Dolomite Recrystallization Research Articles

Widespread dolomite recrystallization and porosity modification of upper Permian Zechstein carbonates, Symra discovery, Utsira High, Norwegian North Sea

The Utsira High is a prominent intrabasinal basement structure in which the eroded remnants of a widespread, Late Permian, Zechstein carbonate shelf are preserved locally. The western margin of the central Utsira High is mainly characterized by weathered basement rocks with a thin Mesozoic cover. However, the recently discovered Symra Field forms an isolated sedimentary inlier basin of the area, where deeply eroded Zechstein shelf carbonates are preserved within a half-graben. The Zechstein carbonates consist of mainly ZS2 marginal marine carbonate facies and are similar to those described elsewhere from the Zechstein Basin. They are unconformably overlain by Paleogene chalks. Based on detailed facies analysis of two recently drilled cores combined with detailed petrographic and stable isotopic analysis and supplemented by age dating of selected carbonate phases using U-Pb geochronology, we show that the Zechstein carbonates have been subjected to several phases of near-surface diagenetic alterations. Volumetrically the most dominant diagenetic product comprises nonplanar dolomites interpreted to have formed by recrystallization of a precursor reflux-type dolomite phase. The recrystallized dolomites retain enhanced reservoir quality in comparison to stratigraphic equivalent ZS2 reflux-type dolomites found elsewhere on the Utsira High. Based on U-Pb-derived age constraints, the recrystallization took place during a long-lived Late Triassic exposure event of the Utsira High, and a near-surface origin for the recrystallized dolomites is proposed. The porosity enhancement occurred contemporaneously with the dolomite recrystallization process and was facies controlled. Reservoir modifications associated with the exposure led to the dissolution of CaSO4 cement, and metastable dolomite phases, in addition to the enlargement of existing pores which had created zones of weakness prone to further dissolution. Overall, the Zechstein carbonates preserved on the Utsira High illustrate the complexity of the diagenetic alterations resulting from the uplift of a carbonate shelf, and its importance for reservoir quality.

Distribution and genesis of the Maokou Formation dolomite in Fengdu-Shizhu area, eastern Sichuan Basin

To investigate the development laws and genesis of the Maokou Formation dolomite in the Fengdu-Shizhu area, we analyzed core, thin section, logging, and geochemical data, and obtained the following understandings: 1. The Maokou Formation dolomite includes layered granular dolomite and leopard-spotted limy dolomite which are fine to medium, and moderately euhedral, and have intergranular pores and intergranular dissolved pores; 2. Vertically, the dolomite is the superimposition of multiple stages, 3–12 m a layer and cumulatively up to 30 m. The distribution of the dolomite is controlled by sedimentary cycles and is commonly found in the middle and upper parts of the cycle; 3. The analysis of rare earth distribution and carbon, oxygen, and strontium isotopes indicates that the dolomitizing fluid is the penecontemporaneous saline seawater, and some pore edges were affected by later hydrothermal dolomitization, resulting in recrystallization of dolomite and cementation of saddle-like dolomite; 4. The relationship between the plane distribution of the dolomite and the paleogeographic pattern during the sedimentary period indicates that the dolomite is concentrated in the granular shoals near geomorphic highlands and slope break zones. In summary, it is proposed that the overlap and migration of granular shoals and isolated seawater promoted the occurrence of reflux infiltration of dolomitizing fluid and dolomitization. Multistage granular shoals on the platform margin provide a good material foundation for the development of dolomite. Karstification is conducive to the occurrence of early dolomitization within the shoals and the preservation of pores. It is found that the early dolomite of the Maokou Formation is best developed in the highlands of faults 15 and 16. The basement faults controlled the sedimentary paleogeomorphology, thereby restricting the distribution of dolomite. This understanding provides a new idea for the exploration of dolomite in the Maokou Formation in the Sichuan Basin.

