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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]

Travertines formed of crystalline crust have been widely reported, but there has not been focus on their geochemical characteristics. We therefore carefully conducted a series of geochemical investigations and U-Th dating on a travertine mound mainly composed of crystalline crust from Sobcha (southwest China) to determine their geochemical features and geological implications. The Sobcha travertines dominantly consist of granular crystals and fan crystals and show δ13C from 3.4‰ to 4.9‰ V-PDB, δ18O from −26.7‰ to −23.7‰ V-PDB, and 87Sr/86Sr from 0.712458 to 0.712951. When normalized to PASS, the Sobcha travertines exhibit MREE enrichment relative to HREE and LREE, HREE enrichment relative to LREE, and positive Eu anomalies. The δ13C signatures and mother CO2 evaluation of the Sobcha travertines show that the Sobcha travertines were thermogene travertines largely receiving mother CO2 from (upper) mantle (i.e., magmatic CO2) or a mixture of soil-derived CO2 and CO2 related to carbonate decarbonation. The 87Sr/86Sr of the Sobcha travertines is out of the 87Sr/86Sr ranges of local deposits exposed at Sobcha and surrounding areas but is well matched with the mean 87Sr/86Sr of Nadi Kangri volcanic rocks which cropped out to the northeast of the studied travertines (over 20 km away). This might indicate the important role of the Nadi Kangri volcanic rocks in suppling Sr to the studied travertines, but more studies are required. The LREE depletion compared to MREE and HREE in the Sobcha travertines was interpreted to be caused by the difference in geochemical mobility between LREEs and HREEs during water–rock interaction at depth, while the MREE enrichment compared to HREE was considered to be most likely inherited from reservoir/aquifer rocks. The positive Eu anomalies of the Sobcha travertines may result from very high reservoir temperatures and/or preferential dissolution of Eu-rich minerals/rocks (especially plagioclase). The Sobcha travertine mounds displays no or very slight vertical variations in δ13C, 87Sr/86Sr, and REE patterns, indicating the compositional stability of mother CO2 and paleo-fluids. However, a significant vertical increase in δ18O was observed and was explained as the result of gradual water temperature decrease related to climate cooling, self-closure of the vents, or mound vertical growth. The findings in this study might help us better understand the deposition of crystalline crust in Ca2+-deficient hot spring systems. [ABSTRACT FROM AUTHOR]

Banded travertines are important parts of fissure ridge systems, but studies on geochemical characterization of banded travertines are limited. This study investigated the lithofacies and stable carbon and oxygen isotopic features of banded travertines from Xiagei (southwestern China) to examine their formation mechanisms. Petrographic analyses of the banded travertines revealed two lithotypes: thick-laminated palisade crystalline crust and thin-laminated composite crystalline crust. δ13C and δ18O of the Xiagei banded travertines range from 2.82‰ to 4.50‰ V-PDB, and from −25.86‰ to −20.90‰ V-PDB. Parent CO2 evaluation shows that the Xiagei banded travertines mainly received CO2 from the decarbonation of marine carbonates, but the contributions of magmatic CO2 and the dissolution of marine carbonates are also unneglectable. Significantly, the magmatic-derived CO2 might indicate that the delamination of the lithosphere along with the asthenosphere upwelling could be taking place in the eastern Tibetan plateau. Paleotemperature calculation shows that the Xiagei travertines were precipitated from moderate- to high-temperature hot springs (44.3 to 86.8 °C). Interestingly, the thick-laminated palisade crystalline crust and thin-laminated composite crystalline crust display calculated paleotemperature between 66.6 and 86.8 °C and between 56.6 and 77.7 °C, respectively, reflecting the great role of water temperature in controlling the lithofacies of banded travertines. A comparison between the banded travertines at Xiagei and other areas also shows temperature is a non-negligible factor controlling banded travertine precipitation. However, this does not mean that water temperature is the decisive controlling factor and more studies on banded travertines are still indispensable to disclose the potential factors controlling the factors/processes affecting banded travertine lithofacies. This study provides a good example for understanding the relationship between lithofacies and stable isotopic geochemical characteristics of travertine deposits. [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]

