Field Geochemistry Research Articles

A new approach to assessment of supergene geochemical gold anomalies, exemplified by the Soldat massif (Murun massif of the Aldan shield)

Lithochemical prospecting by secondary scattering halos is considered in practice as attempts to locate ore body outcrops or to significantly reduce the search area. Even when all commonly accepted methodological principles are followed, the researchers rarely show interest in the identified anomalies, for which the vast majority of forecast resources are calculated. The problem lies in the fact that the calculated resources are usually not confirmed, and many anomalies are recognized as false or related to elevated Clarkes of the elements in the rocks. At the phase preceding the exploration drilling and mining workings, the main task is to determine the prospectivity of a certain anomaly. A methodological approach allowing one to justify the prospectivity of identified anomalies was developed using the Soldat massif as an example. Its essence consists in clarifying the contact interaction between the rocks of the massif and the host rocks by analyzing their covering hypergene analogues. Using factor analysis, based on the division of the main rock-forming elements, the spatial position is determined of the main rock types that make up the geological structure of the area. Within a zone of overlapping factors of the rock-forming elements, anomalies of Au, As, Cu, W, Co, Ni, and other elements are established. The localization of these anomalies within the contour of superposition of the rock-forming factors is considered a potential reflection of specific features of the contact interaction between rocks of different types and compositions. The main goal of the article is to attract the attention of researchers to similar situations so as not to miss weak anomalies. For example, positive significant correlations of ore elements are established in samples taken from the zone of overlapping rock-forming factors, while no correlations are observed in samples taken outside this zone. This indicates a unique geochemical environment that needs to be considered and used to predict potential mineralization. All subsequent constructions are related to geometrizing a detailed area in accordance with the identified zone where the main associations of chemical elements were established and the zonal structure of the secondary geochemical field of the massif was studied.

GEOLOGICAL CONDITIONS OF FORMATION AND DISTRIBUTION OF GAS-GEOCHEMICAL FIELDS IN BOTTOM SEDIMENTS OF THE LAPTEV-SIBERIAN SEA ZONE OF THE EAST ARCTIC SHELF

Gas composition of bottom sediments of the Laptev-Siberian Sea geostructural zone (LSZ) was determined to include: helium and hydrogen in concentrations of 0.0028–0.2092 and 0.0012–0.8727 cm3/kg, hydrogen sulfide – 0.0006–0.0072, carbon monoxide – 0.0016–0.0577, carbon dioxide – 0.1482–21.1602, methane – 0.0017–8.3047 and its homologues (in total) – 0.00001–0.0355 cm3/kg. It was found that the values of average concentrations of gases in bottom sediments increase with the depth of their occurrence and exceed the anomaly criteria identified for the East Arctic shelf by 2.6–27.7 times, including: CO2, CO and H2S – by 2.6, 3.0 and 3.4 times, ΣC2–C5 and He – by 9.3 and 10.8 times, CH4 – by 27.7 times; this actually indicates a high degree of gas saturation of bottom sediments of the region. The formation of CO2, CO, H2S, H2, CH4 and ΣC2–C5 anomalies in sediments of the upper and the base of the middle sampling horizon is determined by relatively high contents of sapropelic and humic organic matter and gas influx from the lower horizon. Distribution of anomalies with maximum concentrations of natural gases and He in sediments of the lower horizon, with minimum contents of Corg, is associated with zones of faults, large dislocations, tectono-magmatic and seismic activity, which are the main routes of diffusion-migration of gas transfer to bottom sediments. Formation of concentrations of gases and their anomalies in sediments of the LSZ follows the rules of additivity, i.e. successive accumulation of migratory natural gases of different genesis with prevalence of gas phase and isotopic indicators of more gas-saturated parental source, epigenetic and syngenetic to sediments. It was found that maximum values of average gas saturation with CH4, ΣС2–С5, He and H2 in LSZ are typical for bottom sediments of the flanks of troughs, CO and CO2 – for the arch parts of uplifts. In sediments of monoclinal folds (structural terraces), minimal values of average gas saturation are observed, with the exception of the South Anyui Fault Zone. In general, gas saturation of LSZ sediments is determined by the complex impact of geological factors, with the major ones being: gas content of rock complexes and gas saturation of underlying sediments, fault and fold tectonics, a high degree of tectono-magmatic and seismic activity, a geostructural position, coal content, oil and gas content, as well as the depth of occurrence, organic saturation and material composition of OM in bottom sediments. The influence of the latter predetermined the formation and distribution of five geochemical fields of CH4, three of CO, ΣС2–С5, He and one of CO2, and one of H2 in LSZ.

