Replacement Dolomite Research Articles

Dolomite formation in the Miocene Kardiva platform, Maldives archipelago: a tale of closed-system and open-system dolomitization by current pumping of seawater

Dolomites from two IODP Expedition 359 sites on the northeastern margin of the Miocene Kardiva platform, Maldives archipelago, were examined to explore the question of open versus closed system dolomitization in a drowned carbonate platform. The uppermost approx. 130 m of platform margin carbonate at site U1465 contains less than or equal to 12% dolomite except for five, meters-thick intervals with up to 65% dolomite. All U1465 dolomites consists of decimicron-sized euhedral cement crystals and mimetically replaced peloids and coralline red algal clasts. The abundance and the petrographic features of these dolomites are similar to periplatform and slope dolomite in many other settings that have been interpreted as the product of hydrologically closed-system diagenesis. In contrast, the recovered platform margin deposits at site U1469 are greater than 99% dolomite. Those dolomites are partially fabric retentive with fine-to-medium crystalline, planar subhedral to euhedral crystal mosaics of replacive dolomite and dolomite cement, all with Sr contents that average 256 ppm. Their characteristics are comparable to Neogene platform carbonates universally interpreted to signify hydrologically open-system dolomitization. Sr-isotope ages indicate Miocene dolomitization at both sites after platform drowning, and 18Odolomite values are compatible with dolomitization by cold (10⁰ to 15⁰ C) seawater when the platform margin was, on average, approx..400 m below sea level (mbsl). A current pumping mechanism for the advection of seawater into the top of the platform at site U1469 is proposed and tested with a computational fluid-flow simulation. Current pumping occurs when strong ocean bottom currents flow over sedimentary bedforms and generate lateral pressure differences along the sediment-water interface. The pressure differentials drive seawater through the underlying sediments. The flow simulation shows that the ocean currents that swept large sediment sand waves over and off the drowned Kardiva platform for many millions of years could have vigorously pumped Miocene seawater to sub-seafloor depths of many tens of meters . Mass balance considerations suggest complete dolomitization of the upper 20 m of the platform within 500 Ky or less. The current pumping mechanism could drive dolomitization and mineralogical stabilization below hiatal surfaces, or very slowly accumulating sediments, in any marine setting characterized by strong bottom currents. The greater acidity of Mioc ene seawater relative to younger seawater possibly made the pumping, by any means, of Miocene seawater of normal salinity an effective dolomitizing agent at site U1469 and in undrowned Miocene platforms globally.

Reservoir evaluation of dolomitized Devonian strata in the Western Canada Sedimentary Basin: implications for carbon capture, utilization, and storage

Abstract Differentially dolomitized carbonate strata in the Western Canada Sedimentary Basin (WCSB) are increasingly targeted for carbon capture, utilization, and storage (CCUS), yet few studies have evaluated the petrophysical characteristics of these conventional hydrocarbon reservoirs for this purpose. To address this, this study uses drill-core analysis (sedimentology, diagenesis, pore morphology, and distribution), together with core-plug and production data, to evaluate the properties of five depleted oil and gas fields in the Middle to Upper Devonian Swan Hills Formation, Leduc Formation and Wabamun Group. The Swan Hills and Leduc formations are composed of reef, shoal, and lagoon deposits that are predominantly fossil-rich (e.g., stromatoporoid-dominated rudstones and boundstones). In contrast, the carbonate-ramp deposits of the Wabamun Group are fossil-poor, consisting instead of variably bioturbated carbonate mudstones, wackestones, and packstones. Replacement dolomitization is variable throughout each stratigraphic unit, but generally occurs within fossil-rich and/or heavily bioturbated intervals. Fracture densities are broadly comparable in limestone and dolostone. Porosity in the Swan Hills and Leduc formations is predominantly moldic and vuggy, occurring where fossils (e.g., stromatoporoids) are partially or fully dissolved. Pore space in the Wabamun Group is mostly restricted to intercrystalline porosity in burrows. In general, burial cements (e.g., calcite and dolomite) are volumetrically insignificant and only partially fill pores. Exceptions to this include porosity-occluding cements associated with fractures and breccias in the vicinity of faults. Dolomitization and depositional facies are found to exert a strong control on pore morphology, distribution, and interconnectivity. Porosity is principally controlled by the relative abundance of skeletal grains and by the presence of burrows. These highly porous facies acted as fluid pathways during burial diagenesis, resulting in their preferential dolomitization, solution enhancement of pre-existing pores, and creation of volume reduction-related porosity. The high CO2 storage capacity and low unplanned plume migration risk (due to depositional and/or diagenetic baffles) of dolomitized reefal reservoirs (e.g., Swan Hills and Leduc formations) make them more attractive targets for CCUS than those with limited capacity and/or potential migration pathways (e.g., fault-related fractures and breccias in the Wabamun Group). These results demonstrate that drill-core analysis, in combination with legacy data, can provide valuable insights into the factors that control reservoir CO2 injectivity, plume migration, and storage capacity.

