Dolomite Formation Research Articles

Controls on the Precipitation of Carbonate Minerals Within Marine Sediments

The vast majority of carbonate minerals in modern marine sediments are biogenic, derived from the skeletal remains of organisms living in the ocean. However, carbonate minerals can also precipitate abiotically within marine sediments, and this carbonate mineral precipitation within sediments has been suggested as a third major, and isotopically distinct, sink in the global carbon cycle, particularly important earlier in Earth history. Here we present a global compilation of pore fluid data and compare the sulfate, calcium, phosphate and magnesium concentrations with pore fluid alkalinity to explore the emerging relationships and explore what drives carbonate mineral precipitation in sediments. Our data compilation shows that the gradient of pore fluid sulfate concentrations correlates strongly with the gradient of alkalinity as well as with the gradient of calcium, and that these correlations improve dramatically in sediments where methane is present. We also note that sedimentary pore fluids that are high in phosphate concentration are also high in alkalinity, which may indicate suppression of carbonate mineral precipitation in the presence of sedimentary phosphate. Our data can be used to highlight sediments where both dolomite formation and dolomitization of previously deposited calcium carbonate minerals is occurring. We explore how carbonate mineral saturation state changes as a function of calcium concentrations, alkalinity, and pH, and suggest a reason why calcium concentrations are never fully depleted in sedimentary pore fluids. We conclude that carbonate minerals precipitate in sediments with methane, where the anaerobic oxidation of this methane helps promote particularly high saturation states for carbonate minerals.

Phanerozoic variation in dolomite abundance linked to oceanic anoxia

Abstract The abundance of dolomitic strata in the geological record contrasts with the general rarity of locations where dolomite forms today, a discrepancy that has long posed a problem for their interpretation. Recent culture experiments show that dolomite can precipitate at room temperature, raising the possibility that many ancient dolomites may be of syngenetic origin. We compiled a large geodata set of secular variations in dolomite abundance in the Phanerozoic, coupled with compilations of genus richness of marine benthic invertebrates and sulfur-isotope variations in marine carbonates. These data show that dolomite abundance is negatively correlated to genus diversity, with four dolomite peaks occurring during mass extinctions. Dolomite peaks also correspond to the rapid increase in sulfur-isotope composition (δ34S), an indicator of enhanced sulfate reduction, in anoxic oceans. These results confirm that variations in dolomite abundance during the Phanerozoic were closely linked with changes in marine benthic diversity, with both in turn related to oceanic redox conditions.

Modern dolomite formation caused by seasonal cycling of oxygenic phototrophs and anoxygenic phototrophs in a hypersaline sabkha

The “Dolomite Problem” has been a controversy for over a century, owing to massive assemblages of low-temperature dolomite in ancient rocks with little dolomite forming today despite favorable geochemical conditions. Experiments show that microbes and their exopolymeric substances (EPS) nucleate dolomite. However, factors controlling ancient abundances of dolomite can still not be explained. To decode the enigma of ancient dolomite, we examined a modern dolomite forming environment, and found that a cyclic shift in microbial community between cyanobacteria and anoxygenic phototrophs creates EPS suited to dolomite precipitation. Specifically, EPS show an increased concentration of carboxylic functional groups as microbial composition cycles from cyanobacterial to anoxygenic phototroph driven communities at low-and high- salinity, respectively. Comparing these results to other low-T forming environments suggests that large turnover of organic material under anoxic conditions is an important driver of the process. Consequently, the shift in atmospheric oxygen throughout Earth’s history may explain important aspects of “The Dolomite Problem”. Our results provide new context for the interpretation of dolomite throughout Earth’s history.

