Gypsum Veins Research Articles

Redistribution of Hydrothermal Resources on Different Elements of the Microrelief of Soils in the Semi-desert Zone and Agrolandscapes under Wheat

In the semi-desert regions of Azerbaijan there are habitats with varying elevations of only a few tens of centimeters. The zonal soils on the territory of the massif are gray-brown soils. Gray-brown soils develop under ephemeral plant cover. The soil is carbonate throughout the entire profile. Below the carbonate horizon lies a layer enriched with sulfates in the form of veins of crystalline gypsum. The content of absorbed sodium in the gray-brown soils of the deluvial plain of the Siyazan-Sumgait massif is 30-50% of the number of absorbed bases in the Solonetz-Columnar soil horizon. The amount of humus in the upper layers of the soil fluctuates between 1-2%, towards the bottom it sharply decreases. In studies carried out on gray-brown soils of the Siyazan-Sumgait massif, hydrothermal potential indicators vary within the range of 426-552 m3/ha. The distribution of temperature and humidity in depth in the soil under wheat differs from the natural habitat. In the arable layer, the temperature is evenly distributed throughout the entire profile, while the humidity gradually increases in the lower layers. The influence of the hydrothermal regime on wheat yield was: 2022 — 24.0-30.0 cwt/ha; 2023 — 16.0-24.5 cwt/ha.

Thermoluminescence dating of gypsum in loess deposits

Gypsum deposits within loess deposits are an important indicator of weathering processes and climate change. We conducted a thermoluminescence (TL) dating study of the gypsum from loess deposits of the Jiuzhoutai section in the western Chinese Loess Plateau. Determination of the equivalent doses (De) of gypsum samples was conducted using several protocols, including single-aliquot regenerative-dose (SAR-TL), multiple-aliquot additive dose (MAAD-TL), and Isothermal TL of SAR (SAR-ITL). The TL glow peak at 280 °C was targeted because of its reported stable signals. The test dose check, estimation of sensitivity changes, and the dose recovery test were applied to assess the applicability of the different protocols, and the characteristics of the thermoluminescence signals for gypsum were evaluated. The De results for the SAR-TL, MAAD-TL, and SAR-ITL protocols were within the error ranges. Among the three protocols, SAR-ITL did not involve high-temperature heating and it provided a more accurate De value, and thus it was used for De determinations for gypsum dating. With a dose rate of 1.74 ± 0.07 Gy/ka, the minimum TL age of the gypsum veins was ∼182 ± 7 ka. U-series dating was also applied to the same samples for comparison. However, the extremely low ratio of 230Th/232Th (≤4 × 10−6) of the gypsum veins deposited within the loess resulted in a large error of the U-series dates. We conclude that TL dating is more appropriate for the dating of gypsum in loess deposits.

Karst collapse and gypsum-filled veins in Eocene outcrops, Dammam Dome, Saudi Arabia: Causes and implications

• Differential weathering of Dammam Dome through uplift and erosion. • Factors affecting Dammam Dome geomorphology. • Gypsum-filled veins on the Dammam Dome due to karst collapse. • Results from this study can be related with water and hydrocarbon reservoirs. A recent road cut through Dammam Formation outcrop on the Dammam Dome in Dhahran, Saudi Arabia exposed an array of open fractures and gypsum-filled veins. The widespread array of fractures in the road cut was mostly contained within Saila Shale member and consisted of horizontal, vertical, and oblique extensional fractures, most were filled with gypsum. The gypsum veins are of varying thickness with the largest exceeding 2.5 inches. Vertical gypsum veins, similar to those encountered at the road cut, were found in adjacent areas. Thin layer of gypsum (about 1 inch) was also observed in the Rus Formation. The open fractures, in the Saila and Midra Shales members, were devoid of gypsum fill. Most of the open vertical and horizontal fractures were interpreted to be recent with indications of triggered by the excavation work. The open fractures and filled veins cover a stretch of about 250 ft. across the road cut that also exposed what was interpreted as a local karst collapse. Horizontally-layered and continuous rocks overlying the collapse indicated that it was a paleo structure. The causal mechanism for fracture formation and subsequent fill was associated with deformations accompanying post-karst collapse. Here, natural fracturing and fluid transport were not synchronous events. Insights from this interpretation offered an explanation for widespread surface structures with major and subtle morphologies on the Dammam Dome. These insights also highlighted the significance of collapse in the modification of the Dammam Dome over a geological timescale. The unstable nature of gypsum, prevalent in Dammam Formation in the Dammam Dome, has potential implications for integrity of anthropogenic structures such as roads and building foundations.