The interplay between dolomitizing fluids, tectonically-controlled saddle dolomite and calcite cements in lower Cambrian to Furongian strata in the Tazhong Uplift

The interplay between dolomitizing fluids, tectonically-controlled saddle dolomite and calcite cements in Lower-Cambrian to Furongian strata in the Tazhong Uplift, Tarim Basin were interpreted from petrography, geochemistry and fluid inclusions microthermometry. Stochiometric replacement dolomites (RD1, RD2 and RD3) and saddle dolomite (DC) cement were identified. Equant-drusy calcite (ECAL) and blocky-to columnar calcite (LCAL) cements were identified. Weak relationships exist between δ13C, δ18O, 87Sr/86Sr, Mg/Ca versus Mn/Sr ratios, which are tools used to evaluate the impact of diagenetic environment on C–O–Sr isotope composition of the carbonates. The replacement dolomites and DC with high Mg/Ca ratios (0.97–1.21) have low depleted δ18O, δ13C values, compared to the ECAL and LCAL with more depleted δ18O, δ13C values, indicating that the C–O–Sr isotope composition suffered diagenetic alteration by higher-temperature and equilibrium isotopic fractionation. The carbonates fluid system is characterized by negative shift in C–O-isotopic composition with hypersaline signatures compared to Early Paleozoic evaporative seawater. Moreover, the increasing trend in (Cl, Br, I)/(CaO + MgO) ratios with Mg/Ca ratios, couple with positive relationship between Cl and Br, suggests fluids contamination by halogen-rich inclusions. The RD1, RD2 and RD3 were precipitated by relatively higher-temperature saline fluids at depth during shallow-deep burial dolomitization. The DC have average Th-values (141.5–199.2 °C) mostly higher than the estimated ambient temperatures (165 °C) of the studied strata, meaning their growth in fractures resulted from higher-temperature Mg-enriched saline basinal fluids conveyed by hydrothermal convection and short-lived geothermal squeegee-type flow. The LCAL were precipitated by Ca/Mg-enriched basinal brine fluids diluted by influx of meteoric water, cementing fractures in host rocks, as dolomite was replaced by calcite (i.e., calcitization). The conceptual model reveals that dolomitizing fluids and calcitization fluids were conveyed through permeable horizons resulting from strong extensional tectonics and weak compressional tectonics, forming irregular networks of saddle dolomite and calcite cement in host rocks.

Diagenetic variability in Tertiary, syn-rift mixed siliciclastic‑carbonate depositional system (Lower Musayr Formation), Red Sea, Saudi Arabia

Mixed siliciclastic‑carbonate depositional systems are prone to differential diagenesis due to lithological heterogeneity. However, unlike pure carbonate and/or siliciclastic counterparts, the diagenesis within mixed clastic‑carbonate successions remains poorly known. This study integrates sedimentological and petrographic analysis with porosity-permeability measurements to understand diagenetic variability and its impact on fluid flow in the Oligocene-Miocene mixed clastic‑carbonate system (lower part of Musayr Fm) in Midyan Basin, Red Sea, Saudi Arabia. Sedimentological observations highlight that siliciclastic intervals comprise conglomerates, coarse- to fine-grained sandstones and subordinate mudstones whereas mixed siliciclastic‑carbonate intervals are composed of shell, ooids and microbialites-dominated facies. Petrographic analysis indicates presence of several diagenetic processes in siliciclastic intervals including dissolution of unstable silicate grains (feldspar), formation of pore filling and/or grain-replacive kaolinite, precipitation of calcite and/or dolomite, and replacement of dolomite by silica. The siliciclastic strata exhibit minimal compaction fabrics with no stylolites and are characterized by higher permeability (average = 1884 md) and porosity (average = 18.7 %). On the other hand, mixed siliciclastic‑carbonate intervals underwent micritization, sparry calcite formation, dolomitization of micrite and bioclasts, and replacement of sparry calcite by pyrite and Fe-oxides. Wavy stylolite seams were also observed in ooid facies reflecting moderate chemical compaction. In addition, the porosity (average = 6.6 %) and permeability (average = 93 md) are magnitudes lower than siliciclastic counterparts. Significant differences between porosity and permeability of siliciclastic and mixed intervals are partly linked to relatively intense diagenetic alteration (higher cementation and chemical compaction) in mixed units. Understanding such diagenetic heterogeneity in mixed systems has important implications for identification of reservoir and non-reservoir zones and may provide useful insights for hydrocarbon exploration and carbon sequestration in the analogous strata in the subsurface.

