Geochemical Studies Research Articles

Flow Profile Estimating in production wells based on chemical composition of fluids (an example on Volga-Ural Petroleum and Gas Province)

Current problems in mature oil fields are high water cut and flow profile estimating of oil and associated brines from different layers. To establish the flow profile in production wells, geophysical research (Production Logging) is traditionally used by lowering special equipment into the well. Production Logging requires production stops and labor costs. Geochemical methods (Production Geochemistry) are used as an alternative solution: sampling is simple and efficient, which makes it possible to cover all the interesting area. Moreover, sampling does not require stopping the well. The geochemical method uses individual indicators of the composition of formation fluids produced from different perforation intervals. In this work, geochemical studies were carried out using wellhead samples from more than 100 wells, with single perforation for carbonate and terrigenous reservoirs. Some wells have joint exploitation of these formations. An automated algorithm was used to identify the distinctive characteristics of each formation based on the composition of the produced brines and oils. Data on the chemical composition of fluids from different development objects made it possible to determine the flow profiles in wells with joint production. Based on the results of the studies, the Devonian reservoir of the field under consideration is divided into 2 parts – northern and southern, which differ in the chemical composition of formation fluids. The same separation of the deposits into 2 parts is noted by field development analysis: over the past 50 years, the main production of oil and associated brines has been concentrated in the southern part of the deposit, confined to the fault, where the active work of the aquifer is assumed. It is recommended to use the obtained data for history matching of the reservoir simulation model.

Contribution of the Lower-Middle Jurassic source rocks in petroleum potential of the Jurassic-Cretaceous series within the central part of West Siberia

In this paper, based on a geochemical study of rock samples and fluids, it is proven that carbon-bearing deposits of the Lower and Middle Jurassic complex of Western Siberia on the southern outskirts of the Khanty-Mansiysk and Yamalo-Nenets Autonomous Okrug can generate liquid hydrocarbons. It has been established that the oil-generation potential of carbon-bearing deposits depends on the paleogeographic conditions of burial of the organic matter and the enrichment of rocks with macerals of the liptinite group. The complex of studies included pyrolysis of coals, coal-rich shale of Lower and Middle Jurassic rocks, study of maceral composition and measurement of reflectivity indicators, hydrocarbon and isotopic composition of extracts from rocks, as well as study of the composition of oils from the studied area, selected both within the Jurassic and Cretaceous complexes. The studied area is located in the Frolov megadepression and the South Nadym megamonoclise. The result of the research was the identification of significant geochemical characteristics of an additional source of hydrocarbons – coals, coal-rich shale of the Lower and Middle Jurassic, which had not previously been proven on the studied area, the contribution of which to the formation of oil and gas potential was confirmed by the correlation of hydrocarbon and isotopic characteristics of fluids and extracts from source rocks. Сluster and the principal component analysis were used for genetic correlation of extracts and oils. The conclusions obtained from geochemical studies suggest the possible prospects of Lower and Middle Jurassic deposits from the point of view of detecting hydrocarbon accumulations.

Lithology and oil content of the Bazhenov Formation in the central part of the Mansi Syneclise (Russia)

The article presents the results of studying four sections of the Bazhenov Formation and the overlying sediments in the central part of the Mansi syneclise, which are based on a comprehensive analysis of the results laboratory lithological and geochemical studies of core material and data from a wide range of geophysical studies of wells. For lithological characteristics, the section of the Bazhenov Formation was divided into five members, which differ in the ratio of carbonaceous, clayey, siliceous contents and carbonate component of rocks. The boundaries of the units were determined by their lithological composition, and then the velocity rectified taking into account GIS. The first two members are characterized by high contents of siliceous and clayey material, the third – a significant increase in the siliceous component, the fourth – increased concentrations of organic matter, which indicates its higher compared to the lower part section oil generation potential. In the fifth member, the content of carbonate material increases, the “coccolithic” interval is divided, and numerous fragments of bivalves are noted. Throughout the section minerals (strontianite, witherite) were discovered in the Bazhenov Formation, indicating that they occurred in diagenesis hydrothermal processes. In the section of one of the studied wells in the middle part of the Bazhenov formation, a reservoir interval was identified. High variability of reservoir properties of Bazhenov rocks was noted even within one oil-field, which is associated with their secondary transformations of rocks in dia- and catagenesis. It is substantiated, that materials of geophysical studies of wells that does not contain nuclear magnetic resonance data can only be used to identify possible productive intervals, the study of which by laboratory methods will allow establish the presence or absence of collectors in them.

