Feldspar Alteration Research Articles

From loose sand to sandstone: An experimental approach on early calcite precipitation in sands of siliciclastic and mixed carbonate-siliciclastic composition.

Artificially cemented sandstones were produced to assess the impact of detrital texture and composition on the precipitation and distribution of early calcite cement, and cement-related degradation in porosity. To simulate early-calcite cementation, loose sediment of variable composition (siliciclastic and calcareous) and grain size was exposed to a calcite supersaturated solution for 35 to 58 days at 23°C. Identification and distribution of the newly precipitated crystals was performed with high resolution 2D optical and scanning electron microscopy. The experimental results show the precipitation of grain-coating, pore-bridging and pore-filling granular calcite cement with up to 100 μm crystal size. Despite a positive correlation between the amount of detrital carbonate grains and calcite crystals, calcite cement does not preferentially nucleate on bioclast surfaces, irrespectively of their favourable mineralogy. Siliciclastic grains show high calcite cement coverage with altered feldspar, particularly plagioclase, displaying coverage of 94.3%. Grain size variations within the sand packs have influence on the precipitation pattern of calcite with coarse-grained layers (500-710 μm) showing minor calcite cementation (6.3%), while medium- (250-500 μm) to fine-grained layers (125-250 μm) comprise average calcite cement contents of 16.3% and 28.2%, respectively. The findings of this study enhance our knowledge regarding the precipitation processes of calcite in porous material with heterogeneous reacting mineral phases, shapes and pore connectivity.

Magmatic evolution and sources of metals and fluids in the large granite-related Xiasai vein-type Ag–Pb–Zn(–Sn) deposit, northern Yidun terrane, SW China

The Xiasai Ag–Pb–Zn–(Sn) deposit in the northern Yidun terrane (eastern Tethys), hosted in Late Triassic metasandstones, is genetically associated with Late Cretaceous granites. We present chemical and Sr–Nd–Pb–Li–B isotope data of Late Cretaceous granites and their mafic microgranular enclaves (MME), late aplites, and Late Triassic metasedimentary rocks. The narrow range of 87Sr/86Sr99 (0.70627–0.70953) and εNd99 (–6.1 to –5.2) values of most Late Cretaceous granites reflects a mixed source with contributions from the mantle and Paleoproterozoic felsic crust. These granites have high SiO2 contents (>69 wt%) and strong depletions in Sr, Ba, and Eu. Major and trace element contents correlate with the fractionation index 1/TiO2, indicating that they experienced extensive fractionation. Local late aplite dikes were derived from similar source rocks, but seem to be younger.The Pb isotopic compositions reflect that the metal Pb was derived from the granite with variable contributions from the Late Triassic sedimentary rocks. Weakly and moderately altered metasandstones have similar δ7Li and δ11B values as most granites (δ7Li: 0.00 to 2.68 ‰; δ11B values: –14.93 to –12.36 ‰), indicating magmatic fluid sources. The magmatic fluids resulted in the initial enrichment of Sn, Cu, Pb, and Zn. Strong alteration of feldspar and biotite resulted in the metasandstones in negligible change of Li concentrations, significant loss of Sr, Na2O, CaO, MgO, and Fe2O3, but significant addition of B. This alteration lowered δ7Li from 1.01 to –3.65 ‰ and lowered δ11B from –9.43 to –19.18 ‰. The low δ7Li values and extremely low δ11B values reflect external fluids from the Late Triassic sedimentary wall rocks, which interacted with granites and mobilized metals (e.g., Sn, Ag, Pb, and Zn) from the granite and/or the sedimentary wall rocks. The location of the Xiasai deposit is controlled by regional NNW-trending faults induced by Late Cretaceous extension.

Weathering-induced organic matter enrichment in marine-continental transitional shale: A case study on the early Permian Taiyuan Formation in the Ordos Basin, China

A comparative analysis of the factors controlling organic matter (OM) enrichment between marine-continental transitional (transitional hereafter) and marine or lacustrine shales is lacking. The early Permian Taiyuan Formation in the Ordos Basin, deposited during a shift from marine to continental settings in northern China, provides a unique opportunity to unravel the differences in OM enrichment mechanisms among these shales. The Taiyuan Formation is characterized by high TOC content (average 4.50%) and kerogen type II2-III. Most samples are thermally mature with a few high to post-mature samples relating to the Late Jurassic–Early Cretaceous Yanshanian magmatism. Rare earth elements and yttrium (REY) are dominated by light- and medium-types enrichments, with distinctly positive Gd anomaly, likely due to seawater incursion. A warm and humid climate prevailed during deposition of the Taiyuan Formation, consistent with the tropical-subtropical location of the North China Plate in the early Permian. The climatic conditions promoted intense continental weathering as reflected by high Th/Sc ratios, chemical index of alteration values, and feldspar alteration to scaly kaolinite. The V/(V + Ni) ratio is inconsistent with the other redox proxies, presumably due to variations in the redox buffer supply in the transitional facies (e.g., OM and pyrite), varying burial rates and dissimilar redox potential of different elements. Hence, this proxy should be interpreted with caution in such settings. Most redox proxies indicate oxic bottom water during deposition of the Taiyuan Formation transitional shale, in contrast to typical OM enriched marine and lacustrine shales where redox stratification or euxinic conditions are common. Instead, the dominant factor for OM enrichment in transitional shales appears to have been a high influx of terrestrial weathering products, including abundant higher-plants OM, associated with preservation of OM due to rapid burial. This process minimizes the detrimental effects of oxic conditions on OM accumulation in the transitional shale facies. This mechanism may hold relevance for analogous basins elsewhere.