Dolomite-Based Concrete: A comprehensive review on mechanical properties

Concrete, an extensively used material contributes to greenhouse gas emissions due to use of Portland cement (PC). The production of PC worldwide is approximately 4 billion tons annually accounting for 8% of global CO2 emissions. To address this environmental challenge and mitigate carbon footprint, various industrial waste such as Fly Ash, Ground Granulated Blast Furnace Slag, Metakaolin, Eggshell Powder, and Sugarcane Bagasse Ash have been employed to partially replace PC, alleviating disposal issues. A waste/byproduct of the dolomite business, dolomite powder is produced yearly in amounts of 1.75–2 million tons (MT), or 25% of the industry's total production (7–8 MT). The current paper intends to summarize the related research on the usage of dolomite powder as a PC replacement in relations to compressive, flexural and split tensile strength. A bibliographic analysis is done using VOSviewer providing relationships between the keywords and countries doing research on dolomite powder. Concrete incorporating dolomite exhibits enhanced mechanical properties, boasting a 5–10% increase in compressive strength, 6–12% higher tensile strength, and a 5–10% boost in flexural strength at a 10% dolomite replacement rate. This is because dolomite has a smaller particle size which fills the miso and macro pores improving the microstructure of concrete and decreasing the porosity Thus, the dolomite as a PC replacement increases overall performance and has proven to be a sustainable option for construction practices.

Ca isotopic gradients in epeiric marine carbonates: Diagenetic origins of and significance for Ca cycle reconstructions

Marine carbonates record geochemical information on the state of Earth’s surface environment in the geological past, but recrystallization during burial can make the records difficult to interpret. Here, we report shore to basin gradients in δ44Ca values in matrix limestone and burrow dolomite across a sequence of Late Ordovician (Katian Stage) burrow-mottled carbonates in the Williston Basin, an intracratonic epeiric marine basin in North America. Both gradients are interpreted to have diagenetic origins. The gradient in limestone δ44Ca values developed in a bathymetrically controlled sediment- to fluid-buffered diagenetic system operating between the center and the edges of the basin. Two dolomitizing events formed the gradient in dolomite δ44Ca values. The sediment-buffered end of the limestone gradient preserves the δ44Ca value of the original carbonate mud (∼–1.28 ± 0.10 ‰). The fluid-buffered end of the gradient conserves the δ44Ca value of contemporaneous seawater (–0.47 ± 0.10 ‰), because: (1) pore water exchange drivers operated with greater variety and intensity in shallow water settings, (2) shallow nearshore deposits exchanged more Ca with seawater during recrystallization to diagenetic calcite than deeper offshore deposits, and (3) diagenetic calcite formed with negligible fractionation under equilibrium conditions. The gradient conserves the local value of Δsed (–0.81 ± 0.20 ‰) despite its diagenetic origin, and the magnitude of Δsed is consistent with primary calcite precipitation in a ‘calcite sea’. Field assessment of the equilibrium Ca isotope fractionation factor between calcite and dolomite of –0.41 ± 0.17 ‰ at ambient temperature is applied to correct basin-centered dolomite for fractionation, yielding –1.62 ± 0.27 ‰, which is very different from the –0.25 ± 0.07 ‰ value of Ca in evaporatively concentrated seawater from which the dolomite originally formed. Applying the same fractionation factor to correct literature data on early diagenetic dolomite in the Great Basin, North America, yields –0.69 ± 0.22 ‰ for the δ44Ca value of seawater, which is ∼0.2 ‰ lower than ‘normal marine’ waters in stratigraphically equivalent deposits in the Willison Basin, and 0.44 ‰ lower than evaporatively concentrated seawater during a brief period of basin restriction and anhydrite deposition. The nearly 0.5 ‰ variation in the δ44Ca value of Katian seawater falls within the range of scatter in the brachiopod reconstruction of seawater Ca isotopes. Here, we propose that the true ocean record of secular change lies towards the bottom of the range of scatter, and that the higher values in the record reflect local Ca cycling and circulation restriction in epeiric seas. Epeiric seas cultivate the fluid-buffered diagenetic conditions required to promote Ca isotope exchange between reactive carbonate sediments and seawater through carbonate mineral dissolution/recrystallization, crystal coarsening by way of Ostwald ripening, and replacive dolomitization. It is not the apportioning of kinetically fractionated aragonite and calcite in the carbonate deposition flux that drove secular changes in global value of Δsed between ‘calcite’ and ‘aragonite’ seas, but rather the replacement of kinetic fractionations with equilibrium fractionations during fluid-buffered diagenesis with seawater.

