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Alkaline lakes (pH > 9) are among the few modern sedimentary environments that are hydrochemically favourable for low‐temperature dolomite formation. While Mg‐clays and Mg‐evaporites also form more easily in alkaline environments, few studies have focused on how the kinetically inhibited dolomite wins the competition for Mg2+. Here, a basin‐wide survey of the distribution, paragenesis and stable C, O and Mg isotopes of main Mg‐rich minerals in the Late Palaeozoic saline alkaline lake deposit of the north‐west Junggar Basin, north‐west China, is conducted to study the influence of the formation and diagenesis of eitelite, northupite and Mg‐clays on dolomite formation. Large, isolated dolomite crystals (20 to 70 μm in diameter), show positive δ13C values (ranging from +1 to +7‰) and a restricted distribution in the mudstones of the lake‐transitional zone. These crystals have been interpreted as organogenic dolomite driven by methanogenesis via fermentation of organic substrates. The δ18O values of dolomitic mudstones (from −7.4 to +3.4‰), calcitic mudstones (from −15.1 to −3.3‰) and bedded Na‐carbonate evaporites (from +0.08 to +3.7‰), together with their Mg isotopic compositions, suggest that dolomite was not enriched in the most concentrated environments or during stages with most Mg sources, but in the organic‐rich deposits containing few other authigenic Mg‐rich minerals. Dolomite is at a competitive disadvantage for Mg2+ ions compared to Mg‐evaporite and Mg‐clay minerals due to its slow crystallization rates and the deficiency of micritic calcium carbonate precursors. However, it can nucleate and progressively grow into large crystals (>20 μm) if bacterial methanogenesis could effectively lower porewater pH (<8.5) and induce the dissolution of generated eitelite, northupite or Mg‐clays. These findings suggest that high salinity and/or high alkalinity are not always favourable conditions for dolomite formation and highlight the active role of pH fluctuations in inducing low‐temperature dolomite formation. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Lacustrine dolomite has received considerable attention during the last twenty years due to its scientific and high economic value in conventional and unconventional hydrocarbon reservoirs. Compilation of lacustrine dolomite in deep time can help to clarify the hydrochemical conditions in which dolomite was formed. In deep time, lacustrine dolomite facies can be divided into two categories: i) lake-marginal facies including dolograinstones and dolomicrobialites, and ii) lake-central facies comprising alternated dolomitic mudrocks/argillaceous dolomites, tuffaceous dolomites/dolomitic tuffites, analcimic/albitic dolomites/dolomitic analcimolites, as well as dolomitic sandstones. Based on lake water salinity and alkalinity, ancient dolomite-rich lakes can be genetically classified into four types: non-alkaline (pH < 9) brackish, non-alkaline saline, alkaline (pH > 9) brackish, and alkaline saline lakes. Dolomites in different lakes show distinctive features in lithofacies, minerology and isotopic compositions (carbon and oxygen). The formation of early dolomite (primary and early diagenetic dolomite) in different lacustrine depositional systems coincidentally resulted from a significant increase of Mg2+ concentrations and alkalinity in bulk lake water and/or porewater. The Mg2+ increment might be due to climate-controlled intense evaporation and/or by excessive input of Mg2+ from hydrothermal fluids and/or basic volcanic tuffs into brackish lakes which increases alkalinity with subsequently precipitation of dolomite. Moderately positive carbon isotopes (δ13C = +2 to +10‰, V-PDB) and negative oxygen isotopes (δ18O = 0 to −20‰, V-PDB) are indicative, not exclusively, of such hydrothermal- or volcanic-related lacustrine dolomite. In alkaline water systems, dolomite can be dominantly primary in origins and the presence of preexisting Ca-carbonate minerals is not very important for dolomite enrichment in deep time. [ABSTRACT FROM AUTHOR]

