Calcite Veins Research Articles

Trace element minerals from carbonatite-related fluids, The Aird, Scotland

Fenitized rocks in Neoproterozoic country rocks adjacent to the Great Glen Fault, Scotland, contain calcite veins with mineral inclusions. The minerals include the rare earth vanadate wakefieldite, sulfur-rich monazite, scandium-rich ilmenite and manganese oxides rich in barium and lead. Each of these phases is recorded in carbonatites elsewhere. An affinity with carbonatites is further suggested by the carbon isotope composition of the calcite veins, and anomalously high levels of manganese and strontium in the veins. These data add weight to a model in which Scotland NW of the Great Glen Fault was affected by carbonatite-related fluids during the Caledonian Orogeny. Thematic collection: This article is part of the Early Career Research collection available at: https://www.lyellcollection.org/topic/collections/early-career-research

Cenozoic evolution of the Tripolitza carbonate platform in the Tethyan realm: new age constraints on deposition, diagenesis, metamorphism and nappe emplacement based on U-Pb and Rb-Sr dating (External Hellenides, Crete)

AbstractWe present kinematic, radiometric, geochemical and PT data, which help to constrain the tectonometamorphic evolution of the Tripolitza Unit (TPU). The age of both the metamorphic peak (P = 0.4 ±0.2 GPa, T = ca. 310 °C) and top-to-the WNW mylonitic thrusting, attributed to the emplacement of the hanging Pindos nappe, has been constrained at 19 ±2.5 Ma using Rb-Sr on synkinematic white mica of a basal mylonite of NW Crete. This early tectonic event is also documented by the oldest generation of veins, which cut through less metamorphic (T = 240 ±15 °C) late Bartonian/Priabonian Nummulite limestone exposed as olistolith in TPU flysch of central Crete. Calcite of these veins yielded a similar U-Pb age at 20 ±6 Ma. U-Pb dating of matrix calcite, on the other hand, reflect the time of sedimentation (38.4 ±5.7 Ma and 37.6 ±1.2 Ma), which is in line with the faunal content of the black limestone. Geochemical data and U-Pb calcite ages of fibres of the Nummulite test (32.3 ±3.1 Ma and 34.6 ±0.9 Ma) suggest unexpected pseudomorphic fibre replacement during late Priabonian/early Rupelian diagenesis. Additional calcite veins, which developed at ca. 10–11 and 7 – 9 Ma (U-Pb on calcite), are attributed to top-to-the S thrusting and subsequent extension, respectively. The resulting anticlockwise rotation of the shortening direction within the TPU from WNW-ESE at ca. 20 Ma to N-S at ca. 10 Ma has significant implications for the geodynamic evolution of the External Hellenides.

Genesis of Calcite Veins in 8# Coal Seam of the Upper Carboniferous Benxi Formation, Southeastern Margin of Ordos Basin

The 8# coal seam in the Benxi Formation of the southeastern margin of the Ordos Basin is a deep coal seam with abundant coalbed methane resources. Calcite veins are commonly developed within the 8# coal seam, and their formation processes and mechanisms have significant implications for the enrichment of deep coalbed methane. Genesis of the calcite veins was analyzed to reveal the impact of the calcite veins formation on coalbed methane accumulation, with an integrated application of petrographic study by thin section, cathodoluminescence analysis, carbon-oxygen isotope analysis, and homogeneous temperature measurements of fluid inclusions. The research findings indicate that the calcite veins in the 8# coal seam can be classified into three stages: C1, C2, and C3. The diagenetic fluids of C1 primarily originated from contemporaneous seawater. The fluids responsible for the formation of C2 primarily consist of organic fluids enriched in biogenic gas, whereas the fluids contributing to the formation of C3 are primarily associated with liquid hydrocarbons originated form decarboxylation of organic matter. Furthermore, the development of both C2 and C3 is influenced by deep hydrothermal fluids resulting from tectonic heating events during the Early Cretaceous. By combining analysis of the hydrocarbon accumulation history and burial history in the study area, it has been established that C2 formation occurred during the Late Triassic to Early Jurassic, while C3 formation took place during the Late Jurassic to Early Cretaceous. The exploration and production practices in the study area have firmly established the crucial significance of the formation and evolution of calcite veins within the 8# coal seam for the migration and accumulation of coalbed methane. The research outcomes provide valuable insights for the exploration of deep coalbed methane enrichment areas.