Analysis of spatial structure and singularity of microbial biogeochemical anomalies in the Taiwan Strait Basin

The abundance of specific hydrocarbon-oxidizing bacteria (MV value) in surface soil is closely linked to the concentration of thermogenic light hydrocarbons found in the underlying layers. The identification of these bacteria using specific detection techniques offers unique insights into microbiological anomalies associated with oil and gas, underscoring the significance of delineating microbiological geochemical anomalies in the exploration process. Accurate determination of the anomaly range is crucial for forecasting potential oil and gas reservoirs. In this study, a variety of fractal and geological statistical methods were employed to analyze microbiological geochemical data from the Taiwan Strait Basin. The multifractal approach allows for the simultaneous assessment of spatial autocorrelation and singularity in geochemical fields, with the fractal distribution reflecting the localized enrichment and depletion patterns of microbiological geochemical elements in rocks and other substrates. Geological statistical methods, such as the variation function, enable the assessment of spatial autocorrelation in geochemical and geophysical fields. The results indicate that microbiological anomalies in the Jiulongjiang Depression are mainly concentrated in the southwestern part of the study area, with some anomalies also observed in the central region. Conversely, microbiological anomalies in the Jinjiang Depression are predominantly located in the southern portion of the study area. Microbiological anomalies in the study area (Jinjiang Depression and Jiulongjiang Depression) exhibit a northeast-southwest strip distribution, aligning with the trend of the depressions and faults, suggesting that faults could influence the distribution of oil and gas resources in the region. Based on microbiological statistical analyses, the Jiulongjiang Depression shows better potential for oil and gas prospects compared to the Jinjiang Depression, guiding future research efforts in the Jiulongjiang Depression.

USE OF DIGITAL MODELS OF GEOLOGICAL STRUCTURE TO IDENTIFY PALEOVOLCANIC STRUCTURES CONTROLLING PORPHYRY ORE OBJECTS

The article is devoted to the implementation of the methodology for paleovolcanic structure analyzing using 3D visualization of the model of the geological territory structure based on the relief digital model. The approach used made it possible to identify two paleovolcanic apparatuses and their structural elements: cone fragments, explosive domes, porphyritic stocks, ring fault zones with localization of dike belts in them. This, within the framework of the evolution model of porphyry ore systems, made it possible to identify promising areas for prospecting on ore objects of a given type. The procedure was checked on the basis of geochemical survey results. The regular control by the selected structural elements of geochemical fields was shown. The analysis of the spatial position of the selected structural blocks with a certain geochemical characteristic rather well into the evolution model of the porphyry ore systems. In all cases the control of geochemical halos by porphyry stocks is noted, the material rock complexes that are fragments of the zonal stone hat of the porphyry ore object, are quite clearly distinguished. Within the area of work, three ore-magmatic systems can be identified at a low level of erosion shear, potentially accompanied by porphyry-type ore mineralization.

Matrix Selection Strategies for MALDI-TOF MS/MS Characterization of Cyclic Tetrapyrroles in Blood and Food Samples.