Impact of sedimentary fabrics on small-scale permeability variations within fine-grained sediments: Early Silurian Qusaiba Member, Northern Saudi Arabia

The main aim of this study is to constrain the range of reservoir properties observed in various depositional environments of the fine-grained sandstones of the early Silurian middle Qusaiba Member, Qalibah Formation, northwestern Saudi Arabia. Facies analysis, micro-X-ray fluorescence (μXRF), lamina-scale permeability measurements, automated helium porosity-permeability measurement, and pore system visualization were integrated to observe variations in these various environments. Seven main facies were recognized and interpreted to be deposited in an offshore “turbiditic-influenced” to lower shoreface paleoenvironments. These facies include current rippled siltstone (F1); Contorted silty sandstone (F2); current rippled fine-grained sandstone (F3); planar-laminated fine-grained sandstone (F4); Bioturbated to current-rippled argillaceous siltstone to silty sandstone (F5); Current rippled silty sandstone (F6); bioturbated sandstone (F7) facies. These facies were clustered into several fabric classes; 1) contorted fabrics associated with load casts flame structure that is characteristic of F2: These typically revealed slightly higher porosity and permeability values than the surrounding media with a spherical and relatively low anisotropy. 2) Planar-laminated fabrics in F4 that overlain by cryptic bioturbation: It comprises of clay mineral-rich laminae with higher porosity and permeability and the quartz-rich laminae with lower porosity and permeability probably due to abundant calcite cement associated with detrital quartz. Higher anisotropy, associated with this fabric, was observed. Cryptic bitourbated interval showed relatively low permeability. 3) In F7, three types of rock fabrics were observed, (1) Vertical sand-filled Piscichnus trace fossil: The burrow fill shows significantly higher porosity and permeability values than the laminated matrix; this fabric reveals an elliptical and highly anisotropic variance map. (2) Horizontal sand-filled Thalassinoides burrow (with iron oxide cement).: The internal core of this fabric was highly porous and permeable compared to the laminated fabric. Thalassinoides fabric had the most elliptical and highest anisotropic variance map in F7. (3) Entirely laminated fabric with bed-parallel gypsum veins: The permeability of the veins was slightly higher than that of the matrix, this resulted in a less anisotropic variance map. μCT scanning was used for F5 and F6; In the F5 facies, more significant porosity associated with quartz-rich lamina than iron oxide (e.g., siderite) and gypsum-rich lamina were observed probably due to higher chlorite alignment in the iron oxide-rich lamina. We found lower porosity and permeability within the vertical mud-filled burrows (Piscichnus) than the laminated fabric; this was also observed in the 3D pore system imaged by μCT scanning. These findings could have significant implications for different fabric types in fluid flow conduits in argillaceous sandstone and siltstone systems.

Contractional fold amplification through bed-parallel gypsum vein (“beef”) formation

Field observations and structural analysis of a 50-m-wavelength gentle anticline document bed-parallel fibrous gypsum veins (“beef”) and calcite veins associated with contractional folding in the Boquillas Formation, east of Langtry, west Texas. Gypsum beef veins with bed-perpendicular fibers accommodated vertical extension of 13% in the anticline, diminishing to 0% in adjacent synclines. Bed-parallel calcite veins accommodated 0–4% vertical extension across the structure, with no clear correspondence of vein distribution to structural position. Homogenization temperatures from two-phase fluid inclusions in calcite veins indicate trapping depths of 2.0–2.7 km. Source rock analysis from unoxidized organic-rich mudrock at the site shows that the rocks never reached hydrocarbon maturation conditions, therefore vein formation cannot be explained by in situ hydrocarbon expulsion. Bed-parallel veins with vertical or bed-perpendicular fibers document a thrust faulting stress regime during contractional folding near the frontal limit of Laramide deformation. Gypsum vein formation may be related to meteoric water interacting with pyrite or H2S to form sulfuric acid, dissolution of carbonate, and deposition of gypsum as an auxiliary mineral. Switching between calcite and gypsum mineralization suggests opening and closing of fault and fracture controlled fluid pathways during deformation.

