Petrographic Studies Research Articles

Space/ground-borne techniques and petrographic microscopic dissection for geologic mapping in Gabal Ras Abda area, Northeastern Desert, Egypt

BackgroundGabal Ras Abda area as a part of the Red Sea Mountain range, is characterized by inaccessible and rugged terrains. The exposed rock units are hardly followed in the field because of the rigid topography. Thus, the present work proposes and develops an integrated approach to map the exposed rock units and extract the geologic structures using satellite imagery data followed by both field and petrographic verification, saving time, efforts and cost.ResultsTo achieve the target, both the measured spectral signature curves with Landsat-8 and Sentinel-2A data were used to develop and create the most enhanced Band Ratios and Principal Components for lithological discrimination and mapping which were (((Band7 + Band 4)/(Band 7), (Band 2)/(Band 2 + Band 5) and (Band 5) in RGB) and ((Band 9 + Band 11 + Band 12)/(Band 1), (Band 4 − Band 2) and (Band 11/Band 6) + (Band 6) in RGB)) with Principal Component Bands ((PC1, PC2 and PC3 in RGB) and (PC3, PC2 and PC1 in RGB)), respectively. Also, georeferenced Google Earth Pro, panchromatic band of Landsat-8 and ALOS PALSAR Digital Elevation Model images were used to extract the structural lineaments. Geologic, petrographic and field structural studies were emphasized the remote sensing results, indicating that the main rock types cropped out in Ras Abda area from the oldest to the youngest are older granitoids (quartz-diorites, tonalites and granodiorites), Dokhan volcanics (andesites, rhyodacites, rhyolites and their related tuffs), younger gabbros, younger granites (monzogranites, syenogranites and alkali-feldspar granites), post-granite dykes and offshoots (acidic, microgranitic and basic types) and Phanerozoic sedimentary rocks. Also, the study emphasized that the E-W trend is the main structural trend controlling the investigated area followed by WNW-ESE and NE-SW directions.ConclusionsThe results of remote sensing achieved compliance with the geologic, petrographic and structural investigation through distinctly differentiating the different rocks and extracting the lineaments, indicating the accuracy of the remote sensing results and emphasizing their importance and effective role in producing a precise and highly accurate geologic map.Graphical abstract

Magmatic–hydrothermal stage recorded by mica zonation and Nb–Ta–W–Sn mineralization: New insight into the genesis of highly evolved Songshugang Nb–Ta deposit (Jiangxi, SE China)

The role of magmatic and hydrothermal processes in niobium-tantalum mineralization is still controversial and has been limited by a lack of mineralogical records in the evolving magmatic–hydrothermal system. Micas as essential rock-forming minerals and comprise the amount of elements (especially rare-metal elements), become one of the ideal indicator. The Songshugang rare-metal granite (RMG) located in the southern part of Jiangxi Province, China, is renowned as the largest tantalum reservoir in South China. The albite–topaz–K-feldspar granite is the major granite, with layers of pegmatite, topaz-K-feldspar granite, and greisen overlay on it. It is the typical highly evolved peraluminous with low phosphorous RMGs (PLP-type) according to geochemistry. The Songshugang pluton preserves analogical characters in topaz-albite-K feldspar granites and topaz-K feldspar granite, and partly reserves in greisen. Detailed petrographic, mineralogy, and chemical composition study of zoned mica and accessory Nb–Ta minerals suggest that zinnwaldite and Nb-Ta oxides have experienced magmatic fractionation with gradually decrease of K/Rb and Nb/Ta ratio in prismatic zinnwaldite and columbite-(Fe). The leap of Rb (Rb2O up to 4.08 wt%) and Ta (Ta# up to 0.29) at the rim of zinnwaldite and columbite is likely attributed to supersaturation. Afterwards the overgrowth of Rb-deficit zinnwaldite (0.56–0.72 wt% Rb2O) is coeval with the Ta-highest wodginite, taking place during the magmatic-hydrothermal transition associated with a fluid-melt interaction in hydrosaline melt. The ascent of fluxing elements contributes to the formation of hydrothermal zinnwaldite and cassiterite through greisenization. The systemic coupling mineralization of zinnwaldite and Nb–Ta oxides altogether record the both the melt-driven and fluid-driven processes in highly evolved Songshugang deposit that is critical for rare-metal mineralization. The Songshugang displays a great similarity to the typical PLP RMGs Cínovec cupola in geochemistry, which are metaluminous to peraluminous, containing low phosphorous, intermediate REE, Zr, Hf, and Th content. But Songshugang has experienced strong supersaturation; the latter is more akin to highly evolved melts alike Yichun or Beauvoir. The strong supersaturation and crystallization demonstrate its discrepant mineralization mechanism in Songshugang RMGs.

