Deep-seated Source Research Articles

Stable isotope imprint of hypogene speleogenesis: Lessons from Austrian caves

Hypogene speleogenesis involves the formation of solution-enlarged permeability structures driven by ascending water acquiring its acidity from deep-seated sources. This process occurs in a wide range of geological settings and in different lithologies. Although hypogene speleogenesis has received increasing attention in the last two decades, the identification of former hypogene activity in caves remains challenging, unless unambiguous morphological, mineralogical and/or hydrogeological evidence is available. Here, we document that geochemical analyses (mainly stable oxygen and carbon isotopes) of cave wall rock samples may fill this methodological gap and provide a useful tool to identify hypogene karstification through geochemical fingerprinting of water-rock interaction.Stable isotope profiles of a comprehensive set of wall rock cores from eight hypogene caves in Austria in both limestone and dolomite settings were investigated. Alteration halos typically a few centimeters wide and characterized by up to 12‰ and 15‰ depletion in δ13C and δ18O, respectively, were identified in limestone and calcite marble caves. Besides isotopic alteration, bleaching and/or staining of the wall rock was observed due to oxidizing conditions of the paleowater. No isotopic change of the wall rock was identified in hypogene caves carved in dolomite, which may be attributed to low Ca/Mg ratios of the paleowater and/or aggressive fluids leading to fast wall rock retreat. On the other hand, dedolomitization was observed in some wall rocks implying the presence paleowaters with elevated Ca/Mg ratios. Similarly to dolomitic caves, no isotopic alteration front was observed in a cave formed by sulfuric acid speleogenesis. A likely explanation is the high rate of wall rock retreat outpacing the rate of isotope exchange in the wall rock. Our study also shows that water-rock interaction during epigene speleogenesis does not lead to isotopic halos.This geochemical fingerprinting approach offers a powerful tool to test the hypogene origin of caves and may provide additional insights into the temperature and provenance of the paleowater and the source(s) of its dissolved inorganic carbon.

Плиоценовый адакитоподобный акцент андезитов и дацитов на Орловском вулканическом поле (о. Сахалин)

Adakite-like geochemical signature (high Sr/Y ratio at a low Y concentration) is recognized in andesites and dacites, associated with intraplate basalts in the Orlov volcanic field of Sakhalin Island. These rocks denote the final (Pliocene) accent of intraplate volcanism in the Lesogorsk zone, which began in the Middle Miocene in an area of its junction with the Chekhov zone of the preceded (Oligocene-Early Miocene) suprasubduction one. The adakite-like accent was related to the Sakhalin folding phase that accompanied the general structural reorganization in the back-side region in the Japan arc system. Such a geological environment differed from the one of classical adakites generation resulted from melting of a young slab in the Aleutian island arc. It is supposed, that the Sakhalin adakite-like magmas were produced in deep-seated sources of the crust-mantle transition displayed in the Sakhalin-Hokkaido-Japan Sea zone of hot transtension due to drastic change of tectonic deformations from the thin crust of the South Tatar Basin to the thicker one of its northeastern extremity.

275. Comparison of Cefazolin versus Nafcillin for Methicillin-Susceptible Staphylococcus aureus Bacteremia with a Deep-Seated Source

BackgroundHistorically, anti-staphylococcal penicillins have been the treatment of choice for methicillin-susceptible Staphylococcus aureus (MSSA) infections. However, cefazolin may have advantages over these agents including convenience and tolerability. Despite several studies finding similar rates of clinical efficacy using cefazolin with fewer adverse drug events, some prescribers remain hesitant to use this agent due to concern for an inoculum effect in deep-seated infections. The purpose of this study was to compare cefazolin and nafcillin for the treatment of MSSA bacteremia with exclusively deep-seated sources.MethodsAdult patients who were admitted with MSSA bloodstream infections (BSI) treated with cefazolin or nafcillin between March 2017 and October 2019 were identified. Patients were included if their BSI had a deep-seated source, defined as endocarditis, osteomyelitis, septic arthritis, pneumonia, prosthetic material, mediastinitis, or abscess. Patients were excluded if they had polymicrobial BSI, central nervous system infection, or received less than 7 days of therapy. The primary efficacy outcome (PEO) was a composite of treatment failure, 60-day mortality, and 60-day infection relapse, and was assessed using multivariate logistic regression. The primary safety outcome (PSO) was discontinuation of therapy due to adverse drug events, which was assessed with a chi-square test.ResultsA total of 164 patients were included in this analysis (141 treated with cefazolin and 23 with nafcillin). There were no significant differences in the baseline characteristics collected (Table 1), and the most common deep-seated sources were prosthetic material and endocarditis. Treatment with nafcillin was not found to be protective against the PEO in multivariate analysis (aOR, 1.19; 95% CI, 0.42 to 3.39; P = 0.75), and the PSO was reached significantly more often among nafcillin recipients compared to those treated with cefazolin (7/23 [30.4%] versus 8/141 [5.7%], P < 0.0001). ConclusionThough the sample size was smaller than desired, cefazolin and nafcillin appeared to have similar efficacy for the treatment of MSSA BSIs with deep-seated sources. Nafcillin was associated with significantly more adverse drug events leading to discontinuation of therapy.Disclosures All Authors: No reported disclosures

