Deep Fault Zones Research Articles

Satellite monitoring of clouds over deep fault zones, short-term earthquake forecast and seismic activity in Ukraine

The article substantiates the possibilities of satellite monitoring of deep fault zones during the formation of a source of strong and catastrophic earthquakes and the manifestations of precursors of strong and catastrophic earthquakes - foreshocks, which manifest themselves in the form of cloud anomalies over a period of time from two weeks to several days. In recent years, cloud cover anomalies have been recorded before relatively weak earthquakes that occurred in southern Ukraine. It is very important that earthquakes occur in the Dnieper-Donetsk Depression (DDZ). An analysis of the tectonic map of the remote sensing data with the position of the epicenters of earthquakes that occurred in 2010 and 2015 showed that these earthquakes are confined to zones of deep faults or their intersections. The article proposed an earthquake prediction technology, which includes two stages. At the first stage, based on the results of spatial-temporal analysis, maps of the parameters of earthquakes (dates, coordinates of epicenters, depths of hypocenters) are built, which occur over a certain period of time, which allows you to determine the area of ​​increased seismic activity. At the second stage, research is being carried out in this area on satellite monitoring of cloud anomalies in order to short-term forecast strong earthquakes.

Evolution of the hydraulic properties of deep fault zone under high water pressure

Repeated water injection tests with varied injection flow rates are conducted on a fault zone under the roadway floor to study the evolution of the hydraulic properties of fault zone under high water pressure. Based on the analysis of test results, the evolution process of the hydraulic properties of fault zone under high water pressure can be divided into three successive stages: the initial infiltration stage, the splitting stage, and the scouring infiltration stage. It is found that in the splitting stage and the scouring infiltration stage, the hydraulic conductivity of fault zone increases rapidly under the condition of sufficient water supply, and this is likely to evolve into a large-flow-rate water inrush accident. Therefore, the safety factor e of fault zone should be defined as the ratio of the splitting pressure of fault zone Pf over the aquifer pressure Ph, i.e., e = Pf /Ph; when e < 1, water inrush may occur in the fault. Based on the results in this study, a new method is proposed for assessing the risk of fault.

Mapping S-Wave Attenuation Field of the North Tien Shan Region Using Seismogram Coda for Local Earthquakes and Quarry Blasts

We have mapped the short-period shear-wave attenuation field in the lithosphere of the North Tien Shan and analyzed seismograms of local earthquakes and quarry blasts, recorded by five stations installed in the area of Almaty and its nearest vicinities (including the site of the WWR-K nuclear reactor). Characteristics of common S coda envelopes for various sectors relative to seismic stations are described. Cross-sections of the attenuation field for the stations considered are constructed. Characteristics of the attenuation field to the north of the Zaili deep fault zone (southern margin of the Kazakh platform) and to the south of it (in mountain areas) are compared. This has allowed us to conclude about the different contents of deep-seated fluids in these areas. The maps of the attenuation field in various depth ranges and at different azimuths for all stations are made. The zones of the highest attenuation, where large earthquake can nucleate, are found. Recommendations are given regarding installation of additional stations in order to provide more operative and accurate determination of epicenter location of a possible earthquake and its magnitude for the safe operation of the WWR-K nuclear reactor.

CLOUD LINEAMENTS – INDICATORS OF FAULT ZONES OF THE EARTH

The results of investigation of the cloud lineaments interrelation with the activated zones of deep faults of the Earth by aerospace data are presented. It is emphasized that remote methods of research allow studying the interrelation of various geodynamic, hydrodynamic, tectonic, geophysical and other processes occurring in the lithosphere, hydrosphere and air envelope of our planet in specific parts of the Earth. It is noted that the existing litho-hydro-atmospheric relationships are determined by the geodynamics of the endogenous parts of our planet and the constantly changing energy states of the system “space – atmosphere – hydrosphere – lithosphere – the core of the Earth”. They are the main driving factors of the earth’s geodynamics, which cause the jet migration of energy, gases, heat, radiation and fluids from the deep shells of the planet to the atmosphere, where cloud ”autographs” of earth deep faults are formed.