Diagenetic alterations induced by lamina-scale mass transfer and the impacts on shale oil reservoir formation in carbonate-rich shale of the Permian Lucaogou Formation, Jimusar Sag

Diagenesis plays a crucial role in shale oil reservoir formation. However, complex organic-inorganic interactions result in pore system formation and heterogeneity changes at the centimeter, or even micrometer scale, which controls the evolution of shale oil reservoirs. In this paper, an integrated approach including core observation, thin section observation, cathodoluminescence thin section observation, scanning electron microscope, μ-XRF analysis, electron microprobe analysis, AMICS analysis, LA-ICP-MS analysis, and isotope analysis was used to clarify the laminae combination carbonate-rich shale, diagenetic alterations in different types of shale laminae and the controls on shale oil reservoir formation. According to the lamina combination, the carbonate-rich shale in the Permian Lucaogou Formation can be divided into three types. They are shale consists of dolomitic lamina and terrigenous felsic lamina (Type A shale), shale consists of calcite-rich tufaceous lamina and terrigenous felsic lamina (Type B shale) and shale consists of calcareous lamina and tufaceous lamina (Type C shale). Dolomite cementation is the major diagenesis in terrigenous felsic lamina of Type A shale. The dolomite can be identified as the early stage forming from recrystallization of micrite dolomite, and the late stage precipitate from adjacent dolomitic lamina. In the terrigenous felsic lamina of Type B shale, however, the calcite cementation is the dominated diagenesis, which was sourced from adjacent calcite-rich tufaceous lamina. Also, some feldspar dissolutions occurred in terrigenous felsic lamina during organic matter evolution. For the Type C shale, the main diagenetic alteration is micrite calcite recrystallization in calcareous lamina. The dissolution and precipitation of carbonate minerals between different laminae of carbonate-rich shale reveal that the organic matter evolution significantly influence the diagenetic alterations of inorganic minerals. Particularly, the diagenetic mass transfer and redistribution at the micro-scale in a relatively diagenetic system were confirmed by organic-inorganic interactions in laminated shale. In this process, dissolution pores and recrystallization pores have provided considerable reservoir pores for shale oil.

Dolomitization of early-post rift Lower Jurassic carbonate platforms along the Moroccan Atlantic Margin: Origin and significance

Dolomitization is the most significant diagenetic process to affect Jurassic carbonate reservoirs along the Central Atlantic Margin (CAM). Despite several studies on dolomitization from different parts of CAM, the origin of these dolomites and their influence on the subsequent diagenetic evolution of Jurassic carbonate systems remains enigmatic. In addition, while dolomitization is evident at the surface and in the subsurface of the Moroccan Atlantic Margin, virtually no detailed studies have been conducted to determine the origin, mechanism, and significance of dolomitization in this basin. Therefore, the principal objective of this study is to assess the origin and occurrence of dolomite in the of Upper Sinemurian-Lower Pliensbachian carbonates of the Arich Ouzla Formation in the Essaouira-Agadir Basin by using petrography and geochemistry.The shallow marine carbonates of the Arich Ouzla Formation have been partially dolomitized and are exposed on the salt-cored Amsittene Anticline. The dolomite is stratabound, and predominantly fabric-retentive, although in some parts it is partially replaced by non-stratabound, fabric-destructive dolomites. From petrographic observations and geochemical proxies, the fabric-preserving dolomites show dolomitization by reflux of mesohaline seawater (δ18Odolomite average = −3.5 ‰ VPDB, and δ13Cdolomite average = 2.0 ‰ VPDB). In contrast, petrographic and geochemical characteristics of the fabric destructive dolomites suggest precipitation from modified seawater/formational brines convected along faults and fractures evidenced by depleted δ18O isotopic values (average = −4.1 ‰ VPDB) with high fluid temperatures (average = 78 °C; range = 66–90 °C) where fluids interacted with the basal Triassic evaporites and siliciclastic sediments.Fabric preserving dolomite has higher porosity (average = 6.0 %) than the precursor limestones (average = 0.4 %), whereas permeability in both rock types (average = 0.48 mD, and average = 0.02 mD, respectively) is low. Fabric destructive dolomite has low porosity in proximity to fracture corridors (average = 1.9 %) due to dolomite recrystallization (overdolomitization), whereas porosity increases to an average of 7.4%, away from fracture corridors. The dolomites are post-dated by calcite cement which occludes vugs, intercrystalline pores and fractures. The calcite is interpreted to be meteoric in origin, because of its non-cathodoluminescence and depleted δ18O (average = −4.7 ‰ VPDB) and δ13C (average = −9.3 ‰ VPDB) isotopic values with respect to Jurassic marine carbonates. The meteoric calcites co-exist with bitumen suggesting that hydrocarbon migration in the basin likely occurred at the same time, most likely during basin inversion and exposure. This work considers dolomitization to be a localised process due to salt diapirism and demonstrates that the coincidence of hydrocarbon emplacement with basin inversion results in degradation and probably leakage of hydrocarbons. This emphasises the importance of local and regional tectonics, including salt diapirism, on patterns of diagenetic overprint in sedimentary basins.

