Chemie Der Erde-Geochemistry最新文献

The interaction between climate and tectonics chiefly governs the erosion and sedimentary budget in the tectonically active Himalayan Mountain chain. In the present study, the relative contribution from the various litho-tectonic units of the Himalaya to the sedimentary budget of the Teesta River System (TRS) (a major tributary of the Brahmaputra system) has been deciphered using the Sr and Nd isotopic compositions (⁸⁷Sr/⁸⁶Sr, ε_Nd) of bed sediments. The possible controlling factors for such observed sedimentary provenance have also been assessed. The discharge-weighted sediment transport amount has been translated into the quantification of the erosion rate in the basin. The ⁸⁷Sr/⁸⁶Sr and ε_Nd values in the silicate fraction of bed sediments range from 0.74867 to 0.90529 and − 24.3 to −13.9, respectively. In the Teesta main channel, ⁸⁷Sr/⁸⁶Sr and ε_Nd show large variability ranging from 0.74867 to 0.82288 and −21.8 to −13.9, respectively along its entire course. This large variability indicates changes in sediment sources i.e., Higher Himalaya and Lesser Himalaya. These data sets show that >60 % of TRS sediments are derived from the Lesser Himalaya, indicating a higher physical erosion over the catchments in the Lesser Himalaya. Our assessment suggests that the sedimentary budget of the Teesta Basin is chiefly governed by higher exhumation as well as the vertical uplift rate of the lesser Himalaya sector of the study area (due to the presence of Rangeet duplex) coupled with focused precipitations. Based on the sediment discharge method, we estimate the erosion rate in the Teesta basin to be 1.7 ± 0.5 mm/yr, which yields a total annual sediment flux of ~41.4 ± 12.4 Mtons. This estimate is comparable to other Himalayan River basins like the Ganga basin, however, lower than the erosion hotspots such as the Brahmaputra basin (Namche Barwa and eastern Syntaxis region) and Indus basin (western Syntaxis region). The obtained erosion rate of the Teesta basin seems to result from the active tectonics in the Sikkim sector of the Himalaya.

Barite can form in a variety of geological environments, as it occurs in a wide range of mineral deposits. To determine the origin and physicochemical conditions under which the Ab Torsh barite deposit formed, an extensive study was conducted using petrographic, Rare Earth Element (REE) geochemical, O and S isotopic, and fluid inclusion methods. Barite mineralization occurs at Ab Torsh as a stratabound vein in the Senonian carbonate rock units. Barite-quartz is accompanied by subordinate malachite, chrysocolla, Fe-Mn oxide-hydroxides, galena, azurite, fluorite, pyrite, and bornite. The ∑REE values are very low in barite (5.32–14.56 ppm), with chondrite-normalized patterns showing enrichment of Light Rare Earth Elements (LREE) relative to Heavy Rare Earth Elements (HREE). The low ∑REE content and REE element ratios (Ce/La, (La/La*)_N, and (Gd/Gd*)_N) indicate that seawater with a highly altered geochemical signature (connate water) acted as a Ba ore-forming fluid. The δ¹⁸O and δ³⁴S values in barite (+10.4–+11.1 ‰ and +27.3–+27.8 ‰, respectively) and the δ³⁴S values in galena (+6.3 and + 7.9 ‰) indicate that the sulfate (and thus sulfur) originated from sulfate-bearing connate waters and/or evaporites. Thermochemical Sulfate Reduction (TSR) was the most likely mechanism for the formation of the reduced sulfur in galena. The salinity and homogenization temperatures in the aqueous fluid inclusions of barite and quartz (2.7–19.3 wt% NaCl equivalent and 110–275 °C, respectively) indicate that basinal fluids containing a meteoric water component were the source of the mineralizing solutions. The fluid inclusion data demonstrate that two fluid mixing have occurred: one between the hot basinal brines and cold meteoric waters, and another between heated and cold meteoric waters. It is estimated that the hot fluids derived from a maximum depth of about 9 km. The Ab Torsh deposit is classified here as a structure (unconformity)-related barite deposit. It is concluded that intense faulting and brecciation of the host rocks caused by post-Cretaceous compressional tectonics probably provided the channels necessary for the upward migration of deep mineralizing fluids from a basinal brine source. Barite formed where these ascending hot, Ba-bearing hydrothermal fluids encountered cooler, sulfate-bearing connate waters trapped in the overlying Senonian strata and/or the descending cold meteoric waters that dissolved evaporite-bearing rock units.

The presence and nature of uranium minerals subject to high leachability is one of the most significant factors in the exploitation of low-grade ores. The detection of these minerals is a challenging task due to their small size and low abundance. This paper presents a novel method of data processing that allows distinguishing and visualization of micron-scale U minerals. For this purpose, core samples of U-bearing sandstones from the Břevniště deposit (Czech Republic) were used. After their study by inductively coupled plasma spectroscopy, optical and scanning electron microscopy (SEM-EDS) and X-ray diffraction analysis, the presence of authigenic grains of U minerals occurring in various associations and their chemical compositions were confirmed by an electron microprobe. However, the use of these conventional instruments for the analysis of heterogeneous ore material with valuable information hidden, showed the necessity for a special data treatment. Matrix transformation of raw SEM-EDS data allowed for even more accurate visualizations such as contour and point maps of elemental distribution. Then, mathematical and visual correlations of transformed data revealed relationships among some measured elements (Ca, P, U, Zr, Nb, Fe, S) and their spectral overlaps. Conditions of ore formation were predicted based on the visualizations, correlations and the paragenesis of ningyoite in uranium ore. Herein suggested approach can help to identify economically significant minerals in complex mineralizations worldwide and increase the mining potential of the deposits.

