Graphitic Schists Research Articles

U-Pb zircon and Sm-Nd geochronology of the dom Silvério group, SE Brazil: Implications for the evolution of the superposed Araçuaí and Minas-Bahia orogens

The Araçuaí orogen, the South American portion of the Brasiliano/Pan-African Araçuaí-West Congo orogenic system (AWCO), formed during the Ediacaran-Cambrian assembly of West-Gondwana. The basement of a substantial portion of the Araçuaí orogen comprises Archean and Paleoproterozoic rocks involved in the 2.1–2.0 Ga Minas-Bahia orogen. The Dom Silvério Group, which consists of mica schists containing layers and lenses of mafic, ultramafic, and Mn-rich rocks, as well as quartzites, graphite schists, marbles, and banded iron formations, defines a ca. 10 km-wide and 150 km-long belt in the southwestern portion of the Araçuaí orogen. This succession, currently interpreted as deep marine deposits or an ocean-floor assemblage of uncertain age - Paleoproterozoic or Neoproterozoic-, potentially records processes involved in critical evolutionary stages of the superposed Minas-Bahia and Araçuaí orogens. Aiming to characterize the age and tectonic significance of the Dom Silvério Group, we performed a geochronological and geochemical investigation on samples collected from selected lithotypes. U-Pb (LA-ICP-MS) determinations on detrital zircons revealed two distinct age spectra. Zircons extracted from the lower Dom Silvério schists define a spectrum characterized predominantly by Neoproterozoic age clusters with a maximum depositional age of 601 ± 10 Ma. Quartzites from the lower and upper sections of the group do not contain zircons younger than 1.9 Ga and show unimodal age distributions around 2067 Ma. The maximum depositional age estimated for these rocks is 2020 ± 6 Ma. The spectrum of the lower section schist indicates the turbidites of the Ediacaran Salinas Formation as their most probable correlative. In contrast, the quartzites and schists of lower and upper sections correlate with the Orosirian Itacolomi Group, the post-collisional assembly of the Minas-Bahia orogen. Sm-Nd isotope analyses and whole-rock geochemistry allow the discrimination of two generations of mafic rocks. The older generation, showing TDM model ages = 1.92 to 3.6 Ga and ƐNd(t) = -12.93 to +4.63, represents components of the basement. The younger generation, with TDM model ages = 1.0 Ga to 1.25 Ga and ƐNd(t) = +0.05 to +2.29, is similar to the ophiolitic rocks that occur imbricated with Ediacaran metasedimentary successions in the central portion of the Araçuaí orogen defining the Ribeirão da Folha accretionary belt. They show Th/Yb and Nb/Yb ratios of the EMORB-OIB mantle array and Nb/Y and Zr/Y ratios compatible with OIB and oceanic plateau basalts. The Dom Silvério Group is thus a complex that involves members of the Araçuaí and Minas-Bahia orogens. Their Ediacaran components materialize the southern extension of the Ribeirão da Folha accretionary wedge and, as such, provide further evidence for oceanic consumption during the development of the southern Araçuaí orogen.

Facile synthesis and characterization of a magnetic biosorbent derived from sodium alginate and activated graphite schist: Experimental and statistical physics analysis for Mn(VII) remediation

H2O2-modified graphite schist (GS) and sodium alginate (SA) interface was loaded by Fe3O4 nanoparticles (MNPs) to prepare a magnetic biosorbent that was employed in removing Mn(VII) from solutions. The prepared GS/SA/MNPs adsorbent was investigated using a variety of techniques, including elemental mapping, TEM, XPS, FTIR, FESEM, EDX, XRD, XPS, and zeta potential. An experimental study supported by statistical physics calculations was carried out to obtain a new outline of the Mn(VII) uptake mechanism. The classical Freundlich and the statistical physical double-layer models adequately described the Mn(VII) uptake process at pH 3.0 and a temperature of 25–55 °C. The removed number of Mn ions (such as Mn+7 and Mn+2) per GS/SA/MNPs active site ranged from 0.70 to 0.84, indicating a mixed adsorption orientation driven by surface complexation and attraction forces mechanisms. The adsorption energies (∆E) calculated by the double-layer model ranged from 18.79 to 24.94 kJ/mol, suggesting that the interaction between Mn(VII) and GS/SA/MNPs was controlled by physical forces. Increasing the adsorption capacity at saturation (Qsat) from 333.14 to 369.52 mg/g with temperature proposed an endothermic capture process. Thermodynamic functions clarified the viability and spontaneity of Mn(VII) uptake on the GS/SA/MNPs adsorbent.