Characterization of arsenic contaminated groundwater from central Bangladesh: Irrigation feasibility and preliminary health risks assessment

To observe the current hydro-geo-chemical condition of shallow aquifers and the response from anthropogenic activities as well as to study the arsenic contents and affiliated health risks, groundwater samples from different parts of north-central Bangladesh (Tangail) were studied. In doing so, analytical parameters, including pH, total dissolved solids, electrical conductivity, dissolved oxygen, NO3−, HCO3−, Cl−, SO42-, Na, K, Ca, Mg, Fe, Cr, Mn, Ni, As, Cu, Cd, Zn, and Pb abundances were utilized. Aquifer hydrochemistry presents mainly Ca-Mg-HCO3 type water, while in some cases appreciable amount of Na+ and Cl− proclaim their presences in the formation of groundwater mineralogy. Therefore, there was significant impact from irrigation return flow to affect the groundwater hydrochemistry of this region. Source rock-water interactions in the form of dolomite and calcite weathering is the chief natural agent to provide minerals in the groundwater. The waters in the aquifer seem to be comparatively young with mostly being unsaturated in terms of carbonate minerals and having the scope of reverse ion exchange. Among the trace elements of this study average values of As, Mn, Fe, and Cd were above the guideline values, where As is the dominant contaminant in deteriorating the water quality. In terms of irrigation water indices, groundwater of the studied site is mostly suitable for agricultural usages. Although, magnesium hazard ratio and total hardness values can exert potential threats. However, As level in the studied groundwater possesses potential health hazardous for both non-carcinogenic and carcinogenic risks where children are more vulnerable than adults. The outcome this study invokes regular monitoring of groundwater quality, especially for As contamination to save the public-health.

Uncommon preservation of dinosaur footprints in a tidal breccia: Eubrontes giganteus from the Early Jurassic Mongisty tracksite of Aveyron, southern France

AbstractA recent excavation yielded 118 large tridactyl footprints in the Lower Jurassic Dolomitic Formation of the Causses Basin, at Mongisty in southern France. Most of the tracks are ascribed to Eubrontes giganteus Hitchcock, 1845. They are preserved on a surface of 53 m2 and form parallel rows with a preferential orientation towards the north. Such an abundance and density of E. giganteus is observed for the first time in the Early Jurassic from the Causses Basin. Sedimentological and ichnotaphonomical analyses show that the footprints were made at different time intervals, thus excluding the passage of a large group. In contrast to all other tracksites from the Dolomitic Formation, where tracks are preserved in fine-grained sediments corresponding to low-energy depositional palaeoenvironments, the tracks from Mongisty are preserved in coarse-grained sediment which is a matrix- to clast-supported breccia. Clasts consist of angular to sub-rounded, millimetric to centimetric-scale (up to 2 cm), poorly sorted, randomly oriented, homogeneous dolostone intraclasts floating in a dolomudstone matrix. Sedimentological analysis shows that the depositional environments of Mongisty varied from subtidal to intertidal/supratidal settings in a large and protected flat marsh. The lithology of the track-bearing surfaces indicates that the mudflat of the Causses Basin was sporadically affected by large mud flows that reworked and redeposited mudstone intraclasts coming from the erosion of upstream, dry and partially lithified mud beds. Throughout the world, this type of preservation of dinosaur tracks in tidal matrix- to clast-supported breccias remains rare.

Evaluation of the economic efficiency of the WAG process based on experimental data for one of the domestic oil fields

The challenge related to the need for an increase of the recovery factor concerns numerous mature, also domestic oilfields, including the most important ones – located in Main Dolomite formation. Satisfactory recovery factor can be ensured only through applying an effective enhanced oil recovery method (EOR). Water Alternating Gas (WAG), as one of the most effective EOR methods, has been tested in conditions characteristic for domestic deposits in carbonate formations. The results of experimental and simulation works carried out at the Oil and Gas Institute (INiG – PIB) indicate significant potential for the application of the WAG method in domestic conditions. An unquestionable advantage of the WAG method is the opportunity to utilize various types of gases, including flue/waste gas or low-energy natural gas. This issue deserves special attention because, as we know, the reduction in the emissions of gases involved in global warming is critical for the future of our planet. Their utilization in EOR methods, coupled with their safe storage in geological structures, constitute measures that reduce the environmental footprint of produced oil. In the article, based on the of experimental results, a simplified economic analysis of the utilization of four gas types in the form of acidic gases (carbon dioxide and its mixture with hydrogen sulfide) and natural gases (high and very high nitrogen content) in the WAG method was carried out. That allowed to identify the most economically optimal variants of the WAG method. The results showed that despite significantly lower effectiveness of nitrogen-rich natural gases in enhancing oil recovery (in the context of recovery factor), their application might be justified in economic terms. The selection of the optimal variant for enhancing recovery is strongly influenced by the assumed (current) cost of acquiring the injected media, and of course by the current (and forecasted) crude oil price.