Evaluation of coconut shell biochar on recycled aggregate concrete through petrographic studies

Recycled Concrete Aggregate (RCA), derived from discarded concrete, substitutes or stabilizes Natural Concrete Aggregate (NCA) in constructing Recycled Aggregate Concrete (RAC). This study investigates Coconut Shell Biochar (CSB) as a potential mitigator of Alkali-Silica Reaction (ASR) in RAC, particularly in tropical regions like Sri Lanka. CSB, noted for its cost-effectiveness, undergoes assessment for its impact on concrete properties. Petrographic analyses, encompassing optical microscopy and SEM, conducted on CSB-modified RAC in Sri Lanka, reveal favorable mechanical outcomes. Grade 25 RAC, supplemented with CSB, demonstrates 28-day compressive, split-tensile, and flexural strengths of 25.4, 2.5, and 4.1 MPa, respectively. Results indicate CSB effectively alleviates ASR-induced alterations and chemical reactions in RAC, prompting further exploration of CSB's role in controlling specific precipitates and chemical reactions associated with ASR.

Microfacies, diagenetic processes and reservoir quality of the Aptian carbonate Dariyan Formation, southeast of the Persian Gulf, South Iran

The Aptian carbonate Dariyan Formation (Shu'aiba equivalent) is one of the most important oil reservoirs in the Zagros Basin and the Persian Gulf (South and Southwest of Iran). The Dariyan Formation with mainly carbonate facies overlies the Gadvan (Hawar) Formation and is overlain by the Kazhdumi (Nahr Umr) Formation. Despite the reservoir importance of the Dariyan Formation, it seems that not much detailed work has been done about the facies characteristics, diagenetic processes and reservoir quality of this formation in the east of the Persian Gulf Basin of Iran.By using the new and valuable data obtained from the core, this study focused on the identification of microfacies, diagenetic processes, determining the sedimentary environment, biostratigraphy and reservoir characterization of this formation in one of the wells of Reshadat oilfield in the southeast of the Persian Gulf. The results obtained from microfacies study, led to the identification of seven carbonate and one mixed carbonate-clastic microfacies that were deposited in four facies belts including the inner ramp, middle ramp, outer ramp, and deep basin. Uniform facies changes and the absence of reef-forming organisms such as rudists and its replacement by Lithocodium algae with the ability to form patch reefs indicate the homoclinal carbonate ramp environment. The study of benthic and planktonic foraminifera, as well as algae, led to the identification of five biozones, which indicated the Aptian age for this formation.Based on petrographic study, the Dariyan Formation is affected by diagenetic processes such as Cementation, dissolution, mechanical and chemical compaction, dolomitization, bioturbation and pyritization. Correspondence of porosity and permeability data of the cores and petrophysical logs indicates that the reservoir quality of the Dariyan Formation in the studied field was affected by facies, and diagenesis factors. Reservoir characterization has revealed that the high quality zone is located in the intervals containing Lithocodium algae. In the Lithocodium-bearing facies (boundstone-floatstone) the presence of many interconnected pores and cavities in the skeletal network structure of Lithocodium/Bacinella algae have created a suitable reservoir quality and a permeable facies.

Geochemistry of the Winneba-Mankoadze pegmatites in Southern Ghana: A clue to the petrogenesis of the pegmatites

The Birimian Supergroup (2195–2135 Ma) of Ghana was affected by the Eburnean Tectono-thermal Event at approximately 2195 Ma. The latest stage of this event resulted in pegmatitic veining in the Birimian Supergroup of Ghana at 2072 Ma. Deposits of pegmatites have been reported by many researchers in the south-western coast of Ghana. However, their mineralogy and economic potential remain fully unexplored. This study determined the geochemical characteristics and mineralogical composition of the pegmatites in the Winneba-Mankoadze area using field observations, petrographic studies, and major and trace element data. From the field work and petrographic data, the pegmatites can be classified as rare-element type, LCT family. The modal composition of the major minerals are spodumene (0.0–30.0 %), muscovite (0.0–20.0 %), garnet (0.0–5.0 %), microcline (0.0–20.0 %), plagioclase (6.0–70.0 %), and quartz (20.0–85.0 %). The geochemical data indicates that the pegmatites are enriched in Rb (2.8–3465 ppm), Cr (22–1803 ppm), Sr (1.2–314 ppm), Ba (13–501 ppm) and the light rare-earth elements (LREE). The pegmatites are highly fractionated accounting for the enrichment in the LREE than the MREE and HREE. They are also depleted in K and Ti. The pegmatites are peraluminous with calc-alkaline trend and are late-orogenic to post-orogenic. Therefore, the source of the pegmatites is the Birimian meta-sedimentary rocks, and the source lithology is upper-to-middle crust supracrustal rocks.

