Group Sedimentary Rocks Research Articles

Integrative mapping of radioactive and alteration zones in Um Had plutons, Egypt: Using Landsat 9, ASTER imagery, and airborne geophysical data

The study aims to assess the effectiveness of using ASTER, Landsat 9, and airborne geophysical data to generate maps depicting hydrothermally altered regions and their correlation with radioactive-mineralized zones in Egypt's Um Had area. Various image processing methods were applied, including color composite images, band ratios, selective principal component analyses, and lineament extraction with enhancement procedures. The ASTER ratios identified the three types of hydrothermal alterations; phyllic, argillic, and propylitic, with other significant alteration zones related to mineralization. Airborne gamma-ray spectrometry contour maps displayed marked varied levels of total count (T.C), eU, eTh, and K, ranging from 1.2 to 21.2 Ur, 0.5–12.34 ppm, 1.45–28 ppm, and 0.13–3.65 %, respectively. The highest anomalies of these radioelements concentration have coincided with the alkali feldspar granite and along their intrusive contact zone with the Hammamat Group sedimentary rocks. Higher anomalies are well recorded in the center and eastern regions of Um Had's, according to the radioelements composite image. The lowest concentrations of these radioelements are associated with gneiss, ophiolitic mélange, metavolcanic, Hammamat Group sedimentary rocks, Taref Formation, and Wadi sediments. To mitigate local magnetic effects, the total aeromagnetic data was reduced to the north magnetic pole (RTP). Power spectrum analysis of the RTP data identified distinct magnetic wavelengths for regional-residual components. In order to identify near-surface magnetic lineaments, such as contacts, shear zones, faults, and dykes, advanced algorithms were applied to the RTP data. The lineaments derived from ASTER and airborne magnetic data revealed dominant fault systems characterized by E–W, NE, NNW, NW and N–S trends, governing the structural framework of the study area. Depth levels of geological contacts and faults that represented pathways for altered and mineralized zones reached more than 1200 m (Euler deconvolution). These findings highlight the consistent results obtained when combining ASTER data with airborne survey data, allowing for the identification of hydrothermally altered zones and primary conductive zones.

Geochemistry and U–Pb zircon geochronology of S-type granites in the Karagwe Ankolean Belt, northwestern Tanzania: Implications for rare-metals mineralization

The Mesoproterozoic Karagwe Ankolean Belt in the Central Eastern Africa region hosts suites of granites that intruded supracrustal rocks and are associated with Sn–W mineralization. In northwestern Tanzania, the granites are S-type, peraluminous and calc-alkalic to alkalic-calcic in composition. Geochemical analyses of the granites reveal the following concentrations: SiO2 (71–77 wt%), Al2O3 (11–15 wt%), K2O (3–7 wt%), MgO (0.06–0.86 wt%), TiO2 (0.07–0.36 wt%) and Fe2O3T (0.71–2.40 wt%). The SiO2 contents correlate negatively with MgO, CaO and K2O and positively correlate with Nb, Ta, Zr, Sr, Ba, Hf, Th and the REE (Eu, Ce and Sm). Primitive mantle normalized spider plots show elevated LILE against depletions in Ba, Nb, Ti and La, suggesting inherited arc-magmatic signatures from the protolith. Chondrite-normalized REE spider plots show elevated LREE and relatively flat MREE and HREE as indicated by (La/Yb)cn = 2–9 and negative Eu* anomalies (0.05–0.24). Modelling of nearby Uha group sedimentary rocks partial melts, supports fractional crystallization and upper crustal contamination for generation of the KAB granites. Trace elemental ratios of Y/Nb, Nb/Ta, Zr/Hf and P205 suggest the granites are weakly mineralized to barren in Sn–W.The U–Pb zircon ages point to two successive and overlapping periods: from 1418 ± 15 Ma to 1395 ± 6 Ma (mean = 1406 Ma) and from 1385 ± 8 Ma to 1376 ± 11 Ma (mean = 1381 Ma). Inheritance U–Pb dates of xenocryst zircons, comprise early to late Paleoproterozoic ages ranging from 2376 ± 30 Ma, 2000 ± 27 Ma to 1901 ± 39 Ma, and from 1795 ± 7 Ma to 1738 ± 7 Ma, implying the participation of the Paleoproterozoic sedimentary rocks in the petrogenesis of the granites. A zircon rim overgrowth age of 1015 ± 26 Ma is interpreted to date a metamorphic/anatexis event that coincided with the c.1000 Ma amalgamation of the supercontinent Rodinia and the emplacement of Sn–W enriched granites. The ages of S-type granite magmatism (1418–1376 Ma) obtained from this study are restricted to the Mesoproterozoic within the widespread bimodal magmatism event c.1375 Ma.