STRATIGRAPHIC FRAMEWORK FOR ZECHSTEIN CARBONATES ON THE UTSIRA HIGH, NORWEGIAN NORTH SEA

The preserved Zechstein succession on the Utsira High in the NE part of the Norwegian North Sea is 25‐100 m thick and is dominated by shelf carbonates. Internal subdivision of the succession is based on the recognition of key surfaces in petrophysical logs and cores, and suggests that the carbonates mainly consist of ZS2 and ZS3 deposits and that younger ZS4 and ZS5 deposits are only locally preserved. The carbonates have undergone early, syn‐depositional dolomitization followed by later dolomite recrystallization and calcitization. Calcitization, interpreted as dedolomitization, is restricted to the upper part of the ZS3 carbonate unit and based on U/Pb dating took place during the Triassic, with a later phase of recrystallization linked to mid‐Jurassic uplift. Both dedolomitization and dolomite recrystallization relate to fresh‐water infiltration with the resetting of δO18 values prior to the Late Jurassic drowning of the Utsira High. The reservoir quality of the carbonates is directly linked to post‐depositional meteoric diagenesis, and the best reservoir properties are recorded in intervals dominated by recrystallized dolomites in ZS2 and lower ZS3 carbonates. Dedolomitization significantly reduced porosity in the upper ZS3 carbonates.

Dolomite recrystallization revealed by Δ47/U-Pb thermochronometry in the Upper Jurassic Arab Formation, United Arab Emirates

Abstract The process of recrystallization affecting dolomitic successions remains a longstanding enigma in carbonate research. Recrystallization influences the accuracy of genetic dolomitization models as well as the prediction of porosity and permeability distribution within dolomitic reservoirs. We investigate early-formed dolomites of the Upper Jurassic Arab Formation reservoir (Arabian Platform, United Arab Emirates), where recrystallization is not easily ascertained based on petrographic and O-C-Sr isotope analyses. Conversely, the application of Δ47/U-Pb thermochronometry revealed the occurrence of burial recrystallization over a temperature-time interval of ~45 °C/45 m.y. during the Early and Late Cretaceous. The process was initially driven by Late Jurassic mixed marine-meteoric fluids, which evolved during burial in a closed hydrologic system and remained in thermal equilibrium with the host rocks. Recrystallization was a stepwise process affecting the succession heterogeneously, so that samples only few meters apart presently record different temperature-time stages of the process that stopped when hydrocarbons migrated into the reservoir. Our results illustrate how Δ47/U-Pb thermochronometry may provide a novel approach to unravel dolomite recrystallization and to precisely determine the timing and physicochemical conditions (temperature and δ18Ow) that characterized the process. Therefore, this study paves the way for better appraisal of recrystallization in dolomitic reservoirs.

Pore Variation Characteristics of Altered Wall Rocks in the Huize Lead–Zinc Deposit, Yunnan, China and Their Geological Significance

The porosity and permeability of the rock surrounding lead–zinc deposits are key factors for controlling the migration and precipitation of ore-forming hydrothermal fluid. In this paper, the Huize super-large lead–zinc deposit was taken as the case study, and variations in the porosity and permeability of the wall rocks and their relationship with the orebody were analyzed by using CT scanning technology. The experimental results showed that the average pore radius of dolomite with a decreasing distance to the orebody ranged from 1.60 to 1.65 μm, increasing to 1.77~2.05 μm. The CT porosity increased from 2.76%–2.81% to 3.35%–3.99%. The average pore throat length decreased from 29.57–39.95 μm to 13.57–16.83 μm. In the research, it was found that the hydrothermal fluids rich in chemical elements changed the properties of the surrounding rocks. Temperature rise will lead to dolomitization of limestone and recrystallization of dolomite. This process led to an increase in the porosity of the wall rocks. During the formation of the orebody, the metal minerals in the hydrothermal fluid entered the pores of the rock. As a result, the pore radius and pore volume of the wall rocks were reduced, along with the pore throat radius and pore throat length. Therefore, the wall rock pores near the orebody were isolated from each other, and the permeability of the surrounding rock decreased. The variation characteristics for the porosity and permeability of the dolomite at various distances from the mine can be used to discover orebodies.

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.