Mineralization of fossil woods with unusual mineral phases remains an underconstrained process despite its relatively common occurrences. Aside from common mineralization agents such as silica or carbonates, there are also atypical mineralization associations, such as zeolite-group phases, but the zeolitization process has not yet been investigated in detail. We studied zeolitized woods collected from two localities in the Cenozoic alkaline České Středohoří Volcanic Complex (Ohře Rift, Czech Republic), where fossil woods of diverse paleobotanical classification were deposited in volcaniclastic rocks of the same origin (lahar) and stratigraphic formation (Upper Oligocene). The identical geological setting allowed the investigation of potential variables influencing this type of mineralization by combining paleobotanical classification, detailed mineralogy, mineral chemistry, geochemistry, Sr isotope analysis and KAr chronology.

The new results demonstrate the significant potential of fossil woods mineralized with zeolite-group minerals to be used to reconstruct the formation and deposition conditions of the lahars in which these woods are contained. The composition of zeolites is strongly dependent on thermal conditions and material exchange between wood and host rocks. Dominant mineral phases are phillipsite and chabazite in variable proportions. The phillipsite/chabazite ratio correlates well with the magnitude of the Eu anomaly, suggesting crystallization of phillipsite at a higher temperature under hot-lahar conditions of deposition. Chabazite lacking an Eu anomaly represents the later, colder mineralization stage. The ⁸⁷Sr/⁸⁶Sr ratios ranging 0.7042–0.7047 provide an additional line of evidence of fluid derivation from volcaniclastic deposits of the Upper Oligocene Děčín Fm.

A-type granite shows unique geochemical characteristics and refers to specific formation conditions and tectonic environments, and therefore plays an important role in reconstructing orogenic events. In this paper, we systematically investigated the zircon UPb, whole-rock geochemistry, in-situ zircon Hf, apatite trace elements, and in-situ Nd data of the Huanren monzogranite to discuss its petrogenesis and the tectonic evolution of Jiao-Li-Ji belt. The studied zircon and apatite grains show typical oscillatory zoning and subhedral texture as well as obvious internal structure without any fluid/mineral inclusions, referring to a magmatic origin. Therefore, zircon UPb dating results suggest that the Huanren monzogranite was formed in the late Paleoproterozoic (1863–1842 Ma). The studied rocks show typical A-type granite affinities as high SiO₂ (64.54–69.06 wt%), Na₂O + K₂O (8.00–9.34 wt%), and Zr + Nb + Ce + Y (381–598 ppm) concentrations, and FeO^T/(FeO^T + MgO) (0.82–0.85) and 10000Ga/Al (2.37–2.65) ratios. Their high whole-rock Zr temperature (812–866 °C, generally >840 °C), and low whole-rock Sr (109–157 ppm), apatite Sr (43.4–79.3 ppm) contents, whole-rock Sr/Y (4.79–8.22) and apatite Sr/Y (0.014–0.033) ratios, combined with slightly depleted in-situ zircon Hf (−1.62 to +2.50) and in-situ apatite Nd (−3.22 to −1.65) isotopic compositions, and ancient zircon Hf two-stage model ages (T_DM2; 2606–2351 Ma), apatite Nd-T_DM2 (2576–2451 Ma), suggest that the studied rocks were formed by partial melting of Neoarchean to Paleoproterozoic granitic gneiss in a high temperature and low-pressure condition. The studied A-type granite, together with regional A-type granite and alkaline syenite in the North China Craton, record the post-collisional event between the Nangrim and Longgang blocks, which combined with coeval worldwide A-type granite magmatisms, representing a response of the initial breakup of Columbia supercontinent.

The research sheds light on the age, origin, and tectonic environment of two significant alkaline plutons, Sullya and Angadimogar, located in the Mercara Shear Zone in the Southern Granulite Terrain of southern India. The zircon U-Pb dating indicates that the plutons crystallized at 829 ± 3.7 Ma and 831 ± 4.7 Ma, respectively. The zircon geochemistry suggests that Sullya and Angadimogar plutons underwent distinct crystallization processes, with zircons from Angadimogar samples crystallizing at varying temperatures over a prolonged period with limited fO₂ levels and zircons from Sullya samples crystallizing at stable temperatures and variable redox conditions. Most of the zircons from both plutons have trace element characteristics indicating formation in the continental crust. In contrast, some zircons from Sullya have trace elements similar to those found in the oceanic crust. The presence of two distinct types of zircons in Sullya samples indicates that the parental melt of the Sullya pluton consisted of magma from multiple sources. The average estimated temperature for zircon crystallization in Sullya was 691 °C, while the mean temperature in Angadimogar was 802 °C. The research implies that the plutons were emplaced parallel to the shear plane during an extensional regime, reactivating the paleo shears during the late Proterozoic era. The study highlights the importance of using multiple geochemical and geochronological techniques to gain a better understanding of the complex geological evolution of the Precambrian terrains.