Metabasites from the Central East Kunlun Orogenic Belt Inform a New Suture Model for Subduction and Collision in the Early Paleozoic Proto-Tethys Ocean

The discovery of eclogite outcrops in the East Kunlun Orogen Belt (EKOB) has confirmed the existence of an Early Paleozoic HP-UHP metamorphic belt. However, the protoliths and metamorphic histories of widespread metabasites remain poorly constrained. We collected three types of metabasites from the central part of EKOB. We present an integrated study of petrography, whole-rock geochemistry, Sr-Nd isotopes, estimated P–T conditions, and zircon U-Pb isotope ages. The results show that amphibolites and retrograde eclogites have clockwise P–T paths with peak conditions of, respectively, 11–12 kbar and 675–695 °C, and 21.5–22.2 kbar and 715–750 °C. Zircon dating of metabasites from Dagele yields Late Ordovician (~449 Ma) to Early Silurian (~440 Ma) protolith ages and Early Devonian (~414 Ma) amphibolite facies metamorphic ages. Retrograde eclogites from east Nuomuhong have a protolith age of ~902 Ma and metamorphic ages of ~418 Ma, consistent with other eclogites from East Kunlun. Our data suggest that the protoliths of Dagele metabasites represent arc-type magmatism during the subduction of a small back-arc oceanic basin. Instead, the protoliths of retrograde eclogites are Neoproterozoic tholeiitic basalts emplaced into continental crust and subsequently deeply subducted. We develop a new model for Early Paleozoic subduction and collision in the East Kunlun region, emphasizing the role of ‘dominant’ and ‘secondary’ suture boundaries. This model helps explain the ages and metamorphic histories of the metabasites studied here and offers new perspectives on the evolution of the Proto-Tethys Ocean.

Mineralogical Characteristics of Greywackes at Lake Bosomtwe Area in the Ashanti Region of Ghana

The mineralogical content and texture of greywackes in the Lake Bosomtwe area of Ghana’s Ashanti Region are examined in this petrographic study. The Graywackes are fine- to medium-grained, poorly sorted, firmly indurated sedimentary rocks. Quartz is the dominant framework component of the studied specimens, and it is both monocrystalline and polycrystalline. The average mineral compositions of the samples are quartz (53%), feldspar (16%), biotite (4%), opaque minerals (2%), and lithic fragments (1%) and classified as feldspathic wacke. The other predominant minerals include orthoclase and plagioclase feldspars. Other constituents include biotite, opaque minerals, and iron oxides. The characteristics of an advanced sedimentary environment with moderate mobility are evident in the rocks. The discoveries broaden our understanding of the Ashanti Region’s geology and have ramifications for resource development, environmental management, and geological study.

Sedimentary Structures and Petrography of Rocks within Akpoha Area of the Afikpo Basin Southeastern Nigeria; Constraints on Provenance

Sedimentary structures and petrographic studies of Akpoha area in Afikpo/Abakiliki basin  was undertaken in an attempt to understudy and determining constraints  in depositional processes in the area . The major outcrops in the area are alternating succession of sandstones and shales striking in a NE-SW direction primarily as a result of folding due to past orogeny in the area. The sandstone outcrops at Akpoha are highly indurated due to age, compaction and tectonism and occur in ridges within the shale unit. The studied shale outcrops were poorly laminated and low in fossil content.Sedimentary structures identified include;Wavy/ripple lamination, Mud cracks, Parallel lamination and Fissility.The results of the petrographic modal analysis from thin section studies show that  the sandstone consists predominantly of quartz (55 - 64%); feldspar (22 - 30%) and rock fragment (1 – 9%). The sandstones were immature, poorly to moderately sorted with angular to sub-angular grain shapes which gives clear indication of high energy of sediment deposition and short transport history from provenance.The abundance of well preserved sedimentary structures  is an indication of prolong calmness of tectonic/orogenic activities in the area. The fissility observed in shale outcrops and the high induration of the Akpoha sandstones was as a result of compaction from overburden pressure before uplift and exposure.

