Spatio-temporal variations of δ<Superscript>2</Superscript>H and δ<Superscript>18</Superscript>O in precipitation and shallow groundwater in the Hilly Loess Region of the Loess Plateau,China

Groundwater is of utmost significance to socio-economic development and ecological recovery for the Loess Plateau. However, studies regarding the mechanism governing groundwater recharge over this area appear to be inadequate. This study is to examine the spatio-temporal variations of δ²H and δ¹⁸O in precipitation and shallow groundwater. On the basis of this, the mechanisms governing shallow groundwater recharge were explored. Precipitation and groundwater were sampled monthly from May to October during the period 2004–2006 at 13 sites in the Chabagou Catchment (187 km²). In the Caopingxigou Experimental Watershed (0.1 km²), meteorological variables were observed and rainfall larger than 5 mm was sampled immediately after each rain event. Across the area, 90% of the precipitation occurred from May to September primarily in the form of heavy rains or rainstorms with great spatial variability. There were about 30 localized rains in each year. It was indicated that there existed notable seasonality and pronounced spatial variability in precipitation isotopic compositions. Contributing factors and indications of isotopic compositions, as well as their climatic indications such as monsoon intensities and mixing processes of water vapor, were investigated. The δ²H–δ¹⁸O relation of groundwater was found to be δ²H = 3.22 × δ¹⁸O − 38.1, deviating from the local meteoric water line δ²H = 7.57 × δ¹⁸O + 3.9. The range of δ values in groundwater is shrunken to be 15–21% of that in individual precipitations, and groundwater in the middle reaches shows a wider range of δ values. Isotopic results showed that groundwater originates from precipitation with hydrogen and oxygen isotopic compositions being −69 and −9.7‰, respectively, and most groundwater experiences serious evaporation and adequate mixing with old water during infiltration or percolation in the aerated zone. It was also founded that obvious fluctuations of isotopic compositions in groundwater mainly appear in the middle reaches especially at sites that are close to valleys, suggesting varying sources of groundwater from precipitation, precipitation runoff, isotopically enriched surface water and/or lateral recharge of adjacent groundwater.     

U–Pb,Re–Os,and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar geochronology of the Nambija Au-skarn and Pangui porphyry Cu deposits,Ecuador: implications for the Jurassic metallogenic belt of the Northern Andes

New U–Pb, Re–Os, and ⁴⁰Ar/³⁹Ar dates are presented for magmatic and hydrothermal mineral phases in skarn- and porphyry-related ores from the Nambija and Pangui districts of the Subandean zone, southeastern Ecuador. Nambija has been one of the main gold-producing centers of Ecuador since the 1980s due to exceptionally high-grade ores (average 15 g/t, but frequently up to 300 g/t Au). Pangui is a recently discovered porphyry Cu–Mo district. The geology of the Subandean zone in southeastern Ecuador is dominated by the I-type, subduction-related, Jurassic Zamora batholith, which intrudes Triassic volcanosedimentary rocks. The Zamora batholith is in turn cut by porphyritic stocks, which are commonly associated with skarn formation and/or porphyry-style mineralization. High precision U–Pb and Re–Os ages for porphyritic stocks (U–Pb, zircon), associated prograde skarn (U–Pb, hydrothermal titanite), and retrograde stage skarn (Re–Os, molybdenite from veins postdating gold deposition) of the Nambija district are all indistinguishable from each other within error (145 Ma) and indicate a Late Jurassic age for the gold mineralization. Previously, gold mineralization at Nambija was considered to be Early Tertiary based on K–Ar ages obtained on various hydrothermal minerals. The new Jurassic age for the Nambija district is slightly younger than the ⁴⁰Ar/³⁹Ar and Re–Os ages for magmatic–hydrothermal minerals from the Pangui district, which range between 157 and 152 Ma. Mineralization at Nambija and Pangui is associated with porphyritic stocks that represent the last known episodes of a long-lived Jurassic arc magmatism (∼190 to 145 Ma). A Jurassic age for mineralization at Nambija and Pangui suggests that the Northern Andean Jurassic metallogenic belt, which starts in Colombia at 3° N, extends down to 5° S in Ecuador. It also adds a new mineralization style (Au-skarn) to the metal endowment of this belt. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Isotopic Composition (δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) and the Origin of Carbonates from Manganese Deposits of the Southern Urals

