Tectonic Contact Research Articles

Сравнительный анализ химического состава породообразующих и акцессорных минералов гранита и дресвяников из динозавровых слоёв укурейской свиты урочища Кулинда в Восточном Забайкалье (Россия)

The relevance of the study lies in determining the sources of clastic material of gruss and sandstones that make up a significant part of the sediments of the Middle Jurassic Ukurey Formation, which contains the remains of the most ancient feathered dinosaur, Kulindadromeus zabaikalicus. These deposits are a part of the section composed of interlayered gruss, sandstones, siltstones, tuff siltstones and chlidolites. They are represented by a fragment of a downed block of the Ukurey Formation preserved from erosion among granites, the tectonic contact with which is in close proximity to the dinosaur layers. The purpose of the study is to determine the source of clastic material from the woodlands. The object of study is the sediments of the Ukurey formation and granites in the Kulinda tract. The subject of the study is the mineral composition of the clastic fraction of these rocks. The objectives are presented by the study of the composition of rock-forming and accessory minerals of granites and gruss deposits of the Ukurey Formation in the Kulinda tract. The method and methodology consist of sampling, comparative study of the mineral and chemical composition of rocks, diagnostics and study of the chemical composition of minerals using electron microscopy and ICP MS. The results of the study are based on a comparative analysis of the chemical and mineral composition of granites and gruss, during which their proximity is determined. Quartz and feldspar of granites and gruss deposits are characterized by similar composition; the average values of the formula coefficients for quartz of granites and gruss deposits are almost identical. Most quartz individuals are characterized by stoichiometric composition, due to the compensation of the Si deficiency by the inclusion of isomorphic Al and, less commonly, Ti and trivalent Fe. For K-Na feldspar, an excess of the sum of cations and a deficiency of Si are typical. Associations of accessory minerals of gruss and granites (zircon, rutile, apatite, magnetite) and their chemical compositions are similar. Zircons are characterized by an excess of Zr with a deficiency of Si, admixtures of Fe and V in rutile, and a deficiency of Ca and P in apatite. The data obtained prove that the gruss deposits are a redeposited product of granite weathering.

Transition Zone from Precambrian Low-grade to High-grade Metamorphic Rocks and the Cauvery Suture Zone in Southern India: Traditional Concepts and Recent Researches

ABSTRACT The contact zone (transition zone) between the low-grade tonalitic and granitic gneisses of the Archaean Dharwar craton and the Neoarchean high-grade granulite facies metamorphic rocks of the Southern Granulite Terrane (SGT) and the existence of Cauvery Suture Zone have been topics of discussion since long. Traditionally, it was strongly believed that there has been gradual transition from low- grade granitic gneisses to high- grade charnockitic rocks from north to south and the concept of transformation finds place even today in some of the latest publications. The existence of Cauvery Suture Zone/shear zone is also being questioned by a few workers. However, accumulation of large volumes of geological, geochemical, geochronological and geophysical data in the form of innumerable publications in the last 2–3 decades has transformed the traditional concepts and several new concepts and innovative ideas have emerged. Recent multidisciplinary studies clearly demarcated the tectonic contact in the place of gradual transition zone and established the existence of Cauvery suture zone and their linkage with the evolution of SGT in terms of fold-thrust tectonics related to convergent tectonic processes.

Replacive IOCG systems in the Ossa Morena Zone (SW Iberia): The role of pre-existing ironstones as a geochemical trap

The central Ossa Morena Zone (SW Iberia) hosts a regionally extensive ironstone level interbedded with bimodal volcanic rocks, limestone and shale of Lower-Middle Cambrian age. The stratabound ironstone includes dominant magnetite and hematite with locally abundant chert and barite. It is interpreted as being (sub-)exhalative at or near the seafloor and formed during a rifting event that postdated the Cadomian orogeny. In some places, such as in the Las Herrerías deposit, the ironstone is irregularly replaced by a chalcopyrite-rich ore; the Cu-rich mineralization is accompanied by the pervasive phyllic alteration of the hosting siliciclastic sediments. The highest copper grades are found when the ironstone is crosscut by WNW-ESE-trending late-Variscan extensional brittle-ductile structures that are interpreted as the feeder channels for deep hydrothermal fluids. A similar nearby copper-rich mineralization (Pallares) is likely controlled by the tectonic contact between limestone and pyrite-rich black shale.Sr-Nd whole-rock isotope geochemistry data suggests that the Sr in the ironstone (87Sr/86Sri ≈ 0.7088) is close to isotopic equilibrium with the local exhalative barite (0.7084–0.7086) and Cambrian seawater. The ironstone has a significantly more crustal εNd initial signature (<-1.8) than the coeval volcanic rocks (+5.2 to + 7.9). The younger sulfide mineralization inherited the Nd isotope composition of the ironstone but shows a significant enrichment in 87Sr (87Sr/86Sr > 0.7091) that is interpreted as related with the input of genetically different and more crustally-derived hydrothermal fluids.39Ar-40Ar dating of the phyllic alteration suggest that the copper mineralization was formed at ca. 332–330 My. These ages are coeval with those of small peraluminous granite intrusions that host Cu-Au vein-like mineralization and dated at 331.8 ± 1.6 Ma (LA ICPMS U-Pb zircon). Our interpretation is that the copper-rich mineralization at the Las Herrerías area is the distal expression of an intrusion-related hydrothermal system.Numerical modelling shows that ironstone is an effective trap for copper precipitation due to the large changes in pH and fO2 that take place when copper-bearing acid and reduced fluids react with the brittle ironstone. The precipitation of chalcopyrite, however, is controlled by the amount of available reduced sulfur in the ore trap. The δ34S values of the sulfides (+12.6 to + 21.6 ‰) suggest that the most likely source for the reduced sulfur is the thermogenic reduction of aqueous sulfate equilibrated with the exhalative barite (δ34S, +31.4 to + 35 ‰) with some minor input of reduced sulfur leached from the metasediments.This system could be considered as a variant of the IOCG clan. The formation of the ironstone and the copper mineralization, however, are separated by more than 200 My. Probably, many IOCG systems have a similar origin as Las Herrerías, with an ironstone being just a passive geochemical trap for the copper–gold mineralization.