To the problem of origin of the Bug series rocks

The age of the Bug series rocks is an object for discussion. Isotopic methods do not give a distinct answer, because in the same outcrop, the same sample and even in the same crystal of zircon several dates of formation are to be found with evident significance. Involvement of additional criteria is required to determine the time of the Bug series formation. While examining the phase composition and the initial source of the material we need taking into account temperature and pressure distribution in the depth of formation of rocks, which are on the surface nowadays. Mineral and chemical composition of rocks and tectonic structure of including patterns have been considered. It has been shown that the composition of the Bug series is an indicator of time of formation. Carbonatites and calcyphyres, carbonate-magnetite rocks are not produced earlier than ~2.0 Ga ago. A considerable part of the stratum is presented by high-clayish and graphite shale, quartzite, and ultrabasite that occur jointly only in this period of time. Arrangement of the structures is subdued to tectonic control by zones of deep faults. Smaller fault zones are determinative in the pattern of structures themselves and in manifestations of metasomatic processes. By the moment of their formation the level outcropped on the modern surface occurred at a depth more than 20 km that makes sedimentary nature of composing rocks doubtful. It has been shown that the most part of rocks attached to the Bug series are crystallization products of magmatic melts as well as carbonate, silicate (water-silicate with chlorine), hydrocarbon fluids in faults (permeable zones) of crystalline basement in diapason of PT -conditions, corresponding to high amphibolites-granulites metamorphic facies. The pattern of these structures in projection is similar to the section of volcano-tectonic apparatuses at a depth not less than 20 km. In the structures small by their area, the rocks formed at different depths (from 300 to 200 km) from melts and fluids in different chemical media are presented. High temperature multiple metasomatism has been superposed. A conclusion has been made that the pattern of the structures of Khashchevato-Zavalye block and the Golovaniv suture zone does not reflect the succession of primary sedimentary rocks occurrence but a result of multiple crystallization and metasomatic re-crystallization of melts and fluids.

Heat flow, heat production, thermal structure and its tectonic implication of the southern Tan-Lu Fault Zone, East–Central China

We present new heat flow and heat production data to constrain the lithospheric thermal structure along the southern Tan-Lu Fault Zone (STLFZ), a continuous active deep crustal fault zone in East Asia, and use these thermal data to infer how the thermal structure of the STLFZ influences regional focal depths. We report five new high-quality heat flow measurements derived from temperature–depth profiles at five well sites and detailed thermal conductivity measurements of 13 outcrop samples and 115 dry core samples from five wells along the STLFZ. We analyze these new data in combination with published data to derive a thermal profile along the STLFZ and map the 350 °C isotherm along the STLFZ, which correlates well with the maximum depth of recorded seismicity. The results indicate that: (1) the thermal gradient along the STLFZ increase from 22.2 °C km–1 in the south to 34.8 °C km–1 in the north; (2) heat flow values are in the 44.0–75.4 mW m–2 range along the STLFZ, with a mean value of 59.9 ± 10.5 mW m–2; (3) the highest heat flow value is located in Lu-Zong Basin, with extremely high radioactive heat production of 15–20 μW m–3 in the upper crust that may be related to local ore bodies; and (4) the bottom boundary of the shallow-earthquake seismic zone in the STLFZ coincides with the 350 °C isotherm.

Paleotemperature research of coals in the Volga-Ural oil and gas province (Russian Federation) in connection with forecasting of non-traditional hydrocarbon sources

Using of thermal methods of influence on the formation allows engaging in the extraction unconventional sources of hydrocarbons. One of the most important unconventional oil and gas objects are highly carbonaceous deposits. On the territory of the Volga-Ural oil and gas province, such objects is the Domanik Formation, which are clayey-siliceous-carbonate shale rocks with a high content of organic matter (OM). To identify and clarify the mechanisms of formation of oil and gas fields, as well as forecast and search for promising areas in the area of distribution of shale strata, the method of measuring the reflectivity of vitrinite is used. However, the absence of residues of plant organic matter in the Domanikites creates difficulties in applying the method. To assess the prospects of individual sections of the Domanik Formation, it is possible to use the paleotemperatures of the field of adjacent and overlying sediments, where coal seams and inclusions of plant organic matter are present. The basis of this approach is the paragenetic association of oil, coal and shale sediments. The formation of shale and coal-bearing formations is due to the primary conditions of sedimentation, and the formation of oil and gas deposits is associated with deep thermal effects during periods of tectonic activation on the territory. The structure of modern thermal fields, due to changes in temperature values, is controlled by the location of zones of deep faults in the crystalline basement of the platforms. Paleotemperatures are determined quite accurately by the degree of catagenetic transformation of organic matter (OM), which can be used to reconstruct the structure of ancient thermal fields in sedimentary formations and identify areas of maximum warming. Regional patterns of change in the degree of coal metamorphism can be used as a basis for forecasting and identifying promising areas of oil and gas shale raw materials.