Travertine mounds are common spring‐constructed landforms with highly variable geometries, but factors controlling their development have not yet been fully understood. To investigate processes linked with their lateral and vertical growth, this study investigated travertine mounds at Heinitang (China) and compared them with typical mounds around the world. Calcium concentration variation modelling and height evaluation of travertine mounds were also performed. The lateral extent of travertine mounds was found to be mainly controlled by Ca2+ and HCO3‐ concentrations and water discharge: relatively small metre‐scale mounds, like those from Heinitang, are largely the products of Ca2+‐deficiency (<150 mg L−1, commonly <100 mg L−1) or HCO3‐‐deficiency (<500 mg L−1) springs with low discharges (<0.1 L s−1), whereas springs with higher Ca2+ and HCO3‐ concentrations and discharges give rise to the formation of wider mounds. Additional factors affecting mound lateral development include infiltration, evaporation, micro‐organisms, water cooling, CO2 degassing and flow paths. Height of travertine mounds cannot be simply considered to represent the piezometric surface, because the vertical growth of travertine mounds is also influenced by the water table change triggered by tectonic pulses and climate fluctuations, and the closing of conduits induced by self‐sealing. The extinction of travertine mounds at Heinitang is, for example, probably caused by the water table lowering triggered by earthquakes or climate drying after 4 kyr bp. These findings offer new inputs for the better understanding of factors controlling travertine mound development and might assist in the reconstruction of fluid properties, hydrodynamics, tectonics and climate fluctuations in fossil geothermal systems with travertine mounds. [ABSTRACT FROM AUTHOR]

Phanerozoic sinter deposits have been reported globally, with their identification mainly based on sedimentological, petrological, and mineralogical studies. In this study, a detailed geochemical investigation, combined with sedimentological characterization, was conducted on the Wugonglilu siliceous deposit, a Cretaceous (106 Ma) sinter in NE China, to examine its genesis. The deposit is inferred as a siliceous hot spring deposit (sinter) owing to its geological setting in an epithermal gold-mining district, its morphologically variable microbial textures typical of subaerial sinters, and nearly pure silica composition. Three lithofacies were recognized in this study, including laminated to thinly bedded sinter, silica-cemented breccia, and epithermal vein, which represent deposits from three contrasting hydrothermal environments. The 87Sr/86Sr ratios of the sinter are similar to the bedrock through which the geothermal fluids passed, namely the Upper Triassic Dajiahe Formation (T 3 dj), a marine siliceous rock unit, and the Lower Cretaceous Datashanlinchang Formation (K 1 d), a rhyolite unit. Results indicate that both underlying units are possible Sr sources of the silica in the Wugonglilu sinter. The REE + Y (rare-earth elements and yttrium) patterns of the sinter exhibit significant variability, primarily due to terrestrial detrital contamination. Differential REE + Y contamination by terrestrial detritus was striking in the white and dark laminae/beds of the laminated to thinly bedded sinter. The white laminae/beds, generally less contaminated, were probably formed during the dry season, whereas the dark laminae/beds exhibiting greater detrital contamination were likely formed during the wet season. When excluding the samples strongly contaminated by detritus, the sinter exhibits Y anomalies comparable to Post-Archean Australian Shale (PAAS), and to the adjacent underlying K 1 d rhyolite and T 3 dj siliceous rock samples, indicating that these anomalies were not inherited from rocks dominating the reservoirs. Furthermore, the sinter shows a (Ce/Ce*) N range close to that of the underlying marine siliceous rock and rhyolite samples, reflecting that the (Ce/Ce*) N of the sinter is largely inherited from the reservoir rock(s). The overall REE + Y patterns of the sinter range from nearly flat to LREE-depleted, similar to some samples of the K 1 d rhyolite and T 3 dj siliceous rock units. However, most samples of the K 1 d rhyolite and T 3 dj siliceous rock display (Pr/Tb) N and (Pr/Yb) N higher than many studied sinter samples. This is likely because of the formation of strong HREE–carbonate complexes during groundwater migration. Based on these results, a generalized formation model of the studied sinter system was constructed. This study suggests that integrated sedimentological and geochemical investigations can aid in interpreting the genesis of sinter deposits, in particular the link between sinter deposits and corresponding reservoir rocks. [ABSTRACT FROM AUTHOR]