Diverse Behaviors of Feldspar Grains during Sandstone Diagenesis: Example from the Xujiahe Formation in the Western Sichuan Basin, China

Feldspar alteration is among the most important processes in clastic rocks during diagenesis, but uncertainty remains about the factors that control feldspar diagenesis under subsurface conditions. Hence, the Upper Triassic Xujiahe formation of the Western Sichuan Basin were examined by an integrated petrographic, mineralogical and geochemical approach to unravel the causes and effects of feldspar diagenesis, with implication for mass transfer and openness of the geochemical system. The sandstones at various depths demonstrate three distinct, separate diagenetic behaviors of detrital feldspar within a single formation including (1) the complete dissolution of both plagioclase and K-feldspar in the upper member; (2) conservation of abundant detrital feldspar grains with minor albitization or overgrowths within the lower member of depths greater than 5 km; and (3) complete disappearance of K-feldspar within the uppermost horizons of the lower member, while plagioclase have survived in significant amounts. The exceptional disappearance of K-feldspar is the result of selective dissolution of K-feldspar during burial, accompanied by illite cementation and substantial K transfer at a scale of tens of meters. It is apparent that the clay diagenesis in the overlying mudstones, rather than porewater chemistry, is the major control of the reactivity of K-feldspar in adjacent deeply buried sandstones.

Timing of Hydrothermal Alteration and Au-Sb-W Mineralization, Stibnite-Yellow Pine District, Idaho

Abstract The Stibnite-Yellow Pine district of central Idaho was mined from the early 1900s until the 1990s, extracting gold, antimony, tungsten, and mercury from veins and disseminated and replacement ores in mountainous terrain along the headwaters of the Salmon River. Mining during the two World Wars supplied critical antimony and tungsten to the war efforts. Recent exploration has delineated mineral resources of over 187 metric tons Au, 274 metric tons Ag, and 93,000 metric tons Sb. Mineralization is hosted in Cretaceous Idaho batholith granitic rocks and a sequence of Neoproterozoic to Paleozoic metasedimentary strata of carbonate and siliciclastic compositions. Historic studies outlined some of the complex paragenesis but debated the absolute age of mineralization. New petrographic and geochronologic work documents a sequence of five hydrothermal events in the Stibnite-Yellow Pine district. Event 1 is related to Cretaceous magmatic and hydrothermal activity and includes events ranging in age from 86 to 75 Ma, including sparse quartz-molybdenite veins dated at 86 Ma. Disseminated gold mineralization of Event 2 is associated with sericitic alteration and sulfidation of igneous biotite and replacement of plagioclase by potassium feldspar, largely in granodiorite. Gold is present in zoned arsenian pyrite in both disseminated ores and in crosscutting carbonate-quartz veins containing pyrite and arsenopyrite. The large Yellow Pine deposit, localized at a dilatant bend in the Meadow Creek fault, hosts such disseminated and vein gold. Event 2 is interpreted as the major gold-forming event; 40Ar/39Ar ages of sericite and potassium feldspar alteration range, respectively, from 70 to 59 and 66 to 56 Ma. The long span is interpreted to reflect the age of gold mineralization and local overprinting by Event 3. A narrower range from 66 to 61 Ma is interpreted to date the peak of gold mineralization and alteration. Event 3, tungsten mineralization with scheelite, is texturally later than Event 2 gold and localized along the Meadow Creek structure. Event 3 scheelite has been dated by isotope dilution-thermal ionization mass spectrometry (ID-TIMS) and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) U-Pb methods at 57 Ma. Event 4, best developed in the West End area, includes gold-silver–bearing quartz-carbonate-pyrite veins and breccias with epithermal textures and potassium feldspar alteration envelopes. Adularia from Event 4 yields 40Ar/39Ar plateau ages of 52 to 51 Ma. Event 5 antimony and mercury mineralization consists of stibnite veins and breccia cements at the Yellow Pine and Hangar Flat deposits as well as cinnabar veins and replacements at the peripheral Fern and Hermes deposits; it is constrained by an LA-ICP-MS U-Pb date on scheelite (ca. 47 Ma) intergrown with stibnite. Minor propylitic and argillic alteration is evident in 47 Ma igneous dikes, which do not contain economic mineralization. The Au-Sb-W ores in the Stibnite-Yellow Pine mining district formed over an extended time period from about 70 to 45 Ma in multiple pulses that were localized along the Meadow Creek fault zone. Each event corresponds to episodes of magmatism and/or hydrothermal activity in the region. Insignificant Event 1 skarn and molybdenum mineralization is similar in age to the Thompson Creek porphyry molybdenum deposit in central Idaho. Event 2 gold mineralization occurred during a magmatic gap in central Idaho but was synchronous with magmatism in the Bitterroot lobe further north; Event 2 is similar in age to orogenic gold-arsenic mineralization at the Beartrack mine in eastern Idaho. Event 3 scheelite mineralization coincides with tungsten mineralization at the Quartz Creek deposit, late magmatism in the Bitterroot lobe, and rapid exhumation of the Atlanta lobe of the Idaho batholith. Event 4 gold mineralization is coincident with the onset of regional Challis magmatism and extension. Event 5 antimony and mercury mineralization is time-equivalent to epithermal gold mineralization in the nearby Thunder Mountain volcanic field and the peak of Challis magmatism.