Timing and distribution of bedding-parallel veins, in evaporitic rocks, Bouhedma Formation, Northern Chotts, Tunisia

Bedding-parallel veins, ‘BPV’, are widespread worldwide especially within sediments of low permeability such as mudstones, marls and evaporitic formations. Gypsum, calcite or quartz fibrous minerals compose these geological features as well as less common components such as bitumen. These structures are heavily studied as they provide significant information about the paleo-stress states, host rock deformation and fluid flow. In the Bouhedma Formation, Northern Chotts Range, Southern Tunisia, although gypsum BPV are clearly observed there is particularly a dearth of published data concerning their characterization. Petrographic examination, tectonic and micro-tectonic analysis of both of the BPV and host rocks were conducted to determine their origin and the formation mechanism of the bedding-parallel fibrous gypsum veins in the Bouhedma Formation.Field studies indicate that the host rocks for the BPV are mostly mudstones and evaporites. The BPV are especially localized within the lower part of the Bouhedma Formation, at boundaries between these sedimentary beds. The deformed gypsum and anhydrite fibers composing the BPV indicate vein growth during the shortening of the basin from the Miocene through Villafranchian times. We argue that the BPV are the result of fluid overpressure and natural hydraulic fracturing in low-permeable sediments due to mineralogical transformations and tectonic compression.

Paleoproterozoic Rajapalot Au-Co system associated with evaporites: Chemical composition and boron isotope geochemistry of tourmaline, and sulfur isotopes of sulfates, Peräpohja belt, northern Finland

The Raja Au-Co mineralization in the Paleoproterozoic Peräpohja belt in northern Finland is part of the wider Rompas-Rajapalot mineralized area with several known Au-Co occurrences. The area is characterized by distinct tourmaline occurrences, spatially associated with Au-Co mineralization. Four texturally distinct tourmaline types were identified although all analysed tourmalines belong to the alkali-group and are classified as dravite. δ 11B values and fractionation modelling indicates that at least two distinct fluids were responsible for the tourmaline formation with initial end member δ11B values of the fluids at −8‰ and −1‰. Possible sources for boron rich fluids are Svecofennian orogeny related (ca. < 1.9 Ga) metamorphic fluids and magmatic-hydrothermal fluids related to the late-orogenic ca. 1.78 Ga granitoids. Structural data from the quartz-tourmaline veins outlines a strong linear trend towards the Raja high-grade mineralization trend. This is interpreted as evidence for structural control of the Raja mineralization, which possibly is contained in shear-zones or within the hinge regions of local high degree folding.A distinctive rock unit with bright purple anhydrite layers and white gypsum veins has been intersected by drilling. While the sulfate-rich unit is unmineralized, the mineralogy and regional geological setting suggests an evaporitic origin. Anhydrite has δ34S values in a narrow range from 8.1 to 9.8 ‰. Gypsum has slightly heavier δ34S from 10.6 to 12.2 ‰. Together with isotope fractionation constraints, textural evidence suggests the authigenic formation of gypsum with SO42- sourced from anhydrite. Significant similarities in textures and sulfur isotope values to well-known evaporite successions of the Onega basin, western Russia, further supports the presence of evaporitic strata within the Peräpohja belt.

Hydrothermal vein distribution in skarn type alteration zone of area X at PT. Freeport Indonesia

Kucing Liar is an economic skarn altered deposited located in Tembagapura, Papua. This deposit was found on Late Cretaceous to Miocene sedimentary rocks of Kembelangan Group and Papua New Guinea Limestone Group. The alteration from this deposit can be divided into groundmass alteration, named penetrative alteration, and selvage alteration forming hydrothermal veins. The penetrative alteration is divided into 9 zones, meanwhile the hydrothermal veins are classified by its mineral composition into 5 types of anhydrite veins, calcite, quartz, sulphide, and gypsum veins. Each vein type will associate the most with different penetrative alteration zone due to its protolith composition and on what stage the vein formed. The anhydrite and quartz veins are found to be associated the most with calc-silicate hornfels zone with the anhydrite sources came from calcite and quartz sources came from the surrounding rocks which the alteration fluid passed through. The sulphide and calcite veins are found to associate the most with dolomitization zone with calcite sources came from the surrounding rock and sulphide deposited filling the open spaces, such as fracture, in crystalline dolostone. Gypsum veins are found to associate the most with fault zone because it formed from the alteration of calcite by faulting process.

