Acta Geochimica最新文献

The Huozhou complex in the Trans-North China Orogen exhibits two events of mafic magmatism (separated by ca. 700 Ma): Neoproterozoic (920 ± 15 Ma) Shimenyu diabase and Late Triassic (217 ± 2.5 Ma) Xingtangsi diabase. Investigations have focused on systematic petrology, zircon U-Pb dating, Lu-Hf isotopes, and lithogeochemistry. The research findings indicate that the Late Triassic Xingtangsi diabase of the Huozhou complex can be classified as a transitional type between intermediate and mafic rocks based on their SiO₂ content. This classification is supported by an average SiO₂ content of 53.94%, ranging from 53.33% to 54.28%. In the Zr/TiO₂ vs. Ce diagram, all samples lie within the range of basalt. The zircons from the Late Triassic Xingtangsi diabase have low ε_Hf(t) values ranging from –12.7 to –8.7, with an average of –11.1. Additionally, the single-stage model age T_DM1 is estimated to be between 1207 and 1701 Ma. These findings suggest that the magma responsible for the dyke originated from either partial melting or an enriched mantle source inside the Meso-Proterozoic lithospheric mantle. The elevated concentrations of Th (thorium) and LREEs (light rare earth elements), as well as the Th/Yb and Th/Nb ratios, suggest the potential incorporation of subducted sediments within the magma source region. The rock displays negative Nb, Ta, Zr, Hf, and Ti anomalies. These geochemical attributes align with the distinctive traits observed in volcanic rocks found within island arcs. The formation of the Late Triassic Xingtangsi diabase is likely associated with the geological context of an arc setting, which arises from the collision between the Yangtze plate and the North China Craton.

The Jiadi and Damaidi gold deposits in southwest Guizhou Province are the largest basalt-hosted Carlin-type gold deposits recently discovered in China. This study uses the Tescan Integrated Mineral Analyzer, supported by detailed field investigations, regional geological data, and extensive sample collections, including mineralized ore, altered wall rock, and unaltered basalt samples, for ore-bearing and geochemical analyses. Comparative analysis between altered and unaltered basalt samples revealed a mineral assemblage of sericite, quartz, and pyrite. This mineral composition forms through the hydrothermal alteration of unaltered basalt, originally containing feldspar, pyroxene, and ilmenite. The wall rock primarily features sericite, quartz, and hematite. During the alteration process, major, trace, and rare earth elements notably migrate. In the Jiadi deposit, K₂O, Rb, Au, and REE significantly increase, while Na₂O, CaO, MgO, and MnO decrease. SiO₂, Al₂O₃, and Fe₂O₃ levels remain relatively stable. In the Damaidi deposit, K₂O, Rb, and Au enrich, contrasting with the depletion of Na₂O, CaO, MgO, and MnO, while SiO₂, Fe₂O₃, Al₂O₃, TiO₂, and REE show no significant changes. In the wall rock, TiO₂, Al₂O₃, K₂O, and REE increase, while Na₂O, CaO, MgO, and MnO decrease; SiO₂ and Fe₂O₃ content remains unchanged. The mineralization process likely originated from mid- to low-temperature, reductive magmatic hydrothermal fluids rich in CO₂, CH₄, N₂, H⁺, S²⁻, HS⁻, H₃AsO₃, and [Au(HS)₂]⁻. These fluids migrated to tectonically weak zones in the Lianhuashan area, where Emeishan basalts are present. They reacted with Fe-bearing minerals in the basalt, such as ferro-hornblende and ilmenite, forming pyrite, arsenic-bearing pyrite, and arsenopyrite, thus enriching Au in these minerals. Additionally, K⁺ and H⁺ in the fluid reacted with plagioclase in the basalt, forming sericite and quartz. As the fluid entered the wall rock from structural weak zones, its oxidation increased, leading to the complete or partial reaction of Fe-bearing minerals in the wall rock, resulting in the formation of hematite or magnetite. This mineralization process is similar to that observed in carbonate-hosted Carlin-type gold deposits in southwest Guizhou, with the primary distinction being the iron source. In carbonate deposits, iron originates from ferridolomite within the wall rock, while in basalt-hosted deposits, it derives from ferripyroxene and ilmenite.

Gallium isotope is a potential geochemical tool for understanding planetary processes, environmental pollution, and ore deposit formation. The reported Ga isotope compositions (δ⁷¹Ga_NIST994 values) of some international geological standards, such as BCR-2 and BHVO-2 basalts, exhibit inconsistencies between different laboratories. During mass spectrometry analysis, we found that δ⁷¹Ga_NIST994 values of geological standards with or without the correction of the interference of ¹³⁸Ba²⁺ (mass/charge ratio = 69) on ⁶⁹Ga show significant isotope offsets, and thus efficient separation of Ba and correcting the interference of ¹³⁸Ba²⁺ are both crucial to obtain accurate δ⁷¹Ga values. By comparing δ⁷¹Ga_NIST994 values (relative to NIST SRM 994 Ga) of the same geostandards from different laboratories, we suggest that the isotopic heterogeneity from NIST SRM 994 Ga is one of the key reasons for the inconsistencies in δ⁷¹Ga_NIST994 values of BCR-2 and BHVO-2. To facilitate inter-laboratory comparisons, we measured the Ga isotopic compositions of 11 geological reference materials (including Pb-Zn ore, bauxite, igneous rocks, and loess) and two Ga solution standards (NIST SRM 3119a and Alfa Aesar). The δ⁷¹Ga_NIST994 and δ⁷¹Ga_IPGP values of these reference materials vary from 1.12 ‰ to 2.63 ‰ and − 0.13 ‰ to 1.38 ‰, respectively, and can be used to evaluate the precision and accuracy of Ga isotope data from different laboratories.