Sedimentology and structure of the Paleogene Nummulitic series of Corsica: Reconstruction of the southern termination of the western Alpine foreland basin

AbstractNummulitic Limestones deposits are preserved along the tectonic contact between the Variscan basement and Alpine units of Corsica. These marine carbonates, dated from the Late Palaeocene to the Middle Eocene, were deposited within a foreland flexural basin that is considered to be the southern continuation of the Alpine foreland basin of southeast (SE) France. However, in contrast with the Nummulitic Limestones of SE France, those of Corsica are far less documented. This field‐based study constrains the sedimentology, stratigraphy and structure of the Nummulitic Limestones of Corsica in three localities (Balagne, Corte and Sari‐Solenzara) to identify factors that controlled foreland basin development and to clarify its significance within the early alpine orogen. The microfacies, microfaunal assemblages and siliciclastic fractions are characterised throughout the succession at each locality. The results indicate the existence of an important Variscan basement relief to the west of the basin (West Corsican Massif) that supplied early alluvial fans found at the base of the foreland succession in the northernmost Balagne area. Continuous high clastic input strongly reduced the development and diversity of the overlying Nummulitic Limestones facies and fauna. Further south, limestones in the Corte and Sari‐Solenzara areas are thicker and contain richer fauna. Three depositional models corresponding to the carbonate ramp system are proposed for the Nummulitic Limestones and used to construct paleogeographic maps illustrating the transgressive evolution of the Corsican foreland basin from the Early to the Late Eocene. Based on our results and available regional tectonic data and LT thermochronological data, we propose that the Nummulitic marine transgression took place within a continuous foreland basin encompassing southern Corsica and SE France during the early development of the western alpine arc.

Geological structure of the Strelnica Massif (Muráň Plateau, Central Slovakia) based on new biostratigraphical data and geological mapping

The Strelnica massif is a part of the Muráň Plateau near Tisovec town. The massif is of prolonged triangular shape and is sharply defined by faults from all sides: the Muráň fault (line) on the SE side, the Mýto-Tisovec fault system from the SW side and a thrust/fault line with unclear character in the Martinová valley on the N side. The massif was supposed to be built by Lower to Middle Triassic carbonate complexes, however our new biostratigraphic data had proven that in fact most of these light grey carbonates belong mostly to Upper Triassic, which makes necessary the reinterpretation of the structure of the whole massif. Despite of new and more complicated structural interpretation, our new age data are in better agreement with earlier works from the Tisovec quarry as it was in earlier geological maps. Our mapping works also confirmed, that the Strelnica massif itself is built by two facially very different blocks: a deep-water type of succession with Reifling and Raming type limestones and a strongly tectonically reduced succesion from probably lower Carnian up to Liassic age. These two blocks are in tectonic contact along a fault line and both are lying on the Lower Triassic shaley complex which forms the basal horizon of the Muráň nappe. The Muráň fault (line) is covered by Quaternary deposits in the aera, but in addition we suppose also at least two newer fault systems: a NNW-SSE normal fault system, parallel with the Mýto-Tisovec fault system in the Rimava valley and a roughly WNW-ESE system with probable sinistral strike-slip kinematics. Both systems were locally known from the Tisovec quarry, or the structural measurements from the Dielik cave in the NE part of the massif, however not mapped in the whole massif.