Isotopic compositions of carbon (δ¹³C from −51.4 to −10.8 PDB) and oxygen (δ¹⁸O from 14.4 to 21.4 SMOW) were studied in rhodochrosite and calcite from manganese ores in the South Faizuly and Kyzyltash deposits of the southern Urals. The geological, petrographic, and isotopic data indicate that the studied carbonates are diagenetic formations. It is suggested that the main ore element (Mn) was delivered to the marine basin with hydrothermal solutions percolating in the oceanic crust. Manganese precipitated on the oceanic bottom as oxides near solution discharge zones. Manganese carbonates formed in sediments as a result of the oxidation of organic matter by manganese oxides. High biological productivity of the environment was caused by proximity to the hydrothermal vent that provided favorable biogeochemical conditions for the development of biocoenosis. Anomalously low ¹³C values in the South Faizuly deposit testify to the large-scale oxidation of methane in the course of manganese carbonate formation.__________Translated from Litologiya i Poleznye Iskopaemye, No. 4, 2005, pp. 416–429.Original Russian Text Copyright © 2005 by Kuleshov, Brusnitsyn.     

Geochemistry of isotopes (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and depositional environment of the upper Kazanian carbonate sediments in the Volga-Vyatka interfluve

This work reports the results of lithological and isotopic study of carbonate rocks from the Pechishchi stratotype section (Kazan) and three adjacent sections of Kazanian rocks of the Volga-Vyatka region at the eastern Russian Platform. These sections were recovered by the Kremeshki, Popovtsevo, and Chimbulat quarries (near the town of Sovetsk, southeastern Kirov district). Lithological features and wide variations of δ¹³C (from −6.0 to 6.8‰) and δ¹⁸ O (from 22.9 to 33.4 ‰) indicate that the rocks were formed in a shallow-marine basin with rapidly varying conditions of sedimentation which characterize different facies (and/or paleoecological) zones: lagoonal, supralittoral, littoral, shoal. They also suggest processes of postsedimentary alteration (mainly, under supergene conditions). Numerous short-term hiatuses are also recorded.     

Rb–Sr and <Superscript>147</Superscript>Sm–<Superscript>143</Superscript>Nd systematics of gabbro and dolerites from fragments of ophiolite complexes of the Middle Urals

The Rb–Sr and ¹⁴⁷Sm–¹⁴³Nd age data obtained for sheeted dolerite dykes and rocks of the Platinum Belt of the Urals within the Tagil segment of the paleoceanic spreading structure (Middle Urals) are discussed. The study of the Rb–Sr isotope systematics of gabbro allowed us to reveal errochronous dependencies, which yielded ages of 415 and 345 Ma at (⁸⁷Sr/⁸⁶Sr)₀ = 0.70385 ± 0.00068 and 0.7029 ± 0.0010, correspondingly. The ¹⁴⁷Sm–¹⁴³Nd isotope age data demonstrate a specific coincidence of the chronometric ages of the sheeted dolerite dyke complex (426 ± 54, 426 ± 34, and 424 ± 19 Ma) and gabbro from the Revda gabbro–ultramafic massif (431 ± 27 Ma) and from screens between dolerite dykes in the sheeted dyke complex (427 ± 32 Ma, 429 ± 26 Ma). The proximity of the ¹⁴⁷Sm–¹⁴³Nd ages of gabbro and dolerite can be explained by the thermal effect of the basaltic melt, which is the protolith for the dyke complex, on the hosting gabbro.     