Enhancing multi-objective mix design for GGBS-based geopolymer concrete with natural mineral blends under ambient curing: A Taguchi-Grey relational optimization

Rising populations and industrial expansion necessitate sustainable alternatives to conventional cement-based concrete. Geopolymers, with their impressive mechanical properties, eco-friendliness, and potential for waste utilization, offer a promising solution. Yet, concerns like rapid setting and low workability limit their widespread adoption. This study addresses these challenges by optimizing a geopolymer concrete incorporating ground granulated blast furnace slag (GGBS) blended with bentonite and dolomite minerals. Employing the Taguchi method, we optimized the mix design considering key factors like binder content, mineral replacements, alkaline-to-binder ratio, and solution ratios. Grey-Relational Analysis identified the optimal mix with 10 % bentonite and dolomite replacement, a 0.55 alkaline-to-binder ratio, 12 M NaOH solution, and 425 kg/m3 cementing materials. This optimized mix exhibited significantly improved setting times, enhanced workability, and the highest compressive strength among all tested mixtures. Experimental validation confirms its effectiveness, highlighting its potential as a sustainable alternative. Importantly, this research explores GGBS as a replacement for commonly used Fly Ash, further advancing sustainable construction practices.

Reactive transport modeling constraints on the complex genesis of a lacustrine dolomite reservoir: A case from the Eocene Qaidam Basin, China

Reactive transport modeling (RTM) is an emerging method used to address geological issues in diagenesis research. However, the extrapolation of RTM results to practical reservoir prediction is not sufficiently understood. This paper presents a case study of the Eocene Qaidam Basin that combines RTM results with petrological and mineralogical evidence. The results show that the Eocene Xiaganchaigou Formation is characterized by mixed siliciclastic-carbonate-evaporite sedimentation in a semiclosed saline lacustrine environment. Periodic evaporation and salinization processes during the syngenetic-penecontemporaneous stage gave rise to the replacive genesis of dolomites and the cyclic enrichment of dolomite in the middle-upper parts of the meter-scale depositional sequences. The successive change in mineral paragenesis from terrigenous clastics to carbonates to evaporites was reconstructed using RTM simulations. Parametric uncertainty analyses further suggest that the evaporation intensity (brine salinity) and particle size of sediments (reactive surface area) were important rate-determining factors in the dolomitization, as shown by the relatively higher reaction rates under conditions of higher brine salinity and fine-grained sediments. Combining the simulation results with measured mineralogical and reservoir physical property data indicates that the preservation of original intergranular pores and the generation of porosity via replacive dolomitization were the major formation mechanisms of the distinctive lacustrine dolomite reservoirs (widespread submicron intercrystalline micropores) in the Eocene Qaidam Basin. The results confirm that RTM can be effectively used in geological studies, can provide a better general understanding of the dolomitizing fluid-rock interactions, and can shed light on the spatiotemporal evolution of mineralogy and porosity during dolomitization and the formation of lacustrine dolomite reservoirs.

New insights into hypogenic non-matrix dissolution mega features driving subsurface fluid movement within the Arab Formation, Saudi Arabia

This paper reports the results of a large integrated study into the classification, distribution and significance of hypogenic non-matrix large-scale dissolution features present in the Arab Formation of Saudi Arabia. The investigation of these features was based primarily on extensive core, wireline caliper, wireline/log while drilling resistivity borehole image logs supported by encountered total lost circulation of drilling fluids and production log data. Dissolution features observed by this study were classified into three main categories: touching vugs, ramiform voids, and cavities. This study is the first to successfully quantify the presence and extent of these non-matrix intervals in this formation, which has greatly improved the 3D modelling efforts. Over 8100 feet of large-scale dissolution features were intersected in both vertical and horizontal wells. The hypogenic genesis of the large-scale dissolution features pre-dates replacement dolomitization in the area. Given the extensive data reviewed, the presented results give insight into the importance of the large-scale dissolution mega features as they are strongly related to total loss of mud circulation during drilling, and will have implications for well-to-well short-cutting during production and impact oil storage capacity.

Basin scale evolution of zebra textures in fault‐controlled, hydrothermal dolomite bodies: Insights from the Western Canadian Sedimentary Basin

AbstractStructurally controlled dolomitization typically involves the interaction of high‐pressure (P), high‐temperature (T) fluids with the surrounding host rock. Such reactions are often accompanied by cementation and recrystallization, with the resulting hydrothermal dolomite (HTD) bodies including several ‘diagnostic’ rock textures. Zebra textures, associated with boxwork textures and dolomite breccias, are widely considered to reflect these elevated P/T conditions. Although a range of conceptual models have been proposed to explain the genesis of these rock textures, the processes that control their spatial and temporal evolution are still poorly understood. Through the detailed petrographical and geochemical analysis of HTD bodies, hosted in the Middle Cambrian strata in the Western Canadian Sedimentary Basin, this study demonstrates that a single genetic model cannot be applied to all the characteristics of these rock textures. Instead, a wide array of sedimentological, tectonic and metasomatic processes contribute to their formation; each of which is spatially and temporally variable at the basin scale. Distal to the fluid source, dolomitization is largely stratabound, comprising replacement dolomite, bedding‐parallel zebra textures and rare dolomite breccias (non‐stratabound, located only proximal to faults). Dolomitization is increasingly non‐stratabound with proximity to the fluid source, comprising bedding‐inclined zebra textures, boxwork textures and dolomite breccias that have been affected by recrystallization. Petrographical and geochemical evidence suggests that these rock textures were initiated due to dilatational fracturing, brecciation and precipitation of saddle dolomite as a cement, but significant recrystallization occurred during the later stages of dolomitization. These rock textures are closely associated with faults and carbonate‐hosted ore deposits (e.g. magnesite, rare earth element and Mississippi Valley–type mineralization), thus providing invaluable information regarding fluid flux and carbonate metasomatism under elevated P/T conditions.