Transverse Faults of the Eastern Caucasus and Their Manifestations in Seismicity

The features of the spatial distribution of earthquake sources in the eastern Caucasus (within the borders of Azerbaijan and the Chechen Republic of the Russian Federation) are analyzed. According to the method developed in recent years to identify seismogenic zones of deep faults by manifestations of weak seismicity, transverse faults extending in the diagonal direction (SW−NE) are found. Like other faults that are longitudinal to the strike of the mountain-folded structures of the greater Caucasus, they are characterized by the different level of seismic activity at the different segments. Only their separate segments of short length are seismically active. Transverse faults are deep and intersect the Earth’s crust through all its thickness. Some of them are associated with the sources of destructive earthquakes. For example, the Kurchaloi earthquake with M = 5.6 occurred on the northern slope of the greater Caucasus, in the eastern part of the Chechen Republic in 2008, which caused human losses and destructions. Its source was confined to the deep NE-striking fault, confining the mountain protrusion of the Dagestan wedge from the northwest. The strong seismic events Anapa of 1996 and Lower Kuban’-II of 2002 also took place on the western periclinal of the greater Caucasus, in the area of Anapa transverse flexural-fault zone. The longitudinal tectonic faults (of Caucasus strike) also demonstrate not only weak but also high magnitude seismicity. Such faults were associated with the sources of the strongest earthquakes in the late 20th and early 21st centuries (1970 Dagestan, 1977 Chernogorsk, 1991 Racha, 1992 Barisakho, 2000 Baku, and 2009 Oni). But the transcaucasian disjunctives for a long time have been underestimated as hazardous seismogenic structures, although their activation may lead to serious consequences. This work shows the role of such faults in the distribution of strong and weak seismicity of the region.

Analysis of the spatial patterns in localization of gold mineralization relative to the system of deep faults in the Chortomlyk greenstone structure of the Ukrainian shield

The results of analysisof the patterns of the spatial relationship between hydrothermal gold ore formations and the zones of metasomatites and systems of deep faults within the Chortomlyk greenstone structure of the Middle Pridniprovie megablock in the Ukrainian Shield are given. As a result of studying the localization conditions of gold mineralization, it was established that the gold-bearing mineralization is confined to tectonically fractured zones and is localized among metasomatically altered rocks. The hydrothermal series of metasomatites in the Chortomlyk greenstone structure is represented by greisens, propylites, amphibole-carbonate metasomatites and listvenite-berezites. The mineralization of Au and Mo is associated with metasomatites of the greisen type. The study of spatial relationship between the fields of metasomatites and the gold mineralization and zones of deep faults revealed that the closest relationship is expressed with systems of faults with azimuths of 0° and 270°, 17° and 287°, 77° and 347°. The results of the studies allowed us to develop a newprospecting criterion, which, in turn allows us to state that the gold mineralization within the Chortomlyk greenstone structure is spatially confined to metasomatites related to the schistosity, fracture, millonitization, and cataclase zones with high content of sulphide mineralization. These zones are localized in nodes of intersecting faults of the first order of the system 77° and 347° with discontinuous violations of high orders of azimuths of 0° and 270°, 17°and 287°. The results of the research can be used to develop a set of predictive criteria and the allocation of promising sites of hydrothermal mineralization of gold within the Chortomlyk greenstone structure of the Middle Pridniprovie megablock in the Ukrainian shield.