Fluid migration along faults and gypsum vein formation during basin inversion: An example in the East Walanae fault zone of the Sengkang Basin, South Sulawesi, Indonesia

In this study, field, petrographic, and sulfur isotope investigations were conducted for gypsum veins that were formed in the sedimentary rocks of the Walanae Formation in the Sengkang basin, South Sulawesi, Indonesia. Four types of gypsum veins developed in the study area: (1) veins sub-parallel or parallel to bedding, (2) veins perpendicular to bedding, (3) vein networks that fill in complex fractures, and (4) veins that surround carbonate concretions. Their internal structures suggest that fibrous gypsum in these veins grew antitaxially from both sides of the median zone toward the vein walls. Judging from the structural relationships of the orientation of veins with the Sengkang anticline, these gypsum veins were most likely formed as a result of hydraulic fractures or layer-parallel compression within a high fluid pressure environment triggered by the activation of the East Walanae fault under the E-W lateral compression during the Pleistocene basin inversion. The relatively wide range of sulfur isotope values indicates that sulfur in gypsum veins was delivered by the fluids that reacted with framboidal pyrites in the sedimentary rocks.

Superimposed mineralization in the deformed Yulekenhalasu porphyry Cu-Mo deposit (Northwest China): Rb-Sr geochronology, S isotope, and trace element analysis of chalcopyrite

The Yulekenhalasu porphyry Cu–Mo deposit is located ca. 40 km southwest of Qinghe, in the Chinese Altay-East Junggar section of the Central Asian Orogenic Belt, NW China. The belt preserves multistage tectonic-hydrothermal and porphyry Cu-Mo mineralization events associated with arc to syn- and post-collision environments over 87 m.y. The 359 Ma deformed granitic porphyries are characterized by positive εNd(t) (+0.62 to + 3.77) and (87Sr/86Sr)initial values (0.704304–0.705960), and old crustal model ages of 747 to 1216 Ma, whereas a 272 Ma sample from late chalcopyrite–molybdenite–pyrite–magnetite–feldspar–calcite–gypsum veins yielded (87Sr/86Sr)initial values of 0.704415 to 0.705764, high εNd(t) values (+5.81 to + 5.88) and young crustal model ages (715 to 614 Ma) suggesting an increasingly juvenile mantle source over time. Seven fragments of a single deformed chalcopyrite grain yielded an Rb-Sr isochron age of 358.6 ± 4.8 Ma (2σ, MSWD = 1.8), which is consistent with the 360 Ma recrystallized zircon grains in the deformed host granitic porphyries. Eight fragments of chalcopyrite from the chalcopyrite–molybdenite–pyrite–magnetite–feldspar–calcite–gypsum veins formed along cleavage planes yielded a younger Rb-Sr isochron age of 272.2 ± 2.6 Ma (2σ, MSWD = 1.8). The younger age is interpreted as the lower age limit for the Mo and Cu mineralization in the post-collisional stage. The heterogeneous initial Sr isotopic compositions of chalcopyrite in the Yulekenhalasu deposit are likely the result of phyllosilicate inclusions. Disseminated chalcopyrite crystals in the granitic porphyry have different trace elemental contents than chalcopyrite in the deformed granitic porphyry and late chalcopyrite veins. Our Rb-Sr and trace element data for chalcopyrite grains constrain the age and geochemistry of multistage mineralization in porphyry Cu-Mo deposits. In-situ S isotope data of chalcopyrite from the deformed granite porphyries and chalcopyrite-feldspar-quartz-muscovite veins display a similar mantle origin. The 272 Ma mineralization event at Yulekenhalasu is coeval with the formation age of the Cu-Ni-(PGE) sulfide deposit in the Chinese Altay, indicating that hydrothermal mineralization like that in porphyry deposits can form proximal to ultramafic–mafic rocks and potentially may have a genetic relationship with mantle-derived Cu-Ni-(PGE) sulfide deposits.

