Chemie Der Erde-Geochemistry最新文献

The giant Cretaceous Jiaodong gold province is the world's third largest gold mining district, while the source of the ore-forming fluids and AuS remains controversial. The elemental sulfur is a key carrier for gold and the isotopic composition is an ideal tracer of the sulfur source, which makes it crucial for understanding gold deposit genesis and guiding ore exploration. Here, we show the Au-bearing pyrite in the gold deposits from Jiaodong bears a non-zero Δ³³S signature from −0.07 ‰ to 0.31 ‰ showing the mass-independent fractionation (S-MIF). This clearly indicates involvement of Archean atmospheric sulfur during the Cretaceous gold mineralization. The MIF-bearing sulfur was likely remobilized from the Archean supracrustal rocks in the basement of the Jiaodong region via fluid-rock interaction while the hydrothermal ore-forming fluids passed through the crustal-scale fault zones. The Phanerozoic gold deposits from Jiaodong have relatively minor S-MIF signals compared with the Archean orogenic gold deposits due to more material (S) cycling and dilution. This new discovery of S-MIF signal in the Phanerozoic ore deposits implies that the recycling of crustal sulfur may be a more common process and play an important role in forming hydrothermal gold deposits.

This study provides new ²²²Rn measurements performed by RAD7 on 31 thermal waters from the Campi Flegrei caldera, the active volcanic-geothermal field close to Naples (Southern Italy). Waters sampled between 2021 and 2023 are characterized for physical parameters, major ions geochemistry and radium content. Rn contents from Somma-Vesuvius, Ischia and Vulcano volcanoes, together with the river plain north to the Campi Flegrei, were obtained for comparison. The Campi Flegrei caldera reaches the highest Rn concentrations respect to the other sites, varying from 0.03 ± 0.02 to ca. 1887 ± 13 Bq/L, although mostly are below 60 Bq/L. We detect a steady-state condition of constant temperature, facies and radon activity that characterizes most sites, with only minor impacts from seasonalilty and Weigel's effects. Just a small fraction of ²²²Rn derives from its ²²⁶Ra parent in solution, while radon activity in local waters is mainly due to emanation from the radium-containing rock reservoir. Our dataset proofs that radon couples with temperature, sulfate and CO₂ in relations to rock-leaching and pore-water phenomena that proceed in the reservoir as it warms up and degasses. Rn and CO₂ are decoupled in deeply and timely equilibrated geothermal fluids.

Two main end-members, i.e., a low radioactive cold diluted and the Rn-richest hypersaline water from the deep geothermal reservoir are recognized; seawater contamination and heating over 70 °C play a major role in radon decrease.

Related radium contents, physical parameters and major ions geochemistry are also presented for a comparison with published data.

In the late Triassic–early Jurassic Nasr-Abad deposit, bauxite ores are bauxitic iron ore and Fe-rich bauxite, grading into bauxite and clayey bauxite upward the succession. The ores mainly consist of a mineral assemblage of diaspore, hematite, kaolinite, and rutile. Conservative elemental ratios of provenance, such as Eu/Eu*, Sm/Nd, and Nb/Ta, suggest a derivation from a mafic protolith, although, in some cases, the Eu/Eu* ratio indicates a felsic protolith, probably derived from Precambrian rock units. R-mode factor analysis indicates that most of the variance of the dataset is explained by a factor related to climatic and redox paleo-environmental conditions. This factor, which controls some critical metals, such as Co, V, and LREE, accounts for the competition between SiO₂ and the Fe₂O₃ + P₂O₅ pool. The second factor is related to the accumulation of Al and Ti (oxyhydr)oxides, and low-solubility critical metals, such as Ga, Nb, Hf, and Ta, during late stages of bauxitization under dry conditions. The Nasr-Abad bauxite ores are remarkably enriched in critical metals Ti, Nb, Ta, V, and Co with respect to the average Upper Continental Crust. Among these, Ti, Nb, and Ta are particularly enriched in the uppermost Al-rich bauxite and clayey bauxite ores, whereas V and Co are particularly enriched in the bauxitic iron ores.

Migmatite formation is dependent of several and inter-related factors, such as P-T conditions, protolith composition and the abundance of volatiles, which usually hinders the full understanding of anatectic progression and how it relates to orogenic evolution. We have constrained the petrological evolution of the anatectic complex (mostly metatexites and diatexites) of the Porto-Viseu Metamorphic Belt in the Central Iberian Zone (CIZ) using petrography, geochemical and isotopic data.