EOCENE MAGMA PLUMBING SYSTEM BENEATH CORTEZ HILLS CARLIN-TYPE GOLD DEPOSIT, NEVADA: IS THERE A DEEP-SEATED PLUTON?

Abstract The magmatic-hydrothermal conceptual model for Carlin-type gold deposit genesis calls upon deep-seated Eocene plutons as the primary source of gold-bearing fluids. However, geophysical surveys, geologic mapping, drilling, geochronology, isotopic tracers, and fluid inclusion chemistry have returned ambiguous evidence for the existence of such plutons. The high-grade Cortez Hills gold deposit in northern Nevada hosts shallow, Eocene syn- and postmineralization intrusions, offering an ideal site to investigate the existence of a deep-seated pluton beneath the district. Here, major and trace element analyses of quartz-hosted melt inclusions from four Eocene rhyolite dikes cropping out within the Cortez Hills pit and results from independent thermobarometers provide a window into the subsurface Eocene magmatic plumbing system to test the existence of a deep-seated source pluton. Dissolved volatile contents, melt inclusion entrapment pressures, and thermodynamic phase equilibria indicate that dike magmas were sourced from ~4- to ≥9-km depth from a polybaric magma reservoir residing as a physically and geochemically interconnected crystal mush with extractable or eruptible magma pockets. Magmas ascended adiabatically (nearly isothermally), exsolving fluids, evolving modestly by fractional crystallization, while trapping quartz-hosted melt inclusions steadily from depth to subvolcanic levels where they were emplaced. These data represent the first unequivocal evidence for a deep-seated magma reservoir from which fluid-saturated magma emanated and released magmatic fluids beneath the Cortez district during gold mineralization. However, further investigation into the specific metallogenic potential and metal budget of parental magmas and the partitioning of gold between silicate melt and aqueous fluids will be necessary to provide evidence that exsolved magmatic fluids may have been gold bearing.

Origin of hydrocarbon and noble gases, carbon dioxide and molecular nitrogen in the Miocene strata of the eastern part of the Polish Carpathian Foredeep: Isotopic and geological approach

Fifteen natural gas samples were collected from the autochthonous Miocene strata between Dębica and Przemyśl towns in the eastern sector of the Polish Carpathian Foredeep. The samples were analysed for the molecular and stable isotope compositions of carbon (δ13C in CH4, C2H6, C3H8, i-C4H10, n-C4H10 and CO2), hydrogen (δ2H in CH4, C2H6 and C3H8), nitrogen (δ15N in N2), and all stable isotopes of noble gases (He, Ne, Ar, Kr and Xe) in order to reveal the origin and migration pathway of these gases. Methane concentrations usually exceeded 90 vol%. Methane was mainly generated by microbial carbon dioxide reduction. The rhythmic and cyclic deposition of clays, muds and sands in the Miocene marine basin facilitated and intensified microbial processes. Microbial ethane can be produced by methanogenic Archaea from acetate via reduction. Ethane partly contains a minor component generated from early mature thermogenic process. Propane and butanes were also generated thermogenically. The hydrocarbon gases in the Miocene horizons IIA and IIIA of the Góra Ropczycka field were generated at higher maturity thermogenic process than the rest of the analysed Miocene gases probably from the Middle Jurassic type III kerogen. They migrated to the Miocene horizons from the underlying Upper Jurassic horizon IV of the same field along a fault. Carbon dioxide of the analysed Miocene gases was mainly generated by primary microbial processes. Moreover, the total carbon constrained in the analysed Miocene gases from 0.1 to 6 wt% contained mantle-derived carbon, and thus carbon dioxide mantle component. Molecular nitrogen was mainly generated during low-temperature thermal transformation of organic matter. It might also involve a relatively insignificant component released from NH4-rich illites of the clayey facies of Miocene strata and an atmospheric component delivered by recharge of air-saturated seawater to the reservoir. Inorganic fluid supply from greater lithospheric depth would not be responsible for the promotion of thermal transformation of organic matter. Molecular nitrogen from Miocene horizons in the Góra Ropczycka field was generated during the thermal transformation of organic matter of higher maturity than the rest of Miocene gases, and migrated from the Upper Jurassic horizon along a fault. Helium is dominated by its radiogenic component, with a minor contribution from the mantle. The almost constant mixing ratios of the two helium components in the natural gases from Miocene horizons of various depths suggest that He migrated from a common inorganic deep-seated source in the lithosphere. The other noble gases are dominated by atmospheric components delivered to the hydrocarbon reservoirs by recharge of air-saturated seawater, with minor nucleogenic 21Ne, radiogenic 40Ar, and sedimentary Kr and Xe contributions. The atmospheric component is isotopically fractionated for Ne, and elementally fractionated with various degrees. The ratios of radiogenic 4He to nucleogenic 21Ne or radiogenic 40Ar are higher than the crustal production ratios, suggesting fractionation during thermal release from their host minerals and/or phase related fractionation during their transport to the gas reservoirs. The striking similarity in noble gas isotopic/elemental ratios of the Góra Ropczycka gas from the Miocene horizon IIIA to the nearby reservoir in Mesozoic strata is consistent with gas migration along a fault system.

