Volcano-sedimentary Deposits Research Articles

Rock density model of the Southern Benue trough in Nigeria from a synthetic gravity and in-situ density data

ABSTRACT The aim of this study is to identify potential mineral deposits beneath the Southern Benue Trough of Nigeria based on apparent density mapping technique of estimating lateral rock densities from modelled gravity data. In this study, we apply the inverse density deconvolution filter to a short-wavelength gravity component in order to estimate the rock densities for mineral exploration applications and tectonic studies at the Southern Benue Trough in Nigeria. The estimated rock densities vary between 2.50 and 2.74 g/cm3, with minimum density values assigned to volcano-sedimentary deposits along the Cameroon Volcanic Line and maximum density values in the eastern and southern parts associated with mafic igneous rocks. Our gravity and density maps reveal the geometry of dominant geological structures within the study area. The density map highlights carbonate-hosted metallic elements with varying intercalations. Some of these identified mineralized zones coincide with locations of past and present producing mines. Intermediate and mafic igneous rocks as well as highly compacted and volcano-sedimentary rocks are mapped as rock-forming minerals for a mineral exploration. Hence, the obtained model of mineral-bearing rocks mainly demonstrates mechanisms of tectonic events, patterns of deformation regimes, and mineral prospectivity map of the study area.

Authigenic Minerals in Phanerozoic Volcanosedimentary Deposits of the Northern Part of the Asian Continent–Pacific Ocean Transition Zone

Authigenic Minerals in Phanerozoic Volcanosedimentary Deposits of the Northern Part of the Asian Continent–Pacific Ocean Transition Zone

Atmosphere and biological impact during sulfide formation in the Archean Central-Vozhma sulfide deposit (Karelia)

Relevance. The need of new knowledge about the early stages of the Earth. Sulfur isotope analysis of sulfide minerals is a powerful tool to understand the processes during the Archaean and Paleoproterozoic. Combined with other data, isotope geochemistry provides an insight into sulfur sources of sulfides from ancient sulfide volcanosedimentary deposits; geochemical factors affecting Archaean sulfide volcanosedimentary ore formation; adjust genetic models and determine the degree of influence of bacteria on the mineral formation. Aim. To identify the sources of sulfur during the formation of sulfide deposits via isotope analysis, and to evaluate bacteria affect mineral formation. Objects. They were obtained from the core of boreholes of Mesoarchaean volcanosedimentary sulfide Central-Vozhma deposit, being a part of the Sumozersko-Kenozersky greenstone belt of the Karelian craton. Methods. Mineralogical studies of rock and ore samples were carried out using optical microscopy; scanning electron microscopy and energy dispersive X-ray spectroscopy. The ratios of four stable sulfur isotopes were analyzed in sulfide minerals of the deposit (33S/32S, 34S/32S, 36S/32S). Results. The results obtained demonstrated the polygenic source of sulfur in sulfides. The sulfides have both positive and negative Δ33S values, indicating the presence of atmospheric sulfur formed under UV photolysis during mineral formation. Sulfide minerals include the following components: Seawater sulfate sulfur of photolytic genesis showed a negative anomaly Δ33S (∼ –0.4‰). It was the source of authigenic pyrite. Sulfides crystallized as a result of biological sulfate reduction demonstrated a narrow range of δ34S values (–2.64‰˂0˂+4.27‰). Elemental sulfur of photolytic genesis mobilized from the host sedimentary rocks by hydrothermal fluids. This sulfur, with a positive Δ33S anomaly (up to +1.6‰) took part in the massive sulfide ores formation.

High-precision U[sbnd]Pb zircon dating of explosive volcanism in an early bi-modal volcano-sedimentary sequence from the Isle of Mull, North Atlantic Igneous Province

The North Atlantic Igneous Province (NAIP) forms one of the best studied Large Igneous Provinces (LIP) on the planet, however, significant uncertainties regarding the age and nature of the early onset of volcanism across the province remain. In order to better understand the onset and timing of volcanism within the NAIP, we present a new study of the mixed volcano-sedimentary deposits exposed at the base of the Isle of Mull lava pile, Inner Hebrides, Scotland. The study area comprises organic rich estuarine sediments, intimately mixed and interdigitated with bi-modal composition volcaniclastic and pyroclastic interlayers, including a prominent stratigraphically constrained c. 30 cm thick water-lain rhyolitic ignimbrite. Pristine magmatic zircons liberated from the ignimbrite reveal a high precision UPb age of 62.035 ± 0.041 Ma (2σ) giving a robust age for early volcanism in this part of the province. Field observations reveal evidence for mafic phreatomagmatic eruptive deposits both above and below the rhyolite documenting the bimodal explosive nature of the initial magmatism in this area and, importantly, that mafic magmatism initiated before the dated unit. The bi-modal explosive eruptions periodically inundated an otherwise quiet and low energy Danian aged estuarine environment prior to its inundation with lava as magmatism took hold in the region.

