Stages Of Dolomitization Research Articles

The Paleogene Forebulge Carbonate Banks, Zagros Foreland Basin, Northern Iraq: their Paleogeography, Basin Evolution, and Economic Implications

The Paleogene sequence of Zagros foreland basin NE Iraq during the Late Paleocene- Middle Eocene is punctuated by linear and disconnected chains of carbonate bank-ramp associations. These carbonate units had developed over the NW-SE trending Paleogene basin forebuldge. Discussion of origin, facies type, and stratigraphic architecture of these banks and associated facies was fully covered by the first part of this research project. In this part the chronostratigraphic synthases of the sequence, the paleogeography, and basin evolution is examined as an integral part of the investigation of the evolution of the Paleogene Zagros foreland basin. Research methods include regional stratigraphic correlation, and biostratigraphic data compilation to infer the chronostratigraphic sequence of the events. The paleogeographic facies mapping and basin history are reviewed accordingly. Research data are collected from 15 master surface sections. In addition to the re-examination and re-interpretation of 40 previously studied surface and subsurface well logs, which are compiled for basin-wide study and regional paleogeographic mapping. The analysis of the compiled biostratigraphic data shows that the inferred age of the associated stratigraphic units is as follows: Sinjar Formation. (E. Thanetian – L. Ypresian), Khurmala Formation (Ypresian - E. Lutetian), Kolosh Formation (Daian -Thanetian), and Aaliji Formation (Danian - Ypresian). Based on this chronostratigraphic synthesis, the basin forebulge history during the Paleogene period is divided into 5 stages: the Pre-bank, Early Bank Development, Bank-ramp Development, Back-bank Lagoon, and Lagoonal dolomitization stage. The paleogeography and paleofacies maps of each stage is discussed in terms of the basin forebulge evolution. An alternative hydrocarbon play of the forebulge-controlled Paleogene sequence is discussed, with a case study from the Sufaiyah oil field of northwest Iraq.

Characteristics and main controlling factors of intra-platform shoal thin-layer dolomite reservoirs: A case study of Middle Permian Qixia Formation in Gaoshiti–Moxi area of Sichuan Basin, SW China

Based on the study of the distribution of intra-platform shoals and the characteristics of dolomite reservoirs in the Middle Permian Qixia Formation in the Gaoshiti–Moxi area of the Sichuan Basin, SW China, the controlling factors of reservoir development were analyzed, and the formation model of “intra-platform shoal thin-layer dolomite reservoir” was established. The Qixia Formation is a regressive cycle from bottom to top, in which the first member (Qi1 Member) develops low-energy open sea microfacies, and the second member (Qi2 Member) evolves into intra-platform shoal and inter-shoal sea with decreases in sea level. The intra-platform shoal is mainly distributed near the top of two secondary shallowing cycles of the Qi2 Member. The most important reservoir rock of the Qixia Formation is thin-layer fractured-vuggy dolomite, followed by vuggy dolomite. The semi-filled saddle dolomite is common in fracture-vug, and intercrystalline pores and residual dissolution pores combined with fractures to form the effective pore-fracture network. Based on the coupling analysis of sedimentary and diagenesis characteristics, the reservoir formation model of “pre-depositional micro-paleogeomorphology controlling shoal, sedimentary shoal controlling dolomite, penecontemporaneous dolomite benefiting preservation of pores, and late hydrothermal action effectively improving reservoir quality” was systematically established. The “first-order high zone” micro-paleogeomorphology before the deposition of the Qixia Formation controlled the development of large area of intra-platform shoals in Gaoshiti area during the deposition of the Qi2 Member. Shoal facies is the basic condition of early dolomitization, and the distribution range of intra-platform shoal and dolomite reservoir is highly consistent. The grain limestone of shoal facies is transformed by two stages of dolomitization. The penecontemporaneous dolomitization is conducive to the preservation of primary pores and secondary dissolved pores. The burial hydrothermal fluid enters the early dolomite body along the fractures associated with the Emeishan basalt event, makes it recrystallized into medium–coarse crystal dolomite. With the intercrystalline pores and the residual vugs after the hydrothermal dissolution along the fractures, the high-quality intra-platform shoal-type thin-layer dolomite reservoirs are formed. The establishment of this reservoir formation model can provide important theoretical support for the sustainable development of Permian gas reservoirs in the Sichuan Basin.