Major and trace element geochemistry allowed us to conclude that the studied metatexites were formed through water-present melting of muscovite from metapelitic schists, whereas diatexites were mostly formed by dehydration-melting of muscovite and, occasionally, by the dehydration-melting of biotite. Additionally, melting evolution from incipient (metatexites) to pervasive (diatexites and anatectic granitoids) included SiO₂ enrichment and elemental depletion (except for Ca, P, Na and K). The geochemical evolution of these elements during melting progression was essentially controlled by fractionation/melting of plagioclase, K-feldspar, mica and apatite, whereas REE abundance was constrained by accessory minerals, such as monazite, apatite and zircon. In both cases, the geochemical control was variable and dependent on the fate of these mineral phases during the progression of melting, as well as the variable degree of melt loss or gain, with diatexites and granites being the result of substantial melt gain.

The ⁸⁷Sr/⁸⁶Sr₃₂₀ values have a large dispersion (0.708–0.766), which is common for rocks that experienced these metamorphic conditions, whereas the εNd₃₂₀ values are less variable, roughly from −3 to −9 (diatexites: −8.24 to −2.96; metatexites: −8.79 to −5.15; staurolite-bearing schists: −7.57 to −5.19; granite: −8.21), being both isotopes independent of lithological type or degree of melting. Therefore, we interpret their Sr isotopic variation as mostly related to secondary alteration processes, whereas Nd isotopes, which are usually more immobile, should correlate to the initial protolith. In fact, their isotopic results are broadly consistent to the values of other CIZ metasediments that did not experience such high metamorphic conditions and melting (Douro-Beiras Supergroup), being most likely derived from these rocks, namely the Douro Group.

Montagna Grande and Monte Gibele represent the northwestern and southeastern portions of a shield volcano (hereafter “MGV”) located entirely within the Cinque Denti caldera structure on Pantelleria. The Cinque Denti caldera formed during the 45.7 ± 1.0 ka eruption of the pantelleritic Green Tuff. Subsequent isostatic compensation was achieved by the eruption within the caldera of ∼3 km³ of trachyte lavas that comprise the MGV and are thought to represent renewed tapping of the Green Tuff magma reservoir. The time interval between the eruption of the Green Tuff and the MGV trachytes has not been well constrained, with previous KAr ages from alkali feldspar yielding ages between 44 ± 8 and 28 ± 16 ka. In this study we report the results of new single-crystal anorthoclase incremental heating and total fusion ⁴⁰Ar/³⁹Ar ages collected from three trachyte lava flows collected at different sites and at different stratigraphic heights that provided ages of 26.2 ± 2.0, 22.5 ± 0.8 ka, and 22.3 ± 2.8 ka (uncertainties at 2σ). The results indicate that there was a repose period of up to ∼20,000 years between the eruption of the caldera-forming ignimbrite and the caldera-filling trachytes. The timing of our oldest dated sample is coincident with renewed basaltic volcanism in the northwest part of the island at ∼28 ka and suggests that the eruption of the trachyte was promoted by mafic recharge. We briefly review the petrogenetic processes operative in the reservoir during the repose period. Modelling of the thermal and compositional evolution of the fractionating system indicates an eruption rate of 7.5 × 10⁻⁴ km³/yr over the ∼4 ka eruption period.

The Acoculco Caldera Complex is considered a promissory hidden high-temperature geothermal system in Mexico. To support the geothermal prospection of this anomalous area, a comprehensive programme of soil CO₂ flux measurements was performed. A long-term measurement programme was conducted to determine the baseline of natural soil CO₂ effluxes. Significant efforts were devoted both to measuring the CO₂ fluxes between 2015 and 2022 and interpreting their origin. Eighteen soil gas surveys of CO₂ were carried out by using the accumulation chamber method. >1200 diffuse CO₂ fluxes were measured in six different areas of the Acoculco Caldera. Two areas (Los Azufres and Alcaparrosa) exhibited cold degassing sites, acid-sulphate springs, and gas bubbling in surface water bodies. The soil CO₂ fluxes ranged from 1 to 26,000 g m⁻² d⁻¹, whereas lower fluxes <29 g m⁻² d⁻¹ were determined as the degassing baseline. A total CO₂ output of 492 t d⁻¹ km⁻¹ was estimated using an integrated SGS-GSA approach, where the highest total soil CO₂ fluxes were obtained for Alcaparrosa (299 t d⁻¹ km⁻²) compared with Los Azufres (164 t d⁻¹ km⁻²), and Surroundings (29 t d⁻¹ km⁻²). Such results agree well with those values measured in other worldwide volcanic and active geothermal ecosystems. The range of CO₂ isotopic composition values from −28.83 ‰ to −3.11 ‰, together with their statistical distribution, suggests multiple CO₂ production sources feeding soil degassing. The combined interpretation of flux and isotopic data allowed us to identify two distinct gas sources: endogenous and biogenic. The present study highlights the importance of using soil CO₂ monitoring to determine baseline emissions at the early exploration stage of geothermal systems.

