Petrographic Descriptions Research Articles

Characterizing the Namorado Sandstone in southern Brazil through the analysis of a multi-borehole database and utilizing a contemporary workflow

The petrophysical characterization of reservoirs requires the identification of sedimentary facies based on the integration of geophysical well logs and rock analyses in the laboratory, which is one of the significant challenges for the petroleum industry. In this work, laboratory analysis results were linked to quality controls of data pre-processing in a new workflow by integrating several techniques. The wells studied were zoned and laterally correlated based on electrofacies characteristic core analysis and petrographic descriptions. This integration resulted in a modeled geologic section for the Namorado Sandstone in the Campos Basin of southeastern Brazil, created based on various data from each well. The borehole NA2 was selected as the key wellbore after creating this section. The logs from this borehole were then analyzed to separate the permo-porous (reservoirs) and non-porous (non-reservoirs) horizons. The zones containing water and hydrocarbons in the permo-porous sections of interest were split along the wells. Subsequently, the identified sectors were interpreted using petrophysical techniques to confirm the preliminary categorization. After combining all quantitative products to quantify the porous layers capable of producing mobile oil, the cut-off factors were introduced. The next step was determining the parameters of the Archie equation, the amount of clay present, and the water saturation from core descriptions, which led to the discovery of deep laminated sandstones. The results strongly correlate with lithology, geological facies, borehole logs, and petrophysical laboratory data. Based on these findings, it is evident that the clay concentration is the primary factor influencing the oil saturation in this reservoir.

Facies variability and depositional cyclicity in central Northern Switzerland: insights from new Opalinus Clay drill cores

The Opalinus Clay, a silty to sandy claystone formation, Early to Middle Jurassic (Toarcian and Aalenian) in age, has been selected as the host rock for deep subsurface disposal of radioactive waste in Switzerland. Over the past thirty years, numerous geotechnical, mineralogical, and sedimentological studies have been conducted on the Opalinus Clay within the framework of the Nagra (National Cooperative for the Disposal of Radioactive Waste) deep drilling campaigns and the Mont Terri Project, an international research program dedicated to the study of claystone. The present study aims to unravel the variability of the lateral and vertical facies of the Opalinus Clay in central Northern Switzerland and to place this variability in a regional and basinal context. Analyses of new cores drilled in central Northern Switzerland, including petrographic, mineralogical (X-ray diffraction, multi-mineral interpretation), geochemical (X-ray fluorescence), statistical (non-metric multidimensional scaling analysis), and bedding dip and azimuth data, shed new light on the depositional facies and the spatial and temporal variability of the Opalinus Clay. Petrographic descriptions encompass nine new drill cores using a revised subfacies/facies classification scheme based on texture (colour, grain size, bedding) and composition (mineralogy). Particularly, one new subfacies (SF6) is described and interpreted as mass-wasting deposits. The drill cores are correlated laterally using specific marker horizons. This correlation is achieved by combining thorough facies investigations with lithostratigraphy, biostratigraphy, and chemostratigraphy. Six to seven small coarsening-upward cycles and two long-term coarsening-upward sequences can be interpreted as regressive trends. The observed trends are influenced by the interplay between sediment supply, eustatic sea level change, synsedimentary subsidence, but also the palaeogeographic configuration in an epicontinental sea, provenance and delivery of sediments, current dynamics and climate change. Finally, combined results show that the current dynamics in the Opalinus Clay has been underestimated until now and new depositional models, including the occurrence of drift deposits, are discussed.

Continental subduction and exhumation of the lower crust in a hot orogen: Insights from high-pressure (ultra) high-temperature granulite in the Pouso Alto Nappe, Southeast Brazil