Multistage ore formation in the world’s largest REE-Nb-Fe deposit of Bayan Obo, North China Craton: New insights and implications

The Bayan Obo deposit in the northern margin of the North China Craton is a world-class REE-Nb-Fe deposit, the complex mineralization history of which remains unresolved. In this study, we employ SEM and Image J software combined with mineralogical data obtained by in-situ EDS and EPMA analyses to analyze petrographic images rapidly and to investigate the mineralogical assemblages of each type of rocks/ores from the Main Orebody in an attempt to understand the mineralization process and ore-formation mechanism of this deposit. We identify three metallogenic periods and six ore-forming stages based on the field occurrence of ores combined with published geochronological data. The Mesoproterozoic magmatic event comprises two stages, which were distinguished as the coarse-grained dolomite stage (stage 1) and the fine-grained dolomite stage (stage 2). Three stages were recognized in the Mesoproterozoic shear deformation-hydrothermal mineralization period, including the disseminated mineralization stage (stage 3), the banded mineralization stage (stage 4) and the massive mineralization stage (stage 5). The Paleozoic hydrothermal period involved the vein mineralization stage (stage 6). Based on detailed studies of the mineral assemblages and paragenesis, as well as the complex textures of the ores, we propose a model of the multi-stage metallogenic process as follows: (1) The REE minerals and rutile occurring as inclusions wrapped within magmatic idiomorphic dolomite grains indicate that the REE-Nb mineralization might have started during the magmatic period. (2) The Nb minerals, such as fergusonite and ilmenorutile, intergrown with REE minerals, such as monazite and bastnäsite, indicate that the Nb and REE were transported by the same ore-forming magma or fluid and precipitated during the same metallogenic process. (3) From the fine-grained dolomite stage to the massive mineralization stage (from stage 2 to stage 5), the mineral assemblages become more complex, with a gradual increase in the degree of hydrothermal alteration suggesting that more hydrothermal fluid was involved in the ore mineralization. (4) The complex mineralogical assemblages and textural relationships indicate multiple formation mechanisms for the REE-Nb-Fe minerals. We propose that the unusually large volume of REE-Nb-Fe resources in the Bayan Obo deposit was the result of combined magma and hydrothermal fluid activities in the Mesoproterozoic (from stage 1 to stage 5), whereas the fluids only caused reactivation of the ore materials previously deposited without the addition of exogenous materials in the Paleozoic stage (stage 6).

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.

Deciphering the diagenetic evolution of the lower Carnian succession in the Gorno and Pian Resinelli sulphide mining districts (Southalpine Domain, N Italy)

The stratabound Mississippi Valley-type (MVT) sulphide deposits of the Gorno and Piani Resinelli mining districts are hosted in the lower Carnian carbonate succession, mainly peritidal limestones, of the Lombardian Basin (Southalpine Domain, Northern Italy). In addition to the formation of the well-known sulphide deposits, these carbonate sediments experienced a complex diagenetic evolution, including different stages of dolomitization, silicification, dissolution, brecciation, calcite cement precipitation. Stratigraphic and petrographic features suggest that, apart from an early calcite cementation and a first penecontemporaneous dolomitization affecting the supratidal beds, diagenetic processes and sulphide precipitation, are intimately and genetically associated. They document the occurrence of a Carnian hydrothermal system developed shortly after deposition at shallow burial depth, confirmed by fluid inclusion microthermometry and U-Pb radiometric datings.

Petrography, sedimentology, palynomorphs and diagenesis of the Paleozoic Khuff and Unayzah formations in Kuwait

Integrated geological investigations using rock cores and well logs were performed on the Paleozoic Khuff and Unayzah formations to study their petrography, sedimentology and diagenesis. The formations occur at depths >15,000 ft, while the Unayzah was not reached in some wells up to 20,000 ft. Their depositional basins seem to be affected by the Kuwait Arch, where the Khuff Formation is fully penetrated only in the wells located over this Arch. The Khuff Formation shows complex lithological facies composed mainly of massive hard, fractured dolostone with frequent anhydrite veins and nodules. The Dolomite concentration ranges from 90 to 100% with varying percentage of anhydrite and low quartz percent (2.5%). Few minerals such as quartz, montmorillonite and pyrite are found at some depths. The Basal Khuff Clastics (BKC) is represented by a sequence of interbedded sand, muddy sand and carbonaceous shale with advanced diagenetic changes, deposited as calcareous grainstone within shallow marine environment (neritic zone) and has been diagenetically altered through several stages of dolomitization. Quartz concentration ranges from 5.3% to 100%, increasing with depth with relatively low percent of muscovite, clay mineral, illite and montmorillonite, microcline, pyrite and hematite. By contrast, the Unayzah Formation shows a fluvial siliciclastic sequence of conglomerate, sandstone, siltstone, mudstone and shale. Four sedimentation cycles of different thickness are recognized within the formation, each cycle starts with fining upward sandstone that grades to mudstone with shale intercalations and ends with red paleosoil sediments. The sandstone is a mixture of quartz, muscovite, and albite minerals. The polynomorphs study suggests that the formation was deposited in alluvial, fluvial, braided streams and a shallow marine environment. Signs of local incipient metamorphism are detected in the Unayzah clastics south of the Burgan Field may interpreted as a local phenomenon due to the lack of matching with the present-day normal thermal regime.