Migration of Natural Hydrogen from Deep-Seated Sources in the São Francisco Basin, Brazil

Hydrogen gas is seeping from the sedimentary basin of São Franciso, Brazil. The seepages of H2 are accompanied by helium, whose isotopes reveal a strong crustal signature. Geophysical data indicates that this intra-cratonic basin is characterized by (i) a relatively high geothermal gradient, (ii) deep faults delineating a horst and graben structure and affecting the entire sedimentary sequence, (iii) archean to paleoproterozoïc basements enriched in radiogenic elements and displaying mafic and ultramafic units, and (iv) a possible karstic reservoir located 400 m below the surface. The high geothermal gradient could be due to a thin lithosphere enriched in radiogenic elements, which can also contribute to a massive radiolysis process of water at depth, releasing a significant amount of H2. Alternatively, ultramafic rocks that may have generated H2 during their serpentinization are also documented in the basement. The seismic profiles show that the faults seen at the surface are deeply rooted in the basement, and can drain deep fluids to shallow depths in a short time scale. The carbonate reservoirs within the Bambuí group which forms the main part of the sedimentary layers, are crossed by the fault system and represent good candidates for temporary H2 accumulation zones. The formation by chemical dissolution of sinkholes located at 400 m depth might explain the presence of sub-circular depressions seen at the surface. These sinkholes might control the migration of gas from temporary storage reservoirs in the upper layer of the Bambuí formation to the surface. The fluxes of H2 escaping out of these structures, which have been recently documented, are discussed in light of the newly developed H2 production model in the Precambrian continental crust.

Fluid inclusion and isotope (O, H, C, Sr) constraints on the orogenic gold mineralization at the Enche Concha and Tunel prospects, Gurupi Belt, Brazil

The gold mineralization at the Enche Concha and Tunel prospects, northwestern portion of the Gurupi Belt, is hosted in dacites and phyllites that belong to the ca. 2160 Ma-old metavolcano-sedimentary Chega Tudo Formation. These host rocks are variably deformed, from a spaced cleavage to schistose and mylonitic varieties. The hosting structures have previously been defined as produced in the Paleoproterozoic. The hydrothermal alteration processes include carbonatization, sericitization, sulfidation, and quartz veining. These processes are both pervasive and fissure-filling in style. Pyrite is largely predominant over chalcopyrite and sphalerite and gold occurs as a refractory phase in pyrite. Fluid inclusion, stable (O, H, C) and Sr isotopes studies indicate that the hydrothermal alteration and gold mineralization occurred between 260 and 370°C (mostly ~328°C), and 0.85 to 2.8 kbar, from a low-salinity (~5% NaCl), aqueous-carbonic (XCO2 = 0.03-0.13), relatively reduced (log fO2 = -32.1), and near neutral (pH = 5.0-6.2) metamorphic fluid (d18O = +9.4 to +11.3‰, dD = -25 to -28‰). Contributions from deep-seated (lower crust and mantle) sources are suggested by a d13CCO2 value of -7.6‰, and 87Sr/86Sr ratios between 0.702699 and 0.705141, which are lower than those found in the country rocks. Gold was transported by the Au(HS)2- complex and precipitation occurred in response to phase separation, identified by fluid inclusions, and fluid-rock interactions, given by the lowering of the d13CCO2 values. The geological and genetic characteristics are equivalent to those of the orogenic gold system previously defined for the Gurupi Belt.