Petrological characteristics, geochemical characteristics, and dolomite model of the lower cambrian Longwangmiao Formation in the periphery of the Sichuan Basin, China

In this paper, the genesis of dolomite in Longwangmiao Formation is analysed through field investigation and geochemical methods, and dolomization model is established, which is very important for the next reservoir prediction in Longwangmiao Formation. According to the crystal size, the Longwangmiao Formation dolomite can be divided into two types: microcrystalline dolomite (MID) and macrocrystalline dolomite (MAD). Both types of dolomite have higher Al content than lime mudstone, indicating that during the deposition process of the Longwangmiao Formation, terrigenous materials are continuously mixed into the sediment, which significantly affects the geochemical characteristics of the Longwangmiao Formation. The rare earth element (REE) composition of the two types of dolomite is consistent with that of lime mudstone (LM), with an obvious δCe positive anomaly and no δEu anomaly, which is a typical distribution pattern of marine carbonate sediments. The microcrystalline dolomite has a higher Na element content, lower δ13C value, and higher δ18O value, suggesting that it is formed in a highly saline evaporation environment. Most of the macrocrystalline dolomite has a residual grain texture. The elements, as well as the isotopic geochemical characteristics of the dolomite, are consistent with those of lime mudstone, indicating that macrocrystalline dolomite is formed early and inherits the geochemical characteristics of the precursor limestone. We believe that the dolomitization model of the Longwangmiao Formation is an evaporative concentration + reflux seepage model. In the late sedimentary period of the Longwangmiao Formation, the sea level declined and a large area of lagoon appeared in the restricted and semi-restricted platform. As the evaporation and concentration continued, precursor minerals such as high-magnesium calcite and aragonite mud experienced chemical precipitation. In this process, a large amount of Ca2+ was consumed, resulting in an increase in the Mg2+/Ca2+ ratio. Then, the precursor minerals underwent dolomitization, and microcrystalline dolomite began to form. At the same time, owing to the difference in density between the high-salinity concentrated seawater and the pore water existing in the early limestone, the concentrated seawater flowed back and penetrated downward, causing the dolomitization of the underlying strata, and then the formation of macrocrystalline dolomite. The degree of dolomitization was mainly affected by the development degree of the granule beach and the thickness of the strata.

Salt precipitation due to supercritical gas injection: II. Capillary transport in multi porosity rocks

Drying of geological formations by supercritical gas injection and the associated precipitation of salts from the formation water may have serious consequences for upstream operations in terms of injectivity/productivity impairment. The potential effect on the formation permeability near the injection well is controversially discussed in literature with some studies finding a strong and others no impairment. In an earlier publication (Ott et al., 2014), we argue that this is related to the microscopic pore structure and capillary-driven brine transport therein, and that in well sorted sandstone reservoirs, permeability may be less affected than in dual- or multi-porosity systems. In the present paper, we investigate the consequences of dolomite-formation drying due to the injection of under-saturated supercritical CO2 into samples from a Middle-East dolomite formation. Compared to earlier reported results on sandstone, the experiments show a complex behavior with a much larger effect on permeability. A permeability reduction of up to three orders of magnitude has been found. This is explained by a simple numerical model on local capillary-driven flow of water from micro- to the macro-porous regions at low brine saturation where evaporation dominates desaturation. The model suggests salt precipitation in the CO2 conductive macropores, which impacts the effective CO2 permeability. The volumetric ratio of micro to macro porosity, the capillary pressure contrast and water salinity and saturation are identified as the controlling parameters.

