundance of carboxylated molecules in EPS which controlled the polymorphism of carbonates. Our study emphasizes the dual importance of organic matter and temperature in dolomite formation and their impact on mineral morphology and chemical composition in sabkhas.

Multiphase carbonatite-related magmatic and metasomatic processes in the genesis of the ore-hosting dolomite in the giant Bayan Obo REE-Nb-Fe deposit

The origin of dolomite that hosts the Bayan Obo REE-Nb-Fe deposit (57.4 Mt.@6% REE2O3, 2.16 Mt.@0.13% Nb2O5, and >1500 Mt.@35% iron oxides) has been controversial for decades, but it is integral to understanding of the genesis of this giant deposit. In this study, based on the textures and in situ major and trace element composition of its carbonates, the dolomite was proved to be initially generated from magnesio-ferro‑carbonatite melts. It subsequently experienced magmatic-hydrothermal alteration and recrystallization in a low strain environment, caused by calcio‑carbonatitic fluids, with formation of finer-grained dolomite, interstitial calcite and increasing amounts of associated fluorocarbonates. Available stable isotope analyses indicate that the recrystallized ore-hosting dolomite has higher δ13C and δ18O ratios compared to its igneous coarse-grained precursor. Rayleigh fractionation during the recrystallization process, rather than crustal contamination, played a major role in the highly-variable stable isotope composition of carbonates in the dolomite. Low-T alteration increased variability with apparently random increases in δ18O within carbonates. The REE, Ba and Sr were added simultaneously with the elevated (La/Yb)cn from magnesio-ferro‑carbonatite melts to calcio‑carbonatitic fluids, and to carbonatite-derived aqueous fluids, through which extensive fluorine metasomatism and alkali alteration overlapped the recrystallization of the ore-hosting dolomite. Therefore, the multi-stage REE mineralization at Bayan Obo is closely related to metasomatism by calcio‑carbonatitic fluids of previously-emplaced intrusive magnesio-ferro‑carbonatite bodies during late evolution of the Bayan Obo carbonatite complex. Then, the ore-hosting dolomitic carbonatite was subjected to compressive tectonics during a Paleozoic subduction event, and suffered intense, largely brittle, deformation, which partially obscured the earlier recrystallization process. The complex, multi-stage evolution of the ore-hosting dolomite is responsible for the uniqueness, high grade and giant size of the Bayan Obo deposit, the world's largest single REE resource with million tonnes of REE oxides.