Re–Os and <Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar ages of the Jiguanshan porphyry Mo deposit,Xilamulun metallogenic belt,NE China,and constraints on mineralization events

The large Jiguanshan porphyry Mo deposit, with more than 100 Mt of ore and grades ranging from 0.08% to 0.11%, is located in the newly identified Xilamulun metallogenic belt along the northern margin of the North China Craton. The Mo mineralization is predominantly disseminated in the host granite porphyry, but locally occurs as stockworks in lithic tuff and rhyolitic rocks. ⁴⁰Ar/³⁹Ar dates of samples from groundmass material in the host granite porphyry, post-ore diabase, and quartz porphyry dikes show plateau ages of 155.1 ± 1.9, 149.4 ± 0.9, and 147.6 ± 0.9 Ma, with inverse isochron ages of 156.0 ± 1.8, 149.3 ± 1.3, and 148.3 ± 1.2 Ma, respectively. Seven samples of disseminated molybdenite yielded a weighted average ¹⁸⁷Re-¹⁸⁷Os age of 155.3 ± 0.9 Ma, whereas six veinlet-type molybdenite samples yielded a weighted average ¹⁸⁷Re-¹⁸⁷Os age of 153.0 ± 0.9 Ma, providing direct timing constraints for the Mo mineralization at 153–155 Ma. The regional geological setting together with the emplacement of post-ore diabase and quartz porphyry dikes in the Jiguanshan deposit, are indicative of an extensional regime in Late Jurassic, which was probably linked to lithospheric extension in northeast China.     

Sr chemostratigrphy, δ<Superscript>13</Superscript>C,and δ<Superscript>18</Superscript>O of rocks in the Crimean carbonate platform (Late Jurassic,northern Peri-Tethys)

The paper presents the results of study of the Sr, C, and O isotope compositions in Upper Jurassic carbonate rocks of the Baidar Valley and Demerdzhi Plateau in the Crimean Mountains represented by different facies of the carbonate platform at the northern active margin of the Tethys. The ⁸⁷Sr/⁸⁶Sr value in them varies from 0.70699 to 0.70728. Based on the Sr chemostratigraphic correlation, the age of massive and layered limestones in the western part of the Ai-Petri and Baidar yailas (pastures) is estimated as late Kimmeridgian–early Tithonian, whereas the age of flyschoids of the Baidar Valley are estimated as late Tithonian–early Berriasian. The nearly synchronous formation of carbonate breccias of the Baidar Valley and Demerdzhi Plateau in late Tithonian–early Berriasian is substantiated. A summary section of Upper Jurassic rocks is compiled based on the Sr chemostratigraphic data. It has been established that δ¹⁸O values in the studied carbonate sediments vary from–2.9 to 1.3‰ (V-PDB). At the same time, shallow-water sediments in the internal part and the edge of the Crimean carbonate platform are depleted in ¹⁸O (–2.9 to +0.1‰) relative to sediments on the slope and foothill (–0.5 to +1.3‰). It is demonstrated that δ¹³C values do not depend on the facies properties and decrease in younger carbonate sediments from 3–3.5‰ to 1–1.5‰ in line with the Late Jurassic general trend. The δ¹³C values obtained for the Crimean carbonate platform turned out to be 0.5–1‰ higher than the values typical of the deep-water marine setting at the western margin of the Tethys. These discrepancies are likely related to peculiarities of water circulation and high bioproductivity in marine waters of the northern Peri-Tethys.     

Behavior of isotope (<Superscript>18</Superscript>O/<Superscript>16</Superscript>O, <Superscript>234</Superscript>U/<Superscript>238</Superscript>U) systems during the formation of uranium deposits of the “sandstone” type

The uneven character of the distribution of ¹⁸O/¹⁶O and ²³⁴U/²³⁸U values was established in the vertical cross section of the productive sequence of the Dybryn uranium deposit (Vitim uranium-ore region, Buryatia). Both a deficiency and an excess of ²³⁴U in relation to the equilibrium ²³⁴U/²³⁸U ratio in the vertical sequence may provide evidence for the extremely low rate of the infiltration water flow. The behavior of oxygen isotope characteristics for different size fractions of terrigenous rocks provides evidence for active uranium redistribution and openness of the isotope system of this element during interaction of terrigenous–sedimentary rocks with infiltration waters.     