Characteristics and control factors of feldspar dissolution in gravity flow sandstone of Chang 7 Member, Triassic Yanchang Formation, Ordos Basin, NW China

To clarify the formation and distribution of feldspar dissolution pores and predict the distribution of high-quality reservoir in gravity flow sandstone of the 7th member of Triassic Yanchang Formation (Chang 7 Member) in the Ordos Basin, thin sections, scanning electron microscopy, energy spectrum analysis, X-ray diffraction whole rock analysis, and dissolution experiments are employed in this study to investigate the characteristics and control factors of feldspar dissolution pores. The results show that: (1) Three types of diagenetic processes are observed in the feldspar of Chang 7 sandstone in the study area: secondary overgrowth of feldspar, replacement by clay and calcite, and dissolution of detrital feldspar. (2) The feldspar dissolution of Chang 7 tight sandstone is caused by organic acid, and is further affected by the type of feldspar, the degree of early feldspar alteration, and the buffering effect of mica debris on organic acid. (3) Feldspars have varying degrees of dissolution. Potassium feldspar is more susceptible to dissolution than plagioclase. Among potassium feldspar, orthoclase is more soluble than microcline, and unaltered feldspar is more soluble than early kaolinized or sericitized feldspar. (4) The dissolution experiment demonstrated that the presence of mica can hinder the dissolution of feldspar. Mica of the same mass has a significantly stronger capacity to consume organic acids than feldspar. (5) Dissolution pores in feldspar of Chang 7 Member are more abundant in areas with low mica content, and they improve the reservoir physical properties, while in areas with high mica content, the number of feldspar dissolution pores decreases significantly.

Natural hydrogen in low temperature geofluids in a Precambrian granite, South Australia. Implications for hydrogen generation and movement in the upper crust

Natural hydrogen (H2) has the potential to be a low-carbon fuel and energy source, playing a crucial role in achieving a clean, secure, and affordable energy future. However, our understanding of the generation, migration, and accumulation of H2 in the subsurface remains poorly constrained. Increasing evidence suggests that H2 is abundantly and widely present in various geological settings, making them viable targets for exploration of this emerging resource. Reports have indicated the presence of hydrogen-dominated natural gas in vintage boreholes located in the Yorke Peninsula and Kangaroo Island, South Australia. These boreholes were drilled in Cambrian sedimentary cover over Precambrian terranes that host iron ore, copper or gold ore deposits. To gain insight into these occurrences, rock samples were examined from the Roxby Downs granite, a Mesoproterozoic pluton associated with the Hiltaba Suite event (1.59 Ga), belonging to the same magmatic origin as the Yorke Peninsula basement.Our investigation revealed metasomatic fluid circulation, evidenced by alteration of feldspar and mafic grains, as well as discrete cemented fractures within the granite. In some of these fracture-fill cements, we discovered water and gas inclusions containing hydrogen. The formation of the hydrogen-bearing cements occurred at paleo-temperatures ranging from 170 °C to the present-day 55 °C. Analysing the oxygen isotope values of the quartz cements and considering their fluid inclusion temperatures, a marine water signature as the source of water in equilibrium with the quartz cements was identified.To constrain the fluid history, neon and argon isotope signatures from the fluid inclusion gases were measured from the granite and separated quartz grains. Nucleogenic neon isotopes increased with depth along specific production lines. While this is consistent with long-lived fluid within the granite and increasing fluid residence with depth, separated quartz grains show less variation with depth and suggest that younger fluids were introduced, triggering alteration of less robust minerals, visible in bulk granite. Petrographic analyses identified hydrolytic alterations of mafic minerals into magnetite, and subsequently magnetite to hematite, as sources of hydrogen and contributors to the observed increase in pore water salinity. Our findings highlight that natural hydrogen can be produced and retained in granites at low temperatures, expanding beyond the commonly reported alteration processes observed in mafic and ultramafic rocks.