Watershed-scale provenance heterogeneity within Eocene nonmarine basin fill: Southern Greater Green River Basin, western USA

Weathering, erosion, and sediment transport in modern landscapes may be investigated via direct observation of attributes such as elevation, relief, bedrock lithology, climate, drainage organization, watershed extent, and others. Studies of ancient landscape evolution lack this synoptic perspective, however, and instead must rely more heavily on downstream records of fluvial deposits. Provenance analysis based on detrital grain ages has greatly enhanced the utility of such records but has often focused broadly on regional to continental scales. This approach may overlook important details of localized watersheds, which could lead to significant misinterpretation of past sediment dispersal patterns. The present study, therefore, explores the impact of geographic and stratigraphic sampling density on detrital zircon provenance, based on a high-density investigation of U-Pb ages (N = 23, n = 4905) obtained from a narrow chronostratigraphic range (∼2 m.y.) within a relatively small (∼25,000 km2) area of an Eocene nonmarine sedimentary basin. Based on multi-dimensional scaling and DZmix modeling, these strata comprise seven distinct, approximately isochronous detrital zircon (DZ) chronofacies, defined as “. . . a group of sedimentary rocks that contains a specified suite of detrital zircon age populations” (Lawton et al., 2010). Four of these DZ chronofacies reflect long-distance transport from extrabasinal source areas. DZ chronofacies CO-1 and CO-2 are interpreted to derive from a primary sediment source in central Colorado (USA), corroborating previously proposed long-distance sediment transport via the Aspen paleoriver. DZ chronofacies ID-1 and ID-2 are interpreted to have been delivered to the basin from central Idaho by the Idaho paleoriver. In contrast, DZ chronofacies UT-1 and UT-2 are interpreted to reflect local drainage from the Uinta Uplift south of the basin, and DZ chronofacies WY-1 is interpreted to have been sourced from the Rawlins, Granite, and Sierra Madre uplifts to the north and east via the Toya Puki paleoriver. Lateral transitions between different DZ chronofacies in some cases occur over distances as little as 5 km, implying that depositional systems carrying sand from disparate watersheds directly competed to fill available basin accommodation. The results of this study reveal a high degree of complexity of Eocene rivers that converged on the Greater Green River Basin, indicating that their deposits contain a rich record of fine-scale landscape evolution across much of the Laramide foreland and Cordilleran orogen. These results illustrate the need for adequate sample density when assessing basin-scale provenance and offer a cautionary consideration for researchers using sandstone (and incorporated authigenic cement) in other nonmarine basins as the basis for paleoaltimetry or detrital thermochronology studies.

Paralava and clinker from the Canadian Arctic: a record of combustion metamorphism dating back to the late Miocene

Outcrops with conspicuous reddish to yellow-colored clinker, blackish paralava, and blends of both with a breccia-like appearance occur across the Canadian Arctic. We examined such rocks on Ellesmere Island, Banks Island, and the Mackenzie Delta area. These rocks are a product from natural combustion of bituminous shale and low-rank coal seams in Cretaceous and Paleogene host sedimentary rocks, respectively. The main mineral phases of clinker and silicate paralava samples are comprised of quartz + hematite ± feldspars ± cristobalite (or tridymite) ± cordierite–sekaninaite ± clinopyroxene ± sillimanite ± glass. Slag-like iron oxide paralava (74–95 wt.% total Fe2O3) consisting of hematite ± magnetite ± clinopyroxene occur in Paleogene host sedimentary rocks, rich in siderite concretions. The whole-rock geochemical composition of clinker and silicate paralava shows similarities for samples from the same outcrop. Regional and local specific elemental enrichments are mainly inherited from the sedimentary protoliths, which are characterized by volcanogenic input (Paleocene sedimentary rocks) or oxygen-depleted depositional conditions (Upper Cretaceous bituminous sedimentary rocks). Spontaneous combustion could take place when the organic-rich sedimentary rocks become exposed to atmospheric oxygen. This process has occurred at least since the Messinian stage (Miocene) on Ellesmere Island (6.1 ± 0.2 Ma; 40Ar/39Ar incremental heating dating on whole-rock paralava) and continues until now. An active combustion process on scree from a coal seam and clastic Eureka Sound Group sedimentary rocks was observed on Ellesmere Island.