Siliceous-sulphate rock coatings were observed at Zhenzhu Spring, an acid sulphate hot spring in the Tengchong volcanic field, China. These rock coatings are mainly formed of gypsum and amorphous silica. Some alum-(K), voltaite, α-quartz and muscovite were also found. Four different laminae are developed in the rock coatings: gypsum layer, tight siliceous layer, tabular siliceous layer and siliceous debris layer. The gypsum layer is located at the top of the rock coatings, while other siliceous layers appear below the gypsum layer. Geochemical modelling of the fluids was performed to identify the mechanisms responsible for the formation of gypsum and amorphous silica. The results indicated that the occurrence of gypsum is related to the acid-fog deposition and amorphous silica mainly originates from spring water. Fog deposition provided the rock coatings with abundant SO42− and Ca, and the subsequent complete evaporation of the condensed fluids produced gypsum. Seasonal climate change (especially variation in rainfall) determines the fluctuations of capillary action and dissolution. Rainfall events in the wet season led to periods of non-precipitating gypsum and promoted the capillary rise of the spring water. Slightly diluted capillary water (a small amount of rainwater) covered the rock coatings, formed a tight siliceous layer on the rock-coating surface and/or filled the pores among the gypsum crystals forming many tabular siliceous aggregates. Heavy rainfall (high dilution), however, resulted in non-precipitating amorphous silica and accelerated the gypsum dissolution, leaving tabular pores around tabular siliceous aggregates and forming a tabular siliceous layer. [ABSTRACT FROM AUTHOR]

The Zhenzhu Spring, located in the Tengchong volcanic field, Yunnan, China, is an acid hot spring with high SO42− concentrations and intense acid aerosol generation. In order to understand the formation mechanism of sulfate minerals at the Zhenzhu Spring and provide a better insight into the sulfur isotope geochemistry of the associated Rehai hydrothermal system, we investigated the spring water hydrochemistry, mineralogy and major-element geochemistry of sulfate minerals at the Zhenzhu Spring together with the sulfur-oxygen isotope geochemistry of sulfur-containing materials at the Rehai geothermal field and compared the isotope results with those in other steam-heated environments. Subaerial minerals include a wide variety of sulfate minerals (gypsum, alunogen, pickeringite, tamarugite, magnesiovoltaite and a minor Mg–S–O phase) and amorphous SiO2. The δ34S values of the subaerial sulfate minerals at the Zhenzhu Spring varied subtly from –0.33 to 1.88‰ and were almost consistent with the δ34S values of local H2S (–2.6 to 0.6‰) and dissolved SO42− (–0.2 to 5.8‰), while the δ18O values (–8.94 to 20.1‰) were between that of the spring waters (–10.19 to –6.7‰) and atmospheric O2 (~23.88‰). The results suggest that most of the sulfate minerals are derived from the oxidation of H2S, similar to many sulfate minerals from modern steam-heated environments. However, the rapid environmental change (different ratio of atmospheric and water oxygen) at the Zhenzhu Spring accounts for the large variation of δ18O. The formation of subaerial sulfate minerals around the Zhenzhu Spring is related to acid aerosols (vapour and acid water droplets). The intense activity of spring water around vents supply the aerosol with H2SO4 (H2S oxidation and acid water droplets formed by bubble bursting) and few cations. Deposition of the acid sulfate aerosol forms the acid condensate, which attacks the underlying rocks and releases many cations and anions to form subaerial sulfate minerals at the Zhenzhu Spring. [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]