Groundwater–rock interactions in crystalline rocks: evidence from SIMS oxygen isotope data

AbstractThe diffusive exchange of dissolved material between fluid flowing in a fracture and the enclosing wallrocks (rock matrix diffusion) has been proposed as a mechanism by which radionuclides derived from a radioactive waste repository may be removed from groundwater and incorporated into the geosphere. To test the effectiveness of diffusive exchange in igneous and metamorphic rocks, we have carried out an investigation of veins formed at low temperatures (<100°C), comparing the oxygen isotopic composition of vein calcite with that of secondary calcite in the wallrocks. Two examples of veins from the Borrowdale Volcanic Group, Cumbria, and one from the Mountsorrel Granodiorite, Leicestershire, UK, have remarkably similar vein calcite compositions, ca. +20‰(SMOW) or greater, substantially heavier than the probable compositions of the host rocks, and these vein calcite compositions are inferred to reflect the infiltrating fluid and the temperature of vein formation. Calcites from the wallrocks are similar to those in veins, with little evidence for exchange with the wallrocks. The results support existing models for this type of vein which suggest low-temperature growth from formation brines originally linked to Permian or Triassic evaporites. The results are consistent with flow through fractures being attenuated through a damage zone adjacent to the fracture and provide no evidence of diffusional exchange with pore waters from wallrocks.

Microstructural characterization of natural fractures and faults in the Opalinus Clay: insights from a deep drilling campaign across central northern Switzerland

The Middle-Jurassic Opalinus Clay is the foreseen host rock for radioactive waste disposal in central northern Switzerland. An extensive drilling campaign aiming to characterize the argillaceous formation resulted in a comprehensive drill core data set. The rheologically weak Opalinus Clay is only mildly deformed compared to the over- and underlying rock units but shows a variety of natural fractures. While these structures are hydraulically indistinguishable from macroscopically non-deformed Opalinus Clay today, their analysis allows for a better understanding of the deformation behaviour in the geological past. Here, we present an overview of the different fracture and fault types recorded in the Opalinus Clay and a detailed microstructural characterization of veins—natural dilational fractures healed by secondary calcite and celestite mineralizations. Macroscopic drill core analysis revealed five different natural fracture types that encompass tension gashes of various orientations with respect to bedding and small-scale faults with displacements typically not exceeding the drill core diameter. The occurrence of different fault types generally fits well with the local tectonic setting of the different drilling sites and with respect to the neighbouring regional fault zones. The microstructural investigations of the various vein types revealed their often polyphase character. Fibrous bedding-parallel veins of presumable early age were found to be overprinted by secondary slickenfibres. The polyphase nature of fibrous bedding parallel veins and slickenfibres is supported by differing elemental compositions, pointing towards repeated fracturing and mineralization events. Direct dating of vein calcites with U–Pb was unsuccessful. Nevertheless, age constraints can be inferred from structural orientations and fault slip kinematics. Accordingly, some of the veins already formed during sediment compaction in Mesozoic times, others possibly relate to Early Cenozoic foreland uplift. The youngest veins are most likely related to Late Cenozoic regional tectonic events, such as the Jura fold-and-thrust belt to the south and the Hegau-Lake Constance Graben to the northeast of the study area. During these latest tectonic events, previously formed veins acted as rheologically stiff discontinuities in the otherwise comparably weak Opalinus Clay along which deformation of the rock formation was re-localized.

Geochemistry of syntaxial calcite veins in ultra-deep sandstone reservoirs from the Kuqa depression, western China

Extensional fractures filled with calcite are widespread in the ultra-deep reservoir sandstones of the Cretaceous Bashijiqike Formation (>6000 m) in the Kuqa Depression, western China. Microstructures of calcite veins provide an excellent opportunity for investigating fracture opening processes in a tectonically active, ultra-deep foreland basin setting. The syntaxial calcite vein is the dominant type and is formed through a single crack-seal event. Calcite crystals in fractures show blocky and elongated blocky shapes. The textural and geochemical characteristics of syntaxial calcite veins were investigated in detail using optical cathodoluminescence, trace element distributions and stable carbon and oxygen isotopes. Calcite veins were inferred to have precipitated from a mixture of seawater and meteoric water. ∑REE + Y and elemental concentrations of Mg2+, Mn2+, Fe2+ and Sr2+ both increase from fracture walls to the center. This process was controlled by temperature, crystal growth rate and variations in elemental concentration in a relatively closed environment. Medium REE-bulge patterns after Post Archean Average Shale (PAAS) normalization, negative δCe and positive δEu anomalies all indicate that calcite wall-rock cements and veins may inherit constituents from seawater and from meteoric water containing feldspar-weathering components, respectively. Both 13δCPDB and 18δOSMOW of the parent fluids imply that calcite veins were primarily precipitated from seawater under a relatively closed diagenetic environment. Considering the scope and timing of the transgression of the Tethyan Ocean in the study area, meteoric water may have been retained prior to the seawater invasion (65 Ma). The mixing of meteoric water and seawater within the reservoir could be the primary origin of the fluids forming the calcite veins.