Multiphase Diagenetic Processes and Their Impact on Reservoir Character of the Late Triassic (Rhaetian) Kingriali Formation, Upper Indus Basin, Pakistan

Multiple episodes of dolomitization of the shallow marine carbonates of the Late Kingriali Formation resulted in regional scale mappable dolostone geobodies in the Kohat and Potwar sub-basins. With the exception of few unaltered patches of the host limestone, more than 90% of the carbonates of the studied formation are diagenetically altered by replacive dolomites with associated dolomite cementation. Petrographical and geochemical data interpretation reveals that during the initial stage of dolomitization, the precursor limestone was significantly modified by the fabric-retentive replacive dolomite (RD-I) and produced bulk dolostones with non-planar-a to planar-s crystals. Neomorphic recrystallization (RD-II) was observed as an overgrowth of the already formed RD-I dolomite crystals during progressive dolomitization. The seawater at shallow depths is enriched with Fe-ions due to its interaction with Fe-rich beds within the studied formation. The modified seawater actively participated in the formation of ferroan replacive dolomites (RD-III). Stable isotopic composition of the unaltered Echinoderm plates, calcite cement (CC-I), and RD-I demonstrates signatures of δ18O and δ13C within the limit of late Triassic marine seawater or modified seawater. Depletion in the stable oxygen isotopic composition (from −0.99‰ to −3.75‰ V-PDB) demonstrates that RD-II and RD-III were formed in a sequence with progressively higher temperature fluids than normal seawater. Precipitation of dolomite cements as cavity filling rhombs (DC-I) and crystal overgrowth (DC-II) with highly depleted δ18O values (−5.44‰ to −7.45‰ V-PDB) illustrates dolomite cementation at higher temperatures and greater depths. The highly depleted values of δ18O (up to −9.16‰ V-PDB) and (up to 0.42‰ V-PDB) for δ13C of saddle dolomite (SD-I) indicate the precipitation of SD-I as a cavity filling dolomite at considerable depth. Calcite cementation and calcitization actively participated in the early, middle, and late diagenetic modifications as interpreted from their petrographic and stable isotopic studies. Porosity enhancement is clearly demonstrated by dissolution, stylolization, fracturing, and replacement dolomitization. Dolomite and calcite cementation had a negative impact on the reservoir character and occluded the dolostone porosity to a greater extent.

Fluid evolution and hydrocarbon accumulation model of ultra-deep gas reservoirs in Permian Qixia Formation of northwest Sichuan Basin, SW China

The fluid evolution and reservoir formation model of the ultra-deep gas reservoirs in the Permian Qixia Formation of the northwestern Sichuan Basin are investigated by using thin section, cathodoluminescence, inclusion temperature and U-Pb isotopic dating, combined with gas source identification plates and reservoir formation evolution profiles established based on burial history, thermal history, reservoir formation history and diagenetic evolution sequence. The fluid evolution of the marine ultra-deep gas reservoirs in the Qixia Formation has undergone two stages of dolomitization and one phase of hydrothermal action, two stages of oil and gas charging and two stages of associated burial dissolution. The diagenetic fluids include ancient seawater, atmospheric freshwater, deep hydrothermal fluid and hydrocarbon fluids. The two stages of hydrocarbon charging happened in the Late Triassic and Late Jurassic–Early Cretaceous respectively, and the Middle to Late Cretaceous is the period when the crude oil cracked massively into gas. The gas reservoirs in deep marine Permian strata of northwest Sichuan feature multiple source rocks, composite transportation, differential accumulation and late finalization. The natural gas in the Permian is mainly cracked gas from Permian marine mixed hydrocarbon source rocks, with cracked gas from crude oil in the deeper Sinian strata in local parts. The scale development of paleo-hydrocarbon reservoirs and the stable and good preservation conditions are the keys to the forming large-scale gas reservoirs.