RESERVOIR CHARACTERIZATION OF AN ISOLATED MIDDLE MIOCENE CARBONATE PLATFORM: CA VOI XANH FIELD, OFFSHORE VIETNAM

The Ca Voi Xanh (CVX) gas field is located offshore Vietnam along the eastern margin of the southern Song Hong Basin. Reservoir rocks are carbonates of Middle Miocene (Langhian and Serravallian) age which developed on an isolated platform (length approximately 100 km, and width approximately 15 km) on top of the Triton Horst structural high. Shallow‐water corals and large and small benthic foraminifera are the main faunal constituents of the Langhian carbonates, whereas overlying Serravallian carbonates, the principal reservoir at CVX, are dominated by deeper‐water coralline red algae (rhodoliths) and large benthic foraminifera (LBF). The Serravallian carbonates consist of rhodolith‐LBF grainstones and packstones to mud‐lean packstones. Langhian carbonates consist of coral‐LBF grainstones‐packstone/rudstones.This paper documents the workflow used to develop an integrated sequence‐stratigraphic and reservoir rock‐type framework for the Ca Voi Xanh reservoir, and the impact of this work on reservoir quality prediction and modelling. Sequence stratigraphic interpretations of the Serravallian carbonates are based on data from three wells including detailed sedimentological core descriptions tied to well‐log character. The carbonate succession consists of three third‐order depositional sequences (Ser1, Ser2 and Ser3). Two well‐developed exposure surfaces can be identified: the base‐Serravallian sequence boundary (Ser1 SB), and the base‐Tortonian sequence boundary (Tor1 SB). The Serravallian shows an overall shallowing‐upward trend from carbonates with more horizontally‐oriented coralline red algae (encrusted and bored pavements/hardgrounds) in the lower part of the section, to carbonates with large, roundish irregular rhodoliths towards the upper part. Petrographic thin section, stable isotope (oxygen and carbon), and fluid inclusion analyses confirm a freshwater (vadose and phreatic) diagenetic overprint of the carbonates below the exposure surfaces (sequence boundaries).Partial dolomitization of the Serravallian carbonates was observed at Wells‐2 and ‐3 in the CaVoi Xanh field. Dolomite formation could be related to hydrodynamic fluid interactions associated with the development of a freshwater lense during the top‐Serravallian sea‐level fall (Tor1 SB), favoring dolomitization by sea water during the subsequent sea‐level rise. Alternatively, dolomitization could be related to CO2 and hydrocarbon charging, causing secondary leaching of the Mg‐rich coralline red algae and large benthic foraminifera, leading to an enrichment of the water with Mg‐ions and thus favoring downward dolomitization.A reservoir rock type (RRT) scheme was developed for the Langhian and Serravallian carbonates at the Ca Voi Xanh field based on a combination of depositional environment interpretations together with diagenetic characteristics and reservoir parameters (porosity and permeability).The Langhian is characterized by two RRTs depending on the degree of cementation (RRT‐3L) and dissolution (RRT‐5L). Serravallian RRTs are separated into dominantly packstone (RRT‐1) and dominantly grainstone textures (RRTs‐2, ‐3, ‐5 and ‐6). RRTs with grainstone texture show varying degrees of cementation (RRTs‐2 and ‐3) and dissolution (RRTs‐5 and ‐6). Early diagenetic dolomitization (RRT‐4) preferentially affects packstones (RRT‐1) but also grainstones (RRT‐2, ‐3 and‐5).The geologic (static) model consists of both matrix and non‐matrix components. Sequence stratigraphic concepts in combination with seismically‐derived palaeo‐reconstructions were used to guide the vertical and lateral distribution of reservoir rock types, which comprise the matrix component of the geological model. Different karst geometries and features consistent with known hydrogeologic processes were identified in the seismic discontinuity (variance) cube, and were used to interpret karst regions with different degrees (and/or types) of karst/fractures. These karst regions were used to populate the non‐matrix component of the geologic model along the Ser1 and Tor1 sequence boundaries.

In situ Sr isotope analysis of mantle carbonates: Constraints on the evolution and sources of metasomatic carbon-bearing fluids in a paleo-collisional setting

Carbonate-bearing wedge peridotites attest the mobilization of carbon (C) by slab fluids/melts circulating in a subduction setting. In general, COH fluids are thought to derive from the dehydration/partial melting of the crustal portions of slabs, especially during the exhumation of crust-mantle mélanges along continental subduction channels. In this study we combined textural observations with in-situ Sr isotope analyses of mantle carbonates occurring in different microstructural sites to test whether the fluids responsible for the carbonation of a mantle wedge are derived from the subducted continental crust or not. We focus on the Ulten Zone peridotites (Eastern Italian Alps) associated with high-grade felsic rocks, where carbonates occur mainly as dolomite and minor magnesite and calcite. In situ laser MC-ICP-MS analysis of peridotites representing different episodes of a complex metasomatic history, indicates that Sr isotopic variations can be linked to the different microstructural positions of carbonates. The C-metasomatism of the UZ peridotites is proposed to have occurred in two stages. The first stage is the HP‑carbonation at peak (eclogite-facies) conditions, with formation of interstitial matrix dolomite in textural equilibrium with hornblende to pargasite amphibole and Cl-apatite. This dolomite exhibits relatively unradiogenic 87Sr/86Sr present day values of 0.70487±0.00010, requiring different sources with respect to the associated migmatites and the overhanging mantle wedge. Carbonation continued during exhumation, with local injection of C-rich fluids forming a dolomite vein in association with tremolite and chlorite. The dolomite vein shows a wide range of 87Sr/86Sr (0.7036–0.7083), reflecting both the primary composition of carbonates and the consequent interaction with crustal fluids as expected in a crust-mantle mélange. The second stage is C-remobilization by dolomite dissolution and precipitation of brucite intergrowths with calcite during the final exhumation. This remobilization event has resulted in a similar Sr composition to the precursor dolomite.The mantle wedge is therefore capable of storing carbonates which have been shown to represent a complex metasomatic evolution from eclogite-facies conditions to very shallow structural levels. Therefore, fluids released from subducting slabs of continental lithosphere might be responsible for the crystallization of metasomatic minerals such as amphibole, phlogopite and zircon in the overlying ultramafic rocks. Conversely, the role of these metasomatic fluids on the carbonation of mantle wedge is likely overestimated. The combination of geochemical, isotopic and textural evidence suggests that dolomite inclusions and interstitial dolomite are derived in large part from a distinct source of C-bearing fluids that could be related to depleted mantle wedge sources and/or trondhjemitic igneous activity. In contrast, at the end of exhumation, residual COH-fluids released by the associated stromatic gneisses and orthogneisses resulted in late-stage dolomite veins having the highest Sr isotope values in the Ulten Zone peridotites.

