Acta Geochimica最新文献

The Huxu Au-dominated polymetallic deposit is a hydrothermal deposit located in the Dongxiang volcanic basin in the middle section of the Gan-Hang tectonic belt in South China. The orebodies primarily occur within the Jurassic-Cretaceous quartz diorite porphyry, while the genesis of this deposit is unclear. This study focused on geological and mineralogical characteristics, in-situ trace elements and S-Pb isotopes of three generations of pyrite of the Huxu deposit to clarify the distribution of trace elements in pyrite, ore-forming fluid and material sources, and genetic types of the deposit. The mineralization stage of the deposit can be divided into quartz-pyrite stage (S1), quartz-pyrite-hematite stage (S2), quartz-polymetallic sulfide stage (S3) and quartz-hematite stage (S4), with the corresponding pyrite being divided into three generations (Py1–Py3). in-situ trace element data of pyrite show that Au in pyrite mainly exists in the form of solid solution (Au⁺), and the content is relatively low at all stages (0.18 ppm for Py1, 0.32 ppm for Py2, 0.68 ppm for Py3), while Pb and Zn mainly exist as sulfide inclusions in the pyrite. S-Pb isotopes show that the sulfur and ore-forming material of this deposit are mainly sourced from magma. The mineral association, mineral textures and trace elements in different stages of pyrite indicate that fluid boiling and fluid mixing are the key factors of native gold precipitation in S2 and S4, respectively, while water-rock interaction controlled the precipitation of Pb-Zn sulfides. These integrating with geological characteristics suggests that the deposit should be an intermediate sulfidation epithermal deposit.

Geological setting, facies characteristics, and geochemistry, including TGA (thermo-gravimetric analysis) of Paleogene deposits in east Beni Suef region (Egypt), were studied in the present work. Lithostratigraphically, the area consists of three rock units, arranged from oldest to youngest: Tarbul Member of Beni Suef Formation (Middle-Late Eocene), Maadi Formation (Late Eocene), and Gebel Ahmar Formation (Oligocene), this last formation registered for the first time in the east of Beni Suef area (Egypt). Seven microfacies types (F1–F7) were determined by the microscopic examination of the studied samples in low- to high-energy and shallow-subtidal marine conditions. The lithostratigraphic, petrological, and geochemical results revealed that the Eocene succession in the present area is composed mainly of carbonates as well as siliciclastics. The Oligocene Gebel Ahmar Formation consists mainly of silica and iron oxides. The enrichment of the rock units with iron oxides in the studied area, as well as the high proportions of trace elements such as Zr, Ba, V, and Sr, in particular in the Gebel Ahmar Formation, reflects the influence of the hydrothermal solutions during the Oligocene. TGA, which monitors weight changes during heating at a constant rate, was used to determine the thermal stability and volatile component content of the materials. The ferruginous sandstone of Gebel Ahmar Formation exhibits various decomposition phases when exposed to thermal influences, with TGA indicating an initial mass decrease starting at 61.8 °C. In contrast, the ferruginous limestone of the Maadi Formation shows a single-phase mass decrease between 650 and 875 °C.

The Guanfang large-scale W deposit is located in the W polymetallic ore concentration area of Bozhushan in southeastern Yunnan, China. Despite extensive research, the fluid evolution process of the deposit remains ambiguous, leading to controversy regarding its genesis. This study conducted a detailed field geological survey, with systematic sampling of the KT6 orebody, to delineate mineralization stages. Fine mineralogy work, including the use of CL images of scheelite, in-situ LA-ICP-MS trace elements, and Sr isotopes, was carried out on different generations of scheelite formed in various stages. The findings identified the evolution of fluids in the mineralization process, shedding light on the genesis of the deposit. The study revealed four mineralization stages at the Guanfang W deposit: prograde skarn stage, retrograde skarn stage, quartz-sulfide stage, and carbonate-fluorite stage. Different generations of scheelite (Sch I, Sch II, Sch III) were observed in the first three stages, displaying distinct chondrite-normalized REE patterns. The REE of Sch I mainly substituted into the Ca site by REE³⁺  + □_Ca, and there may be a similar substitution of Nb for REE, whereas it is not the main substitution method. The REE of Sch II mainly enter the scheelite lattice in the form of REE³⁺  + Na⁺, and there may be a substitution of Nb for REE isomorphism. In the early stage, The REE of Sch III was mainly replaced by Nb for REE isomorphism, while in the later stage, the replacement mode of REE³⁺  +  □_Ca coexisted with it. The Mo content in scheelite, along with the corresponding Eu anomalies in both scheelite and garnet, collectively imply that the ore-forming fluids during various mineralization stages were predominantly oxidizing, with only slight reducibility observed in Sch II. The in-situ Sr isotope ratios of scheelite concentrates ranged from 0.7093 to 0.7153, resembling those of the Bozhushan granite, indicating a relationship between W mineralization and granite. In addition, the Y/Ho ratios of scheelite from various mineralization stages exhibit a narrow range (19–31), with a pronounced correlation between the contents of Y and Ho and a similar trend in their variation. This consistency suggests that the Guanfang deposit has undergone a uniform or comparable evolutionary process, implying a stable ore-forming fluid across different mineralization stages.