Petrography and Geochemistry of Hydrothermal Alteration in the Low-Sulfidation Epithermal Kestanelik Au Deposit, Biga Peninsula, NW Turkey

In this study, the facies and degrees of hydrothermal alteration related to the low-sulfidation epithermal Kestanelik Au deposit in the Biga Peninsula metallogenic province are identified through petrographic studies and analysis of geochemical characteristics, such as mass changes, molar element ratios, and alteration indices. The gold mineralization is located in silicified zones containing veins and stockwork veinlets of silica. In the Kestanelik Au deposit, common hydrothermal alteration is mainly found in the Permian-Upper Cretaceous Çamlıca basement metamorphics and the Eocene granodiorite, and less often in the Eocene Şahinli volcanic rocks of the Karabiga Massif on the Peninsula. Based on mineralogical and geochemical studies conducted on altered samples, four different alteration facies are defined as silicic, sericitic, argillic, and propylitic, which show remarkable differences in the behavior of REEs, Si, K, Al, Na, and Ca elements. The hydrothermal fluids that caused alteration in the Kestanelik Au mineralization and host rocks had low REE contents because of REE mobilization. In addition, the kaolinization of feldspars and micas, and the chloritization of biotite and feldspars, may have caused negative Eu anomalies. The characterization of rocks subjected to hydrothermal alteration that are most influenced by diverse K-metasomatism with the largest K gains and losses in Na–Ca is illustrated by molar element ratio plots. Depending on the intensity of K-metasomatism, gold mineralization increases with increasing K trends toward gold ore veins. In the Kestanelik Au field, the argillic, sericitic, and propylitic alteration types from the zones enclosing the Au ore veins are revealed using the Ishikawa alteration index and chlorite–carbonate–pyrite index. Mass changes in the altered rocks indicate that there are gains in Si, K, and Al, and losses in Na and Ca with the increasing intensity of alteration toward the ore veins. The results confirm the presence of silicic and K–metasomatic (sericite and argillic) and propylitic (Fe-rich chloride) alteration zoning extending from the inner regions to the outer regions, which characterize the epithermal ore systems.

Tourmaline growth in the border and wall zones of the Emmons pegmatite (Maine, U.S.A.): Evidence for disequilibrium crystallization and boundary layer formation

Abstract The anisotropic textures, including unidirectional solidification textures and graphic intergrowths, characteristic for pegmatites, are interpreted to result from disequilibrium crystallization at high degrees of undercooling. Experimental studies have revealed the existence of thin boundary layers surrounding the rapidly growing crystals. Here, tourmaline-bearing samples from the outer zones of the Emmons pegmatite (Maine, U.S.A.) are used to examine if a boundary layer can also occur in natural samples. Crystal morphology is linked with geochemistry to understand the evolution of pegmatite melts and to constrain disequilibrium conditions at large degrees of undercooling. Petrographic studies and semiquantitative micro-X-ray fluorescence element mapping were conducted to identify crystal morphology and zonation, complemented with electron microprobe analyses to determine major and minor element compositions and LA-ICP-MS analyses of selected trace elements. Three textural groups were identified: comb-like tourmaline, quartz-tourmaline intergrowths, and radiating tourmaline. The intergrowths are optically coherent and are split into three different morphologies: central, second tier, and skeletal tourmaline. Most tourmaline is schorl, but chemical variation occurs on three different scales: between textural groups, between different morphologies, and intracrystalline. The largest scale geochemical variation is caused by the progressive evolution of the melt as it crystallized from the borders inwards, while the intracrystalline variations are attributed to sector zoning. A model is suggested where the systematic variation of Mg, Mn, and Fe within individual intergrowths is proposed to be the result of crystallization from a boundary layer, rich in water and other fluxing elements (e.g., Li, P, B), formed around the rapidly growing central tourmaline. Here, we show the first examples of boundary layers in natural pegmatites. Furthermore, the results bring into question whether boundary layer tourmaline can be used as a bulk melt indicator in pegmatitic melts.