Formation, diagenesis and palaeoenvironmental significance of upper Ediacaran fibrous dolomite cements

AbstractNeoproterozoic marine dolomite cements represent reliable, albeit complex, archives of their palaeoenvironment. Petrological and high‐resolution geochemical data from well‐preserved fibrous dolomite and pyrite in the upper Ediacaran (ca 551·1 to 548·0 Ma) Dengying Formation in south‐west China are presented and discussed here. The aim of this research is to reconstruct the redox state of late Ediacaran shallow seawater and porewater in the Sichuan Basin using early marine diagenetic fabrics. Based on crystalline texture and axis, four basic types of fibrous dolomite cements formed penecontemporaneously in a microbialite reef setting at the platform margin: (i) bladed dolomites (replacement from a high‐Mg calcite precursor); (ii) fascicular fast dolomites (replacement from an aragonitic precursor); (iii) fascicular slow dolomites; and (iv) radial slow dolomites. The latter two fabrics are considered direct marine porewater precipitates due to their length‐slow character, cathodoluminescent zonation, and enriched copper and cobalt concentrations. Marine cements yield rare earth element and yttrium patterns comparable to modern seawater and represent a refined set of archive data relative to previously published bulk dolostones. Redox‐sensitive elements and cathodoluminescence indicate that the fascicular fast dolomites formed in suboxic seawater, while fascicular slow and radial slow dolomites formed in euxinic marine porewaters. Microbial sulphate reduction during the formation of fascicular slow and radial slow dolomites is recognized by nanometre‐scale spheroidal ankerite and sulphur‐containing dolomite, and intergrown pyrite grains with U‐shaped δ34S transects. Data shown here suggest predominantly suboxic shallow late Ediacaran seawater and euxinic marine porewaters, with microbial activity promoting the direct precipitation of dolomite.

New isotope constraints on the Mg oceanic budget point to cryptic modern dolomite formation

The oceanic magnesium budget is important to our understanding of Earth’s carbon cycle, because similar processes control both (e.g., weathering, volcanism, and carbonate precipitation). However, dolomite sedimentation and low-temperature hydrothermal circulation remain enigmatic oceanic Mg sinks. In recent years, magnesium isotopes (δ26Mg) have provided new constraints on the Mg cycle, but the lack of data for the low-temperature hydrothermal isotope fractionation has hindered this approach. Here we present new δ26Mg data for low-temperature hydrothermal fluids, demonstrating preferential 26Mg incorporation into the oceanic crust, on average by εsolid-fluid ≈ 1.6‰. These new data, along with the constant seawater δ26Mg over the past ~20 Myr, require a significant dolomitic sink (estimated to be 1.5–2.9 Tmol yr−1; 40–60% of the oceanic Mg outputs). This estimate argues strongly against the conventional view that dolomite formation has been negligible in the Neogene and points to the existence of significant hidden dolomite formation.