The Pouso Alto Nappe (PAN) is part of the Southern Brasília Orogen (SBO) hinterland and primarily comprises K-feldspar + garnet + kyanite + rutile-bearing paragneiss, interpreted as a residual high-pressure (HP) granulite. The PAN granulite was studied by combining detailed microstructural and petrographic descriptions with monazite petrochronology, thermodynamic modeling, and trace element thermometry. The P–T–t trajectory of the PAN granulite reveals that it experienced HP, and possibly ultrahigh temperature (UHT), metamorphic conditions during the SBO evolution. The prograde subsolidus metamorphic conditions occurred between ca. 670 and 620 Ma. Subsequently, the granulite underwent water-undersaturated partial melting during heating and burial, reaching peak conditions of ≥1000 °C and 18–19 kbar at a maximum age of ca. 620 Ma. The proposed prograde path indicates that the granulite was involved in the continental subduction channel, reaching depths of ca. 70 km, corresponding to the interface between the lower crust and lithospheric mantle in a double-thickened crust. The granulite facies metamorphic conditions would have been attained primarily through the accumulation of heat production elements and mantle heat flow. Decompression and cooling of the melt-weakened rocks commenced after the delamination of denser material of lithosphere. Subsequently, the previously melt-weakened granulite initiates the in-sequence ductile flow, driven by gravitational force exerted by the weight of the orogen. The PAN granulite likely records the transition from a wedge-shaped orogenic belt to an orogenic plateau during continental collision development.

Geology and characteristics of petrographic rocks in the region of Trepça, Kosovo

Purpose. The purpose of this paper is to accomplish a thorough identification of rock types occurring within the Mitrovica region by describing in detail all encountered varieties. The authors aim to determine the discontinuity or continuity of all inter-formational boundaries, crucial for accurate delineation on the ground and complete reflection on a 1:25000 scale map. Additionally, the objective is to identify the nature of contact between rock types and provide a detailed their description. Methods. Field exploration in the Mitrovica region was conducted for several months. Rock identification involved detailed sampling and petrographic analysis, including thin section preparation of magmatic rocks. The method included sample preparation, polarized light source interactions with minerals and observation of optical properties. Key properties observed are birefringence, interference colors, extinction angles, identification and analysis. Findings. Based on the study of stratigraphic units and geological descriptions of mineral outcrop areas, various types of rocks through petrographic microscope preparation, as well as chemical and geochemical analyses, have been differentiated. The Mitrovica area encompasses the following lithostratigraphic units: harzburgite, gabbro, diabase, metasandstone, sandstone, quartzite and grainstone with calcium (Aeolisaccus sp.; Bioclastic grainstone with dacycladal algae and small milliolides; Salpin-goporella sp.). Originality. The originality of the research lies in employing an optical microscope for the precise identification of rocks. Through the re-search carried out in the study area, we obtained a comprehensive petrographic description of mineral composition, texture, and mineralization, facilitating the assessment of the area exploitation potential. Practical implications. The petrographic exploration yielded the conclusion that the Pb-Zn mineralization is present in the study area, which is a sig-nificant finding for the advancement of the mining sector and the local community, provided that environmental preservation measures are upheld and responsible methods of area exploitation are implemented.

Dark pottery from the early Iron Age in southern Veneto, Italy: The provenance issue under the adoption of specific technological procedures

One of the earliest known settlements of Pre-Roman Padua, Veneto Region (north-eastern Italy) was located in the very centre of the present-day city. The archaeological excavations revealed structures related to craft activities (metallurgical and pottery workshops) and residential areas datable from the late 9th to the 1st century BCE. A large pyrotechnic structure formed by an earthen block densely filled and covered by broken ceramic vessels was found, bringing to light the earliest foundry of the Pre-Roman Padua known to date. Many potsherds found in the site correspond to a regional ceramic class that included coarse and fine wares mainly composed of very dark ceramic bodies. A multi-analytical study based on macroscopic and petrographic descriptions as well as mineralogical, geochemical and microstructural analyses, was carried out in order to state the technological choices adopted for producing these dark vessels and the provenance of the raw materials. A poorly manufacturing was adopted to produce the common pottery, resulting coarse wares uneven textured, while the fine wares were produced by adopting the purification of the base clays and/or the tempering of the clay pastes. Common geo-resources were used, consisting in illitic-chloritic clays rich in quartz and feldspars, the firing regime conditions covered a range of maximum temperatures (normally under 850 °C) and the temper was constituted by silicate (chiefly quartzite, rhyolite and trachyte rock fragments) and/or carbonate (calcite and/or dolomite) inclusions, that were added after having been grounded and/or sieved. The specific mineral and lithic markers of provenance identified in the sherds confirmed the correspondence of this pottery with a local production. However, the specific technological choices that were carried out must be taking into consideration, since the purification removed the larger grains originally present in the base clays and the tempering influenced the chemical composition of the sherds. The different degree of purification of the base clays and the differential tempering procedures that were accomplished point out the adoption of very specific recipes to produce the fine wares, suggesting the correspondence of this local ceramic class with diverse sub-productions and the high level of specialization achieved by the potters in southern Veneto during the Early Iron Age.