Diagenetic History and Timing of Cu and Zn-Pb Sulfide Mineralization in the Permian Kupferschiefer System, Saale Subbasin, Eastern Germany

Abstract The Southern Permian basin in central Europe contains a number of important high-grade sediment-hosted Cu deposits. Laterally extensive stratabound Cu and Zn-Pb sulfide mineralized rocks are located at a major stratigraphic redox boundary, where coarse-grained continental sandstones of the uppermost Rotliegend Group are overlain by carbonaceous mudstones (T1) and limestones (Ca1) of the Zechstein Formation. This study investigates the diagenetic evolution and style of sulfide mineralization in three drill cores that intersect Cu and Zn-Pb sulfide mineralized rocks at three locations (Sangerhausen, Allstedt, and Wallendorf) in the Saale subbasin (Eastern Germany), which is located at the southern margin of the Southern Permian basin. We combine macro- to microscale petrographic data (binocular, transmitted and reflected light, and scanning electron microscopy) with quantitative X-ray diffractometry and bulk-rock geochemical analyses. Petrographic results show extensive, primary-porosity-occluding, early diagenetic calcite cementation that predates both the diagenetic alteration of detrital clasts and sulfide mineralization. The highest-grade Cu and Zn-Pb sulfides (bornite, sphalerite, and galena) replace the calcite cement, with subordinate replacement of dolomite and detrital clasts. Quantitative mineralogical and geochemical data demonstrate that the highest base metal (Cu, Zn, and Pb) concentrations are associated with carbonate-rich samples, mostly as disseminated mineralization in the middle T1. Bulk-rock geochemical results show enrichment and covariation of redox-sensitive trace elements (RSTEs, e.g., Mo) with total organic carbon content toward the lower T1, consistent with highly reducing depositional conditions. Overall, the distribution and dissolution of calcite cement across this stratigraphic redox boundary provided the main control on the lateral migration of base metal-bearing fluids and high-grade Cu and Zn-Pb sulfide mineralization in the Saale subbasin.

Sedimentary control on diagenetic paths of dolomite reservoirs in a volcanic setting: A case study of the Permian Chihsia Formation in the Sichuan Basin, China

The formation mechanisms of multi-stage dolomites and hydrocarbon reservoirs associated with volcanic activity are complex. This study investigated the Permian Chihsia Formation in the Sichuan Basin, China, which is the location of recent discoveries of giant natural gas reserves. We undertook a petrological and geochemical study, including trace element and C–O–Sr isotopic analysis. The quality of the dolomite reservoirs associated with volcanic activity was controlled by different diagenetic pathways associated with the sediment types and fault systems. Outcrop, core, and thin-section observations identified three types of replacement dolomite: very finely to finely crystalline dolomite (Rd1), medium crystalline patchy dolomite (Rd2), and coarsely to medium crystalline dolomite (Rd3). The tidal flat sediments only experienced limited seepage reflux dolomitization, which resulted in the formation of the dense very finely to finely crystalline dolomite (Rd1). The bioclastic grain shoal sediments deposited in a high-energy shallow-water environment contain abundant intergranular and dissolution pores, and vugs, which allowed convection of seawater and dolomitization during the initial phases of volcanic activity of the late early Permian. Subsequently, the bioclastic grain shoal sediments were mostly replaced by coarsely to medium crystalline dolomite (Rd3), which formed the highest-quality reservoirs in the study area. In addition, the incompletely dolomitized medium crystalline patchy dolomite (Rd2) was formed due to the low original porosity of micritic, peloidal, grain shoal sediments during a transgression in the middle and late depositional stages. During the peak of volcanism at the end-Guadalupian, deep faults allowed seawater to mix with ascending hydrothermal fluids, which resulted in the precipitation of dolomite cement in pores near the fault fracture zones, which blocked the pores. Therefore, these high-energy shoal sediments deposited in the slope break zone far from strike-slip faults have a relatively high porosity. Our results provide new insights into the origin of dolomite reservoirs formed in association with a volcanic setting, and are applicable to deep dolomite reservoirs worldwide.