Biomarkers and Re–Os geochronology of solid bitumen in the Beiba Dome, northern Sichuan Basin, China: Implications for solid bitumen origin and petroleum system evolution

Solid bitumen is ubiquitous in the Sinian Dengying Formation dolomite, a crucial gas reservoir in the Sichuan Basin. We use biomarkers and Re–Os geochronology, coupled with burial-thermal evolution history to trace the source and formation timing of the solid bitumen, and to reconstruct the petroleum system evolution in the Dengying Formation of the Beiba Dome, northern Sichuan Basin. The bitumen reflectance (Rb) values of solid bitumen range from 1.92 to 3.95%, with the solid bitumen showing no fluorescence, indicating that the solid bitumen in the Dengying Formation dolomite has a high thermal maturity, which consequently belongs to pyrobitumen formed by thermal cracking of crude oil. The isoprenoids and terpanes indicate that the solid bitumen was sourced from a marine shale deposited under a reducing environment. The C27–C29 regular sterane distribution of the solid bitumen is consistent with the Lower Cambrian Guojiaba Formation (Є1g) organic-rich shale, indicating that the solid bitumen was derived from the Є1g shale, rather than dolomite or shale within the Dengying Formation. The burial-thermal evolution history indicates that initial oil generation began in the Late Cambrian to the Early Silurian. Since the Early Permian, the Є1g organic-rich shale re-entered the oil generation window, and formed a giant paleo-oil accumulation during the Late Triassic. Thermal cracking of crude oil occurred from the Late Triassic to the Early Cretaceous forming the extensive solid bitumen. The Re–Os isotopes of solid bitumen yield two similar isochron ages of 128.1 ± 7.3 Ma and 126.2 ± 8.0 Ma, which represents the timing of thermal cracking termination, i.e. the final timing of dry gas generation. This work highlights that the Re–Os geochronology has great potential to constrain the absolute timing of evolution of the petroleum system.

Precipitation of low-temperature disordered dolomite induced by extracellular polymeric substances of methanogenic ArchaeaMethanosarcina barkeri: Implications for sedimentary dolomite formation

AbstractA correlation between methanogenesis and dolomite formation has been reported; however, the mechanism underlying this association is not fully understood. In this study, we conducted forced carbonate precipitation experiments at room temperature in calcite-seeded Ca/Mg carbonate solutions containing either purified non-living biomass or bound extracellular polymeric substances (EPS) of the methanogen Methanosarcina barkeri. Purified non-living biomass and bound EPS was used so as to avoid the possible influence of the complex components of the growing microbial culture on carbonate crystallization. Our results demonstrated that non-living biomass of M. Barkeri can enhance the Mg incorporation into calcitic structure and induce the crystallization of disordered dolomite. In the presence of ~113 mg L–1 of non-living biomass, disordered dolomite with ~41 and 45 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. A systematic increase in the MgCO3 contents of the precipitated Ca-Mg carbonates was also observed with the increased non-living biomass concentration. Bound EPS was shown to be the component of non-living biomass that catalyzed the precipitation of disordered dolomite. At only ~25 mg L–1 of bound EPS, disordered dolomite with ~47 and 48 mol% of MgCO3 was precipitated in solutions with initial Mg:Ca ratios of 5:1 and 8:1, respectively. We propose that adsorption of bound EPS to growing carbonate surfaces through hydrogen bonding is the key to catalyzing disordered dolomite crystallization, and that this mechanism is also applicable to natural EPS-induced dolomite formation. This study provides significant insight into the formation mechanism of microbial-induced dolomite with high δ13C values.