Conditions of localization and criteria for kimberlites exploration (on the example of Igyatta diamondiferous region, Western Yakutia)

The main source of mined diamonds in Russia is the Yakutia diamond-bearing province (YAP). To date, all open areas within the prospective Vilyuy-Markha deep fault zone (VMZ) of the YAP, which is a mineragenetic and which hosted diamond deposits of Mir, International, Nyurbinskaya, Botuobinskaya, have been studied satisfactorily. It is unlikely to reveal new kimberlite fields within its' opened part. In this case, the presence of direct signs of new objects beneath the Mesozoic-Cenozoic strata indicates that the diamond potential of the territory is not exhausted. This was confirmed by the discovery in 2015 of a kimberlite body in the Suldyukar field, which confirmed the presence of the Igyatta diamondiferous region and allows us to view the overlapped areas of the VMZ as the most promising for discovering of the new deposits. However, the search for buried deposits requires improvement of the criteria.

Dynamics of Metasomatic Transformation of the Rocks of the Lithospheric Mantle and Earth’s Crust in Deep-Fault Zones Controlling the Siberian Platform Trap Magmatism

Abstract —For a multirate approximation, we have determined the dynamics of rock heating by a magmatic-fluid flow in a flat permeable zone cutting the cratonic lithosphere of the Siberian Platform from a magma chamber at a depth of 50 km to the Earth’s surface. This dynamics is compared with the dynamics of infiltration metasomatism in a three-layer lithosphere section: (1) harzburgitic mantle (depth 50–40 km), (2) crystalline basement (39–7 km), whose composition was simulated by the section of rocks hosting the skarn deposits of the Aldan Shield, and (3) platform cover (6–0 km), with its simplified rock compositions specified on the basis of the rock compositions in the southern and northern parts of the trap area of the Siberian Platform. Numerical modeling of the metasomatic transformation of rocks was performed in a multireservoir flow reactor, using the Selektor software. The initial composition of fluids in a magmatic source varied from highly reduced (water–methane) to ordinary (water–acid) (lg pO2 from –13.0 to –12.0). The obtained balances of the interacting phases show no significant change in the mass of aluminosilicate rocks in the mantle and Earth’s crust sections and a significant loss of their mass under replacement of carbonate and sulfate deposits.

Geological-structural control and criteria of forecasting the types of primary sources of placer gold on the East of Siberian platform

Identification of indicators on placer gold allowed predicting formation types of primary sources of gold and their location in the East Siberian platform, overlapped by thick cover of Meso-Cenozoic deposits, where traditional methods of prospecting of gold deposits are ineffective. Each gold-ore formation is known to have the corresponding geological-structural control. Based on study of mineralogical-geochemical features of placer gold and regularity of its distribution, it has been determined that primary sources of low-sulfide gold-quartz, gold-copper-porphyritic, gold-ferruginous-quartzite, gold—rare-metal and gold-platinoid formations are confined to basement outcrops and their framings; gold-sulfide-quartz formation — to metasomatites, developed within the zones of deep faults, tracing the terrigenous-carbonate series; and manifestations of gold-silver formation are localized in the zones of intracontinental paleorifts. In general, identification of geological-structural location of specific types of gold sources favors more correct selection of prospecting methods for gold deposits in areas with thick sedimentary cover, and evaluating their potential.

3‐D Modeling of Induced Seismicity Along Multiple Faults: Magnitude, Rate, and Location in a Poroelasticity System

AbstractUnderstanding of the potential to injection‐induced seismicity along faults requires the response of fault zone system to spatiotemporal perturbations in pore pressure and stress. In this study, three‐dimensional (3‐D) model system consisting of the caprock, reservoir, and basement is intersected by vertical strike‐slip faults. We examine the full poroelastic behavior of the formation and perform the mechanical analysis along each fault zone using the Coulomb stress change. The magnitude, rate, and location of potential earthquakes are predicted using the spatial distribution of stresses and pore pressure over time. Rapid diffusion of pore pressure into conductive faults initiates failure, but the majority of induced seismicity occurs at deep fault zones due to poroelastic stabilization near the injection interval. Less permeable faults can be destabilized by either delayed pore pressure diffusion or poroelastic stressing. A two‐dimensional (2‐D) horizontal model, representing the interface between the reservoir and the basement, limits diffusion of pore pressure and deformation of the formation in the vertical direction that may overestimate or underestimate the potential of earthquakes along the fault. Our numerical results suggest that the 3‐D modeling of faulting system including poroelastic coupling can reduce the uncertainty in the seismic hazard prediction by considering the hydraulic and mechanical interaction between faults and bounding formations.