Geophysical Investigation of Mount Nemrut Stratovolcano (Bitlis, Eastern Turkey) Through Aeromagnetic Anomaly Analyses

Quaternary Mount Nemrut stratovolcano, having a spectacular summit caldera and associated lakes, is located north of the Bitlis–Zagros suture zone, Eastern Turkey. Although much attention has been paid to its geology, morphology, history and biology, a detailed geophysical investigation has not been performed in this special region. Thus, we attempted to characterize the stratovolcano and the surroundings using total field aeromagnetic anomalies. Potential field data processing techniques helped us to interpret geologic sources causing magnetic signatures. Resulting image maps obtained from some linear transformations and a derivative-based technique revealed general compatibility between the aeromagnetic anomalies and the near-surface geology of the study area. Some high amplitude magnetic anomalies observed north of the Nemrut caldera rim are associated with the latest bimodal volcanic activity marked by lava fountains and comenditic-basaltic flows occurred along the rift zone. After minimizing the high-frequency effects, a pseudogravity-based three-dimensional inversion scheme revealed that the shallowest deep-seated sources are located about 3.0 km below the ground surface. Two-dimensional normalized full gradient solutions also exposed the depths of these anomaly sources, in good agreement with the inversion results. This first geophysical study performed through aeromagnetic anomalies clearly gave insights into some main magnetized structures of the Mount Nemrut stratovolcano.

The Franconian Basin thermal anomaly: testing its origin by conceptual 2-D models of deep-seated heat sources covered by low thermal conductivity sediments

This study presents conceptual 2-D models for coupled fluid flow and heat transport simulations of the Franconian Basin in SE Germany to verify the plausibility of different hypothesis on the origin of a local temperature anomaly. The simulated geothermal systems consist of a deep-seated heat source within an impermeable basement (heat-producing granite or enhanced background heat flow), covered by low thermal conductivity sediments. Solely conductive or additional convective heat transport including the presence of a permeable fault was applied. We found that heat transfer in the model setups is strongly controlled by (1) the volume of the heat-producing granite, (2) the amount of the background heat flow, (3) the permeability of the basement rocks, (4) the thermal conductivity contrasts between the sedimentary cover and the basement, and (5) the type of heat transport. If there is no reliable information on these model parameters, a high degree of uncertainty with regard to quantitative statements on the heat transfer in the specific geothermal system can be expected. An equilibrium temperature log from the study area could only be reproduced by (1) an enhanced background heat flow of 0.115 W m−2, in combination with a permeable fault zone of permeability 1.0 × 10−13 m2 or (2) a heat-producing granite of large cross-sectional area (300 km2) in combination with an average background heat flow of 0.070 W m−2. Moreover, high temperatures were only achieved in the presence of a low conductive, insulating cover above the heat source.

Geochemical Characterization of the Uranium Mineralization in the Cenozoic Basin in the Central Anatolia, Turkey

Uranium contents were measured in the siltstones and siliceous limestone interbeds within sandstone of Kubaca member of the Eocene of Ayhan formation, with chemical values ranging from 175 to 2537 ppm and radiometric values ranging from 85.40 to 660.40 cps. The highest chemical values of uranium and Th/U were obtained from the siltstones. Mineralogical studies were also conducted, and these indicated that uranium is associated with fluorapatite and hydroxyapatite in the siltstone and as discrete rutherfordine in fractures of siliceous limestone. The uranium contents in the siltstones are higher than those in the siliceous limestones due to the cation-exchange capacity and adsorption of smectite/kaolinite group clay minerals. The D-factor determination of the equilibrium state of the studied rocks indicated that chemically analyzed uranium values are greater than radiometrically determined uranium values, reflecting a disequilibrium state characterized by the addition of uranium into the system. The study shows two types of the uranium mineralization: (1) syngenetic phosphate cement controlled and (2) epigenetic, fluid driven, fracture controlled. The potentiality of the area with respect to uranium is enhanced in view of the addition of uranium into fractures and fault planes, after remobilization from the overlying phosphatic siltstone unit and other deep-seated sources.

Sources of the lithosphere magnetic field based on magnetic data obtained at different heights

Our investigations are based on the property that the fields of sources, whose depths are numerically equal to survey heights, are most brightly presented in the data of magnetic survey. Therefore, the magnetic field created by the upper boundary of the magnetically active layer is well presented in the data of magnetic surveys (survey heights are up to the first kilometres), whereas the geomagnetic field of the centre mass is well presented in the data of gradient magnetic surveys at heights of 20–40 km. These data were used separately for the interpretation of the depths of the upper and lower boundaries of the lithospheric magnetically active layer by spectral methods. This fact is especially valuable for estimating the positions of deep sources. For the central part of the East European platform, we obtained by spectral methods, the following values: the depth of the upper boundary of the layer is 8.5 km and that of the lower boundary of the layer is 64.3 km. The discrete localisation of the source depths along the profile is performed by the methods of converting the initial information into transformed fields, continuation upward, and reduction to the pole with the determination of singular points. The Poisson integral, representing the solution of the outer Dirichlet problem for the plane, served as a theoretical base for such an interpretation. These approaches made it possible to determine more exactly the localisation of deep sources along the profile and showed that the published magnetic maps based on aeromagnetic data do not contain in full measure the fields of deep-seated magnetic sources.