Cyclic tetrapyrrole derivatives such as porphyrins, chlorins, corrins (compounds with a corrin core), and phthalocyanines are a family of molecules containing four pyrrole rings usually coordinating a metal ion (Mg, Cu, Fe, Zn, etc.). Here, we report the characterization of some representative cyclic tetrapyrrole derivatives by MALDI-ToF/ToF MS analyses, including heme b and c, phthalocyanines, and protoporphyrins after proper matrix selection. Both neutral and acidic matrices were evaluated to assess potential demetallation, adduct formation, and fragmentation. While chlorophylls exhibited magnesium demetallation in acidic matrices, cyclic tetrapyrroles with Fe, Zn, Co, Cu, or Ni remained steadfast against demetallation across all conditions. Phthalocyanines and protoporphyrins were also detectable without a matrix using laser desorption ionization (LDI); however, the incorporation of matrices achieved the highest ionization yield, enhanced sensitivity, and negligible fragmentation. Three standard proteins, i.e., myoglobin, hemoglobin, and cytochrome c, were analyzed either intact or enzymatically digested, yielding heme b and heme c ions along with accompanying peptides. Furthermore, we successfully detected and characterized heme b in real samples, including blood, bovine and cod liver, and mussel. As a result, MALDI MS/MS emerged as a powerful tool for straightforward cyclic tetrapyrrole identification, even in highly complex samples. Our work paves the way for a more comprehensive understanding of cyclic tetrapyrroles in biological and industrial settings, including the geochemical field, as these compounds are a source of significant geological and geochemical information in sediments and crude oils.

Biogeochemical lichen indication study of formation and temporal changes of Oleksandriia Arboretum atmospheric pollution

Using the method of biogeochemical lichen indication the levels of the average long-term content of K, Ca, Al, Fe, Mg, Na, Mn, Zn, Sr, Ti, Ba Cu, B, Pb, Ni, V, Cr, Co, Cd, Se, Sb in surface air were measured on the territory of Olexandriia Arboretum and adjacent territories (Bila Tserkva, Kyiv Region, Ukraine). The purpose of the study is to determine the state and dynamics of the atmospheric geochemical field on the territory of Olexandriia Arboretum and its surroundings. The method of biogeochemical lichen indication based on the values of micro- and macroelements content in epiphytic lichens was used as a research tool. Application of the ICP-OES spectroscopy method allowed the analysis for chemical elements content of the samples of epiphytic deciduous Parmelia sulcata, Xanthoria parietina and bushy Evernia prunastri lichens collected in 2009 (20 points) and 2021 (23 points).
 In order to identify possible sources of entering of certain elements into lichens the researchers used the factor analysis method, i.e. a method of principal components with Varimax factors rotation. Six obtained factors represent: the impact of land cover (the most significant factors F1–F3) and man-made components (minor polygenetic, factors F4–F6). As a factor's number decreases, its partial explanatory variance decreases too. Factor F1 represents a common dust loading on the atmosphere with a predominance of the terrigenous component (mostly coarse-dispersed particles). Factor F1 is responsible for the dust-aerosol loading that is largely caused by the microclimatic circulation of the city breeze type thus forming a peculiar atmospheric and geochemical association that consists of chemical elements related to the building industry and building materials production. Powerful, however, local (impactful) atmospheric man-made influences manifest themselves through factor F3. Minor factors' discrimination ability reduces if a source of atmospheric pollution is less significant. Both division of factors into subfactors and noticeable temporal dynamics are observed more often. For instance, factor F4 has two geochemical sub-associations affected by the impact on the Olexandriia's landscapes of the 1990s' ecological disaster (fall-out of oil products, geochemical marker – V) and galvanic waste causing soil pollution (Zn, Mg, Cr, Ni and Ti), as well as discharges of agricultural enterprises (K). One of the peculiarities of F4 is a gradual decrease in its intensity over time. Factor F5 should be interpreted as the only load on the environment associated with galvanic pollution with cobalt being replaced by copper as a leading pollution marker. We suggest considering Factor F6 as a factor associated with oil pollution only.