The Tepsi Ultrabasic Intrusion, the Northern Part of the Lapland–Belomorian Belt, Kola Peninsula, Russia

The Tepsi ultrabasic body is located in the northeastern Fennoscandian Shield close to the junction of the Serpentinite Belt–Tulppio Belt (SB–TB) with suites of the Lapland–Belomorian Belt (LBB) of Paleoproterozoic age. The body is a deformed laccolith that has tectonic contacts with Archean rocks. Its primary textures and magmatic parageneses are widely preserved. Fine-grained olivine varies continuously from Fo90.5 to Fo65.4. The whole-rock variations in MgO, Fe2O3, SiO2, and other geochemical data are also indicative of a significant extent of differentiation. Compositional variations were examined in the grains of calcic and Mg-Fe amphiboles, clinochlore, micas, plagioclase, members of the chromite–magnetite series, ilmenite, apatite, pentlandite, and a number of other minor mineral species. Low-sulfide disseminated Ni-Cu-Co mineralization occurred sporadically, with the presence of species enriched in As or Bi, submicrometric grains rich in Pt and Ir, or diffuse zones in pentlandite enriched in (Pd + Bi). We recognize two series: the pentlandite series (up to 2.5–3 wt.% Co) and the cobaltpentlandite series (~1 to ~8 apfu Co). The latter accompanied serpentinization. The two series display differences in their substitutions: Ni ↔ Fe and Co → (Ni + Fe), respectively. Relative enrichments in H2O, Cl, and F, observed in grains of apatite (plus high contents of Cl in hibbingite or parahibbingite), point to the abundance of volatiles accumulated during differentiation. We provide the first documentation of scheelite grains in ultrabasic rocks, found in evolved olivine-rich rocks (Fo77–72). We also describe unusual occurrences of hypermagnesian clinopyroxene associated with tremolite and serpentine. Abundant clusters of crystallites of diopside display a microspinifex texture. They likely predated serpentinization and formed due to rapid crystallization in a differentiated portion of a supercooled oxidized melt or, less likely, fluid, after bulk crystallization of the olivine. We infer that the laccolithic Tepsi body crystallized rapidly, in a shallow setting, and could thus not form megacycles in a layered series or produce a well-organized structure. Our findings point to the existence of elevated PGE-Au-Ag potential in numerous ultrabasic–basic complexes of the SB–TB–LBB megastructure.

Using U-Pb detrital zircon systematics to constrain the architecture and provenance of the Ronda peridotites’ crustal hanging wall, S Spain

Detrital zircon geochronology is frequently used for sediment provenance analysis in orogenic belts, but it can also be applied to locate the boundary between basement and sedimentary cover in successions and tectonometamorphic units where the paleontological record is missing, and primary stratigraphic relationships have been erased by intense deformation and/or metamorphism. This is critical to differentiate metamorphic mineral associations, P-T conditions and deformational events associated with superposed orogenic cycles. In this work, detrital zircon geochronology has been applied to the Ronda peridotites’ crustal hanging wall (hinterland of the Alpine Betic Cordillera, southern Spain), apparently made up of two major tectonic units – the Jubrique unit and the Maláguide complex – separated by a major Alpine tectonic contact camouflaged in between low-grade metamorphic rocks.Our results show that pre-Paleozoic peaks (c. 650, 1000, 2000, and 2600 Ma) are common along the entire studied crustal sequence, and Paleozoic ages are only recorded in the upper part of the Jubrique unit (c. 290, 350, and 475 Ma) and in the Maláguide complex (c. 350 and 475 Ma). Thus, the Paleozoic rocks of the Maláguide complex, constituting the highest crustal unit overlying the Ronda peridotites, represent syn-to-late orogenic Variscan sediments deformed and slightly metamorphosed prior to the Alpine cycle. In contrast, the underlying Jubrique unit is made of a post-Variscan sedimentary cover deposited on a Variscan basement, with the contact between both (hidden unconformity?) located within low- to medium-grade schists. These data support the presence of a major tectonic contact between the Maláguide complex (Late Carboniferous maximum depositional ages) and the Jubrique unit (Early Permian maximum depositional ages), i.e. the Maláguide complex would have overthrust the Jubrique unit during the Alpine orogeny. These results indicate a Permian or younger exhumation/emplacement up to crustal levels of the Ronda peridotites subcontinental mantle slab.Altogether, these detrital zircon age spectra support a relatively, but not entirely local derivation of the studied crustal sequences (fragments of the Iberian Massif) and suggest some Mesozoic drifting with respect to its present position. Nonetheless, the different age distribution of the Paleozoic populations between the Jubrique (Alpujárride complex) and Maláguide samples suggests that the former deposited relatively close to other samples from the eastern-central Alpujárride complex and Nevado-Filábride complex, which, in turn, show strong similarities with the Cantabrian zone and NE Iberia (Iberian Massif). In contrast, the Maláguide samples were probably deposited close to the Ossa-Morena Zone (Iberian Massif) . Thus, Alpujárride/Nevado-Filábride and Maláguide complexes would have been juxtaposed during the Triassic-Cretaceous Neotethys rifting/drifting and/or the subsequent Alpine collisional events.