Proterozoic Deep Carbon—Characterisation, Origin and the Role of Fluids during High-Grade Metamorphism of Graphite (Lofoten–Vesterålen Complex, Norway)

Graphite formation in the deep crust during granulite facies metamorphism is documented in the Proterozoic gneisses of the Lofoten–Vesterålen Complex, northern Norway. Graphite schist is hosted in banded gneisses dominated by orthopyroxene-bearing quartzofeldspathic gneiss, including marble, calcsilicate rocks and amphibolite. The schist has major graphite (<modality 39%), quartz, plagioclase, pyroxenes, biotite (Mg# = 0.67–0.91; Ti < 0.66 a.p.f.u.) and K-feldspar/perthite. Pyroxene is orthopyroxene (En69–74) and/or clinopyroxene (En33–53Fs1–14Wo44–53); graphite occurs in assemblage with metamorphic orthopyroxene. Phase diagram modelling (plagioclase + orthopyroxene (Mg#-ratio = 0.74) + biotite + quartz + rutile + ilmenite + graphite-assemblage) constrains pressure-temperature conditions of 810–835 °C and 0.73–0.77 GPa; Zr-in-rutile thermometry 726–854 °C. COH fluids stabilise graphite and orthopyroxene; the high Mg#-ratio of biotite and pyroxenes, and apatite Cl < 2 a.p.f.u., indicate the importance of fluids during metamorphism. Stable isotopic δ13Cgraphite in the graphite schist is −38 to −17‰; δ13Ccalcite of marbles +3‰ to +10‰. Samples with both graphite and calcite present give lighter values for δ13Ccalcite = −8.7‰ to −9.5‰ and heavier values for δ13Cgraphite = −11.5‰ to −8.9‰. δ18Ocalcite for marble shows lighter values, ranging from −15.4‰ to −7.5‰. We interpret the graphite origin as organic carbon accumulated in sediments, while isotopic exchange between graphite and calcite reflects metamorphic and hydrothermal re-equilibration.

Biogenic signatures in the Cryogenian Brusque Metamorphic Complex, Brazil: Geochronological and paleoclimatological implications

The Cryogenian was a period of drastic environmental changes across the entire planet, with glacial erosion limiting the preservation of life forms in most of the geological record. Therefore, to explore more of the biogenic evidence and its implications for this period, here we investigate some metavolcano-sedimentary sequences from the Brusque Metamorphic Complex in Brazil, that record a mid-latitude basin so far unexplored from a paleontological point of view. On the one hand, field discoveries reveal the occurrence of Conophyton-type stromatolites preserved in dolomitic marbles that are isotopically incompatible with cap carbonates. On the other hand, 13C-depleted graphitic schists interspersed in these marbles indicate an accumulation from exceptional volumes of organic matter. Raman spectroscopy and transmission electron microscopy also highlight disordered structures, heterogeneous morphologies, and structures similar to ultra-small microorganisms in this graphite, in compatibility with a biogenic origin. Combined with geochronological data, these records reflect formation during the late Cryogenian greenhouse, between the Sturtian and Marinoan Snowball Earth glaciations. From this, we suggest deposition under conditions of warmer and brighter waters without ice cover, favoring the growth of some stromatolitic barriers. Sturtian nutrient-rich meltwater pulses may also have led to exceptional phytoplankton blooms followed by mass mortality, promoting the exceptional deposition of organic-rich horizons during the transition window to a Precambrian algae-dominated world.

Using an Inventory of Unstable Slopes to Prioritize Probabilistic Rockfall Modeling and Acid-Base Accounting in Great Smoky Mountains National Park

Abstract An important first step in the geotechnical asset management of Great Smoky Mountains National Park (GRSM) is the creation of an unstable slope inventory along major transportation corridors. Slope-stability problems are frequent in GRSM, often initiated in highly weathered and fractured metasedimentary rocks. In this study, an unstable slope inventory was created using the Unstable Slope Management Program for Federal Land Management Agencies protocols. Hazards and risks were evaluated for 285 unstable slopes along 243.67 km of roadway. Kernel density estimation was used to identify unstable slope hotspots and establish 14 sites for site-specific investigations to evaluate potential impacts of discrete unstable slopes along major roadways. Two-dimensional probabilistic rockfall simulations and acid-base accounting tests were used to predict rockfall pathways and evaluate the acid-producing potential of rocks. Simulations indicated that rock material would likely enter the roadway at all 14 sites. Acid-base accounting test results indicated that slaty rocks of the Anakeesta Formation and graphitic schist of the Wehutty Formation are primary acid-producing rocks in rockfall-prone areas. This research illustrates an approach for prioritizing areas for site-specific investigations towards the goal of improving safety in GRSM, including developing mitigation strategies for rockfall by widening ditches, installing barriers, and encapsulating acidic rockfall material.