The identification of fault zones in deep karst aquifer of North China coal mine using parallel directional well logs

Water inrush from Ordovician karst aquifer in North China coal mine has become a key issue on underground coal mining. To address this problem, it is necessary to identify fault zones, because fault zones might connect limestone aquifers and coal seams, enabling Ordovician karst water to enter the mine. In the study area, a series of parallel directional holes were drilled along Ordovician limestone at depths between 70 and 90 m under Ordovician limestone boundary. To conveniently detect fault zones and govern mine water disasters, a series of natural gamma-ray logging while drilling (GRLWD) were undertaken. The entire detecting region can be comprehensively covered by several directional borehole groups. Then, fast Fourier transform and short-time Fourier transform approaches were employed on the basis of GRLWD data and geological data to extract faults information. A segmented identification method for deep fault zones was established in this study. This method can be used to markedly improve the identification of fault zones within Ordovician limestone or the unitary lithology formation and provide crucial information relevant for deep coal mining safety.

Heating mechanism of the Abgarm–Avaj geothermal system observed with hydrochemistry, geothermometry, and stable isotopes of thermal spring waters, Iran

In this study, the Abgarm–Avaj geothermal system in Iran is investigated by analyzing hydrochemistry and stable environmental isotopes of water samples collected from cold and thermal water springs and the Khare-Rud River together with tectonic settings. The findings reveal that the geothermal system is associated with the deep fault zone of Hasanabad and can be categorized into a convection dominated and non-magmatic geothermal system in accordance with the catalog of geothermal play types presented by Moeck (Renew Sustain Energy Rev 37:867–882, 2014). In fact, local rainfall that is occurred over the Kuhe-Bozorg limestone highlands percolates into a high depth along the main active fault of Hasanabad and then heats and emerges finally at the lowest topographic elevation of the fault in the form of thermal springs characterized by temperature ranges from 30 to 52 °C. The water samples from the thermal springs are of a high electrical conductivity value (ranges from 6585 to 11265 µS/cm) with the chloride water type. The lower circulation depth of meteoric water in the geothermal system is estimated to be about 3000 m by considering the possible maximum geothermal gradient of about 46 °C/km. The stable isotopes ratios analysis suggests that thermal water originates predominantly from rainfall occurring over the higher elevations, since the oxygen-18 ratios of the thermal spring waters are depleted than that of the cold spring waters. The equilibrium temperatures of the geothermal system are estimated via using the Na–K (Truesdell, Summary of section III: geochemical techniques in exploration. In: Proceedings of the 2nd U.N. symposium on the development and use of geothermal resources, vol 1. U.S. Government Printing Office, Washington, DC, pp liii–lxxx, 1976) and Na–K (Tonani, Some remarks on the application of geochemical techniques in geothermal exploration. In: Proceedings of advances in European geothermal research, 2nd symposium, Strasbourg, pp 428–443, 1980) geothermometers are 142–148 and 146–153 °C, respectively, which fall within the temperature range suggested by the mineral saturation indices (137–160 °C) and by the warm spring mixing model (135–164 °C) for the thermal spring waters. Furthermore, the results show that geothermal hot water mixes predominantly with shallow cold groundwater during ascending, where the portion of the cold shallow and deep-hot waters is about 70 and 30%, respectively.

The relation between heat flow obtained by curie depth point (CDP) and the faults and gas leaks in the black sea

CDP values were measured. They range from 22km to 38km. The deepest CDP values are observed in the Black Sea basins where the sediment thickness is high, while the shallowest values are observed in the middle Black Sea ridge, and in the thin sedimentary regions of the coast. The Central Black Sea ridge generally ranges from 22 to 28 km. From the CDP values, the heat gradient and corresponding heat flow values were also obtained. Heat flow values of the Black Sea basin range from 22 to 41mW/m2. When the relationship between the deep faults and gas leaks with the heat flow is examined, it is observed that the possible deep fault zones and gas migration places correspond to a higher heat flow value than the average heat flow.