Geochronology, geochemistry, and geological evolution of the Troiseck-Floning and Rosskogel nappes (Eastern Alps): unraveling parallels between the Eastern Alps and Western Carpathians

The Troiseck-Floning and Rosskogel nappes are part of the Austroalpine Unit in the eastern part of the Eastern Alps. The nappes are in tectonic contact and comprise Permian to Mesozoic lower greenschist facies metamorphic metasediments, but only the Troiseck-Floning Nappe consists of a pre-Permian crystalline basement (Troiseck Complex) as well. LA-ICP-MS U–Pb zircon ages, Rb–Sr biotite ages and geochemical data unravel the geological evolution of these tectonic units from Neoproterozoic to Mesozoic times. Detrital U–Pb zircon analyses from siliciclastic metasediments of the Troiseck Complex indicate a late Ediacaran to early Cambrian deposition age of the volcanoclastic sequence. The age distribution correlates with a position along the northeastern Gondwana margin. A late Cambrian crystallization age (502.4 ± 6.8 Ma) of granitic intrusions together with evidence for Late Cambrian/Ordovician magmatism and metamorphism indicate a position at an active plate margin. Polyphase overprinting during the Variscan orogeny is recorded by Late Devonian/early Carboniferous pegmatite dikes (~ 353 Ma) that formed after an early Variscan event, while Pennsylvanian ages of overgrowth rims and inherited grains (~ 320 Ma) are evidence for late Variscan metamorphism. Rhyolitic to andesitic volcanic rocks from the Troiseck-Floning and Rosskogel nappes (271–264 Ma) concomitant with intrusions of porphyric granitoids now transformed to augen gneiss (271 Ma) yield evidence for Permian rift-related magmatism that is widely reported from the Eastern Alps and Western Carpathians. Rb–Sr biotite ages (75–74 Ma) indicate Late Cretaceous cooling below c. 300 °C. This relates to Late Cretaceous exhumation of the Troiseck-Floning Nappe following an Eo-Alpine metamorphic overprint at lower greenschist-facies metamorphic conditions. Based on the similar lithostratigraphy, analogous geological evolution and structure, the Troiseck-Floning Nappe represents the lateral extension of the Seckau Nappe. The new dataset also allows for correlations with other basement complexes that occur in the Western Carpathians.

A direct observation of a hydrogen-rich pressurized reservoir within an ophiolite (Tișovița, Romania)

Subsurface natural hydrogen accumulations that can be economically and safely recovered are a major target of the present roadmap towards the use of hydrogen as an alternative and C-free energy resource. The existence of H2-rich reservoirs in ophiolites, where H2 is produced via serpentinization, was theoretically predicted based on the intensity of gas seeps in Turkey and Albania. Here, we examine the geological bases supporting the potential existence of H2 reservoirs within ophiolites in general, also considering available data on ophiolitic petroleum reservoirs, and we document a direct observation of a H2-rich accumulation within an ophiolite. We studied the case of a borehole, drilled in the 1970s for metal mining exploration purposes within the Tișovița–Iuți ophiolite, in Romania, which accidentally encountered pressurized gas with ∼29 vol% H2 at a depth of about 800 m. We reconstructed the geometry of the ophiolite sequence in correspondence with the well and suggested that the reservoir is within cataclastic dunite near the tectonic contact between layered dunite and the harzburgite-dunite peridotites; the upper, non deformed portion of serpentinized layered dunite, likely with the contribution of tonalite veins, acted as a fluid trap. Although plugged, we detected H2 leakage from this well and from a second well in the same area, with H2 concentrations exceeding 100 ppmv in the air and 0.1 mg H2/L in the water within the wellhead open casing. We propose a broad conceptual model in which deformed, fractured, and non-deformed, impermeable sections of serpentinized peridotites can determine ophiolitic H2-rich reservoirs and traps, respectively, as factors that shall be considered in the guidelines for H2 reservoir exploration.

New discovery of 3.84–3.64 Ga diverse granitoids in eastern Hebei, North China Craton: Petrogenesis and significance

Abstract Eoarchean rocks are scarce worldwide, limiting our understanding of Earth’s early history. This contribution presents the field geology, sensitive high-resolution ion microprobe (SHRIMP) U-Pb zircon dating, Nd-Hf-O isotopic analysis, and whole-rock geochemical study of Eoarchean granitoids newly discovered in the Labashan area, eastern Hebei, North China Craton. Eoarchean rocks, including 3838–3754 Ma trondhjemitic gneisses (4 samples), 3786–3773 Ma granodioritic gneisses (4 samples), 3775 Ma quartz monzonite gneiss (1 sample), and 3786–3640 Ma potassic granite gneisses (3 samples), are identified in a tract 1 km long. They are in tectonic contact with 3.4–3.1 Ga supracrustal rocks, with both occurring as enclaves in 2.5 Ga potassic granite. The trondhjemitic gneisses show low (La/Yb)N values (23.4–56.8) and insignificant Eu anomalies (Eu/Eu* = 0.86–1.24). Compared with the trondhjemitic gneisses, the granodioritic gneisses and quartz monzonite gneiss have lower (La/Yb)N ratios (9.2–11.3) and lower Eu/Eu* values (0.62–0.79). The potassic granites have (La/Yb)N ratios of 20.7–55.2 and Eu/Eu* values of 0.75–1.84. Zircon saturation temperatures of the granitoids range from 667 °C to 878 °C. They have whole-rock εNd(t) of −4.6 to 1.0 and tCHUR(Nd) of 4.3–3.6 Ga, and zircon εHf(t) values of −6.9 to 0.4 and tCHUR(Hf) ages of 4.1–3.5 Ga (mainly 4.1–3.8 Ga) and δ18O of 4.6‰–7.5‰ (CHUR—chondritic uniform reservoir). It is speculated that different amounts of low-K mafic rocks and tonalite-trondhjemite-granodiorite rocks with prior crustal residence times were the source of the spectrum of trondhjemitic to granitic compositions at Labashan. Two possible models, magma underplating and convergent plate boundary processes, are explored for the tectonic regime forming these Eoarchean granitoids.