Textures, in situ trace element and sulfur isotope of pyrite from the sediment-hosted Cu–Co deposits in the Zhongtiao Mountains, Trans-North China Orogen: Implications for ore genesis

Sediment-hosted Cu–Co deposits in the Zhongtiao Mountains are hosted by the metasedimentary rocks of the Paleoproterozoic Zhongtiao Group in the southern part of the trans-North China orogen, North China Craton. The ore genesis is still disputed with proposed genetic models including metamorphosed sedimentary exhalative (M−SEDEX), metamorphosed sediment-hosted stratiform copper (M−SSC) and metamorphogenic mineralization type. Pyrite occurs as a ubiquitous mineral throughout all ore-forming stages and is ideal for clarifying this issue via the integration of in situ (LA–ICP–MS and EPMA) elemental and LA–MC-ICP–MS sulfur isotope composition analyses. Two main types of pyrite are identified based on petrographic and SEM observations: fine- to medium-grained, variably deformed and inclusion-rich Py1 (including Py1a and Py1b subtypes) in the disseminated–veinlet mineralization stage (S1), and coarse-grained, fractured Py2 (including Py2a and Py2b subtypes) in the fracture-controlled vein-type mineralization stage (S2, main ore stage). Deformed and inclusion-rich Py1a is overgrown by inclusion-poor Py1b, and nonporous Py2a is rimed by porous Py2b. Geochemical results show that the studied pyrite samples have considerable contents of Co (mean > 1000 ppm) and Ni (mean > 100 ppm). These features could be related to both the Co and Ni-rich sedimentary environment (e.g., a volcaniclastic-rich sulfidic marine) and metamorphic enrichment. The content of Se (mean > 200 ppm) in Py2 is significantly higher than that in Py1, indicating the extensive interaction of mineralizing fluids with Se-bearing graphite schist. LA–ICP–MS mapping and textural studies indicate the presence of coupled dissolution-reprecipitation (CDR) reactions that could lead to trace metal compositions (e.g., Co, Ni, Se, Pb, Bi) in the product phases (Py1b, Py2b) that differ from those of their parent phases (Py1a, Py2a). These variations were most likely controlled by fluid temperature and precipitation of specific minerals. The grain-scale chemical zoning of Py2a revealed by elemental mapping indicates fluctuating fluid parameters (e.g., temperature, fS2, and/or parent fluid compositions). The bimodal distributions in Py1 (10–14 ‰ and 18–24 ‰) are consistent with the inheritance of sulfur from sedimentary pyrite and the input of thermochemical sulfate reduction (TSR) -related sulfur from reworked evaporite sulfates in an increasingly open system. The relatively lighter and wider variation in δ34S values (3.2–22.4 ‰, mean = 15.1 ‰) of Py2 implies that the evolution of the ore fluid in S2 was far more complicated, probably as a result of fluid–rock interactions and fluid cooling. During this process, hydrothermal rims with lighter δ34S values (8.4 ‰) grew on older metamorphic pyrite cores with relatively heavier δ34S values (17.4–18.2 ‰). Chalcopyrite displays overlapping S isotopic compositions with pyrite from different mineralization stages, suggesting that copper sulfides inherited reduced sulfur from earlier formed pyrite. Cooling and fluid–rock interactions serve as the critical controls triggering Cu precipitation from the fluid. The combined textural and compositional data of pyrite and chalcopyrite are suggestive of a syn-orogenic copper deposit model, in which all the ore-grade copper was introduced during the Zhongtiao orogeny.

Evaluation of the relationship between the collapsed mechanism and excavation method in tunnels excavated in schists

The problems experienced in tunnels excavated under high cover in graphitic schists generally vary according to the squeezing mechanism. During the tunnel excavation, slips occur on the slickenside surfaces in the tunnel face from time to time and collapse occurs. Most of the time, failures in the support systems are observed due to the squeezing mechanism in the long term in the sections whose tunnel excavations have been completed. In addition, if tunnel excavations are carried out from both entrance faces, it is possible to encounter excessive deformations at the junction points of the tunnels. Especially on weak ground, the importance of the distances between the stages increases in the case of opening the tunnels in the form of top heading, bench, and invert gradually. As a tunnel excavation method, excavation in stages directly affects the stability of the tunnel. Within the scope of this study, the collapsed mechanism in the junction area of the tunnels of the T6 tunnel is examined. For this purpose, 3-dimensional numerical analysis are performed with the Flac3d program. Analysis results are compared with site deformation measurements. As a result, an excavation methodology is proposed for the junction area of tunnels in weak ground.