Petrology of volcano-sedimentary deposits from Mayo Mbota, west of the Cretaceous Babouri-Figuil basin, North Cameroon

Petrology of volcano-sedimentary deposits from Mayo Mbota, west of the Cretaceous Babouri-Figuil basin, North Cameroon

Paleogeotectonic position of the Kyzyk Chadr porphyry copper ore field, Republic of Tyva

Regional-scale localization conditions of the Kyzyk-Chadr molybdenum-copper porphyry ore field in the southern folded frame of the Siberian Platform are considered. The porphyry mineralization of the field formed at the Salairian (Early Caledonian) phase of development of the Altai-Sayan segment of the
 Central Asian Orogenic Belt (CAOB), in connection with small intrusions of the gabbro-diorite-granodioritegranite Kyzykchadr Complex (Є2–3). The formation of the ore-bearing complex was associated with an accretionalcollisional magmatic belt superimposed on a Vendian–Early Cambrian volcanic-sedimentary island-arc substrate. The complex is only known within the local Ozhu uplifted block with a “batholitic” massif of the gabbrotonalite-plagiogranite Tannu-Ola Complex (Є2), but it is suggested that there could existed some analogous accretional-collisional Cambrian intrusions productive for Mo-Cu porphyry mineralization in some other areas, that were later overlapped by Ordovician–Silurian and Early Devonian volcanosedimentary deposits or broken by
 intrusive massifs of the Caledonian collisional volcano-plutonic belt. The rocks of the Salairian and Caledonian magmatic belts were deformed during subsequent geotectonic stages, primarily the Late Paleozoic collision, and overlain by younger sedimentary deposits. The position of economic-grade Mo and Mo-Cu porphyry deposits in the southern Altai-Sayan segment of the CAOB is assumed to be confined to areas of spatial superposition
 (overlapping) of magmatic belts of the three geotectonic stages: 1) island-arc volcanic belts (V–Є1), 2) “batholithic” intrusions and small intrusions of the magmatic belt of the accretional-collisional stage (Є2–3), 3) the collisional volcano-plutonic belt (O–D1).

Uranium Isotope Characterization in Volcanic Deposits in a High Natural Background Radiation Area, Mamuju, Indonesia

Mamuju is an area of high natural radiation in Indonesia with high natural radiation levels (average 613 nSv h−1). Mamuju is anomalous due to its high average 238U and 232Th concentrations of 22,882 and 33,549 Bq kg−1, respectively, in laterite and rock. High natural radionuclide concentrations of 238U, 232Th, and 40K have also been reported in soil samples from several locations in Mamuju, including Botteng, Northern Botteng, Takandeang, Ahu, and Taan. High radiation levels are related to radioactive mineral occurrences in the Adang volcanic complex, comprised of phonolitoid and foiditoid lithologies. According to the International Atomic Energy Agency (IAEA), uranium deposits can be classified into several types, among them a volcanic-related deposits, which include three sub-types: stratabound, structure-bound, and volcano-sedimentary deposits. This study aims to characterize volcanic rock deposit sub-types in the Mamuju area based on uranium radioisotope measurements. The uranium isotopes were measured using a tandem quadrupole inductively coupled plasma mass spectrometer combined with chemical separation by extraction chromatography using UTEVA resin. The analytical results for the 234U/238U ratios are used to determine the formation characteristics of minerals in each deposit sub-type based on mineral formation age, post-formation processes, and disturbances that affected the formation processes. Based on geochronological calculations using 234U/238U mineralization age, the deposits in the Mamuju area are 0.914–1.11 million years old and are classified as recent mineralization. These data have important implications for tracing uranium source rocks in the Mamuju area and may explain the anomalously high radiation levels in the Mamuju area.