A Formula of Potassium Polymer Sulfonated Drilling Fluid Developed Based on Rock Microscopic Analysis

During the rapid coring drilling in the Wenchuan earthquake Fault Scientific Drilling project, the deep fault zone is characterized with extremely broken strata and strong creeping, which made the drilling tasks very difficult. The key technology to drilling success is a suitable drilling fluid system. To design such a drilling fluid system with desired properties, the mineral composition of rock samples from deep fault zone was analyzed using X-Ray Diffraction (XRD) and Fourier Transform Infrared (FTIR) absorption spectrum. Based on the results, a potassium-based polymer sulfonated drilling fluid was developed. This drilling fluid system possesses a variety of advantages including good quality of filter cake, strong anti-pollution ability, anti-collapse ability and inhibition ability, and great compatibility with treating agent. Also, the composition of this drilling fluid is simple, easy to prepare and at a relatively low cost. The study of potassium sulfonated drilling fluid system is of great significance for the rapid core drilling in deep fault zone.

Fluid regime and ore water of bitumo-hydrothermal mineral associations in the conditions of Western Donetsk graben

The fluid regime of bitumen-hydrothermal mineral associations on the ore fields of the Western Donetsk graben, which are part of the Dnieper-Donetsk paleorift, has certain geochemical features. It is conditioned by the peculiarities of geological development, structure and the current level of tectonic activity of the region. The purpose of the article is to study the fluid regime of bitumen-hydrothermal mineral associations and the participation of hydrocarbons in the hydrothermal process. The research methodology consisted of a theoretical interpretation of laboratory studies, in particular chemical analyzes of rocks, ore minerals, bitumen, groundwater and gases, as well as isotopic analysis of carbon of solid bitumen. Research results. Field studies have found that bitumen-hydrothermal mineralization in the rocks is naturally timed to tectonically activate anticlinal structures. The energy source of its formation are the polychronous processes of heat-and-mass transfer, the power of which is determined by the tectonic activity of the zones of deep faults. In the rocks of the region, with the specified type of ore mineralization (mercury and mercury-polymetallic), the halo waters of a certain geochemical specialization are formed, which include chemical elements of ore and impure minerals and bitumen, as well as endogenous fluids. Among such fluids is the carbon of solid hydrocarbon compounds, the isotopic composition of which indicates its inorganic origin. The presence of bitumen-hydrothermal mineral associations of chemical elements and gases in halo waters of deep origin indicates that hydrothermal systems of the ore fields of the Western Donetsk graben are on the modern, post-hydrothermal stage of development. The scientific novelty of the material presented in the article is based on several substantive theoretical conclusions, which are based on the actual material of the research carried out by the authors. The natural affinity of bitumen-hydrothermal associations to tectonically activate structures located in the zones of deep faults has been proved. The affinity of inorganic carbon bitumino-hydrothermal mineral associations in solid bitumen and chemical elements present in gas jets and groundwater to metamorphic and mantle processes (He, H2, Hg, CO2, CH4, Li, Rb, Cs, etc.) is substantiated. , which are manifestations of modern tectonic activity of alpine tectogenesis. It was substantiated that the formation of the chemical composition of the halo waters of bitumen-hydrothermal mineral associations is provided both by exchange chemical processes in the system mineral-water-gas and by the income of fluids of deep genesis. The practical significance of the work is conditioned, on the one hand, by the use of ore hydrothermal mineralization in the rocks as a criterion for the search of hydrocarbon accumulations, and on the other hand, by the prediction of the possibility of hydrocarbon compounds upcoming from large depths in which, in abnormal thermodynamic conditions, an inorganic synthesis of hydrocarbons takes place.

A stand-alone Co mineral deposit in northeastern Hunan Province, South China: Its timing, origin of ore fluids and metal Co, and geodynamic setting