The process of data-driven clustering to uncover geochemical anomalies linked to sample catchment basins (SCBs) includes a comprehensive framework to discern areas exhibiting unique geochemical attributes within a specified study area. The Ordering Points to Identify the Clustering Structure (OPTICS) method can serve as a robust methodology for detecting geochemical anomalies in SCBs. This is attributed to its capacity to effectively manage varying cluster densities, adaptively identify cluster numbers, exhibit resilience to noise, and display minimum sensitivity to parameters. A comparison was conducted in this research between the outcomes of the OPTICS clustering algorithm and two traditional clustering techniques, namely the Gaussian Mixture Model (GMM) and K-means clustering. In the following, the Expectation-Maximization (EM) technique is employed to train the GMM for clustering. Moreover, the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) as two validate statistical metrics implemented to ascertain the optimal number of components (clusters) belong to the GMM. It should be noted that the effectiveness of the clustering algorithms was further assessed using the Calinski-Harabasz (CH) index and the success-rate curves. OPTICS, a density-based clustering approach, was confirmed to be more effective than K-means and GMM for identifying MVT PbZn anomalies in Varcheh district, western Iran. Furthermore, the specified anomalies show a geo-spatial correspondence with the geological facts, and it has been observed that strong anomalies are more discoverable in close proximity to MVT PbZn occurrences. This work suggests a novel anomaly detection approach based on OPTICS, which exhibits superior performance and data-modeling efficiency. The main emphasis is on effectively distinguishing geochemical anomalies from sample data originating from populations with uncertain distributions.

Composite mantle xenoliths from the Cima Volcanic Field (CA, USA) contain a variety of melt (now glassy) inclusions hosted within mantle phases. The compositions and textures of these melt inclusions have the potential to constrain their trapping processes, melt sources, and the rates of ascent of their parent xenoliths. Here we focus on unusual spinel-hosted melt inclusions from one composite xenolith, reporting glass and daughter mineral compositions and textures and attempting to reconstruct inclusion bulk compositions. The xenolith contains spinel-hosted melt inclusions in its harzburgite, olivine-websterite and lherzolite layers; there are none in its orthopyroxenite layer.

The glass compositions and reconstructed bulk compositions of the partly-crystallized inclusions correspond to alkaline intermediate melts, mostly trachyandesites. Such melts are most likely to be generated and trapped by vapor-undersaturated phlogopite or amphibole dehydration melting to an assemblage of liquid + spinel + olivine ± pyroxenes. We modeled the near-liquidus phase relations of the inclusion bulk compositions and noted the closest approach of each inclusion to simultaneous saturation with spinel and either phlogopite or amphibole, resulting in estimated trapping pressures of ∼0.5–1.5 GPa and temperatures of ∼1000–1100 °C. The large size of the hosting spinel grains suggests a slow process associated with these breakdown reactions, probably thinning of the lithosphere and steepening of the geotherm during regional extension.

A linear correlation between the vesicle area and inclusion area (as proxies for volume) suggests an in-situ exsolution process from melts of relatively uniform volatile initial contents, consistent with trapping of vapor-undersaturated melts that later exsolve vapor during cooling and daughter crystal growth. A negative correlation between the glass content in melt inclusions and the size of the inclusion itself suggests a control on the degree of crystallinity with the size. There appears to be a two-stage cooling history captured by the inclusions, forming first prismatic daughter crystals and large round vesicles at the wall of the inclusion, followed by quenching to form a mat of fine crystallites and small vesicles in most inclusions. We connect the final quench to rapid ascent of the xenolith in its host melt, which also triggered partial breakdown of remaining amphibole to fine glassy symplectites.