Petrographic, Rare Earth Elemental and Isotopic Constraints on the Dolomite Origin: A Case Study from the Middle-Upper Cambrian Xixiangchi Formation in Eastern Sichuan Basin, Southwest China

The Middle-Upper Cambrian Xixiangchi Formation in the Sichuan Basin is regarded as an important reservoir with great potential for hydrocarbon exploration. It is previously indicated that the Xixiangchi carbonates have experienced extensive dolomitization, however, the origin of dolomitizing fluids and the dolomitization mechanism still remain uncertain. In this study, a set of petrographic and geochemical examinations, including rare earth elements (REE) and isotopic (C, O, and Sr) compositions were used to trace the origins of dolomitizing fluids and associated diagenetic processes. The petrographic examination revealed three types of matrix dolomites (D1, D2, D3) and one cement saddle dolomite (SD). These phases have crystal size ranges of less than 30 μm (very fine to fine crystals, D1), 30–100 μm (fine to medium crystals, D2), 100–300 μm (medium to coarsely crystalline dolomite, D3), and 0.3–4 mm (fracture filling cements, SD), respectively. D1 is characterized by non to very weak luminescence, weakly negative Ce anomalies (Ce/Ce* = 0.84 ± 0.02), strongly negative Eu anomalies (Eu/Eu* = 0.65 ± 0.03), and high 87Sr/86Sr ratios (0.71062 ± 0.00122). In combination with δ13C (−1.5‰ ± 0.2‰) and δ18O (−9.7‰ ± 0.5‰) compositions, D1 is interpreted to be formed by penecontemporaneous dolomitization in the near-surface environment with seawater as the dolomitizing fluid. In contrast, D2 is characterized by intercrystalline pores, dirty crystal surfaces, similar δ13C (−1.4‰ ± 0.4‰) compositions but higher δ18O (−8.9‰ ± 0.7‰) compositions, and lower 87Sr/86Sr ratios (0.70992 ± 0.00035), similar Ce anomalies (Ce/Ce* = 0.87 ± 0.04) and higher Eu anomalies (Eu/Eu* = 0.85 ± 0.04). The coarser D2 is regarded to be formed by the post-penecontemporaneous seepage-reflux dolomitization or by recrystallization of D1 dolomite in a near-surface or shallow burial environment. D3 is distinguished by a cloudy core with clear rims, showing slightly higher Eu anomalies (Eu/Eu* = 0.88 ± 0.02) and similar Ce anomalies (Ce/Ce* = 0.88 ± 0.02) than those of D1 and D2. Combined with the δ18O compositions (−10.4‰ ± 0.4‰) and 87Sr/86Sr ratios (0.70989 ± 0.00048), D3 is thought to be formed by the overgrowth or recrystallization of D1 and D2 dolomites in a shallow to moderate burial environment. The fractures filling SD dolomite consists of nonplanar and much coarser crystals with undulatory extinctions and brighter red luminescence. The lower δ18O (−11.1‰ ± 0.3‰) compositions, lower negative Eu anomalies (Eu/Eu* = 0.70 ± 0.01) of SD than the matrix dolomites, and similar Ce anomalies (Ce/Ce* = 0.83 ± 0.01) are indicative of hydrothermal dolomitization, with possible fluids associated with the magma during the period of Emei taphrogenic movement. In addition, the 87Sr/86Sr ratios (0.70941 ± 0.00003) of SD suggest probable origin from the coeval seawater partially. Therefore, SD dolomite is interpreted to be formed by hydrothermal dolomitization with mixed dolomitizing fluid of seawater and hydrothermal fluids. In summary, all the matrix dolomites have almost the same ΣREE concentrations and exhibit similar near-flat REE partition patterns with weak LREE enrichments, weakly negative Ce anomalies, and negative Eu anomalies. Such characteristics of REE compositions are indicative of similar evolved dolomitizing fluid, such as seawater or seawater- derived fluids. By contrast, SD dolomites have a different REE partition pattern with left-leaning characteristics, LREE depletions, and negative Eu anomalies, thus suggesting a different dolomitizing fluid source from the matrix dolomites. In addition, the development of intercrystalline pores associated with D2 dolomite makes it more likely to be a potential reservoir, indicating that the dolomitizing history of carbonate has a strong influence on the quality of potential dolomite reservoirs.

