Chemie Der Erde-Geochemistry最新文献

In the last few years, many efforts have been devoted to the factors controlling the distribution of CMs in karst bauxites, residual deposits hosted in carbonate rocks. Most of these efforts regard Mediterranean-type karst bauxite deposits of Cretaceous age occurring in southern Italy. Further, there is an increasing interest in assessing the usefulness of machine learning applications devoted to geochemically based datasets. With this in mind, we explored a data-centric machine learning arrangement aiming to find the proper input, limited to Al₂O₃, Fe₂O₃, TiO₂, and SiO₂, the most abundant major oxides occurring in these ores, for predicting the HREE distribution in southern Italy karst bauxite deposits.

Among the machine learning techniques used, Artificial Neural Network (ANN), Support Vector Machine (SVR), Random Forest (RF) and Extreme Gradient Boosting (XGBoost) are those that effectively predict HREE concentrations. A predictive model based on just Al₂O₃, Fe₂O₃, and SiO₂, is one conducing at the worst performance impact suggesting that TiO₂ is a relevant input variable in order to predict HREE concentrations in considered karst bauxite deposits. The XGBoost model was found to deliver the highest accuracy in predicting HREE for the validation data records (R² ~ 0.830, RMSE~7.299, MAE ~ 5.091).

Moreover, Fe₂O₃ is the highest correlated input variable with the output variable and is a significant predictor in our model suggesting iron oxyhydroxides play a relevant role in distributing HREE, likely through a scavenging mechanism at the expense of soil solutions.

A further step of our research will involve comprehensive cross-validation studies across multiple areas where Mediterranean-type karst bauxite deposits occur, thus providing a thorough assessment of the model's performance. By addressing these tasks and exploring avenues for improvement, the data-centric approach can advance its potential as a cheap and fast technique to perform a preliminary economic evaluation of potentially HREE abundance, as well as other CMs, in karst bauxite ores benefiting applications reliant on these critical resources.

In this study, we investigate the mineralogical and petrochemical characteristics of maskelynite occurring in a shocked basalt boulder from a terrestrial impact crater on a basaltic target – the Lonar impact crater in India, and the martian and lunar meteorites. The majority of Lonar maskelynite experienced solid-state transformation and maintained almost a uniform chemical composition, consistent with the unshocked feldspar. The locally flow-like texture and marginal vesiculation in feldspathic glass are needed in interaction with the impact-melt. The vesiculated melt occasionally occurring at the margins of maskelynite is characterised by Na-loss due to the shock-induced volatility. A shock pressure of ≤42 GPa and at a temperature of ≤1000 °C appear consistent for the formation of Lonar vesiculated melt/ feldspathic glass. Under the impact-induced shock metamorphism, maskelynite samples from the moon retain both the crystalline and amorphous domains with a distinct chemical heterogeneity attributed to different shock metamorphism effects of the plagioclase. In contrast, the martian maskelynites exhibit a smooth, homogeneous composition. The estimated shock pressure is relatively higher at ∼42–45 GPa based on experiments and models. The difference in Si/Al ratio in lunar (1–1.3) and martian maskelynite (1.5–1.9) suggests its inherent difference in composition of the crust, whereas the Lonar maskelynite shows overlapping composition with the martian maskelynite contending Lonar basalt as a potential terrestrial analogue to the martian crust.

Deep-ocean ferromanganese deposits represent one of the most important strategic reservoirs for rare and critical metals. In particular, Mn-oxyhydroxides, such as asbolane and lithiophorite, concentrate large amounts of Li, Ni, and Co into polymetallic nodules and crusts. However, because of their poor crystallinity and the presence of finely intermixed additional phases, these minerals cannot be unambiguously identified by standard X-ray powder diffraction methods. In addition, Li cannot be routinely detected by standard X-ray spectroscopy techniques.

In this work we show how the spatial distribution of asbolane (the Ni-Co-rich Mn-oxide) and lithiophorite (the Li-rich Mn-oxide) across strongly inhomogeneous ferromanganese mineralizations can be investigated at high-resolution (∼ 1 μm) via fast and easily accessible Raman scattering measurements. Because of the strong selectivity of these minerals to the incorporation of critical metals, the obtained micro-Raman maps provide also an indirect map of the Co and Ni vs. Li distribution in the crusts. The described results thus show that our spectroscopic approach could represent an efficient and valuable in situ tool for mineral chemistry and resource evaluation of these elements in ferromanganese deposits from deep-ocean environments. This research opens a new frontier for the application of Raman spectroscopy in ore prospecting for critical minerals and metals.