Assessment of NO<Subscript>3</Subscript><Superscript>−</Superscript> contamination in a karstic aquifer,with the use of geochemical data and spatial analysis

Kopaida plain is a cultivated region of Eastern Greece, with specific characteristic related with the paleogeographic evolution and the changes in land use. The present article examines the contamination that derives from nitrates, in terms of contaminant levels, definition of sources and spatial distribution of contaminant plume. For this purpose, 50 water samples were collected from the karstic aquifer and analyzed for 15 parameters including major ions, trace elements, physicochemical parameters, and stable isotopes. The assessment of the above parameter values along with the notes derived by the statistical process revealed the existence of nitrate contamination which has been spatial defined with the aid of spatial interpolation techniques. The correlation of NO₃ ⁻ concentrations with the stable isotope values, defined the infiltration conditions and showed contaminant transport. Nitrate values revealed the potential environmental threat for local people, as 10% of the samples exceeded the parametric value of 50 ppm and 54% of them are above 25 ppm, indicating no optimal quality conditions. The origin of nitrate contamination seems to derive exclusively from the application of N-fertilizers, since the rest of potential sources were not verified by analytical data and field works.     

Isotopic composition (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genesis of carbonates from the famennian manganiferous formation of Pai-Khoi

The results of isotope-geochemical studies of carbonates of different mineral types from manganese and host rocks of the Famennian manganiferous formation of Pai-Khoi are reported. Kutnahorite ores are characterized by δ¹³C values from–6.6 to 1.3‰ and δ¹⁸O from 20.0 to 27.4‰. Rhodonite–rhodochrosite rocks of the Silovayakha ore occurrence have δ¹³C from–5.2 to–2.9 and δ¹⁸O from 25.4 to 24.3‰. Mineralogically similar rocks of the Nadeiyakha ore occurrence show the lighter carbon and oxygen isotopic compositions: δ¹³C from–16.4 to–13.1 and δ¹⁸O from 24.8 to 22.5‰. Similar isotopic compositions were also obtained for rhodochrosite–kutnahorite rocks of this ore occurrence: δ¹³C from–13.0 to–10.4‰ and δ¹⁸O from 24.6 to 21.7‰. Siderorodochrosite ores differ in the lighter oxygen and carbon isotopic compositions: δ¹⁸O from 18.7 to 17.6‰ and δ¹³C from–10.2 to–9.3‰, respectively. In terms of the carbon and oxygen isotopic compositions, host rocks in general correspond to marine sedimentary carbonates. Geological-mineralogical and isotope data indicate that the formation of the manganese carbonates was related to the hydrothermal ore-bearing fluids with the light isotopic composition of oxygen and carbon dissolved in CO₂. The isotopic features indicate an authigenic formation of manganese carbonates under different isotopegeochemical conditions.     

Isotopic (Sm–Nd,Pb–Pb,and δ<Superscript>34</Superscript>S) and Geochemical Characteristics of the Metasedimentary Rocks of the Baikal–Patom Belt (Northern Transbaikalia) and Evolution of the Sedimentary Basin in the Neoproterozoic

This paper reports the results of a detailed isotopic (Sm–Nd, Pb–Pb, and δ³⁴S) and geochemical studies of Neoproterozoic metasedimentary rocks from the Patom and Bodaibo domains of the Baikal–Patom belt (northern Transbaikalia). It was shown that the metasedimentary rocks of these domains are strongly variable in their geochemical and isotope geochemical characteristics. Regular variations in these characteristics were observed, and their correlation with the main stages of the evolution of the sedimentary paleobasin in the Neoproterozoic was established.     