Mineral chemistry and P–T conditions of the winchite-bearing metabasic rocks in the NE edge of the Menderes Massif (Western Türkiye)

The Menderes massif is a large metamorphic crystalline complex located in western Turkey. The massif consists of late Neoproterozoic basement (core) rocks and Palaeozoic - Cenozoic cover units that have undergone by high- and low-grade polyphase metamorphisms. Palaeozoic to Mesozoic cover units cropped out in the NE edge of the massif (east of Sivaslı) are overlaid tectonically by upper Cretaceous meta-ophiolitic rocks. The meta-ophiolitic unit comprises blocks mainly of metabasalt, metagabbro, metadiorite and metaultramafite within an intensively sheared serpentinite matrix. It also includes blocks of epidote-actinolite-schist and tremolite-actinolite-schist, which originated from basic rocks, as well as chlorite schist blocks, which originated from ultramafic rocks. Sodic-calcic amphiboles recorded in the samples of metabasalt block taken from the marble-metabasalt block boundary, at the tectonic contact where the meta-ophiolitic rocks overlap the underlying marble sequence. Sodic-calcic amphiboles were classified into winchite and ferri-winchite with relatively homogeneous TSi (7.35–8.01 a.p.f.u.), and XMg (0.69–0.80) values. P-T conditions were estimated to be around 300–400 °C and 5–6 kb based on the mineral chemical analyses of the sodic-calcic amphiboles. According to these values, the NE edge of the Menderes Massif must have undergone metamorphism under a medium-pressure (MP) greenschist facies.

Cenozoic Pb–Zn–Ag mineralization in the Western Alps

Abstract Metallogenic models of polyphase mountain belts critically rely on robust geochronology. We combine petrology with Rb–Sr and U–Th–Pb in situ geochronology, paired at thin-section scale, to date mineralization in deformed hydrothermal Pb–Zn–Ag deposits along an east-west transect in the Western Alps, France. The Pb–Zn–Ag veins occur in shear zones with kinematic structures consistent with the mylonitized host rocks. The ore consists mainly of galena in a quartz-phengite gangue. The paragenesis can be related to hydrothermal crystallization during periods of variable strain. Both isotope systems yield only Cenozoic ages (ca. 35 Ma and 15–20 Ma) without any pre-Alpine inheritance, clearly indicating orogenic mineralization. The metallogenic model proposed here includes significant fluid circulation along major tectonic contacts between basement and sedimentary cover during Alpine convergence.

Is the Inverted Field Gradient in the Catalina Schist Terrane Primary or Constructional?

AbstractNew geothermometry using laser‐Raman data on carbonaceous material from low and intermediate grade rocks on Santa Catalina Island, California, together with existing thermobarometric data, show that there is a quasi‐continuous increase in peak metamorphic temperature from 327 ± 8°C in lawsonite blueschist facies rocks at the lowest structural levels, through ∼433°C in overlying epidote blueschists, 546 ± 20°C in albite‐epidote amphibolite facies rocks, to 650–730°C in amphibolite facies rocks at the top of the sequence. Rocks of different metamorphic grade are separated from one another by tectonic contacts across which temperature increases by ∼100°C in each case. Previously published geochronological data indicate that peak metamorphism in the highest grade rocks at 115 Ma preceded deposition of blueschist facies metasediments by ∼15 million years, so that the present inverted grade sequence does not represent an original inverted temperature gradient. The present structure results from progressive underplating of oceanic rocks in a cooling subduction zone following a high‐T metamorphic event at 115 Ma. An inverted temperature gradient of ≥100°C/km across the subduction channel likely existed during the high‐T event, decreased during underplating, and reached zero by ∼90 Ma.