AN UNUSUAL CORROSION PRODUCT, KOBYASHEVITE, FROM ANCIENT EGYPTIAN COPPER ARTIFACTS: A TECHNICAL NOTE

Kobyashevite is one of copper sulfate hydroxide hydrate minerals “devilline group”. It forms as a thin coating crust in calcite and gypsum veins and it takes flower-like morphology and its color varies between the bluish-green to turquoise-blue. In the laboratory studies, it has been synthesized in humid atmospheric air containing sub-ppm of SO2. In the corrosion studies of archeological metals, it has not been identified before. Fortunately, the case study objects have never been undergone any conservation processes before, which gives originality of the analyzed samples and more accurate results. Scanning electron microscopy-energy dispersive X-ray spectroscopy, X-ray diffraction, X-ray fluorescence, and Raman spectroscopy have been carried out in order to identify the corrosion samples. The present study identified a kobyashviete corrosion product for the first time in the archaeological studies on some archeological copper artifacts from the Egyptian Museum in Cairo. The study has been suggested that the formation of the unusual corrosion product kobyashviete is due to exposure the case study to uncontrolled long–term humid storage environment which has been contaminated with a high percentage of sulfur gases.

Multiscale and Multispectral Characterization of Mineralogy with the ExoMars 2022 Rover Remote Sensing Payload

AbstractIn 2022, the European Space Agency and Roscosmos will launch the ExoMars rover, with the scientific objective to detect evidence of life within the Martian surface via the deployment of a 2 m drill. The ExoMars Pasteur payload contains several imaging and spectroscopic instruments key to this objective: the Panoramic Camera (PanCam), Infrared Spectrometer for ExoMars (ISEM), and Close‐UP Imager (CLUPI). These instruments are able to collect data at a variety of spatial (sub‐mm to decimeter) and spectral (3.3 to 120 nm) resolutions across the 440 to 3,300 nm wavelength range and collectively will form a picture of the geological and morphological characteristics of the surface terrain surrounding the rover. We deployed emulators of this instrument suite at terrestrial analog sites that formed in a range of aqueous environments to test their ability to detect and characterize science targets. We find that the emulator suite is able to effectively detect, characterize, and refine the compositions of multiple targets at working distances spanning from 2 to 18 m. We report on (a) the detection of hydrothermal alteration minerals including Fe‐smectites and gypsum from basaltic substrates, (b) the detection of late‐stage diagenetic gypsum veins embedded in exposures of sedimentary mudstone, (c) multispectral evidence of compositional differences detected from fossiliferous mudstones, and (d) approaches to cross‐referencing multi‐scale and multi‐resolution data. These findings aid in the development of data products and analysis toolkits in advance of the ExoMars rover mission.

Origin of the gypsum veins associated with the diagenetic dolomite nodules from the Neogene Marls of South-East Spain

The late Neogene marls from the Lorca, Murcia and Vera basins in S-E. Spain contain abundant dolomite nodules that were formed due to intense methane-rich fluid migration. The pathways for these fluids are evidenced by dense networks of fractures that are crossing the sedimentary layers and eventually the dolomite nodules. The fractures are generally filled by secondary fibrous gypsum that form veins of a few cm thick.The oxygen and sulfur isotopic compositions of the gypsum veins exhibit wide ranges (−2.2 < δ18O‰ VSMOW <6.7; −22.3 < δ34S‰ VCDT <10.5) that are far away from the average δ values of Tortonian-Messinian gypsum from the basins of Lorca, Fortuna and Sorbas (δ18O = 13.8 ± 2.3‰; δ34S = 21.3 ± 1.0‰) precipitated from the late Neogene seawater sulfate. These low δ values of the gypsum veins clearly indicate their diagenetic origin that would have resulted from sulfide oxidation with 18O-depleted water. The isotopic compositions of the diagenetic dolomite nodules close to the gypsum veins (−0.7 < δ18O‰ VPDB <3.7; −11.9 < δ13C‰ VPDB <7.3) are indicative of their formation in methane-rich fluids. In this context, the formation of sulfide is related to the bacterial sulfate reduction that is associated with the anaerobic oxidation of methane. During this process, pyrite coprecipitated with dolomite, either within the dolomite framework or within conduits where the fluids were seeping. The pyrite oxidation occurred later during diagenesis when more or less oxygenated groundwaters circulated within the fractures during the sealevel drawdown of the Messinian Salinity Crisis and after the regional uplift of the sedimentary deposits in relation with the Betic cordillera tectonics.