Lithological controls influence river form in contrasting sub‐catchments: The Scamander and Avenue Rivers, Tasmania

AbstractBedrock influence on river channel form is difficult to assess, with many catchments dominated by glacial erosion and with blanketing sediments from both pluvial and fluvial sources. The Scamander catchment in Tasmania lacks glacial history and features two bedrock‐confined sub‐catchments of similar area and maximum flow length, but one dominated by Mathinna Group sedimentary rocks, and the other with large areas in granite lithology. Lithology types, stream network parameters and near stream slope angles were analysed using geographical information systems, and results of stream cross sections, channel form and planforms were compared for each lithology. Results showed that granite features low gradient, shallow streams with channel slope cross sections of <10°. Granite channels are irregular and dominated by large boulders that create channel roughness and resist incision. Weathering products of coarse quartz sand provide anchorages for in‐channel vegetation. By contrast, sedimentary Mathinna Group rocks feature steeply incised trough‐like channels, with near‐channel hillslope gradients mostly between 30° and 50°. Rectangular blocks disintegrate to gravel cobbles on the channel bed, providing poor anchorage for vegetation. Mathinna lithology influences steep, low roughness channels, and stable, incised recurved meanders formed by river interaction with vertical layers of resistant rock. The resistant bedrock lithology is demonstrated in this study to influence varied river planforms, near‐channel slope gradients, channel sediments and cross sections.

Provenance of Ordovician Malieziken Group, Southwest Tarim and Its Implication on the Paleo-Position of Tarim Block in East Gondwana

Tarim is inferred to have a close connection with East Gondwana during the Ordovician, but the position in East Gondwana remains controversial. In this study, we report 316 detrital zircons U-Pb data from three samples of Ordovician Malieziken Group sedimentary rocks, collected in the Qiate Section, Southwest Tarim, provided new insight into the position of Tarim in East Gondwana. Detrital zircons data indicated the maximum depositional age for the three samples is 489.5 Ma, 478.1 Ma, and 465 Ma, respectively, indicating the Qiate and Kandilike Formation of the Malieziken Group was deposited in Early—Middle Ordovician. The detrital zircons are characterized by two main peaks at ~490 Ma and ~1100 Ma, and three subordinate peaks at ~880 Ma, ~1400 Ma, and ~1650 Ma, suggesting most of the detritus of Malieziken Group from the South Kunlun Terrane (SKT) itself. However, the source of the ~1650 Ma peak is not found in the Tarim block, and the ~1400 Ma and the ~1650 Ma peak are absent in the middle of the three samples, which implied that there is an exotic source. The Paleoproterozoic sediment strata in the Albany–Fraser belt shows dominant peaks at ~1400 Ma and ~1650 Ma may have been transported to SKT and redeposited in the Malieziken Group during the Ordovician. The Malieziken Group shows detrital zircon age patterns resembling those of East Sumatra, Lhasa, and Western Australia which, in combination with the Albany–Fraser belt provenance, enables us to propose that the Tarim block has a close linkage with Western Australia, East Sumatra, and Lhasa in East Gondwana.

The Kaiser effect in sedimentary rocks on the example of deposits of the Vertoletnaya area in Tomsk region

The acoustic emission memory effect (Kaiser effect) discovered in 1953 by the German scientist Joseph Kaiser is widely used as one of the methods of non-destructive testing of materials. In addition, according to acoustic emission data, it is possible to estimate the level of stresses experienced by rocks in reservoir conditions. In this paper, we consider the results of recording acoustic emission in sedimentary rock samples from the Mesozoic part of the section of Western Siberia. The studying features of the manifestation of acoustic emission signals was carried out on the main lithological varieties (medium-grained sandstone, mudstone, siltstone, sandstone with carbonate cement). In total, 43 samples were analyzed under uniaxial loading and 1 sample under pseudo-triaxial loading. The results of the study are graphs of the dependence of the total number of acoustic emission signals from the stress. The dependence of the number of acoustic emission signals on the group of sedimentary rocks is determined.

Phanerozoic orogeny in the South China Block traced by clastic components from Cambrian to Triassic sedimentary rocks

Systematic detrital zircon U-Pb dating was applied to Cambrian to Triassic sedimentary rocks in the Yong’an basin, southeastern South China, and yields four age groups. Cambrian and Ordovician quartz schists display prominent Grenvillian (906–1199 Ma) and minor late Neoarchean to earliest Paleoproterozoic (2418–2619 Ma) populations, showing characteristics of an East Gondwanan origin. Devonian and Carboniferous sandstones feature euhedral Neoproterozoic (733–999 Ma) and early Paleozoic (405–468 Ma) detrital zircons, suggestive of a proximal accumulation sourced from the nearby Wuyi terrane. Middle Permian to early Triassic sandstones contain abundant late Paleoproterozoic (1743–1979 Ma), Grenvillian (893–1271 Ma), and early Paleozoic (403–471 Ma) detrital zircons, together with increasing late Paleozoic (264–374 Ma) populations, indicating multiple provenances. Late Triassic sandstones show dominant proportions of late Paleoproterozoic (1805–1938 Ma) zircons, derived mainly from the Wuyi terrane. Three Phanerozoic orogenic events in the South China Block (SCB), i.e. early Paleozoic (Kwangsian or Wuyi-Yunkai), late Paleozoic, and early Mesozoic (Indosinian) orogenies, probably are responsible for the changes in clastic provenance among the four groups of sedimentary rocks. The late Paleozoic orogeny in the SCB was previously thought to be a limited minor event. This study suggests that the impact of late Paleozoic orogenesis on the SCB could be much wider. The uplift of the Wuyi Mountains probably started after the early Paleozoic orogeny, much earlier than previously proposed, and has provided long-term clastic supply to the Yong’an basin and nearby basins.