First report of a nephrite deposit from Somaliland, Africa: Characterization and geological and archaeological implications

Nephrite jade is of wide interest, not only for its value as a highly valued precious stone, but also for its significance in tracing geological and petrological processes. Here, we report the first discovery of a nephrite deposit in Africa, located in Somaliland. Field investigations revealed that two open nephrite pits at Goodieood and Laasmacaane are composed of a series of discontinuous vein-shaped nephrite bodies stretching for ∼2 km and defining a nephrite belt. Dolomitic marble and epidote-amphibole are the major rock types in this nephrite belt. Most of the nephrites show grayish green color, while a few are grayish white to light-grayish green, with an occasional brownish rind mainly in nephrite gravels from the secondary deposits. The nephrite is composed of tremolite and actinolite with minor diopside, pargasite, edenite, and apatite. Most nephrites display fine fiber microstructures, and the tremolite-actinolite aggregates show crystallographic preferred orientation. Parallel to the nephrite bodies, epidote-amphibole rocks appear as a metasomatic dark-colored boundary zone, composed mainly of actinolite-tremolite, epidote, and titanite with minor rutile, pargasite, edenite, diopside, and zircon. The dolomitic marble displays dolomite replacement partially by antigorite. Textural observations suggest a sequence from Na-, Al-, and Fe-rich toward Mg-rich compositions, leading to the possibility of prospecting for white nephrite. The various features suggest that the deposit belongs to a fluorine-bearing Ca-Fe-P fluid-dominant skarn. This deposit is estimated to have more than 1000 tons of nephrite jade reserves and has important significance for the exploration for more economic nephrite deposits in Africa, South America, and even Antarctica, as well as potentially important archaeological implications in establishing a connection between the nephrite jades and materials for the Hargeisan lithic industries, as part of the social development in eastern Africa. Comparison with a typical white nephrite deposit in Saidikulam provides insights into the formation of white and nonwhite nephrites.

Textural and chemical evolution during dedolomitization: A case study of the Benassal Formation, Maestrat Basin, Spain

The process of dedolomitization (dolomite calcitization) has been the subject of several studies, but the mechanisms that control it are still not fully understood. Dedolomitization, i.e. the replacement of dolomite by calcite, is an important diagenetic process that affects dolostones, in certain cases decreasing the local porosity and altering the rock mechanical properties, reservoir quality and fluid flow pathways. In this contribution we report the petrographic and geochemical evidence of the progressive dedolomitization and its impact on the Lower Cretaceous dolostones of the Benicàssim area (Maestrat Basin, Spain) that host Mississippi Valley Type (MVT) ore deposits. Textural observations and geochemical data suggest that replacement was induced by the interface coupled dissolution-precipitation mechanism. Moreover, quantitative chemical content measurements with mass balance calculations show that dedolomitization caused a ∼5% gain of mass during the reaction, associated with 5 vol% of solid volume increase. The lack of microfracture development suggests that the compensation of such volume increase is achieved by microporosity being filled during calcite precipitation. Considering the meteoric origin of the dedolomitizing fluids, the mass balance calculation allows estimating the element mass transfer during the reaction. The fact that the fluid was enriched in Zn supports that the fluid was acidic, and likely percolated through the local MVT deposits. Calculation of the required volume of the reactive fluid (102–104 m3/m3 dolostone) also shows that thermodynamic calculations overestimate the volume of fluid. This study shows the importance of quantifying mass transfer in diagenetic conditions to better understand metasomatic processes and constrain the fluid pathway in the basin history. As a societal impact, this diagenetic process releases ∼11% of the carbon mass but seems to preserve all rare earth elements.

Ediacaran cap dolostones from south China: Petrography, dolomite stoichiometry, and crystal architecture

Using the global Ediacaran cap dolostones to understand the post-Marinoan Earth is based on the premise that the original environmental features have not been diagenetically altered. On the southwest side of the Yangtze Block in southeast China, the Doushantuo cap dolostones that are part of the shelf-to-slope transition are characterized by distinctive petrographic, stoichiometric, and crystal architectural features that recorded their genesis and evolution. Standard petrographic and X-ray diffraction analyses show that these cap dolostones are formed of populations of near-stoichiometric (average molar %Ca = 50.8 %; n = 73), highly ordered (average cation ordering of 0.83; n = 73) dolomite. Six microfacies recognized in the cap dolostones indicate that the rocks have experienced early cementation, dolomitization, fracture filling, recrystallization, and late-stage dolomite precipitation. All of the matrix dolomite crystals from the Liujing, Duodingguan, and Chuanyandong sections, and >95 % of those from the Wuhe section, are formed of unzoned dolomite crystals that are each formed of numerous subcrystals (200–700 nm wide, <2 μm long). In contrast, the zoned dolomite crystals from the Wuhe section are characterized of different degrees of etching between their cores and cortices, and/or between different growth zones. The zoned dolomite crystals demonstrate that the Doushantuo cap dolostones were initially formed via replacive dolomitization followed by multiple phases of dolomite precipitation in a manner similar to many Phanerozoic marine dolostone counterparts. The homogeneity in the dolomite populations and crystal architecture and the increases in dolomite stoichiometry and cation ordering in the Doushantuo cap dolostones suggests that they have been pervasively recrystallized.