Detrital mode and geochemical constraints on the provenance, paleoweathering and tectonic setting of the Surma sediments of Chittagong-Tripura fold belt, Tripura, India

The Chittagong Tripura Fold Belt (CTFB), the folded eastern part of the Bengal Basin is critical for understanding sediments origin and tectonic evolution linked to interactions among the Indian, Eurasian, and Burma plates. The upper Bhuban and Bokabil sediments (Surma Group) from Atharamura anticline, Tripura, India were analyzed using whole-rock geochemistry and petrography to assess their provenance, tectonics, weathering, and depositional milieu in this least explored area. Petrographic study indicates sub-lithic arenite to sub-arkosic composition, with moderate-high quartz, feldspar poor containing metamorphic and sedimentary lithic fragments (Bh Q88.3F3.0L8.6; Bk Q83.5F7.6L8.8), indicating significant recycled orogen source. Geochemical characteristics such as Eu/Eu* (0.6, 0.7), La/Sc (3.2, 2.4), Th/Sc (1.7, 1.4), Th/Co (1.9, 1.8), CIA/WIP (∼1–2), high Hf, Zr/Sc, low K2O/Al2O3 and enriched LREEs, depleted HREEs, and negative Eu anomalies, indicate a recycled provenance of felsic origin that underwent moderate to intense chemical weathering in active continental margin settings. Radiolarian chert, volcanic lithic fragments with Barail-like recycled sediments strongly suggests an additional detritus from uplifted Indo-Burman Ranges from the east. Elevated Th/U and Th/Rb ratios, sediment immaturity and poor sorting suggest first-cycled detritus. This study indicate that Tripura Neogene sediments are chiefly Himalayan-derived, with minor inputs from IBR and Meghalaya Plateau. Sedimnets deposited under brackish near shore to shallow marine oxic basins to the west of the uplifted IBR during the Miocene. The study offers insights into regional geological processes, facilitating correlation of foreland Neogene basins and enhances understanding of the geological evolution of the CTFB.

Petrogenesis of cassiterite-rich greisens from Guengue hill, Mayo Darlé, Northwestern Cameroon

The studied cassiterite-rich greisens of Mayo Darlé are hosted in granites located on the NE slope of the Guengue hill. They have three various structural forms including massive, vein, and stockwork. This study aims to describe all the different structural forms of greisen and also explains the main processes of greisen deposition related to the tin mineralization. The petrographic study shows that the massive form presents two facies: the massive greisens are rich and barren in cassiterite respectively; the stockworks and veins are all mineralized. The mineralogical composition recorded for each of the forms are Quartz + Muscovite + Cassiterite + Zinnwaldite + Plagioclase for the cassiterite-bearing greisen; Quartz + Muscovite + Tourmaline + Chlorite for the barren greisen and Quartz + Muscovite + Cassiterite + Tourmaline + Chlorite for the veins and stockworks. This mineralogy could indicate a varied chemical nature of the hydrothermal/magmatic fluids and the host granite. Also, the presence of minerals such as tourmaline, zinnwaldite and chlorite can equally indicate that there were three main stages in the granitic process: (i) tourmalinisation, (ii) lepidolitisation, and (iii) chloritisation. The greisens have a chemical composition rich in SiO2 (over 71%) and Al2O3 (5.23-43%). The chondrite-normalized REE pattern shows a negative Eu anomaly (0.0005 to 0.15) resulting from the disappearance of feldspars and biotite from the geochemical system, which is gradually transformed to quartz and chlorite respectively during leaching. The ratios of Rb/Sr, Zr/Hf, and Nb/Ta indicates an M-type tetrad effect, which also characterizes the variable values of the Eu anomaly. From a mining point of view, the areas most suitable for further prospection would be those hosting the massive, cassiterite-rich greisens. The significant occurrence of tourmaline and zinnwaldite in the studied greisens could indicate a possible Sn-W-Li polymetallic deposit.

Implications of the depositional and diagenetic attributes on the reservoir properties of the siliciclastic mangahewa formation, Taranaki Basin, New Zealand: Integrated petrographical and petrophysical studies

The siliciclastic Mangahewa reservoir in the Pohokura Gas Field presents a formidable challenge for gas exploration due to its diverse petrophysical properties. To address this challenge, our study employs a comprehensive approach integrating petrography and petrophysical analyses for core and well logging data. In the present study, we uncover an association of sandstone microfacies, in the key wells like Pohokura South-1, Pohokura-1, and Pohokura-2 wells, which change from fine to medium-grained subfeldsarenites to lithic feldsarenites microfacies. The petrographic investigation pinpoints the critical factors influencing the reservoir quality, including facies types, grain size, pore types, sedimentary structure, detrital clay content, and mineral composition. Both the primary intergranular and secondary dissolution pore spaces are responsible for the influential large intergranular pore volumes causing a superior reservoir quality. Mineral composition, influenced by the source and lithofacies, significantly impacts the reservoir properties. Diagenetic processes further control the reservoir quality, where the abundant silica cement, the illitic clays, and the rare pervasive carbonate cementation render the low reservoir quality. Compaction significantly reduces porosity during diagenesis. High-quality reservoir sandstones are characterized by coarse grain size, minimal detrital clay presence, predominantly quartz composition, limited carbonate and moderate silica cementation, low authigenic illite levels, and minor compaction impact. Based on the petrophysical data, the Mangahewa reservoir is composed of five RRTs (reservoir rock types) with the RRT1 samples having the best reservoir characteristics due to their fair porosity, very good permeability, and macro pore sizes. On the other side, the RRT5 samples are characterized by the lowest reservoir quality (poor porosity and permeability and micropore sizes) due to cementation and compaction. Our study provides valuable insights into the intricate petrophysical properties of the Mangahewa reservoir, enhancing the predictability of the reservoir behavior and optimizing hydrocarbon production in this challenging geological setting.