Mineralogy and paragenesis of the Meso-Proterozoic Rohil uranium deposit, North Delhi Fold Belt, Rajasthan, India

The Rohil uranium deposit, located about 70 km south of the Khetri copper deposit in the North Delhi Fold Belt, Rajasthan, India, is albitite-hosted. This deposit is associated with Na-metasomatism, mainly affecting the metasedimentary wall rocks. Quartzite, quartz biotite schist, quartz amphibole schist, and graphite schist underwent variable degrees of albitization, as documented by a wide range of altered wall rocks, including weakly altered metasediments to almost pure albitites. These host rocks consist of metamorphic mineral assemblages such as Ca plagioclase – quartz – biotite – Ca amphibole – graphite – garnet – sillimanite – corundum – ilmenite. The hydrothermal alteration assemblages contain variable amounts of albite, chlorite, biotite, sericite, orthoclase, sphene, rutile, allanite, actinolite, and calcite. The textural relationships between primary host rock minerals and hydrothermal alteration products suggest four main stages of metasomatism: (1) an early pre-albitization stage, (2) a Na-metasomatic stage, that are followed by (3) a Na‐Ca-Mg- and (4) K- metasomatic stage. Albites are of two types: albite-1 is subhedral to euhedral and equidimensional, formed by the alteration of metamorphic plagioclase during the Na-metasomatic stage. Albite-2 is feathery and formed in fractures/cavities by precipitation of alkali-rich fluid during Na-Mg-Ca metasomatic stages. Albitization is pervasive on a regional scale. The hydrothermal metasomatic fluids utilized structures such as shears and breccias in the metasediments as conduits and as the loci of uranium mineralization. Major uranium minerals are uraninite, U-silicate, and brannerite. Uranium-bearing minerals occur as disseminations, in veins and as minute inclusions in biotite and titanite within albitized metasediments. The petrographic studies suggest that the metasomatic alteration of host rocks is characterized by albitization, actinolitization, carbonatization, chloritization, sericitization, biotitization, sulfidation, and silicification. The close association of U, REE-bearing minerals, and albite-2 indicates that the main U-REE mineralization stage was coeval with the Na-Ca-Mg and K-metasomatic stages.

Affinity and Petrogenesis of the Huzyk Creek Metal-Enriched Graphite Deposit: A Metamorphosed Metalliferous Black Shale in the Trans-Hudson Orogen Of Manitoba, Canada

ABSTRACT The Huzyk Creek area is situated along the boundary between the Reindeer Zone and the Superior Boundary Zone of the Paleoproterozoic Trans-Hudson Orogen, where the Precambrian rocks are overlain by Phanerozoic cover. Two drill holes intersect graphite schist that is enriched in V, as well as U, Zn, Mo, and Cu, and is hosted by a metamorphosed wacke-mudstone sequence interleaved with variably altered mafic rocks. Whole-rock lithogeochemistry and Sm-Nd isotope chemistry suggest that the wacke-mudstone package is related to the turbidite-derived Burntwood Group of the Kisseynew Domain and was likely deposited relatively proximal to the Flin Flon arc-collage. A model is proposed in which redox-sensitive metals were leached from rocks of the Flin Flon arc-collage during weathering under oxidizing conditions. The metals were transported in oxygenated surface run-off draining the arc-collage and discharged into the Kisseynew Basin. Shallow waters of the Kisseynew Basin were likely oxygenated and biologically productive; however, the basin was likely euxinic at mid-depths. The mixing of the metal-enriched, oxygenated water with organic matter and euxinic water resulted in the reduction of the redox-sensitive metals and the formation of insoluble organometallic complexes and particles. A highstand, or period of tectonic quiescence, likely halted turbidite deposition and allowed for the settling organic and metal-rich particles to create relatively thick deposits. Burial and metamorphism resulted in the organic-rich material being transformed into graphite, while Mo, Cu, and Zn were partitioned into sulfides. The mineral hosts of V and U are not known at this time. The model calls for the fractionation of redox-sensitive metals from the water column shortly after discharge into the Kisseynew Basin and implies that graphitic horizons in relatively close proximity to the Flin Flon arc-collage have a greater potential for metal enrichment than graphite deposits farther removed from the arc. This model could apply to basins of similar metamorphic grade, age, and tectonic setting around the globe.