The dolostone units in the Cambrian Changlangpu Formation of the Sichuan Basin have demonstrated substantial potential for natural gas exploration. A systematic understanding of the various dolomite (and non-carbonate) phases that build these rocks and the diagenetic pathways of the Canglangpu Formation carbonates still needs to be developed. This study combines the petrographic properties of dolomite (and non-carbonate) phases with their geochemical evidence (carbon, oxygen and 87Sr/86Sr isotope data). Moreover, fluid inclusion evidence and in situ rare-earth element (REE) data are presented. The samples studied were taken from hand specimens collected in the field and plugs from cores to explore the properties and evolutions of dolomite in the Cambrian Canglangpu Formation in the central-northern Sichuan Basin. Six dolomite phases are identified: silty crystalline dolomite (D1), fabric-retentive dolomite (D2), fabric-destructive dolomite (D3), fine crystalline dolomite (D4), medium to coarse crystalline dolomite cement (C1a and C1b) and saddle dolomite cement (C2). Silty crystalline dolomite represents the earliest dolomitization and shows δ18O values comparable to those of Cambrian seawater. Fabric-retentive dolomite, fabric-destructive dolomite and fine crystalline dolomite represent reflux dolomitization during shallow burial. Specifically, Fine crystalline dolomite with a turbid-core and clean-rim may have recrystallized from secondary dolomitization of marine-derived porewater. Fabric-destructive dolomite shows the depleted δ13C values, resulting from the secondary dolomitization involving organics degradation in medium to deep burial environment. The fluid inclusion data demonstrates that the medium to coarse crystalline dolomite cement (C1a) formed in relatively elevated temperature and salinity brines. Intrusion of sulfur- and metal-rich (Zn and Fe) medium-temperature hydrothermal fluids gave rise to the formation of medium to coarse crystalline dolomite cement (C1b) and sphalerite mineralization. Cementation of saddle dolomite was promoted by sulfate-rich reducing hydrothermal fluids. δ13C depletion, extremely low REE content and positive δEu in the saddle dolomite cement suggest the presence of thermal sulfate reduction. • Six dolomite phases were identified, including matrix dolomites and dolomite cements. • Dolomitization occurred from near the surface to deep burial. • In situ elements data reveal the properties of variously dolomitizing fluids in one dolomite phase. • The dolomite crystal zoned as fluid reducibility increased from core to rim section. • Medium temperature hydrothermal dolomitization associated with sphalerite mineralization. [ABSTRACT FROM AUTHOR]

Based on comprehensive studies in petrography, petrofabric analysis and geochemistry, this paper describes a unique and rare laminated micritic ferruginous primary dolostone crystallized and precipitated from the alkaline hot brine under the conditions of the Mesozoic faulted lake basin. The main rock-forming mineral of this dolostone is ferruginous dolomite with a micritic structure. This dolomite mostly exhibits laminae of 0.1–1 mm thick and is often discovered with other minerals, such as albite, analcite, barite and dickite, which have at least two types of interbedded laminae. Petrogeochemistry reveals that this dolostone contains a large number of typomorphic elements of hydrothermal sedimentation, including Sb, Ba, Sr, Mn, and V. In addition, the LREE is in relatively high concentrations and possesses the typical REE distribution pattern with negative Eu anomaly. Oxygen isotope values (δ 18O PDB) range from 5.89‰ to 14.15‰ with an average of 9.69‰. The ratio of 87Sr/ 86Sr is between 0.711648 and 0.719546, with an average of 0.714718. These data indicate that the depositional environment is a stable, blocked, anoxic low-lying hot brine pool in the bottom of deep lake controlled by basement faults. The hydrothermal fluid is the alkaline hot brine formed by the combination of the infiltration lake water and mantle-derived magmatic water, consisting of many ions, including Ca 2+, Mg 2+ and Fe 2+. Under the driving flow power of magmatic heat, gravity and compaction, the hydrothermal fluid overcame the overburden pressure and hydrostatic pressure of the lake water body, and boiled to explosion, and then the explosion shattered the original laminated micritic ferruginous primary dolostone near the vent and then formed a new type of dolostone called shattered “hydroexplosion breccias”. In the low-lying, unperturbed hot brine pool, far from the vent, the laminated micritic ferruginous primary dolostone was quickly crystallized and chemicals precipitated from the hydrotherm. This study of special rocks contributes to research into the causes of the formation of lacustrine carbonate rocks and dolostone. In particular, it provides new examples and research insights for future studies of the lacustrine dolomite from the similar Mesozoic and Cenozoic basins in China. [ABSTRACT FROM AUTHOR]