A Multi-Method Approach to Geophysical Imaging of a Composite Pluton in North Portugal

Potassium (K), thorium (Th), and uranium (U) are good markers of magmatic or alteration processes and the surface concentrations of these radioelements can be mapped at the regional or local scale through radiometric (gamma) surveys. In this study, a radiometric survey was performed in a post-orogenic pluton located in North Portugal, namely the Lamas de Olo Pluton, composed by three granitic facies. This pluton has already been intensively studied, including magnetic susceptibility, gravimetric, geochemical, and petrographic studies. The main objective of this work is to evaluate the radiometric data and combine them with other characteristics, such as magnetic susceptibility, and gravimetry, as well as to elucidate structures such as faults and fractures, outline geological boundaries, and identify alteration zones within various granites of the pluton. The radiometric approach reveals the spatial distribution of radioelements, offering a more distinct portrayal of the geology in the studied area. The radioactive heat production rate was calculated for the studied pluton, showing that the mean value is 4.09 µW m−3, surpassing the known mean values for granites. Our study highlights that radiometric measurements unveil compositional variations within granitic pluton and aid in identifying feeder zones. Furthermore, these measurements can be correlated with each type of granites, demonstrating associations with surface concentrations of K-Th-U. Our findings indicate a spatial alignment between the NE feeder root and a U-rich granite (Barragem granite), as evidenced by its elevated concentration of this radioelement. Conversely, the other root displays a notable relative concentration of Th, consistent with the Th-rich characteristics observed of the two other granites (Lamas de Olo and Alto dos Cabeços granites).

Sanidinite facies metamorphism at Nagarahal Village of Bilgi taluk, Bagalkot district, Karnataka, India


 
 
 Nagarahal village in the Bilgi taluk of Bagalkot district in Karnataka state shows the contact between the younger Bilgi granites and the Deccan basalts. The thermal impact that has happened on the pre-existing granites by the basalts has been studied by identifying the field occurrence of the contact zone of granites and basalts. The unaffected portion of the granites and the basalts that are far away from the contact are studied as well. The petrographic studies of the contact rocks between the granites and basalts show the polygonal recrystallized quartz with the triple point contact with other grains indicating the decussate texture indicating the high-temperature conditions, crushed quartz grains having contact with each grain appear as sugary crystals from the granoblastic texture, the irregular oriented mafic and felsic crystals show the hornfelsic texture and micrographic, and intergrowth textures showing the effect of thermal metamorphism near to the contact between the granite and basalt. The post-hydrothermal solution activity is at the peripheral region of the contact between granite and basalt which is seen by the formation of tourmaline. The mineral assemblages that are identified by the petrographic studies are muscovite, quartz, orthoclase, sanidine, sillimanite, chlorite, and Fe-Ti oxide minerals. The mineral chemistry data shows the high potash K-feldspar that consists of Or from 84% to 97.97%, Ab2%- 4.26%, and An0.09%-1.34% was identified as sanidine variety of K-feldspar, the plagioclase feldspar shows the dominant composition of Ab ranging from 84.92% to 97.10%, An content in plagioclase was found to be ranging from 0.87% to 14.02%, and the Or content is ranging from 1.06%-7.41%. indicating the presence of the albite variety of plagioclase, the Fe2+ dominating chlorite, and the presence of tourmaline indicates the hydrothermal effects at the periphery of the contact. The two feldspar thermometry indicates the formation temperature was 750°C at the contact and the chlorite formation temperature calculated is found to be 313°C which shows a decrease in the temperature away from the contact of the basalt and the granitic rocks. Whereas the granite and the basalts that are far away from the contact do not show any changes. The mineral assemblages, mineral chemistry data, and thermometric results indicate the high temperature and low-pressure sanidine facies of contact metamorphism in the studied area.
 