Application of blocky calcite vein LA-MC-ICP-MS U–Pb dating and geochemical analysis to the study of tectonic–fault–fluid evolutionary history of the Tabei Uplift, Tarim Basin

In this work, blocky calcite veins in drilling cores were studied by integrating petrography, in-situ laser ablation multiple collector inductively coupled plasma mass spectrometry U–Pb dating and trace element analysis to investigate whether calcites without obvious syn-kinematic features can constrain the timing of local fault/fracture activity and far-field tectonic movement. On this basis, a synergistic ‘tectonic–fault–fluid’ evolution model in the Tabei Uplift, Tarim Basin was developed. Cathodoluminescence characteristics, rare earth elements and U–Pb ages allow the subdivision of calcite veins into three categories. The much shorter timescales for sealing the studied calcite vein than the uncertainties of current carbonate U–Pb dating (<5 %) attest to a temporal link between calcite precipitation and fracture activity. The consistency between calcite U–Pb ages and the timing of terminal collisions of the Tarim Craton with surrounding terranes demonstrates the potential of blocky calcites to record far-field tectonics. The information on the origin of fluids and the timing of fracture activity revealed by the calcite veins is placed within the established tectonic evolutionary history of the study area, allowing the tectonic–fracture–fluid evolutionary history of the study area to be recovered. From ~466 Ma to ~454 Ma, accompanying the formation of strike-slip faults under compressional stresses resulting from the closure of the Kudi Ocean, seawater and formation fluids seeped down into the carbonate strata and eventually precipitated along the fracture walls. From ~383 Ma to ~366 Ma, continued northward subduction of the eastern Kunlun–Altyn Tagh terrane caused tectonic uplift and fault reactivation. Circulating meteoric fluid and downward seeping seawater transported by advection eventually precipitated blocky calcites. From ~287 Ma to ~232 Ma, activation of the early faults along with the closure of the Paleo-Asian Ocean promoted the formation fluids buffered by the host limestones to flow into open fractures and precipitated. This work, in a practical geological context, confirms that centimeter-scale blocky calcite veins have the potential to constrain the timing of fault slip and associated fracturing, which further boost confidence in the applicability of calcite U–Pb geochronology in dating of brittle faulting episodes.

Gold remobilization of the Sanshandao gold deposit, Jiaodong Peninsula, Eastern China: Perspective from in-situ sulfide trace elements and sulfur isotopes

The Sanshandao is a typical altered-rock-style gold deposit (>260 t @ 3.96 g/t). The deposit is located at the northernmost end of the NE-trending Sanshandao fault in the northwestern Jiaodong Peninsula. The orebodies are mainly hosted by the footwall (minor in hanging-wall) of the fault, which cut Jurassic granites. According to the mineral assemblage and crosscutting relationship, five hydrothermal/mineralization stages were identified: (I) veinlet-disseminated quartz + pyrite ± chalcopyrite, (II) quartz + pyrite + chalcopyrite + native Au veining, (III) quartz + pyrite + galena + sphalerite + native Au + chalcopyrite veining, (IV-a) veinlet-disseminated siderite + pyrite + galena + sphalerite, (IV-b) ore-barren siderite, and (V) ore-barren calcite veining. Stages II and III auriferous pyrite contains visible native gold, and can be divided into four generations (Py2a, Py2b, Py3a, and Py3b). Py2a contains little native gold, whereas Py2b, Py3a, and Py3b contain much coarse-grained native gold. Gold in these pyrite generations correlates positively with the As content. Typical Py2a and Py3a porous/sieve-like texture indicates that Au remobilization has occurred, which may have controlled mainly by coupled dissolution-reprecipitation (CDR). Relative Sb-Te enrichments in Py2b and Py3a imply that a melt with low-melting point chalcophile elements (LMCEs) also facilitated Au remobilization by efficiently scavenging Au from preexisting minerals. Combined with the trace element compositions, we show that CDR may have liberated Au and LMCEs, and the subsequent fluid and LMCEs melt have altogether caused Au remobilization. The narrow δ34SV-CDT range of the pyrite from stages II-III (11.75–12.22‰ (Py2a), 10.83–12.39‰ (Py2b), 11.63–12.88‰ (Py3a), 11.91–12.67‰ (Py3b)), together with the compiled S-isotope dataset (from literature data) (δ34SV-CDT = mostly 7–10‰), indicate that reduction of marine sulfates represents the main sulfur source.