Volcanoclastic and epiclastic diagenesis of sandstones associated with volcano-sedimentary deposits from the upper Jurassic, Lower cretaceous, Paraná Basin, southern Brazil

The opening of South Atlantic Ocean produced an immense volume of lava that covered an active aeolian system during Lower Cretaceous. The contact of Botucatu Formation sandstones with the Serra Geral volcanic flows generated a variety of volcano-sedimentary features and deposits. During volcanism, the temperature contrast between lava and wet or water-saturated sediments formed volcanoclastic features and related sedimentary deposits. In moments of magmatic quiescence, epiclastic deposits were generated due to the return of sedimentation. Volcanoclastic deposits include volcanic breccias with sandy matrix, and epiclastic deposits include conglomerates and conglomeratic sandstones with intraclasts. Quantitative petrography of the sandy matrix samples of these deposits was analyzed, in order to discuss the paragenesis of volcano-sedimentary interactions. Regular burial diagenetic processes were discussed for epiclastic sandstones and contact diagenesis processes for volcanoclastic sandstones. The original composition of volcanoclastic and epiclastic sandstones is heterogeneous. However, the main difference between them is the presence of textural fluidization indicators found in the volcanoclastic sandstones, along the lava-sediment interface. These textural characteristics indicate that the unconsolidated sediments were probably wet or saturated in water during the interaction with the volcanic flow. Epiclastic sandstones have infiltrated clays and neoformed smectites as their main diagenetic constituent due to alteration of available volcanic lithoclasts. For volcanoclastic sandstones, the formation of opal, chalcedony and megaquartz cements through burial is unlikely, due to the intergranular volume occupied by these cements, indicating near-surface early diagenetic conditions. Therefore, the mechanisms of dissolution and repreciptation of the siliceous fluids, incorporated in the system, formed mainly the chalcedony cementation. Furthermore, zeolite cementation, which occurs along the lava-sediment contact, is another important indicator of contact diagenesis for volcanoclastic sandstones. During telodiagenesis, the formation of secondary porosity by dissolution of volcanic lithoclasts and detrital K-feldspar grains was of great importance in volcanoclastic and epiclastic sandstones.

<i>XENOXYLON</i> (CONIFERALES) FOSSIL WOODS FROM THE LOWER CRETACEOUS DEPOSITS OF THE FRANZ JOSEF LAND ARCHIPELAGO

Fossil wood species Xenoxylon phyllocladoides and X. jakutiense of the Mesozoic genus Xenoxylon (Coniferales) are described from the Lower Cretaceous volcano-sedimentary deposits of Mac-Cult and Salisbury islands of the Franz Josef Land Archipelago. Fossil woods from Mac-Cult and Salisbury islands are described for the first time. Record of X. jakutiense from Salisbury Island represents the first record of this species from the island part of the Arctic. The presence of abietinean pitting on the radial walls of tracheids indicates that these species belong to the “phyllocladoides” group sensu Philippe et al. (2013), evolved from ancestral forms of Xenoxylon in the Early Jurassic. The discovery of X. phyllocladoides and X. jakutiense fossil woods in the Lower Cretaceous deposits both on the island and mainland parts of the Russian Arctic confirms the conclusions about floristic links between Franz Josef Land and the northern part of the Siberian Platform in the Early Cretaceous.