Application of geochemical methods for direct prediction of hydrocarbon deposits in Western Yakutia

Based on the materials previously carried out by various types of geochemical surveys (water-gas, hydrochemical, litho-gas-geochemical, snow, atmochemical, bituminological sampling), an analysis of the oil and gas potential prospects of the eastern part of the Siberian platform within the Sakha Republic (Yakutia) established a direct relationship between the anomalies and the proven oil and gas potential of the studied territories. The indicators of geochemical field anomalies are an order of magnitude higher within territories with proven oil and gas potential (Nepa-Botuobinskaya anteclise) than in the territories where they are absent, including oil and gas fields that have been discovered to date (Syugdzherskaya structural low). A striking example is the geochemical research carried out within the Chayanda oil and gas condensate field, where the confirmation of deposits by drilling, using geochemical data, increased by 30 %, which indicates the validity of the use of geochemical analysis methods in the search for oil and gas. Based on the analysis of the data presented, when predicting and searching for hydrocarbon accumulations, the most rational approach is to carry out mandatory advanced zoning of large territories according to the level of geochemical background.

Analysis of marine diversity and anthropogenic pressures on Seixas coral reef ecosystem (northeastern Brazil)

Coral reefs, vital and ecologically significant ecosystems, are among the most jeopardized marine environments in the Atlantic Ocean, particularly along the northeastern coast of Brazil. The persistent lack of effective management and conservation has led to fragmented information on reef use and pressures, hindering the understanding of these ecosystems' health. Major difficulties and challenges include inadequate data, diverse anthropogenic pressures, and the complex interaction between marine species. This study sought to bridge this knowledge gap by conducting a comprehensive analysis of marine diversity and anthropogenic pressures, specifically focusing on Seixas coral reef near João Pessoa city, an area notably impacted by tourism. Utilizing 25 monitoring transects, subdivided into 1 m2 quadrants, the marine diversity was meticulously evaluated through innovative procedures including (a) sedimentological and geochemical field surveys, (b) application of Shannon-Weaver diversity and Simpson dominance indices, (c) cluster analysis, (d) species identification of macroalgae, coral, and fish, and (e) an examination of anthropogenic interactions and pressures on the coral reef. The assessment encompassed three distinct zones: Back Reef, Reef Top, and Fore Reef, and identified a total of 25 species across 15 genera and 10 fish families. The findings revealed the prevalence of brown macroalgae, fish, and coral, with heightened abundance of red macroalgae in the Fore Reef, which also exhibited the greatest diversity (2.816) and dominance (0.894). Original achievements include the identification of specific spatial variations, recognition of the anthropogenic factors leading to ecological changes, and the formulation of evidence-based recommendations. The study concludes that escalating urbanization and burgeoning daily tourist visits to the reef have exacerbated negative impacts on Seixas coral reef's marine ecosystem. These insights underscore the urgent need for strategic planning and resource management to safeguard the reef's biodiversity and ecological integrity.

Editorial: Volcanism in the Central Volcanic Zone of the Andes

This Research Topic was born out of the 1 st Symposium on the Central Volcanic Zone of the Andes 21 (CVZA; carried out in virtual mode during 2021) which counted for the presence of geoscientists 22 working in the volcanological, petrological and geochemical fields, among the others. Our objective 23 was to bring together contributions focused on the evolution and impact of volcanism in the CVZA 24 from its formation (Late Oligocene) up to present times, including specific and multidisciplinary 25 studies, with both detailed and regional approaches. The nine articles published in this Research Topic 26 cover a spectrum of topics applied in different areas of the CVZA, including poly-and monogenetic 27 volcanism, the influence of the crust in magmatic processes, fluid geochemistry, volcano seismicity, 28 petrology, geochronology, hazards assessment, relationships between volcanoes and 29 archeology/anthropology/sociology. 30 Barry et al. (2022) compare the degassing processes between the Central and Southern volcanic zones 31 of the Andes using He and carbon isotopes, and CO2/ 3 He ratios obtained from gas and water samples 32 related to fumaroles and thermal springs. They conclude that crustal thickness exerts a primary control 33 on the extent of fluid-crust interaction, as helium and other volatiles rise through the upper plate in the 34 Andean convergent margin, with more significant crustal inputs in the CVZA due its thicker crust. 35

