Acta Geochimica最新文献

Bulk geochemistry, Sr, Nd, and O–H isotope systematics are reported for the first time on banded iron formation (BIF)-hosted high-grade iron ore at the north-western segment of Congo Craton (CC). Located in Mbalam iron ore district, Southern Cameroon, Metzimevin iron ore deposit is a hematite-magnetite BIF system, dominated by SiO₂ + Fe₂O₃ (97.1 to 99.84 wt%), with low concentrations of clastic elements e.g., Al₂O₃, TiO₂, and HFSE, depicting a nearly pure chemical precipitate. The REE + Y signature of the iron deposit displays strong positive Eu anomaly, strong negative Ce anomaly, and chondritic to superchondritic Y/Ho ratios, suggestive of formation by mixed seawater-high temperature hydrothermal fluids in oxidising environment. The ⁸⁷Sr/⁸⁶Sr ratios of the BIF are higher than the maximum ⁸⁷Sr/⁸⁶Sr evolution curves for all Archean reservoirs (bulk silicate earth, Archean crust and Archean seawater), indicating involvement of continentally-derived components during BIF formation and alteration. The Ɛ_Nd(t) (+ 2.26 to + 3.77) and Nd model age indicate that chemical constituents for the BIF were derived from undifferentiated crustal source, between 3.002 and 2.88 Ga. The variable and diverse O and H isotope data (− 1.9‰ to 17.3‰ and − 57‰ to 136‰ respectively) indicate that the Metzimevin iron ore formed initially from magmatic plumes and later enriched by magmatic-metamorphic-modified meteoric fluids. Mass balance calculations indicate mineralisation by combined leaching and precipitation, with an average iron enrichment factor of > 2.67 and SiO₂ depletion factor of > 0.99. This is associated with an overall volume reduction of 28.27%, reflecting net leaching and volume collapse of the BIF protholith.

The Yueguang gold deposit is located in Fengjia, Xinhua County, Hunan Province, South China. It represents a recently discovered small-scale gold deposit situated in the southwestern region of the Jiangnan Orogenic Belt, west of the Baimashan granitic batholith. In order to discern the characteristics of the ore-forming fluids, the underlying mineralization processes, and establish a foundation for the origin of the Yueguang gold deposit fluid inclusion microthermometry, as well as quartz hydrogen and oxygen isotope analysis, have been carried out on samples obtained from various stages of mineralization. The hydrothermal mineralization stages within the Yueguang gold deposit can be categorized into three stages: (i) the barren, pre-ore quartz-pyrite stage (Stage I), the quartz-pyrite-gold stage (Stage II), and the post-ore quartz-carbonate stage (Stage III), with the second stage being the main mineralization stage. The fluid inclusions identified in samples from the main mineralization stage can predominantly be described with the NaCl–H₂O and CO₂–NaCl–H₂O systems. These inclusions display homogenization temperatures ranging from 158.8 to 334.9 °C, and the fluid salinity ranges from 0.3% to 4.0% (wt.% NaCl equiv.). Laser Raman spectroscopy analysis of individual inclusions further reveals the presence of gas-phases such as CO₂, CH₄, and N₂. Isotopic analysis indicates δ¹⁸O fluid values ranging from 3.95 to 6.7 ‰ and δD_H2O values ranging from − 71.9 to − 55.7 ‰. These results indicate that the ore-forming fluid of the Yueguang gold deposit belongs to metamorphic hydrothermal fluids of middle-low temperature and low salinity. In the process of ore formation, gold is transported in the form of Au (HS)²⁻ complexes, with gold deposition being driven by fluid immiscibility. Therefore, the Yueguang gold deposit is categorized as an orogenic gold deposit dominated by metamorphic hydrothermal fluid. It may become a new target for gold exploration in the Baimashan region, central Hunan Province.

It is a challenge to make thorough but efficient experimental designs for the coupled mineral dissolution and precipitation studies in a multi-mineral system, because it is difficult to speculate the best experimental duration, optimal sampling schedule, effects of different experimental conditions, and how to maximize the experimental outputs prior to the actual experiments. Geochemical modeling is an efficient and effective tool to assist the experimental design by virtually running all scenarios of interest for the studied system and predicting the experimental outcomes. Here we demonstrated an example of geochemical modeling assisted experimental design of coupled labradorite dissolution and calcite and clayey mineral precipitation using multiple isotope tracers. In this study, labradorite (plagioclase) was chosen as the reactant because it is both a major component and one of the most reactive minerals in basalt. Following our isotope doping studies of single minerals in the last ten years, initial solutions in the simulations were doped with multiple isotopes (e.g., Ca and Si). Geochemical modeling results show that the use of isotope tracers gives us orders of magnitude more sensitivity than the conventional method based on concentrations and allows us to decouple dissolution and precipitation reactions at near-equilibrium condition. The simulations suggest that the precise unidirectional dissolution rates can inform us which rate laws plagioclase dissolution has followed. Calcite precipitation occurred at near-equilibrium and the multiple isotope tracer experiments would provide near-equilibrium precipitation rates, which was a challenge for the conventional concentration-based experiments. In addition, whether the precipitation of clayey phases is the rate-limiting step in some multi-mineral systems will be revealed. Overall, the modeling results of multi-mineral reaction kinetics will improve the understanding of the coupled dissolution–precipitation in the multi-mineral systems and the quality of geochemical modeling prediction of CO₂ removal and storage efficacy in the basalt systems.

