Acta Geochimica最新文献

Numerous locations in Eastern Arabia contain organic-rich Cenomanian-Turonian (C/T) sediments, including Qatar, UAE, and Oman. The contrast between organic-rich and organic-lean anoxic facies, as well as the local to regional distribution of the C/T facies, is shown here. These regions are confirmed to be rich in organic material using bore-hole data, and our study has precisely identified their limits. The inorganic geochemical investigations focus on the Cenomanian-Turonian Mishrif Member of the upper Wasia Formation. This elemental chemostratigraphy approach is investigated as a timely and cost-effective substitution for carbon isotope determination during hydrocarbon exploration and development phases. The study reveals information about how the palaeoceanography of intrashelf basin changed throughout the Cenomanian-Turonian time. The use of discrete redox-sensitive and primary productivity along with sensitive trace element data and detrital-related elements is demonstrated as one of the best methods to emphasise the reducing conditions that reigned during the OAE-2 in the area. The redox-sensitive proxies show three broad pulses of anoxic to euxinic conditions associated with the bottom water, separated by dysoxic to oxic conditions in the Cenomanian-Turonian Mishrif Member. Either the basin geometry or the expansion of an oxygen minimum zone (OMZ) is the main factor that controlled the bottom redox condition distribution during the deposition of the Mishrif Member. Elemental chemostratigraphy has been utilised to identify and characterise the OAE-2 phases in the study area in the absence of carbon isotope perturbation data.

The Baïbokoum syenitic pluton (BSP), located in southern Chad, to the NE of the Adamawa-Yadé domain, is one of the few strongly potassic magmatic bodies in the southern part of the Central African Fold Belt (CAFB) in Chad. It has been previously studied petrologically, but its petrogenesis has remained poorly known. Petrographic and whole-rock geochemical data presented in this article highlight their magma genesis and geodynamic evolution. The BSP consists of medium- to coarse-grained syenites associated with minor microdiorites, which occur as syn-plutonic dikes and mafic microgranular enclaves (MME) coarse- and medium-grained syenites outcrop respectively to the core and the border of the BSP. The syenite displays high-K and alkaline to trans-alkaline affinity. Petrographic and geochemical data suggest that medium- to coarse-grained syenites are from single magma source that evolved and differentiated by fractional crystallization in a magma reservoir. REE profiles show enriched LREEs (La_N/Yb_N = 6.19–45.55) while HREEs show an almost flat profile (Dy_N/Yb_N = 1.0–2.23), and the La/Sm and Sm/Yb ratios have led to propose that the aforementioned rocks derived from the partial melting of a garnet-spinel-lherzolite mantle source. Negative Nb and Ta anomalies indicate that this mantle source was modified by the addition of subduction-related material. Th/Yb ratios associated with high Ba/La ratios indicate that enrichment of the source could be related to slab-derived fluids. The parental magma of the BSP was generated by partial melting of the metasomatized lithospheric mantle that was modified into arc-magmatism material in a subduction setting. Its emplacement took place in two successive stages: a static stage of fractional crystallization and crystal settling in a deep magma source and a dynamic stage in a shear deformation setting during which stratified magma rises towards the upper crust, with evolved syenite magma being emplaced first and diorite later. The emplacement of the BSP was probably controlled by the evolution of the Tcholliré-Banyo Fault and M'Béré Shear Zone during the Pan-African orogeny.

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.

The Jianglang Dome has integral tectonostratigraphic units and contains a suite of high-grade stratiform Cu deposits. However, the formation mechanism of this dome and genetic model of Cu mineralization remain a matter of debate. The resolution of these problems hinges on the presence of magmatic intrusions in the core. Here, we report bulk geochemical and zircon U-Pb data of a newly discovered syenite intrusion as well as chalcopyrite Re-Os dating results. We aim to explore genesis of the Jianglang Dome, genetic model of the stratiform Cu deposits, and rare metal mineralization potential of the syenite intrusion. The dated syenite sample yields an emplacement age of 207.1 ± 2.0 Ma, which matches post-collisional extension in the Songpan-Ganze Orogen. The syenite rocks have average high (Zr + Nb + Ce + Y) concentrations of 512 ppm, 10000×Ga/Al ratios of 3.97, and crystallization temperatures of 827 °C, together with low Mg# values of 1.73; they fit the A-type granitoid definition and a crustal origin. Chalcopyrite separates yield a Re-Os isochron age of 207.1 ± 5.3 Ma, which markedly postdates the formation age of their ore-hosting rocks (the Liwu Group, ca. 553 Ma). Our new age determination, together with previous chalcopyrite Re–Os isochron age of ca. 151.1 Ma and sulfide sulfur isotope (δ³⁴S_V-CDT = 8.7‰–5.6‰) and tourmaline boron isotope (δ¹¹B = − 15.47‰ to  − 5.91‰) data, confirms multistage epigenetic Cu mineralization related to magmatic-hydrothermal fluids. Compared with regional ca. 209–207 Ma fertile granitoids, the studied syenite intrusion shows unevolved and barren affinities and negligible rare metal mineralization potential. Combined with residual gravity low anomalies in the core of the Jianglang Dome, which suggest a large deep-seated granitic batholith, we prefer thermal doming resulting from magma-induced uplift for the nature of this dome.