Study on influencing factors of radon exhalation from coal measures in the northern margin of Ordos Basin

Coal-uranium synergistic development has become an emerging research area in recent years. Radon gas emitted by uranium decay in the stratum may be used as an indicator to study the uranium-bearing area. Investigating the relationship between radon exhalation law and the depth, lithology, and mineral composition of coal measures is very important. This paper analyses the influencing factors and mechanism of radon release in coal measure strata in the Ordos Basin which is rich in coal and uranium co-occurrence deposits, by drilling a core at a depth of 18.3–84 m in the mining area, and using low-field nuclear magnetic resonance (NMR), polarizing microscope and environmental radon meter tests. The results show that the proportion of micropores decreases with an increase in particle size, and the proportion of macropores and porosity increases. High porosity has a greater effect on the release of radon gas, resulting in a higher exhalation rate in samples with coarse particle sizes. The radon exhalation rate of sandstone is higher than that of coal, and the radon concentration increases with the improved sealing property of deep rock and soil masses. The radon exhalation rate of sandstone is strongly influenced by feldspar and is positively correlated with each other. The radon exhalation was aided by the submicron structure created by feldspar alteration. The research results may be used as a reference for rock radiation risk assessment in mining areas.

Effects of fluorine on dynamic reaction interfaces in hydrothermal feldspar alteration

Fluid-induced mineral reactions exert an important control on the physico-chemical properties of the crust and hydrothermal plumbing systems. Feldspars are the most abundant minerals in the Earth's crust and a major host of the main cations found in geofluids (Na, K, Ca). In previous work, we discovered that K-feldspar-albite-sanidine zonation textures developed dynamically when sanidine ((K,Na)AlSi3O8) was reacted with NaCl or NaF in H216O- and H218O-enriched solutions at 600 °C and 2 kbar in a closed system. Based on a detailed SEM and TEM characterisation, the present study aims to constrain the reaction mechanism and the effect of fluorine on reaction kinetics and overall reaction textures in alkali feldspar replacement reaction interfaces. Replacement of high-sanidine (Sa) by albite (Ab) and/or K-feldspar (Kfs) proceeded via an interface-coupled dissolution-reprecipitation reaction, with the products preserving the crystallographic orientation of the parent Sa in both NaCl- and NaF-bearing solutions. Mineral interfaces with different micro-textures were observed depending on the nature of the interface (Ab-Sa, Kfs-Ab, Kfs-Sa) and whether the reaction occurred in NaCl-only or NaF-bearing solutions. Abundant amorphous domains (25–50 nm thick) occur both within Ab and at Ab-Sa and Kfs-Ab interfaces formed in NaF-bearing solutions. At the Kfs-Sa interface formed in NaCl-only solutions, a complex interfacial zone characterised by a high density of defects may represent sealed fluid pathways. These results reveal that the nature of the reaction interface depends on both mineral properties, in particular microtextures (i.e. polysynthetic twinning in Ab); and solution chemistry; this directly affects porosity development, the chemical environment at the interface, and chemical exchanges between the reaction front and the bulk solution. Fluorine aids the formation of amorphous layers that modify nucleation mechanisms; and facilitates element transport by forming Na-Si-Al-fluoride complexes, resulting in accelerated reaction rates relative to chloride-solutions. These nm- to μm-level processes contribute to efficient coupling between fluid-flow and mass transfer, explaining why sodic and potassic alteration associated with metal and fluid transport in some of the world's largest ore deposits can develop extensively, and subsequently access metals locked within silicate mineral assemblages.

Unravelling Source Area Composition for the Paleoproterozoic Basal Conglomerate (Vindhyan Supergroup) around Dalla, Sonbhadra District, Central India

Abstract The present investigation reveals that the Basal Conglomerate is ~ 2-3 m thick forming basal part of the Vindhyan Supergroup in the Sonbhadra district, central India. The Basal Conglomerate consists of fragments of different rock types supported by the sandy matrix. It contains pebble-sized clasts of quartzite, jasper, phyllite and slate dominated by the quartzite clasts.The matrix is composed of sandy material that is rich in quartz, chert and iron oxides. The occurrence of quartzite clasts suggests its derivation from the Chhotanagpur Granite Gneiss Complex (CGGC) and the phyllite, slate and jasper suggest their derivation from the Mahakoshal Group. The presence of polycrystalline and monocrystalline quartz suggests their derivation from Chhotanagpur Granite Gneiss Complex and the presence of chert suggests its derivation from the Mahakoshal Group. The iron oxides were produced as a result of leaching of iron during chemical weathering. The X-ray diffractometry of the matrix suggest ubiquitous presence of quarz, muscovite and kaolinite. The kaolinite has formed due to alteration of feldspars under warm and humid conditions. This is also supported by the presence of ferruginous matter in the matrix.