Induced Polarization as a Tool to Assess Mineral Deposits: A Review

Disseminated ores in porous or fractured media can be polarized under the application of an external low-frequency electrical field. This polarization is characterized by a dimensionless property that is called the “chargeability”. Induced polarization is a nonintrusive geophysical sensing technique that be used in the field to image both the electrical conductivity and the chargeability of porous rocks together with a characteristic relaxation time. A petrophysical model of the induced polarization of metallic ores immersed in a porous conductive and polarizable material is reviewed, and its predictions are compared to a large dataset of experimental data. The model shows that the chargeability of the material is linearly dependent on the volume fraction of the ore and the chargeability of the background material, which can, in turn, be related to the conductivity of the pore water and the cation exchange capacity of the clay fraction. The relaxation time depends on the grain sizes of the ores and on the conductivity of the background material, which is close to the conductivity of the porous rock itself. Five applications of the induced-polarization method to ore and metallic bodies are discussed in order to show the usefulness of this technique. These applications include: (i) A sandbox experiment, in which cubes of pyrite are located in a specific area of the tank; (ii) The tomography of an iron slag at an archeological site in France; (iii) A study of partially frozen graphitic schists in the French Alps; (iv) The detection of a metallic tank through the tomography of the relaxation times; and (v) The detection and localization of a deep ore body that is associated with a tectonic fault. We also discuss the possibility of combining self-potential and induced-polarization tomography to better characterize ore bodies below the seafloor.

Geological and Geo-electrical Outlining of Graphite Deposits Within Kagara, North-Central Nigeria

The specific locations of graphite exposures in Kagara, their host rocks, as well as their spatial extent and gross volume have hitherto remained unaddressed. This study focused on spatially outlining the deposits and conservatively ascertaining their gross volume, using combined surface geological mapping and geoelectrical surveying techniques. The geoelectrical surveying aspect comprised 1D geoelectrical (electrical resistivity, spontaneous potential, induced polarization) sounding, and 2D geoelectrical (electrical resistivity and induced polarization) tomographic surveying conducted using ABEM Terrameter (SAS 4000). The 1D electrical resistivity data were processed and interpreted using WinResist version1.0 resistivity inversion software. The 2D geoelectrical tomographic data were processed and interpreted using RES2DINV computer program. The rock outcrops found are quartzose - micaceous as well as graphitic schists, amphibolite, quartzite, coarse grained biotite granite, and exposures of graphite bodies. The graphite bodies are hosted within the schist and quartzite. The schist, amphibolite and quartzite rock bodies appear to be lateral metasediment equivalents, and therefore are synchronous. The geoelectrical attributes of the graphite bodies are 2-90 Ωm resistivity value, combined with 1-20 ms induced polarisation and negative spontaneous potential values. The deposits exist within 0.7 – 12 m depth interval, occupy 1470509.0 m² surface area, and constitute 14705090.0 m³ conservative gross volume. The graphite deposits appear to be sheet like bodies interlaid with the quartzite and schist bodies.

Late Cretaceous to Paleocene magmatic record of the transition between collision and subduction in the Western and Central Cordillera of northern Colombia

Deformational field relations, compositional and U–Pb geochronological data of magmatic rocks of the northwestern Colombian Andes provide insights to recognize the different pre to post-collisional stages in an arc-arc collisional scenario until the re-initiation of a new subduction zone. A western belt of undeformed to mylonitized volcanic rocks with flat REE patterns and a weak LREE depletion present similar geochemical features of adjacent oceanic plateau basalts that were intruded by arc-related dykes at ca. 82 Ma, before their accretion to the continental margin. A sequence of greenschist and quartz + muscovite + graphite schist, and their associated mylonitized dikes, are part of the eastern continental arc, and present ages between 69 and 71 Ma, show tholeiitic affinity, flat rare-earth patterns, and Nb–Ti anomalies. These characteristics mark a decrease in the mantle wedge and older crustal signatures during magma genesis, as the oceanic-continent collision was advancing. Both suites were metamorphosed in a high geothermal gradient with temperatures between 490 and 400 °C and pressures between 2 and 4 Kbar. After this metamorphic event associated to the end of the collision a series of dykes intrude the different units at ca. 58 Ma, including basalts and andesites with Nb–Ti anomalies, adakite-like, and high niobium compositional signatures, suggesting that they were probably formed associated with slab melting in a young subduction environment that evolved after 62 Ma, following a ≤7 Ma magmatic hiatus. This magmatic and metamorphic record can be related to an arc-arc collision scenario in which mantle involvement in the late stages of collision influence both the magmatic and thermal regimes.

Genesis of the Tianping flake graphite deposit at the western margin of Yangtze Block, SW China

The Tianping flake graphite deposit is located in the western margin of the Yangtze Block, South China. It is one of the biggest and representative flake graphite deposits with 8 million tons of flake graphite in the Kangdian Axis graphite metallogenic belt. The graphite orebodies are hosted in graphite schist and mica quartz schist. Carbon isotope data for graphite ore show light carbon values between −26.3‰ to −28.8‰ which suggests a biogenic origin. The carbonaceous materials of the Tianping graphite deposit were deposited younger than ca. 833 Ma according to U-Pb dating results of detrital zircons in graphite samples. The calculated metamorphic temperature of graphite from Raman spectra is between 414 and 436 ± 50 °C, indicating a greenschist to amphibolite facies metamorphism is responsible for the transformation of disordered carbonaceous matter into well-crystallized graphite. After carbonaceous material deposition and graphite formation by regional metamorphism, the graphite may recrystallize and form large graphite flakes in a heated environment brought by the intrusion of plutons in the region. Based on previous studies and new data, a three-stage genesis model including processes of material deposition, regional metamorphism, later magmatism, and contact metamorphism for Tianping flake graphite deposit is proposed.