[Display omitted] • A Paleozoic borosilicate-bearing deposit of potential economic interest is analyzed. • Reedmergnerite is a temperature-controlled, replacive diagenetic mineral largely generating at 90–110 °C. • Boron for reedmergnerite was inherited from the primary evaporitic lake. • Reedmergnerite can indicate primary depositional environments, and burial histories of a basin. Reedmergnerite (NaBSi 3 O 8) and searlesite (NaBSi 2 O 6 ·H 2 O) are rare borates minerals over the world. In the Late Paleozoic Fengcheng Formation of the NW Junggar Basin, NW China, the two borates, especially reedmergnerite, occur as locally abundant minerals of potential economic interest. Here, petrology, minerology, and boron isotopes of different reedmergnerite occurrences in the Fengcheng Formation were systematically studied to explore its enrichment mechanism in sedimentary rocks. Reedmergnerite in the Fengcheng Formation is a temperature-controlled (crystallizing at 55–180 ℃, mainly at 90–110 ℃) and diagenetic replacive mineral. In the lake center deposits of the Fengcheng Formation, reedmergnerite is confined within sodium carbonate beds or nodules and shows a clear replacement of sodium carbonate minerals. In the lake slope deposits, reedmergnerite is also very abundant and mainly replaces calcite or dolomite-ankerite within sub-horizontal fractures or shortite/eitelite crystal molds, occasionally seen replacing bedded searlesite or growing both replacively and displacively in organic-rich matrix. No significant reedmergnerite is found in the lake margin deposits. Boron isotopes of reedmergnerite-bearing samples (–0.04‰ to + 7.69‰) are higher than those of lacustrine mudstone samples (–14.2 to + 2.79‰) and overlap those of ignimbrite samples (+2.37 to + 3.87‰). Boron for diagenetic reedmergnerite formation was primarily inherited from the original hydrothermal fluids that flowed into lakes and its concentration was influenced by lake water pH and salinity. Potential early boron reservoirs were searlesite, sodium carbonates, organic matter and tuff materials. Fluids responsible for reedmergnerite replacement were weakly acidic. CO 2 generated by organic matter degradation-maturation and carbonate dissolution acted as a mineralizer to promote reedmergnerite formation at a lower temperature range than the previously synthetic temperatures (>270 ℃). Enrichment of reedmergnerite in sedimentary rocks requires a primary volcanic-related evaporated lake with high organic productivity, and meanwhile a deep burial environment, at best reaching the depth of oil generation window. [ABSTRACT FROM AUTHOR]

Travertines are generally considered as carbonate deposits related to Ca-rich hot springs. Ca-deficiency hot springs, however, can also produce travertines under suitable conditions and little is known about the travertine deposition and diagenesis in these systems. Therefore, we studied a fossil perched Ca-deficiency spring system at Shihuadong, China to discuss its genesis and diagenesis. This system is a platform-like build-up with travertines developing mainly in the steep frontal area. The travertines were deposited from high-temperature fluids (dominantly 45 °C to 55 °C) with strong interaction mostly with underground Gaoligong Group metamorphic rocks. Their parent CO 2 was originated from magmatic CO 2 and a little soil CO 2. The travertine-depositing paleofluid was predominantly Ca-deficiency hot spring water, but stream water possibly from cold springs on surrounding hills might make a small contribution. The low calcium levels and flat substrate on the top area of the studied system handicapped the travertine deposition, only developing a small perched travertine channel entirely composed of crystalline crust. On the contrary, progressive CO 2 degassing in the top area and increasing CO 2 degassing and water flow rates in the frontal area induced the formation of abundant travertines. However, water scattering/dispersion in the frontal area might not heavily increase the water flow rate and only induced boundstone deposition. After the travertine deposition, underground thermal water, meteoric water, and microbial activity progressively altered the primary fabrics, forming mosaic and need fiber calcite cements, dissolution pores, microborings, and dissolution-induced and microbe-induced micritization. The studied perched spring system shows limited scales, notable progradation but slight aggradation, and the dearth of distal autochthonous travertines, unlike those formed by Ca-rich hot springs, which might extend widely and produce striking distal travertine deposits away from the frontal slope/waterfall. These findings suggest the significant roles of water chemistry and local topography in travertine deposition and the influence of slowly underground thermal water seeping on the diagenesis of travertines in hilly areas and might be used to recognize fossil Ca-deficiency and Ca-rich hot spring systems. [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]