The Hatu gold deposit is the largest historical gold producer of the West Junggar, western China, with an Au reserve of about 62 t. The orebodies were controlled by NE-, EW-, and NW-trending subsidiary faults associated with the Anqi fault. This deposit exhibits characteristics typical of a fault-controlled lode system, and the orebodies consist of auriferous quartz veins and altered wall rocks within Early Carboniferous volcano-sedimentary rocks. Three stages of mineralization have been identified in the Hatu gold deposit: the early pyrite-albite-quartz stage, the middle polymetallic sulfides-ankerite-quartz stage, and late quartz-calcite stage. The sulfur isotopic values of pyrite and arsenopyrite vary in a narrow range from − 0.8‰ to 1.3‰ and an average of 0.4‰, the near-zero δ³⁴S values implicate the thorough homogenization of the sulfur isotopes during the metamorphic dehydration of the Early Carboniferous volcano-sedimentary rocks. Lead isotopic results of pyrite and arsenopyrite (²⁰⁶Pb/²⁰⁴Pb = 17.889–18.447, ²⁰⁷Pb/²⁰⁴Pb = 15.492–15.571, ²⁰⁸Pb/²⁰⁴Pb = 37.802–38.113) are clustered between orogenic and mantle/upper crust lines, indicating that the lead was mainly sourced from the hostrocks within the Early Carboniferous Tailegula Formation. The characteristics of S and Pb isotopes suggest that the ore-forming metals of the Hatu orogenic gold deposit are of metamorphogenic origin, associated with the continental collision between the Yili-Kazakhstan and Siberian plates during the Late Carboniferous.

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.

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 Wadi Natash volcanic field (WNVF) in the south of the Eastern Desert of Egypt is a typical example of well-preserved intraplate alkaline magmatism during the Late Cretaceous, i.e., prior to the Oligo-Miocene Red Sea rift. We compiled stratigraphic sections at two sectors; namely East Gabal Nuqra and West Khashm Natash (WKN) where the volcanic flows are intercalated with the Turonian Abu Agag sandstone with occasional paleosols when volcanic activity is intermittent. Peridotite mantle xenoliths are encountered in the first sector whereas flows in the second sector are interrupted by trachyte plugs and ring dykes. On a geochemical basis, the mafic melt originating from the lithospheric mantle beneath the WNVF practiced ~ 5% partial melting of phlogopite-bearing garnet peridotite. Basalts dominate in the two sectors and highly evolved (silicic) rocks are confined to the WKN sector. Rejuvenation of ancient Precambrian fractures following the NW–SE and ENE-WSW trends facilitated the ascend of Late Cretaceous mantle-derived alkaline magma. Structurally, the WNVF developed at the eastern shoulder of the so-called “Kom Ombo-Nuqra-Kharit rift system” that represents a well-defined NW-trending intracontinental rift basin in the southern Eastern Desert. In such a structural setup, the Natash volcanic are confined to half-grabens at the East Gabal Nuqra sector whereas the West Khashm Natash sector is subjected to extensional stresses that propagated eastwards. The WNVF is a typical example of fluvial clastics (Turonian) intercalation with rift-related alkaline volcanic rocks in northeast Africa.

Isotope effects are pivotal in understanding silicate melt evaporation and planetary accretion processes. Based on the Hertz–Knudsen equation, the current theory often fails to predict observed isotope fractionations of laboratory experiments due to its oversimplified assumptions. Here, we point out that the Hertz-Knudsen-equation-based theory is incomplete for silicate melt evaporation cases and can only be used for situations where the vaporized species is identical to the one in the melt. We propose a new model designed for silicate melt evaporation under vacuum. Our model considers multiple steps including mass transfer, chemical reaction, and nucleation. Our derivations reveal a kinetic isotopic fractionation factor (KIFF or α) α_{our model} = [m(¹species)/m(²species)]^0.5, where m(species) is the mass of the reactant of reaction/nucleation-limiting step or species of diffusion-limiting step and superscript 1 and 2 represent light and heavy isotopes, respectively. This model can effectively reproduce most reported KIFFs of laboratory experiments for various elements, i.e., Mg, Si, K, Rb, Fe, Ca, and Ti. And, the KIFF-mixing model referring that an overall rate of evaporation can be determined by two steps jointly can account for the effects of low P_H2 pressure, composition, and temperature. In addition, we find that chemical reactions, diffusion, and nucleation can control the overall rate of evaporation of silicate melts by using the fitting slope in ln(− lnf) versus ln(t). Notably, our model allows for the theoretical calculations of parameters like activation energy (E_a), providing a novel approach to studying compositional and environmental effects on evaporation processes, and shedding light on the formation and evolution of the proto-solar and Earth-Moon systems.

Theoretical studies of the diffusional isotope effect in solids are still stuck in the 1960s and 1970s. With the development of high spatial resolution mass spectrometers, isotopic data of mineral grains are rapidly accumulated. To dig up information from these data, molecular-level theoretical models are urgently needed. Based on the microscopic definition of the diffusion coefficient (D), a new theoretical framework for calculating the diffusional isotope effect (DIE_(v)) (in terms of D^*/D) for vacancy-mediated impurity diffusion in solids is provided based on statistical mechanics formalism. The newly derived equation shows that the DIE_(v) can be easily calculated as long as the vibration frequencies of isotope-substituted solids are obtained. The calculated DIE_(v) values of ¹⁹⁹Au/¹⁹⁵Au and ⁶⁰Co/⁵⁷Co during diffusion in Cu and Au metals are all within 1% of errors compared to the experimental data, which shows that this theoretical model is reasonable and precise.