Carbonate U-Pb Geochronology and Clumped Isotope Constraints on the Origin of Hydrothermal Dolomites: A Case Study in the Middle Permian Qixia Formation, Sichuan Basin, South China

Reservoirs in the dolomites of the Middle Permian Qixia Formation in the Sichuan Basin are currently important oil and gas exploration objects in China. However, the questions concerning the sources of the dolomitized fluids and the control factors of the Qixia hydrothermal dolomites remain unclear. In this study, the original hydrothermal dolomites (the replacement dolomites (RDs) and saddle dolomites (SDs)) from the Qixia Formation in the southwestern Sichuan Basin (the PR1 well and Baoxing section) were mainly examined using novel in situ carbonate U-Pb dating with clumped isotopes (∆47). Our results show that the U-Pb ages of the latest SDs from the PR1 well (located in the middle zone of the Emeishan large igneous province (ELIP) and distanced from the Orogenic Belt of Longmenshan) are 257.9–251.0 Ma, coincident with the period of main activity of the ELIP. Combined with the previous U-Pb dating, we propose that the high-temperature T∆47 (82.2–108.4 °C and 127.5–205.9 °C) recorded for SDs from the PR1 well and Baoxing section may have responded to ELIP activity and Longmenshan orogeny activity, respectively. In addition, in the entire southwestern Sichuan Basin, the RDs and SDs yield similar δ13C and δ18O values, indicating that the dolomites were formed by hydrothermal fluids of similar sources, with marine hydrothermal fluids being a highly possible source. Finally, this study proposes a new hydrothermal dolomite genesis model for the Qixia Formation, emphasizing that the formation of hydrothermal dolomites mainly depends on the proximity to tectonic thermal events in space and time.

δ26Mg-δ13C-δ18O systems as geochemical tracers for dolomite recrystallization: A case study of lower Ordovician dolomite from Tarim Basin

Widespread replacement dolomite occurs commonly in the geologic record. These dolomites are generally characterized by different crystal sizes and shapes that have at times been interpreted to represent multistage dolomitization; although the variation of dolomite texture could also have resulted from the recrystallization of precursor dolomite. It is important, but challenging, to differentiate these two scenarios to better understand the nature of dolomite and the associated processes of dolomitization. Our study explores this problem by using δ26Mg, δ13C, and δ18O isotopes to characterize the different crystal morphologies exhibited by Ordovician dolomites from the Tarim Basin, China.Although the dolomites show distinct textures and crystal morphologies, there are no discernible trends in their δ26Mg values (from −1.66 to −2.39‰), suggesting that these dolomites were formed by the same dolomitizing fluid. This interpretation is supported by an overlapping range of δ13C values (from 0.46 to −1.89‰). By contrast, the δ18O data demonstrate a wide range of values, from −3.8‰ to −8.8‰, reflecting the different degree of recrystallization with increasing burial temperatures. We suggest that the Mg and C isotopes remained unchanged during recrystallization because Mg and C were rock-buffered, so the recrystallized dolomite inherited Mg and C from the precursor dolomite. Based on these results, it appears that Mg isotopes, together with conventional OC isotopes, can provide a diagenetically robust geochemical tracer for identifying dolomite recrystallization in the geological record.

The Ediacaran Hiyam Formation: A zoom through the diagenetic and structural complexity of the metamorphic Hi2 Member, Saih Hatat Dome, Oman Mountains

This is the first diagenetic study of the metamorphic upper Hiyam Member “Hi2” of the Saih Hatat Dome. Despite metamorphism, texture and paragenesis of the Hi2 Member are so well preserved that it is possible to unravel its diagenetic evolution. We applied lithological and structural field methods, transmission and cathodoluminescence microscopy to establish the paragenetic sequence and vein-forming deformation. Our work reveals a complex diagenetic and structural history, including several alternating dolomitization, calcite precipitation, silicification and fracturing events. Most of the replacive dolomite and silica are linked to early diagenesis under intertidal to sabkha and restricted marine as well as to terrestrial conditions. Evaporation of sea water concentrated Mg2+ and triggered extensive early dolomitization while silica-bearing fluids caused local silicification. Various generations of dolomite and calcite cements pervasively occluded macropores, micropores and veins. Late burial is reflected by pressure solution seams/stylolites, non-ferroan to weakly ferroan saddle-type dolomite cement and non-ferroan blocky and drusy calcite cement, filling veins and macropores. The various generations of calcite cements (e.g., in veins) are considered to have formed during burial or possibly meteoric conditions. The latter potentially resulted from tectonic uplift, while late dolomite cement displays hydrocarbon residues from coeval oil migration. A later minor oil migration event has also been recognized. Initial fracture/fissure formation is most likely related to folding during the Early Cambrian Angudan Orogeny (open NW-SE fractures) and to divergence during the Late Paleozoic Pangean/Neo-Tethys rifting (open NW-SE fractures). Metamorphism impacted the Hi2 Member, resulting in phyllite formation, phyllosilicate crystallization and ductile shearing. Metamorphism occurred during shallow Late Cretaceous subduction. Striking lithological similarities between the Hi2 Member and that of the upper Kharus Formation of the Jabal Akhdar region could indicate their lateral equivalence.