Genesis of Fibrous Calcite in the Chang 7 Member of the Yanchang Formation, Ordos Basin

AbstractTo explore the genesis of the laminated calcite veins developed in the black shale of Chang 73 submember of the Ordos Basin, the petrology, microstructure and geochemistry of calcite veins are studied using thin sections, fluid inclusions, trace elements and isotopic geochemistry. The source, the time of formation, the mechanism of formation, and the dynamic background of the veins are discussed. The veins are mostly made up of calcite, mixed with lenticular or spindle‐shape solid wall rock inclusions. Three structures are identified in the calcite minerals: fibrous, rhombic cleavage, and wedge‐like structure. Trace elements and isotopes of carbon and oxygen confirm that the calcite veins were formed from a high density hydrothermal fluid. It is assumed that calcite veins formed prior to wall rock consolidation during the formation of the Qinling Orogenic Belt in the Middle and Late Triassic. The results show that the sedimentary sources of Chang 73 submember were influenced by hydrothermal materials besides terrigenous detritus. The fibrous calcite is different from the fibrous calcite reported in the morphology and mechanism of formation. In this way, the research complements and improves the morphology and the mechanism of formation of fibrous calcite.

Jewelry Morocco septaria. Gemological characteristics and mineral composition

Background. Septaria, comprising calcareous-clay nodules with calcite veins, are widely used as jewelry and ornamental materials. Septaria from Morocco represented on the world and Russian markets of jewelry and ornamental stones remain gemologically and mineralogically underexplored. In this article, the gemological characteristics and mineral composition of Morocco septaria, as well as the conditions of their formation, are described for the first time.Aim. To determine the gemological characteristics and mineral composition of Morocco septaria, as well as their formation conditions. To conduct their comparison with septaria from other regions of the world.Materials and methods. The author studied the assortment of Morocco septaria presented at international and Russian exhibitions. A collection (15 samples) representing the main decorative septarium varieties was compiled. The materials underwent determination of microhardness, density, luminescence (10 samples), optical-petrographic analysis (4 sections), quantitative determination of mineral and chemical composition (2 samples), electron probe studies (2 samples).Results. The septaria were found to consist of (wt %): quartz — 38, goethite — 16, chlorite (shamosite) — 28, kaolinite — 6, siderite — 5, dolomite — 4, calcite — 1, pyrite, zeolite, apatite, hydrosludes less than 1. Dispersed inclusions of leucoxene, barite, and microcline were discovered. The septarium veins consisted mainly of dolomite, quartz, chlorite with inclusions of siderite, calcite and pyrite. The Zn, Sr, and Ba impurities ranged within 0.01—0.05 wt %, while Cr, V, Ni, Cu, Rb, Zr, Y, and Pb ranged within 0.001—0.01 wt %. The mineral composition of the studied Morocco septaria was found to differ significantly from that of septaria in other regions of the world, where calcite plays a decisive role.Conclusion. Morocco septaria of small size (from 2 to 6 cm) are used as interior decorations and jewelry cabochons. The mineral and chemical (including micro-inclusions and impurities) composition of Morocco septaria was established for the first time, which allows their identification. The mineral composition of the septaria under study is associated with the regional low-temperature metamorphism, which altered the original, predominantly calcite composition.

Applicability of carbonate clumped isotope thermometry in the Tuchang-Jentse geothermal field

Temperature and oxygen isotope composition of fluids, which are critical for reconstructing thermal structures and characterizing hydrothermal fluids in a geothermal system, can be obtained using carbonate clumped isotope analysis. Ideally, carbonate clumped isotope compositions (Δ47) recording the thermal fluid temperatures and compositions for fault-associated veins and rhombic calcite crystals inside open cracks demonstrate previous tectonic activities and present reservoir conditions, respectively. We studied the clumped isotope geochemistry of veins and travertines in outcrops and cuttings from exploration wells in the Tuchang–Jentse geothermal field in Taiwan. The Δ47-based temperatures (Δ47-T; 161−8+8 to 238−14+16 °C) and calculated δ18Ofluid values (+2.0 to +5.0‰) of vein calcites were the highest, indicating mixing of the magmatic/metamorphic fluids with the meteoric water after water–rock interaction in the deep thermal fluid cycle and migration to shallow areas by fault activity. Rhombic calcites with high Δ47-T (142−6+6 to 185−4+5 °C) and low calculated δ18Ofluid (−9.1 to −5.3‰) correspond to downhole temperatures and measured δ18Ofluid of thermal fluids in the exploration wells, respectively, suggesting that the latest thermal fluid was of meteoric water origin. Travertines near faults have low calculated δ18Ofluid (−8.9 to −1.0‰) and Δ47-T that are above air temperatures (32−9+9 to 95−9+10 °C), implying that the local groundwater was subsequently heated by or mixed with deep fluid. Carbonate clumped isotope thermometry application demonstrates that the carbonate clumped isotope thermometer not only precisely estimates temperature in the carbonate-dominated system using drill cutting samples during the exploration phase, but also reveals the spatial and temporary evolution of geothermal fluids used as a reference for future site selection planning.