Petrography and isotopic geochemistry of bedding-parallel fibrous gypsum veins in the Neogene Qingshuiying Formation of the Ningnan Basin, North China: Insights for growth model of antitaxial fibrous veins

Bedding-parallel fibrous gypsum veins, which are widely developed in the Neogene Qingshuiying Formation of the Ningnan Basin, a Tertiary basin located in the northeastern margin of the Tibetan Plateau, provide valuable information on stress field, fluid origin and host rock deformation during their formation. The systematic petrographic observation and isotope analysis of both of veins and wall rocks were carried out to investigate the formation mechanism of bedding-parallel fibrous gypsum veins. The distribution of fibrous gypsum veins shows vertical heterogeneity, and mainly occurs in the middle and lower part of the Qingshuiying Formation with relatively high gypsum content. Single fibrous gypsum veins consist of a median zone and two fibrous zones. The median zone is characterized by blocky gypsum crystals and multiple host rock fragments formed by crack-seal events. The fibrous zone is composed of parallel gypsum fibres a high angle to the vein walls, indicating shear component. Vein opening is considered to be the result of fluid overpressure caused by horizontal compression in low-permeability mudstone during basin inversion. Fibrous gypsum crystals grow continuously without obvious growth competition between adjacent crystals. This indicates that growth of gypsum fibres occurs in limited space, even without fractures. The main driving force for vein dilation is force of crystallization of gypsum fibre growth, and fluid pressure also has a certain contribution. The sulfur and strontium isotopes of gypsum veins exhibit a high degree of consistency with wall rocks, which indicates the material of the fibrous gypsum veins is derived from the wall rock. A suitable growth model of bedding-parallel fibrous gypsum veins in the Ningnan Basin is established, which explains the process of vein interaction, and the formation of V-shaped or U-shaped solid inclusions.

Mesozoic porphyry Cu–Au mineralization and associated adakite-like magmatism in the Philippines: insights from the giant Atlas deposit

The Atlas porphyry Cu–Au deposit is located in central Cebu Island in the Philippines, with a proven mineral reserve of 1420 Mt at 0.45% Cu, 0.24 g/t Au, 0.018 g/t Mo, and 1.8 g/t Ag. It is associated with the Lutopan quartz diorite porphyry stock that was emplaced into the Cretaceous volcano-sedimentary sequences of the Cansi Formation. Two stages of hypogene alteration have been identified at Atlas: stage I quartz–magnetite ± chalcopyrite ± pyrite veins associated with potassic and propylitic alteration and stage II anhydrite–pyrite–chalcopyrite ± specularite veins associated with phyllic alteration. Supergene gypsum veins are also present. Mineralization is mainly associated with the stage I and stage II veins. The Lutopan quartz diorite porphyry yielded a zircon U–Pb age of 108.5 ± 1.6 Ma and is characterized by high Sr/Y (54–69), with the corresponding magma having high oxygen fugacity (avg. ΔFMQ +2.7, avg. Ce4+/Ce3+ of zircon = 439) and H2O concentrations (presence of amphibole and biotite phenocrysts) and low magmatic temperatures (avg. Ti-in-zircon thermometry = 706 °C). The relatively low La/Yb ratios (9–11), listric-shaped normalized REE patterns with weak or absent Eu anomalies, together with zircon eHf(t) values of 3.4 to 10.0, whole-rock ISr values of 0.70373–0.70382, eNd(t) values of 2.4–3.4 and Mg numbers of 49–53 suggest that the Lutopan quartz diorite porphyry was likely derived from partial melting of young lower continental crust or interaction between asthenospheric melts and lower crust melts, that evolved to adakite-like trace element compositions through amphibole fractionation. Stage I veins yielded a pyrite Re–Os isochron age of 110.2 ± 5.6 Ma, making Atlas the only Mesozoic porphyry deposit discovered in the Philippines.