 

Petrography and lithology of the lower Vendian sediments of the Vishera diamondiferous area

The article presents the results of a study of the mineral composition, structural and textural features of sediments of the Ustchurochinskaya and Churochinskaya formations in the Krasnovisherskiy diamondiferous region. Rocks were studied with the petrographic analysis. According to the results of petrographic study, the composition of rocks of the Ustchurochinskaya and Churochinskaya formations was clarified. The rocks of the Ustchurochinsky formation were represented by siltstones, sandstones and gravelly sandstones. The rocks of the Churochinskaya formation are represented by tillite with interlayers of siltstones. The roof is represented micrograined dense dolomites. A detailed study of rock fragments of different genesis gives an idea of the sources. The Vendian deposits consist of erosion products of the underlying sediments of the Upper Riphean formations, volcanic rocks, granitoids and gneisses, the source of which are the Timan uplifts with outcrops of crystalline basement rocks of the Russkaja Plate.

Geology and geochronology of the Yingqian granite-type uranium deposit in the Taoshan-Zhuguang Belt, South China

The Yingqian uranium deposit in the Taoshan-Zhuguang belt, South China is a typical granite-type uranium deposit. The geological characteristics and timing of mineralization are not clear, which hinders the understanding of the ore genesis of the Yingqian deposit as well as regional uranium metallogeny in the Taoshan-Zhuguang belt. These scientific problems are examined in this study through detailed field work and petrographic study, as well as geochemical and geochronological analyses on apatite, rutile and zircon. The orebodies of the Yingqian uranium deposit are mostly present in the medium–coarse grained biotite granite and fine grained two-mica granite, and controlled by the NE-trending faults. The principle U-bearing minerals are pitchblende and brannerite, with the presence of metal minerals like pyrite, hematite and minor galena and sphalerite. The predominant hydrothermal alterations are hematitization, fluoritization, illitization, silicification, chloritization and carbonatization. Gangue minerals are mainly quartz, fluorite, illite and calcite. Moreover, four hydrothermal stages can be recognized in the Yingqian deposit, and the second and third stages are the uraniferous ones. The alterations and mineral assemblages indicate ore fluids are acidic, oxidized, and rich in F, P, CO2, which is favorable for dissolving uranium. Abundant apatite and rutile are discovered in the uraniferous third-stage veins. Geochemical analyses indicate that rutile is plotted in the hydrothermal area in the Ti-100 (Fe + Cr + V)-1000 (W) ternary diagram, and apatite is classified as fluorapatite. In-situ LA-ICP-MS dating indicates that the ore-hosting medium–coarse grained biotite granite and fine grained two-mica granite obtain the zircon U-Pb ages of 160.0 ± 0.3 Ma and 164.1 ± 0.6 Ma, which are consistent with many Jurassic granites in the South China Block. The hydrothermal apatite and rutile yield the U-Pb ages of 96.4 ± 11.6 Ma and 85.6 ± 10.8 Ma, respectively, indicating that uranium mineralization is significantly younger than the ore-hosting granites and generally coeval with the regional mafic magmatism. Consequently, the uraniferous fluids in the Yingqian deposit are independent from the magmatic system. Combined with previous research, it is concluded that fluid convection promoted by mantle–crust interaction would leach uranium from various lithologies, and fluid-rock interactions, fluid boiling and cooling are the causes for uranium precipitation.

Reservoir characteristics of the middle Eocene Avanah Formation in Erbil governorate, northern Iraq: Integration of outcrop and subsurface data

The current work investigate the petrophysical characteristics of the Middle Eocene Avanah Formation in the Erbil Governorate using IP software to analyze the well data, integrated with the 
 petrographic investigation of the formation in the nearest outcrop in the Gomaspan section. Well logging data revealed that the main lithology of the formation is limestone and dolostone while the lithology in the Gomaspan section is composed of limestone, dolomitic limestone, marly dolomitic limestone, and thin beds of shale. The lower dolomitic unit in the subsurface section of the formation is believed to be the most suitable reservoir unit due to good petrophysical characteristics including, low water saturation, high porous medium, and the presence of movable hydrocarbon. To measure porosities, a variety of well logging techniques were used in this study. The investigated formation was divided into Avanah dense (limestone unit) and Avanah porous (dolostone unit) based on their porous components. The petrographic study shows that most pore types of the formation are secondary and represented mainly by vuggy, moldic, intercrystalline, and fracture types. Most of the fractures in the upper limestone units are filled by calcite cement. The study claimed that the lower part of the formation (dolostone unit) in both sections is considered a good reservoir.