Zircon and monazite U-Pb ages of the Mashan Complex of the Jiamusi Block of NE China: a link to Gondwana?

ABSTRACT The tectonic affinities of the microcontinents in the eastern Central Asian Orogenic Belt of NE China, particularly the Jiamusi Block, have been hotly debated in the last decade. The Mashan Complex, which is the oldest exposed basement of the Jiamusi Block and consists of khondalitic rocks (felsic granulite, marble, and graphite schist) and orthogneisses, is the key geological unit to look into the tectonic affinity of the Jiamusi Block. In this study, we carried out LA-ICP-MS U-Pb dating on zircon and monazite minerals from the granitic gneisses in the Mashan Complex, to constrain the affinity of the Jiamusi Block. Magmatic zircons from two granitic gneisses yielded weighted mean 206Pb/238U ages of 517 ± 7 Ma and 527 ± 4 Ma, respectively, which are interpreted as the protolith ages. These age data verify a late Pan-African magmatic event in the Jiamusi Block. Metamorphic zircons from one granitic gneiss gave a weighted mean 206Pb/238U age of 510 ± 3 Ma, which constrained the metamorphic time of the Mashan Complex. This metamorphic age is further supported by monazite dating results – the monazites from three granitic gneiss samples yielded weighted mean 206Pb/238U ages of 489 ± 3 Ma, 498 ± 3 Ma, and 500 ± 3 Ma, respectively, which represented the metamorphic recrystallization ages of these monazites. These data consistently indicate a regional metamorphic event immediately after the late Pan-African magmatism in the Jiamusi Block. Our new data, combined with previously published late Pan-African geochronological data from the Jiamusi Block, support that the Jiamusi Block has an affinity to the Gondwana.

Comparative assessment of slope stability in weathered schists using Q-slope and LEM

The slope stability in weathered schists formations has always been of great importance for studies in rock mechanics. The majority of the methods used in the geotechnical practise for estimating the slope stability are based on the traditional limit equilibrium methods, analysing the possibilities of circular and non-circular failures. On the other side, very few empirical methods exist for assessment of the slope stability. Q-slope is a relatively new methodology for the assessment of slope stability in terrains built from rock masses. This method was developed over the last decade by Barton and Bar (2015, 2017), with modification of the original Q-system and it is based on the results of more than 500 case studies all over the world in reinforcement-free slopes. This paper presents results of the slope stability assessment with the limit equilibrium and Q-slope method at four characteristic cut sections with unusual general heights (ranging from 30 m to 92 m), along the route of the A2 motorway, section Kicevo-Ohrid in the Republic of N. Macedonia, which is under construction. The subject cuts presented in this paper were excavated in the period 2014 – 2018 without the application of special measures for protection. The research area is built mainly from phyllite, sericite schists, quartz-sericite schists and graphitic schists, which are folded, tectonically disturbed and affected by the process of degradation. According to the results of the analyses, it can be emphasised that both limit equilibrium method and Q-slope are applicable at terrains built from weathered and tectonically disturbed schists because the stability conditions on site were confirmed with these two methodologically different approaches. As a general conclusion, we can summarize that the best way to assess the slope stability condition is when we combine different tools and methodologies, especially when we face highly variable rock mass conditions.

Stability evaluation of cut slopes in highly weathered schists using limit equilibrium, kinematical and empirical methods