Genesis of Cambrian Dolomites in the Bachu Area, Tarim Basin, NW China: Constraints from Petrology, Geochemistry, and Fluid Inclusions

The dolomitization of carbonate rocks has always been a hot topic in the study of the dolomite reservoir. In this study, the genesis of Cambrian dolomite in the Bachu area, Tarim Basin, was assessed through petrographic examinations, isotope compositions (C, O, and Sr), trace and rare earth elements, and fluid inclusion microthermometry. Microscopic analysis revealed three types of dolomites: very fine-crystalline, nonplanar dolomite (D1); fine-crystalline, nonplanar to planar-s dolomite (D2); and medium- to coarse-crystalline, planar-e to planar-s dolomite (D3). D1 dolomite exhibits well-preserved original sedimentary features, such as algal laminae, stromatolite, and evaporite streak, and is characterized by the 87Sr/86Sr value and δ18O value in equilibrium with the coeval seawater, its high Sr and Na content, and its low Mn content. This indicates that D1 dolomite is primarily a penecontemporaneous dolomite in tidal flat or lagoon environments, and its dolomitizing fluid is mainly evaporated mesosaline to penesaline seawater. D2 dolomite shows ghosts of precursor particles; features δ13C values in equilibrium with the coeval seawater, high 87Sr/86Sr values, low Sr content, and positive Eu anomaly; and is widely distributed close to stylolite. This illustrates that D2 dolomite was principally formed by seepage–reflux dolomitization, and is closely related to hydrothermal activity and pressure dissolution. D3 dolomite displays a crystal texture with a cloudy core and compositional zoning, and the original sedimentary fabrics cannot be identified. It has similar δ13C values and REE patterns to the calcite precipitated from coeval seawater, high 87Sr/86Sr values, low Sr contents and high Mn/Sr ratios, which suggests that D3 dolomite is chiefly related to the recrystallization of the precursor dolomite during the deep burial stage, and the deep circular brine provides Mg ions through the fluid–rock reaction. This study shows that the Cambrian dolomite in the Bachu area is mainly formed in the coeval seawater environment during the penecontemporaneous and shallow burial stages, and has extensively suffered from recrystallization and burial diagenesis due to long-term deep burial, which was further strengthened in the fracture-enriched area.

Mineralization of the Bayan Obo Rare Earth Element Deposit by Recrystallization and Decarbonation

AbstractThe genesis of the Bayan Obo giant rare earth element (REE) deposit has been debated for several decades. Here, we report the isotopic effects of dynamic recrystallization in the H8 carbonatite, which is the principal ore carrier in the deposit. We studied fresh drill core to a depth of 1.78 km and documented the elemental and C-O-Sr isotope evolution of rock-forming dolomite during its deformation and reaction with fluids. The precursor dolomite and the products of its recrystallization differ in δ13CVienna-PeeDee Belemnite (V-PDB) (–1.09 to 2.37 vs. –3.59 to 0.79‰, respectively) and 87Sr/86Sr (0.70241–0.70394 vs. 0.70288–0.71409, respectively), and show a similar δ18Ovienna-standard mean ocean water (V-SMOW) range (10.3–16.9‰). The strong negative shift in δ13CV-PDB indicates that, locally, there was as much as 40% loss of CO2 from the precursor dolomite, although most of the recrystallized dolomite experienced decarbonation on a smaller scale. Clumped monazite grains associated with apatite in paragenetically similar samples yielded variable in situ Th-Pb dates (980–340 Ma), whereas those in monomineralic veinlets give a consistent age of ~400 Ma and consistent initial Nd isotope ratios. This indicates that the wide range of dates may not represent real REE depositional events and that the primary REE minerals deposited in the Mesoproterozoic underwent isotopic reequilibration and REE remobilization in the mid-Paleozoic. Recrystallization and decarbonation of dolomite in the H8 unit were facilitated by its reaction with subduction-derived silica- and halogen-rich fluid, genetically linked to plate-convergence processes along the northern margin of the North China craton, and did not require an influx of REEs from an external source.

Extensive recrystallization of Cenozoic dolomite during shallow burial: A case study from the Palaeocene–Eocene Umm er Radhuma formation and a global meta‐analysis