ОСОБЕННОСТИ ГАЗОГЕОХИМИЧЕСКИХ ПОЛЕЙ МЕТАНА В РАЙОНЕ ЗАЛИВА ПЕТРА ВЕЛИКОГО (ЯПОНСКОЕ МОРЕ)

The paper presents the comparison results of detailed gas-geochemical surveys carried out in the Alekseev Bight (Peter the Great Bay, Sea of Japan) in 1989 and 2022 aimed at studying the distribution of methane and carbon dioxide in waters of the Alekseev Bight. CTD sounding was performed to measure temperature and salinity using an YSI CastAway CTD probe. Based on the available geological information and prior research, the main patterns were identified in the distribution of gas geochemical fields in the Alekseev Bight (Peter the Great Bay). The results obtained are in demand both in theoretical studies of gas geochemical field distribution on the earth’s surface in connection with seismotectonic and geological features of the region, and in the practice of development of methods and technologies for measuring balance between major greenhouse gas emissions and uptake. The results are also important for the implementation of research programs within the «climate agenda».

Prospecting terrain for surface geochemical exploration of oil and gas in West Siberia

ABSTRACT The relevance of this research is necessitated by the need to advance geochemical prospecting methods for raw hydrocarbon deposits. The study aimed to design new migration models by the interpretation of surface geochemical survey data. In particular, the relationship behavior needs to be identified between the hydrocarbon migration intensity and the presence of oil-bearing reservoirs within the section. To reveal quantitative relationships between the oil show and the oil-bearing capacity of the section, a substantial body of geochemical data across different fields in West Siberia was processed herein. Field-type and in-office operations were being performed by the snow survey procedure. The СН4–С6Н14 saturated hydrocarbons were employed as the major measured parameter. No direct quantitative connection was found between the intensity of surface hydrocarbon anomalies and the section productivity. Nonetheless, the two basic factors were found to affect the structure of the surface geochemical field: first, oil-bearing reservoirs present in the section and, second, fluid-resistant properties of the above-productive strata. Despite a direct quantitative relationship between the oil-bearing capacity of the section and the intensity of surface hydrocarbon anomalies being absent, the large sample made it possible to determine quantitative measures of the surface geochemical field that credibly characterize territories whose sections hold oil-bearing horizons.

Unraveling overprinted formation mechanisms of massive dolostone in the Lower Triassic sequence of an isolated carbonate platform in Nanpanjiang Basin, south China

Massive dolomitization is common in carbonate platforms but determining the causes of dolomitization remains challenging. A particular difficulty lies in identifying cases where petrographic and geochemical attributes of dolostone related to one mechanism could be obscured by a later, different one. To better understand whether traditional approaches are sufficient to unravel the origins of dolostone resulting from successive, different mechanisms, this study investigates the formation mechanism(s) of dolostone along a platform-to-basin transect of a Permian-Triassic isolated platform in the Nanpanjiang Basin. The dolostone in the Lower Triassic succession comprises three dolomite phases that can be distinguished through field relationships, petrography, 87Sr/86Sr ratios, and microthermometry. Dolomite type 1 formed due to the reflux of platform-top evaporated seawater that flowed through the platform interior in the Early Triassic. Dolomite types 2 and 3 are interpreted to have formed at elevated temperatures during or after Late Triassic platform burial and to have played a secondary role in forming the dolostone. The dolomitizing fluids that resulted in the formation of dolomite types 2 and 3 were derived from Early Triassic seawater-like fluid that was expelled from the Lower Triassic basinal carbonate sediments and moved updip to the platform interior. Dolomitized clasts in partially or non-dolomitized slope breccias demonstrate pre-burial timing of dolomite type 1, and distinguish the earlier dolomitization from later, post-burial dolomitization represented by dolomite types 2 and 3. Dolomite type 1 retains its Early Triassic seawater δ13C and 87Sr/86Sr signatures, whereas overlapping geochemical fields of the three types of dolomite (trace element concentration, δ18O) imply that burial dolomitizing fluids locally reset the geochemistry of dolomite type 1. This finding suggests that the same dolomite archive may retain well-preserved or altered data depending on the specific geochemical proxy and that identifying individual dolomitization mechanisms using geochemical proxies is possible only in some cases.