Precise U-Pb dating of incremental calcite slickenfiber growth: Evidence for far-field Eocene fold reactivation in Ireland

Abstract The Variscan orogen in southern Ireland and Britain is characterized by an intensely deformed, E-W–trending fold-and-thrust belt. Farther north in Ireland, the Carboniferous North Dublin Basin exhibits tight chevron folds and kinematically linked en echelon vein sets, along with bedding-parallel veins with slickenfibers. This deformation is assumed to be Variscan in age, despite lying 150 km north of the supposed Variscan “front.” The laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb dating of these calcite veins undertaken for this study showed that relict Variscan U-Pb ages are very poorly preserved. Instead, late Eocene ages were obtained from many calcite veins, including fold hinge breccias and bedding-parallel slickenfiber veins associated with N-S shortening (flexural slip). Also, U-Pb ages from one bedding-parallel vein showed protracted flexural slip over ~5 m.y. during late Eocene times. Detection of the growth domains within this vein was facilitated by the imaging approach to LA-ICP-MS U-Pb dating adopted in this study, which can identify age-homogeneous domains by integrating spatial U-Pb isotopic information with maps of petrogenetically diagnostic major and trace elements. The late Eocene fold reactivation phase was hitherto undetected on the Irish mainland, but regional Cenozoic N-directed shortening has been documented in Mesozoic–Cenozoic sequences of the southern Irish Sea, Celtic Sea, southern England, and the Paris Basin. We attribute late Eocene fold reactivation to far-field, N-directed shortening associated with the Alpine/Pyrenean orogenies. It is likely that many Variscan or Caledonian folds, particularly in southern onshore Ireland, were reactivated during Eocene–Oligocene shortening, which has not been recognized to date because of the lack of post-Variscan markers (e.g., dikes, Mesozoic–Cenozoic cover sequences).

The role of uranyl complex decomposition and redox conditions in the precipitation of hydrothermal uranium deposits: Insights from chlorite mineralogy and geochemistry in the Shazhou uranium deposit, Xiangshan, SE China

Uranium precipitation involves the decomposition of uranyl complexes and the reduction of hexavalent uranium, which can occur sequentially or simultaneously within one redox reaction. The redox condition of hydrothermal fluid plays a vital role in controlling the migration and precipitation of uranium in hydrothermal uranium deposits. However, little attention has been paid to the role of uranyl complex decomposition in uranium precipitation. In this study, chlorite mineralogy and geochemistry were examined to clarify the process of uranium precipitation in the Shazhou deposit, Xiangshan uranium orefield, Southeast China. Based on comprehensive petrographic and mineral chemistry studies of chlorites obtained from altered granite porphyries and uranium ores, five types of chlorites were identified: (1) chlorite present in the form of pseudomorphous biotite, which was produced by the hydrothermal alteration of biotite in rocks that underwent hematitization and chloritization (Chl-I); (2) chlorite filling the cleavage cracks in biotite in rocks that underwent hematitization and chloritization (Chl-II); (3) chlorite occurring in pyrite veins (Chl-III); (4) chlorite intergrown with pitchblende in ore veins (Chl-IV); and (5) chlorite occurring in calcite veins (Chl-V). Chlorite geothermometry revealed that the formation temperatures of the five types of chlorites ranged from 219 °C to 254 °C. Mineral chemistry analyses revealed that the five types of chlorites formed in a reductive fluid environment, where the oxygen fugacity at different stages is similar, with log fO2 values ranging from −41.6 to −39.1. Uranium precipitation started only in stage Chl-IV. The examination of the mineral assemblage revealed that the ore-forming fluid was rich in F−, HPO32−, and CO32−. Comprehensive investigation of chemistry and physicochemical conditions of the ore-forming fluid revealed that oxidized uranium (UO22+) could be complexed with HPO42− and F−, and uranyl phosphate and the uranyl fluoride complexes were the main uranium species when uranium precipitation and the decomposition occurred at stage Chl-IV. However, the assessment of oxygen fugacity of the solution equilibria between the UO2(s) and the uranyl phosphate and uranyl fluoride complexes revealed that the reducibility of the fluid favoring the reduction of uranyl ions (UO22+) to U4+ was insufficient to reduce the uranyl phosphate and uranyl fluoride complexes. This indicates that the breakup of uranyl phosphate and the uranyl fluoride complexes to release uranyl ions should occur first at stage Chl-IV, reducing uranyl ions to U4+, and leading to uranium precipitation. Hence, the decomposition of uranyl complexes played an important role in uranium precipitation. With the increase in pH, uranyl phosphate and the uranyl fluoride complexes gradually decomposed and became reducible. Moreover, the decrease in ligand concentration was conducive to the decomposition of uranyl phosphate and the uranyl fluoride complexes. During the formation of the Shazhou deposit, the fluid boiling process induced the loss of volatiles, such as CO2, CH4, HF, and H2S, leading to an increase in pH and decrease in HPO42−, F−, and CO32− concentrations in the fluid. These factors also led to the decomposition of uranyl fluoride and the uranyl phosphate complexes and the precipitation of fluorapatite and calcite. Uranium was then reduced by the action of Fe2+ and S− (pyrite) from a hexavalent to a tetravalent, and finally, uranium was precipitated to form uranium ores.