Supergene mineral assemblage from sediments affected by contact and hydrothermal metamorphism, locality Dobrá – Staré Město near Frýdek-Místek (Czech Republic)

At locality Dobrá – Staré Město, series of outcrops occur on both banks of the Morávka River. The river cuts through the sediments on the tectonic contact between the Silesian and Subsilesian units of the Outer Western Carpathians. Clayey facies of the Hradiště Formation (Early Cretaceous, Valanginian to Aptian) contains numerous tectonic fragments reaching hundreds meters in size of strongly altered (carbonatized, smectitized) subaquatic volcanic rocks of the Teschenite Association. These volcanic rocks are dominated by fine-grained to aphanitic rocks, possibly of both effusive and intrusive origin. The host sediments affected by the contact metamorphism (contact adinole) also occur. Framboidal pyrite is abundant in majority of sediments of the studied area, giving rise to efflorescences of gypsum, baryte, celestine, and a mineral of alunite group (Figure 1). A more varied assemblage of supergene minerals was recognized on a small rock outcrop at GPS coordinates N 49° 39.992’ E 018° 23.810’ (Figure 2), where it developed relatively recently after the big floods in 2010 that washed away the previous mineralization. During the first years, only gypsum crusts were detectable, while in 2023, we recognized 4 macroscopic and 5 microscopic minerals forming thin botryoidal crusts. These crusts show faint zoning, with gooey, gel-like X-ray amorphous material in the humid upper part , middle zone with abundant fibroferrite and the lower part dominated by gypsum. Gypsum forms well-developed, but frequently corroded crystals with various habitus (Figure 4A, B). Fibroferrite is present as fibrous aggregates with individual crystals of up to ca. 100–150 µm long and &lt;1 µm thick (Figure 4C). It forms directly from the gel-like substance (Figure 4D). Rhombohedral crystals and rossete aggregates of hydroniumjarosite vary from 2 µm to 5 µm in size (Figure 4E), but the most common are dust-like coatings on rock fissures with particle size &lt;1 µm. Rusty coatings of schwertmannite (Figure 4F), in some places accompanied by gypsum, with Fe/S at. % ratios near 4–5 are common. In some cases, the Al/Fe at. % ratio exceed 1, which might indicate presence of yet undescribed Al-analogue of schwertmannite, ill-defined alumogel, or an amorphous precursor of aluminite. Presence of ferrihydrite is possible but not confirmed. The above-mentioned Fe-rich phases are frequently accompanied by diplobacilli-shaped bacteria (Figure 4F). Slavíkite, pickeringite-halotrichite, alunogen, copiapite group mineral, hexahydrite, and melanterite (Figure 5) were also recognized by EDS and PXRD. Slavíkite and copiapite group minerals seem to be alteration products of fibroferrite. Unit-cell parameters refined from the X-ray powder diffraction are presented for all recognized phases (Table 1). Surprisingly, secondary sulfates are present on the metasediments with abundant calcite (Figure 3), which should serve as a neutralization buffer. The reason is the restricted amount of calcite available on the rock fissure surfaces, and water infiltration partially blocked by newly-formed gypsum. The albite in contact adinole (Figure 3) is unaffected by the acid rock drainage, since Na ions are not present in significant quantities in supergene phases. Since there is still enough available unaltered pyrite, we expect a shift towards more stable mineral assemblage in future. The limited amount of secondary sulfates does not present any significant harm to the environment.

New data on the geological structure and zonality of the Vorontsovka gold field in the Northern Urals

Spatial distribution of gold, copper, and iron ore deposits in the Northern Urals with the largest Turya-Auerbakh ore region is controlled by north-west trending linear zones of a probably fault nature. The influence has been demonstrated of regional factors (including the tectonic, magmatic, and geochemical ones) favoring formation of the large Au-As-Sb-Hg-Tl Vorontsovka deposit located within a volcanogenicsedimentary, substantially carbonate rock sequence. Closely spaced swarms of Devonian pre-ore and Carboniferous post-ore mafic dykes have been identified within the deposit. The primary geochemical halos of the Vorontsovka deposit have a multielement composition and, coupled with the ores, form a thick ore-halo zone confined to the gentle tectonic contact of sandstone-siltstone and limestone units. The main halo-forming elements are zonally distributed in the section. The footwall side of the gold deposit (rear zone) is dominated by Zn, Cd, Cu, Pb, Ag, and Bi; the axial zone, by Sb, Hg, Tl, and As; and the hanging side (frontal zone), by&#x0D; As, Zn, Hg, Pb, Ag, (Cu). In the east (closer to the Auerbakh granitoid massif), Ag, Zn, Pb, Mn, Y, Mo, and W relatively accumulate, while in the west (most remotely from the intrusive), Sb, Ba, As, Hg, Co, Ni, and Sn dominate. The 40Ar/39Ar age of ~391.1 million years, obtained for hydromica of the gold-arsenic ores, corresponds to the period of emplacement of granodiorite of the final Auerbakh massif intrusive phase. The conclusion is made on the crucial magmatic contribution to the formation of gold ores of the deposit.

Regional-scale 3D modelling in metamorphic belts: An implicit model-driven workflow applied in the Pennine Alps

Leveraging a high resolution geological and structural dataset acquired over decades of fieldwork, we build the 3D structural model of a portion of the highly deformed core of the Alpine orogen, in the Northern Aosta Valley. The model represents tectonic contacts separating the tectono-metamorphic units outcropping along the section between Mont Blanc and Monte Rosa, and it covers an area of ca. 1500 km2. The input source data include original 1:10,000 geological surveys synthetised in a 1:75,000 tectonic map, and a dense database of structural stations. After a first orientation statistics study of the structural field database, our workflow develops through structural interpretation in vertical cross-sections that allow including in the modelling process structural drivers such as crosscutting relationships, interference patterns, kinematic constraints and fold morphology from detailed field studies. Three-dimensional interpolation on a tetrahedral mesh using the implicit Discrete Smooth Interpolator method follows, using also foliation and fold axes data as interpolation constraints. After describing the workflow and the model, we discuss the difficulties of modelling in polydeformed metamorphic complexes. In particular, we address the issue of modelling shear zones, refolded, isoclinal and/or recumbent folds and dense networks of faults, that characterise the geology of the Northern Aosta Valley.