Projevy interakcí vulkanické aktivity Velkého Roudného s vodním prostředím

Velký Roudný scoria cone represents the most prominent volcanic edifice of the Plio-Pleistocene Bruntál Volcanic Field (according to Cajz et al. 2012). Various types of volcaniclastic deposits and volcanic features were interpreted in various ways in the past. However, several features remained overlooked, and the investigated ones required critical revision. Velký Roudný is an asymmetric horseshoe-shaped cone, with a crater open towards the southeast (Fig. 1a). The lava flows emitted from this cone flew southwards and also to the east/ southeast, where the lava filled a paleo-canyon of the Moravice River. In the paleo-canyon, the lava (up to 60 m thick) buried earlier fluvial sediments (Horský et al. 1972; Fig. 2). The best exposure of the lava flow is accessible in the active Bílčice quarry. The two levels of columnar jointing separated by a platy- to blocky-jointed domain represent the upper and lower colonnade of a single lava flow (Fig. 3a, b). The columnar jointing domains are interrupted by subvertically platy-jointed, piston-like features filled with breccia (Fig. 3c, d). These monomictic breccias are dominated by basaltic clasts with signs of abrupt quenching, like cauliflower structures (Fig. 3e, f ). The shapes of these breccia bodies, together with signs of magma/water interactions, suggest their origin from rootless explosions. Their extension through the complete thickness of the lava, as well as their distribution in the lava flow were earlier documented by geophysical survey (Štainbruch 2019), whose results are re-interpreted in this contribution (Fig. 4). Another effect of the lava/water interaction is represented by “sunburn” decomposition (spheroidal decomposition of the rock; Fig. 5a). The development of the spheroidal cracks seems to be restricted to the glass-containing groundmass (Fig. 5b). As the sunburn decomposition develops mainly at the base, around the rootless craters, and less frequently on the top of the lava. It seems that these irregular features reflect gentle hydration of glass-containing rock, similarly to perlitic cracks in silicic lavas (e. g., McPhie et al. 1993). Subsequently to cracking, the glass tends to analcime recrystallization (Fig. 5c, d), enhancing the decomposition of the rock. Pyroclastic deposits exposed in two small abandoned quarries near Razová and Karlovec, respectively, were interpreted as tuffites (mixed volcano-sedimentary deposits) in the past. Despite that these deposits contain abundant fragments of basement greywackes, the fragments display no signs of rounding or sorting (Fig. 6). Poorlysorted clast-supported deposits well correspond to the near-vent ballistic accumulation of phreatomagmatic eruption. At Karlovec, predominance of the finer-grained facies with diagonal bedding suggests that these rocks originated by base-surge deposition. The potential source of the phreatomagmatic eruption can be found in the circular depression underneath Velký Roudný (Fig. 1b, c), as supported by a magnetic anomaly (Fig. 7). The phreatomagmatic character of the volcanic activity possibly results from an eruption through a lake dammed by the Bílčice lava of the Velký Roudný scoria cone (Fig. 8).

Tectonics of the Basement of the Kara-Bogaz Arch (Turan Plate)

The Kara-Bogaz geoblock is interpreted by many researchers as a structure underlain by the Precambrian sialic crust, incorporated in the young (epi-Hercynian) Turan Plate. The paper presents the results of a detailed study of the material composition of igneous and metamorphic rocks making up the basement of the Kara-Bogaz Arch and recovered by deep boreholes. To subdivide and correlate the sections, we employed data of geophysical borehole surveys, including all types of logging. Microscopic examination of rocks was also performed to determine their composition, genesis, and degree of postsedimentation alterations. Data on absolute ages of rocks and paleontological data were also employed. The results of the work of our predecessors have been studied and critically analyzed. It is established that there are no direct indications of the continental crust older than Paleozoic in the basement of the Near-Kara-Bogaz region. The metamorphic units of the Kara-Bogaz Arch are represented by primary sedimentary and volcanosedimentary deposits that were altered at the stage of regional greenschist metamorphism. These rocks are intruded by granitoid bodies corresponding to the final stages of Hercynian tectogenesis. In parts spatially close to intrusions, the degree of secondary alterations in metamorphic rocks increases due to the thermal effect. Amphibolites of the Kara-Bogaz Arch are metamorphosed igneous rocks, which are closely associated with primary terrigenous deposits and have undergone subsequent metamorphosis (greenschist facies regional metamorphism). Gneisses of the discussed region refer to the marginal facies of granitoid plutons that formed as a result of metasomatic reworking of the host strata (protomagmatic gneissic banding). We think that the Kara-Bogaz Arch is a Hercynian megaanticlinorium of the young platform, and this does not exclude the possibility of fragments of more ancient crust in the structure of this arch. The sedimentary–metamorphic and volcanosedimentary rocks making it up and intruded by multiple granitoid bodies are units of the active margin.