Siliceous mudstones and chert-shale couplets are primary source rocks for many large paleo-oil reservoirs over the world. It is well established that photosynthetic organisms can be exceptionally well preserved in silica precipitates, but the fate of the trapped organic matter (OM) during burial remains poorly studied. Here, dense unicellular microbial fossils have been recently discovered in the cherts of the Late Paleozoic alkaline saline lake deposit in NW Junggar Basin, NW China. We use petrographic observations and organic geochemical analysis to study the origin of unicellular microbes and how silicified organic matter generate and expel hydrocarbons. A 300-m-long oil shale core shows overall low levels of porosity, permeability and total organic carbon contents (generally <0.5 wt%), but very high extracts (on average 0.26 wt%), suggesting that the rich extracts were mostly indigenous and the original organic matter had a high hydrocarbon conversion. Analysis of biomarkers and silicified fossils both suggest that the bio-precursor for the organic matter is a special green alga. Reconstruction of silica diagenesis of cherts indicates that both the trapped lipids and hydrocarbons in silicified microbial cells and extracellular polymeric substances had been effectively expelled out during the transformations of Opal-A to microcrystalline quartz and during OM thermal maturation. Exceptional OM preservation and effective oil expelling ways render siliceous source rocks as excellent unconventional hydrocarbon reservoirs. • Dense unicellular microbe fossils are found in Early Permian lacustrine cherts. • Microbe preserved by cherts is a special green alga. • Two stages of bitumen expulsions were associated with silica diagenesis. • Organic biosignatures in cherts have already been largely lost before metamorphism. [ABSTRACT FROM AUTHOR]

The Archaeocyathid Extinction Carbon Isotope Excursion (AECE) and Mingxinsi Carbon Isotope Excursion (MICE) are major events in the Early Cambrian sedimentary records but controversially heterogeneous in their magnitudes, as well as local and global correlations. This paper presents a high-resolution δ13C carb profile from the Shiliu section in the western Yangtze Platform (China) that spans the lower part of the Lower Cambrian. The investigated Lower Tianheban interval (∼110 m) is dominated by marine carbonates (lime mudstones) alternating occasionally with thinly-bedded black shale and mudstone interbeds. The investigated micritic carbonates were examined through a multi-technique protocol, combining petrographic and geochemical assessments, to evaluate the degree of sample preservation. The δ13C carb values (−2.4‰–1.8‰) show an insignificant correlation with the δ18O carb values (−12.6‰ to −8.7‰, R2 = 0.012), the Mn/Sr ratios (R2 = 0.001), and Al concentrations (R2 = 0.041). This supports the preservation of predominantly at least near-primary δ13C carb signatures, which can be utilized to reconstruct a reliable, high-resolution δ13C carb profile for chemostratigraphic correlations in the area and beyond. The profile displays a lower positive excursion of ∼3‰ overlain immediately by a negative excursion of ∼2.3‰. The archaeocyath zonation scheme suggests that the lower excursion is correlated with the global MICE event and the upper excursion with the AECE event. The results provide an alternative option for interpreting the carbon isotope excursions in the Lower Cambrian within the Yangtze Platform as well as other equivalent sections at a global scale. • δ13C carb composition of the Lower Cambrian carbonates in northwestern Yangtze Platform. • Distinct δ13C carb excursions correlated with the MICE and AECE events. • Alternative interpretation of the MICE and AECE events in the Yangtze Platform. • A useful C-isotope profile for chemostratigraphic correlations in Yangtze Platform and beyond. [ABSTRACT FROM AUTHOR]