Influence of temperature, salinity and Mg2+:Ca2+ ratio on microbially-mediated formation of Mg-rich carbonates by Virgibacillus strains isolated from a sabkha environment

Studies have demonstrated that microbes facilitate the incorporation of Mg2+ into carbonate minerals, leading to the formation of potential dolomite precursors. Most microbes that are capable of mediating Mg-rich carbonates have been isolated from evaporitic environments in which temperature and salinity are higher than those of average marine environments. However, how such physicochemical factors affect and concur with microbial activity influencing mineral precipitation remains poorly constrained. Here, we report the results of laboratory precipitation experiments using two mineral-forming Virgibacillus strains and one non-mineral-forming strain of Bacillus licheniformis, all isolated from the Dohat Faishakh sabkha in Qatar. They were grown under different combinations of temperature (20°, 30°, 40 °C), salinity (3.5, 7.5, 10 NaCl %w/v), and Mg2+:Ca2+ ratios (1:1, 6:1 and 12:1). Our results show that the incorporation of Mg2+ into the carbonate minerals is significantly affected by all of the three tested factors. With a Mg2+:Ca2+ ratio of 1, no Mg-rich carbonates formed during the experiments. With a Mg2+:Ca2+ ratios of 6 and 12, multivariate analysis indicates that temperature has the highest impact followed by salinity and Mg2+:Ca2+ ratio. The outcome of this study suggests that warm and saline environments are particularly favourable for microbially mediated formation of Mg-rich carbonates and provides new insight for interpreting ancient dolomite formations.

Bovine-derived hydroxyapatite coatings deposited by high-velocity oxygen-fuel and atmospheric plasma spray processes: A comparative study

Bovine-Derived Hydroxyapatite (BHAp) is Ca-deficient natural hydroxyapatite with several ions substitutions that play a crucial role in its biomimetic behavior. Natural quantities of ions such as Mg2+ and CO32– increase the bioactivity and the biological performance of BHAp compared to stoichiometric hydroxyapatite (HAp). In this contribution, BHAp powder was obtained from two years old bovines and thermally sprayed by Atmospheric Plasma Spray (APS) and High-Velocity Oxygen Fuel (HVOF) on 304L stainless steel substrates. BHAp coatings morphology and structure were analyzed by scanning electron microscopy (SEM), Fourier Transformed Infra-Red (FTIR) spectroscopy and Rietveld refinements of the coatings grazing incident X-ray diffraction (GIXRD) patterns. The bioactivity of the coatings was evaluated following the modifications of its structure and surface morphology after immersion in simulated body fluid (SBF) during 3, 5 and 10 days, by SEM and GIXRD. The Ca, P and Mg concentration in SBF was also measured by inductively coupled plasma optical emission spectrometry (ICP-OPS). The obtained BHAp powder was highly crystalline and mainly a B-type HAp. The main differences between BHAp and HAp are CO32– and the Mg2+ contents, which lead to the formation of dolomite in the crystalline content (48.75 wt%) during HVOF spraying. APS coating did not exhibit any secondary phases in its crystalline content. The typical polyhedral grain apatite layer was evident after 3 days of immersion in SBF for both coatings. However, SEM micrographs of HVOF coatings show delamination after 5 days of immersion. Thus, the dolomite phase is detrimental for coating stability and bioactivity.

Geochemistry and mineralogy of serpentinization-driven hyperalkaline springs in the Ronda peridotites

We present a detailed study of the water geochemistry, mineralogy and textures in serpentinization-related hyperalkaline springs in the Ronda peridotites. Ronda waters can be classified into hyperalkaline fluids and river waters that are broadly similar to Ca2+–OH- and Mg2+–HCO3- water types described in serpentinite-hosted alkaline springs elsewhere. At the discharge sites of the fluids (fractures or human made outlets) and ponds along the fluid flow paths, the fluids are hyperalkaline (10.9 < pH < 12) and characterized by low Mg and high Na, K, Ca, and Cl- concentrations. River waters, occurring near the spring sites, are mildly alkaline (8.5 < pH < 8.9) and enriched in Mg and DIC compared to Na, K, Ca and Cl-. The chemistry of Ronda Mg–HCO3 river waters is likely due to the hydrolysis of ferromagnesian peridotite minerals in equilibrium with the atmosphere by infiltrated meteoric water and shallow groundwater in the serpentinized peridotite. The Ronda Ca–OH hyperalkaline fluids are generated by the combination of low temperature serpentinization reactions from infiltrated surface Mg–HCO3 river waters —or Ca–HCO3 waters from near karst aquifers— and deep carbonate precipitation isolated from atmospheric CO2. Mass balance calculations indicate that the weathering of Ca-bearing peridotite silicates, such as diopside, is a feasible source of Ca in Ronda Ca–OH hyperalkaline fluids; however, it requires steady-state dissolution rates substantially greater than those determined experimentally.Travertine, crystalline crusts and sediment deposits comprise the types of solid precipitates observed in Ronda hyperalkaline spring sites. Calcite, aragonite, dolomite and Mg-Al-rich clays are the main mineral phases identified in the spring sites. As illustrated in the Baños del Puerto spring site, (i) calcite-dominated precipitation is due to uptake of atmospheric CO2 by Ca–OH hyperalkaline fluids during discharge, and (ii) aragonite-dominated precipitation is due to mixing of Ca–OH hyperalkaline fluids with Mg–HCO3 river waters. Aragonite and dolomite contents increase away from the springs and toward the river waters that uniquely reflects the effect of Mg ions on the precipitation of aragonite versus calcite. Other potential factors controlling the precipitation of these CaCO3 polymorphs are the Mg/Ca ratio, the CO2 content, and the temperature of the fluids. Dolomite forms during lithification of travertine due to periodic flooding of river water combined with subsequent evaporation.