Carbonate Rocks from the Riphean–Vendian Boundary Deposits of the Anabar Uplift: Isotope (<Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>13</Superscript>C, δ<Superscript>18</Superscript>O) Systematics and Chemostratigraphic Consequences

New ⁸⁷Sr/⁸⁶Sr, δ¹³C, and δ¹⁸О chemostratigraphic data were obtained for carbonate rocks of the Lower Riphean Yusmastakh and the Vendian Starorechenskaya formations. The δ¹³С values in dolomites of the Yusmastakh Formation varies from–0.6 to–0.1‰ and in dolomites and dolomitic limestones of the Starorechenskaya Formation, from–1.2 to–0.4‰ PDB, and δ¹⁸О values, from 24.4 to 26.4‰ and from 25.3 to 27.6‰ SMOW, respectively. The Rb–Sr systematics of carbonate rocks was studied using the refined method of stepwise dissolution of samples in acetic acid, including chemical removal of up to one-third of the ground sample by preliminary acid leaching and subsequent partial dissolution of the rest of the sample. Owing to this procedure, secondary carbonate material is removed, which enables one to improve the quality of the Sr-chemostratigraphic data obtained. The initial ⁸⁷Sr/⁸⁶Sr ratios in carbonate rocks of the Yusmastakh (0.70468–0.70519) and Starorechenskaya (0.70832–0.70883) formations evidence the Riphean–Vendian boundary in the Precambrian sequence of the Anabar Uplift.     

Isotope (δ<Superscript>13</Superscript>C and δ<Superscript>18</Superscript>O) and genetic features of manganese carbonates of the Mazul deposit (Krasnoyarsk region)

New isotope and mineral data on manganese carbonates of the Mazul deposit (Krasnoyarsk region) in combination with morphology of ore bodies suggest that the ores were formed in several stages with the involvement of meteoric solutions through infiltration and, possibly, exfiltration mechanisms. Based on the geological–geochemical data, manganese carbonates of the Mazul deposit may be ascribed to a new genetic subtype of the catagenesis (epigenesis) zone.     

Geochronological (U–Pb,U–Th–total Pb,Sm–Nd) and geochemical (REE, <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δ<Superscript>18</Superscript>O, δ<Superscript>13</Superscript>C) tracing of intraplate tectonism and associated fluid flow in the Warburton Basin,Australia

The Warburton Basin of central Australia has experienced a complex tectonic and fluid-flow history, resulting in the formation of various authigenic minerals. Geochemical and geochronological analyses were undertaken on vein carbonates from core samples of clastic sediments. Results were then integrated with zircon U–Pb dating and uraninite U–Th–total Pb dating from the underlying granite. Stable and radiogenic isotopes (δ¹⁸O, Sr and εNd), as well as trace element data of carbonate veins indicate that >200 °C basinal fluids of evolved meteoric origin circulated through the Warburton Basin. Almost coincidental ages of these carbonates (Sm–Nd; 432 ± 12 Ma) with primary zircon (421 ± 3.8 Ma) and uraninite (407 ± 16 Ma) ages from the granitic intrusion point towards a substantial period of active tectonism and an elevated thermal regime during the mid Silurian. We hypothesise that such a thermal regime may have resulted from extensional tectonism and concomitant magmatic activity following regional orogenesis. This study shows that the combined application of geochemical and geochronological analyses of both primary and secondary species may constrain the timing of tectonomagmatic events and associated fluid flow in intraplate sedimentary basins. Furthermore, this work suggests that the Sm–Nd-isotopic system is surprisingly robust and can record geologically meaningful age data from hydrothermal mineral species.     