Microfacies Analysis and Diagenetic Assessment of the Late Oligocene- Early Miocene Succession in Khabaz Oilfield, Northern Iraq

The Jeribe and Azkand formations, which were deposited during Late Oligocene-Early Miocene are a major reservoir interval in the Khabaz oilfield where it is, for the most part, a carbonate reservoir. The study area is located in the north of Iraq within the Foothill Zone. Through the thin-section examination for Azkand/Anah carbonates in the Khabaz oil field (Kz-3, Kz-4, Kz-10, and Kz-43), the petrographic and microfacies have been studied, which show six major carbonate microfacies, each of them is representing a distinctive deposition environment, and these are: restricted and semi-restricted for the Jeribe Formation, and semi-restricted, back reef, reef, fore reef, shallow open marine and slope for Azkand and Anah formations. There are several types of diagenetic processes; some of these processes enhance the porosity and the other reduces it. Porosity reduction by: Cementation (Isopachous, meniscus, syntaxial calcite, Drusy calcite, botryoidal, blocky calcite), Recrystallization; Compaction and late stage of dolomitization. This appeared near the upper and lower contact of the Jeribe Formation and in the lower part of the Azkand Formation. While the porosity enhancement which represented by depositional process (primary porosity) and dissolution (secondary porosity), in addition to the early and middle stage of dolomitization. This appeared in the middle part of the Jeribe Formation and the upper part of Azkand.

Diagenesis of Palaeozoic-Mesozoic Tethys Ocean Carbonate succession- Oman Mountains

This paper studies the uppermost unit of Kharus Formation (Cambrian) and the Autochthonous Akhdar Group (Permian-Triassic), which unconformably covers the pre-Permian strata. The petrographic and geochemical as well as field observations indicate that the succession underwent different stages of dolomitization that produced rocks inheriting the original host rock textures and structures (fabric-preserving dolomitization) and rocks with complete obliteration of the pre-existing textures (fabric-destroying dolomitization). Dolomites that retain the original fabric of the limestone are indicators of the host rock mineralogy, i.e., whether it was made up of high Mg-calcite or aragonitic allochems and indicate early dolomitization. The top part of the Kharus Formation consists of pervasively dolomitized units, whereas dolomites belonging to the Autochthonous Akhdar Group display variable degrees of structural and textural preservation. The evidence suggests very early dolomitization in a relatively short time interval for the Permian-Triassic carbonates. The preserved depositional features in the Permian-Triassic carbonates indicate deposition in shallow marine environments with variable energy levels. Seven facies are inferred: stromatolites, mudstones, wackestones, intraformational breccias, grainstones, packstones and grain/packstones. Petrographic as well as field observations exclude evidence of evaporites within Palaeozoic-Mesozoic rocks. Five paragenetic phases are determined to explain the type of dolomitization and to indicate the type and severity of diagenesis that affected the Palaeozoic-Mesozoic Tethys Ocean carbonates from the Oman Mountains.

Dolomitization of the lower cretaceous carbonate reservoir in the Euphrates Graben, Syria

Results of this study are based on core materials description, thin sections, Cathodoluminescence (CL), and Scanning Electron Microscope (SEM) examinations. The Lower Cretaceous over the Euphrates Graben area was characterized by carbonate sedimentation in shallow marine environments. The low energy lagoonal to inner shelf sediments of the Judea Formation includes micritic mudstone to wackestone texture, dolomitic limestones and dolostones. Two types of dolomites recognized in the carbonates of the Judea Formation, the preserving microcrystalline dolomites which commonly founded in the partially dolomitized micritic limestones, and the destructive coarsely crystalline dolomites which commonly founded in the dolostones and dolomitic limestones. Petrographic examinations indicate that the preserving microcrystalline dolomites represent subtidal cycles developed in a shoal to open marine depositional environments, they probably formed under conditions of the shallow burial diagenesis. The destructive coarsely crystalline dolomites may develop in more basinward, open marine environments under conditions of the deep burial diagenesis that accompanied by rising in temperature, pressure, and burial depth. It is believed that evolution of the diagenetic history of the Judean Formation sediments occurred in two diagenetic stages; the shallow burial diagenesis, and the deep burial diagenesis. Compaction processes, early fracturing, micritization, early calcite, and the early phase of dolomitization were part of the multiple diagenetic alterations during the shallow burial diagenesis. The deep burial diagenesis was marked by dissolution, late stage of dolomitization and replacement, mechanical and chemical compaction, and the late calcite precipitation. It is believed that dolomitization of the Judea Formation carbonates in the Euphrates Graben has contributed to improving the reservoir properties by increasing the porosity and thus the permeability.