Diagenesis of Subang Formation in Cibeet River, Bogor, West Java

Subang Formation is one of many turbidite flow-induced sediment that built Bogor Trough, Java. Despite its marine origin, the exposure to the meteoric environment and burial process caused this formation to go under diagenesis, thus altering its primary characteristics. Intensive cementation by equant calcite, feldspar alteration, and grain dissolution indicates diagenesis in a meteoric environment. The cementation process filled primary and secondary pores, decreasing the primary porosity. On the other hand, the dissolution of the grain formed new secondary porosities. The presence of stylolites and fractured fragments indicated the formation had experienced burial. Although compaction often decreases rock’s porosity, the fracturing of the rocks due to compaction leads to fracture porosity formation, increasing rock’s porosity. Analyzing the compaction signs that destroyed the cementation and dissolution features in the rock samples, it is assumed that the diagenesis had occurred in the meteoric environment before it occurred in the burial environment. This study showed that the heterogeneity of the Subang Formation in the Cibeet River resulted from all these diagenesis processes mentioned above. Furthermore, the diagenesis study of the Subang Formation can be implemented as a preliminary stage to determine the quality of the petroleum reservoir. Diagenesis of Subang Formation in Cibeet River, Bogor, West Java.

Multianalytical diagnostic approaches for the assessment of materials and decay of the archaeological sandstone of Osiris Temple (The Abaton) in Bigeh Island, Philae (Aswan, Egypt)

• Osiris Temple wasn't rescued during UNESCO saving campaign of Aswan temples. • Yellowish and pinkish sandstones are the building materials of the temple. • Fluctuations of Nile water level decayed the temple building materials severely. • Main intrinsic decay factors is the clayed matrix of sandstones. The ancient Osiris Temple at Philae/Aswan, also known as Abaton, was considered one of the most sacred and mysterious temples in Egypt. In 1960, UNESCO rescued Philae temples because the rise of the water level after the construction of the Old and New Dam of Aswan caused the submergence of all temples underwater. Rescue camping was performed to protect all Aswan temples. Unfortunately, Osiris Temple was not rescued and was forgotten in its original place on Bigeh Island. Currently, this temple is subjected to Nile water effects and is partially submerged underwater due to Nile water level changes. This study aims to unravel and assess the conservation state of the temple sandstone-building material through multianalytical and examination techniques (X-ray diffraction, X-ray fluorescence, Raman spectroscopy, polarizing microscopy, binocular microscopy, and scanning electron microscopy). Furthermore, in situ records were obtained, and degradation maps were drawn to establish the patterns of damage and decay of the building material of the Osiris temple. The results showed that blackening, soiling, and disintegration are the dominant decay patterns in the building material of the temple. In addition, the Nile water as an extrinsic factor severely affected the conservation state of the temple due to the continuous exposure to water force. The mineralogical composition of sandstone played an important role in its degradation as an intrinsic decay factor due to mainly dispersion of clayed and/or carbonate matrix dissolution, quartz hydrolysis, and feldspar alteration.

The Behavior of Water in Orthoclase Crystal and Its Implications for Feldspar Alteration

The phenomenon of feldspar alteration that occurs in the interior of feldspar crystals remains poorly understood. We observed experimentally that water can go into orthoclase crystals under pressures of up to 600 MPa at room temperature. With increasing pressure, the FTIR spectra of colorless orthoclase show a sharp increase in integral absorbance from 1.50 cm−1 to 14.54 cm−1 and normalized integral absorbance from 120 cm−2 to 1570 cm−2; the pink orthoclase saturates quickly with no significant change in either the integral absorbance or normalized integral absorbance. The different responses to the pressure between colorless orthoclase and pink orthoclase might be related to the K content in the structure. Moreover, FTIR spectra at atmospheric pressure collected in different crystallography directions show different absorbance intensities, which illustrates the characteristic of preferred crystallographic orientations. These results reveal that H2O molecules can occur as structural constituents entering the crystallographic channels of alkali feldspar crystals, preferentially along (001) orientation. These findings provide clues into the mechanism of feldspar alteration occurring in the interior of feldspar crystals, as well as the formation of micropores and microstructure in feldspar minerals. This study also provides important insights into the behavior of water molecules in nominally anhydrous minerals in the upper crust of the Earth.

Epidote U-Pb geochronology and H isotope geochemistry trace pre-orogenic hydration of midcrustal granitoids

Abstract Hydrothermal veins and altered feldspar are evidence for fluid circulation in granitic rocks in the continental crust. The hydrothermal alteration of feldspar affects the deformation behavior of granitoids, especially if it occurs before orogeny. Geochronology can establish the timing of fluid circulation to determine if this fluid-driven alteration plays a role in crustal deformation. Although existing dating techniques cannot be applied to feldspar alteration directly, absolute ages of fluid circulation can be obtained from hydrothermal veins. We combined U-Pb geochronology and hydrogen isotope data (δD) from epidote [Ca2Al2(Al,Fe3+) Si3O12(OH)] to unravel the hydration of post-Variscan granitoids in the Alpine orogen. The recent protocol for epidote U-Pb dating proves for the first time that fluids of meteoric origin infiltrated the granitoids in Permian times by exploiting synrift faults, consistent with the δD values of the epidote-forming fluids. This hydration event caused at least some degree of feldspar hydration and weakening of the granitic rocks ~260 m.y. before their pervasive structural overprint by the Alpine orogeny. The preservation of Permian U-Pb ages despite Alpine orogenic processes confirms epidote as a powerful tool with which to unveil pre-orogenic hydration events in metagranitoids. Our analytical approach broadens insights into the water cycle in the middle continental crust in orogens.