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 Bijigou intrusion is one of the largest and most well-differentiated Fe–Ti oxide-bearing layered intrusions in the Hannan massif located in the northwestern margin of the Yangtze Block, South China. Besides the mineralization-related mafic-ultramafic rocks, the intermediate-acid intrusive rocks are also exposed in the mining area, which is of great significance for the understanding the Neoproterozoic tectonic evolution of the Yangtze Block, but studies on these intermediate-acid rocks are scarce. The Bijigou mafic-ultramafic layered intrusion is surrounded by granite and cut by syenite veins. Here, we report new zircon U-Pb ages, Lu-Hf isotope composition and bulk rock geochemical data of the Bijigou syenite vein and wall-rock granite in the northwestern margin of the Yangtze Block. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb dating results show that the Bijigou syenite vein and wall-rock granite formed at 770 ± 3.5 Ma (MSWD = 0.17, n = 28) and 810 ± 4 Ma (MSWD = 0.84, n = 26), respectively. The zircon εHf(t) values of the syenite veins range from + 1.52 to + 5.33 (average of + 3.05), combined with its high potassium contents, negative Nb–Ta anomalies and positive Pb anomalies, suggesting that they may have originated from mantle-derived basaltic magma, which was modified by materials from subducting oceanic slab. The zircon εHf(t) and T_DM2 ages of the wall-rock granite range from + 0.71 to + 5.71 (average + 3.06) and 1344 to 1659 Ma (average 1519 Ma), respectively, indicating that the granite was produced by partial melting of juvenile crust. The geochemical characteristics of the Bijigou syenite and granite indicate that they were formed in a continental margin arc setting. Thus, combined with previous studies, it suggests that there was a major subduction system along the northwestern margin of the Yangtze Block during 824–720 Ma, and the magmatism in the Hannan massif was divided into two episodes: (1) early magmatism (824–790 Ma), such as the Bijigou, Hongmiaozhen and Huangguanzhen granitoids, was derived from partial melting of the juvenile or pre-existing crust in a continental arc setting; (2) later magmatism (789–718 Ma), including Bijigou syenite, Wudumen, Erliba and Zushidian granitoids, formed in a subduction-related back-arc extensional environment. The long-term subduction system along the northwestern margin of the Yangtze Block during 824–720 Ma suggests that the Yangtze Block was previously located at the periphery of the Rodinia supercontinent.

The offshore Tanzanian Basin contains numerous igneous intrusions emplaced at various stratigraphic levels. Previous studies indicate these intrusions have impacted petroleum systems, affecting key elements such as source rocks, reservoirs, seals, migration pathways, and trapping mechanisms. However, due to the limited number of wells drilled in the region, there have been few studies reporting the associated thermal effects on source rock maturation and their role in hydrocarbon generation. To gain a comprehensive understanding of the intricate relationship between intrusions and the petroleum system, particularly source rock, an integrated geochemical and resistivity log analysis was carried out. The geochemical results show that the Cretaceous-Cenozoic sediments of the study area have low total organic carbon contents (TOC < 1 wt%), kerogen yield (< 1 Mg HC/g), and Hydrogen Index (< 100 Mg HC/g), primarily composed of Type III (gas-prone) to Type IV (inert) kerogens. These sediments have undergone varying levels of thermal maturity, ranging from post-mature (within Cretaceous), matured (in Paleocene) to immature (in Eocene) thermal states. The Cretaceous strata located proximal to the intrusions exhibit significant thermal alteration, resulting in a reduction of both organic matter (OM) content and source potential compared to the Eocene and Paleocene samples. This observation is consistent with the estimated paleotemperature (T) and resistivity log (ILD) along the depth profile, which have mapped local thermal alteration increasing from base Paleocene to Cretaceous. These findings have implications for source rock potential and thermal evolution history in the offshore Tanzanian Basin. This study highlights the necessity for thorough subsurface mapping in the area to identify both younger and older intrusive rocks. These intrusions pose a potential risk in petroleum exploration, especially when they intrude into matured source rock intervals.

In many places across the globe, including the Wassa District of Ghana, groundwater provides a significant supply of water for various purposes. Understanding the groundwater origin and hydrogeochemical processes controlling the groundwater chemistry is a major step in the sustainable management of the aquifers. A total of 29 groundwater samples were collected and analysed. Ionic ratio graphs, multivariate statistical analysis, mineral saturation indices, stable isotopes, and geostatistics methods were used to examine the sources and the quality of the groundwater. The findings describe the water types in the district as Ca–Mg–HCO₃–Cl, Ca–Na–HCO₃, Na–Ca–HCO₃, Ca–Na–HCO₃–Cl, Na–Ca–HCO₃–Cl, mix water type, Na–HCO₃–Cl, with possible evolution to Ca–Na–Cl–HCO₃, and Na–Ca–Cl–HCO₃. According to the IEWQI for drinking water, around 53.6% of the samples have good quality, whereas 10.7% have very low-quality groundwater. Only 3.45% of the samples are suitable to use for irrigation without treatment, whereas 41.4% are somewhat safe with minimal treatment. Water-rock interactions, including the dissolution and weathering of silicate minerals, cation exchange processes, and human activities like mining and quarrying, are some of the main factors influencing groundwater chemistry. Principal component analysis revealed that groundwater chemistry is influenced by a combination of natural and anthropogenic sources. The APCs-MLR receptor model quantifies the factors that play important roles in groundwater salinization, including mineral dissolution and weathering (19.4%), localised Cd (16%), Ni (14.6%), Pb (12.8%), and Fe (11.4%) contamination from urbanisation while unidentified sources of pollution account for about 26.0%. The stable isotopes revealed groundwater is of meteoric origin and water-rock interaction the major mechanism for groundwater mineralization. The results of this research highlight the need of implementing an integrated strategy for managing and accessing groundwater quality.