Mineralogical and Geochemical Studies on the Daum Vent Field, Central Indian Ridge

Mineralogical and Geochemical Studies on the Daum Vent Field, Central Indian Ridge

Zhaman-Koitass granite-leukogranite massif (Northern Kazakhstan): U-Pb age, mineralogical and geochemical features, geodynamic analysis

Research subject. The Zhaman-Koitass granite-leucogranite massif of the Borovsk complex in Northern Kazakhstan. In the platform cover of the West Siberian plate, overlying granite-leucogranites, a large uranium deposit Semizbai of sandstone type is localized. Materials and methods. Geochemical, mineralogical, and geochronological studies of granite-leucogranites were conducted by X-ray fluorescence analysis, ICP-MS, electron probe microanalysis, scanning electron microscopy, and LA-ICP-MS (Analytical Center for multi-elemental and isotope research SB RAS). Results and conclusions. The Zhaman-Koitass massif is composed by A-type subalkaline leucogranites (SiO2 = 72.8–75.4 wt %, Na2O + K2O = 7.5–8.8 wt %, K2O/Na2O = 1.11–1.25, ∑REE = 120–231 ppm, (La/Yb)n = 10–22, Eu/Eu* = 0.2–0.4), which are characterized by titanite-ilmenite-magnetite accessory specialization. U-Pb isotope dating confirms the silurian age of the Zhaman-Koitass granite-leucogranites (426–420 Ma). The obtained geochronological data, along with the similarity of the chemical composition of the rocks, confirm that the granite-leucogranites of the Zhaman-Koitass massif belong to the Borovsk complex. The significant age interval between the ordovician granitoids of the krykkuduk complex (448 ± 2 Ma) and the silurian granite-leucogranites of the karabulak and borovsk complexes considered in the article (431–426 Ma) suggests a change in the geodynamic situation from suprasubduction to transform marginal-continental.

Coastal upwelling and redox variations in the northwestern Tarim Basin (northwest China) during the Middle-Late Ordovician: implication for paleo-depositional conditions of the organic matter enrichment in the Saergan Formation

The black shales of the Saergan Formation, which represent one of the main hydrocarbon source rocks in the Tarim Basin, witnessed a time span of organic matter enrichment by profound changes in the Earth System. A multi-proxy geochemistry study was carried out on the samples of the Saergan Formation to reconstruct the depositional environment and to explore the mechanism of organic matter enrichment of the unit at the Yingshanbeipo section, Keping area, northwest Tarim. Elemental and TOC data are suggestive of an upwelling setting, with a less pronounced oxygen minimum zone (OMZ) compared to the highly productive Peru and Namibian margins. Ferruginous anoxic bottom water conditions prevailed during most time of the deposition, with suboxic conditions dominating the basal and the top parts of the studied unit. As a whole, primary productivity seems to be the dominating factor that controlled the micronutrients and OM accumulations in the Saergan Formation whereas the role of benthic redox conditions may have been subordinate. The variations in primary productivity and bottom water redox conditions were resulted by the multiple, interacting environmental factors including nutrient supply regulated by oceanic circulation and climate changes, and relative sea-level fluctuations.