AbstractRecrystallization of dolomite can alter textural, mineralogical and geochemical attributes used to infer environmental conditions of initial dolomitization. A meta‐analysis of the published literature shows that extensive recrystallization is most common in old and deeply buried dolomites, but data from the Palaeocene–Eocene Umm er Radhuma Formation in Qatar show that this can also happen in geologically young carbonates that have never been deeply buried. Evidence of extensive recrystallization comes from a principal component analysis of published mineralogical and geochemical data that are integrated with new clumped isotope (∆47) measurements and cathodoluminescence petrography. The observations indicate that dolomite stoichiometry and cation ordering correlate with dolomite texture and depth: shallow mimetic dolomites are less stoichiometric and poorly‐ordered whereas coarser and deeper planar‐e and planar‐s to nonplanar dolomites with mottled cathodoluminescence signatures are more stoichiometric and well‐ordered, suggesting they have recrystallized. Dolomite δ18O data (Vienna Pee Dee Belemnite) also covary with texture, as mimetic dolomites have the narrowest range (−0.45‰ to +0.38‰), planar‐e dolomites are generally ≤0‰ (−2.4‰ to +0.68‰) and planar‐s to nonplanar dolomites exhibit the widest range (−2.3‰ to +1.4‰). The principal component analysis results indicate one component with positive loadings for dolomite stoichiometry (+0.752) and cation ordering (+0.813), and negative loading for δ18O (−0.833), trends predicted in recrystallized dolomites. The ∆47‐derived temperatures (32 to 46°C) and δ18Ow (+0.18 to +2.6‰ Vienna Standard Mean Ocean Water) reflect recrystallization by warm, slightly evaporated seawater at shallow depths. In the context of a meta‐analysis of recrystallized dolomites in the literature, this study implies that extensive dolomite recrystallization in Cenozoic dolomites and shallow‐burial conditions is an under‐appreciated phenomenon. The results of this study also demonstrate that initial petrological attributes can be significantly altered after dolomitization, hindering interpretations of past seawater chemistry and environmental conditions of dolomitization.

Stylolites and stylolite networks as primary controls on the geometry and distribution of carbonate diagenetic alterations

There is ongoing debate on whether stylolites act as barriers, conduits, or play no role in fluid transport. This problem can be tackled by examining the spatial and temporal relationships between stylolites and other diagenetic products at multiple scales. Using the well-known Lower Cretaceous Benicàssim case study area (Maestrat Basin, E. Spain), we provide new field and petrographic observations of how bedding-parallel stylolites can influence different diagenetic processes during basin evolution. The results reveal that stylolites can serve as baffles or inhibitors for different carbonate diagenetic reactions, and act as fronts for dolomitization, dolomite recrystallization and dolomite calcitization processes. Anastomosing stylolites that pre-date burial dolomitization probably acted as a collective baffle for dolomitization fluids in the study area, resulting in stratabound replacement geometries at the metre-to-kilometre scale. The dolomitization front coincides with stylolites, and can be traced along consecutive anastomosing ones. Such anastomosing stylolites are typical of mud-dominated facies that characterize limestone-dolostone transition zones. Conversely, dolostone bodies tend to correspond to more grain-dominated facies characterized by parallel (non-anastomosing) stylolites. Stylolites subsequently acted as fluid flow conduits and barriers when the burial and stress conditions changed. Stylolitic porosity enhanced by dissolution within dolostones close to faults appears filled with saddle dolomite riming the stylolite pore, and high-temperature blocky calcite cements filling the remaining porosity. The fluids responsible for these reactions were likely released from below at high pressure, causing hydraulic brecciation, and were channelised through stylolites, which acted as fluid conduits. Stylolites are also found acting as baffles for subsequent dolomite calcitization reactions during meteoric diagenesis and occasionally appear filled with iron oxides likely released by calcitization. This example demonstrates how the same type of stylolites (bedding-parallel) can act as barriers/inhibitors and/or conduits for different types of diagenetic reactions through time, and how important it is to consider their collective role when they form networks.

Dolomitization of Paleozoic Successions, Huron Domain of Southern Ontario, Canada: Fluid Flow and Dolomite Evolution