Qualitative Mineralogical Analysis of Deep-Sea Sediments Based on the Lithology in the Offshore Mannar Basin by using FTIR and XRD Techniques

The Mannar Basin, with a total area of 45,000 km2, is a key sedimentary basin for petroleum prospecting in Sri Lanka. It stretches from southeast India to southwest Sri Lanka and the basin is made primarily of Precambrian high-grade metamorphic rocks. In 2011, Cairn Lanka Private Limited (Cairn) drilled three exploration wells (Dorado, Dorado North, and Barracuda) and one exploration well (Wallago) in the recent petroleum exploration in Mannar basin. Source rock quality and maturity are important factors to consider when evaluating oil and gas potential. As a result, mineralogy plays an important role in the geochemical characterization of source rocks. In this study sediment samples of Barracuda exploration well in Mannar basin were collected from Petroleum Resources Development Secretariat in Sri Lanka (PRDS) at depth ranging from 2,260 to 4,740 m, and the samples were cleaned before analysis to eliminate drilling mud. The powdered material was made by crushing the cleansed sediment samples with a motor and petal, then screening them through a 53 µm standard sieve. After that sediment samples were analyzed through Fourier Transformation Infrared Spectroscopy (FTIR) and X-Ray Diffractometer (XRD) to obtained the available mineralogy qualitatively. The observed data were interpreted based on sample age, depth, and lithology, as well as their impact on the Mannar basin. According to previous research, the detected FTIR band wavelengths were used to determine the accessible mineralogy of the samples. The XRD peaks were determined using Crystal Impact Match 3.0 software and literature. The presence of quartz, feldspar, kaolinite montmorillonite, and calcite in the entire marlstone and mudstone samples is confirmed by FTIR analysis. The XRD analysis revealed that calcite, and quartz are dominant in marlstone with black carbon sediment samples and marlstone sediment samples. Quartz, illite, and calcite dominate in argillaceous marlstone sediment samples. Illite, montmorillonite, kaolinite, and calcite are dominant in slightly calcareous mudstone sediment samples. Quartz overgrowth and cementation were caused by the presence of quartz. In the Mannar Basin, quartz cementation can reduce reservoir rock porosity and have an effect on permeability by increasing rock brittleness and occupying pore space. Overgrowths of quartz reduce reservoir porosity and permeability. Feldspar alteration changes porosity and permeability, whereas dissolution increases secondary porosity and permeability of source rock in the basin. Clay minerals have an impact on hydrocarbon formation, accumulation, migration, and reservoir quality in the basin. Also, tectonics and sedimentation modify the abundance and crystal structure of clay minerals. Furthermore, the presence of carbonate minerals and hematite is responsible for the cement's creation. Calcite cement has an effect on the basin's reservoir quality, while hematite cement is uncommon.
 Keywords: Mannar basin, Sediment, Mineralogy, FTIR, XRD

Late-magmatic processes in the St. Lawrence Granite: Implications for fluorite mineralization

The St. Lawrence Granite (SLG), located in the Avalon Zone of the Appalachian Orogeny, is a peralkaline to metaluminous granite, best known for its association with extensive, and economically significant, fluorite deposits. Genetic models for the fluorite mineralization have been proposed, but major gaps and misconceptions still remain, such as the lack of fluorite in the country rocks, the source of Ca to form fluorite and the locally abundant calcite in the fluorite veins and the granite. Petrography, geochemistry and electron microprobe analysis revealed new details on the late-magmatic processes in the granite that played a significant role in fluorite mineralization.The SLG intruded at shallow levels and comprises several phases, each having different textures, mineralogy, and geochemistry. Most of the fluorite deposits are associated with the East lobe (E lobe), which is the only peralkaline and one of the most intensely altered phases of the granite. Alteration and fluorite mineralization are the results of devolatilization of the granite. The volatiles included F, Cl, CO2 and water. Early exsolution of a F-rich vapour, followed by a F-rich acidic fluid led to the fluorite mineralization. In the later stages, a F-poor and CO2-rich fluid was left and was unable to leave the granite, migrated along the upper contact of the granite and accumulated in the cupolas resulting in late carbonatization and extensive autometasomatism.Exsolution of a F-rich vapour, coupled with the shallow depth of intrusion of the SLG, allowed the F to leave the granite leading to fluorite mineralization in the country rocks, as demonstrated by the AGS fluorite deposit, which is partially hosted in sedimentary rocks overlying the granite; this encourages further exploration in the surrounding rocks. The source of the Ca is from the alteration of primary Ca-bearing feldspar and amphiboles in the granite, implying the lack of spatial correlation between fluorite mineralization and Ca-containing country rocks. This is significant for pursuing further exploration for fluorite in the St. Lawrence area, and elsewhere.