Genesis of Dolomite in Middle Permian Maokou Formation in Eastern Sichuan: Constraints from In Situ Geochemistry, Sr-Mg Isotopes, and Fluid Inclusions

The dolostone reservoir of the Middle Permian Maokou Formation in Eastern Sichuan has good prospects for oil and gas exploration. Study of dolomitizing genesis of the Maokou Formation is essential for predicting the distribution of the dolostone reservoir. Petrography, in situ geochemistry, Sr-Mg isotopes, and fluid inclusions were carried out on samples from the Maokou Formation in Eastern Sichuan in order to discuss the dolomitizing process. Based on mineral and textural characteristics, dolomites were divided into four components: partially clouded dolomite (PCD), mosaic-like dolomite (MLD), cloudy-centered and clear-rimmed dolomite (CACD), and saddle dolomite (SDD). Results indicate that the Maokou Formation in Eastern Sichuan mainly experienced two stages of dolomitization. PCD, MLD, and cloudy-centered dolomite (CCD) were formed during the early dolomitization. They all show turbid crystal planes and bright orange-red CL and have similar trace element contents, 87Sr/86Sr ratios, and rare-earth patterns, indicating that they might be formed in the same fluid. This is a period when dolomitizing fluids mainly migrated along pores or microcracks and replaced protogenetic calcites, which occurred in the shallow burial stage of the Maokou Formation before the Late Permian. Clear-rimmed dolomite (CRD) and SDD were formed in the late stage of dolomitization. They all have clean crystal planes and darkly red CL. CRD of the ERY profile has trace element contents, 87Sr/86Sr ratios, and rare-earth patterns similar to SDD of the HLCH profile and Well TL6, inferring that both may be formed in the same fluid. Combined with high SrO contents and homogenous temperatures of fluid inclusions of CRD and SDD and Mg-isotopic compositions, they were generated by hydrothermal dolomitization. The hydrothermal fluid stage is related to the movement of the Emeishan Large Igneous Province, which was made up of basaltic magmatic fluids mixing with the surface water. The hydrothermal fluid mainly migrated upwards along structural fractures or faults and filled in structural fractures, occurring in the Late Permian to Middle-Late Triassic.

Structural control and timing of evaporite-related Mississippi Valley-type Zn–Pb deposits in Pucará Group, northern central Peru

The South America MVT belt comprises the deposits of San Vicente, Shalipayco, Florida Canyon, and some minor Zn–Pb occurrences hosting in the carbonates and evaporites of the Pucará Group, in an extension of more than a thousand kilometers from the center to the north of Peru. Structural constraints of the MVT Peruvian deposits are conditioned by deep sub-vertical secondary extension structures with general N, NNE or NNW direction related to strike-slip movements superimposed on previous Andean NW thrust structures. Recent studies on Shalipayco and Florida Canyon deposits point to similar processes of rock formation, diagenesis and mineralization acting in province scale. New isotopic data of Pb–Pb and Rb–Sr in sulfides summed with a compilation of the available data for the evaporite-related MVT deposits and studies on the structural and tectonic evolution of the Peruvian Andes allowed establishing correlations of local process of the deposits to the regional Andean tectonic events. The Florida Canyon saline dome structure was attributed to Juruá Orogeny (157–152 Ma). The stage of dolomitization forming the porous dolostone and evaporite breccia during burial diagenesis is attributed to the period between Juruá and Mochica stages. The oil migration to these reservoir rocks occurred probably during Mochica event (100–95 Ma). The Rb–Sr dating of sphalerite from the Florida Canyon deposit together with temporal field relations indicated that the Zn–Pb sulfide ore formed in or just after Peruvian Orogeny (86-83 Ma).

Hydrothermal Dolomitization on Devonian to Carboniferous Carbonates in Kinta Valley, Perak, Malaysia: A Petrographic Study

DOI:10.17014/ijog.7.1.25-39Thick Devonian carboniferous carbonates had been deposited in a shallow marine before they were uplifted by granitic intrusion during Triassic. This carbonate platform was selectively dolomitized along N-S oriented deep-seated fault that presented in the carbonate platform. Dolostones are characterized into significant matrix dolomites and several cement dolomites. In petrography point of view, matrix dolomites comprise very fine- to fine-crystalline nonplanar-anhedral dolomite (Dolo-I), fine- to coarse-crystalline nonplanar-anhedral to planar dolomite (Dolo-II), and fine- to medium-crystalline planar- dolomite (Dolo-III). Cement dolomites also consist of medium- to coarse-crystalline planar saddle dolomite (Dolo-IV) which halfway or totally fill disintegration vugs and breaks and coarse- to very coarse-crystalline nonplanar to planar dedolomite (Ded-I). Matrix dolomites predated cement dolomites, later infilled with sepiolite and calcite. The origins of matrix and cement dolomites and other diagenetic minerals are interpreted based on the petrography and isotopic signatures by a previous worker. Dolo-I dolomite was initiated by the early stage of dolomitization with the replacement of calcite by first driving fluids from low to high temperature of magmatic source. While Dolo-II dolomite was formed with slightly modified from Dolo-I and more Mg concentration inputs. Dolo-III dolomite was likely the consequence of hydrothermal fluids causing a brecciation textures as dolomite precipitated rapidly. Subsequently, in a high temperature, the Dolo-III dolomite was formed by precipitation of cement dolomite, hydrothermal fluids as evidenced by highly depleted values of isotopic δ18O. This study intends to provide useful information for understanding the dolomitization processes in alteration of hydrothermal related to Palaeozoic carbonates within Kinta Limestone.