The Hengdong cobalt (Co) deposit, located in northeastern Hunan Province of South China, is hosted by the low-grade metamorphic volcaniclastic sedimentary rocks of the early Neoproterozoic Lengjiaxi Group. The Co orebodies strictly controlled by the NE- to ENE-trending Changsha-Pingjiang deep fault zone (CPDFZ) and its secondary structures. Occurring in altered breccias and cataclasites with similar mineral assemblages, Co mineralization is characterized by zoned alteration with predominant silicification and chloritization proximal to the orebodies, and sericitization and carbonatization distal from the mineralization. The integrated field and microscope observations reveal three hydrothermal stages marked by quartz + pyrite + muscovite ± chalcopyrite of the early-stage mineralization (E-stage), quartz + polymetallic sulfides + chlorite of the middle-stage mineralization (M−stage), and quartz + chlorite + carbonate of the late-stage mineralization (L-stage). Muscovite from both the E-stage Co-bearing altered breccia and the CPDFZ mylonite yield 40Ar–39Ar plateau ages of 124.7 ± 0.6 Ma (1σ) and 130.3 ± 1.4 Ma (1σ), respectively, indicating an early Cretaceous mineralization likely associated with the last movement of CPDFZ strike-slip shearing. The δ34S values of pyrite and chalcopyrite ranging from −1.5 to −15.9% with a majority between −7.5 and −15.9%, and the lead isotope compositions of the pyrite (206Pb/204Pb = 18.156–18.761, 207Pb/204Pb = 15.645–15.662 and 208Pb/204Pb = 38.469–39.172) overlapping with those of upper crust, indicate a main crust-derived source. The chemical compositions of pyrite further indicate the ore fluids and metal Co of the Hengdong deposit are most likely linked to the meta-mafic and volcanic rocks of the Neoarchean to Paleoproterozoic Lianyunshan Group, but with a contamination by the wall rocks of Lengjiaxi Group and Lianyunshan granitoids. Fluid inclusion investigations from Hengdong deposit reveal the decreasing homogenization temperatures from 250 to 320 °C (peak of 280–300 °C) at the E stage, through ∼220–320 °C (peak of 270–300 °C) at the M stage, and to ∼150–230 °C at the L stage without obviously salinity (7.0–15 wt% NaCl equiv.) changed. In the M−stage, the presence of coexisting LV inclusions and V-rich inclusions with the similar homogenization temperature, consistent with the chlorite geothermometry data, is interpreted to be the result of fluid immiscibility, which was caused by cyclic pressure release during fault-zone movement. Combined with the Late Mesozoic tectonism of South China, the present data support the Hengdong deposit formed under an extension-associated tectonic regime most likely induced by slab roll-back of the subducted Paleo-Pacific Plate during the early Cretaceous. This extensional event not only caused the reactivation of the pre-existing structures as manifested by the CPDFZ characteristic of stress transformation from compression to extension but also likely resulted in the release of large amounts of Co-bearing ore fluids from the Proterozoic or older volcanogenic rocks. When the ore fluids migrated along the CPDFZ and its secondary faults, the decompression (adiabatic cooling) of the hydrothermal fluids shifted the ore fluid to the immiscibility field, significantly reduced the degree of cobalt undersaturation, and caused cobalt to precipitate, which finally formed the Hengdong Co mineral deposit.

Heterogeneous structure of the lithosphere in the Black Sea from a multidisciplinary analysis of geophysical fields

Magnetic, gravity, geothermal, seismic and tomographic data from the lithosphere were first jointly examined. A multidisciplinary interpretation has resulted in a new and consistent model for lithospheric density, magnetic, thermal and velocity heterogeneities. Faults of different orders for the crystalline crust have been mapped in details. Large deep fault zones were recognized. Among them is the most prominent Odessa-Sinop-Ordu (OSO) fault zone, which played a key role in the opening and development of the Black Sea Depression. A fundamental difference was revealed between the crustal and mantle structure and geophysical parameters of the Western Black Sea Basin (WBSB) and Eastern Black Sea Basin (EBSB). These dissimilarities are in the size of «non-granitic» crust, pattern and intensity of heat flow, topography of the lower boundary of the thermal lithosphere, mantle seismic velocity and structure of magnetic and residual gravity anomalies. Based on new information it was demonstrated that the WBSB and EBSB were diachronously formed on two large distinct continental blocks with independent post-rift development of the sub-basins. The rifting of the western sub-basin commenced earlier than that of the eastern one. The EBSB is characterized by younger thermal activity than the WBSB and consequently it was stabilized later. The Mid Black Sea High (MBSH) is not a single tectonic unit but is formed by two ridges of various crystalline crustal structure and age shifted relative to each other by the faults of the OSO zone.