The Ploučnice River region (polzenite group) is uniquely characterized by its melilite-bearing subvolcanic rocks located in the northern section of the Bohemian Massif. They are the crystallization products of ultracalcic melts during the pre-rift evolution of the Ohře/Eger Rift, which is the easternmost part of the European Cenozoic Rift System. The melt was produced by low-degree partial melting of carbonate-bearing garnet peridotite and pyroxenite at a depth of approximately 100 km (P ~ 3.0 Gpa). The rapid ascent of the ultracalcic melts through the lithosphere was accompanied by fractional crystallization of olivine + spinel ± clinopyroxene mainly within the upper to middle crustal storage zone at depths between 12 and 24 km (0.3–0.6 Gpa). Notably, olivine crystallized generally at higher temperatures (1257–1356 °C) compared to clinopyroxene (1156–1203 °C) and plagioclase (1099–1112 °C). The calculated oxygen fugacity during fractional crystallization (perovskite, −4.8 to +3.9 ΔNNO) decreases at the late-stage of crystallization due to residual magma exsolving oxidizing fluids and decreased fO₂ (oxygen fugacity) of the magmas from which monticellite was crystallized (ΔNNO −6.0 to −3.9). The rounded shapes and chemical composition (Cr/(Cr + Al) 0.52–0.82) of partially resorbed chromite xenocrystic cores in subhedral to euhedral spinel grains indicate that they originated in the mantle. The first stage of magmatic evolution for the studied rocks is related to the Cr-spinel (Cr/(Cr + Al) 0.35–0.50) crystallization, which successively changed to a high-alumina composition (Cr/(Cr + Al) 0.25–0.30). Magnetite (magnetite–ulvöspinel solid solution) forms an atoll texture or small euhedral crystals in the groundmass. Both textural types of magnetite crystallized during the late-stage magmatic evolution of the ultracalcic melt. Carbonate or quartz-rich xenoliths were incorporated during magma emplacement under the upper crust. Sr-Nd isotopic data, mineral composition, and whole-rock chemical composition all verified that the assimilation of the xenoliths only affected the chemical composition of the host magma in the immediate neighborhood of the contract (up to a few millimeters around the xenolith).

Rare Earth Elements (REE) form critical raw materials in environment-friendly, high-technology devices and components, and therefore have been classified as “critical minerals and metals” by most countries. About 62% of the global resources of REE occur associated with carbonatite-alkaline complexes; however, the entire production of REE in India currently comes from secondary deposits, even though India contains a variety of REE-enriched primary source rocks, particularly carbonatites and alkaline complexes. There is, therefore, a significant potential in the county for new REE deposit discoveries associated with carbonatite-alkaline complexes. This research attempts to identify exploration targets for REE associated with carbonatite-alkaline complexes in northern India utilising a Self-Organising Maps (SOM)-driven workflow. This unsupervised machine-learning-based workflow eliminates the hand-crafting of input predictor features. The algorithm creates clusters of features directly from primary gridded geophysical and topographical datasets. The obtained clusters are then analysed based on available geological knowledge and empirical spatial associations with known occurrences in the study areas to identify prospective clusters and generate prospectivity maps. Nine new targets are identified across the Shillong plateau in northeastern and Western Rajasthan in northwestern India. These new targets, in addition to the known carbonatite-alkaline complexes, are recommended for further data collection and follow-up exploration. It is noteworthy that these targets conform to the targets identified by Aranha et al. (2022a, 2022b) using mineral systems-guided fuzzy inference systems.

Deposition of the Liantuo Formation is closely associated with the evolutionary history of the Rodinia supercontinent, as indicated by the breakup of the Yangtze Craton in South China. In this paper, we also carried out a detailed UPb zircon examination of the Liantuo Formation. Radiometric dating of sandstone within the Liantuo Formation suggested that it was deposited ca. 790 Ma, which coordinated with the age-paleopole contradictions in the Liantuo Formation for exploring the evolutionary history of the Yangtze Block during the breakup of the Rodinia supercontinent. All the Liantuo sandstones and shales have variable Sr isotopic compositions, with initial Sr isotope ratios ranging from 0.6920 to 0.7217, which support significant fluvial contributions during the deposition of the Liantuo Formation. Samples of the Liantuo Formation show distinct variations in NdHf isotopic compositions, suggesting that the Nantuo Formation received relatively juvenile materials (e.g., the newly formed Neoproterozoic rocks from the northern margin of the Yangtze Block), whereas the Liantuo Formation received relatively mature inputs (e.g., the Huangling granitoids and/or Kongling complex). The samples from the Liantuo Formation also contain various Pb isotopes (²⁰⁶Pb/²⁰⁴Pb = 17.56–17.76, ²⁰⁷Pb/²⁰⁴Pb = 15.05–15.43 and ²⁰⁸Pb/²⁰⁴Pb = 35.51–36.79). These values are also close to the Pb isotopic compositions of the Kongling complex, which suggest that the sedimentary rocks were enriched (the Kongling complex) and are consistent with the NdHf isotope data. That is, the major provenance of the Liantuo Formation is from the South China Block basement, which is the local Kongling complex. In contrast, the Luoquan diamictite can represent the well-mixed composition of the upper continental crust (UCC) in the North China Block during the Neoproterozoic.