Geochemical constrains on dolomitization pathways of the Upper Jurassic carbonate rocks in the Geneva Basin (Switzerland and France)

The Kimmeridgian-Tithonian carbonate rocks of the Geneva Basin represent potential reservoirs for geothermal energy exploitation. Based on petrographic data, a previous study (Makhloufi et al. 2018) reported three different stages of dolomitization affecting these carbonate rocks, followed by dedolomitization. The present study focuses on the geochemical characterization of these stages based on O, C and Sr isotopes of dolomites and dedolomites. The oxygen isotopic values of early dolomite are depleted compared to the values of Late Jurassic marine cements. This is interpreted to reflect dolomite precipitation from an oxygen-enriched fluid, likely evaporitic. Carbon isotopic values are close to the composition of well-preserved Late Jurassic cements suggestive of abiotic precipitation. These findings are consistent with a scenario of reflux type dolomitization induced by high-frequency sea-level changes producing pulses of dolomitizing brines. Late dolomitization represents an advanced level of replacement. Isotopic data exhibits depleted oxygen composition pointing towards burial diagenesis and is interpreted as the results of shallow burial over-dolomitization. Ages provided by radiogenic strontium isotopes data are consistent with an early first stage of dolomitization followed by late burial dolomite. Dedolomitization is observed at different orders of magnitude and might results from the interaction with meteoric water initiating the dissolution of both early and late dolomites. This dedolomitization would have taken place during long-term emersion events or after the exhumation. The results presented in this work provide further understanding of the processes involved in dolomitization under the influence of high-frequency sea-level fluctuations and the evolution of dolomitic fabrics during burial.

Hydrothermal dolomitization and porosity development: An example from Precambrian dolomitic rocks of Água Clara Formation, Ribeira Belt, southern Brazil

Dolomitic rocks from the Rio Bonito Quarry, located in the Água Clara Formation, record multistage dolomitization processes linked with porosity development in the southern Brazilian Shield. Dolomitic marble and breccia samples from this unit contain three textural types of dolomite that either are fabric destructive or void-filling phases. To study the role of dissolution in the porosity development in these dolomite types - from sedimentary to metamorphic domains - we combine petrographic methods, cathodoluminescence, Raman spectroscopy, micro-computed tomography, stable isotopes of carbon and oxygen, and fluid inclusion analysis. The protoliths of the dolomitic rocks of Rio Bonito Quarry are assumed as Mesoproterozoic (ca. 1.5 Ga) marls presenting shallow shelf to deep water lithofacies. The first dolomitization stage, forming a nonmimic replacement of the primary carbonate (Dol1) is fabric destructive and related to a burial setting (δ18O: -12.42 to - 9.68‰; δ13C: -2.29–0.40‰). The development of bedding-parallel stylolites of low amplitude led to increasing of porosity (up to 0.2%) and promoted the fluid circulation. During the Neoproterozoic Brasiliano tectonic events (ca. 650-550 Ma), the metamorphism of marls into dolomitic marble is intimately connected to the development of second replacement dolomite (Dol2). Penecontemporaneous brecciation is assumed as triggering factor to the fluid change, where Dol2 is akin to dolomites from MVT-deposits (δ18O: -19.12 to −12.03‰; δ13C: -0.07 to −2.17‰), and the total porosity is enhanced to 2%. The ubiquitous tectonic stress boosted the hydraulic fracturing, providing conditions to saddle dolomite (Dol3) precipitation, occurring as a replacement and void-filling phase. Lower isotopic values of Dol3 (δ18O: -14.93 to −10.77‰; δ13C: -3.36 to −1.89‰) suggest disequilibrium between the host-rock and the dolomitization fluid, while the porosity is augmented to 9%. The subsequent dedolomitization, generating void-filling calcite, is associated to meteoric phreatic input (δ18O: -8.31 to −5.56‰; δ13C: -5.59 to −2.56‰), similarly to the development of calcretes; therefore, diminishing the porosity towards 4%. Microthermometric data shows different signatures in the dolomitization fluid, which started at 100–150 °C and moderate-salinity (3–4 wt% eq.NaCl) in a burial setting, reaching up to 270 °C and low-salinity (0-2 wt% eq.NaCl) as a response to a shifting tectonic environment or fluid mixing. Additionally, the temperature shift between the host rock (120–130 °C) and the different dolomitization stages (from 140-150° to 210–270 °C), above 10 °C offset, point to a hydrothermal setting.

