International Journal of Earth Sciences最新文献

The Pipe 200 kimberlite in northern Lesotho on the southeast margin of the Kaapvaal Craton is a diamond-poor deposit despite its proximity to economically viable kimberlites like the Liqhobong kimberlite. We study harzburgite xenoliths' mineral composition, geochemistry, and pressure–temperature evolution to understand factors influencing diamond destruction. The xenoliths are classified into five types based on their petrography and geochemistry. The diversity in the mineral assemblage correlates with a sampling depth of ~ 100 to 175 km (~ 2.8 to < 5.0 GPa). The signatures of metasomatism are evident in type 3 and 4 xenoliths, where garnet breaks down to form a cluster (henceforth assemblage) of phlogopite, chromite, and diopside. Fine-grained melts associated with an uplift in the mantle to shallower depths of < 90 km (< 2.3 GPa) encompass the minerals in the assemblage, which display resorption at the boundaries. Water contents (structural hydroxyl) of olivine and possibly orthopyroxene are lower in the xenoliths with metasomatism-induced breakdown of garnet. The structural hydroxyl distribution in the nominally anhydrous minerals shows flat distribution profiles of re-equilibration due to residence in the kimberlite magma. It is supported by the disruption of the inter-mineral water partition coefficient due to olivine water diffusion during residence in the kimberlite magma. The barren nature of the Pipe 200 kimberlite is attributed to the signatures of mantle metasomatism and residence in kimberlite magma, which led to the diamond destruction.

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Under Priabonian (latest Eocene) far-field compression, the major Jurassic graben systems and their proximal areas experienced domal uplift in the Netherlands and northeastern Belgium. It has been proposed that this flexural uplift was simultaneous with flexural subsidence of the surrounding areas. This hypothesis is, however, only based on the correlation of few widely spaced boreholes without supporting biostratigraphic or seismic data. For this study, a seismic line and cross-boundary borehole log correlation panel, comprising three biostratigraphically analyzed boreholes, were used to critically examine the abovementioned hypothesis. The seismic line and correlation panel run from the southern flank of the uplifted West Netherlands Basin, also known as the Voorne Trough, towards the south. They both show a gradual thickening of the Priabonian sequences towards the Voorne Trough, confirming the flexural subsidence hypothesis. The underlying Bartonian and overlying lowermost Rupelian units do not share the abovementioned thickness trend, which highlights the particularity of Priabonian basin dynamics. Within the Priabonian depocenter, plenty of reworked organic material was deposited, mostly of Bartonian age, providing further evidence of the simultaneity of subsidence and uplift in nearby regions. We consider it very likely that subsidence and uplift were part of the same vertical surface deformation process. The results of this study are consistent with the mechanism of lithospheric folding under far-field compression by the convergence between Africa and Europe. A Priabonian N-S compressional stress-field would also be consistent with fault, joint and vein analyses performed at the British Isles west of the study area. As the regional stress-field changed around the Eocene–Oligocene boundary, the vertical surface movements of the study area also changed.

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The relationships between metallogenic capacity and geochemical features of adakitic rocks along the South Tan-Lu Fault (STLF) remain unclear. In this study, the ore-barren adakitic rocks (Tuncang, Guandian and Wawuliu) exhibit higher K₂O/Na₂O ratios and lower Sr/Y ratios than the ore-bearing adakitic rocks (Chuzhou and Shangyaopu). These differences strongly suggest that the ore-barren adakitic rocks originated from the thickened lower continental crust (LCC), whereas the ore-bearing adakitic rocks were derived from oceanic slabs. Notably, the Tuncang granite exhibits higher Y/Yb and (Ho/Yb)_N ratios than the Guandian granodiorite and Wawuliu intrusion. Accordingly, we suggest that the Tuncang granite likely originated from a delaminated eclogitic LCC, whereas the Guandian and Wawuliu intrusions were derived from a thickened basaltic LCC sources. The occurrence of diorite and gabbro mafic microgranular enclaves (MMEs) within the Tuncang granite strongly suggests a magma-mixing process. Considering their MgO contents and εNd(t) and (⁸⁷Sr/⁸⁶Sr)_i values, we suggest that the gabbro MMEs were likely derived from an enriched mantle source previously metasomatized by subduction-related components and that the diorite MMEs were subsequently formed by magma mixing. Due to the slightly younger ages of the ore-bearing adakitic rocks, we propose a model in which the ore-barren adakitic rocks formed through LCC delamination at 130 Ma and the ore-bearing adakitic rocks formed through oceanic slab remelting at 125 Ma. Consequently, the exploration of Cu–Au mineralization along the STLF should focus on younger oceanic slab-derived adakitic rocks.