Granitoids of the western Himalaya and Karakoram as potential geothermal reservoirs – A petrological, geochemical and petrophysical study

Hot springs in various granitoid and gneissic complexes in northern Pakistan indicate elevated geothermal gradients, which warrants their evaluation for geothermal applications. Evaluation of such prospects requires extensive knowledge of rock properties and their interaction with reservoir fluids. Our contribution provides first-order information on petrophysical, geochemical, and petrographic descriptions derived from outcrop analogs. About seventy samples of (mostly) granitoids and gneisses from the Nanga Parbat Massif (NPM), the Kohistan-Ladakh Batholith (KLB), and the Karakoram Batholith (KB) were collected. Biotite, K-feldspar, and plagioclase in the granitoid and gneiss show weak to moderate alteration, which increases in shear zones, suggesting fracture-assisted fluid interaction. Geochemical results indicate that the gneisses and granites of the NPM are mostly peraluminous S-type and show enrichment in most LREEs and depletion in HREEs. In addition, depleted Sr and Ba and enriched Rb and U in granites indicate partial melting and high fractionation. On the contrary, granitoids from the KLB are of calc-alkaline I-type and characterized by the depletion of REEs and the enrichment in Ba and Sr. The granitoids of the KB, due to their different magmatic histories, are more diverse, ranging from older I-type calc-alkaline granodiorite, and younger I-type alkaline syenite and S-type calc-alkaline granite. They show an overall enrichment in HREEs along with Ba, Th, Ta, Sr, and Lu. Allanites from syenite of the KB show significant concentrations of Th and U.Petrophysical measurements reveal low matrix porosities (0.6–3.5 %), with primarily average thermal conductivities (1.48–3.37 W m−1K −1) and thermal diffusivities (0.68–1.95 ∙10–6 m2 s−1), with minor variation in specific heat capacities (744–767 J kg−1 K−1). The NPM displays higher average thermal conductivity, thermal diffusivity, and lower porosity than the KB, while the petrophysical properties of the KLB range between these two domains.The geothermal systems in the area operate due to the thickening of a granitoid-dominant crust, which is enriched with radiogenic elements. The fault zones provide channels for meteoric water to access deeply these hot and thermally conductive rocks in the subsurface. After heat exchange, the water is discharged back to the surface as hot springs. The present data set provides a better understanding of regional geothermal regimes from petrological, geochemical, and petrophysical perspectives and assists in numerical modeling for potential geothermal assessment.

Integrated petrophysical, sedimentological and well-log study of the Mangahewa Formation, Taranaki Basin, New Zealand

This study aims to address the problem of porosity preservation in the Mangahewa Formation of five main hydrocarbon fields covering onshore and offshore of the Taranaki Basin. An integrated reservoir characterization of the Middle to Late Eocene Mangahewa Formation is achieved through petrophysical evaluation, sedimentological and petrographical descriptions, and well log analysis methods. Petrophysical parameters (porosity and permeability) were acquired from the available core analysis and using mathematical equations to obtain other petrophysical matrices such as normalized porosity index (NPI) and reservoir quality index (RQI). Factors that affected Mangahewa reservoir were studied through thin-section microscopy and well-log analysis helped to measure the reservoir and hydrocarbon potentiality in the Mangahewa Formation. The Mangahewa Formation is dominated by sandstone and a range of marginal to shallow marine facies with varying hydraulic flow units (HFU). The Mangahewa Formation has a high positive correlation in porosity-permeability relationship and has a maximum of 4.67 μm RQI and 20.08 μm FZI (Well Kapuni-14) which reflect potential reservoir. The Mangahewa Formation observed from Wells Kapuni-14, Maui-A1G, McKee-16A, and Mokau-1 are dominated with 59.6%, 56.4%, 79.3%, and 68% of macro- and megapores, respectively. The presence of authigenic clay and calcite cement has greatly reduced the reservoir quality; however, primary and secondary pores are still observed within the Mangahewa sands. Moreover, well log analysis was carried out on four wells in Taranaki Basin, to run a qualitative and quantitative analysis of the Mangahewa reservoir. Eight potential reservoir zones were examined, revealing that the Mangahewa Formation has a very low shale volume of less than 6%, good effective porosity ranging between 11.0% and 13.3%, up to 36.2% of average water saturation and maximum of 69.8% average hydrocarbon saturation. In conclusion, from this comprehensive study, it can be deduced that the Mangahewa Formation possesses fair to good reservoir quality and hydrocarbon potentiality.