Multiphase dolomitization in the Jutana Formation (Cambrian), Salt Range (Pakistan): evidences from field observations, microscopic studies and isotopic analysis

Excellent dolomite exposures are observed in the eastern Salt Range (Pakistan), where the Cambrian Jutana Formation consists of two distinct units (i.e. oolitic – pisolitic unit and massive dolomite unit). Field observations revealed that the lower, oolitic-pisolitic unit mostly comprises medium to thick bedded, interlayered brown yellowish dolostone containing ooids/pisoids and faunal assemblages, and grey whitish sandstone with distinct depositional sedimentary features (i.e. trough-, herringbone- and hhummocky crossbedding). The upper massive dolostone unit consists of thick bedded to massive dolostone. These two units are separated by shale. Petrographic studies identified three dolomite types, which include: fine crystalline dolomite (Dol. I), medium-coarse crystalline dolomite (Dol. II) and fracture associated, coarse crystalline dolomite (Dol. III). Stable isotope studies indicate less depleted δ 18 O values for Dol. I (-6.44 to -3.76‰V-PDB), slightly depleted δ18O values for Dol. II (-7.73 to -5.24‰V-PDB) and more depleted δ 18 O values for Dol. III (-7.29 to -7.20‰V-PDB). The δ 13 C values of the three dolomite phases are well within the range of Cambrian sea-water signatures. Furthermore, δ 26 Mg-δ 25 Mg signatures (Dol. I; δ 26 Mg=-1.19 to -1.67, δ 25 Mg=-0.61 to -0.86 and Dol. II; δ 26 Mg=-1.34 to -1.59, δ 25 Mg=-0.70 to -0.83) indicate three phases of dolomitization in different diagenetic settings. First, an initial stage of dolomitization during the early Cambrian resulted from altered marine, Mg-rich fluids associated with the mixing zone mechanism. Second, a late stage of dolomitization was associated with burial during late Permian. A third dolomitization phase was related to post-Eocene times.

Natural factors forming the quality of carbonate raw materials

The article is devoted to the study of natural factors that influence the regularities in the distribution of the quality of carbonate raw materials in the deposits of the Articulation Zone of the Donbass Basin with the Priazovskiy block of the Ukrainian Shield. Paleogeographic conditions of sedimentation and the processes of secondary quality change were established by comparing the lithologic-stratigraphic sections of deposits in different parts of the Articulation Zone: from the west (Novotroitskoe deposit) through the center (Elenovskoe deposit) and to the east (Karakubskoe deposit). The variability in the quality of carbonate raw materials was estimated from the results of statistical processing of the chemical analysis of exploratory samples. Geochemical associations were established for carbonate rocks within the stratigraphic horizons of each deposit. The best in quality of all deposits of the Donbass Articulation Zone are the carbonate rocks of the Cl t c horizon. The conditions for their formation were the most stable. They are characterized by a negative connection of the main quality component (CaO) with all other quality indicators. At the same time, the association of indicators associated with a significant positive connection is retained: SiO2, R2O3, S.The process of forming carbonate strata are accompanied by intense silicification processes that relate to climatic changes in temperature, salinity and depth of the sea basin. This is confirmed statistically, because the significant negative correlation between the main component of quality and silica is established on the horizons of carbonate rocks with an increased silica content. The process of dolomitization of the carbonate stratum took place in two stages. At the beginning of the accumulation of the carbonate layer, the sea basin was characterized by lower temperatures and higher magnesium contents than in the subsequent period. Therefore, the lower stratum of the Novotroitskoe and Elenovskoe deposits include sedimentary dolomites. Later, the conditions changed, so there are no dolomites in the Karakubskoe deposit. The second stage of dolomitization is associated with the post-carbon andesite-trachyandesite magmatic complex. The rocks of this complex are found in the carbonate stratum in the form of interplastic intrusions, dikes and stock-like bodies. This contributed to the appearance of sulphide mineralization, zones of silicification and recrystallization of carbonate minerals in spot-dolomitized limestones. The structural-tectonic factor, which is manifested by intensive leaching processes, exerts the greatest influence on the degradation of the quality of carbonate raw materials. In zones of tectonic disturbances, they have maximum extension areas. The structural transformationsthat began in the Donbas after the early Carboniferous disrupted the primary stratified occurrence and quality of the carbonate thickness. The upper Visean horizons are subject to intensively leached, similar to tectonic,melange.The contribution made by these studies to scientific knowledge lies in the identification of the natural factors that determined not only the quality of carbonate rocks, but also indicators of the development stages of the Donbass Articulation Zone of two major tectonic structures - Donbass and the Ukrainian Shield. The practical significance of the results of the work is connected with the fixed possibility of conducting geological and technological mapping and further construction of varietal maps of the deposits of carbonate raw materials.