Age and genesis of the Tongshan banded iron formation in the Zhongtiao region, North China Craton

Banded iron formations (BIFs) occur extensively within the late Neoarchean supracrustal successions of the North China Craton (NCC). The Zhongtiao region is located on the southern margin of the NCC and is a terrane that may host BIFs. The region is well known for exposed early Precambrian rocks, such as the Sushui Complex, which can be divided into three lithological units: Neoarchean tonalite-trondhjemite-granodiorite (TTG) rocks, Paleoproterozoic multi-phase granitoids, and supracrustal rocks (i.e., the Sushui Group). Despite previous studies focused on the granites within the Sushui Complex, limited information is available on the supracrustal rocks and the potential for associated BIFs that have undergone amphibolite-facies metamorphism. Here, for the first time, we conduct petrological, geochronological, and geochemical studies on a newly discovered BIF layer (i.e., the Tongshan BIF) and the associated metamorphosed volcanic and sedimentary rocks in the Sushui Group. This study constrains the formation age of the Tongshan BIF, its source characteristics, and the redox state of the depositional environment. SHRIMP zircon U–Pb analyses on biotite plagioclase gneiss (the footwall rock of the BIF) demonstrate that the Tongshan BIF was formed at ca. 2583.1 ± 9.8 Ma. The Tongshan BIF, occurrs in a volcano-sedimentary succession, and may have been deposited in a continental margin arc along the southern segment of the NCC and may best be considered as an Algoma-type BIF. Given a 2.58 Ga depositional age for the Tongshan BIF and the Sushui Group, and the more restricted depositonal nature of Algoma-type BIFs, this work highlights the potential for the identification of additional BIF layers in the Zhongtiao region.Two distinctive sedimentary facies are recognized in the Tongshan BIF based on mineral composition: an oxide facies BIF, composed of magnetite and quartz, and a silicate facies BIF, composed of magnetite, amphibole, and quartz. The Tongshan BIF displays distinct seawater-like, shale-normalized REY characteristics, including super-chondritic Y/Ho ratios, HREE enrichment relative to LREE, and positive La, Gd, and Y anomalies. Consistently positive Eu anomalies are also observed, indicating the influence of a high-T hydrothermal input. No true negative Ce anomalies occur in the Tongshan BIF, suggesting a dominantly anoxic water column contemporaneous to deposition. Further, the lack of true negative Ce anomalies extensively occurring in the Tongshan BIF is consistent with other BIFs observed globally, and confirms that ferruginous (anoxic and Fe-rich) conditions were maintained in deep seawater before to the GOE.

Mineralogical and geochemical features of heavy concentrates from creeks of the North-Western end of the Оkhotsk terrane (Khabarovsk Krai)

The work presents the mineralogical and geochemical features of heavy concentrates from eight gold-bearing tributaries of the Anmandykan 2nd Creek in Khabarovsk krai: West otvilok, East otvilok, Somnitelny, Smezhny, Perevalny, Sobol, Vstrechny and Khvory. The mineral composition of heavy concen-trates of the right creeks differs from those of the left ones by a signifcantly higher percentage of minerals of the garnet group, sulfdes and native gold and a lower content of zircon and amphiboles. Garnets in heavy concentrates have different compositions and belong to the almandine and grossular-andradite series. Native gold grains in placers are yellow, small, mostly weak and medium-rounded, often with inclusions of quartz, feldspar, pyrite, iron oxides and hydroxides, and zircon. The native gold grains have low fneness and contain only Ag. The native gold grains are rimmed by thin, intermittent, porous aggregates of high-fneness gold. Two gold placer mineral assemblages are identifed, which include native gold, chalcopyrite, sphalerite and pyrite in assemblage 1 and native gold, scheelite and minerals of the garnet group (mainly of the grossular-andradite composition) in assemblage 2. Our data allow the forecast of the presence of two primary gold sources of gold-quartz-sulfde and skarn types. The comparison of results of mineralogical and geochemical studies showed their suitability and expressiveness for the interpretation of geological and metallogenic features of the studied area.