TO EXPLAIN THE NATURE OF CORE POROSITY USING RESULTS OF PETROGRAPHY ANALYSIS

The Permian to Triassic Tirrawarra Sandstone succession in the Cooper Basin of CentralAustralia is characterized by its low permeability. Ambient core porosity averages8.96% and ambient permeability 0.9 mD. Most samples studied have permeabilities lessthan 3mD. Despite its overall poor reservoir characteristics, the Tirrawarra Sandstone isone of the major oil and gas targets in Australia. A total of 17 core plugs from 6 wellswere studied petrographically using optical petrography, SEM and XRD. These resultswere integrated with core analysis data.Petrographic study revealed the diagenetic events, mainly mechanical and chemicalcompaction, cementation and alteration have modified the reservoir quality. Ductile componentssuch as rock fragments, clay and matrix influence mechanical compaction, whichare the main cause of reservoir quality reduction. Quartz cementation and clay distributionalso affected the porosity, but particularly permeability. Mechanical compaction aswell as quartz cementation have reduced and blocked pore-throats to isolate intergranularpores. The alteration of feldspar to kaolin has changed intergranular porosity tomicroporosity. Illite occurs as either cement, alteration of rock fragments or kaolinite. Allof these diagenetic events also affect fluid movement in the reservoir.This paper presents the evaluation of the determination of effectiveness of porosity inthe delivery of gas from sandstone reservoir in the Cooper Basin using integrated petrographyanalysis and core measurements.

Diagenetic evolution of the lower Yaojia Formation of Songliao Basin, China: Impact on reservoir quality

The oil-bearing layer in the first member of the Upper Cretaceous Yaojia Formation (K2y1) in the middle of the Changling Sag of the southern Songliao Basin, northeast China, has low-permeability and is deemed to be an unconventional reservoir. Therefore, better evaluation of reservoir quality and related controlling factors are crucial for the reduction in exploration risk and sustainable development. The current study integrates diagenesis, sequence stratigraphy, and sedimentary facies to decipher their influence on reservoir quality using core and thin section observations, X-ray diffraction, scanning electron microscopy, fluid inclusion, and carbon and oxygen isotope analyses. The K2y1 is composed of shallow delta sediments of transgressive system (TST) and lowstand system (LST) tracts. The depositional microfacies of the sandstones include TST deltaic front underwater distributary channels and sheet sands, and LST deltaic plain distributary channels, distributary channel intersections, and crevasse splays. The sandstones are dominantly fine-grained moderately sorted feldspathic litharenites and lithic arkoses with silt. Early diagenetic events included compaction, development of smectite, and alteration of feldspar and kaolinite whereas mesogenetic alteration included feldspar dissolution, precipitation of quartz and carbonate cements, and clay mineral alteration. The reservoir quality of the K2y1 sandstones is generally poor, exhibiting variations among different deltaic depositional facies within the LST and TST. The (underwater) distributary channel (intersection) sandstones, particularly those of the LST, have relatively higher reservoir quality owing to their larger grain size, better sorting, lower carbonate cement content, mixed-layer I/S and illite, and greater occurrence of chlorite.

The Coles Hill Uranium Deposit, Virginia, USA: Geology, Geochemistry, Geochronology, and Genetic Model