The regular hydrochemical monitoring of groundwater in the Mila basin over an extended period has provided valuable insights into the origin of dissolved salts and the hydrogeochemical processes controlling water salinization. The data reveals that the shallow Karst aquifer shows an increase in TDS of 162 mg L⁻¹ while the thermal carbonate aquifer that is also used for drinking water supply exhibits an increase of 178 mg L⁻¹. Additionally, significant temperature variations are recorded at the surface in the shallow aquifers and the waters are carbogaseous. Analysis of dissolved major and minor elements has identified several processes influencing the chemical composition namely: dissolution of evaporitic minerals, reduction of sulphates, congruent and incongruent carbonates’ dissolution, dedolomitization and silicates’ weathering. The hydrogeochemical and geothermometric results show a mixing of saline thermal water with recharge water of meteoric origin. Two main geothermal fields have been identified, a partially evolved water reservoir and a water reservoir whose fluid interacts with sulphuric acid (H₂S) of magmatic origin. These hot waters that are characterized by a strong hydrothermal alteration do ascend through faults and fractures and contribute to the contamination of shallower aquifers. Understanding the geothermometry and the hydrogeochemistry of waters is crucial for managing and protecting the quality of groundwater resources in the Mila basin, in order to ensure sustainable water supply for the region. A conceptual model for groundwater circulation and mineralization acquisition has been established to further enhance understanding in this regard.

This study predicts favorable oil and gas source-rock formation conditions in the Aryskum Depression of the South Turgay Basin, Kazakhstan. This study assesses the thermal maturity and characteristics of organic matter by determining its environmental conditions using data from geochemical analysis of core (pyrolysis) and oil (biomarkers and carbon isotopic compositions) samples. According to the geochemical parameters obtained by pyrolysis, the oil generation potential of the original rocks of most studied samples varies from poor to rich. The facies–genetic organic matter is predominantly humic and less frequently humus–sapropel, indicating organic matter accumulation in the studied samples were under moderately reducing conditions (kerogen III and II types) and coastal–marine environments (kerogen type I). The carbon isotopic compositions of oils derived from the Jurassic deposits of the Aryskum Depression also indicate the sapropelic and mixed humic–sapropelic type of organic matter (kerogen II and I). Biomarker analysis of oils indicates original organic matter formation in an anoxic environment.

The post-collisional Cenozoic basic volcanic rocks in NE Turkey show temporal variations in whole-rock lithophile element and highly siderophile element (HSE) systematics that are mainly associated with the nature of sub-continental lithospheric mantle (SCLM) sources and parental melt generation. So far, the traditional whole-rock lithophile geochemical data of these basic volcanic rocks have provided important constraints on the nature of SCLM sources. Integrated lithophile element and HSE geochemical data of these basic volcanic rocks also reveal the heterogeneity of the SCLM source, which is principally related to variable metasomatism resulting from previous subduction(s) and post-collisional mantle-crust interactions in an extensional setting. Lithophile element geochemical features suggest that the parental magmas have derived from metasomatized spinel- to garnet-bearing SCLM sources for Eocene and Miocene basic volcanic rocks with subduction signatures whereas originated from spinel- to garnet-bearing SCLM sources for Mio-Pliocene and Plio-Quaternary basaltic volcanic rocks without the subduction signature. Lithophile element and HSE geochemistry also reveal that Eocene and Miocene basic volcanic rocks were affected by more pronounced crustal contamination than the basaltic volcanic rocks of Mio-Pliocene and Quaternary. Furthermore, the integrated lithophile element and HSE compositions of these basic volcanic rocks, together with the regional asymmetric lithospheric delamination model, reveal that the compositional variation (especially due to metasomatism) was significant temporally in the heterogeneity of the SCLM sources from which parental magmas formed during the Cenozoic era.