Hydrochemical and Isotopic Characteristics of CO2-rich Groundwater in the Gyeongsang Sedimentary Basin, South Korea: A Natural Analogue Study on the Potential Leakage of Geologically-stored CO2

The potential of geologic carbon storage has been considered in the Gyeongsang sedimentary basin (GSB) at the southeastern part of South Korea. In order to understand the long-term behaviors of geologically-stored CO2, hydrochemical and multi-isotopic data (δ18O-δD of water and δ13C of dissolved inorganic carbon) of groundwater samples were collected from naturally seeping CO2-rich springs and ambient shallow groundwater wells in the GSB. The collected samples were classified into three hydrochemical groups: 1) dilute and acidic, CO2-rich springs, 2) high TDS, CO2-rich springs, and 3) CO2-poor groundwater. CO2-rich springs (i.e., Groups 1 and 2) were differentiated from Group 3 by their high carbon isotope values and partial pressure of CO2, suggesting that the CO2 in Groups 1 and 2 originates from a deep-seated (magmatic) source. Compared to the CO2-rich springs of Group 1, major cations and HCO3-, together with Fe, Mn and Sr, are enriched in CO2-rich springs of Group 2 and these constituents are highly correlated with TDS. This indicates that, during the ascent of CO2-rich fluids via fractures, CO2-rich springs of Group 2 experienced substantial CO2-water-rock interactions, possibly through longer or restricted pathways, whereas CO2-rich springs of Group 1 acquired the chemistry through lower degrees of water-rock interaction via the rapid ascent through more permeable pathways. The results of principal component analysis (PCA) with compositional data demonstrated that pH can be used as a prior monitoring parameter to detect the direct leakage of CO2 as represented by Group 1. In addition, the changes of ionic composition can be used as a supplementary tool to monitor the indirect leakage of CO2 accompanying a substantial CO2-water-rock interaction.

The geochemistry and origin of the hydrothermal water erupted at Lusi, Indonesia

The spectacular Lusi mud-eruption started in northeast Java the 29th of May 2006. Despite extensive research, the origin of the erupted water remains elusive and poorly constrained. Here we present a comprehensive study of the geochemistry of Lusi waters compared with those collected from surrounding areas, all collected between 2006 and 2013, including data from mud volcanoes and volcano-hosted hydrothermal springs. Within this broad context, the geochemical characteristics of the fluids expelled in the Lusi region suggest that we can classify the waters in three groups: 1) meteoric waters expelled in cold springs and artesian wells, 2) hydrothermal waters typically mixed with meteoric waters, and 3) formation water from marine sediments altered by diagenetic processes such as clay-mineral dehydration. Samples collected from the Lusi crater are Cl and Na dominated (up to 527 mM and 471.7 mM, respectively) similar to seawater indicating that altered sedimentary formation waters are predominant in this system. In addition they are enriched in Sr (up to 808.4 μM) and other elements commonly associated with hydrothermal systems, such as Li (up to 877.6 μM compared to 26 μM in seawater). Some of these elements are up to ten times enriched compared to seawater values. High-temperature fluid mineral interactions in the subsurface appear to have facilitated the transfer of Li and other mobile elements into the fluids. High temperature fluid-mineral interaction reactions are also supported by Si concentrations significantly higher compared to other sampled mud volcanoes in the island. Crater samples also show the highest δ18O values (+5‰ after correction for evaporation compared to +1‰ at the MV localities). 87Sr/86Sr ratios vary between of 0.7077 and 0.7083 and seem to reflect a general mixture of fluids from clay-mineral dehydration, carbonate recrystallization, alteration of volcanic rocks and hydrothermal imprint. Eight years of geochemical monitoring indicate that the composition of the deep-sourced Lusi fluids remain fairly constant through time. Thus our findings show that the Lusi crater waters represent a regional geochemical anomaly, and we suggest that a combination of high temperatures in the source region, and fluid-rock interactions with silicates and, possibly, carbonate-rich lithologies can explain the data. This is consistent with a model where the emitted gases migrate from a deep-seated (>4 km) source region, likely associated with the presence of hot igneous intrusions and/or high T reactions related to the presence of neighbouring active volcanoes.