Integrated petrographic, isotopic, fluid inclusion microthermometry, and geochemical analyses of Paleozoic carbonate successions from multiple boreholes within the Huron Domain, southern Ontario were conducted to characterize the diagenetic history and fluid composition, on a regional scale, and evaluate the nature and origin of dolomitized beds. Multiple generations of non-stochiometric dolomite have been observed. These dolomites occur as both replacement (D1 and D2) and cement (saddle dolomite; SD) and formed either at near-surface to shallow burial zone (D1) or intermediate burial (D2 and SD). Petrographic and geochemical data of dolomite types and calcite cement suggest that these carbonates have experienced multiple fluid events that affected dolomite formation and other diagenetic processes. Cambrian and Ordovician strata have two possibly isolated diagenetic fluid systems; an earlier fluid system that is characterized by a pronounced negative shift in oxygen and carbon isotopic composition, more radiogenic Sr ratios, warm and saline signatures, higher average ∑REE compared to warm water marine brachiopods, negative La anomaly, and positive Ce anomaly; and a later Ordovician system, characterized by less negative shifts in oxygen and carbon isotopes, comparable Th, hypersaline, a less radiogenic, less negative La anomaly, and primarily positive Ce anomaly but also higher average ∑REE compared to warm water marine brachiopods. Ordovician, Silurian, and Devonian Sr isotopic ratios, however, show seawater composition of their respective age as the primary source of diagenetic fluids with minor rock/water interactions. In contrast, the isotopic data of the overlying Silurian and Devonian carbonates show overlaps between δ13C and δ18O values. However, δ18O values show evidence of dolomite recrystallization. D2 shows wide Th values and medium to high salinity values. Higher Th and salinity are observed in SD in the Silurian carbonates, which suggest the involvement of localized fluxes of hydrothermal fluids during its formation during Paleozoic orogenesis. Geochemical proxies suggest that in both age groups the diagenetic fluids were originally of coeval seawater composition, subsequently modified via water-rock interaction possibly related to brines, which were modified by the dissolution of Silurian evaporites from the Salina series. The integration of the obtained data in the present study demonstrates the linkage between fluid flux history, fluid compartmentalization, and related diagenesis during the regional tectonic evolution of the Michigan Basin.

Statistically learning Archean carbonate diagenesis

Geochemical data are often very noisy due to the large natural heterogeneity of the geological materials as well as oversimplification of rock classification. This has serious repercussions on the precision of our knowledge of the deep past as we often rely solely on geochemical proxies to investigate the geological evolution of Archean and Proterozoic environments. Here statistical learning procedures were applied to achieve unbiased classification of Neoarchean stromatolitic dolostone textures on the basis of the distribution of their trace elements and rare earth elements (REE) investigated through laser ablation induced coupled plasma – mass spectrometry (LA-ICP-MS). Multivariate statistical analyses and supervised statistical learning have revealed that different dolomite fabrics, thought as products of aggrading diagenesis and recrystallization, are in fact chemically indistinguishable. The diagenetic processes that cause the re-crystallization of dolomite and the consequent change of textures, is not affecting the distribution of major and trace elements inherited by the depositional environment or during early stages of diagenesis. At the same time the algorithm has revealed that an optically homogeneous microcrystalline dolomite sample may in fact be geochemically inhomogeneous because of processes of ripening and recrystallization occurred at an early stage of marine diagenesis and that have contributed to element mobilization. Statistical learning has succeeded in recognizing chemofacies which not always overlap with dolomite textures and fabrics highlighting the importance of crystallographic and diagenetic studies before any study of carbonates as geochemical proxies.

The interaction of tectonics, climate and eustasy in controlling dolomitization: A case study of Cenomanian–Turonian, shallow marine carbonates of the Iberian Basin

abstractDuring the Cretaceous, high global sea‐level and low latitudinal temperature variations led to the growth of epeiric carbonate platforms. Platform‐scale dolomitization of these platforms is not common, reflecting the low Mg/Ca ratio of seawater and a humid climate. This study describes the processes governing pervasive dolomitization of a land‐attached carbonate platform within the Iberian Basin. Dolomite is planar to sub‐planar with a geochemical signature consistent with dolomitization from penesaline seawater. Dolomitization was most pervasive during a 1 Myr period in the middle Cenomanian, by repeated reflux of seawater from brine pools formed on the top of a southward‐prograding carbonate platform. Tilting and structural reorganization in the Upper Cenomanian led to a reversal in polarity of the platform, and dolomitization was restarted by the northward reflux of seawater. Rising relative sea‐level and oceanic acidification led to back‐stepping of the platform such that the supply of dolomitizing fluids was cut off. In the Lower Turonian, pervasively dolomitized rudist rudstone facies in the south of the study area indicate that dolomitization restarted, either penecontemporaneously or later, from highly evaporated Campanian–Maastrichtian seawater. A systematic increase in dolomite crystal size up‐section ties broadly, but not entirely, to stratigraphy. It is possible that these textural differences reflect changes in fluid chemistry, limestone permeability or precursor rock texture. However, the lack of stratigraphic conformance, and the preservation of the earliest‐formed dolomite only in the oldest sediments, could indicate a progressive recrystallization of early‐formed dolomite through repeated reflux of brines. As such, the succession appears to preserve a fossilized record of dolomite recrystallization through time during the Cenomanian–Turonian. The results of this study therefore provide a record of the progressive dolomitization of a carbonate platform and demonstrate the important interplay of climate and basin‐scale tectonics on dolomite distribution and crystallinity.