Application of electrical Tomography for the hydrogeological study of the Fez-Taza corridor, Case of Bir Tam-Tam(Morocco)

The present work contributes to the multidisciplinary geological, hydrogeological, and geophysical exploration of groundwater in the Bir Tam-Tam region of the Fez-Taza corridor. Our main objective is to release water resources in our study area to supply drinking water in the region and irrigation of agricultural areas. Potential zones are located at the Lias fracturing zones. This study is interested in understanding the hydrogeological context of the study area through geophysical methods such as Electrical Tomography. The comparison of geological data, Drilling data, and the interpretation of the results of electrical Tomography have made it possible to highlight the geoelectric levels likely to constitute a potential aquifer and to locate possible structural accidents (faults) affecting the dolomitic limestone formations of Lias that could drain groundwater.

Hyperspectral analysis of shale gas drilling in southern Huangling anticline

In this paper, the visible near infrared, short wave red, and thermal infrared spectral data and high-definition core images of three wells in Huangling anticline in Western Hubei are obtained by hylogger. Through comprehensive analysis, it can be seen that the thickness of Niutitang Formation is decreasing from west to East, However, the thickness of Doushantuo Formation is basically the same, ranging from 210m to 220m. Quartz, clay minerals, carbonate and limonite are the main minerals in Niutitang formation, while in Doushantuo Formation dolomite is the main mineral. Dolomite fracture is developed in this area, which is a favorable area for shale gas development.

Cenozoic coastal carbonate deposits of Qatar: Evidence for dolomite preservation bias in highly‐arid systems

AbstractIn the ancient rock record, early replacement of metastable marine calcium carbonate deposits by dolomite has long been associated with evidence of arid depositional environments. Such associations led to the development of the seepage reflux dolomitization model, whereby magnesium‐rich marine waters concentrated by evaporation descend into underlying sediments, replacing primary aragonite and calcite deposits with dolomite through rock–water interaction. In the modern arid coastal systems of Qatar, where marine waters concentrate to halite saturation, both aqueous geochemical measurements and mineralogical investigations show that replacement of calcium carbonate deposits by dolomite is not occurring to any consequential degree. At best dolomite formation is minor and localized. Instead, modern and mid‐Holocene coastal deposits retain their original mineralogy, showing little evidence of carbonate precipitation reactions with the notable exception of beachrock formation. Pleistocene coastal deposits are mostly absent in comparison with both Qatar Holocene coastal deposits, and Pleistocene coastal deposits from more humid settings. The lack of onshore Pleistocene‐aged carbonate outcrops in Qatar, as well as regionally, is interpreted to reflect coastal sediment denudation during emergence due to: (i) the absence of coastal marine lithification; and (ii) the absence of meteoric cementation and root stabilization in the highly‐arid realm. In contrast, marine Palaeogene and Miocene carbonate deposits are preserved in outcrop in Qatar, having suffered marine lithification by dolomite, thus promoting retention of carbonate strata through subsequent lowstands. These older deposits formed during times of ocean acidification, which, based on natural system and laboratory investigations, is interpreted to promote metastable carbonate dissolution and dolomite formation. The highly‐arid Holocene and Pleistocene coastal systems of Qatar represent limestone factories, but these deposits are not being retained to the rock record. Based on these observations, the association between the occurrence of dolomite and arid depositional settings may be to some degree a preservation bias.

Trace Elements in Sphalerite from the Dadongla Zn‐Pb Deposit, Western Hunan–Eastern Guizhou Zn‐Pb Metallogenic Belt, South China