Assessing the importance of H2O content in the tectono‐metamorphic evolution of shear zones: A case study from the Dora‐Maira Massif (Western Alps)

AbstractMetamorphic reactions are commonly driven to completion within shear zones thanks to fluid circulation, making the re‐equilibration of the mineral assemblage one of the dominant processes. Despite the important role of H2O in such processes, forward thermodynamic modelling calculations commonly assume either H2O‐saturated conditions or only fluid loss during prograde evolution to peak conditions. These assumptions influence the understanding of shear zones during the retrograde evolution. Here, we investigate the P–T–MH2O retrograde evolution of the Mt. Bracco Shear Zone (MBSZ), an Alpine ductile tectonic contact which marks the boundary between two HP units in the Dora‐Maira Massif (Western Alps, Italy). After the eclogite‐facies peak (at 500–520°C and 1.8–2.2 GPa), the subsequent mylonitic event is constrained at amphibolite‐facies conditions, continuing its evolution at decreasing pressure and temperature during rock exhumation, from ~590°C, 1.0 GPa down to ~520°C, 0.7 GPa. The P/T–MH2O forward modelling highlights different behaviour for the two analysed samples. After reaching a minimum H2O content at the transition from eclogite‐ to amphibolite‐facies conditions, a significant fluid gain is modelled for only one of the two analysed samples just before the mylonitic event. The MBSZ then evolves towards H2O‐undersaturated conditions. This work thus underlines the necessity of investigating the H2O evolution within shear zones, as the H2O content is susceptible to change through the P–T path, due to dehydration reactions or fluid infiltration events. Furthermore, lithological heterogeneities influence possible different fluid circulation regimes in shear zones, resulting in externally or internally derived fluid gain.

Petrology, U-Pb titanite dating and Sr-Nd isotope geochemistry of a shoshonitic lamprophyre dyke near the Western Dharwar Craton-Southern Granulite Terrane contact, southern India: Implications for long-lived enriched mantle, widespread Tonian-Cryogenian rifting, and Rodinia configuration

We present petrology, U-Pb titanite geochronology, bulk-rock and Sr-Nd geochemistry of an undeformed and unmetamorphosed shoshonitic lamprophyre (spessartite) from the Mysuru area (Halaguru- Harohalli alkaline province) located approximately 80 km north of the tectonic contact between the Western Dharwar Craton and the Southern Granulite Terrane, southern India, to understand its origin and to evaluate its geodynamic significance. The lamprophyre shows a typical porphyritic-panidiomorphic texture with amphibole phenocrysts. Clinopyroxene occurs only as xenocrysts with feldspar, apatite, titanite and titano-magnetite confined to the groundmass. Amphibole mineral composition classifies them as pargasite and tremolite, and the groundmass is essentially constituted of alkali feldspar. High whole-rock Mg# (56–65), Ni + Cr (245 to 730 ppm), a lack of correlation between SiO2 and Rb/Sr as well as ɛNd(i) indicates that crustal contamination experienced by the lamprophyre magma is limited. U-Pb titanite (n = 18) SHRIMP-II dating yielded a Neoproterozoic Tera-Wasserburg concordia age of 820 ± 15 Ma (MSWD = 5.9). Whole-rock derived 87Sr/86Sr(i) (0.70425–0.70530) and ɛNd(i) (-6.3 to −4.3) suggest origin of the lamprophyre from an enriched lithospheric mantle source. Paleoproterozoic depleted-mantle model ages of ca. 1.8 Ga suggest the existence of a long-lived enriched mantle which correspond to the timing (ca.1.9–1.7 Ga) of widespread mafic dyke swarms’ emplacement in the Dharmapuri-Agali-Tiruvannamalai domain in the Salem block at the northernmost portion of the Southern Granulite Terrane. The Tonian emplacement age of the studied lamprophyre is synchronous with several co-spatial Neoproterozoic (i) alkaline complexes ± carbonatite ± lamprophyre from the northern part of the Southern Granulite Terrane, and (ii) other mafic alkaline dykes of the Halaguru-Harohalli alkaline province from the southern part of the Eastern Dharwar Craton. Our results highlight the importance of this lamprophyre dyke, present close to the’Fermor Line’ as well as near the boundary between Eastern and Western Dharwar Cratons, which provides a rare opportunity to understand its spatio-temporal link with alkaline dykes of the Southern Granulite Terrain. Our findings also help understand the role of repeated large-scale rifting, associated with the Rodinia break-up, leading to the generation of a wide spectrum of Tonian-Cryogenian alkaline magmas in southern Indian Shield. The new age data also have implications for the configuration of India’s paleoposition during the Neoproterozoic and support the idea that South China Block-India were located on the periphery of Rodinia or were already detached from the supercontinent.