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.

Origin and characteristics of grain dolomite of Ordovician Ma55 Member in the northwest of Ordos Basin, NW China

The origin of grain dolomite in M55 Member of Ordovician Majiagou Formation in northwestern Ordos Basin was studied by geochemical and petrological tests on core samples. Observation of cores, thin sections and casting thin sections, analysis of cathodoluminescence, X-ray diffraction, microscopic sampling of trace elements, laser sampling δ18O and δ13C, and fluid inclusion homogenization temperature were conducted. The results show that the dolomite is the product of recrystallization of micritic to crystal powder dolomite rather than the product of dolomitization of grain limestone. In the spherical grains are residual gypsum and halite pseudo crystals identical with those in the host micritic dolomite. The spherical particles of dolomite has similar trace elements and δ18O and δ13C characteristics to micritic dolomite. Furthermore, Mn/Sr ratio of the fine—medium dolomite between the dolomite grains is about 5-8, while Mn/Sr ratios of calcite in limestone, micritic dolostone in micritic dolomite, and micritic and powdery dolomite are about 0-2, indicating that the dolomite experienced strong diagenesis. Homogenization temperature of inclusions of fine—medium dolomite is about 148. 19 °C, higher than that of inclusions in micritic to crystal powder dolomite (about 122.60 °C), which also supports the conclusion that the grain dolomite experienced burial diagenesis and negative shift of δ18O and δ13C. The δ18O, δ13C values of micritic to crystal powder dolomite match with the negative migration, but those of calcite in limestone don't. It is of great significance to elucidate the genesis of “dolomite recrystallization” for the prediction of such dolomite reservoirs.

Dolomite-illitic rocks (dolillite) – the product of hydrothermal replacement of carbonate rocks in the Holy Cross Mts., Poland – a possible guide to ores

The dolomites altered to dolomite-illitic rocks were found accompanying marcasite-pyrite-hematite mineralization within the Middle Devonian carbonate rocks in the eastern part of the Holy Cross Mts. (Poland) in the fault zones and their close vicinity. The alteration consist of dolomite recrystallization and replacement by illite, accompanied by small amounts of silica, K-feldspars and disseminated pyrite or hematite. The final product of dolomite replacement by illite are massive, dolomite-illitic rocks unevenly distributed within the unaltered dolomites or in fault breccia. They form irregular metric-sized nest-like bodies most often with blurred boundaries, or in some places sharp delineated veins and lenses. Illite crystallinity, demonstrates its hydrothermal origin. Altered rocks are characterized by elevated contents of REE, Rb, and sometimes other trace metal (Zr, Ti, Zn, U, Th, Ba and al.), compared to unaltered dolomites. The enrichment in REE, Zr and Ti implies their mobility in hydrothermal solutions. The lamprophyre intrusions present in the close vicinity may be considered as the probable source of potassium rich hydrothermal fluids. The peculiar features of dolomite-illitic rocks such as: their composition and occurrence close to the sulphide and hematite mineralization, allow supposing, that they may be a guide to deeply seated unknown ore deposits. Normal 0 21 false false false PL X-NONE X-NONE /* Style Definitions */ table.MsoNormalTable {mso-style-name:Standardowy; mso-tstyle-rowband-size:0; mso-tstyle-colband-size:0; mso-style-noshow:yes; mso-style-priority:99; mso-style-parent:; mso-padding-alt:0cm 5.4pt 0cm 5.4pt; mso-para-margin-top:0cm; mso-para-margin-right:0cm; mso-para-margin-bottom:10.0pt; mso-para-margin-left:0cm; line-height:115%; mso-pagination:widow-orphan; font-size:12.0pt; font-family:Arial,sans-serif; mso-bidi-font-family:CG Times 12pt; mso-fareast-language:EN-US;}

Rock-buffered recrystallization of Marion Plateau dolomites at low temperature evidenced by clumped isotope thermometry and X-ray diffraction analysis

Abstract Much debate exists on the extent to which early dolomites recrystallize and preserve the signature of their primary diagenetic setting. Here, we combine clumped isotopes thermometry with X-ray diffraction and thin section petrography to study dolomite recrystallization under shallow burial (