Alkaline igneous rocks represent one of the most economically important resources of radioactive minerals and rare metals. New field observations and petrographic studies are integrated with whole-rock geochemical analyses and Gamma ray spectroscopy data of alkaline rocks associated with the Amreit complex. The fieldwork was achieved by the collection of more than forty samples from alkaline granites and alkaline syenites. The youngest rocks cropping out in the study area are the cogenetic alkaline rocks, ranging from alkaline granite to alkaline syenite. These alkaline rocks are composed essentially of K-feldspar, alkali amphiboles (arfvedsonite), and sodic pyroxene, with accessories such as zircon, apatite, and ilmenite. Mineral characterization of the highly radioactive zones in both alkaline granite and alkaline syenite displays enrichment in monazite, thorite, zircon, ferro-columbite, xenotime, and allanite minerals. Geochemical analyses indicate that the Amreit rocks are alkaline with peralkaline affinity and have high concentrations of total alkalis (K₂O + Na₂O), large ion lithophile elements (LILEs; Ba and Rb), high field strength elements (HFSEs; Y, Zr and Nb), rare earth elements (REEs) and significantly depleted in K, Sr, P, Ti, and Eu, typically of post-collision A-type granites. Typically, the Amreit alkaline igneous rocks are classified as within plate granites and display A2 subtype characteristics. The fractionation of K-feldspars played a distinctive role during the magmatic evolution of these alkaline rocks. The geochemical characteristics indicate that the studied alkaline igneous rocks which were originated by fractional crystallization of alkaline magmas were responsible for the enrichment of the REE and rare metals in the residual melt. The high radioactivity is essentially related to accessory minerals, such as zircon, allanite, and monazite. The alkaline granite is the most U- and Th-rich rock, where radioactivity level reaches up to 14.7 ppm (181.55 Bq/kg) eU, 40.6 ppm (164.84 Bq/kg) eTh, whereas in alkaline syenite radioactivity level is 8.5 ppm (104.96 Bq/kg) eU, 30.2 ppm (122.61 Bq/kg) eTh. These observations suppose that these alkaline rocks may be important targets for REEs and radioactive mineral exploration.

The main objectives of this study were to investigate the distribution features of the ²¹⁰Po in abiotic (water and bottom sediments) and biotic (zooplankton, mollusks, fish) components of the North Crimean Canal (NCC) aquatic ecosystem and adjacent irrigated soils as well as assessment of the doses received by water organisms from α-radiation of absorbed ²¹⁰Po. The samples were processed using standard radiochemical methods accepted in international practice. The activity of ²¹⁰Po in the samples was measured using the alpha-spectrometric OCTETE Plus complex (ORTEC-AMETEK, USA). The measurement error did not exceed 20%. Activity concentration of ²¹⁰Po in the studied objects decreased in the following rank: suspended matter (73.6 Bq/kg d.w.) > soils (32.5 Bq/kg d.w.) ≈ bottom sediments (32.1 Bq/kg d.w.) > mollusks (23.4 Bq/kg w.w.) > fish (6.4 Bq/kg w.w.). The ²¹⁰Po distribution coefficient (K_d) values in water between suspended matter and its dissolved parts varied within the 1.4 × 10⁴–1.4 × 10⁵ L/kg range. The concentration factors (CF) of ²¹⁰Po for hydrobionts of the NCC were in the range 10³–10⁴ L/kg. The calculated absorbed radiation doses from ²¹⁰Po alpha radiation for the hydrobionts of the North Crimean Canal were significantly below the recommended dose limits.