Origin and evolution of gold-bearing fluids in a carbon-rich sedimentary basin: A case study of the Algamarca epithermal gold-silver-copper deposit, northern Peru

Sediment-hosted gold deposits account for the major part of economic gold in the Earth's crust. However, the origin of the gold-bearing fluid and its evolution in sedimentary basins in the presence of organic carbon and its metamorphosed products such as graphite are poorly known. In an attempt to clarify these issues, we performed an integrated mineralogical, geochemical, and fluid-inclusion study of the Algamarca epithermal Au-Ag-Cu deposit, hosted by Mesozoic sediments corresponding to an over-mature petroleum system within the Marañón fold and thrust belt (northern Peru). Results show that mineralization started with a pre-gold stage characterized by quartz veins containing gold-poor pyrite and chalcopyrite. Most gold was deposited afterwards, during the main gold stage in an “invisible” form within arsenian pyrite, followed by minor visible native gold with sulfosalts and chalcopyrite at a later stage. Fluid inclusions in quartz from the pre-gold and gold stages show features analogous to those observed in porphyry Cu-Au systems such as vapor–liquid immiscibility, enrichment in K, Rb, Cu, As, and Sb, a wide range of salinity (5–35 wt% NaCl eq.), and similar elemental (atomic) ratios (Zn/Pb ∼ 4, 0.1 < K/Na < 5, Br/Cl ∼ 0.06), all consistent with a fluid of magmatic origin. In addition, the fluid inclusions from the pre-gold stage are highly enriched in CO2 (∼60 mol% in gas phase), CH4 (∼10 mol%) and H2S (∼30 mol%). Such high volatile contents are rather unusual for typical porphyry-epithermal systems and likely reflect reactions between the magmatic fluid and carbon-bearing sediments. This conclusion is independently supported by the temperature values of graphite metamorphic peak determined by Raman spectroscopy, which are similar to those derived by fluid-inclusion microthermometry in quartz veins. Our findings imply that strong interactions of magmatic fluid with carbonaceous matter favored gold transport through the sedimentary basin and its subsequent concentration in arsenian pyrite. Furthermore, our results point to a possible presence of porphyry-style mineralization beneath the sedimentary sequence hosting the epithermal Algamarca deposit, thereby providing new potential for exploration.

Lead in paint-contaminated residential soils with varying physicochemical properties from three large US cities: assessment of geochemical forms and recommendations on amendments for immobilization

Lead in yard soils has been recognized as the principal source of excess lead absorption among young children. The hazard imposed by soil lead is dependent on the geochemical forms of lead in soils. Soil properties such as pH, soil organic matter, clay, and carbonate content influence the geochemical forms of lead in soil. This study was conducted to investigate the correlation between soil properties and the geochemical speciation of lead in lead paint-contaminated residential soils from three major US cities. A comprehensive field survey was conducted, involving the collection of soils from ten houses in each of the cities: Baltimore, San Antonio, and Detroit. The influence of soil properties on geochemical speciation was analyzed to identify effective immobilization amendments for each soil type. Results showed that soils collected from San Antonio were slightly alkaline, whereas those from Baltimore were slightly acidic. Soils collected from Detroit were neutral to mildly alkaline in pH. San Antonio soils had relatively high soil salinity, high clay content, moderate to high soil organic matter (SOM), and high total carbon (TC). In contrast, soils collected from Baltimore had lower salinity and clay content, low SOM, and total carbon. Soils from Detroit exhibited relatively high salinity, clay, SOM, and TC contents. The average total soil lead concentrations were as follows; San Antonio 4,073 mg/kg, Baltimore 2,706 mg/kg, and Detroit 850 mg/kg. Geochemical speciation studies revealed significant differences in lead distribution among the studied soils. San Antonio soils exhibited high carbonate-bound and organic matter-bound fractions, while Baltimore soils had elevated soluble + exchangeable fractions. Detroit soils showed substantial lead in organic matter-bound fractions. Correlation analysis showed that the soil properties influencing exchangeable lead, were pH, total Al, and total Ca for San Antonio soils; pH and total P for Baltimore soils; and SOM and total Al for Detroit soils. Correlation analysis showed that there is a significant negative correlation (p &amp;lt; 0.05) between exchangeable lead and total Al (r = −0.653), and total Ca (r = −0.438) for San Antonio soils; pH (r = −0.286) and total p (r = −0.314) for Baltimore soils; and SOM (r = −0.628) and total Al (r = −0.408) for Detroit soils. Based on these results, the best potential immobilization amendments for each of these cities were predicted.