1. Extended abstractThe traditional limit equilibrium method and the kinematic analyses are the first choices in the geotechnical practice for the assessment of the slope stability of a particular rock slope. The reason is that very few empirical methods exist for this purpose.This paper presents results of the slope stability assessment with limit equilibrium, kinematical and empirical methods for over 50 deep cut slopes in highly weathered schists along the A2 motorway section Kicevo-Ohrid in RN Macedonia. The research area is mainly composed of phyllite, sericite schists, quartz-sericite schists and graphitic schists. These rocks are highly weathered, tectonically disturbed and often folded, which means they are prone to local deformations, especially in aspects of the long-term impact of atmospheric influences and the existence of unfavourable joint sets. Although most of the failures that occurred had local character, on several cut slopes global failure occurred. The slope stability assessment of the subject rock formations is a difficult challenge because the highly weathered phyllite and schists are difficult to be characterised, considering the uncertainties associated with the material anisotropy and the network of discontinuities.This study employs different failure mechanisms of cut slope stability in complex geological media. The rock masses were modelled according to the generalized Hoek-Brown criterion while the joints behaviour was modelled using the Barton-Bandis shear strength criterion. Rocscience software package was used for executing the limit equilibrium analyses and the stability analyses along discontinuities. The relatively new and intuitive empirical method Q-slope was applied for the stability assessment of the excavated rock slopes.According to the results and the findings, it can be emphasized that although the limit equilibrium analyses showed relatively stable slopes, significant failures on several cut slopes occurred on site. The stability conditions on site were confirmed with the kinematical analyses and the Q-slope method.The final decision of the slope geometry should be made after the inspection of the excavated surfaces because it is very difficult to register the discontinuity conditions before the construction phase starts. The best way to assess the stability condition is with a combination of different tools and methodologies, especially when we face highly weathered rock formations.The next step of this research will be to propose a methodology, where the long-term rock slope stability and the associated level of risk will be evaluated using graphical diagrams.

Evaluation of collapse mechanism and portal interaction of a High-Speed Railway Tunnel (T29 Tunnel, Turkey)

Within the scope of the Ankara-Istanbul High Speed Train Project, the T29 tunnel was opened to traffic after completing excavation and support implementation. The tunnel was excavated in weak to very weak graphitic schists causing severe face stability problems and deformations on the tunnel supports during excavation stage. In the progress of the excavation stage, a serious face failure occurred at Km: 225 + 784, and the effects of this face failure were observed on the surface. Since the cover thickness is approximately 35 m in this section of the tunnel, it was decided to pass this zone with a cut and cover structure. The portal excavation was carried out from the failure zone, and it was planned to provide stability with the closure of the surface by constructing a cut-and-cover structure immediately afterwards. However, slope failures occurred during the portal excavations. This section was reworked, and it was decided to support this area with a pile system. Consequently, in this study, the reasons for the face failure and slope failures are investigated, and the construction stages are explained and discussed.

Geology, mineralogy, and sulfur isotopes of the Mowana copper deposit, Matsitama Schist Belt, NE Botswana

AbstractThe Mowana hydrothermal Cu deposit is located within the Matsitama–Motloutse Complex in the southwestern part of the Zimbabwe Craton in the northeastern part of Botswana. This study aims to document the characteristics of the mineralization based on geology, quartz textures, ore mineralogy, chlorite geothermometry, and sulfur isotope analyses. The deposit is hosted by the NNE‐striking and nearly vertically dipping (70–80°) Bushman Lineament, within the graphitic schist lenses in the carbonaceous and argillaceous metasedimentary rocks of the Neoarchean to Paleoproterozoic Matsitama Sedimentary Group. The hydrothermal alteration of the host rocks is characterized by silicification, chloritization, epidotization, sericitization, hematite, and calcite alteration. Based on the alteration mineral assemblage, the main mineralization stage is attributed to near neutral pH fluids at temperatures between ~200 and ~340°C. The base metal mineralization of the Mowana deposit was evolved in at least two vein types. The first mineralization type, represented by the quartz+calcite±K‐feldspar veins and breccias is characterized by the precipitation of principal chalcopyrite with pyrite, minor bornite, and trace amounts of galena. The Type 2 veins represented by the quartz+calcite±fluorite veins, host appreciable amounts of galena. The supergene mineralization widely distributed in the shallow levels of the deposit is manifested by the significant presence of chalcocite, bornite, covellite, anglesite, malachite, and hematite. The temperature obtained from the chlorite geothermometry in the Type 1 veins indicate that the mineralization associated with chlorite alteration formed at a temperature ranging from 340 to 400°C. The ore mineral assemblage: pyrite, bornite, and chalcopyrite, paired with the chlorite geothermometry data indicate that the Type 1 veins formed at an intemediate to high sulfidation state. Sulfur isotopic ratios determined on the sulfides indicate the magmatic S and/or leaching of the host metasedimentary rocks and closed system reduction of seawater sulfate as the sources of S.