Limited expression of the Paleoproterozoic Oklo natural nuclear reactor phenomenon in the aftermath of a widespread deoxygenation event ~2.11–2.06 billion years ago

The only known case of natural fission reactors is hosted by high-grade uranium (U) deposits at Oklo-Okelobondo and Bangombé in sandstones of the ~2.1 Ga Francevillian Group, Gabon. However, the geochemical influence of the depositional environment on this unique natural nuclear phenomenon has not been clearly established. Localized, unusually high vanadium (V) enrichments are thought to have prevented such natural nuclear fission reactors from occurring in other Francevillian U deposits (e.g., Mounana, Boyindzi, and Mikouloungou). However, while U-bearing detrital monazite derived from Archean rocks surrounding the Francevillian basin is viewed as the main source of U, the source of V remains poorly constrained. Here, we combine petrographic and whole-rock geochemical data for the Francevillian Group sedimentary rocks, coupled with previously documented geochemical data for the Archean basement. These data suggest that, although ultramafic to mafic igneous rocks of the Mesoarchean Bélinga Group, and to some extent Archean granitoids, were likely important sources of V to the Francevillian U deposits, they were not the only source of the abnormally high V concentrations in the U deposits that did not produce natural nuclear reactors. Instead, hydrocarbon migration from V-rich black shales of the Upper Francevillian Group, deposited during widespread and protracted deoxygenation of the Paleoproterozoic ocean at the end of the Lomagundi Carbon Isotope Excursion (LE) at ~2.11–2.06 Ga, resulted in a redox front that precipitated the U deposits. These migrated V-rich hydrocarbons likely account for the high V concentrations, which ultimately prevented natural fission reactions from occurring in these U deposits. Similarities with other pyrobitumen-bearing Paleoproterozoic U deposits worldwide suggest that organic-rich source rocks, which deposited in open-marine settings under widespread hyper-euxinic conditions in the aftermath of the LE, played a key role in preventing the Oklo natural nuclear reactor phenomenon from reaching a larger extent.

Geochemistry of Barail sandstone in Champhai, Mizoram: Implications on provenance and weathering history

Mizoram is geologically young comprising entirely of sedimentary rocks. The Oligocene age of Barail group of sedimentary rocks, exposed near the Indo Myanmar mobile belt, i.e., Champhai area, Mizoram have been studied to infer the provenance and paleoweathering history. The significance of the work is that it will uncover and reconstruct origin of Barail sandstone and restore the tectonic and paleoclimatic conditions based on petrography and geochemistry of the sandstone. The geochemical classification based on diagrams of log(SiO2/Al2O3) vs. log(Na2O/K2O) and log(SiO2/Al2O3) vs. log(Fe2O3/K2O) of the Barail sandstones indicated that the sandstones belong to litharenite and wacke. The geochemical characteristics of the rock are plotted in various binary and ternary diagrams and selected ratios of elements are interpreted in the light of source rock characteristics. The Chondrite normalised REE pattern show similar pattern to UCC (Taylor and McLennan 1985) with an enrichment of LREE and depletion of HREE. The ratio of Al2O3/TiO2 and the ternary diagram of La–Th–Sc indicate felsic to intermediate source. Similarly, the high LREE/HREE ratios and negative Eu anomaly (La/Lu)cn, ratios of Eu/Eu*, La/Sc, La/Co, Cr/Th, Th/Sc, Th/Co and Cr/V ratio indicate felsic source. The binary diagram between ratios of Zr/Sc vs. Th/Sc inferred volcanic source. The sediments are also found to be derived from recycled sedimentary rock which is proved by ratios of Zr/Sc and Th/U. The A–CN–K diagram and weathering indices such as CIA, PIA shows moderate to intense weathering in the source area. The investigated Barail sandstones are chemically mature as indicated by the ICV while the ratio of SiO2/Al2O3 indicated moderate sediment maturity.