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This study presents the trace element geochemistry of an 851-m thick basaltic succession from the central part of the southeastern Ethiopian plateau (Chole section) to assess the petrogenesis and temporal geochemical evolution of the Chole basalts. The composition of trace and major elements in the Chole section varied temporally, and three distinct magma types were identified; lower, middle, and upper basalts. The lower and middle basalts are highly enriched in incompatible trace elements with strong positive Nb and negative K anomalies in the primitive-mantle-normalized multi-elemental variation diagram. Both exhibit similar middle to heavy rare earth element fractionation patterns. However, the middle basalt shows distinctive negative U, Th, and positive Ba anomalies, with higher Ba/Th, Ba/Nb ratios, and more pronounced light-to-heavy rare earth element fractionation, suggesting varying degrees of mantle metasomatism and partial melting in the lithospheric mantle. The trace element signatures indicate that these basalts originated from the melting of garnet-bearing lithospheric mantle mixed with rising plume component. The upper basalt reveals two groups: the lower flows show moderate enrichment with a strong negative K anomaly, while the upper flows display highly enriched trace elements with crustal influence. Trace element signature in the uncontaminated upper basalt indicates partial melt contribution from the spinel-garnet transition zone of the depleted component, probably the melting of the sub-lithospheric mantle and hydrated lithospheric mantle or partial melting of the previously depleted part of the lithospheric mantle.

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Numerous Priabonian and Oligocene tuff layers within the Eastern and Central Europe Paratethys and Tethyan sedimentary basins attest for extensive volcanic activity during that period. However, the sources of these eruptions are completely unknown. Using precise radioisotopic dating, along with their stratigraphic relationships, we correlate some of these tuff layers with Rupelian eruptions from the Rhodope Massif, South Bulgaria. Three major silicic eruptions of ignimbrites and ash falls, namely Dazhdovnitsa, Borovitsa and Perelik, with ages of 33.2 Ma, 32.8 Ma and 31.6 Ma, respectively, have been identified in the Rhodope Massif. The ejected tephra was dispersed over more than 1,000,000 km² and the combined estimated volume of thousands cubic kilometers, which marks this period as one of the most voluminous sequence of volcanic eruptions/explosions in Europe in the Rupelian Stage. The areal coverage and estimated volumes meet the requirements for volcanic supereruptions and the wide geographic occurrence of the products can be used as stratigraphic isochronous marker horizons for correlating, dating, and synchronizing deposits and events in geologic and paleoenvironmental studies. Further, the volcanic products were deposited during the early Rupelian anoxic Paratethys events and we explore the possible influence of volcanic eruptions on the process of anoxia.