Apatite is increasingly used as a tracer for petrogenetic and metallogenic processes. We present the cathodoluminescence (CL) texture, and composition of igneous apatite from the tungsten-bearing Zhuxi biotite granite and the nearby Zhenzhushan granite porphyry in the Late Mesozoic Jiangnan porphyry-skarn tungsten belt in South China. Most apatite grains show homogeneous CL-dark texture, but some apatite grains have CL-dark cores and CL-bright rims, or CL dark-bright-dark-bright (from core to rim) growth zoning bands. The CL-bright bands are Mn depleted. Apatite from the Zhuxi biotite granite has elevated W contents (mean: 0.16 ppm; n = 75) compared to the Zhenzhushan granite porphyry (mean: ≤0.02 ppm; n = 32). The CL-bright rims of apatite from Zhuxi have more W than the CL-dark cores, i.e. average 0.58 ppm W versus average 0.07 ppm W, respectively, implying that the late-stage magmatic system was enriched in W. The chondritic Y/Ho ratios (22−33) of Zhuxi apatite, and the similar or even slightly more elevated REE contents of the CL-bright rims, compared to the cores, rule out hydrothermal effects. The Zhuxi apatite samples have distinctly negative δEu, F-rich and sulfur-depleted features, which are interpreted as signature of igneous apatite from magmatically evolved granitic intrusions associated with W deposits, while the inverse trend applies to apatite from Cu porphyry systems. In-situ microanalysis of apatite may be a useful exploration tool.

Egypt hosts numerous rare-metal granites, i.e., highly evolved granites enriched in rare metals (Ta, Nb, Be, Sn, Zr, Th, and REE). However, the processes involved in the rare-metal enrichment are not fully understood. We present new data on the textural characteristics and chemical composition of rare-metal mineralization associated with microgranite dikes in the Ras Abdah area of the Egyptian Eastern Desert. These dikes are garnet-bearing leucogranites (GLG) composed of perthitic alkali-feldspars and quartz. When compared to other Egyptian A-type granites, microgranite dikes are alkaline rocks with particularly higher HREE contents. Zircon, huttonite, fergusonite (Y), and Fe-Ti-Zn oxides (magnetite, Zn-bearing ilmenite and pyrophanite) are largely associated with the altered domains, which are also enriched in Nb, Zr, Y, Ta, Th, and REE. However, similarities between the chondrite-normalized REE patterns of the altered and unaltered domains of the GLG dikes favor the hypothesis of a unique magmatic signature. Moreover, the chemical and textural features of rare-metal minerals indicate that the alteration of primary minerals was caused by deuteric fluids or aqueous residual melt exsolved from the parental granitic magma (autometasomatism). Garnet compositions are rich in the spessartine component (up to 84 %), which is typical of garnet in highly evolved pegmatitic rocks. Furthermore, garnet exhibits no major element zoning but shows chemical fluctuations in trace element concentrations, suggesting correspondingly abrupt changes in melt composition due to sequential magma pulsing. This magma emplacement may cause crystal nucleation and oscillatory crystallization followed by magmatic segregation. Overall, parental magma type, dike injection, and magmatic-hydrothermal processes all play a role in the unusual enrichments of rare metals.

The closure time of the Jinshajiang Ocean (a branch ocean of the Paleo-Tethys) has been widely debated due to the complex structure and strong deformation of the northern-central Tibetan Plateau. In this paper, the U-Pb-Hf isotope compositions of detrital zircons from Tongtian River sand sediments from the northern-central Tibetan Plateau were analyzed via laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). The UPb ages of the detrital zircons from Tongtian River sand sediments were distributed in six major groups: 2685–2346 Ma, 2035–1676 Ma, 1220–580 Ma, 544–407 Ma, 297–157 Ma, and 46–20 Ma. The ε_Hf(t) values (range from −30 to +19) of the detrital zircons varied within a wide range from negative to positive, indicating that the zircons were sourced from diverse host magmas. The 46–20 Ma zircons and their Hf isotope compositions reveal that the far-field effect of the Cenozoic India-Eurasia plate collision triggered reworking of the Neoproterozoic basement of the North Qiangtang terrane. The U-Pb-Hf isotope compositions of these detrital zircons indicate that the Tongtian River sand sediments are composed of a mixture of materials from the Bayan Har-Songpan Ganzi terrane and the North Qiangtang terrane, with a contribution ratio of 3:7. The UPb ages and Hf isotope compositions of the zircons from the Tongtian River sediments and regional igneous rocks suggest that the ε_Hf(t) values of the Permian zircons are predominantly positive while those of the Triassic, especially the Late Triassic, are negative. This marked shift most likely indicates that the closure of the Jinshajiang Ocean occurred by at least the Late Triassic.