AbstractThe western Hunan–eastern Guizhou Zn‐Pb metallogenic belt is one of the important Zn‐Pb mineralization regions in China. The Dadongla deposit, located in the northeast of Guizhou Province, is one of the typical Zn‐Pb deposits in the region and has estimated resources more than 12 million metric tons (Mt) with an average grade of 4.11 wt% Zn+Pb. Its orebodies are hosted in the lower Cambrian Aoxi Formation dolomite, occurring as bedded, para‐bedded in shape, and in conformity with the wall rock. The ore mineral assemblage is simple, dominated by sphalerite with minor pyrite and galena, and the gangue minerals are composed of dolomite, calcite with minor bitumen and barite. In view of the lack of geological and geochemical researches, the genesis of Zn‐Pb ore is still unclear. Laser ablation‐inductively coupled plasma mass spectrometry (LA‐ICPMS) spot and mapping analyses were used to obtain sphalerite trace element chemistry in the Dadongla Zn‐Pb deposit in Guizhou, China, aiming to constrain its ore genesis. The results show that sphalerite is characterized by the enrichment of Cd, Fe, Ge and Hg, corresponding with that of typical MVT deposits. Four zones were identified in the sphalerite crystal from Dadongla from the center to margin according to the color bands, in which the zone in the center, representing the early ore‐stage sphalerite, is characterized by enrichment of Cd relatively, while the zone forming at late ore‐stage is enriched in Ge and Hg relatively. The finding was controlled by differential leached metals content in ore‐forming fluid from its source rock. Notably, critical element Ge trends to be enriched at the late ore‐stage and follows a substitution of 2Zn2+↔ Ge4++□ (vacancy). Moreover, the calculated ore‐forming temperature ranges from 79.9°C to 177.6°C by the empirical formula, which is similar to that of typical Mississippi Valley‐type (MVT) deposits. Compared with the features of trace elements in sphalerite from different types of deposits, together with the geology, the Dadongla deposit belongs to an MVT Zn‐Pb deposit.

Geochemical evidence for forearc metasomatism of peridotite in the Xigaze ophiolite during subduction initiation in Neo-Tethyan Ocean, south to Tibet

Subduction initiation is one of the key processes in the operation of plate tectonics. Our understanding of this process is largely derived from geochemical studies of basaltic rocks in ophiolites at continental margins. Because of the petrological inheritance in forearc basalts, it is important to acquire geochemical signatures for fluid metasomatism of the nascent mantle wedge at forearc depths. This may be provided by the study of peridotite in ophiolites, which is least affected by tectonic emplacement of the ophiolites onto continental margins. For this purpose, we have carried out a comprehensive study of whole-rock and mineral geochemical compositions for harzburgites from the Xigaze ophiolites in the Yarlung Zangbo suture zone, southern Tibet. The results provide insights into the forearc metasomatism during subduction initiation before the closure of Neo-Tethyan Ocean for the India-Asia continental collision in the Cenozoic. The harzburgites show high spinel Cr# values and strong whole-rock depletion in CaO, Al2O3 and REE, suggesting high degrees of melt extraction (~10–20%) during seafloor spreading to form the Neo-Tethyan mid-ocean ridge basalts. They exhibit positive LILE anomalies in trace element distribution patterns, indicating metasomatism by subduction zone fluids. This is supported by the occurrence of tremolite and dolomite in some of the harzburgites. Based on the presence or absence of carbonate minerals, two types of metasomatic fluids are distinguished. One is the aqueous solutions, leading to the formation of tremolite and the enrichment of LILE in clinopyroxene. The other is the carbonate-bearing fluids, resulting in the formation of dolomites, tremolite and low Mg# (79–82) olivine. The metasomatism also results in the high contents of structural hydroxyl in pyroxenes and elevated δ18O values for the harzburgites. It is inferred that the fluid metasomatism was induced by incipient subduction of the juvenile Neo-Tethyan oceanic slab to forearc depths shortly after conversion of the Neo-Tethyan mid-ocean ridge into the trench. During this period of subduction initiation, the fluids were produced by heating dehydration of the sinking ridge crust to the forearc depths. It is these fluids that would metasomatize the overlying nascent mantle wedge, leading to superimposition of the geochemically enriched signature on the lithochemically depleted harzburgites in the ophiolites. Therefore, the forearc metasomatism of peridotite in ophiolite can be used as a geochemical proxy for subduction initiation. Furthermore, the Xigaze ophiolites were produced by the ridge-trench conversion for subduction initiation in the Mesozoic and they were emplaced onto the southern margin of the Asian continent in the Cenozoic.