Geochemical study of rare earth elements, uranium and thorium in mylonitic rocks of north Abu Rusheid, Egypt using inductively coupled plasma mass spectrometry

Geochemical study of rare earth elements, uranium and thorium in mylonitic rocks of north Abu Rusheid, Egypt using inductively coupled plasma mass spectrometry

Zircon U-Pb-Hf Isotopes and Whole-Rock Geochemistry of the “Kulumudi Formation” from the Laofengkou Area (West Junggar): Implications of the Construction of a Juvenile Arc in the Junggar–Balkhash Ocean

The properties of ancient magmatic arcs are crucial for understanding the tectonic evolution of the Central Asian Orogenic Belt. The Middle Devonian Kulumudi Formation in the Laofengkou area of West Junggar lacks accurate chronological data constraints, which hampers the knowledge of the nature of the Late Paleozoic magmatic arcs in the West Junggar and circum-Balkhash areas. In this contribution, samples of pyroclastic rocks and sedimentary rocks were collected from the volcano–sedimentary strata of the Kulumudi Formation. Petrography, zircon U-Pb-Hf isotopic analysis and whole-rock geochemistry were carried out to constrain the age and the tectonic setting of the Kulumudi Formation. The zircon U-Pb age of the lithic crystal tuff from the Kulumudi Formation on the northeast side of the Alemale Mountains was 386 ± 2 Ma, accurately indicating that this rock unit formed during the Middle Devonian. However, the fine sandstone near the Huojierte Mongolian Township, originally assigned as the “Kulumudi Formation”, yielded a maximum depositional age of 341 ± 3 Ma. Combined with the stratigraphic contact, this rock unit was redefined to belong to the Lower Carboniferous Jiangbasitao Formation. According to the whole-rock geochemistry study, the lithic crystal tuff of the Kulumudi Formation was characterized as medium potassium–calc–alkaline series rock, which is relatively enriched in light rare earth elements and large ion lithophile elements (i.e., Rb, Ba, K) and depleted in high-field-strength elements (i.e., Nb, Ta, Ti), showing similar geochemical characteristics to the volcanic arc rocks. By contrast, the fine sandstone from the Jiangbasitao Formation had Al2O3/SiO2 (0.25–0.29) and K2O/Na2O (1.29–1.72) ratios close to those derived from the continental arc and active continental margin and was characterized as part of the continental arc field in the La-Th-Sc and Th-Sc-Zr/10 tectonic discrimination diagrams. Zircon Hf isotope analysis showed that the εHf(t) values of the Kulumudi Formation were +5.6–+12.8, and those of the Jiangbasitao Formation were +11.43–+15.48, both of which show highly positive juvenile characteristics. The above data indicate that the Kulumudi Formation and Jiangbasitao Formation both formed in a juvenile arc setting with ocean–continent subduction. Combined with the previous work, it was concluded that the southward subduction of the ocean basin represented by the Darbut–Karamay ophiolitic mélanges beneath the newly accreted arc crustal segments produced a juvenile arc with positive Hf isotope characteristics.

Genesis and tectonic setting of an Early Palaeozoic skarn in the East Kunlun Orogenic Belt, China: constraints from adakitic rocks and garnet megacrysts