Fluid flow evolution revealed by carbonate clumped isotope thermometry along the fractures in a complex salt dome setting: Study case (Jebel Madar, Oman)

A better understanding of constraining paleo fluid circulation along fractures in tectonically complex regions is essential from the scientific perspective and in exploiting sustainable geo-energies such as geothermal resources, but it remains a challenge. Jebel Madar in Oman as a salt dome is an ideal case study because of the complexity of its tectonic evolution and diagenetic process history. Here, we investigated the Cretaceous carbonate-encasing rock of salt domes and calcite-infilling fractures, making them an ideal case study for paleo fluid flow. The objectives were to test two different hypotheses: (1) to verify the previous study based on seismic data that the Hawasina Nappe had been priorly emplaced at the study location and was subsequently completely eroded, and (2) that the complex history of paleo-fluid flow existed in the fractures of the salt dome, and that this system was not yet fully constrained by previous studies, and also including accounting for the presence of any kinetic fractionation. To achieve our objectives, we applied carbonate clumped isotopes on different generations of veins as well as on the carbonate cement of the host rock itself to reconstruct paleo temperatures and the isotopic composition of the paleo-fluids. The carbonate clumped isotope temperatures indicate that calcite vein precipitation occurred between 48 and 171 °C. Assuming that our carbonate clumped isotopes data faithfully capture the recrystallization temperature of the limestone, the geothermal gradient during the Campanian can be estimated at 33.5 °C km−1, implying that the Hawasina Nappe must indeed have been present at the location. We also argue that the evolution of fluid flow in the study area comprises two distinct episodes of fluid circulation. The first episode led to the precipitation of high-temperature calcites followed by closed-system recrystallization during the local emplacement of the Hawasina Nappe. The second episode of fluid circulation occurred at lower temperatures with calcite members representing meteoric diagenetic fluids with evidence of the kinetic fractionation of the clumped isotopes signature in our samples. Our result demonstrates that the new paleotemperature data present new constraints of paleo fluid circulation along the fractures in a salt-domed setting and, importantly, confirms for the first time that the Hawasina Nappe was present at the site of Jebel Madar in the Late Cretaceous.

Geochronology, Geochemistry, Fluid Inclusion and C, O Isotope Compositions of Calcite Veins in the Paleogene of the Jiangling Basin, South China: Implications for Fluid Evolution and Brine Potash Mineralization

AbstractDeep‐seated potassium‐rich brines were identified in the Jiangling Basin, South China. Although magmatic‐hydrothermal sources have been proposed, the relationship between brine‐type potash mineralization and volcanism remains unclear. In this study, U‐Pb geochronology, geochemistry, fluid inclusion and C‐O isotopic compositions of hydrothermal vein minerals in the Jiangling Basin are examined. Laser ablation U‐Pb dating of calcite veins indicates that the ages are slightly younger than the formation age of the Balingshan basalt. Fluid inclusions in hydrothermal minerals show medium–low homogenization temperatures (160–220°C) and low salinities (0.14 to 4.9 wt% NaCl eqv.) and densities (0.882–0.944 g/cm3). The liquid compositions of fluid inclusions in calcite veins from sedimentary strata have higher contents of potassium, compared with those from basalt. The coupled negative δ13CPDB (–10.3‰ to −8.0‰) and positive δ18OSMOW (17.4‰ to 20.7‰) values imply that calcite precipitation resulted from CO2 degassing of the basaltic magmatic fluids, as indicated by the gas composition of these inclusions in hydrothermal minerals. Rare earth element patterns indicate that water‐rock interaction between hydrothermal fluids and sedimentary wall rocks contributed to the calcite precipitation in sedimentary strata. It is proposed that high‐temperature water‐rock interaction between magmatic fluids and sedimentary strata resulted in the potassium enrichment in fluids, interpreted as one of the sources of potassium‐rich brines in the Jiangling Basin.