The Petrography and geochemistry of iron-bearing units from Mingo’o area (Ntem complex, southern Cameroon)

The banded iron formation (BIF) of the Mingo’o region is located on the northern edge of the Congo Craton. They constitute a significant component of the southern Cameroonian Archean to Paleoproterozoic. Petrographic description indicates that the most characteristic facies of the Mingo'o BIFs are quartz-magnetite BIFs (QMB), which are mostly composed of magnetite and quartz. Geochemistry analyses show that the major elements of this BIF are very simple, with SiO2 and Fe2O3 representing 95.25 wt. % of the bulk rock on average. The low concentrations of Al2O3, TiO2, and HFSE reveal that these chemical sediments are detritus-free. According to Paerson's major element correlation matrix, there is a slight contribution of detrital material to chemical sediment, as confirmed by the strong positive correlation (r = 0.72) of Al and Ti, also by the binary diagrams Al vs. Σ(Y + Nb + Zr) with a weak positive correlation (r2 = 0.31) and Al vs. ΣREE with a zero correlation (r2 = 0.08), indicating that the detrital input was insignificant. The transition metals Zn, Cr, Sr, and V are among the trace elements with low enrichments. This suggests the direction of the volcanogenic hydrothermal input in chemical precipitates. The mean ∑REE concentration of the studied BIF is 26.74 ppm, with a range of 8.82 to 36.74 ppm. Pure chemical sediments are comparable to that. The shale-normalized patterns display minor positive europium anomalies, a sharp decrease in heavy REE, and enrichment in light REE. These geochemical characteristics suggest that the hydrothermal activity in the deep ocean, coupled with seawater, was the source of the Fe and Si. Low-temperature hydrothermal solutions play a key role in the studied BIF, as shown by the absence of a notable positive Eu anomaly. Ce anomalies are seen in the chrondrite-normalized REE patterns, which are characterized by LREE-enriched (mean LaCN/YbCN = 5.28) and HREE depletion (mean TbCN/YbCN = 1.04) patterns. This may indicate that the BIF within the Mingo’o region was formed in place of the basin towards redoxcline, alternating at different times and under various influences from the influx of seawater that has been oxidized.

Assessment of crushed rock aggregates addition to granular lateritic materials in sub-Saharan extensive urbanization context (Yaoundé Cameroon) for road construction purposes

Granular lateritic materials (GLM) suitability for road construction after the addition of crushed rock aggregates (CRA) in the sub-Saharan extensive urbanization context was evaluated. The urbanization rate was made by using Landsat7 and Landsat8 radar images. The studied materials were subject to petrographic description and geotechnical analysis. The urbanized surface in the Yaoundé area increased by 41.88% from 2004 to 2020. The studied rock are gneisses composed of quartz, garnet, biotite, pyroxene, and feldspars. The gneisses obtained CRA have fines content of less than 10%, maximum dry density value of more than 2.00 g/cm3, good bearing capacity (CBR = 88%), and excellent mechanical characteristics (Los Angeles < 40%). This recommend their use as base course for low to medium volume traffic pavements. Gneisses-derived GLM, less load-bearing (CBR = 39 to 42%) than CRA, have limited behaviour in road construction and can be used as sub-base for all volume traffics. Their use in base courses requires prior treatment. CRA addition to these GLM result in the plasticity index decrease and compaction parameters improvement. CBR values increase shows a perfect correlation (R2 > 0.90) with the addition of CRA. Litho-stabilised materials (CBR ≥ 60%) can be used as base course for low-volume trafficked pavements. The addition of a CRA percentage greater than or equal to 25% is efficient for the optimization of their use. By reducing the amount of quarry aggregates, the improved material could be an economical alternative for road projects. This reduces and controls the haulage of materials within crowded urban area.