Zircon U-Pb ages and Hf isotope analysis of Neoproterozoic Yaolinghe Group sedimentary rocks in the Chengkou area, South Qinling: Provenance and paleogeographic implications

The Yaolinghe Group sedimentary rocks exposed along the Dabashan fault zone are rich in pyroclastics and are thousands of meters thick. They provide key information for studying the tectonic location relationship between the Yangtze and Qinling blocks and the tectonic-sedimentary environment of the late Neoproterozoic continental margin. This study reports U–Pb and Lu-Hf isotope data of 438 and 408 detrital zircons from five samples of Yaolinghe Group sedimentary rock, collected in the Chengkou area, China. Combining the youngest detrital zircon U–Pb age for gravelly sandstone with data from previous studies, we constrained the maximum depositional time of the Yaolinghe Group sedimentary rocks in this area to ca. 650–635 Ma (i.e., Neoproterozoic Cryogenian). In the Chengkou area, the predominantly Neoproterozoic (1000–635 Ma) detrital zircons, with wide εHf(t) values ranging from −31.03 to 13.07 and an average of 2.00, were primarily derived from the adjacent regions of exhumed igneous rock in the South Qinling and Northern Yangtze blocks. Several minor age peaks at more than 1000 Ma indicate that this formation contains a small amount of recycled zircon. Further study indicates that the Yaolinghe Group sedimentary rocks were formed by near-source accumulation in a rift setting, located at the north margin of the Yangtze Block, during the breakup of Rodinia. As in previous studies, the retreat of the subducted slab may have led to continental rifting and extension of the Northern Yangtze Block, and the Yaolinghe Group sedimentary rocks in the Chengkou area recording the history of the last stage of Rodinia breakup.

The western boundary between the Yangtze and Cathaysia blocks, new constraints from the Pingbian Group sediments, southwest South China Block

The Pingbian Group of southeast Yunnan Province is the only exposed Precambrian unit in the southwestern corner of the South China Block (SCB). The nature of this group is crucial to reveal the basement component in the area, and to further understand Precambrian evolution of the SCB and to constrain the boundary between the Yangtze and Cathaysia blocks. Zircon U-Pb ages indicate that the Pingbian Group sedimentary rocks were deposited later than 772–710 Ma. “Cambrian” Langmuqiao formation sedimentary rocks near the Pingbian Group have the maximum deposition age and age spectra of detrital zircons similar to the Pingbian Group, suggesting that they probably belong to the Pingbian Group. The Pingbian Group sedimentary rocks have variable compositions with low SiO2/Al2O3 (3.7 ∼ 9.7), implying relatively low maturity. Their REEpatterns are characterized by enrichment in LREE with (La/Yb)n of 5.3–10.3. Geochemical and petrologic characteristics of these sedimentary rocks are similar to those Neoproterozoic sedimentary rocks in the southern Yangtze Block. The Pingbian Group sedimentary rocks show unimodal age spectra of detrital zircons with a peak at 844–808 Ma and minor old detritus. The age spectrum resembles those of Neoproterozoic sedimentary rocks in the southern and western Yangtze Block, but different from those in the Cathaysia and Indochina blocks, suggesting the Pingbian Group’s affinity with the Yangtze Block.The integration of U-Pb ages and Hf-isotopes of detrital zircons indicates that there were five main magmatic events occurring in the southwestern Yangtze Block. Early Paleoproterozoic, late Paleoproterozoic and late Mesoproterozoic magmatism mainly involved the reworking of ancient crusts, whereas 1.7–1.5 Ga and 0.9–0.7 Ga magmatic activities were accompanied by the input of significant mantle-derived materials. Continuous Neoproterozoic mantle-derived magmatism suggests a subduction-related tectonic setting. Geochemistry of sedimentary rocks and age distribution of detrital zircons suggest that the Pingbian Group was deposited in the back-arc foreland basin.All observations suggest that the Pingbian Group is similar to those Neoproterozoic sedimentary rocks in the western Yangtze Block, and consequently the Pingbian area should belong to the Yangtze Block. Thus, the boundary between the Yangtze and Cathaysia blocks must be located at east of the study area.

GEOLOGIA DA FOLHA TOPOGRÁFICA GUAPIARA NA ESCALA 1:50.000

A região da Folha Topográfica de Guapiara situa-se no extremo sul do estado de São Paulo e compreende na porção central da Província Mantiqueira a faixa centro-sul do Cinturão Ribeira. O arcabouço geológico local envolve rochas da sequência metavulcanossedimentar do Supergrupo Açunguí, de idade meso- a neoproterozoicas deformadas e metamorfizadas no neoproterozoico, associadas a rochas granitoides geradas durante os episódios colisionais ligados ao Ciclo Brasiliano e à formação do Supercontinente Gondwana. As principais unidades litoestratigráficas são constituídas pelos metassedimentos da Formação Água Clara e dos grupos Votuverava e Itaiacoca de idade meso- a neoproterozoica, corpos graníticos neoproterozoicos representantes do Complexo Granito Três Córregos e do Maciço Granito Capão Bonito, rochas sedimentares do Grupo Itararé, efusivas básicas associadas ao Magmatismo Serra Geral e sedimentos recentes quaternários. O quadro estrutural-metamórfico é determinado pelo arranjo tectônico final neoproterozóico, evidenciado nas rochas epimetamórficas por uma evolução estrutural do tipo polifásica, com quatro fases de deformação (Sn, Sn+1, Sn+2 e Sn+3) e três eventos metamórficos (M1, M2 e M3), de caráter regional progressivo, contato e regional a localmente dinâmico de natureza retrometamórfica.
 Palavras-chave: geologia, estrutural, mapeamento, Guapiara.