Progressive accretion of antitaxial crystal fibres: Implications for the kinematics and dynamics of vein dilation

We made field and petrographic observations of antitaxial crystal fibres in the gypsum veins of the Mercia Mudstone (SW England) to examine the kinematic and dynamic implications of these fibres. The gypsum fibres are predominantly oriented oblique to vein walls, indicating that the veins were formed in a shear-extension mode rather than pure tension mode. Fibres in single veins are parallel aligned, regardless of the vein-wall curvatures, except those in the tapering tip zones. Field data demonstrate that there is no clear correlation between fibre plunges and vein dips. Fibres in many vein steps exhibit the same orientations as those in the main vein segments, suggesting that these steps were formed as reactivated pre-existing faults/fractures instead of dilatational jogs. Vein tips that propagated progressively, commonly curved towards the planes of neighbouring larger veins, or interacted with the tips of adjacent sub-parallel veins, suggesting that vein propagation may have been dominated by the local stress field rather than remote stresses. Fibres in vein branches may not share the same orientations as those in the parent veins. The host marls are commonly segmented into multiple parts by interconnected vein arrays that contain differently oriented fibres in each vein. Our study suggests that the gypsum veins lack evidence for crack-seal increments, and the crystal fibres in the veins had a continuous accretion towards vein walls. The fibres are parallel to vein opening directions as indicated by the correlative vein-wall irregularities, and are of great potential for kinematic analysis and deformation restoration of the host rocks. Vein dilation could be driven by force of crystallization of veins exerted on the enclosing rocks, which could have caused a growth-related compaction and fracturing rather than uplifting the overlying strata that was accompanied with a bulk extension.

Quantifying vein attributes in massive mudstones (Triassic, SW England): Implications for progressive evolution of opening-mode fracture networks

Abstract Attributes of gypsum vein arrays in the Triassic massive mudstones of SW England were analyzed to infer the process of vein network evolution. Field data demonstrate that there is no clear correlation between vein orientation and thickness; however, sub-horizontal veins have a higher average thickness than the inclined veins. Vein thickness distribution is best characterized by a log-normal distribution. The fractal dimensions ‘D’ of vein networks analyzed range from 1.44 to 1.71. Vein networks, which contain abundant bridging veins between neighbouring sub-horizontal veins to form an anastomosing pattern, exhibit a higher D value. The fraction of vein volume derived from image analysis ranges from 6.75 to 17.94%. A positive correlation is revealed by plotting the fractal dimensions of the vein networks versus the number of vein-intersection points, suggesting that an anti-clustering distribution of veins could contribute to more interactions and also intersections of the gypsum veins. The vein attribute data presented indicate that mineralization in the massive mudstones is mainly localized to some of the largest veins as a result of mechanical interaction of neighbouring veins during vein propagation, hindering the nucleation of new veins as a consequence. Accommodation of the bulk rock strain could be facilitated mainly by expansion of the large, backbone veins, which follow a positive feedback mechanism of growth. This study reveals a strong link between the geometric parameters of opening-mode fractures with the history of their formation, propagation and intersection.

Spectral and chemical characterization of gypsum‐phyllosilicate association in Tiruchirapalli, South India, and its implications