Interactions between Clays and Carbonates in the Aptian Pre-Salt Reservoirs of Santos Basin, Eastern Brazilian Margin

The giant Pre-salt reservoirs represent most of the oil production in Brazil. The main Aptian sag reservoirs were deposited in a unique and highly complex hyper-alkaline lacustrine setting. These deposits are essentially constituted by fascicular and spherulitic calcite precipitated in a magnesian clay matrix (stevensite, kerolite, and saponite/hectorite). Although vital for understanding the origin and main reservoir quality control, the genesis and interactions of clays and carbonates are still poorly constrained. The detailed petrographic description was focused on 812 thin sections from five wells drilled in the Santos Basin Aptian Barra Velha Formation, combined with cathodoluminescence, UV epifluorescence, and X-ray diffraction analyses. The main syngenetic processes were the deposition of finely laminated peloidal and ooidal Mg-clays, the formation of fascicular calcite crusts on the sediment–water interface, and the redeposition of these materials as intraclasts. Abundant clay peloids engulfed in syngenetic shrubs indicate that calcite and clay precipitation was concomitant, though with highly variable rates. Eodiagenetic phases include matrix-replacive and -displacive spherulites and fascicular shrubs; matrix-replacive blocky calcite and dolomite; lamellar carbonates filling matrix shrinkage pores; and microcrystalline calcite, dolomite, and silica replacing the Mg-clay matrix. The preferential dolomitization and calcitization of peloidal layers were most likely due to their higher permeability and larger specific surface. Matrix-replacive saddle dolomite, macrocrystalline calcite, and dawsonite are interpreted as mesodiagenetic or hydrothermal phases after significant matrix dissolution. Unraveling the processes of the formation and alteration of the carbonates and clays and their interactions in the Pre-salt deposits is essential for constraining the depositional and diagenetic conditions in their unique environments and their diagenetic overprinting and for decreasing the exploration risks and increasing the production of those extraordinary reservoirs.

Diagenesis processes of the Gulneri Shale, Kometan, Mushorah formations within the upper Turonian - lower Campanian Supersequence from Bai Hassan area, North of Iraq

The upper Turonian - lower Campanian Supersequence is characterized with fine-grained, mixed siliciclastic-carbonate middle shelf to sub-basinal depositional system. The present study included the study of vertical and lateral changes of the diagenesis processes of the Gulneri shale, Kometan and Mushorah Formations within the sequences of the (upper Turonian-lower Campanian) cycle in two subsurface sections within the Dome of Daoud in the Bai Hassan field - northern Iraq. The first is represented by a section (BH-16) with a thickness of (213) m. In contrast, the second is represented by a section (BH-86) and a thickness of (233) m. The lithostratigraphy of the rock units of this study are consisting (206) slides were studied, (80) for well (BH-16) and (126) for well (BH-86).The study showed that the sequences of this cycle are unconformity with the lower Dokan Formation and upper contact with Hartha. The petrographic study showed that the rocks of the study were exposed to many diagenesis processes, like cementation, dolomitization, neomorphism, dissolution and compaction according to the degree of their prevalence with a large spread of locally authigenic minerals represented by the widespread pyrite mineral during the two sections of the study, in addition to a few presence of glauconite mineral. Depending on the petrographic description of the diagenesis processes, the vertical and lateral changes of the diagenesis processes were studied, and the diagenesis environments of the formations under study were deduced, where it was found that all the rocks of the two sections of the study were formed in the early stages of the diagenesis processes within the vadose environments as a result of exposure to tectonic processes that helped uplift their rocks to the surface and be affected by meteoric water.

Did the Troodos ophiolite of Cyprus form during subduction inception?