ABSTRACT The East Kunlun Orogenic Belt (E-KOB) in the northern Tibet Plateau has undergone a long multi-stage history of drifting, accretion, and collision of the Proto- and Palaeo-Tethyan oceans. It has been addressed a trench-arc-back-arc basin system of the Proto-Tethys in the E-KOB, and the tectonic history of this major ocean has been well constrained by previous studies. However, the evolution of the back-arc basin still remains unclear. In this study, the Yugusayi monzodiorite, quartz-bearing syenite, and gradually contacted skarn rocks characterized by garnet megacrysts, in the North Qimantagh Belt were investigated. To reveal the tectonic setting and skarnization mechanism, geochemical and geochronological studies were conducted on the intrusions and garnet skarn, and the elemental compositions of the garnet megacrysts were also analysed. These monzodioritic rocks have high MgO (6.87–8.31 wt.%), Sr and La contents, low Y and Yb contents, and relatively high La/Yb (31–55) and Sr/Y (41–51) ratios, resembling to high-Mg adakites. They display right-sloping chondrite-normalized REE patterns, enriched in LILEs, and depleted in Ti with significant Nb-Ta troughs, which are similar to those of continental arc rocks. Zircons from monzodiorite and skarn samples have contemporaneous U-Pb ages of 459 ± 3 Ma and 454 ± 4 Ma with εHf(t) values of − 8.53 to − 4.64 and − 7.7 to + 12.8, respectively. In addition, the chemical zoning garnet megacrysts are composed mainly of grossular and andraditic grossular, and are characterized by spoon-shaped REE patterns. Based on our new data and the regional geology, it can be concluded that the Yugusayi intrusive rocks formed in a subduction-related tectonic setting and resulted in the skarnization. These data not only provide key constraints on the initial subduction of the Qimantagh back-arc basin’s oceanic crust, but also offer a unique opportunity to explore the Caledonian skarnization in the Qimantagh Belt.

Precise Determination of Eu Concentration in Coal and Sedimentary Rock Samples Using High-Resolution Inductively Coupled Plasma Mass Spectrometry (HR-ICP-MS)

Europium (Eu) in coal and sedimentary rocks has important mineral resource potential as well as being a crucial parameter in geochemical studies that can represent changes in the depositional environment during coal deposition and identify the depositional source region. Therefore, it is essential to realize the precise measurement of Eu in coal as this could be a useful parameter for paleoenvironmental reconstruction studies and the exploration of mineral resources. During inductively coupled plasma mass spectrometry (ICP-MS) analysis, polyatomic ions of Ba may interfere with Eu, causing the observed value to be higher than the actual value. This paper develops a new approach for Eu determination by using a high-resolution inductively coupled plasma mass spectrometry (HR-ICP-MS). The mass spectral interference and correction of Eu in the coal and sedimentary rock samples at low, medium, and high resolutions were investigated. The results showed that in the high-resolution mode (resolution = 10,000 amu), the interference of polyatomic ions of Ba could be distinguished from Eu; hence, Eu was determined under this circumstance. Under the optimal experimental circumstances, the detection limit was 0.006 μg/mL, the relative standard deviation was 0.80%–1.22%, and the linear correlation coefficient of the standard curve was over 0.9999. The recoveries of the 103Rh internal standard solution ranged from 94.41% to 100.10%. This method was verified using standard reference materials and selected samples, which demonstrated its high sensitivity, accuracy, and reliability, and a low detection limit, making it appropriate for detecting Eu in samples of coal and sedimentary rocks.

Petrogenesis of the Weideshan Pluton in Jiaodong and Its Implications for Gold Polymetallic Mineralization: Constraints from Zircon U-Pb-Hf Isotopes, Petrogeochemistry, and Whole-Rock Sr-Nd Isotopes

The Early Cretaceous Weideshan granites are associated with large-scale Au and polymetallic Cu-Mo-Pb-Zn mineralization. To investigate the petrogenesis of the Weideshan granite and constrain its tectonic setting during the Early Cretaceous, we conducted a zircon U-Pb-Hf isotope and whole-rock geochemical and Sr-Nd isotopic study of the granite. In situ zircon U-Pb dating of three granite samples yielded Early Cretaceous ages of 112.83 ± 0.80, 112.64 ± 0.91, and 111.82 ± 0.78 Ma. The samples had high-K calc-alkaline compositions and were enriched in the light rare earth and large-ion lithophile elements (e.g., K, Rb, Ba, Th, and U) and depleted in high-field-strength elements (e.g., Nb, Ti, and P). The samples had small negative Eu anomalies and initial 87Sr/86Sr and εNd(t) values of 0.70853–0.71029 and –18 to –14, respectively. The zircon εHf(t) values varied between −16 and −12, with corresponding two-stage model ages (tDM2) of 2180–2000 Ma. These characteristics indicated that the Weideshan pluton was formed in a back-arc extensional environment caused by subduction of the Paleo-Pacific Plate toward the Asian continent during the early Cretaceous. The magma was generated by crust–mantle interaction during lithospheric thinning. The diagenetic age of the Weideshan granites was the same as the formation age of gold and polymetallic ores in the Jiaodong area. The extensive fluid circulation induced by the magma emplacement may be the main source of ore-forming materials for the gold and polymetallic Cu-Mo-Pb-Zn deposits.