Effects of Deep Fluids on Middle Permian Dolomite of the Western Sichuan Basin

Deep fluid activity is closely related to carbonate reservoir transformation as well as oil and gas accumulation. The large-scale deep fluid activity caused by Emeishan large igneous province (ELIP) brought a lot of deep material and energy to Sichuan Basin and changed the paleogeothermal and geochemical field of the region they flowed through, which had an important impact on the formation of the Middle Permian dolomite reservoir in Western Sichuan. Through comprehensive analyses of petrographic, geochemical, and fluid inclusion data from two drill cores and three outcrop sections of the Middle Permian Formation in the Western Sichuan Basin, the dolomitizing fluid, dolomite genesis, and effect of deep fluid were discussed. The dolomite was divided into four types which are matrix dolomites (Md1, Md2, and Md3) and cement dolomites (Cd). Among them, the Md1 was formed in penecontemporaneous seawater by seepage reflux dolomitization, while Md2 was formed in heated Permian seawater through thermal convection. In the northwest of the study area, the Md3 and Cd were formed by higher-temperature thermal convection without hydrothermal. In the southwest of the study area, the lower δ13C value and Sr contents, higher Mn contents and 87Sr/86Sr ratios, and positive Eu anomalies as well as much higher Th and salinity values of inclusions show that hydrothermal participated in the dolomitization, forming the Md3 and Cd through structure-hydrothermal dolomitization and the thermal convection of seawater mixed with hydrothermal. The deep fluid activities of ELIP provided both the material source (Mg2+) for the dolomitization process and the migration motive power for the dolomitization fluid. From the southwest to northwest, with the deep fluid activity decreasing, the abundance of Md3 and Cd, which are closely related to the hydrothermal solution, reduced.

A New look at the nature of the geochemical field and the halo-forming system

A New look at the nature of the geochemical field and the halo-forming system

Introducing a Novel Approach for Oil‐Oil Correlation based on Asphaltene Structure: X‐ray Diffraction