Origin of the ore-forming fluids and metals of the Hetai goldfield in Guangdong Province of South China: Constraints from C-H-O-S-Pb-He-Ar isotopes

The Hetai goldfield, located in the southern segment of the Qinzhou Bay-Hangzhou Bay Juncture Orogenic Belt (QHJB), is one of the largest gold mining districts in South China. Although the goldfield shares some similarities in ore geology and geochemistry to orogenic gold deposits, there has been controversy on the origin of the ore-forming fluids and metals. An integrated analysis on C-H-O-S-Pb-He-Ar isotopes has been carried out to trace the origins of the ore fluids and metals, thereby to evaluate the genesis of the Hetai goldfield. The 3He/4He and 40Ar/36Ar ratios of fluid inclusions in sulfides from the Hetai ores exhibit a range of 0.27–0.62Ra and 932.4–6324.2, with the 4He and 40Ar concentrations varying from 1.99 to 96.80 (10−7cm3 STP/g) and 0.51–7.75 (10−7cm3 STP/g), respectively. This suggests a dominant crust-derived fluid mixed with significant mantle-derived fluids for the Hetai gold mineralization. The calculated δ18OH2O-VSMOW and measured δDH2O-VSMOW values of fluid inclusions in gold-bearing quartz range from 0.5 to 10.4‰ and from −83.1 to −52.4‰, respectively. The δ13CPDB and δ18OPDB of calcite range from −3.53 to −1.91‰ and −21.21 to −19.67‰, respectively, suggesting a deep-seated source of the C-bearing portion of the fluid. The δ34SCDT values of sulfides intergrown with native gold range from −4.0 to −1.3‰, with an average of −2.8‰, indicating that the majority of the sulfur came from granitic magma by partial melting of the host rocks, but likely with minor input from the mantle. The Pb isotopic components of sulfides from the Hetai ores have 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb ratios of 18.044–19.404, 15.568–15.793, and 38.426–39.597, respectively, indicating a main upper continental crust source of ore metals, but probably with minor contribution from the mantle. Linked to the ore geology and regional tectonomagmatic events, the integration of the isotopic compositions obtained in this study with previously published fluid inclusion and geochronologic data suggests that the ore fluids and metals may have undergone two stages of evolution, i.e. from the Indosinian metamorphic water-dominated fluids mixed with minor magmatic waters, to the Yanshanian magmatic water-dominated fluids mixed with mantle-derived fluids and meteoric waters.

Numerical models for continental break-up: Implications for the South Atlantic

We propose a mechanism that explains in one unified framework the presence of continental break-up features such as failed rift arms and high-velocity and high-density bodies that occur along the South Atlantic rifted continental margins. We used 2D and 3D numerical models to investigate the impact of thermo-rheological structure of the continental lithosphere and initial plume position on continental rifting and break-up processes. 2D experiments show that break-up can be 1) “central”, mantle plume-induced and directly located above the centre of the mantle anomaly, 2) “shifted”, mantle plume-induced and 50 to 200 km shifted from the initial plume location or 3) “distant”, self-induced due to convection and/or slab-subduction/delamination and 300 to 800 km off-set from the original plume location. With a 3D, perfectly symmetrical and laterally homogeneous setup, the location of continental break-up can be shifted hundreds of kilometres from the initial position of the mantle anomaly. We demonstrate that in case of shifted or distant continental break-up with respect to the original plume location, multiple features can be explained. Its deep-seated source can remain below the continent at one or both sides of the newly-formed ocean. This mantle material, glued underneath the margins at lower crustal levels, resembles the geometry and location of high velocity/high density bodies observed along the South Atlantic conjugate margins. Impingement of vertically up-welled plume material on the base of the lithosphere results in pre-break-up topography variations that are located just above this initial anomaly impingement. This can be interpreted as aborted rift features that are also observed along the rifted margins. When extension continues after continental break-up, high strain rates can relocalize. This relocation has been so far attributed to rift jumps. Most importantly, this study shows that there is not one, single rift mode for plume-induced crustal break-up.

Late Paleozoic and Early Mesozoic rare-metal granites in Central Mongolia and Baikal region: review of geochemistry, possible magma sources and related mineralization