Comparing two arms of an orogenic belt during Gondwana amalgamation: Age and provenance of the Cuiabá Group, northern Paraguay Belt, Brazil

The Cuiabá Group is the basal part of the sequence of passive margin sediments that unconformably overly the Amazonian Craton in central Brazil. Despite these rock's importance in understanding Brazil's path in the supercontinent cycle from Rodinia to Gondwana and their potential record of catastrophic glaciation their internal stratigraphy and relationship to other units is still poorly understood. The timing of deposition and source areas for the subunits of the Cuiabá Group sedimentary rocks are investigated here using integrated U-Pb and Sm-Nd isotope data. We sampled in the northern Paraguay Belt, a range that developed in response to the collision between the Amazonian Craton, the Rio Apa Block, the São Francisco Craton and the Paranapanema Block. 1125 detrital zircon LA-ICPMS U-Pb ages were calculated and 22 whole rock samples were used for Sm-Nd isotope analysis. The U-Pb ages range between Archean and Neoproterozoic and the main source is the Sunsás Province. Moving up stratigraphy there is a subtle increase in slightly younger detritus with the youngest grain showing an age of 652 ± 5 Ma, found at the top of the sequence. The age spectra are similar across each of the sampled units and when combined with the Sm-Nd data, indicate that the source of the detritus was mostly similar throughout deposition. This is consistent with the analysis here that indicates sedimentation occurred in a passive margin environment on the southern margin of the Amazonian Craton. The maximum depositional age of 652 ± 5 Ma along with the age of the overlying cap carbonate of the Mirassol d’Oeste Formation suggests that part of this section of sediments were deposited in the purportedly global ∼636 Ma Marinoan glaciation, although we give no sedimentological evidence for glaciation in the study area. Compared to the southern Paraguay Belt where no direct age constraints exist, the glacial epoch could be either Cryogenian or Ediacaran. In addition, available data in the literature indicates a diachronous evolution between the northern and southern arms of the Paraguay Belt in the final stages of deposition and deformation.

U-Pb and Hf isotopes of detrital zircons from the Pongola Supergroup: Constraints on deposition ages, provenance and Archean evolution of the Kaapvaal craton

The c.3.0–2.9 Ga Pongola Supergroup located on the south-eastern margin of the Kaapvaal Craton preserves some of the most pristine Archean successions but the timing of volcanic and sedimentary processes, and sediment sources remain relatively poorly known. Combined zircon U-Pb ages and Hf isotope data of sandstones from the Nsuze and Mozaan Groups are presented from key locations of the Northern, Central and Southern Domains of the 270 km long belt. These data indicate important lateral and temporal variations and therefore changes in source areas. The basal Pongola sedimentary rocks (Mantonga Formation) in the Northern Domain were derived from the c.3.4 Ga basement, and also most likely from reworked Moodies Group sedimentary rocks of the Barberton greenstone belt. Zircon detritus in Nsuze and lower Mozaan Groups sandstones shows similar age-Hf isotope spectra, and a remarkable overlap with data from magmatic and (meta)sedimentary rocks of the southeastern Kaapvaal craton. The data point to a proximal source for Nsuze sandstones in the Northern Domain and a different provenance for the Southern Domain sandstones. A possible signature of the extensive 3.10–3.07 Ga Mpuluzi granite is seen only in the Nsuze sandstones and basal Mozaan of the Central Domain. The data indicate for the first time that Cr-rich sandstones of the Ndikwe Thrust Segment of the Southern Domain were deposited at <2.92 Ma, and therefore belong to the uppermost Mozaan Group. Furthermore, they reveal a significant switch in provenance from the lower to upper Mozaan Group from a reworked Archean basement towards a more juvenile hinterland with εHft = +0.9 to +6.3 between 3.07 and 2.92 Ga; similar to the source previously reported for the Central Rand Group of the Witwatersrand Basin. The juvenile zircon detritus in the upper Mozaan Group requires sources as far as the Pietersburg and/or Kimberley Blocks, which became amalgamated to the central block of the Kaapvaal Craton at 2.97–2.87 Ga, but perhaps also a source outside of the present day Kaapvaal Craton, and possibly as far as the Pilbara Craton.