The Lower Triassic Jialingjiang Formation, characterised by a carbonate–evaporite succession, is one of the important pay layers in the eastern Sichuan Basin. Several diagenetic events were identified in this study, including meteoric dissolution, dolomitisation, anhydrite/gypsum replacement/cementation, TSR, BSR, and multiple calcite cementation (C1–C3). Dolomitisation and anhydrite/gypsum replacement/cementation are the major diagenetic events affecting reservoir porosity. The dolostone is composed of D1, D2, and D3 dolomite. D1 dolomite is very finely crystalline, dominating in the low-energy, restricted depositional facies. D2 dolomite is finely crystalline, constituting a major portion of the dolostones. D3 dolomite is depositional fabric retentive dolomite, including dolo-wacke-/packstone, dolo-grainstone, and dolo-boundstone. Four types of anhydrites (A1–A4) were identified in the core and thin sections. A1 is stratabound anhydrite. A2 is the anhydrite in dolostones, filling in the interparticle spaces in dolo-grainstone and the intraparticle spaces in dolo-wackestone. A3 is the anhydrite in limestones, which occurs non-fabric-selectively. A4 is the anhydrite filling in the fractures. The diagenetic anhydrites (A2–A4) are interpreted to postdate to major dolomitisation. D1 and D2 dolomites are characterised by δ13C VPDB values of −3.4–+7.3 ‰ and δ18O VPDB values of −4.7−-2.4 ‰. The REE patterns of D1–D3 dolomites are characterised by light rare earth element (LREE) depletion relative to heavy rare earth elements (HREE) ((Pr/Tm) N of 0.54–1.16, 0.81 ± 0.20), middle rare earth elements (MREE) enrichment (BSI values of 1.06–1.38, 1.26 ± 0.10), and negative Ce anomalies (Ce/Ce* ratios of 0.74–0.94, 0.83 ± 0.07). The diagenetic anhydrites (A2–A4) show δ34S V-CDT values of +25.9–+45.7 ‰, similar to those of the stratabound anhydrite (+29.6–+43.0 ‰). The large variations in the δ13C VPDB values of D1 and D2 dolomites are attributed to methanogenesis and BSR. The δ18O VPDB values and REE characteristics of D1–D3 dolomites suggest that the carbonates are dolomitised by the mesohaline fluid. The δ34S V-CDT values of A1–A4 indicate that the diagenetic anhydrites are formed by redistribution in a closed system. The petrographical and geochemical data suggest that the Jialingjiang carbonate evolved in a closed system during burial diagenesis. Primary pores dominate in the Jialingjiang carbonate reservoirs, and the boundstone and grainstone show the best porosities based on the porosityversus lithofacies data. Therefore, we propose that the lithofacies, instead of the diagenetic events, are the key factors of the Jialingjiang carbonate. The resisting compaction of dolomitisation helps preserve the reservoir porosity, and anhydrite is the major diagenetic event destroying the reservoir porosity. • Lithofacies are the key factor of the reservoir qualities. • Petrography and geochemical work suggest the closed diagenetic systems. • Jialingjiang carbonate is dolomitized by mesohaline fluids predating diagenetic anhydrite. • Anhydrite selectively replace calcitic part in Jialingjiang carbonate. [ABSTRACT FROM AUTHOR]

This study describes and interprets the mineral and facies assemblages of lacustrine shales that occurred in an ancient volcanic-related alkaline playa-lake, considering the importance of diagenetic pathways (calcification, dolomitization, borosilicate replacement, and silicification). Three laminated lithofacies (LF) have been differentiated in the Early Permian Fengcheng Formation, NW Junggar Basin: an alginite-poor, dolomite-lean, and argillaceous-rich lithofacies deposited during high lake level stages (LF-A), an alginite-moderate, dolomite-moderate, and evaporite-present lithofacies deposited during low lake level stages (LF-B), and an alginite-dense, dolomite-rich and reedmergnerite (NaBSi 3 O 8)-rich lithofacies deposited during periods of intense hydrothermal input (LF-C). Depositional environments influenced subsequent diagenetic pathways of the three laminated lithofacies. In the shallow-water LF-B sediments, extensive evaporite crystallization, dissolution and subsequent calcification or dolomitization occurred during eodiagenesis. The deep-water LF-A sediments underwent slight eodiagenetic modifications, but developed abundant calcite-filling sub-horizontal fractures during moderate inversion and uplift in mesodiagenetic regime. With increasing depth of burial, the previous vugs-filling calcite and dolomite were preserved well or only slightly replaced by reedmergnerite in the LF-A and LF-B sediments, but intensely replaced by reedmergnerite in the LF-C sediments