Volcano-sedimentary processes at Las Derrumbadas rhyolitic twin domes, Serdán-Oriental Basin, Eastern Trans-Mexican Volcanic Belt

Abstract The eruption of the c. 10 km 3 rhyolitic Las Derrumbadas twin domes about 2000 years ago generated a wide range of volcano-sedimentary deposits in the Serdán-Oriental lacustrine basin, Trans-Mexican Volcanic Belt. Some of these deposits have been quarried, creating excellent exposures. In this paper we describe the domes and related products and interpret their mode of formation, reconstructing the main phases of the eruption as well as syn- and post-eruptive erosional processes. After an initial phreatomagmatic phase that built a tuff ring, the domes grew as an upheaved plug, lifting a thick sedimentary pile from the basin floor. During uplift, the domes collapsed repeatedly to form a first generation of hetero-lithologic hummocky debris avalanche deposits. Subsequent dome growth produced a thick talus and pyroclastic density currents. Later, the hydrothermally altered over-steepened dome peaks fell to generate second-generation, mono-lithologic avalanches. Subsequently, small domes grew in the collapse scars. From the end of the main eruptive episode onwards, heavy rains remobilized parts of the dome carapaces and talus, depositing lahar aprons. The Las Derrumbadas domes are still an important source of sediments in the basin, and ongoing mass-wasting processes are associated with hazards that should be assessed, given their potential impact on nearby populations.

НОВЫЕ ДАННЫЕ ПО ВУЛКАНОГЕННО-ОСАДОЧНОМУ КОМПЛЕКСУ НЕОГЕНА ИЧИНСКОЙ ВПАДИНЫ: РАЗРЕЗ ТЫРКАЧИН (ЗАПАДНАЯ КАМЧАТКА)

In the 90-m river bank of the middle reaches of the Icha River, thick strata of volcano-sedimentary deposits represented by pyroclastic rocks and siliceous tuffites with layers of diatomites were studied. Representative diatom assemblages assigned to the Middle Miocene Denticulopsis hyalina Zone (14.9–13.1 Ma) indicate relatively deep-water conditions of the marine basin. Comparison with the data on diatom assemblages from reference sections of Western Kamchatka indicates that the deposits of the Tyrkachin section belong to the upper part of the Kakert Formation. The petrographic composition and lithological structure of volcanogenic formations (members 1, 2, and 4) suggest that they are the product of pyroclastic flows associated with volcanic activity in central Kamchatka. It is assumed that Members 1 and 2 were formed in subaerial conditions, as indicated by the complete absence of marine microfossils, signs of primary volcanic stratification, and the absence of layering. The sediments of Member 4 with eroded contacts and pronounced layering apparently accumulated in a subaqueous setting

Les problèmes géologiques et typologiques des sites paléolithiques dans le Nord-Est du Japon

The northeastern area of Honshu is composed of six departments: Aomori, Iwate, Akita, Miyagi, Yamagata and Fukushima. The study of Paleolithic began in 1927 and has given place to the discovery of the site of Hanaizumi. Subsequently, one pushed the research below on land and the discovery of the first paleolithic site in Iwajyuku happened in 1946. Then, H. Nakagawa took back a success of survey of the geomorphological processes and the ordering of the fluviatile terraces with some layers from the volcano-sedimentary deposit of the Pleistocene in the basin of Kitakami and the one of Mabechi. Therefore, we understood that the archaeological levels of the region contain several sedimentary deposits volcano of quaternary origin from the Mounts Öu. In the basin of Kitakami, one noted the deposit of fall pyroclastic of Aiçra (AT) above the layer of the local volcanic ash from Kurosawajiri. Especially, it is one key of an important layer, the last horizon located under AT and above the tephra of Daïfudô from the local volcano, the pyroclastic stream of Towada and the involution of the layer of the volcanic ash from Kurosawajiri. More than 610 paleolithic sites has been counted in the Northeast.

Geology of Las Minas: an example of an exhumed geothermal system (Eastern Trans-Mexican Volcanic Belt)

ABSTRACT The Las Minas area corresponds to an exhumed geothermal system considered a proxy for the deep part of the nearby Los Humeros active geothermal system. The stratigraphic succession is made up of: Palaeozoic-Miocene granitoids, a thick Jurassic- Cretaceous carbonate succession, Neogene lava flows and volcano-sedimentary deposits. Linked to a Miocene magma intrusion, marble and skarn rock-volumes developed by contact metamorphism and geothermal fluid flow. Faults are arranged in SW- and NNW-striking systems. These controlled the morphological evolution and favored Neogene-Quaternary dyke emplacement. Faulting gave rise to a tectonic depression where lacustrine sediments and pyroclastics deposited. Skarn rocks are mainly located at fault intersections and along pre-existing discontinuities, suggesting the role of bedding and/or fractures in channeling deep fluids. Results give inputs for exploration at depth of Los Humeros geothermal system.