Behavior of Sandstones Under Heat Treatment

Knowledge of materials behavior under heat treatment is of high importance in construction and safety engineering; tunnels represent a special field because of their specific safety issues. In the case of fire, tunnel structure and surrounding rock are subjected to extreme temperatures which induces irreversible changes in the material’s microstructure and consequently its mechanical properties. Significant portion of the Earth’s crust is formed by sandstones; this group of sedimentary rocks is highly variable in structure, composition and engineering properties. Quartz grains (alternatively together with other minerals) form the clastic part of sandstones; the space between clasts is filled by variable amount of cement and matrix which can contain particularly clay minerals, quartz and calcite. The porosity of sandstones is again highly variable from a nearly compact material to a highly porous one. The paper aims to find out and explain differences in response of various kinds of sandstones to heat treatment. The behavior of a representative set of sandstones under heat treatment was studied by TG/DSC, thermodilatometry and residual strength measurement. These experiments were accompanied by SEM and porosimetry measurement. The effect of increased temperature on the compressive strength was found to be crucially dependent on the nature of the cement and matrix present in the individual rock. The rocks with calcite cement which had high initial strength and low porosity were damaged by calcite decomposition. The siliceous sandstones were damaged by cracking due to thermally induced volume changes. In contrary, the strength of the clayey sandstones was even improved after the heat treatment. It can be concluded that behavior of sandstone under heat treatment is controlled by its composition and diagenesis.

Evidence for Two Stages of Mineralization in West Africa’s Largest Gold Deposit: Obuasi, Ghana

The supergiant Obuasi gold deposit is the largest deposit in the Paleoproterozoic Birimian terranes of West Africa with 62 Moz of gold (past production + resources). The deposit is hosted in the Paleoproterozoic Kumasi Group sedimentary rocks composed of carbonaceous phyllites, slates, psammites, and volcaniclastic rocks intruded by different generations of felsic dikes and granites. A three-stage deformation history is defined for the district. The D1Ob stage is weakly recorded in the sedimentary rocks as a layer-parallel fabric and indicates that bedding parallel shearing occurred during the early stage of deformation at Obuasi. The D2Ob is the main deformation stage affecting the Obuasi district and corresponds to a NW-SE shortening. Tight to isoclinal folding, as well as intense subhorizontal stretching, occurred during D2Ob, parallel with the plane of a pervasive NE-striking subvertical foliation (S2Ob). Finally, a N-S shortening event (D3Ob) refolded previously formed structures and formed a distinct ENE-striking, variably dipping S3Ob cleavage that is domainal in nature throughout the deposit. Two economic styles of mineralization occur at Obuasi and contribute equally to the gold budget. These are (1) gold-bearing sulfides, dominantly arsenopyrite, mainly disseminated in metasedimentary rocks and (2) native gold hosted in quartz veins that are as much as 25 m wide. Microstructural evidence, such as strain shadows surrounding gold-bearing arsenopyrite parallel with S2Ob, but folded by S3Ob, indicates that the sulfides were formed during D2Ob. Concentrations of as much as 700 ppm Au are present in the epitaxial growth zones of the arsenopyrite grains. Although the large mineralized quartz veins are boudinaged and refolded (indicating their formation during D2Ob), field and microanalytical observations demonstrate that the gold in the veins is hosted in microcracks controlled by D3Ob, where the S3Ob cleavage crosscuts the quartz veins in the main ore zones. Thus, these observations constitute the first evidence for multiple stages of gold deposition at the Obuasi deposit. Futhermore, three-dimensional modeling of stratigraphy, structure, and gold orebodies highlights three major controls on oreshoot location, which are (1) contacts between volcaniclastic units and pre-D1 felsic dikes, (2) fault intersections, and (3) F3Ob fold hinges. The maximum age for the older disseminated gold event is given by the age of the granites at 2105 ± 2 Ma, which is within error of hydrothermal rutile in the granites of 2098 ± 7 Ma; the absolute age of the younger gold event is not known.

Linked thermal convection of the basement and basinal fluids in formation of the unconformity‐related uranium deposits in the Athabasca Basin, Saskatchewan, Canada