Tin Mineralization in the Triassic Chacaltaya District (Cordillera Real, Bolivia) Traced by In Situ Chemical and δ18O-δ11B Compositions of Tourmaline

Abstract We present a petrographic and geochemical study of tourmaline from the Triassic Chacaltaya Sn-polymetallic district in the Cordillera Real of Bolivia. Tourmaline is associated with greisens, breccias, and veins, which occur around the Triassic Chacaltaya peraluminous granitic stock hosted by Silurian metasedimentary rocks. Three main petrographic types of hydrothermal tourmaline have been identified: pre-ore greisen-related (Tur-1), syn-ore breccia-related (Tur-2), and syn-ore vein-related (Tur-3). The three types of tourmaline belong to the alkali group and have Fe-rich compositions mostly close to the schorl end member. Overlapping Fe/(Fe + Mg) ratios suggest broadly similar compositions of the hydrothermal fluids during the deposition of tourmaline. The most notable differences in minor and trace element contents include relative enrichment in Zn and Li in Tur-1 and relative enrichment in Ca, Sc, V, Cr, Sr, Sn, Y, Cs, Be, and Zr in Tur-3, with Tur-2 showing intermediate compositions between those of Tur-1 and Tur-3. The progressive enrichment in Sn from Tur-1 (avg = 14 ppm) through Tur-2 (avg = 311 ppm) and Tur-3 (avg = 476 ppm) indicates an increase of Sn concentrations in the hydrothermal system coinciding with cassiterite deposition in breccias and veins. The transition from high Li and Zn contents in Tur-1 to elevated Ca, Sr, V, and Cr contents in Tur-3 is interpreted as reflecting interaction between a hydrothermal fluid of magmatic origin and the metasedimentary country rocks. Strong and relatively steady positive Eu anomalies in all tourmaline types suggest dominantly reduced hydrothermal conditions. In situ δ18O and δ11B analyses of greisen-related Tur-1 reveal crystallization in isotopic equilibrium with magmatic water derived from a peraluminous S-type granite. In contrast, higher δ18O values of breccia-related Tur-2 and vein-related Tur-3 indicate crystallization in isotopic equilibrium with a fluid of metamorphic origin or a magmatic fluid that variably interacted with the metasedimentary host rocks. Geochemical modeling reproduces interactions between a fluid of magmatic origin and the host metasedimentary rocks at moderate water/rock ratios between 0.1 and 0.5. We conclude that cassiterite mineralization in the Chacaltaya district was formed primarily through interaction between B-Sn–rich magmatic fluids and the metasedimentary country rocks.

Paleoenvironmental evolution preceding the end-Permian mass extinction in the Lower Yangtze region (South China) and its controls on extreme enrichment of organic matter

The Changhsingian stage preceding the end-Permian mass extinction (EPME) was not only a significant period of global climatic–environmental change, but also an important period of widespread formation of organic-rich deposits. However, no consensus has been reached regarding the direct cause of mortality of the EPME and the main forcing mechanisms responsible for organic matter (OM) enrichment. As such, we conducted a petrological and geochemical study of the organic-rich deposits in the Talung Formation (Changqiao section), Lower Yangtze region, South China. Results show that the Lower Yangtze region was dominated by climatic fluctuations between cool–arid and warm–humid conditions during most of the Changhsingian. A paleoenvironmental reconstruction suggests that the early–middle Changhsingian was characterized by long-term anoxic to intermittent euxinic conditions, seasonal coastal upwelling, and weak basinal restriction. By contrast, the late Changhsingian was characterized by intensified euxinic conditions, weakened upwelling, and enhanced basinal restriction. The entire Changhsingian was also marked by high productivity, low detrital input, weak hydrodynamic conditions, and limited hydrothermal and volcanic activity. Quantitative analysis shows that the upwelling-driven high productivity and intensified anoxia–euxinia caused the OM enrichment, while other factors such as climate-controlled detrital inputs were less important. Two stages of climatic–oceanic dynamics were identified that reflect the relative roles and contributions of these different controlling factors. The paleoenvironmental reconstruction also indicates that euxinic conditions formed slightly earlier in deep waters, which may have been due to the combined effects of the persistent high productivity, weaker oceanic circulation, and intensified basinal restriction. Shoaling of these euxinic waters, combined with enhanced volcanism, climate warming, and ocean acidification occurred during the Permian–Triassic transition interval, could have contributed to the main stage of the EPME.