AbstractAsphaltenes have always been an attractive subject for researchers. However, the application of this fraction of the geochemical field has only been studied in a limited way. In other words, despite many studies on asphaltene structure, the application of asphaltene structures in organic geochemistry has not so far been assessed. Oil‐oil correlation is a well‐known concept in geochemical studies and plays a vital role in basin modeling and the reconstruction of the burial history of basin sediments, as well as accurate characterization of the relevant petroleum system. This study aims to propose the X‐ray diffraction (XRD) technique as a novel method for oil‐oil correlation and investigate its reliability and accuracy for different crude oils. To this end, 13 crude oil samples from the Iranian sector of the Persian Gulf region, which had previously been correlated by traditional geochemical tools such as biomarker ratios and isotope values, in four distinct genetic groups, were selected and their asphaltene fractions analyzed by two prevalent methods of XRD and Fourier‐transform infrared spectroscopy (FTIR). For oil‐oil correlation assessment, various cross‐plots, as well as principal component analysis (PCA), were conducted, based on the structural parameters of the studied asphaltenes. The results indicate that asphaltene structural parameters can also be used for oil‐oil correlation purposes, their results being completely in accord with the previous classifications. The average values of distance between saturated portions (dr) and the distance between two aromatic layers (dm) of asphaltene molecules belonging to the studied oil samples are 4.69Å and 3.54Å, respectively. Furthermore, the average diameter of the aromatic sheets (La), the height of the clusters (Lc), the number of carbons per aromatic unit (Cau), the number of aromatic rings per layer (Ra), the number of sheets in the cluster (Me) and aromaticity (fa) values of these asphaltene samples are 10.09Å, 34.04Å, 17.42Å, 3.78Å, 10.61Å and 0.26Å, respectively. The results of XRD parameters indicate that plots of dr vs. dm, dr vs. Me, dr vs. fa, dm vs. Lc, Lc vs. La, and fa vs. La perform appropriately for distinguishing genetic groups. A comparison between XRD and FTIR results indicated that the XRD method is more accurate for this purpose. In addition, decision tree classification, one of the most efficacious approaches of machine learning, was employed for the geochemical groups of this study for the first time. This tree, which was constructed using XRD data, can distinguish genetic groups accurately and can also determine the characteristics of each geochemical group. In conclusion, the obtaining of structural parameters for asphaltene by the XRD technique is a novel, precise and inexpensive method, which can be deployed as a new approach for oil‐oil correlation goals. The findings of this study can help in the prompt determination of genetic groups as a screening method and can also be useful for assessing oil samples affected by secondary processes.

Reconstructing the middle to late Pleistocene explosive eruption histories of Popocatépetl, Iztaccíhuatl and Tláloc-Telapón volcanoes in Central México

The Sierra Nevada Volcanic Range (SNVR), which includes Popocatépetl, Iztaccíhuatl and Tláloc-Telapón volcanoes, has been the source of multiple large explosive eruptions that have dispersed tephra across central México. Several eruptions since 40 ka have previously been described, particularly from Popocatépetl, the southernmost volcano of the range. However, the longer-term eruption history of the SNVR is poorly understood, due to challenges with correlating limited exposures of older pyroclastic sequences, and in discriminating between tephras derived from different sources. Here we describe two extensive exposures located between Popocatépetl and Iztaccíhuatl volcanoes, which provide a more complete and longer-term explosive eruption record of the SNVR: the Nepopualco and Xalitzintla tephra sequences. A detailed tephrostratigraphic survey, together with new 40Ar/39Ar geochronological analyses and glass geochemistry, has permitted the characterization of identified eruption units further leading to the determination of geochemical fields for each volcano and the subsequent discernment of volcanic sources. Our results show that, since the collapse of the Los Pies Cone, which destroyed the Paleo-Iztaccíhuatl edifice at 631 ± 44 ka (2σ), Iztaccíhuatl has produced at least 6 explosive rhyolitic eruptions. After coeval activity with Popocatépetl, between ~600 and ~500 ka, Iztaccíhuatl's explosive activity ceased while Popocatépetl's continued until present day. Popocatépetl has produced at least 27 medium to large explosive eruptions (inferred VEI 4–6), commonly of andesitic to dacitic compositions. Some of these eruptions deposited pumice fallout of >1 m thick in both the Nepopualco and Xalitzintla sequences (e.g. the 339 ± 16 ka [2σ] NT-23/WRT-7 eruption), suggesting that Popocatépetl has produced several eruptions similar in magnitude to the well-known ~14 ka Tutti Frutti Pumice (a VEI 6 eruption with a ~5 km3 tephra volume). The Popocatépetl and Iztaccíhuatl tephras are interbedded with deposits from more distal volcanoes, including some mafic to intermediate products of unknown sources (possibly from nearby monogenetic cones) and tephras related to the late Pleistocene eruptions of Tláloc-Telapón (including the tephra layer produced by the San Valentin Ignimbrite, recently 40Ar/39Ar dated in this study at ~102 ka; 2σ). Our new chemical, stratigraphic and geochronologic investigations of these pyroclastic deposits, predominantly from Popocatépetl and Iztaccíhuatl, provide information on the scale and frequency of medium to large magnitude explosive eruptions over a longer-time period than currently known and that have had potential to disperse tephra across central México since the middle to late Pleistocene. This new data can be used to determine the source of further unknown tephras in the region as well as to better assess the volcanic hazard to the densely populated megalopolis of México City.