The Central Asian Orogenic Belt (CAOB) was a scene of intense granitoid magmatism during the Phanerozoic with formation of vast batholiths: Angara-Vitim and Daurian-Khentei. In the Late Paleozoic and Early Mesozoic times, the peripheral zones of batholiths underwent granitic magmatism associated with rare-metal mineralization. Petrological and geochemical studies show that the rare-metal Li–F granites formed, with a gap about 100 My, large igneous provinces of the Mongol-Okhotsk Belt. Late Paleozoic rare-metal granites build a series of multiphase plutons in the Baikal region (e.g. Kharagul 318 ± 7 Ma, Bitu-Dzhida 311 ± 10 Ma and Urugudei 321 ± 5 Ma). The early medium-grained biotite granites and leucogranites were followed by topaz-bearing microclineand amazonite–albite granites and a series of dikes. The Early Mesozoic epoch was marked by the formation of the Daurian-Khentei Batholith (230–190 Ma) in the center of area and rifting zones with alkaline and rare-metal granite plutons on the peripheries. In contrast to the Late Paleozoic, small Early Mesozoic intrusions (e.g., Avdar Pluton ~10 km2, 212–209 Ma) of rare-metal Li–F granites within the Avdar-Khoshutul series of granitoids coexisted with sizable plutons (e.g., Janchivlan Pluton ~70 km2, 227–195.3 Ma). Rare-metal Li–F granites of the Janchivlan Pluton produced small domal intrusions composed of microcline–albite, amazonite–albite and albite–lepidolite granites. The Sn–Ta–Nb mineralization is associated with albite–lepidolite granites. The rare-metal granites of the Baikal region and Central Mongolia of contrasting ages show an identical geochemical signature of Li–F granites. It is expressed by increase in F, Li, Rb, Cs, Sn, Be, Ta and Pb and decrease in Sr, Ba, Zn, Zr, Th and U contents in course of multiphase intrusions formation. The geochemical data confirm the magmatic model for genesis of the studied rare-metal Li–F granites. However, this process of magma differentiation was terminated with formation of albitites, microclinites and muscovite greisens. The whole-rock geochemistry and isotopic composition of the granites points to the Precambrian crust of the Baikal region (T2DМ = 1.0–1.3 Ga) as the most likely source. We propose the formation of the initial granitic melts due to anatexis of the higher levels of the continental crust, with fluids released during granulite-facies metamorphism in the lower crust. The rare-metal Li–F granites of the studied provinces are intraplate formations geochemically different from the Early Paleozoic collisional granitoids. This could be caused by the influence of deep-seated source on the occurrence of rare-metal magmatism.

Cenozoic lithogenesis in the Baikal rift zone

New data on the geological history and Cenozoic lithogenesis in depressions of the Baikal rift zone are considered with areas adjacent to Lake Baikal as example. In this region, rifting developed during the plain (Late Oligocene‒Early Pliocene) and orogenic (Late Pliocene‒Holocene) stages and was accompanied by the accumulation of plain coaliferous fan and orogenic molasses formations, respectively. The examination of Quaternary sequences in the Baikal region reveals that deposition and postsedimentary transformations of riftogenic sediments were intensely influenced by deep-seated water sources of the so far undivided stratal‒infiltration, elision, and exfiltration types, according to the classification in (Kislyakov and Shchetochkin, 2000). Deep processes in this region determined the elevated heat flow, volcanism, and extensive discharge of hydrothermal solutions and gas fluids. In our opinion, gaseous‒hydrothermal activity stimulated the formation of hydrothermal‒sedimentary rocks (dolomitic and calcitic travertines, geyserites, aluminosulfates), the accumulation of diatomaceous and carbonaceous oozes in Baikal, and the formation of a large methane gas hydrate deposit.

Hydrothermal Transport of Ag, Au, Cu, Pb, Te, Zn, and Other Metals and Metalloids in New Zealand Geothermal Systems: Spatial Patterns, Fluid-Mineral Equilibria, and Implications for Epithermal Mineralization