Due to their high density, the ilmenite-bearing cumulates (IBC) (with or without KREEP) formed during the late-stage lunar magma ocean solidification are thought to sink into the underlying lunar mantle and trigger lunar mantle overturn. Geophysical evidence implied that IBC may descend deep inside the Moon and remain as a partially molten layer at the core-mantle boundary (CMB). However, partial melting may have occurred on the mixed mantle cumulates during the sinking of IBC/KREEP and the silicate melt may be positively buoyant, thus preventing the IBC/KREEP layer from sinking to the CMB. Here, we perform thermodynamic simulation on the stability of lunar mantle cumulates at different depths mixed with different amounts of IBC/KREEP from an updated LMO model. The modeling results suggest that the sinking of IBC/KREEP will cause at least 5 wt% partial melting in the shallow (~ 120 km) and a much larger degree of partial melting in the deep lunar mantle (~ 420 km). Due to the density contrast with the surrounding mantle, IBC/KREEP-bearing melts could potentially decouple under certain conditions. The modified lunar mantle by sinking of IBC/KREEP can better explain the formation of different kinds of lunar basaltic magma than the primary lunar mantle formed through differentiation of lunar magma ocean. Sinking of IBC/KREEP back into the lunar mantle may introduce plagioclase, clinopyroxene, garnet, and incompatible radioactive elements into the deep lunar mantle, which will further affect the thermal and chemical evolution of the lunar interior.

The North Crimean Canal is a watercourse originating from the Kakhovka Reservoir and flowing into the Crimean Peninsula. The canal is an important source of drinking water supply and is also used to irrigate agricultural lands and fill fish farms. Due to its location, in recent years its functioning has not been stable, and the processes occurring along the canal have been poorly studied. In this study, we determined the content (with a safety assessment), features of spatial and seasonal distribution, and potential sources of hydrocarbons in the water of the North Crimean Canal, Crimea. During the study period (from March to November 2023) in the primary canal, the content of aliphatic hydrocarbons did not exceed sanitary standards (0.05 mg·L⁻¹). Their increased concentrations in the secondary canals could be associated with the input of organic substances into the canal water as it moves across the Crimean Peninsula. The composition of n-alkanes had temporal and spatial variability. In the period from spring to autumn, the content of autochthonous compounds decreased sharply. The share of allochthonous compounds increased as a consequence of the natural processes. The analysis of biogeochemical markers showed that autochthonous compounds produced by phytoplankton predominated in the spring–summer period. Subsequently, they had less importance, and the main share was accounted for allochthonous n-alkanes.

Background

The Bundelkhand Craton is significant for preserving the multiphase granitoids magmatism from Paleoarchean to Neoarchean periods. It consists of a variety of granite rocks, including TTGs, sanukitoids, and high-K granitoids. This study presents geochemical characteristics of high-silica (68.97 wt.%–73.99 wt.%), low-silica (58.73 wt.%–69.94 wt.%), and high K₂O (2.77 wt.%–6.16 wt.%) contents of granitoids.

Objective

The data on Bundelkhand Craton's granitic magmatism and geodynamics is not sufficiently robust. Geochemical data from this study will be used to further understand the origin, source, and petrogenesis of granitoid rocks and their implications for the evolution of geodynamics.

Methodology

Twenty-one samples were collected and analyzed for major, trace, and REE elements. Major elements were measured using X-ray fluorescence spectrometry (XRF), and trace and REE elements were analyzed by ICP-MS. Standard procedures from the Geological Survey of India were followed.

Results

The geochemical analysis presents high-silica (68.97-73.99 wt. %), low-silica (58.73-69.94 wt. %), and high K2O (2.77-6.16 wt. %) contents in granitoids, classified as granite-granodiorite. The rocks are calcic to calcalkalic, magnesian, and range from peraluminous to metaluminous composition. REE patterns showed strong LREE enrichment relative to HREEs, with prominent negative Eu anomalies corresponding to earlier plagioclase fractionation. Multi-element patterns revealed negative anomalies in Nb, Sr, P, and Ti and positive anomalies in Pb.