Processes Associated with Multiphase Dolomitization and Other Related Diagenetic Events in the Jurassic Samana Suk Formation, Himalayan Foreland Basin, NW Pakistan

Foreland fold and thrust belts always represent a complex diagenetic history of carbonate succession, particularly multiphase dolomitization, due to the multi-sourcing nature of fluids affecting syn-to post-depositional successions. The present work documents a comprehensive study on the diagenetic changes, particularly dolomitization patterns in the Jurassic carbonates (Samana Suk Fm) in the Lesser Himalayan fold and thrust belt, NW Pakistan. To better understand the processes involved, integrated field/petrographic, geochemical, isotopic, and micro-thermometric studies were carried out. Field observations indicate that dolostones appear as light grey to brown stratabound and patchy units within the formation. Petrographic analysis reveals that fabric destructive matrix dolomite (RD-I) and fabric preserving replacive dolomite (RD-II) phases are present as distinct units. In addition, saddle dolomite cement (SD) and fracture-filling calcite (CC) are also observed in association with replacive dolomite cement. Geochemical analysis (EPMA) showed that Fe, Mn, and Ba concentrations in matrix dolomite are relatively less than those of replacive/saddle dolomite and fracture-filling calcite, suggesting a hydrothermal source of replacive/saddle dolomite in reducing conditions. Furthermore, stable isotope studies of RD-I showed non-depleted δ18O values, which represent coeval seawater signatures of Jurassic carbonates. RD shows depleted δ18O values and non-depleted δ13C, respectively, indicating burial or elevated temperature of dolomitization. Fracture-filled calcite represents lighter δ18O values and δ13C, indicative of relatively high temperatures. 87Sr/86Sr ratios of all diagenetic phases range from 0.707718 to 0.710747, showing more radiogenic values indicates interaction fluids with more radiogenic sources. Fluid inclusion micro-thermometry data of saddle dolomite shows TH ranging from 102.8 to 186 °C, and salinity ranging from 11.7 to 19.4 eq. wt.% NaCl, suggesting hot saline brines are responsible for the dolomitization. Fracture-filling calcite shows TH ranging from 68.0 to 98.4 °C and salinity ranging from 6.9 to 13.1 eq. wt.%. NaCl suggests moderately hot and saline solutions are responsible for their formation. In conclusion, the above-mentioned studies indicate two distinct processes of dolomitization are involved in the formation of matrix and replacive/saddle-type dolomites. RD-I has formed in the evaporative setting, whereas RD has formed due to the interaction of hydrothermal fluids during burial. Fracture-filling calcite is produced from hot subsurface solutions during uplift related to the Himalayan orogeny.

Geochemical evolution of structure-bedding controlled hydrothermal dolomites of the Kinta Valley, Western Malaysia

Study of secondary dolomitization and documentation of the evolution of basinal fluids and host rock, especially that of fine-grained deepwater carbonate mudstone are of interest toward of reservoir characterization. From this perspective, field characteristics in relation to mineralogy, petrography and geochemistry of the Paleaozoic carbonates of the Kinta Valley, Peninsular Malaysia, were investigated and documented. A total of 33 samples were collected across the various degree of dolomitization and analyzed for their petrographic properties (n = 55), while mineralogical and geochemical compositions (n = 33) were analyzed using XRD, ICP and ICPMS analysis, respectively. These analyses were performed to elucidate the diagenetic events, episodic dolomitization and development of fluid evolution as a function of regional tectono-thermal events during the Permian to Cenozoic. It is interpreted that the host calcareous mudstones (HCMs) were deposited in oxygenated, open marine seawaters. The studied samples retain the original seawater characteristics despite negligible, minor alterations of REE characteristics due to diagenesis and detrital input. Though the concentrations and trends of REEs of the dolomitic facies are modified to a certain extent, their affinities with HCMs are preserved. Enrichments of Mg, Mn, Na and depletion of Sr are linked to the effect of dolomitization under the open system of diagenesis. The redox conditions fluctuated to show distinct, recognizable variations between different dolomite phases. The MREE enrichments with other trace elements such as Mn, Fe and low Sr content indicate the association of diagenetic fluids with deep-circulating crustal hydrothermal fluids. Hydrothermal influence has been recorded along the fractures, which might have originated from the sub-surface litho units and facilitated episodic dolomitization, especially during significant tectono-thermal events. This study affirms that the occurrence, geochemical and mineralogical evolution of the dolomites of the Kinta Valley were controlled by magmatic events, whereas the intensity of replacement dolomitization was controlled by host rock texture, varied bulk chemistry of host rock, circulating fluid and temperature.