Ultra-deep carbonate basement reservoirs formed by polyphase fracture-related karstification in the Offshore Bohai Bay Basin, China

The Palaeozoic carbonate basement of the Offshore Bohai Bay Basin (OBBB) presents considerable potential for hydrocarbon exploration. However, the multistage tectonism and complex superimposed palaeo-karstification in the area are unclear, which leads to a lack of understanding on the formation mechanism and distribution of the deep carbonate basement reservoirs. In this study, the occurrence of a fracture-vug network and its fillings in carbonate reservoirs were investigated based on borehole cores, thin sections, and image logs from the southwestern slope of the OBBB's Bozhong Sag. Then the diagenetic fluid properties of the carbonate matrix and fillings were analysed via the data of carbon, oxygen, and strontium isotopes, and major, rare elements from coring intervals. The results revealed that fracture-related karst reservoirs have lithologic selectivity inclined toward dolomite strata. The intersecting relationships, widths, and strikes of the fractures and the regional tectonic background indicate three structural fracture families: NW-, NNE-, and NNW- trending, related to the Indosinian, middle Yanshanian, and late Yanshanian orogeny, respectively. The Indosinian NW- and end-Mesozoic NNE-trending fractures produced by compressional tectonic stress mainly contributed to the formation of the basement reservoirs. The geochemistry of the calcite veins filling these fractures suggests two main types of diagenetic fluids. The fluid of autogenic recharge related to the earlier fills is karstification diffuse flow dominated by internal runoff from rainfall in the highland setting of the Indosinian thrusting orogenic belt. The other fluid of allogenic recharge related to the later fills is the main lateral freshwater flow dominated by external runoff from the catchment in the setting of the horst-lowland within the rifting basin, induced by the Yanshanian destruction of the North China Craton. Finally, the relationship between the three fracture families and two kinds of related fluids is revealed. This allows us to propose a model to understand the polyphase-superimposed fracture-related karst reservoir complexes within the deep carbonate basement of tilting fault blocks that neighbour the Bozhong hydrocarbon kitchen and predict the formation of potential plays with high accuracy.

The influence of Cenozoic Eurasia-Arabia convergence on the Southeast Arabian Foreland Basin: new geochronological and geochemical constraints from syn-kinematic carbonate mineralization

The Cenozoic succession of the Jabal Hafeet anticline yields the most complete surface expression of the deformation that affected the Southeast Arabian Foreland Basin (SEAFB). The carbonate rocks of the Eocene Rus Formation comprise the core of the Jabal Hafeet anticline and host a network of fractures and carbonate veins associated with dynamic fracture opening and sealing events. These fracture networks developed during the propagation of compressional stresses from the Makran and Zagros fold-and-thrust belts into their foreland basin system (the SEAFB) and are associated with Arabia-Eurasia convergence. Syn-kinematic calcite veins associated with the Cenozoic folding events in the SEAFB were dated by U–Pb LA-ICP-MS carbonate geochronology and characterized further by Raman fluid-inclusion geochemistry. The U–Pb data show that Cenozoic compression linked to the propagation of the Makran fold-and-thrust belt into the SEAFB took place from c. 20 Ma (early Miocene) to c. 2 Ma (mid Pleistocene). Raman fluid-inclusion data reveal the presence of complex hydrocarbons within the parent carbonate-bearing fluids, reflecting a fluid transport pathway between the upper Cenozoic rocks and deeper hydrocarbon-bearing Mesozoic sequences. Combined isotopic and geochemical datasets show that the deformational history of the SEAFB is likely related to the reactivation of inherited deep-seated structures in the upper Cenozoic stratigraphic sequence due to the far-field stress propagation from the Makran belt into the Arabian peninsula, rather than the propagation of a thin-skinned deformation architecture.

Trace Element and Sulfur Isotopic Analysis of Pyrite from the Luyuangou Gold Deposit, Xiong’ershan Au-Ag Polymetallic District, Central China: Implications for The Origin and Evolution of Ore-Forming Fluids

The Luyuangou gold deposit is located in the eastern section of the Xiong’ershan Au-Ag polymetallic district (XESPMD) and consists of a few gold-bearing veins found in the EW-striking faults located in the Archean Taihua and Mesoproterozoic Xiong’er Groups. The gold deposits contain numerous gold-bearing pyrites in thin quartz veins, representing an ideal tool for explaining the enigmatic genesis of gold deposits in the XESPMD. The distributions of trace elements and the sulfur isotopes of gold-bearing pyrite in the Luyuangou gold deposit were investigated to define the origin and evolution of ore-forming fluids. Five generations of pyrite have been identified: coarse-grained euhedral pyrite cores (Py1-1) and margins (Py1-2) in milky quartz veins, fine-grained pyrite (Py2) in quartz veins and host rocks, pyrite (Py3) in quartz + polymetallic sulfide veins, and pyrites (Py4) in quartz calcite veins. The distributions of trace elements indicated that Py2 and Py3 represented the main gold-bearing minerals and contained high concentrations of As, Au, Ag, Pb, Zn, and Cu, and the distributions were controlled by the micro/nanoinclusions. The δ34S values in the five pyrite generations ranged from −19.5 to 3.4‰. Py2 (−15.4 to −6.1‰) and Py3 (−19.5 to −12.4‰) had the lowest δ34S values, indicating that the sulfur originated from an oxidizing fluid. Py1 showed δ34S values (−0.3 to 1.9‰) corresponding to a magmatic origin. Py4 (1.1–3.4‰) displayed the highest δ34S values, indicating that the sulfur originated from the host rock under the action of meteoric water cycles. Analyses of the pyrite’s trace elements and sulfur isotopes, in combination with geological evidence, indicated that magmatic ore-forming fluids contributed to the formation of the Luyuangou gold deposit. The magmatic ore-forming fluids interacted with meteoric water during the main mineralization period. The changing physicochemical conditions of the mineralized fluids caused the precipitation of a large amount of gold and other mineralized elements.