Superorbital variability of the X-ray flux in the Be-donor binaries SXP 138, GX-304, and <Emphasis Type="Italic">γ</Emphasis> Cas

RXTE observations of the X-ray binary systems SXP 138, GX-304, and γ Cas in 1997–2011 have shown for the first time that these objects (X-ray binaries with Be donors) display X-ray flux variations on timescales of ～1000 days. This timescale is about 10 times longer than their orbital periods, and is comparable to the total time of the observations. The observed variations are apparently not strictly periodic and represent stochastic variability, as is characteristic of such systems in the optical. γ Cas is considered as an example. The series of optical observations of this system available in the AAVSO database covers 78 years, and is much longer than the timescale of the variability studied. Our analysis of this series has shown that γ Cas variability on a timescale of tens of years is predominantly stochastic with a power-law spectrum.     

Origin and evolution of oilfield brines from Tertiary strata in western Qaidam Basin: Constraints from <Superscript>87</Superscript>Sr/<Superscript>86</Superscript>Sr, δD, δ<Superscript>18</Superscript>O, δ<Superscript>34</Superscript>S and water chemistry

Chemistry of major and minor elements, ⁸⁷Sr/⁸⁶Sr, δD, δ¹⁸O and δ³⁴S of brines were measured from Tertiary strata and Quaternary salt lakes in the western Qaidam Basin. The water chemistry data show that all oilfield brines are CaCl₂ type. They were enriched in Ca²⁺, B³⁺, Li⁺, Sr²⁺, Br⁻, and were depleted in Mg²⁺, SO₄ ²⁻, which indicated that these brines had the characteristics of deeply circulated water. The relationship between δD and δ¹⁸O shows that all data of these brines decline towards the Global Meteoric Water Line (GWL) and Qaidam Meteoric Water Line (QWL), and that the intersection between oilfield brines and Meteoric Water Lines was close to the local spring and fresh water in the piedmont in the western Qaidam Basin. The results suggest that oilfield brines has initially originated from meteoric water, and then might be affected by water-rock metamorphose, because most oilfield brines distribute in the range of metamorphosing water. The ⁸⁷Sr/⁸⁶Sr values of most oilfield brines range from 0.71121 to 0.71194, and was less than that in salt lake water (>0.712), but close to that of halite in the study area. These imply that salt dissolution occurred in the process of migration. In addition, all oilfield brines have obviously much positive δ³⁴S values (ranging from 26.46‰ to 54.57‰) than that of salt lake brines, which was caused by bacterial sulfate reduction resulting in positive shift of δ³⁴S value and depleteed SO₄ ²⁻ in oilfield brines. Combined with water chemical data and δD, δ¹⁸O, ⁸⁷Sr/⁸⁶Sr, δ³⁴S values, we concluded that oilfield brines mainly originate from the deeply circulated meteoric waters, and then are affected by salt dissolution, water-rock metamorphose, sulfate reduction and dolomitization during the process of migration. These processes alter the chemical compositions of oilfield brines and accumulate rich elements (such as B, Li, Sr, Br, K and so on) for sustainable utilization of salt lake resources in the Qaidam Basin.     

The origin of high δ<Superscript>18</Superscript>O zircons: marbles,megacrysts, and metamorphism