Multiphase evolution of a Li-pegmatite field from the Tashisayi area, Altyn Tagh, NW China: insights from a petrological, geochemical, and geochronological study

Multiphase evolution of a Li-pegmatite field from the Tashisayi area, Altyn Tagh, NW China: insights from a petrological, geochemical, and geochronological study

Magmatic evolution of the Schiel Alkaline Complex, Bushveld large igneous province, South Africa

The Paleoproterozoic Schiel Alkaline Complex (SAC) in the Limpopo Belt is a composite pluton that coevally formed with the layered intrusion of the Bushveld large igneous province. The SAC is mainly composed of alkaline felsic rocks, pyroxenite, gabbro, syenodiorite/−gabbro, and minor carbonatite and gabbroic dikes. Here we present a new geochemical, geochronological, and isotopic study of the felsic and (ultra)mafic rock suites of the western SAC. The whole rock SmNd isotope study of the felsic rocks revealed εNd2059 Ma values between −6.5 and − 3.8, while those of the syenodiorite/−gabbros and gabbros vary from −7.9 to −8.8, and − 4.3 to +1.1, respectively. The pyroxenites have εNd2059 Ma values of +7.4 to +2.7. The LuHf isotope study displays εHf2059 Ma values of −15.9 to −9.0 for the syenites, alkali granites and syenodiorite/−gabbros, and significantly higher εHf2059 Ma of −3.1 to +3.4 for the gabbros. LA-SF-ICPMS UPb zircon dating yields 2079 ± 10 Ma and 2088 ± 30 Ma for the syenodiorite/−gabbros, and 2086 ± 51 Ma and 2107 ± 30 Ma for the gabbros. In conjunction with published data from the eastern SAC, we propose a close petrogenetic link between the mafic and felsic rocks of the SAC. The positive εNd and εHf values of the pyroxenite and gabbro favor a depleted mantle source, while variable εNd and εHf values, and trace element ratios of the gabbros are ascribed to crustal contamination during magma ascent. The felsic rocks, however, mainly formed by fractional crystallization from the gabbros as revealed by major and trace element correlation trends, isotopic data and thermodynamic modelling.

SOURCES OF THE PALEOZOIC SEDIMENTARY ROCKS IN THE NORA-SUKHOTINO TERRANE: RESULTS OF Sm-Nd ISOTOPIC-GEOCHEMICAL STUDIES

The paper presents the first results on Sm-Nd isotopic-geochemical studies of the Paleozoic sedimentary rocks in the Nora-Sukhotino terrane at the northeastern flank of the South Mongolia-Khingan orogenic belt.According to the studies, the sedimentary rocks of the Zeya-Selemdzha and Amur fragments of the Nora-Sukhotino terrane are characterized by the two-stage Mesoproterozoic Nd-model age (TNd(DM2)=1.62–1.08 Ga) at negative εNd(0)=–9.5…–3.0 and εNd(T)=–5.8...–0.2. Based on previous geochemical and isotopic (U-Pb, Lu-Hf) studies of sedimentary rocks of the Nora-Sukhotino terrane, as well as on the available models for the formation of the South Mongolia-Khingan orogenic belt, it can be assumed that most of the Paleozoic sediments of the Nora-Sukhotino terrane were carried from the Mamyn terrane of the Argun superterrane with the participation of the island arc formations.