Carbonate metasomatism in sub-continental lithospheric mantle: Insights from decoupled Nd[sbnd]Hf isotopes of Mesozoic gabbros in Jinan, eastern North China Craton

The ancient sub-continental lithospheric mantle (SCLM) of the eastern North China Craton (NCC) underwent metasomatism in the Triassic due to fluids derived from subducted Yangtze continental crust, followed by further metasomatism during the Jurassic–Cretaceous caused by fluids originating from subducted Paleo-Pacific oceanic slabs. However, the occurrence of carbonate metasomatism during these two geological events is a subject of debate, and the origin of carbonate-bearing metasomatic agents remains ambiguous. Here, we present an integrated study of petrography, whole-rock trace element data and Sr–Nd–Hf–Pb isotope data for the Jinan gabbros in the Luxi block of the eastern NCC. The Jinan gabbros show arc-like trace element patterns and enriched Sr–Nd–Hf–Pb isotopic compositions, suggesting that they originated from partial melting of the enriched ancient SCLM. In addition, there are remarkable similarities in Sr–Nd–Hf–Pb isotopes between the Jinan gabbros and HP–UHP metamorphic rocks in the Dabie–Sulu orogenic belt, indicating the involvement of continental crustal components from the subducted Yangtze block in the mantle source of the Jinan gabbros. The gabbro samples show decoupled NdHf isotopes with positive ∆εHf values (∆εHf = εHf – 1.55 × εNd – 1.21; Vervoort et al., 2011), which were mainly caused by a metasomatic agent with high Nd/Hf ratios. The increase in εNd values with increasing Hf/Hf* (Hf/Hf* = HfPM/[(NdPM × SmPM)0.5]) and Nb/La further indicates that the metasomatic agent in the mantle source was depleted in high field strength elements (HFSEs) but enriched in rare earth elements (REEs), resembling the mantle-derived carbonatites in the Luxi area. The Jinan gabbros show significant variations in εNd and εHf, which cannot be explained by the involvement of C–O–H fluids and pure carbonatitic melts in their mantle source. Instead, carbonate-bearing felsic melts from the deeply subducted Yangtze continental crust are proposed to have acted as metasomatic agents. Carbonate metasomatism can significantly weaken the strength and decrease the melting temperature of the overlying ancient SCLM. Therefore, carbonate metasomatism that occurred in the lithospheric mantle of the eastern NCC during the Triassic, likely associated with deep subduction of the Yangtze block, might have played a significant role in promoting lithospheric thinning and mafic magmatism in the late Mesozoic.

Relationship of lithological data with the sedimentation environment of the Upper Batian deposits of the Kogalym region

Research subject. Upper Batian deposits (formation U2) of the Eastern part of the Kogalym region, within the North Nizhnevartovsk monocline. Aim. Reconstruction of the sedimentation environment of the U2 formation in the Kogalym region. Materials and methods. The reconstruction of sedimentation environments was conducted based on a macroscopic description of core samples with identification of structural and textural features of rocks from 35 wells. The clarification of sedimentation settings was carried out based on the description of 41 thin sections and a granulometric analysis of the clastic part of the studied rocks. Results. According to the macroscopic study, the U2 layer is composed of an uneven alternation of silty-argillaceous and silty-sandy rocks with coal interlayers. The rocks are of a polyfacial nature, represented by deposits of continental, transitional, and marine genesis. The thickness of the deposits varies markedly depending on the paleorelief of the area. The optical petrographic study revealed the polymictic composition of the studied rocks, which, according to the classification by V.D. Shutova, correspond to the arkose (graywacke arkose), less often graywacke (feldspar-quartz, quartz-feldspar greywackes) group. Cement most often exhibits a carbonate-clay composition. Rocks of marine genesis are characterized by a decrease in the proportion of rock fragments and an increase in the proportion of mechanically stable accessory minerals (zircon, garnet, tourmaline). The rocks of continental genesis are characterized by the maximum content of micas and plant detritus. An increase in the content of pyrite is noted in the sediments transitional from the continent to the sea. The grain-size analysis data of reservoir rocks of the U2 formation showed that both coastal-marine and channel deposits were formed in environments with active hydrodynamics of the medium, when the dynamic processing of clastic material exceeded the intensity of its supply. Conclusions. The performed lithofacies studies made it possible to reconstruct the sedimentation environment, as well as to reveal specific features in the distribution of the silty-sandy rock components of the U2 formation.