Gas detecting of the Lower Permian dolomite reservoirs in Sichuan Basin using amplitude ratio attribute of PP-wave to PS-wave

At present, multi-wave multi-component seismic exploration has a good effect in the gas-bearing structural imaging improvement, fracture detection, clastic rock lithology description, reservoir monitoring, etc., but in gas detection, especially for the deep carbonate reservoir, there are not many successful cases. The paper takes the 5000m deep Lower Permian Qixia Formation dolomite gas reservoir in the central part of Sichuan Basin as the research target. Aiming at the differences in gas production characteristics of different wells and difficulty in gas detection, based on the high-precision matched 3D multi-wave seismic data by seismic processing and fine horizon interpretation of target formation, the differences between the PP- and PS-wave reflection amplitudes of high-production wells, low-production wells and poor gas wells are compared and analyzed, and multi-wave forward modelling and analysis are carried out. It is clear that the high production gas layer has the characteristics of strong PP-wave reflection and weaker PS-wave reflection. With the gas production decreasing from high to low, the reflection amplitude of PP gradually weakens, while that of PS change little and nearly have no obvious abnormalities. Finally, attribute of amplitude ratio of PP to PS were extracted for gas detection, which is in good agreement with the gas characteristics of the known wells and consistent with current geological understanding. The amplitude ratio attribute of PP to PS could provide a quick and reliable gas detection method for the deep dolomite reservoir.

The influence of geological fractures on karstic sinkhole development in eastern areas of Centurion, South Africa

Centurion is a fast developing area facing severe karstic sinkhole development. The area is predominantly underlain by fractured dolomite formations that include Oaktree, Monte Christo, Lyttelton, and Eccles of the Malmani Dolomite Group (~ 2600 and ~ 2500 Ma). Fractures of the underlying dolomite formations were investigated as one of the most influencing factors in karstic sinkhole formation using fieldwork mapping, petrology, and remote-sensing techniques (automated lineament extraction methods). Fieldwork results indicate that the study area has two prevalent fracture orientations trending N–S and ENE–WSW with average orientations of 178°/86° E and 073°/67° N. The fieldwork mapping results coincide with those acquired using remote-sensing techniques by automated extraction of lineaments from colour-shaded image and from four Sobel directional filtered images. Remote-sensing techniques indicate that there are extra two prevalent lineament directions; NE–SW and E–W in the study area and majority of the sinkholes develop in areas with high lineament density. Eccles, Monte Christo, and Oaktree dolomite formations exhibit a unidirectional fracture system while Lyttelton dolomite formation exhibit bidirectional fracture system. It is, therefore, concluded that (1) geological lineaments extracted using remote sensing and fractures present in the underlying dolomite formations play a significant role in controlling fluid circulation and thus influencing karstic processes such as dissolution, and therefore, (2) sinkhole development is primarily structurally controlled; however, it is exacerbated by anthropogenic activities due to the influence of water ingress.

Sabkha dolomite as an archive for the magnesium isotope composition of seawater

Abstract Recent studies have uncovered the potential of Mg isotopes (δ26Mg) for studying past ocean chemistry, but records of such data are still scarce. Dolomite has been suggested as a promising archive for δ26Mg of seawater. However, its enigmatic formation mechanism and the difficulty in precipitating dolomite in the laboratory at surface temperatures decrease confidence in the interpretation of δ26Mg values from the rock record. To evaluate factors determining the δ26Mg of dolomite, we studied pore water and sediment from Dohat Faishakh Sabkha, Qatar—one of the rare environments where dolomite is currently forming. The δ26Mg values of the dolomite (−2.56‰ to −1.46‰) are lower than that of seawater (−0.83‰), whereas δ26Mg values of pore water (−0.71‰ to −0.14‰) are higher. The isotope fractionation accompanying dolomite formation is generally in accordance with an empirical fractionation from the literature, extrapolated to the sabkha’s temperature (−1.84‰ to −1.51‰). The results suggest that evaporated seawater is the sole source of Mg, and isotopically light dolomite is the major sink, so that the δ26Mg of the dolomite-forming pore water is equal to or greater than that of seawater. Thus, provided that the lowest δ26Mg value among several dolomite samples is used, and the formation temperature is known, similar sabkha-type dolomites can be utilized as an archive for δ26Mg values of ancient seawater.