In order to analyze the concentrations of Ag, As, Au, Cd, Cu, Hg, Mn, Mo, Ni, Pb, Se, Sb, Te, Tl, W, and Zn in deep hydrothermal chloride waters at 195° to 320°C, and in shallow boiled chloride waters at 160° to 230°C, we sampled wells drilled to 3-km depth in geothermal systems of the North Island, New Zealand. Six of the systems are located in a segment of volcanic arc in the central Taupo Volcanic Zone, and the seventh system is associated with an intraplate mafic (felsic) volcanic center. The concentrations of metals range widely from 0.1 to >1,000 μ g/kg with a large degree of intersystem variability. Some of the largest contrasts in Au, Ag, Pb, and Te concentrations are observed in the two nearest systems, Rotokawa and Wairakei, which are only 10 km apart. The correlations between metals are poor, except for Ag-Au-Pb-Te, and As-Sb. The correlations between metals, Cl, and H2S are also poor, with the exception of Rotokawa where the highest concentrations of Ag, Au, Cu, and Te correlate with the highest concentration of aqueous H2S. Speciation calculations indicate that the dominant aqueous species of Ag, Au, Cu, Pb, and Zn involve HS− complexes. The calculations also show that the states of saturation range from undersaturated conditions for acanthite, arsenopyrite, and gold to oversaturated conditions for chalcopyrite, sphalerite, and tellurides. Notably, the Au- and Ag-transporting capacities of the deep chloride waters are much larger than the measured aqueous concentrations. These results suggest that fluid-mineral equilibria and the concentrations of ligands exert weak influence on metal concentrations at the temperatures and depths of sampling. The complex trends in hydrothermal metal concentrations strongly suggest that the deep-seated sources of metals, comprising magmatic intrusions, deep country rock, and their related fluids, limit the hydrothermal supplies of metals. Between the geothermal systems, hydrothermal fluxes of Ag (6–8,000 kg/y), Au (0.9–66 kg/y), Cu (30–23,500 kg/y), and Te (2–10,400 kg/y) are variable. The highest concentrations and fluxes of Ag, Au, and Te in Rotokawa and Mokai are attributed to direct fluid inputs from intrusions of andesitic and basaltic magmas, respectively. Compared to their deep counterparts, boiled chloride waters are strongly depleted in Ag, Au, Cd, Cu, Pb, and Te, because these metals deposit in sharp response to gas loss and cooling in the well. By comparison, the As, Mn, Mo, Ni, Sb, Tl, and Zn concentrations are measurably less depleted in boiled waters, making them available to form metal anomalies at shallow depths and in the peripheral parts of the epithermal environment. Periods of strong metal flux through the geothermal system combined with deep boiling favor epithermal ore formation and the development of large precious metal deposits. However, even moderate metal fluxes can produce Au and Ag mineralization as long as the duration of focused fluid flow and boiling can be sustained. There is no evidence that the compositions of deep magmatic intrusions, mafic or intermediate, limit the ore-forming potential of a geothermal system.

Monitoring of CO2-rich waters with low pH and low EC: an analogue study of CO2 leakage into shallow aquifers

The geochemistry of CO2-rich springs and wells was monitored as a natural analogue study of CO2 leakage into shallow aquifers. In result, within the relatively small study area, diluted CO2-rich waters (DCWs), concentrated CW (CCW), and ordinary groundwaters were observed. DCWs showed an exceptionally low pH (mean 4.8) and low EC (mean 150 μS/cm). The low pH and EC as well as the time-invariant geochemistry of DCWs were probably due to continuous CO2 inputs into the open system, which had a short reaction time and rarely consisted of reactive minerals. DCWs showed the low concentrations of SiO2 (mean 20.4 mg/L) and the average tritium concentration of 3.4 TU, which indicates the low CO2–H2O–rock interaction. In addition, \(\delta^{13} {\text{C}}_{{{\text{CO}}_{ 2} {\text{g}}}}\) in equilibrium with water was estimated using the mass balance equation and δ13CDIC measured in water samples. The results of the stable carbon isotope analysis showed that the CO2 originated from a deep-seated source to the shallow DCW aquifer (<80 m deep below the surface), whereas the deeper CCW aquifer was affected by soil organic CO2 near the surface. To detect the CO2 leakage from CO2 storage sites, the geochemistry of shallow aquifers should be monitored. This study result suggests that at least pH, EC, DIC, and carbon isotopes should be monitored because the monitoring of pH is helpful in an aquifer with low buffering capacity; while EC can be low despite CO2 leakage and the subsequently low pH at early stages, depending on the subsurface environment. Above all, this present study indicates that understanding the characteristics of aquifer conditions is of great importance for CO2 leakage detection.

Collision and mountain building

The spatial, chronological, and genetic relationships of recent (Late Alpine) collisions to mountain building are considered at three levels of scale: (i) in separate zones of the Arabian–Caucasus segment of the Alpine–Himalayan Orogenic Belt, (ii) throughout the central segment of this belt from the Alps to the Himalalayas, and (iii) in Central Asia and other mountain belts of continents. Three stages of mountain building are distinguished at all three levels. The first stage starts with widespread collision and similar plate interactions from the end of the Eocene to the middle Miocene and is expressed in the formation of uplifts, commonly no higher than the moderately elevated level in regions that concentrate deformations of transverse shortening induced by compression. The second short stage, which embraces the Pliocene–Quaternary and occasionally the end of the Miocene, differs in general, though differentiated in the value and intensification of vertical movements, when the height of mountains increases by 2–3 times. Elevations are spread over certain platform territories and even frameworks of rift zones. This is related not so much to the intensity of compression and shortening as to the compositional transformation of the upper mantle and the lower crust, leading to their decompaction. Comparison with the Hercynian and Caledonian orogenic stages shows that the second phase, predetermined by widespread collision, reflects a more important geodynamic event expressed in a change of the global plate interaction system and its deep-seated sources.