during mesodiagenesis. The reedmergnerite enrichment processes in the LF-C sediments were closely related to organic matter production and thermal evolution. Algae blooms induced by spring input functioned as an important media in sinking hydrothermal boron from the not very concentrated waters. Later degradation and thermal evolution of alginite released boron and organic acids into interstitial waters, which caused carbonate dissolution and promoted reedmergnerite formation. The formation of reedmergnerite during mesodiagenesis restrained silicification process of carbonate minerals in ancient volcanic-related alkaline lake deposits. • Tuff-rich lithofacies is algae-poor and dolomite-rich lithofacies is algae-rich. • Algae, primary dolomite, and diagenetic reedmergnerite are genetically related. • Primary environments determine subsequent diagenetic pathways of dolomitic shales. • Mesodiagenetic formation of reedmergnerite restrained late silicification process. [ABSTRACT FROM AUTHOR]

The marine carbonate rock in the Changxing-Feixianguan Formation of Xuanhan-Daxian area, northeastern Sichuan, the Sichuan Basin holds abundant natural gas resource, where the dolomite includes two types, the one with original texture (grain dolomite, reef dolomite) and the one without original texture (idiomorphic to anhedral-crystal dolomite, porphyritic dolomite). The analysis on the isotopes of C, O, Sr and other geochemical features shows that the dolomitizing process happened in fluid rich in Mg2+ and lean in Ca2+ and brought in Fe2+, Mn2+ and took out Sr2+; with the rise of recrystallization, the Fe2+, Mn2+ contents tend to increase while δ18O PDB and Srcontent tend to decrease. According to the variation of 87Sr/86Sr and other geochemical features, it is considered that the dolomitizing fluid participating in the reservoir diagenesisis the high salinity sea water later than the end of Feixianguan stage, and the dolomitization takes place mainly through the multiple thermal convective cycle effect. The micritic dolomite is of evaporative dolomitization origin with the characteristics of high Sr2+, Na+ and low Mn2+ which reflects the open environment of the surface exposed in the quasi syngenetic period, while the reef dolomite is of high Mg/Ca dolomitizing fluid origin with the characteristics of lowSr2+, Na+ and Mn2+, whose diagenetic environment changed from near surface to shallow burial environment; crystalline dolomite is formed by metasomatism of low Mg/Ca dolomitizing fluid at deep burial depth and high temperature, with the increase of burial depth,in the high temperature and high pressure deep sealing system, its Fe2+ and Mn2+ elements increased while Sr2+ decreased. Through reconstruction of dynamic diagenesis-pore evolution, it is concluded that the low porosity of shallow beach grain dolomite is resulted from early compaction and medium-term cementation, yet the reservoir quality has been apparently improved by the later hydrocarbon dissolution; compared with shallow beach grain dolomite,the reef dolomite has smaller initial porosity, but stronger compressive capacity. The early compaction and medium-term cementation have less impact on the reef dolomite, and the later dissolution strength is slightly weaker than that of the granular dolomite, thus the change range of porosity is smaller than that of the grain dolomite; the micrite-silt dolomite mainly suffered porosity reduction caused by compaction, and the later burial dissolution and recrystallization are key to the quality improvement of this kind of dolomite reservoir. • There are two types of dolomite in the study area: the original fabric and without original fabric. • The high salinity marine source fluid of late period is involved in the dolomitization process of dolomite reservoir. • Dissolution is the key to the formation of high quality reservoir. • Compaction and cementation are the key factors to reduce the porosity of dolomite reservoir. [ABSTRACT FROM AUTHOR]