Geological and geochemical specialization of ore-bearing formations

The geochemical and metallogenic specialization and zoning of structural-material complexes are analyzed. In the general systematic operational circuit of metallogenic forecasting and geological prospecting, processing the data on the geochemical specialization of geological formations and their constituents make it possible to perform a formational interpretation of anomalous geochemical fields revealed in the course of multipurpose geological and geochemical mapping; to subdivide the explored areas by the types of geological formations differing by their ore-bearing potential, with allocation of the most productive subformations, phases, and facies; and to provide quantitative estimates of the forecasted resources. Geochemical criteria of the ore-bearing potential of geological formations consist, first of all, in stable correlations between petrochemical features of ore-bearing rocks and their corresponding fluctuations of the ore composition (for example, the correlation of the potassium content of ore-hosting silicic volcanic rocks of the ore-bearing volcanogenic geological formations with the Cu/Pb ratio in ores of various types of deposits of the VMSD formational family). These criteria are to be taken into account in regional and local assessments of the perspective ore-bearing potential of geological formations.

APPLICATION OF STATISTICAL METHODS FOR PROCESSING THE RESULTS OF GROUND GEOCHEMICAL SURVEY WITH THE PURPOSE OF OIL AND GAS FORECASTING

To solve the problems of forecasting oil and gas content on one of the local structures of the South Tatar arch (Volzhsko-Kama anteclise), discriminant and neural network analyzes with training on reference wells were applied during the processing the results of geochemical surveys. Comparison with the results of the classical quantitative description of the geochemical field showed mainly the coincidence of areas of high hydrocarbon concentrations in near-surface sediments and anomalies identified by statistical methods. Based on the integration of the results of statistical processing of geochemical data and their geological interpretation, the structure was characterized as promising.

Vertical Zoning and Geochemical Anomaly Fields of the Hamama Gold Deposit in the Central Part of Egypt’s Eastern Desert

Vertical Zoning and Geochemical Anomaly Fields of the Hamama Gold Deposit in the Central Part of Egypt’s Eastern Desert

Stream Sediments of the Pestrinsk Silver-Bearing System (Northeastern Russia)

The composition, structure, and formation features of the exogenous anomalous geochemical fields (AGCFs) identified through stream sediments (SSs) are considered here within the Pestrinsk silver-bearing system and the Goltsovy silver-polymetallic deposit. The research was performed in the southern part of the Balygychan-Sugoy trough (northeastern Russia). The exogenous AGCFs of the main indicator elements of ores, formed in cryolithogenesis zone conditions, were studied. We used the results of multi-scale areal geochemical surveys of SSs. A survey of SSs at 1:200,000 scale was found to be effective at the stage of regional forecasting. Indeed, it is characterized by simplicity and the possibility of obtaining information operatively regarding the metallogeny of the area. It was found that at the local forecast stage, when prospecting for mineralization, the most effective was a survey of SSs at 1:50,000 scale. The AGCFs identified during this survey were distinguished by a richer component composition, higher contrast, and closer relationship with ores. During the lithochemical sampling of the watercourse heads, where alluvial sediments were found to be almost completely absent, a positive result was obtained by the bryolithochemical method, which is based on moss sampling together with a fine fraction of alluvium held by a moss cushion. The method enabled the sampling of watercourse heads and thus yielded information about the presence or absence of anomalous concentration fields of ore elements.