Petrographic and Physiomechanical Investigation of Late Cretaceous Kawagarh Formation Kahi Section, Nizampur Basin

The late Cretaceous Kawagarh Formation has been investigated in terms of field observation, and petrographic analysis, to understand the petrography and its impact on the geotechnical properties. The Kawagarh Formation is well exposed among the upper Indus Basin, and has been studied by various workers in different aspects. Kawagarh Formation exposed in Kahi section of Nizampur Basin has been selected in this study to know the behavior of carbonate rocks for engineering purposes. Lithologically, this formation is composed of thick to medium bedded, highly fractured limestone, marls, and dolomitic limestone which has undertaken diagenetic alteration including dolomite, calcite veins, and stylolites. Followed by petrographic analysis which reveals that the Kawagarh limestone is mostly fossiliferous comprised of a large number of planktonic foraminifera fossils like Globotruncana Hilli and Globotruncana Linneana fossils. Furthermore, to know the impact of petrographic minerals on engineering behavior, mechanical properties in terms of uniaxial compressive strength (UCS) and uniaxial tensile strength (UTS) were also computed by using a universal testing machine (UTM). The resultant mechanical values lie in the strong compressive strength and suggest their usage for various construction purposes. Aggregate degradation tests including water absorption, specific gravity, aggregate impact value, Los angles abrasion, and soundness was also computed according to the International standard organization, ASTM (American Society for testing materials) and British standard. The aggregate values of the Cretaceous Kawagarh Formation are within the defined standard limits and can be used as an aggregate source for different construction engineering projects.

Rare Earth Partition Characteristics and Sedimentary Diagenetic Response in Layered Argillaceous Limestone: Taking the Shale of Upper Es4 in the Nx55 Well Area as an Example

Taking the layered argillaceous limestone in the upper Es4 in the Dongying Sag as the research object, the geochemical analysis of major, trace, and rare earth elements (REEs) established the response relationship between REE distribution characteristics and sedimentary diagenesis. The average values of total light REE ( Σ LREE )/total heavy REE ( Σ HREE ) of micrite calcite and argillaceous laminae are 6.75 and 4.06, respectively. The LREEs and HREEs are differentiated, consistent with the distribution pattern of REEs in the crust. Th and U elements are more enriched in the sediments in the lacustrine sedimentary environment than in the diagenetic calcite veins. In primary sediments (argillaceous clay and micrite calcite laminae), LREEs are more enriched, HREEs are depleted, and Eu shows positive anomaly-enrichment characteristics. The LREEs and HREEs of the sparry calcite veins are lower than those of the original sediment argillaceous clay and micrite calcite, showing characteristics of a negative anomaly depletion. Sparry calcite veins originate from diagenetic fluid crystallization and precipitation and have the characteristics of low Th and U, evident positive anomalies of Sr and Eu, and substantial depletion of La. The distribution patterns of REEs within the four components of the laminated argillaceous sparry limestone reflect the order of REE distribution from primary sediment laminae (argillaceous clay and micrite calcite) to diagenetic laminae (calcite veins). Compared with the North American shale, the four components of the contact surface between the argillaceous and bright crystalline laminae, the micrite calcite, the calcite veins, and the argillaceous laminae all showed weak negative δ Ce anomalies and positive δ Eu anomalies. The fractionation degree between LREEs and HREEs reflected by La / Sm cn and Gd / Yb cn is in descending order: the interface between the argillaceous lamina and sparry calcite lamina, micritic calcite, calcite vein, and argillaceous lamina. The argillaceous laminar material has the characteristics of basalt REEs, indicating that the terrestrial debris and argillaceous lacustrine shale in the upper Es4 member of the Niuzhuang subsag are primarily derived from the basic extrusive rocks of the Qingtuozi bulge. REE differentiation is most noticeable at the interface between the argillaceous lamina and calcite vein, proving the directionality of REE differentiation from the original sedimentary lamina to the diagenetic lamina. Shale in the study area is primarily deposited below the redox interface of water at a certain depth, and the deposition rate is stable and slow, providing good conditions for preserving organic matter.