Fracture dolomite as an archive of continental palaeo-environmental conditions

The origin of Quaternary dolomites in continental environments (e.g. karst and lakes) is barely constrained compared to marine dolomites in sedimentary records. Here we present a study of dolomite and aragonite formations infilling young fractures of the ‘Erzberg’ iron ore deposit, Austria, under continental-meteoric and low temperature conditions. Two dolomite generations formed shortly after the Last Glacial Maximum (~20 kyr BP): dolomite spheroids and matrix dolomite. Clumped isotope measurements and U/Th disequilibrium ages reveal formation temperatures of 0–3 °C (±6 °C) and 3–20 °C (±5 °C) for the both dolomite types, and depositional ages around 19.21 ± 0.10 kyr BP and 13.97 ± 0.08 kyr BP or younger, respectively. Meteoric solution and carbonate isotope compositions (δ18O, δ13C and 87Sr/86Sr) indicate the dolomites formed via aragonite and high-Mg calcite precursors from CO2-degassed, Mg-rich solutions. Our study introduces low temperature dolomite formations and their application as a sedimentary-chemical archive.

Geochemistry of the Yudong bauxite deposit, south-eastern Guizhou, China: Implications for conditions of formation and parental affinity

The Yudong bauxite deposit is located in the Kaili–Huangping bauxite-concentrated area, in south-eastern Guizhou Province, China. The ore-bearing rock series consists of the lower part (Fe-bearing member) and middle part (Al-bearing member) of the Middle Permian Liangshan Formation, covering the Upper Devonian Gaopochang Formation. The elemental content in the ore-bearing profile is variable. In general, Al, Ti, V, Cr, Zr, Nb, Hf, Th, and U increase from the Fe- to the Al-bearing member, whereas Si, Fe, Rb, Cs, Ba, and total rare earth elements (REE) decrease. These observations indicate the occurrence of bauxitisation, which corresponds to a process of elemental variation during which Si and Fe were dissolved and migrated, and Al was enriched. Compared with Ordovician–Silurian (O–S) mudrock, the ore-bearing rock series are enriched in Al, Ti, V, Zr, Hf, Nb, Ta, Th, and U, but depleted in Si, Fe, Rb, Cs, and Ba as a result of weathering. The REEs show negative correlations with Si, Fe, K, Na, Ca, Mg, and Y, indicating that the REEs may be associated with clay, Fe-bearing, and Y-bearing minerals.The La/Y ratio, (La/Yb)N ratio, and Ce anomalies show that the Fe-bearing member formed in an alkaline reducing environment, whereas the Al-bearing member formed in an acidic oxic environment. The immobile element ratios, Eu anomalies, REE distribution patterns, and Cr–Ni diagram, as well as the comparison with worldwide bauxite deposits, show that the ore-bearing rock series may have originated in the O–S mudrock, with contributions from the dolomite of the Gaopochang Formation. Furthermore, the position of the samples in the Al2O3–Fe2O3–SiO2 ternary plot, the chemical index of alteration, and the strong positive correlations of some stable elements (such as Al, Ti, Zr, Hf, Nb, and Ta) indicates that the degree of weathering gradually increases from the O–S mudrock to the bauxitic mudrock, and further increases into the bauxite; the bauxitic mudrock and bauxite therefore originated from the O–S mudrock. The laterisation products of the O–S mudrock from the northern part of the ore collection area were transported to the karst depression in the early stages of the Middle Permian. The weathering products of the O–S mudrock and a small amount of the Gaopochang Formation dolomite accumulated on the karst interface, forming the original bauxite layers through cycles of compaction and concretion. These bauxite layers were then uplifted to or near the surface during the Late Cretaceous to Paleogene, eventually, forming karstic bauxite.