Proto-dolomite formation in microbial consortia dominated by Halomonas strains.

Microbes can be found in hypersaline environments forming diverse populations with complex ecological interactions. Microbes in such environments were found to be involved in the formation of minerals including dolomite, a mineral of economic importance and whose origin has been long-debated. Various reports on in vitro experiments using pure cultures provided evidence for the microbial role in dolomite formation. However, culturing experiments have been limited in scope and do not fully address the possible interactions of the naturally occurring microbial communities; consequently, the ability of microbes as a community to form dolomite has been investigated in this study. Our experiments focused on examining the microbial composition by culturing aerobic heterotrophs from the top hypersaline sediments of Al-Khiran sabkha in Kuwait, a modern dolomite-forming environment. The objectives of this study were to assess the ability of two microbial consortia to form dolomite using enrichment culture experiments, mineralogy, and metagenomics. Proto-dolomite was formed by a microbial community dominated by Halomonas strains whereby degradation of the extracellular polymeric substances (EPS) was observed and the pH changed from 7.00 to 8.58. Conversely, proto-dolomite was not observed within a microbial community dominated by Clostridiisalibacter in which EPS continuously accumulated and the pH slightly changed from 7.00 to 7.29.

Thermodynamic analysis of theoretical dolomite formation from seawater and captured carbon dioxide

In this study, an integrated system is proposed to form dolomite chemically from seawater and carbon dioxide. The system contains three main chemical processes: formation of magnesium hydroxide and calcium hydroxide, the reaction of magnesium hydroxide and calcium hydroxide with carbon dioxide, and the formation of dolomite. Based on the fundamental thermodynamic laws, energy and exergy analyses, and performance assessment of the proposed system is carried out. Furthermore, the effects of varying temperatures of each process on various energy and exergy efficiencies are assessed through parametric studies. The results show that formation of dolomite is favorable at temperature ranges of 313–470 K based on the thermodynamic study. The exergy efficiency of dolomite formation is found to be about 23.8% and the results reveal that the dolomite formation process has the specific exergy destruction value of 29.6 kJ/mol among other sub-processes. The results suggest that the proposed system may provide enhanced options for low-temperature dolomite formation using seawater and captured carbon dioxide once the reaction kinetics is favored.

The identification and mitigation of geohazards using shallow airborne engineering geophysics and land-based geophysics for brown- and greenfield road investigations