Stratigraphy and facies architecture of the Neoproterozoic syn- and inter-eruptive succession: An example from Gabal El Urf, Northeastern Desert, Egypt

A new integrated stratigraphic, sedimentological and geochemical study was carried out on the Gabal El Urf deposits to clarify their facies architecture, sequence stratigraphic framework, and evolutionary stages. The Gabal El Urf in the Northeastern Desert of Egypt is a well-preserved example of the Neoproterozoic volcano-sedimentary deposits. It comprises inter- and syn-eruptive facies that were generated by diverse depositional processes including debris flows, sheet floods, lacustrine flood flows, and pyroclastic ash falls/density currents. These facies correspond to alluvial fan, aeolian -fluvial, lacustrine, and pyroclastic facies associations that are arranged in four depositional cycles. They reveal variations in accommodation to sediment supply (A/S) ratio. Unconformities and lithofacies assemblages define six stages of volcanism and sedimentation and three unconformity-bounded tectonic sequences that should be allied to tectonic subsidence and basin uplift. These sequences may be subdivided in Low-Accommodation Systems Tracts (LAST) having negative A/S (A/S ratio close to zero) and High-Accommodation Systems Tracts (HAST) with fluctuation in A/S ratio between zero and +1. Sequences 1 and 3 are characterized by clast-supported conglomerates and pyroclastics, overlain by fluvial sand bodies and debris flow deposits, respectively denoting a LAST (positive A/S ratio close to zero). Sequence 2 is marked by pyroclastic ash fall facies at the base, passing to lacustrine deposits that are interlayered with fan-delta conglomerates at the top, defining a HAST (A/S ratio >1). The defined sequences are superimposed by thick fissure-fed ignimbrites which are capped by a co-ignimbritic breccia lag deposits with concentrated pyroclastic fountains. Clastic petrofacies of the El Urf succession indicate an adjacent miscellaneous rock sources involving volcanic, plutonic, and metamorphic terrains during the chief sedimentation processes in rift-related paleotectonic regime. The low to moderate magnitude of the CIA and PIA together with low SiO2/Al2O3 ratio indicate sediment recycling and moderate weathering of the protolith near uplifted basin flank(s) under dominant semi-arid to arid conditions. The development of the El Urf basin is akin to the post-amalgamation Ediacaran volcano-sedimentary basins that are widely distributed in the northern part of the Arabo-Nubian Shield, and is a key tectono-stratigraphic marker for the Cryogenian-Ediacaran shift along a major unconformity surface.

Early–Middle Ordovician Graptolites from the Argentine Puna: Quantitative Paleobiogeographic Analysis Based on a Systematic Revision

The updated taxonomic revision of the Early–Middle Ordovician graptolites from the eastern Argentine Puna allows describing Sigmagraptus praecursor, Baltograptus extremus, B. geometricus, B. vacillans, Cymatograptus protobalticus, Expansograptus constrictus, E. pusillus, E. similis, and Corymbograptus v-fractus tullbergi for the first time in this region. The analyzed material was collected from the volcano-sedimentary deposits assigned to the Cochinoca-Escaya Magmatic-Sedimentary Complex and exposed at the Munayoc and Santa Rosa sections, Jujuy Province. This taxonomic analysis confirms the occurrence of 23 taxa in the studied region, from which S. praecursor, B. extremus, and E. pusillus were not previously documented in South America. Additionally, it contributes to the clarification of the faunal graptolite affinities earlier postulated for Northwestern Argentina. Quantitative paleobiogeographic analyses of clusters and principal coordinates were carried out, including the described species and previous certain graptolite assignations for the Puna region, to quantify its faunal affinities with Baltoscandia, Great Britain, North America, and Southwestern China. Finally, our results are discussed and compared with those formerly obtained in paleobiogeographic analyses based on different fossil groups from Northwestern Argentina.