• The thermal decomposition of marine carbonates and metasediments is a source of CO 2. • The radioactive decay of U and Th within crustal granite is the dominant source of He. • Three thermal reservoirs were identified using multiple gas geothermometers. • The parent geothermal fluid had a temperature of ∼320 °C at an ∼8-km depth. • Yangbajing has no supercritical fluids, given its small, deep-seated magma chamber. Exploring and exploiting high-temperature (even supercritical) geothermal resources are significant to meet energy demands and reduce carbon emissions. The Yangbajing geothermal field is the most exploited in China, with the currently highest temperature (329.8 °C) measured in a geothermal well. However, whether there are supercritical geothermal fluids beneath the deep parts of this geothermal field is under controversy. In this paper, the water isotope, chemical compositions, and C–He isotopes of gas samples were collected and analyzed. The geothermal water originated from the mixing of meteoric water and magmatic water (25%). The sources of CO 2 in the geothermal field were dominated by the thermogenic degassing of carbonates and metasediments in the crust while the radioactive decay of U and Th in granite is the dominated source of He. The temperatures of three different reservoirs are 150 ± 15 °C, 250 ± 10 °C, and ∼320 °C (with a depth of ∼8 km), respectively. These were obtained using dissolved gas, soil CO 2 flux, and noble gas geothermometers. Unlike other supercritical geothermal fields worldwide with larger, shallower, basaltic magma chambers, the Yangbajing geothermal field has a deep-seated, small-scale, granitic magma chamber. Thus, its geological conditions are not conducive for gestating supercritical fluids. These results are of great significance for exploring and developing high-temperature (even ultra-high-temperature) geothermal resources in China. [ABSTRACT FROM AUTHOR]

Authigenic silicates are typical minerals of alkaline lake deposits. Their mineral composition and lateral distribution can be used for reconstructing paleoclimates, paleoenvironments, and diagenetic subsidence histories of hydrocarbon reservoirs. In particular the examples of pre-Mesozoic alkaline lake deposits were widely missing in the literature. To unveil this, a basin-wide high-resolution mineralogical investigation is conducted on the Late Paleozoic volcanic-related alkaline lake deposit of the Fengcheng Formation in the NW Junggar Basin, NW China. The investigated Late Paleozoic shales mainly contain stable silicates like K-feldspar, albite and reedmergnerite. Laterally, the authigenic silicates are mostly distributed in the lake-central and transitional zones and are especially abundant in organic-rich shales. Authigenic K-feldspar and albite were mainly formed by replacing detrital clay minerals and K-feldspar or directly precipitating from alkaline porewaters. Reedmergnerite was later formed by replacing acid-sensitive carbonate or feldspar minerals when organic matter was mature. Organic matter thermal maturation and hydrocarbon generation helped lower porewater pH and periodically open diagenetic systems. These processes facilitated fluid migration and element exchanges and promoted the transformation of thermodynamically unstable silicates and acid-sensitive carbonates to more stable silicates and quartz. A main result of this study is thus to emphasize the control of organic matter diagenesis and temperature on authigenic silicate evolution. This model can help revisit the genesis of the dominant felsic shales in alkaline lake deposits. • K-feldspar, albite, searlesite and reedmergnerite are typical authigenic silicates. • Organic matter maturation promoted formation of stable authigenic silicates. • A temperature-controlled diagenetic model for authigenic silicates is proposed. • Felsic shales of alkaline lake may contain both detrital and authigenic feldspars. [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]