Understanding whether ophiolites formed during subduction inception or above long-lived subduction zones poses an outstanding challenge as modern examples of incipient subduction zones are rare. It is generally agreed that the Troodos ophiolite of Cyprus was formed at a spreading center above a subduction zone within the closing Neo-Tethys Ocean during the Late Cretaceous; beyond this, however, details of its geodynamic origin have been widely debated. To explore this problem, we examine further the analogy previously made between the Troodos ophiolite and the Mariana fore-arc, by here comparing extrusive rocks from the Troodos ophiolite to Pliocene-aged basalts from the southeastern Mariana fore-arc rifts (SEMFR), which formed by near-trench spreading above a highly dehydrating, mature subducted slab. The SEMFR basaltic glasses and olivine-hosted melt inclusions possess the highest markers of water-rich, slab-fluids (Rb/Th, Cs/Th, H2O/Ce, Ba/Th) of any worldwide arc magmas, a signature that may be consistent with efficient slab dehydration within ∼90–100 km from the trench. Troodos ophiolite lavas, which have preserved fresh glasses, recorded similar, elevated slab-fluid markers to those of the SEMFR glasses. Although geodynamic comparisons cannot be direct, we nevertheless propose that the chemical similarities in these slab-fluid markers support the formation of the Troodos ophiolite above a highly dehydrating, shallow subducted plate in a near-trench setting. Unlike the southern Marianas, lack of juvenile arc volcanism and associated volcaniclastics, but presence of an extensive boninitic sheeted dyke complex with associated extrusive section, demonstrates that Troodos presumably developed by organized seafloor spreading in a near-trench setting, immediately following subduction inception of the Neo-Tethyan plate. As such, the chemical similarities between the SEMFR magmas and the Troodos ophiolite can only be used to provide constraints upon the geodynamic and the petrogenetic conditions of magma formation. Hence, deciphering precisely the original setting of ophiolites requires us to integrate thoroughly accurate geology and petrographic descriptions, along with comparison with modern analogues of infant subduction zones.

Quartz-tourmaline metasomatites in gold deposits in Aktyuz-Boоrdinsky ore district (Northern Tien Shan, Kyrgyzstan)

In the Aktyuz-Boordinsky ore district in the Northern Tien Shan, numerous deposits and ore occurrences of gold, rare and rare earth metals and base metals are discovered. Gold deposits are characterized by the presence of a number of metasomatites: beresites, listvenites, K-feldspatites, etc. Many of them are also characterized by quartz-tourmaline gold-bearing metasomatites. A mineralogical and petrographic description of the metasomatic zoning of quartz-tourmaline metasomatites from weakly to highly altered zones is given. Quartz-tourmaline metasomatites form vein-like bodies and metasomatic breccias, where they act as cement, and the fragments can consist of intrusive rock, beresites, K-feldspatites, and listvenites. For the Aktyuz-Boordinsky ore district, quartz-tourmaline metasomatites are a prospecting feature for gold, and for other areas they are a prospecting criterion.

Geological and Geomechanical Characterization of Phosphate Mine Waste Rock in View of Their Potential Civil Applications: A Case Study of the Benguerir Mine Site, Morocco

Sedimentary phosphate extraction in open-pit operations generates large volumes of waste rock (WR), which are mainly overburdens and interburdens. Traditionally, the WR is mixed and stored on the surface in waste rock piles (WRPs). This paper presents a case study of the Benguerir mine site in Morocco. It investigates the potential valorization of each WR lithology based on the geological and geomechanical properties to reduce their environmental footprint and create added value to “waste.” The WR samples (soils and rocks) were collected from drill cores and mining trenches in the Benguerir mine. The geological characterization results using petrographic descriptions indicate the presence of nine phosphate layers and, in addition to the overburdens, eight interburdens. Four types of WR are identified: carbonate, siliceous, marly clay, and phosphate. The geomechanical characterization of soil-like samples showed an average plasticity index (PI) of 50% according to the methylene blue value (MBV) of 7.1, classifying them in the A3–A4 categories as plastic and clayey marl soils. The hard rock samples have excellent mechanical properties in terms of their uniaxial compressive strength (UCS), Los Angeles abrasion value (LA), and micro-Deval value (MD). The average compressive strength is 104 MPa for the flint, 35 MPa for the phosphate flint, 32 MPa for the silexite, 26 MPa for the limestone, 11 MPa for the indurated phosphate, and 8 MPa for the marly limestone. Based on the obtained results, these WRs can be considered as an excellent alternative secondary raw material for use in civil engineering applications, ceramics, and cement industries.