Conclusion

The geochemical signatures attributed to the post-collisional magma generation and continental crustal contamination. The studied rocks show A-type and A2-type lineage, suggesting they originated from the melting of continental crust during transitional/post-collisional tectonic activity. The formation of hybrid granitoids in the Bundelkhand Craton is connected to the fractionation of hybrid magmas in shallow-seated magma chambers during these tectonic processes.

Leucogranite, pegmatite, and aplite from selected areas in the Wadi El Gemal area in the southern Eastern Desert of Egypt were investigated geochemically for their petrogenesis. These rocks represent a significant episode of felsic magmatism during the late stage of the Pan-African orogeny in the evolution of the Arabian–Nubian Shield (ANS) during the Late Neoproterozoic. On a petrographic basis, the leucogranite is sometimes garnetiferous and can be distinguished into monzogranite, syenogranite, and alkali feldspar granite. The analyses of muscovite, biotite, garnet, and apatite reveal the magmatic nature of the studied leucogranite. The investigated leucogranite, pegmatite, and aplite are alkali-calcic, calc-alkaline, and peraluminous. The peraluminous nature of these rocks is evidenced by using the chemical analyses of biotite. These studied rocks show a slight enrichment in light rare-earth elements (LREEs) and large-ion lithophile elements (LILE, especially Rb and Th), with an insignificant depletion of heavy rare-earth elements (HREEs). On a geochemical basis, the leucogranite, pegmatite, and aplite in the study area crystallized from multiple-sourced melts that include mafic, metagraywake, and pelitic. They were derived from melts generated at crystallization temperatures around 568–900 °C for leucogranite, 553–781 °C for pegmatite, and 639–779 °C for aplite based on the Zr saturation geothermometers, and at a pressure around 0.39–0.48 GPa, i.e. shallow depth intrusions. The studied felsic rocks have strong negative Eu anomalies, which are very consistent with an upper crust composition, indicating fractionation of feldspar cumulates. Also, they show a moderate La/Sm ratio indicating combined magmatic processes represented by partial melting and fractional crystallization. Integration of whole-rock chemical composition and mineral microanalysis suggests that felsic magmatism in the west Wadi El Gemal area produced voluminous masses of syn- to post-collisional granite, pegmatite, and aplite. An evolutionary three-stage model is presented to understand late magmatism in the ANS in terms of a geodynamic model. Such a model discusses the propagation of felsic magmatism in the ANS during syn-collisional to post-collisional stages.

The onset of the big mantle wedge (BMW) structure beneath the North China Craton remains debated. Research on the genesis of Late Mesozoic granites associated with gold deposits in the Jiaodong Peninsula above the BMW could provide fresh insights into this question. The monzogranite from the Zhaoxian-Shaling gold district was intruded during 154–148 Ma. This I-type granite has high-K calc-alkaline and metaluminous characteristics. The monzogranite formed at medium temperatures (718–770 °C) and was generated in a thickened lower crust at depths within the stability field of garnet. The monzogranite's high zircon Ce⁴⁺/Ce³⁺ and Eu_N/Eu_N* values and low FeO^T/MgO ratios, suggest that it formed in a high oxygen environment. Its variable ε_Hf(t) values with T_DM2 of 1.93–2.87 Ga imply that it originated from the melting of ancient crust basement, with contributions from mantle-derived materials. The granite's enrichment in LREEs and LILEs, and depletion in HREEs and HFSEs, along with its trace element tectonic discrimination diagrams and medium Sr/Y, indicate an adakite affinity in an active continental margin setting. The transition from S-type granites to I-type granites and finally to A-type granites observed in the eastern part of North China Craton suggests a shift in the tectonic environment from compression to extension. This change is also reflected in the transition from flat subduction to steep subduction. Therefore, the monzogranite was formed in a tectonic transition setting triggered by a change in the subduction angle of the Paleo-Pacific Ocean slab during the Late Jurassic. This event may have marked the initiation of the BMW above the North China Craton.