The Lower Cambrian Longwangmiao Formation is a significant deep carbonate (dolomite) reservoir that has great hydrocarbon potential in the Sichuan Basin, South China. However, the origin of the Longwangmiao dolomite and the mechanism of dolomitization remain a debate that needs better understanding due to its crucial importance in determining reservoir quality. In the current study, a multi-technique approach of petrography, geochemistry (C- and O-isotopes, major and trace elements including rare earth elements), and microthermometry has been applied to reconstruct the mechanisms of dolomitization. Petrographic examinations revealed three types of matrix dolomite: (1) near-micritic (dolomicrite), fabric retentive texture (Md1, 10–50 μm), (2) finely to medium crystalline dolomite, subhedral to euhedral crystals (Md2, 60–120 μm), and (3) medium to coarsely crystalline dolomite, subhedral to nonplanar crystals (Md3, 100–300 μm). The similarity of PAAS-normalized REE patterns of the investigated dolomites to that of their precursor lime mudstone and their δ13C (C1 = 0.3 ± 0.3‰, Md1 = −1.3 ± 0.6‰, Md2 = −0.6 ± 1.6‰, and Md3 = 0.3 ± 1.5‰) and δ18O (C1 = −9.4 ± 0.2‰, Md1 = −7.8 ± 0.8‰, Md2 = −8.7 ± 0.5‰, and Md3 = −8.9 ± 0.6‰) values suggest that the parent dolomitizing fluids of Md1, Md2, and Md3 dolomites were likely modified seawater. The significant enrichment of Na (1266 ± 1316 ppm) and K (4649 ± 3199 ppm), tight structure and presence of moldic pores in Md1 and its slightly elevated δ18O values suggest that it formed in a near-surface (i.e., mesosaline to penesaline) environment from slightly evaporated seawater, which is consistent with its Sr/Ca molar ratios (0.0075−0.0090). The Md2 dolomite is crosscut by stylolites and has a coarser grain size, higher Mn (295 ± 172 ppm), Nd SN /Yb SN (1.33 ± 0.06) and U (0.49 ± 0.11 ppm), and lower Ce/Ce* (0.83 ± 0.02) values than Md1. This suggests a shallow to mid-burial dolomitization with possible downwards seepage-reflux of Mg2+-rich evaporated brines. The Md3 dolomite has the largest crystal size and mosaic contacts, lowest Ce/Ce* (0.81 ± 0.02), and highest U (0.64 ± 0.15 ppm) and Mn (487 ± 240 ppm) values, which imply a deeper dolomitization environment than Md2. Thus, it precipitated likely in a mid-to deep burial setting at a higher temperature (T h = 120−135 °C), from the residual Mg2+-rich fluids stored in the strata pores. Considering the regional geology, we suggest the sedimentary facies and paleogeomorphology controlled the spatial distribution of early dolomites, while sealevel fluctuations influenced the initial time and scale of dolomitization. • Origin of dolomites reflected by stable isotopic, elemental, and fluid inclusion constraints. • Dolomitization of Md1 to Md2 and Md3 took place from early to late. • Controls of depositional environment and sea-level fluctuation on dolomitization. • Dolomitizing model for the Longwangmiao carbonates in the eastern Sichuan basin. [ABSTRACT FROM AUTHOR]

The Middle−Upper Xixiangchi Formation of the Sichuan Basin consists of mixed carbonates and siliciclastics. The carbonates are dominated by dolomitized lime-mudstones and dolo–grainstones of shallow warm-water environments. Petrographic examination of carbonates reveals two types of dolomite: (i) fabric retentive dolomicrite (D1, micritic to near-micritic, 4–50 μm) and (ii) fabric-destructive dolomite (eu-to subhedral crystals with cloudy cores and clear rims, 50–300 μm). The mean ∑REE values of D1 (15.3 ± 6.3 ppm) and D2 (14.6 ± 7.1 ppm) and the nearly identical mean PAAS-normalized patterns that coincide at some points, suggest an origin from similar parent fluids that were not dramatically changed by water-rock interaction through basinal sedimentary rocks. The estimated Sr/Ca molar ratios of D1 (0.0043–0.0055) parent fluids and their estimated δ18O values imply an origin from a mixture of meteoric and seawaters, which is also consistent with the petrographic evidence. The partially coinciding normalized REE pattern of D2 with that of the earlier D1 suggests that the latter was formed at a mid-burial setting rather than deep burial, which is consistent with higher homogenization temperatures of primary two-phased fluid inclusions retained in D2 (T h = 118.1 ± 5.3 °C). Therefore, D1 is suggested to have formed (1) during a penecontemporaneous period that resulted from a slight evaporative pumping effect within a near-surface diagenetic environment, and/or (2) during post-penecontemporaneous or penecontemporaneous seepage-reflux dolomitization, induced by sea-level fluctuations, during early stages of diagenesis at near-surface conditions. D2 formed likely through the recrystallization of D1 or a continuation of reflux dolomitization of the same precursor at mid-burial rather than deep burial settings. • Origins of D1 and D2 dolomites are reflected by their geochemical compositions. • Dolomitizing fluids were derived from slightly evaporated seawater and basinal brines. • Similar parent fluids of D1 and D2 are reflected by their nearly identical PAAS-normalized REEs patterns. • Recrystallization of D1 likely accounts for comparable δ18O values with D2. [ABSTRACT FROM AUTHOR]