South Africa is a mineral-rich country with a diverse geology and a long history of mining. The rich history of mining activities includes the extraction of coal from the Ecca Group Sediments of the Karoo Supergroup (250 Ma), gold and uranium from the Witwatersrand Supergroup (2900 Ma), as well as platinum, uranium, tin and lead from the layered Bushveld Igneous Complex (BIC) (2150 Ma). The extraction of gold, copper, tin, lead and rare earth minerals also took place in the Archean rocks of Swazium age (3500–3000 Ma). The historical mining records have either not been accurately recorded or have been lost over time. This has resulted in significant geohazard risk during infrastructure development, especially in and around historical mining towns, such as Johannesburg and Ermelo. These geohazard risks require careful appraisal and quantification prior to any infrastructure design or construction. This case study aims to set out the development aspects of the Multi-Faceted Geophysical Modelling Systems approach, which was used by the South African National Roads Agency SOC Ltd (SANRAL) during an investigation of undermined ground for the historical coal-mining town of Ermelo in South Africa. The N11/N2 ring road was planned to go around Ermelo to ensure mobility between major routes, whilst still maintaining town access. The systems approach used a combination of airborne geophysics (Versatile Time Domain Electromagnetic System (VTEM TM ) and magnetics), generally used in mining exploration, land-based and borehole geophysics, borehole water testing, and ground-truthing. The approach was continuous and iterative, building on the data at hand and reducing unnecessary investigations while eliminating the possibility of anomalies being missed, as in the case of conventional discrete drilling. The investigation ensured that 100% of the route was comprehensively investigated with a high confidence in the geological and geophysical data, and concomitant mitigation of infrastructure risk. The Multi-Faceted Geophysical Modelling Systems approach was successfully used to identify a previously unknown 1 × 1 m mining stope cavity at 90 m depth and a 3 × 5 m access tunnel at 24 m depth in a timely and cost-effective manner. Seven reverse-circulation percussion boreholes confirmed the structural integrity of these underground cavities, as well as the structural geology along the centreline. Based on the great success achieved in identifying shallow anomalies, this Multi-Faceted Geophysical Modelling Systems approach is now being considered for field trails on the dolomitic formations and the Wild Coast greenfields road project where there are large historical slumps and many fault lines. Thematic collection: This article is part of the Ground-related risk to transportation infrastructure collection available at https://www.lyellcollection.org/cc/Ground-related-risk-to-transportation-infrastructure

Improving acid fracture design in dolomite formations utilizing a fully integrated acid fracture model

Acid reactivity with dolomite is more sensitive to temperature as compared to calcite formations, resulting in different dissolution profile. Acid fracture fluids are heated inside the fracture, as it flows away from the wellbore, due to the reservoir heat flux. This causes a sharp increase in dolomite reactivity, resulting in a bell-shaped conductivity profile along the fracture half-length. The productivity of such conductivity distributions will be wrongly estimated if a single average conductivity value is used. In the present research, a reservoir simulator that includes variable fracture conductivity was coupled with an integrated acid fracture model. Also, design guidelines for acid fractures in dolomite formations have been proposed.Case studies are presented, showing that in conventional acid fracture designs, the maximum dissolution in dolomite usually occurs away from the wellbore. This negatively affects productivity, as maximum conductivity is desired near the wellbore. This effect is especially significant for fracturing candidate wells in higher permeability reservoirs. This study shows that in good permeability formations, productivity can be improved by acid fractures designed at a lower injection rate. In cases where the desired injection rate is not sufficient to propagate the fracture, good productivity can be obtained by using two acid stages with different injection rates. A higher injection rate is employed to create the desired fracture dimensions, and a smaller rate used to obtain good conductivity in the near-wellbore region. In low permeability reservoirs, it is desirable to alternate injections of retarded acid systems with pad fluid to create longer acid penetration distances. The results show that, in certain conditions, designs following the proposed guidelines can result in substantial productivity improvement compared to the conventional approach.

Characterization and environmental impact assessment of gold mine tailings in arid regions: A case study of Barramiya gold mine area, Eastern Desert, Egypt