The Role of Black-Shale Strata in the Formation of the Natalka and Pavlik Gold Deposits (Yana–Kolyma Orogenic Belt)

Abstract —Data on geochemistry, distribution of ore and rare-earth elements and precious metals, and micromineralogy are presented. The objects of study are late Permian sedimentary and volcanosedimentary deposits of the Tikhonya Brook (Atkan (P3at) and Omchak (P3om) formations) and hydrothermally metamorphosed rocks of the Natalka and Pavlik gold deposits of the Omchak ore–placer cluster. Analysis of the deposit ores showed enrichment in chalcophile trace elements Au, Ag, As, W, and Sb relative to their average contents in the upper crust and the host Permian rocks. The high contents of W and Bi in the ores suggest the participation of a magmatic fluid. The absence of abnormal contents of Ni, Co, Sb, Mo, Cr, and Se indicates the redeposition of these elements from ore-bearing rocks, without their input by ore-forming fluids, which is confirmed by the isotopic composition of sulfide sulfur and the characteristics of carbonaceous ore material. The formation of deposits proceeded with a change in REE contents. All objects show similar trace-element patterns: The rocks are enriched in LREE and lack a Ce anomaly. The identical REE patterns of ores reflect their inheritance from unaltered late Permian deposits. It has been established that the ores formed under different redox conditions, mainly with the participation of a relatively oxidized fluid enriched in LREE of the hydrothermal system NaCl–H2O, with domination of Cl over F. The studies have shown that the host carbonaceous sedimentary complexes, which served as additional sources of precious and associated metals, played a crucial role in the formation of the Natalka and Pavlik gold deposits. Some of the ore elements in the unaltered deposits form their own minerals.

Structural characteristics of the “Puquios chaos” and its relationship with the Andean middle Cretaceous extensional tectonics at 27°S, northern Chile

In the Precordillera of the Atacama region (~27°S), the Puquios chaos corresponds to a singular tectonic unit traditionally ascribed to a mid-Cretaceous extensional stage affecting the Mesozoic formations that were deposited between the arc and the backarc settings of the Andean margin. The mélange-like unit consists of blocks of limestones in a mainly volcanic matrix that includes andesitic lavas, volcanic breccias and red vulcarenites of Upper Jurassic to Lower Cretaceous age. These rocks constitute a conformable succession of andesites, limestones and vulcarenites (from the base to the top) in the Cerro Águila strata, observed to the south of the study area.The Puquios chaos is characterized by often isolated, tight to isoclinal folds and a set of low- and high-angle normal faults that were responsible for the block-in-matrix mesoscopic structure of the tectonic unit. These structures are easily observable in the calcareous rocks as the volcanic matrix shows a tectonically brecciated texture. The detailed structural study presented in this work allows us to propose the deformational mechanisms that finally constituted the apparently chaotic unit, including the restoration of the structures related to late contractive stages. Thus, we suggest that the singularity of the Puquios chaos is associated with a local, wide, extensional shear zone linked to a regional detachment surface. The intimate relation between recumbent folds and low- and high-angle normal faults, besides the similar orientation of fold hinges and fault planes, point to the actuation of a brittle-ductile shear zone where the calcareous competent layers gave rise to decametric-scale folds as the volcanic matrix responded to strain by brecciation. Stretching calculations carried out in a domino fault system yielded horizontal extension of around 30% for the high-angle faults.We obtained a relative age of the Puquios chaos formation according to the field relations and the structural determinations. A NE-directed intense extensional stage promoted a significant crustal thinning that exhumed but not affected the late Paleozoic basement. The Puquios structures and the related regional detachments affected the pre-Barremian arc and back-arc formations and were unconformably covered by the latest Cretaceous and Paleocene thick volcanic and volcanosedimentary deposits.

New U–Pb Ages of the Volcano–Plutonic Association of the Oloi Belt (Alazeya–Oloi Folded System, Western Chukotka)

The Egdygkych hypabyssal rocks of the Nichan and Vuknei massifs, as well as the host volcanites and tuffs of the Oloi volcanic belt of the Alazeya–Oloi folded system have been dated by U–Pb (SIMS and LA–ICP–MS). The obtained ages of the Egdygkych complex correspond to the Early Cretaceous (Berriasian–Valanginian), while those of their host rocks range from the Late Jurassic (Tithonian) to the beginning of the Early Cretaceous (Berriasian). These new U–Pb geochronological data confidently suggest a unity of the volcanic–plutonic association of the Late Jurassic (Tithonian)–Early Cretaceous (Berriasian–Valanginian) age of the Oloi volcanic belt. The data obtained refine the age range of Au–Mo–Cu ores confined to the contacts between the Egdygkych complex rocks and host volcano-sedimentary deposits or contacts of individual intrusive phases within the complex.