Meso-Neoarchean fuchsite quartzites are present in different stratigraphic positions of Dharwar Craton including the oldest (~ 3.3 Ga) Sargur Group of western Dharwar Craton. The present study deals with the petrographic and geochemical characteristics of the fuchsite quartzites from the Ghattihosahalli belt to evaluate their genesis, depositional setting and the enigma involved in the ancient sedimentation history. Their major mineral assemblages include quartz, fuchsite, and feldspars along with accessory kyanite and rutile. The geochemical compositions are characterized by high SiO₂, Al₂O₃, low MgO, CaO, strongly enriched Cr (1326–6899 ppm), Ba (1165–3653 ppm), Sr (46–210 ppm), V (107–868 ppm) and Zn (11–158 ppm) contents compared to the upper continental crust (UCC). The UCC normalized rare earth element (REE) patterns are characterized by depleted light REE [(La/Sm)_UCC = 0.33–0.95] compared to heavy REE [(Gd/Yb)_UCC = 0.42– 1.65)] with conspicuous positive Eu-anomalies (Eu/Eu* = 1.35–18.27) characteristic of hydrothermal solutions evidenced through the interlayered barites. The overall major and trace element systematics reflect a combined mafic-felsic provenance and suggest their deposition at a passive continental margin environment. The comprehensive field, petrographic, and geochemical studies indicate that these quartzites are infiltrated by Cr-rich fluids released during high-grade metamorphism of associated ultramafic rocks. The Sargur and the subsequent Dharwar orogeny amalgamated diverse lithounits from different tectonic settings, possibly leading to the release of Cr-rich fluids and the formation of fuchsite quartzite during or after the orogeny. These findings suggest a pre-existing stable crust prior to the Sargur Group and the link between orogenic events and various mineral deposits in the Dharwar Craton.

The Baidi Au-Sb deposit, which contains 8 t of Au and 10,979 Mt of Sb, is a typical and rare paragenetic deposit located in southwestern Guizhou Province, China. Previous studies have focused on individual ores, but have not combined them to identify their paragenetic mechanism or metallogenic regularity. Therefore, we used field investigations, microscopic observations, and in situ analyses to identify the spatial distribution, mineral paragenesis, compositional evolution, and metallogenic material sources of the ore bodies. We also determined the Au and Sb paragenetic characteristics and the metallogenesis of the deposit. The main Au-bearing minerals in the deposit were early (Apy1–2) and late (Apy3) stage arsenopyrites, as well as pre-mineralization (Py1), mineralization (Py2–5), and late mineralization (Py6–7) stage pyrites. The main Sb-bearing minerals were stibnite (Snt), skinnerite, bournonite, and valentinite. The minerals formed in the order of Py1, Py2–3 + Apy1, Py4–5 + Apy2, Snt, and Py6–7 + Apy3. The δ³⁴S values of the arsenopyrites and pyrites ranged from − 5 to 5‰, while those of stibnite were mostly less than − 5‰ in the later mineralization stages. Sulfur was provided by deep magmatic hydrothermal fluids, but sedimentary sulfur was added in the later stages. Moreover, the trace elemental contents fluctuated and eventually became similar to those of the sedimentary strata. By comprehensively considering the ores along with the geological characteristics of the deposit, we determined that deep magma provided the Au during ore formation. Later tectonic changes provided Sb from the sedimentary strata, which precipitated along fault expansion areas and produced Au and Sb paragenesis.

The sandstones of the Late miocene–Pliocene Dibdibba Formation in the Najaf–Karbala Plateau and Basra were examined to determine their source rocks and origin. The rare earth elements (REE) and trace elements (Sc, Co, V, and Th) concentrations in these sandstones revealed that they likely derived from a single source. The steep light rare earth elements (LREE) and flat, heavy rare earth element (HREE) patterns, negative Eu anomaly, and high ΣREE contents in sandstones suggest its derivation from a suggests that a passive continental margin environment and originated from felsic source rocks. The average concentration of ΣREE is 93.5 ppm, which is lower than that of the average crustal compositions like Upper Continental Crust and Post Archean Australian Shale. The higher proportion of LREE compared to HREE implies that these sandstones were recycled and derived from a distal source. The Th/Co, Th/Sc, La/Sc, La/Co, Eu/Eu* and (La/Lu)cn elemental ratios indicated that these Late Miocene–Pliocene sandstones were derived from felsic rocks located in the marginal region of the Arabian Shield.

The Ain El Bey abandoned mine, in North-West Tunisia, fits into the geodynamic context of the European and African plate boundary. Ore deposit corresponds to veins and breccia of multiphase Cu–Fe-rich mineralization related to various hydrothermal fluid circulations. Petro-mineralogical studies indicate a rich mineral paragenesis with a minimum of seven mineralization phases and, at least, six pyrite generations. As is also the case for galena and native silver, native gold is observed for the first time as inclusion in quartz which opens up, thus, new perspectives for prospecting and evaluating the potential for noble metals associated with the mineralization. Scanning Electron Microscope—Energy Dispersive Spectroscopy and Transmission electron microscopy analyses show, in addition, a large incorporation of trace elements, including Ag and Au, in mineral structures such as fahlores (tetrahedrite-tennantite) and chalcopyrite ones. The mineral/mineral associations, used as geothermometers, gave estimated temperatures for the mineralizing fluids varying from 254 to 330 °C for phase III, from 254 to 350 °C for phase IV, and from 200 to 300 °C for phases V and VI. The seventh and last identified mineralization phase, marked by a deposit of native gold, reflects a drop in the mineralizing fluid’s temperature (< 200 °C) compatible with boiling conditions. Such results open up perspectives for the development of precious metal research and the revaluation of the Cu–Fe ore deposit at the Ain El Bey abandoned mine, as well as at the surrounding areas fitting in the geodynamic framework of the Africa-Europe plate boundary.