The oxygen isotope ratios (δ¹⁸O) of most igneous zircons range from 5 to 8‰, with 99% of published values from 1345 rocks below 10‰. Metamorphic zircons from quartzite, metapelite, metabasite, and eclogite record δ¹⁸O values from 5 to 17‰, with 99% below 15‰. However, zircons with anomalously high δ¹⁸O, up to 23‰, have been reported in detrital suites; source rocks for these unusual zircons have not been identified. We report data for zircons from Sri Lanka and Myanmar that constrain a metamorphic petrogenesis for anomalously high δ¹⁸O in zircon. A suite of 28 large detrital zircon megacrysts from Mogok (Myanmar) analyzed by laser fluorination yields δ¹⁸O from 9.4 to 25.5‰. The U–Pb standard, CZ3, a large detrital zircon megacryst from Sri Lanka, yields δ¹⁸O = 15.4 ± 0.1‰ (2 SE) by ion microprobe. A euhedral unzoned zircon in a thin section of Sri Lanka granulite facies calcite marble yields δ¹⁸O = 19.4‰ by ion microprobe and confirms a metamorphic petrogenesis of zircon in marble. Small oxygen isotope fractionations between zircon and most minerals require a high δ¹⁸O source for the high δ¹⁸O zircons. Predicted equilibrium values of Δ¹⁸O(calcite-zircon) = 2–3‰ from 800 to 600°C show that metamorphic zircon crystallizing in a high δ¹⁸O marble will have high δ¹⁸O. The high δ¹⁸O zircons (>15‰) from both Sri Lanka and Mogok overlap the values of primary marine carbonates, and marbles are known detrital gemstone sources in both localities. The high δ¹⁸O zircons are thus metamorphic; the 15–25‰ zircon values are consistent with a marble origin in a rock-dominated system (i.e., low fluid_(external)/rock); the lower δ¹⁸O zircon values (9–15‰) are consistent with an origin in an external fluid-dominated system, such as skarn derived from marble, although many non-metasomatized marbles also fall in this range of δ¹⁸O. High δ¹⁸O (>15‰) and the absence of zoning can thus be used as a tracer to identify a marble source for high δ¹⁸O detrital zircons; this recognition can aid provenance studies in complex metamorphic terranes where age determinations alone may not allow discrimination of coeval source rocks. Metamorphic zircon megacrysts have not been reported previously and appear to be associated with high-grade marble. Identification of high δ¹⁸O zircons can also aid geochronology studies that seek to date high-grade metamorphic events due to the ability to distinguish metamorphic from detrital zircons in marble.     

<Superscript>40</Superscript>Ar/<Superscript>39</Superscript>Ar age of gold mineralization of the Malomyr deposit (eastern part of the Mongolian–Okhotsk foldbelt)

Reliable age estimation was obtained originally in this study for gold mineralization of the Malomyr deposit (the eastern part of the Mongolian–Okhotsk foldbelt), which is one of the most well-known deposits in the Russian Far East. The data obtained show that the age of hydrothermal process that resulted in the formation of the Malomyr deposit may be estimated as ～133–132 Ma. Data on magmatism of the same age within the considered region are absent. In the opinion of the authors, mobilization, redistribution of the ore material, and the formation of the Malomyr deposit were mostly controlled by dislocation processes accompanied by hydrothermal activity, which is supported by the results of structural studies.     

δ<Superscript>18</Superscript>O shifts of geothermal waters in the central of Weihe Basin,NW China

The thermal-waters resources in Weihe Basin of Shaanxi province, NW China are historically classified as middle- to low- temperature thermal-waters in China. Recent exploitation of the deep thermal reservoir in the centre part of the basin (i.e. Xi’an and Xianyang) had observed plentiful supply of thermal fluid with higher measured maximum temperature (120°C) and higher hydraulic pressure (80.50 MPa) in the deeper (more than 4,000 m deep) sedimentary basin. A recent isotope study shows that deep geothermal waters in the cities of Xi’an and Xianyang are characterized by an observable horizontal oxygen-18 (δ¹⁸O) shift and minor deuterium (²H) enrichment. The considerable oxygen shift is possibly due to the following four reasons: water–rock interaction at high temperature, slow circulation rate of water, low water-to-rock ratio, and old age. On the end number of the δ¹⁸O shift, minor δ²H enrichment occur when there is higher concentrations of H₂S, CH₄, I and Br with lower rate of rSO₄ ²⁻/rCl⁻ and r _Na⁺/r _Cl⁻ suggesting relatively isolated geological environment. In a few thermal waters points, r\textNa^\text+ \text/r\textCl^-r{\text{Na}}^{{\text{+}}} {\text{/}}r{\text{Cl}}^{-} < 0.85. This shows possible presence of formation waters. Combining the results from isotopic study and chemical analysis, we can classify the types of geothermal waters into three groups, the shallow and fast circulating system, the semi-circulating system and the deep and slow circulating system.