Combining methane clumped and bulk isotopes, temporal variations in molecular and isotopic composition, and hydrochemical and geological proxies to understand methane's origin in the Ronda peridotite massifs (Spain)

In serpentinised peridotite and ultramafic rock systems, methane (CH4) origin is frequently considered abiotic, but variable microbial and thermogenic components can also exist. Typically, the origin of CH4 is studied using bulk, 13C/12C and 2H/H isotopic composition, molecular gas composition, occasionally radiocarbon (14C), microbiology and geological context. Recent advances in CH4-clumped isotope methods have yielded novel insights into the formation of CH4: nonetheless, their interpretation in natural gas samples is often uncertain and requires additional research. Here, we study the origin of the gas released in hyperalkaline (pH > 10) springs in the Ronda Peridotite Massifs (southern Spain), combining bulk and clumped CH4 isotopes with molecular gas composition, hydrochemical (Total Organic Carbon and Platinum Group Elements in water), geothermal and geo-structural data. Five springs analysed in 2014 have been re-examined for changes in gas chemistry over time, and three newly discovered gas-bearing springs are analysed for the first time. Regardless of whether springs have microbial or abiotic isotopic fingerprints, we find that bulk CH4 isotopes are fairly stable over a seven-year period. This suggests that the CH4 source(s) or postgenetic processes (such as oxidation and diffusion) have not undergone significant temporal changes. Major variations in H2 and CH4 concentrations in certain springs may be the result of changes in gas pressure and migration intensity. Paired CH4 clumped isotopes (Δ12CH2D2 - Δ13CH3D) were analysed in two bubbling springs, where the presence of CH4 can be interpreted as non-microbial based on 13C enrichment, absence of 14C, and the presence of ethane and propane. However, these isotopes are in disequilibrium, which prevents the quantification of the gas formation temperature. Within the Δ12CH2D2 - Δ13CH3D diagram, the data lie within both the microbialgenic zone, suggested by previous authors, and the abiotic zone that results combining data from laboratory gas synthesis and other natural gas samples. Therefore, attributing a microbial origin to CH4 based only on clumped isotopes is less definite than previously assumed. The amount of Total Organic Carbon appears to be correlated with the origin of CH4, as it is higher in 13C-depleted CH4 samples and lower in 13C-enriched samples. Palladium (Pd) and Rhodium (Rh) dissolved in water (the more soluble Platinum Group Elements) can be a proxy for the chromitite ore deposits contained in plagioclase tectonite layers throughout the investigated area, which may act as catalysts for abiotic CO2 hydrogenation. Clumped isotope disequilibrium and the reported absence of diffuse CH4-bearing fluid inclusions in the peridotites appear to rule out high temperature gas genesis in post-magmatic inclusions. These observations, along with the moderate temperatures at the base of the peridotite massifs and the consistent occurrence of gas along tectonic contacts between serpentinised (H2-bearing) peridotite and carbon-bearing rocks, are compatible with the theory of low-temperature CO2 hydrogenation.

Geophysical Research on an Open Pit Mine for Geotechnical Planning and Future Land Reclamation: A Case Study from NW Macedonia, Greece

In open pit mining areas, knowledge of geotechnical conditions (e.g., overburden thickness, background slope, and fault locations) ensures geotechnical safety during exploitation as well as reclamation planning. The Greek Public Power Corporation initiated a research program after stability issues emerged on the southern side of the Mavropigi open pit mine in NW Macedonia. Geotechnical wells revealed steeply dipping bedrock and thin tectonic contact, indicating the need for the detailed imaging of the subsurface for future stability measures. For this purpose, a geophysical investigation aimed to extract information mostly for the dip of the interface between schist bedrock and overlaying Neogene sediments and/or limestones. Based on the high contrast of electrical properties between schists and limestones, as well as the differences in acoustic impedance and formation thickness, the seismic reflection and electrical resistivity tomography (ERT) methods were selected. The suitability of the seismic reflection for its application in this area was checked by generating synthetic seismic data, which resulted from the simulation of seismic wave propagation for geological models of the area. The acquisition parameters were determined after the noise test. Field seismic data processing produced a depth-migrated section, which revealed the existence of a fault. The use of dipole–dipole and gradient arrays, in 2D and 3D electrical resistivity measurements, ensured both the lateral and vertical mapping of schist bedrock and detected limestone bodies within the overburden. Also, the tectonic contact zone between limestone and schist formations was properly imaged. The comparison between geoelectrical and seismic sections indicated that the seismic reflection method provided a more accurate estimate of fault inclination. Finally, the geophysical survey enriched the geotechnical models necessary for sustainable mining (e.g., rational exploitation, the optimization of productivity, and zero accidents) including the planning of future reclamation.