Cobalt concentration in a sulfidic sea and mobilization during orogenesis: Implications for targeting epigenetic sediment-hosted Cu-Co deposits

The origin of sediment-hosted copper-cobalt deposits (SCDs) within metamorphic terranes remains contentious, particularly in regard to the timing of mineralization relative to basin evolution. Here, we link the timing of Cu-Co mineralization in the Zhongtiao Mountains district, central China, to basin closure during development of the Trans-North China Orogen. Metamorphic apatite from meta-evaporite has a U-Pb age (1844 ± 25 Ma) within error of a Re-Os age for molybdenite from Cu- and Co-bearing veins (1819 ± 10 Ma), implying that mineralization and metamorphism were coincident. In situ trace element and Fe-S isotope analyses of deformed sedimentary pyrite (Py I) and younger euhedral metamorphic pyrite (Py II) in pyritic graphite schist indicate that Co was mobilized from Py I via fluid-mediated dissolution and reprecipitation. In the graphite schist, representing a sulfidic shale metamorphosed at upper greenschist facies conditions, relic Py I has high δ34S values (22.9 ± 0.4‰, n = 10) and δ56Fe values (0.90 ± 0.16‰, n = 8). These values are consistent with bacterial sulfate reduction and pyrite formation in a Paleoproterozoic sulfidic sea (i.e., after the first great oxygenation event). In addition, Py I has unusually high Co contents (0.8–3.0 wt.%), suggesting that transition metals, including Co and Ni, were originally concentrated in sedimentary pyrite in an evaporative setting. The evaporitic setting is supported by the presence of sylvite in sulfidic schists and adjacent scapolitic calc-silicates. Younger Co-poor Py II has slightly lower δ34S values (21.4 ± 0.2‰, n = 19) and higher δ56Fe values (1.61 ± 0.15‰, n = 9), supporting the hypothesis that Co was released from sedimentary sulfide during dissolution and reprecipitation. The great variability of δ34S (14.8 to 22.9‰) and δ56Fe (0.13 to 2.30‰) in hydrothermal pyrite (Py III) associated with Cu-Co mineralization suggests that an external Cu-rich oxidizing fluid was involved in the ore-forming process.Copper and Co in this type of deposit are typically both considered to be derived from fertile basement rocks. This study, however, highlights another potential source for the Co, with its enrichment in a sulfidic sea along an evaporative margin and its upgrading through mobilization during orogenesis that leads to formation of a relatively high-temperature group of SCDs. Tectonometamorphic processes causing such Co enrichments took place during the Paleoproterozoic and Neoproterozoic oxygenation events, implying that the most prospective targets for Co resources are concentrated in ca. 2.2–2.0 Ga and 0.9–0.7 Ga deformed passive margin marine sequences.

Phreatic uranium mineralisation hosted by Neogene sediments from the Taunsa area, Dera Ghazi Khan, Eastern Sulaiman Range, Pakistan: unique exploration targets in a deformed geological setting

The Taunsa uranium mineralisation is hosted by the upper Miocene–Pliocene Litra Formation, part of the molasse sediments (Siwalik Group) deposited in the Himalayan foreland basin of the eastern Sulaiman Range, Dera Ghazi Khan, Pakistan. The Litra Formation is the only rock unit hosting economic-grade uranium occurrences in the stratigraphic record of Pakistan. Surface radioactivity is mainly associated with cemented layers of the host sandstone, known as hard-bands, whereas weakly cemented sandstone is non-radioactive. The outcropping sandstone is dominantly whitish grey owing to bleaching, which is probably related to hydrocarbon migration from the underlying marine black shales. The main sandstone-type uranium mineralisation is phreatic owing to its parallelism with the water-table, whereas the concordantly oriented tabular (roll-type) ore is subordinate. The host sandstone layers have high dips ranging from 70 to 85°E indicating a deformed geological setting that makes the Taunsa uranium ore unique. Uranium is trapped mostly by scant organic matter, probably related to hydrocarbons, and also by Fe–Ti-oxide phases, phyllosilicates (chlorite and biotite), graphitic schist and black shale clasts in the host sandstone. Uranium ore minerals are mainly coffinite, pitchblende and minor brannerite. Uranium in the ore phases is accompanied by the high contents of Ca, Fe, Ti, Si and Al. Radiometric disequilibrium in the phreatic ore is strongly positive, whereas that in the tabular (roll-front) is moderately positive, which in turn suggests the former is younger than the latter. The coexistence of the two ore-types, characterised by different intensities of radiometric disequilibrium within the same uranium deposit, suggests that the phreatic uranium ore has possibly resulted from remobilisation of the tectonically uplifted tabular (roll-type) ore and its reprecipitation at the horizontally oriented redox interface following the water-table in the host sandstone aquifers. KEY POINTS The Taunsa uranium resource is dominantly phreatic that has resulted from oxidation and remobilisation of the earlier tabular (roll-type) mineralisation. The phreatic uranium mineralisation is very young, which is indicated by low radioactivity and high positive disequilibrium in comparison with that within the tabular (roll-type) ore. Uranium phases are mainly pitchblende, coffinite and brannerite, associated with organic matter of hydrocarbon origin, with phyllosilicates and Fe–Ti-oxide phases. Outcrop of the host sandstone in the Taunsa area is pervasively bleached and is marked by flat topography.