Beryllium mineralization was studied by EPMA and XRD techniques in the beryl-columbite pegmatite D6e from the Maršíkov District, Bohemian Massif, Czech Republic. A detailed study of microtextures in BSE images revealed a complex formation of fine-grained secondary Be-silicates at the expense of primary beryl and earlier secondary Be-minerals in the following proximal and distal assemblages: (A) primary magmatic beryl; (B) proximal secondary beryl; (C) proximal bertrandite + K-feldspar and minor muscovite, chamosite, gismondine-Ca and quartz; (D1) proximal assemblages of milarite + gismondine-Ca and bavenite-bohseite + epidote; (D2) distal assemblages on brittle tectonic cracks including milarite, bavenite-bohseite, albite, K-feldspar, quartz and rare phenakite, and (D3) epidote, bavenite-bohseite, quartz, albite, K-feldspar and minor milarite. A formation of secondary Mg,Fe,V,Na-enriched beryl (B) is connected with a mixing of residual (pegmatite) and external Ca,Mg,Fe,V-enriched fluids from the host amphibole gneiss at T ~ 300–400 °C and P ~ 200–400 MPa. The assemblage (C) formed due to an income of K,Mg,Ca-enriched fluids (residual + external) at T ~ 150–300 °C. The subsequent proximal (D1) and distal (D2, D3) assemblages formed during an moderate to strong income of Ca-rich external fluids from the host rocks related to retrograde hydrothermal-metamorphic overprint manifested by the Alpine-type hydrothermal veins. A common presence of epidote in the assemblages with bavenite-bohseite suggests crystallization at T < ~200–300 °C. Detailed textural and paragenetic study of primary and secondary Be-minerals is a useful tool to recognize and study various processes proceeded during subsolidus evolution of granitic pegmatites.

The Lancangjiang tin belt is located on the eastern margin of the Tibetan Plateau, which is in the northern part of the Southeast Asian tin belt that contains Sn, W, and base metal ore deposits. Tourmaline alteration and high B contents are typical features of the magmatic–hydrothermal systems in the Lancangjiang tin belt. Our new data for tourmaline intergrown with cassiterite reveal complex geochemical features that provide important insights into the origins of ore-forming fluids in this B-rich tin belt. Most of the tourmaline in the Sn deposits and metamorphic rocks belongs to the alkali-group dravite series, except for some that is part of the X-vacant group. The tourmaline compositions differ among the ore districts, but all exhibit Fe-poor (0.61–1.08 apfu) and Mg-rich (1.43–2.06 apfu) compositions, with Fe/(Fe + Mg) = 0.18–0.46. Most trace elements in the tourmaline occur at low contents (<50 ppm), including the large-ion lithophile elements (e.g., Cs and Ba) and high-field-strength elements (e.g., Nb, Ta, Hf, and Th). However, some trace elements (e.g., Zn, Sr, and V) and Sn have high contents (up to several hundreds or thousands of ppm). The δ¹¹B values of tourmaline from the Sn deposits range from −14.7 ‰ to −11.3 ‰, except for those in the Man Makhsan Sn deposit, which range from −12.0 ‰ to −8.1 ‰. The δ¹¹B values (−14.7 ‰ to −8.1 ‰) and the positive correlation between Sn contents and Nb/Ta, Al/Ga, and K/Cs ratios in the ore-related tourmaline indicate that the exsolved ore-forming fluids were derived from highly fractionated S − type granitic magmas. The fluids were mainly B-, Sn-, and Al-enriched. The low Fe/(Mg + Fe) ratios, high oxygen fugacity, and Al saturation were favorable for cassiterite precipitation, and are proxies for the ore-forming potential of large-scale Sn deposits.

To investigate the mobility of elements during extreme weathering of basalts, major and trace elements concentrations along with the mineralogical abundance of weathered products, are reported for lateritic profiles developed over the ~118.1 ± 0.3 Ma old Rajmahal Trap basalts in Eastern India. Special emphasis has been given to studying the re-distribution of rare earth elements during extreme chemical weathering of relatively less explored Rajmahal flood basalt, which results in the development of lateritic profiles. Here, we conduct a systematic attempt to decipher the alteration of the composition of laterite developed over the Rajmahal flood basalt.

We demonstrate that the chemical index of alteration (CIA) and the mafic index of alteration (MIA) are less effective in deciphering extreme chemical weathering events, where desilication dominates the weathering process. In the weathered residues, the retention of REEs generally follows the trend of light rare earth elements (LREE) > middle rare earth elements (MREE) > heavy rare earth elements (HREE). Except for La and Ce, all other REEs show a depleting trend with reference to the parent rock composition. The ∑REE did not show any significant relationship with the commonly used weathering indices, which shows that the retention of REEs is not a function of the extent of weathering. The covariation of Ce anomaly with the Fe and Mn distribution in the samples reflects a redox-controlled mechanism. The negative correlation between the Eu anomaly and kaolinite abundance suggests that Eu mobilization from parent rock happens during initial weathering phases when plagioclase weathering dominates. The samples show an interesting positive Gd anomaly, which is explained by enhanced retention of Gd than neighboring Eu and Tb. During the chemical weathering of basalt, the retention of Ho was found to be greater than its geochemical twin Y. The results of this study show that significant differences exist between the REEs' distribution of parent basalt and that of the weathered residue, which has implications for the application of REEs as a provenance indicator and/or weathering proxy. The enrichment of Th and U in the weathered samples is explained through the addition of aeolian dust of upper continental crustal composition.