Prograde metamorphism provides gold and base metals to orogenic gold deposits in southern Tibet: Insights from thermodynamic modeling

The metamorphic devolatilization model is presently the most widely accepted model for the formation of orogenic gold deposits. According to this model, Au is likely derived from metamorphic rocks that experienced metamorphism from greenschist to amphibolite facies. Nevertheless, in recent years, some researchers have suggested that Au may have originated from different source regions, such as the mantle. As one of China's most significant orogenic gold provinces, the Indus-Yarlung Zangbo suture zone (IYZSZ) in southern Tibet plays a critical role in this discussion. Orogenic gold deposits within the IYZSZ are typically hosted in Triassic metasedimentary rocks called the Langjiexue group. This paper aims to investigate the suitability of metasedimentary rocks of the Langjiexue group as a source region of Au and base metals. Furthermore, we also examine whether the metamorphic devolatilization model is applicable to the IYZSZ. Our study involves reporting the mineral compositions and evolution of metamorphic zones (chlorite to sillimanite zones) in metasedimentary rocks obtained from the IYZSZ and providing detailed petrographic descriptions of these rocks. Thermodynamic modeling based on metamorphic mineral compositions was used to reveal the behaviors of H2O, Au, S, Cu, Pb, and Zn within prograde metamorphism. Our modeling results proved that the breakdown of chlorite and muscovite, both of which are the main mechanisms of Au and base metals release in metasedimentary rocks, occurs under different geothermal gradients. Given the observed metamorphic mineral compositions, it is the breakdown of chlorite that is the predominant reaction responsible for releasing Au and base metals under the real geothermal gradient of lower crustal metasedimentary rocks (∼25°C/km). This gradient is capable of releasing at least 1.5 ppb Au, 5.2 ppm Cu, 4.3 ppm Pb, 5.0 ppm Zn, and 20 ppm S (as part of the bulk rock) into the ore fluids. Through the mass balance calculations, we found that over 4.05 t of Au, 14040 t of Cu, 11600 t of Pb, and 13500 t of Zn can be mobilized from 1km3 of metasedimentary rocks. Therefore, lower crustal metasedimentary rocks can play a significant role in contributing to Au and base metals in the IYZSZ and other metasediment-hosted orogenic gold deposits worldwide.

Stone Decorations from the Chalcolithic Sites of the Upper and Middle Kama Region (an attempt at complex analysis)

The purpose of the article was a comprehensive study of stone decorations from the Chalcolithic sites of the region. The authors used petrographic description, typological and use-wear analysis. The authors examined 40 items from 6 archaeological sites located in the Upper (2) and Middle (4) Kama region. The jewelry was made of serpentinite of varying degrees of hardness. Serpentinite could originate from the Saranovsky and Moivinsky deposits in the Perm region. The shape of the products was divided into 5 groups, among which the leading ones were oval and teardrop-shaped. Most of the jewelry had a single through hole, which was more often located in the upper part of the product and was drilled on one side. The use-wear analysis made it possible to identify traces from the manufacture of jewelry, traces from use and traces that appeared as a result of the archeologization of objects. Most products were treated as patches on clothing. The shape and quality of jewelry could depend on both the hardness of the raw materials and the skills of the master. They don't create standard groups. The closest analogies can be found in the ornaments studied at the Ust-Kama burial grounds. The population that left these burial grounds could take part in the formation of the Chalcolithic cultures of the Kama forest.

Genesis and evolution of the post-caldera pyroclastic rhyolites from La Primavera caldera, Jalisco, Mexico: A crystal mush perspective

La Primavera is a Quaternary rhyolitic volcanic complex located in the northwestern part of the Trans-Mexican Volcanic Belt. It consists of lavas, domes, and composite volcanoes emplaced between 143.5 and 25.5 ka, both inside and outside the caldera rim. Post-caldera volcanism occurred between ∼95 and 25.5 ka, erupting 2 lavas, 22 domes, and 5 composite volcanoes. Here, we present new petrographic descriptions, mineral chemistry, and whole-rock chemical compositions of juvenile fragments from the post-caldera pyroclastic units originated from the Nejahuete, San Miguel, Planillas, and Tajo composite volcanoes. Based on their major, trace, LREE (light rare earth elements), and isotopic compositions, we identify four compositional groups: 1) Colli dome (low-LREE and low-Zr), 2) E to N pyroclastic units (low-LREE and intermediate-Zr), 3) B to D pyroclastic units, and 4) GP to A pyroclastic units (high-LREE and high-Zr). Based on their composition, we propose a three-step model to explain the genesis of the post-caldera magmas: 1) injection of a calc-alkaline subduction-related melt modified by an OIB mantle-derived component (e.g., the Gachupín basaltic-andesite) into the magmatic chambers and generation of at least two crystal mushes, one formed from crystallization of a mantle-derived magma (i.e, Gachupín), and the other from a differentiated residual liquid derived from the same mantle-derived magma, 2) formation of the REE-poor and REE-rich compositions of the La Primavera magmas by different degrees of partial melting of the crystal mushes, and 3) generation of the REE-intermediate compositions of the La Primavera magmas by mixing of the REE-poor and REE-rich melts produced from partial melting of the crystal mushes to generate the LREE and Zr variations of the La Primavera magmas.

