Acta Geochimica最新文献

Permanently shadowed regions (PSRs) on the Moon are potential reservoirs for water ice, making them hot spots for future lunar exploration. The water ice in PSRs would cause distinctive changes in space weathering there, in particular reduction-oxidation processes that differ from those in illuminated regions. To determine the characteristics of products formed during space weathering in PSRs, the lunar meteorite NWA 10203 with artificially added water was irradiated with a nanosecond laser to simulate a micrometeorite bombardment of lunar soil containing water ice. The TEM results of the water-incorporated sample showed distinct amorphous rims that exhibited irregular thickness, poor stratification, the appearance of bubbles, and a reduced number of npFe⁰. Additionally, EELS analysis showed the presence of ferric iron at the rim of the nanophase metallic iron particles (npFe⁰) in the amorphous rim with the involvement of water. The results suggest that water ice is another possible factor contributing to oxidation during micrometeorite bombardment on the lunar surface. In addition, it offers a reference for a new space weathering model that incorporates water in PSRs, which could be widespread on asteroids with volatiles.

Carbonic acid produced by the dissolution of atmospheric and soil CO₂ in water is usually the most dominant catalyst for chemical weathering, but a sulfuric acid-driven phenomenon, different from usual, was found in the orogenic belt watersheds dominated by silicate bedrock. This study, rooted in comprehensive field investigations in the Manas River Basin (MRB) north of the Tianshan Mountains, delves into the mechanisms and impacts of sulfuric and carbonic acid as catalysts driving different types of chemical weathering in the Central Asian Orogenic Belt. Quantitative analyses elucidate that carbonate weathering constitutes 52.4% of the total chemical weathering, while silicate and evaporite account for 18.6% and 25.3%, respectively, with anthropogenic activities and atmospheric precipitation having little effect. The estimated total chemical weathering rate in MRB is approximately 0.075 × 10⁶ mol/km²/year. Quantitative findings further suggest that, preceding carbonate precipitation (< 10⁴ year), chemical weathering can absorb CO₂. Subsequently, and following carbonate precipitation (10⁴–10⁷ year), it will release CO₂. The release significantly surpasses the global average CO₂ consumption, contributing to a noteworthy climate impact. This study underscores the distinctive weathering mechanisms, wherein sulfuric acid emerges as the predominant catalyst. The quantity of sulfuric acid as a catalyst is approximately three times that of carbonic acid. Sulfuric acid-driven carbonate rock weathering (SCW) is identified as the sole chemical weathering type with a net CO₂ release effect. SCW CO₂ release flux (5176 mol/km²/year) is roughly 2.5 times the CO₂ absorption by Ca–Mg silicate weathering, highlighting the pivotal role of chemical weathering in sourcing atmospheric CO₂ over the timescales of carbonate precipitation and sulfate reduction. Lastly, this study posits that catalyst and transport limitations are the most plausible critical factors in MRB. The interplay between sulfuric acid and dissolved CO₂ competitively shapes the types and rates of chemical weathering reactions.

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.

Unraveling the precise mineralization age is vital to understand the geodynamic setting and ore-forming mechanism of the sediment-hosted Pb-Zn deposit; this has long been a challenge. The Sichuan-Yunnan-Guizhou (SYG) triangle in the southwestern margin of the Yangtze Block is a globally recognized carbonate-hosted Pb-Zn metallogenic province and also an essential part of the South China low-temperature metallogenic domain. This region has > 30 million tons (Mt) Zn and Pb resources and shows the enrichment of dispersed metals, such as Ga, Ge, Cd, Se, and Tl. During the past 2 decades, abundant data on mineralization ages of Pb-Zn deposits within the SYG triangle have been documented based on various radioisotopic dating methods, resulting in significant progress in understanding the geodynamic background and ore formation of Pb-Zn deposits hosted in sedimentary rocks at SYG triangle. This paper provides a comprehensive summary of the geochronological results and Pb-Sr isotopic data regarding Pb-Zn deposits in the SYG triangle, which identified two distinct Pb-Zn mineralization periods influencing the dynamic processes associated with the expansion and closure of the Paleo-Tethys Ocean in the western margin of the Yangtze Block. The predominant phase of Pb-Zn mineralization at SYG triangle spanned from the Middle Triassic to Early Jurassic (226–191 Ma), which was intensely correlated with the large-scale basin fluid transport triggered by the closure of the Paleo-Tethys Ocean and Indosinian orogeny. The secondary Pb-Zn mineralization phase occurred during the Late Devonian to Late Carboniferous and was controlled by extensional structures associated with the expansion of the Paleo-Tethys Ocean. Further investigation is necessary to clarify the occurrence and potential factors involved in the Pb-Zn mineralization events during the Late Devonian to Late Carboniferous.

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.

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.