Geochemical and mineralogical characterization and environmental impact assessment were performed on Barramiya gold mine tailings, Eastern Desert, Egypt. The Barramiya gold mine is situated within ophiolite-decorated Nugrus suture zone, which composed of dismembered sequence of highly deformed and metamorphosed mafic to ultramafic rocks and their hydrothermal alteration products (carbonate rocks), interbedded with meta-volcano clastic rocks of the Nubian Shield. Morphometric parameters were quantified using ArcGIS 10.0 with Shuttle Radar Topography Mission (SRTM) maps. A total of four tailings samples were gathered and analysed by XRF, XRD, ICP-OES/MS and ESEM. Tailings are characterized by homogeneous silty fine sand, high carbonates in the form of dolomite and ankerite, and low sulfides content in the form of arsenopyrite and pyrite relics. High concentration averages of heavy elements are found for As (2936 mg/kg), Cr (237 mg/kg), Sr (219 mg/kg), Ni (150 mg/kg), V (131 mg/kg), Zn (64 mg/kg), Cu (51 mg/kg), Co (19 mg/kg) and Pb (12 mg/kg). They reflect mafic and ultramafic rocks, graphite schists, and quartz veins of the study area. Gypsum is present as secondary phase, identified in the shrinkage crack fillings. Morphometric drainage basin parameters for wadi Barramiya basin show rectangular to subdendritic drainage pattern with a moderate hazard degree. This study recommends designing multiple-layered dry cover, overlain by plant cover at uppermost part for treatment of the gold mine tailings; otherwise, wind and water erosion can easily remove all dissolved and unconsolidated material that absorb heavy elements and would create a potential risk to the surrounding ecosystem.

Controls of temperature, alkalinity and calcium carbonate reactant on the evolution of dolomite and magnesite stoichiometry and dolomite cation ordering degree - An experimental approach

Natural and synthetic dolomites exhibit large scatter of structural and compositional characteristics with respect to variations in their stoichiometry and cation ordering degree (COD), but the physicochemical factors controlling dolomitization reactions are still poorly understood and by far not quantified. In the present study, dolomite was synthesized in batch reactors at temperatures of 150°C, 180°C and 220°C by reacting calcite or aragonite with an artificial solution containing MgCl2 and NaHCO3 over a 360day period. The obtained results on the evolution of the stoichiometry of dolomite and magnesite and on the COD of dolomite indicate that the formation of stoichiometric and well ordered dolomite and of stoichiometric magnesite follow a ripening process and proceed through dissolution and re-precipitation of a sequence of intermediate phases, such as Ca-magnesite, huntite, very-high-Mg-calcite (VHMC) and disordered dolomite. The initial occurrence and the metastability of the precursor phases were dependent particularly on reaction temperature and to a lesser extent on the CaCO3 source used. Temperature is the major rate-limiting parameter of the evolution of dolomite and magnesite stoichiometry and dolomite COD, corroborating results from prior experiments and natural deposits. From our data, we determined kinetic rates for the stoichiometric ripening of magnesite and dolomite and for the dolomite COD for the first time using a first-order reaction kinetic approach. The kinetics increase with temperature following the linear Arrhenius equation, in turn allowing for activation energies of the respective processes to be determined (44.0, 57.4 and 9.3kJ·mol−1, respectively). Interestingly we found the reaction rate for magnesite stoichiometric ripening is approximately one third of that for dolomite regardless of the temperature, carbonate alkalinity or CaCO3 phase used for reaction. Furthermore, we see that the activation energies required for the formation of ideal dolomite decreases in sequence from the precipitation of an initial VHMC phase (∼133.5kJ·mol−1; Arvidson and Mackenzie, 1999), followed by stoichiometric ripening (57.4kJ·mol−1) to cation ordering (9.3kJ·mol−1; for perfect ordering).Extrapolation of the obtained dolomitization rates indicates that about 1.4 and 6.8Myr are required to approach ideal dolomite at 50 and 25°C, respectively, explaining the large absence of ordered dolomite in modern sedimentary successions. The present dataset provides new insights into dolomitization pathways and rates and contributes to a better understanding of the wide range of stoichiometries and COD values of dolomite and its evolution versus magnesite observed in the geological record and in laboratory studies.

Hydrostratigraphy of the Malmani Subgroup dolomites within the northeastern escarpment (Limpopo and Mpumalanga, South Africa)