Magnetic susceptibility as an indication of reservoir properties of the Nubia Group in Aswan-Komombo, Southern Egypt

The magnetic susceptibility is a reflection of minerals like the iron oxide that affect the pore system. This work applied the magnetic susceptibility as an effective indicator of reservoir quality. A total of (38) sandstone samples were collected from the Nubia Group exposed in Northeast Aswan, representing the Sabaya, Abu Agag and Um Bramil Formations. The samples were petrophysically examined in terms of porosity, grain density, bulk density, permeability and magnetic susceptibility (κ). Reservoir quality index (RQI) and distribution of the sample within the Global Hydraulic Elements (GHEs) were determined. Several relations constructed between (κ) as a function of iron oxides and the measured properties. The results showed Sabaya and Abu Agag samples are characterized by the highest average values of κ, (287 & 219SI) respectively, matched with the lowest average values of porosity, permeability, RQI and the highest average values of grain & bulk density, hence most samples lie in the lower quality zones (GHE-1, 2, 3 and 4). On the contrary, the lowest average value of (κ) is (37.9 SI) for Um Bramil samples, matched with the highest average values of porosity, permeability, RQI and the lowest average values of grain & bulk density, hence most samples lie in the higher quality zones (GHE-6, 7 and 8). Petrographic description showed the tight and wide pore system relation to the magnetic susceptibility values. The novelty here is utilizing the magnetic susceptibility as an effective pointer for the reservoir quality expectation and classification into hydraulic units.

The use of OBIA and petrography in the study of stone masonry: The case of La Palma, Sierra Nevada de Santa Marta

AbstractIn this case study, rubble masonry walls from the Sierra Nevada de Santa Marta in Northern Colombia were investigated. We applied a combined object‐based image analysis (OBIA), photogrammetry, and petrography method to characterize and determine the provenance of the materials used in three targeted terraces of the archaeological site of La Palma. In situ data acquisition included detailed photographic records and petrographic descriptions of a selected section of masonry walls, as well as mapping of local geological units. Photographs were processed using OBIA. The data obtained allowed characterization and systematization of the constructive elements, via parameters that include architectural function within the construction (headers, stretchers, and wedges), roundness, and lithotype. This methodology enabled us to identify the source of the lithotypes from local rock outcrops (metagranodiorite and schist) as well as rock boulders and cobbles from the nearby riverbeds. The results suggest that pre‐Hispanic communities had knowledge of the properties of the geological resources available in the region, which they used to build masonry that has endured more than five centuries. Vertical differences identified in segments of different terrace walls suggest changes in the constructive process and therefore may serve to determine a construction chronology.

The mafic volcanism of Meseta de Coli Toro, Somún Cura massif, Patagonia, Argentina

The Meseta de Coli Toro is a mafic volcanic plateau located in the Río Negro province, north of Patagonia, Argentina. It belongs to the Somún Cura Massif geological province and its origin remains uncertain.The previous information concerning the volcanic rocks of Meseta de Coli Toro is limited to field outcrop and petrographic descriptions and one bulk rock geochemical analysis. Based on that information, the lavas were included in the Basalto Meseta Coli Toro unit. This contribution presents new petrographic and geochemical information from three different areas of the plateau: northeastern, central, and western, contributing to the knowledge of the origin and evolution of this volcanic plateau. Most of the studied lava flows classify as alkali basalts with OIB signature. Regarding their petrological features and geochemical signature the lava flows were linked to the Oligocene-Middle Miocene volcanism of the Meseta de Somuncurá in northern Patagonia, specifically to the Vulcanitas Corona Chico unit. Based on a published isotope analysis (87Sr/86Sr = 0.70439 and 143Nd/144Nd = 0.51275) and the petrological modelling proposed by other authors for the Vulcanitas Corona Chico unit, the lava flows of Basalto Meseta Coli Toro unit derived from 1 to 8% of partial melting of a spinel-bearing pyroxenite source, with an isotopic composition equivalent to 96% of DM and 4% of EM1.