Geology of the ‘Sénia stone’ from Ulldecona, Catalonia (Aptian, Maestrat Basin, Iberian Chain) and its implications for regional stratigraphy

The municipality of the town of Ulldecona (Catalonia) is notable for extensive quarrying activities, which exploit limestone, popularly named Stone from Ulldecona, for ornamental and building purposes. The Stone from Ulldecona, commercially known as Sénia stone, is one of the most important ornamental and building stones quarried in Catalonia, and is used worldwide in all kinds of public and private buildings. Little is known about the geological nature of this stratigraphic interval of commercial value. Therefore, this study explores the geology of the Stone from Ulldecona in open pit quarries and natural outcrops. The Stone from Ulldecona consists of limestones of upper lower Aptian age, including wackestone, packstone and grainstone textures containing peloids, miliolids, Palorbitolina lenticularis, Orbitolinopsis simplex, Paracoskinolina maynci, Lithocodium aggregatum, Choffatella decipiens, Salpingoporella muehlbergi, Chondrodonta, Toucasia carinata, Polyconites sp. and Mathesia darderi. These platform carbonates rich in orbitolinids and rudists belong to the Villarroya de los Pinares Formation of the Maestrat Basin. Locally, the limestones are highly bioturbated and/or dolomitized. Dolomitic limestones, calcitic dolostones and dolostones are stratabound tabular geobodies with thicknesses of up to 60 m. Dolomitic limestones and calcitic dolostones corresponding to initial and intermediate stages of dolomitization mainly exhibit isolated euhedral dolomite crystals or idiotopic mosaics. Dolostones (advanced dolomitization stages) are sucrose, exhibit vacuolar and cave porosities, and are characterized by idiotopic and hypidiotopic mosaics, which indicate temperatures lower than 50-60 °C during dolomitization. Dolomite textures are mainly fabric-destructive and pervasive, but locally retentive and/or selective fabrics also occur. The limestones of the Villarroya de los Pinares Formation, the underlying marls of the Forcall Formation and the overlying marls and platform carbonates of the Benassal Formation examined can be arranged into two high-rank, low-frequency transgressive-regressive sequences. Similar coeval long-term transgressive-regressive trends have been reported from other basins, indicating that eustatism largely controlled accommodation of the Aptian succession studied in Ulldecona.

Discussion of multiple formation mechanisms of saddle dolomites—Comparison of geochemical data of Proterozoic–Paleozoic dolomites

As important gas-bearing strata, Proterozoic–Paleozoic marine carbonate successions in the Sichuan Basin have experienced a long diagenetic history and multiphase hydrothermal activities. Some hydrothermal fluids were closely linked to reservoir dolomitization, which dramatically affected the physical properties of the Proterozoic–Paleozoic marine carbonates in the region. Based on comprehensive petrology, mineralogy, and geochemistry analyses, as well as analyses of the structural and depositional settings, we took as study objects three sets of typical saddle dolomites from the Upper Sinian, Middle-Upper Cambrian, and Middle Permian to understand the mechanism of hydrothermal dolomitization during the Proterozoic–Paleozoic. First, we analyzed the dolomitization stages and mineral characteristics. Then, we performed comprehensive geochemical analyses to discriminate between different saddle dolomites and utilized a fluid reconstruction method to reveal the formation mechanisms of different hydrothermal dolomitizations. We present the three sets of saddle dolomites as being characterized by obviously different lithologies, diversified mineral features, varying paragenetic mineral combinations, and distinguishable horizontal distributions. The Upper Sinian saddle dolomites originated from mantle-derived materials of high temperature and high salinity and experienced continuous and stable crystallization. Moreover, their hydrothermal dolomitization was related to the hydrothermalism of rock intrusion resulting from stress transformation during the Caledonian–Hercynian. The Middle Permian saddle dolomites originated from multisource fluids of moderate-high temperature and high salinity, and their hydrothermal dolomitization was linked to the hydrothermalism of extrusive rocks during the main body of the Hercynian. The Middle-Upper Cambrian saddle dolomites originated from fluid of moderate-low temperature and moderate salinity, and instead of hypomagma activity, their hydrothermal dolomitization might be related to the differential compaction of bottom mudstone rich in hydrothermal minerals during the diagenetic period. In summary, in this paper, we clarify the fluid characteristics and mechanisms of three-stage hydrothermal dolomitization and provide theoretical support for the exploration of hydrothermally dolomitized reservoirs of the Proterozoic–Paleozoic marine carbonate successions, which can also be used as reference for investigations of the multiple mechanisms of hydrothermal dolomitization.