Abstract The Late Archaean to Early Proterozoic Malmani Subgroup comprises of dolomites and limestones forming part of the Chuniespoort Group within the Transvaal Supergroup, outcropping as an arc structure east of the Pretoria Group along the Limpopo and Mpumalanga escarpment. These rocks form a fractured karst aquifer in the area and have a high degree of heterogeneity and anisotropy. The aquifers are unconfined to semi-confined, with compartmentalisation by dolerite dykes being a possible effect (if the dykes are large and extensive enough) due to the dykes acting as aquitards or barriers to groundwater flow. The contact zones between the dolomite formations and dolerite dykes are usually fractured however, and along with any other faults and fractures result in preferential dolomite dissolution and the development of groundwater flow paths in the area. Borehole yields ranges between 2 to 5 l/s and potentially &amp;gt;10 l/s per borehole in the vicinity of large regional fractures or dolerite intrusions. Groundwater from the Malmani Subgroup generally meets the drinking water quality standards for major constituents and it is of Mg-Ca-HCO3 nature. Groundwater development within this particular hydrostratigraphy is linked to potential well field target zones that take cognisance of various surface water-groundwater interaction affecting surface water discharge rates as well as groundwater over-abstraction concerns. Preliminary results have indicated that given a groundwater potential of 44 hm3/a, the aquifer will be able to support abstractions of up to 29 hm3/a if systematically developed adaptively and could be used and managed conjunctively with surface water to alleviate the pressure on the already stressed Olifants Water Management Area.

Early dolomitization and recrystallization of the Lower-Middle Ordovician carbonates in western Tarim Basin (NW China)

Abstract The Lower-Middle Ordovician shallow-marine carbonates in western Tarim Basin were dolomitized to different degrees. The resultant dolomites are mainly concentrated in the successions of platform interior to shallow subtidal shoal, and display an upward decrease in abundance. Based on petrographic textures, three types of matrix dolomites were identified: (1) very finely to finely crystalline, nonplanar-a to planar-s dolomite (Md1), (2) finely to medium crystalline, planar-e(s) dolomite (Md2), and (3) finely to coarsely crystalline, nonplanar-a dolomite (Md3). Origins of matrix dolomites are interpreted on the basis of field investigation, petrography and isotopic (O, C and Sr) geochemistry. Md1 dolomite was formed from slightly evaporated (i.e., mesosaline to penesaline) seawater in near-surface to very shallow burial settings, possibly facilitated by microbial mediation (i.e., bacterial sulphate reduction; BSR). Md2 dolomite was precipitated from connate seawater stored in coeval limestone in shallow burial realms. In contrast, Md3 dolomite was likely formed as a consequence of intense recrystallization of early-formed dolomites (e.g., Md1) at depths. The formation of early matrix dolomites (i.e., Md1) was likely determined by depositional facies and sea-level variations of variable orders. This study provides a useful analogue to understand the dolomitization processes related to depositional facies and variable-order sea-level fluctuations in the Tarim Basin and elsewhere.

Characterization of the extracellular polymeric substances (EPS) of Virgibacillus strains capable of mediating the formation of high Mg-calcite and protodolomite

The origin of dolomite –a common mineral in the geological record– is the subject of an ongoing debate. Among different hypotheses, it has been proposed that extracellular polymeric substances (EPS) excreted by microbes include organic molecules that catalyze the incorporation of Mg in the carbonate mineral. However, limited information exists on the composition of the EPS produced by CaMg carbonate-forming microbes, which in turn hampers a precise understanding of their role in the mineralization mechanism. Here, we present the results of laboratory experiments in which we cultured different strains of microbes, characterized their EPS, and identified components associated with carbonate minerals with high mol% Mg. Two Virgibacillus strains known to mediate the formation of Mg-rich carbonates, as well as a strain of Bacillus licheniformis–a negative control that does not mediate mineral formation but produces EPS, were grown under different salinities and temperatures, which caused them to produce EPS with different compositions. The EPS were subsequently characterized by measuring total carbohydrate (TCHO) and total protein (TP) contents, as well as by Fourier-transform infrared spectroscopy (FTIR). At the tested conditions, we found that Mg-carbonates with a mol% Mg higher than 40% (i.e., potential dolomite precursor phases) formed exclusively in association with EPS rich in carbohydrates (TCHO > than 75% of the total mass). FTIR spectra of CaMg carbonate-forming strains were distinct from those of the non-mineral-forming strain in areas associated with the protein structures responsible for the formation of hydrogels, which contribute to hydration or dehydration of ionic clusters; further differences have been observed in the regions of phosphoryl functional groups. These results provide insight on which fraction of organic molecules and specific functional groups are, among the many constituents of EPS, important for mineral nucleation and incorporation of Mg into carbonate minerals, a crucial step for the formation of dolomite in natural environments.