Abstract The Coles Hill uranium deposit, with an indicated resource of about 130 Mlb of U3O8, is the largest unmined uranium deposit in the United States. The deposit is hosted in the Taconian (approx. 480–450 Ma) Martinsville igneous complex, which consists of the Ordovician Leatherwood Granite (granodiorite) and the Silurian Rich Acres Formation (diorite). The host rock was metamorphosed to orthogneiss during the Alleghanian orogeny (approx. 325–260 Ma), when it also underwent dextral strike-slip movement along the Brookneal shear zone. During the Triassic, extensional tectonics led to the development of the Dan River Basin that lies east of Coles Hill. The mineralized zone is hosted in brittle structures in the footwall of the Triassic Chatham fault that forms the western edge of the basin. Within brittle fracture zones, uranium silicate and uranium-bearing fluorapatite with traces of brannerite form veins and breccia-fill with chlorite, quartz, titanium oxide, pyrite, and calcite. Uranium silicates also coat and replace primary titanite, zircon, ilmenite, and sulfides. Sodium metasomatism preceded and accompanied uranium mineralization, pervasively altering host rock and forming albite from primary feldspar, depositing limpid albite rims on igneous feldspar, altering titanite to titanium oxide and calcite, and forming riebeckite. Various geothermometers indicate temperatures of less than ~200°C during mineralization. In situ U-Pb analyses of titanite, Ti-oxide, and apatite, along with Rb/Sr and U/Pb isotope systematics of whole-rock samples, resolve the timing of geologic processes affecting Coles Hill. The host Leatherwood Granite containing primary euhedral titanite is dated at 450 to 445 Ma, in agreement with previously obtained ages from zircon in the Martinsville igneous complex. A regional metamorphic event at 330 to 310 Ma formed anhedral titanite and some apatite, reequilibrated whole-rock Rb/Sr and U-Pb isotopes, and is interpreted to have coincided with movement along the Brookneal shear zone. During shearing and metamorphism, primary refractory uranium-bearing minerals including titanite, zircon, and uranothorite were recrystallized, and uranium was liberated and mixed locally with hematite, clay, and other fine-grained minerals. Uranium mineralization was accompanied by a metasomatic episode between 250 and 200 Ma that reset the Rb-Sr and U-Pb isotope systems and formed titanium oxide and apatite that are associated and, in places, intimately intergrown with uranium silicate dating mineralization. This event coincides with rifting that formed the Dan River Basin and was a precursor to the breakup of Pangea. The orientation of late-stage tectonic stylolites is compatible with their formation during Late Triassic to Early Jurassic basin inversion, postdating the main stage of uranium mineralization and effectively dating mineralization as Mesozoic. Based on the close spatial and temporal association of uranium with apatite, we propose that uranium was carried as a uranyl-phosphate complex. Uranium was locally reduced by coupled redox reactions with ferrous iron and sulfide minerals in the host rock, forming uranium silicates. The release of calcium during sodium metasomatic alteration of primary calcic feldspar and titanite in the host rock initiated successive reactions in which uranium and phosphate in mineralizing fluids combined with calcium to form U-enriched fluorapatite. Based on the deposit mineralogy, oxygen isotope geochemistry, and trace element characteristics of uranium silicate and gangue minerals, the primary mineralizing fluids likely included connate and/or meteoric water sourced from the adjacent Dan River Basin. High heat flow related to Mesozoic rifting may have driven these (P-Na-F-rich) fluids through local aquifers and into basin margin faults, transporting uranium from the basin or mobilizing uranium from previously formed U minerals in the Brookneal shear zone, or from U-enriched older basement rock.

Predicted CO2 water rock reactions in naturally altered CO2 storage reservoir sandstones, with interbedded cemented and coaly mudstone seals

Geological storage of CO2 captured from industrial processes such as coal combustion or from direct air capture is part of the transition to low emissions. The Jurassic Precipice Sandstone of the southern Surat Basin, Queensland, Australia, is undergoing feasibility studies for industrial scale CO2 geological storage, however regional data has so far been lacking. Precipice Sandstone reservoir drill core samples from the Southwood 1 and Tipton 153 wells in the southern Surat Basin include favourably quartz rich sandstone regions with quartz grain fracturing. A mudstone layer is also present in the reservoir. The overlying lower section of the Evergreen Formation seals consist of clay rich sandstones, interbedded mudstones, coal layers, Fe-Mg-Mn siderite, and Mg-calcite cemented sandstones. K-feldspars are weathered creating localised secondary porosity and pore filling kaolinite and illite. Layers of coal, pore filling cements, and framework grain compaction introduce vertical heterogeneity. Heavy minerals including pyrite, mixed composition sulphides, and barite are associated with disseminated coals in mudstones. Precipice Sandstone mercury intrusion porosities (MIP) ranged from 9 to 22% with favourably low reservoir injection threshold pressures, and the QEMSCAN measured open porosity between 2 and 22%. Evergreen Formation seal porosities were 7.5 to 16% by MIP or 1 to 19% by QEMSCAN, with the smallest pore throat distribution associated with the low permeability coal rich mudstone. Synchrotron XFM shows Rb mainly hosted in K-feldspars and muscovite, with metals including Mn mainly hosted in siderite. Zn and As are present in sulphides; and calcite and apatite cements mainly hosted Sr. Twenty kinetic geochemical CO2-water-rock models were run for 30 and 1000 years with Geochemist Workbench, with calcite and siderite initially dissolving. In the Precipice Sandstone reservoir variable alteration of carbonates, feldspars and chlorite to kaolinite, silica, siderite and smectite were predicted with the pH remaining below 5.5. CO2 was mineral trapped through alteration of chlorite to siderite in three of the four cases, with −0.02 to 1.43 kg/m3 CO2 trapped after 1000 years. In the calcite and siderite cemented Evergreen Formation seal, plagioclase conversion to ankerite trapped the most CO2 with 2.6 kg/m3 trapped after 1000 years. The Precipice Sandstone in both wells appears to be generally suitable as a storage reservoir, with mineral trapping predicted to mainly occur in the overlying lower Evergreen Formation and in interbedded mudstones. Heterogeneity in interbedded sandstone, mudstone, and coal layers are likely to act as baffles to CO2 and encourage mineral trapping. Quartz grain fractures may influence preferential migration pathways in the reservoir but this would need future experimental investigation. Experimental CO2 water rock reactions to understand porosity and permeability changes were out of scope here but are recommended in future validation, along with investigating the potential for CO2 adsorption trapping in coal and mudstone layers.