Here, we present the detailed chemical and spectral characteristics of gypsum‐phyllosilicate association of Karai Shale Formation in Tiruchirapalli region of the Cauvery Basin in South India. The Karai Shale Formation comprises Odiyam sandy clay and gypsiferous clay, well exposed in Karai village of Tiruchirapalli area, Tamil Nadu in South India. Gypsum is fibrous to crystalline and translucent/transparent type with fluid inclusions preserved in it. Along some cleavage planes, alteration features have been observed. Visible and near infrared (VNIR), Raman, and Fourier transform infrared techniques were used to obtain the excitation/vibration bands of mineral phases. VNIR spectroscopic analysis of the gypsum samples has shown absorption features at 560, 650, 900, 1,000, 1,200, 1,445, 1,750, 1,900, 2,200, and 2,280 nm in the electrical and vibrational range of electromagnetic radiation. VNIR results of phyllosilicate samples have shown absorption features at 1,400, 1,900, and 2,200 nm. Further, we have identified the prominent Raman bands at 417.11, 496.06, 619.85, 673.46, 1,006.75, 1,009.75, ∼1,137.44, ∼3,403, and 3,494.38 cm−1 for gypsum due to sulphate and hydroxyl ion vibrations. We propose that gypsum veins in Karai may have precipitated in the fractures formed due to pressure/forces generated by crystal growth. The combined results of chemical and spectral studies have shown that these techniques have significant potential to identify the pure/mineral associates/similar chemical compositions elsewhere. Our results definitely provide the database from a range of spectroscopic techniques to better identify similar minerals and/or mineral‐associations in an extraterrestrial scenario. This study has significant implications in understanding various geological processes such as fluid‐rock interactions and alteration processes involving water on the planets such as Mars.

Fault and fracture patterns around a strike-slip influenced salt wall

The trends of faults and fractures in overburden next to a salt diapir are generally considered to be either parallel to the salt margin to form concentric patterns, or at right angles to the salt contact to create an overall radial distribution around the diapir. However, these simple diapir-related patterns may become more complex if regional tectonics influences the siting and growth of a diapir. Using the Sedom salt wall in the Dead Sea Fault system as our case study, we examine the influence of regional strike-slip faulting on fracture patterns around a salt diapir. This type of influence is important in general as the distribution and orientation of fractures on all scales may influence permeability and hence control fluid and hydrocarbon flow. Fractures adjacent to the N-S trending salt wall contain fibrous gypsum veins and injected clastic dykes, attesting to high fluid pressures adjacent to the diapir. Next to the western flank of the salt wall, broad (∼1000 m) zones of upturn or ‘drape folds’ are associated with NW-SE striking conjugate extensional fractures within the overburden. Within 300 m of the salt contact, fracture patterns in map view display a progressive ∼30°–35° clockwise rotation with more NNW-SSE strikes immediately adjacent to the salt wall. While some extensional faults display growth geometries, indicating that they were syn-depositional and initiated prior to tilting of beds associated with drape folding, other fractures display increasing dips towards the salt, suggesting that they have formed during upturn of bedding near the diapir. These observations collectively suggest that many fractures developed to accommodate rotation of beds during drape folding. Extensional fractures in the overburden define a mean strike that is ∼45° anticlockwise (counter-clockwise) of the N-S trending salt wall, and are therefore consistent with sinistral transtension along the N-S trending Sedom Fault that underlies the salt wall. Our outcrop analysis reveals fracture geometries that are related to both tilting of beds during drape folding, and regional strike-slip tectonics. The presence of faults and fractures that interact with drape folds suggests that deformation in overburden next to salt cannot be simply pigeon-holed into ‘end-member’ scenarios of purely brittle faulting or viscous flow.

Lithological control on fracture cementation in the Keuper Marl (Triassic), north Somerset, UK

AbstractThe spatial arrangement of gypsum veins as preserved natural hydraulic fractures have been characterized in the Triassic Keuper Marl Formation (UK), a caprock for hydrocarbon reservoirs and CO2sequestration. The marls cropping out are subdivided into five discrete fracture units based on the presence and abundance of gypsum veins. The nodular gypsum in evaporite horizons provides excess gypsum for nodule-rooted horizontal gypsum veins. Our petrographic observations demonstrate that the development of gypsum veins in beds lacking macroscopic evaporites is closely associated with disseminated gypsum cement in the marls. We interpret that the gypsum veins in marl are sourced from disseminated gypsum cements in the host rocks, based on stratigraphic correlations, and much lower Sr concentrations than gypsum nodules. Gypsum was transported to adjacent veins mainly through diffusion in the low-permeability marls. The localization of gypsum veins and varied Sr concentrations of veins and nodules indicate that the diagenetic fluids are a mix of connate water with meteoric water rather than brines transported from evaporite beds along faults to non-evaporite beds. This results in the absence of gypsum fillings in fractures in rocks without primary gypsum cements. The study implies that the cementation of natural fractures in low-permeability rocks can highly depend on the presence of cement minerals in the host rock.