AbstractWorld‐class unconformity‐related U deposits in the Athabasca Basin (Saskatchewan, Canada) are generally located within or near fault zones that intersect the unconformity between the Athabasca Group sedimentary basin rocks and underlying metamorphic basement rocks. Two distinct subtypes of unconformity‐related uranium deposits have been identified: those hosted primarily in the Athabasca Group sandstones (sediment‐hosted) and those hosted primarily in the underlying basement rocks (basement‐hosted). Although significant research on these deposits has been carried out, certain aspects of their formation are still under discussion, one of the main issues being the fluid flow mechanisms responsible for uranium mineralization. The intriguing feature of this problem is that sediment‐hosted and basement‐hosted deposits are characterized by oppositely directed vectors of fluid flow via associated fault zones. Sediment‐hosted deposits formed via upward flow of basement fluids, basement‐hosted deposits via downward flow of basinal fluids. We have hypothesized that such flow patterns are indicative of the fluid flow self‐organization in fault‐bounded thermal convection (Transport in Porous Media, 110, 2015, 25). To explore this hypothesis, we constructed a simplified hydrogeologic model with fault‐bounded thermal convection of fluids in the faulted basement linked with fluid circulation in the overlying fault‐free sandstone horizon. Based on this model, a series of numerical experiments was carried out to simulate the hypothesized fluid flow patterns. The results obtained are in reasonable agreement with the concept of fault‐bounded convection cells as an explanation of focused upflow and downflow across the basement/sandstone unconformity. We then discuss application of the model to another debated problem, the uranium source for the ore‐forming basinal brines.

Mineralisation footprints and regional timing of the world-class Siguiri orogenic gold district (Guinea, West Africa)

Siguiri is a world-class orogenic gold district hosted in the weakly metamorphosed Upper Birimian to Lower Tarkwa Group sedimentary rocks of the Siguiri Basin (Guinea). The district is characterised by a protracted deformation history associated with four main deformation events: D1S is a N-S compression; D2S is an E-W compression progressively evolving into an early-D3S transpression and then into a late-D3S NNW-SSE transtension and D4S is a NE-SW compression. Field observations, petrography and geochemistry at three key deposits of the Siguiri district (Bidini, Sintroko PB1 and Kosise) suggest a polyphase hydrothermal history that can be subdivided into four hydrothermal events. The first hydrothermal event was associated with the development of barren bedding-parallel and en-echelon V2S quartz-dominated-(pyrite) veins. The second hydrothermal event is characterised by the development of V3A pyrite-ankerite veins late during D3S. Laser ablation-ICP-MS data show that this vein set contains high gold contents of up to 43.3 ppm, in substitution in pyrite crystal lattice, representing a minor first gold mineralisation event. The third and most prominently developed hydrothermal event is late D3S and represents the second and principal gold mineralisation event. This mineralisation event led to two distinct mineralisation textures. The first texture is best exposed in the Kosise deposit and is characterised by gold-bearing quartz-ankerite-arsenopyrite conjugate V3B veins. Although the bulk of the gold is hosted in native gold grains in V3B veins, LA-ICP-MS analyses show that gold also substitutes in the arsenopyrite crystal lattice (up to 55.5 ppm). The second mineralisation texture is best expressed in the Sanu Tinti deposit and consists of disseminated barren pyrite hosted in a polymict conglomerate. The second and third hydrothermal events are both structurally controlled by a series of early-D3S N-S, NE-SW, WNW-ESE and E-W sub-vertical incipient structures expressed as fracture zones of higher V3S vein density. A composite geochemical cross section across fracture zones from the Kosise deposit indicates that gold mineralisation in the Siguiri district is associated with enrichments in Ag, Au, As, Bi, Co, Mo, (Sb), S, Te and W relative to background. Geochemical variations associated with the ore shoots in the Siguiri district are consistent with petrographic observations and highlight an albite-carbonate-sulphide-sericite alteration. The fourth and last hydrothermal event is associated with the development of a late penetrative S4S cleavage during D4S deformation, which overprints all pre-existing hydrothermal features and is associated with the deposition of free gold, chalcopyrite and galena along fractures in V3A pyrite and V3B pyrite and arsenopyrite. Mineralogical and geochemical footprints as well as timing of the gold-mineralising events in the Siguiri district, when compared with other deposits of the West African Craton, highlight the synchronicity of gold mineralisation in Siguiri (syn-D3S and syn-D4S events) with other similar events in this part of the craton, such as the early Au-Sb-Bi-(Te-W) mineralisation at the Morila deposit in Southeast Mali. Our results support the hypothesis that late Eburnean-age gold mineralisation in the Siguiri district and in the West African Craton as a whole was polyphase.

Development of a conceptual geohydrological model for a fractured rock aquifer in the Karoo, near Sutherland, South Africa

An aquifer in the semi-arid environment of Sutherland, South Africa is examined. The characterization of the saturated geological unit was completed by mapping associated dolerite dykes and fractured outcrops of Beaufort Group sedimentary rocks. This was complimented by pumping tests in order to better understand the fracture connectivity that was found to decrease with distance from a prominent dyke. Groundwater samples and precipitation samples in the study area were analysed for stable isotopes. Drill chips of selected boreholes were examined in order to also better understand the nature of the subsurface at the contact of the dolerite and a suspected subsurface feature identified through satellite imagery. The results are combined into a conceptual geohydrological model to gain insight into the groundwater flow dynamics of the area. Findings suggest preferential recharge occurs in the study area with higher hydraulic infiltration related to the river beds and fracture sets in the study area.