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The current study reports the occurrence of a deformed Paleoproterozoic K-rich metasedimentary rock unit at the eastern margin of Bastar Craton, India. Geochemical studies of the metasedimentary rocks suggest the presence of an active continental margin at the eastern part of the Bastar Craton. Detailed petrological and mineralogical studies suggest that K-feldspar, quartz, chlorite, muscovite, kaolinite, and ± biotite represent the primary mineral assemblage in the rock unit. Combined mineral thermobarometric and P–T pseudosection estimate the peak metamorphic condition (i.e. M3 metamorphic) at T ~ 550 °C, P ~ 0.5 GPa. Furthermore, monazite grains (about 20–60 μm in diameter) were dated using the in-situ U–Th–Pb_total dating method under EPMA. Based on composition and texture, monazite spot ages can be categorized into four groups: Group I, Group II, Group III, and Group IV. The mean age of the oldest monazite population (Group I) at ~ 2100 Ma suggests vertical sagging of the sedimentary basin and the associated M1 metamorphism. The M2 metamorphic event is represented by the Group II monazite population’s mean age of ~ 1629 Ma. Rhyolite magmatism in the eastern margin of the Bastar Craton may be connected to this specific event. The Group III monazite mean age of ~ 1533 Ma represents the M3 metamorphic event (peak metamorphic condition). This event could be linked to the accretionary growth at the Bastar Craton's eastern boundary during the assembly of the Columbia supercontinent. Furthermore, the M2 and M3 events may correlate with orogenic events recorded in the Central Indian Tectonic Zone at ca 1.62 and 1.5 Ga, respectively. Finally, the mean age of ~ 528 Ma for the Group IV monazite population represents Pan-African orogenic activity along the eastern margin of the Bastar Craton.

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Deltaic siltstones and sandstones from the Pennsylvanian (Upper Carboniferous) in the Ruhr Basin are currently being examined for post-mining applications (e.g., geothermal) but are also an important tight-gas reservoir analog in NW Germany. Core material from two wells in the eastern Ruhr Basin, comprising Bashkirian delta deposits of the Langsettian and Duckmantian substages (Westphalian A and B), were studied using petrographic and petrophysical data to assess their reservoir properties and factors controlling these. The samples have low porosities and permeabilities (mean porosity 5.5% well Bork-1 and 5.1% well Haidberg-1; mean permeability 0.41 mD and 0.16 mD, respectively). Grain size and detrital mineralogy are the main factors affecting reservoir properties. The change in mineralogy from litharenites to lithic subarkoses corresponds to an increase in grain size from silt to sandstone and is associated with a general increase in porosity and permeability. Dissolution porosity largely contributes (up to 6%) to measured plug porosity. The dissolution porosity mostly is caused by the break down of detrital K-feldspar and plagioclase grains and affects low present-day feldspar contents (6.0 to 6.8%). Ductile rock fragments, such as shales and phyllites, cause porosity reduction due to facilitated mechanical compaction and are especially present in siltstones (ICOMPACT > 0.99). The study also used SiO₂ and Al₂O₃ contents from XRF analyses as proxies for estimating reservoir properties and distinguishing between sandstones and siltstones. These findings help identify sections with better reservoir properties for potential exploration and production strategies in similar geological settings.

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Porphyry indicator zircons (PIZ) are often used as an exploratory guide to distinguish fertile granitoid-hosted porphyry copper deposits (PCD). Alumina-saturated granitoids with higher zirconium (> 200 ppm) contents are often the hosts of PCD. REE profiling of zircon is used here to evaluate the chemistry of zircon from a blend of I- and A-type granitoids that hosts the ‘Kaziranga’ porphyry copper mineralization in the Karbi Hills of the Shillong Plateau. Zircon Hf(t) (1.69–9.88; av.4.65) and whole rock ⁸⁷Sr/⁸⁶Sr (0.703) of the Kaziranga granitoids suggest magma derivation in an intraplate or primitive island arc tectonic setting, in which the active mechanism of magma derivation was partial melting of a metasomatically enriched lithospheric mantle that was probably fertilized by an intraplate mantle plume. The fertility indicators with PIZ are grouped with respect to the distribution pattern of normalized REEs, as they exhibit low LREE and enriched HREE, large + Ce, and small -Eu anomalies. The hypogene alteration zones have developed on a regular basis, the potassic alteration zone being close to the central zone around Kuthori in the north Karbi Hills. Although the hosts demonstrate expansion and development of the hypogene alteration zones, the zircons from the mineralized granitoids of Kuthori central zone and granitoids of the nearby Panbari–Dallamara shear zone have similar REE pattern. The target zone’s zircon has higher values of ΣHREE, U, Th, Y, Nb, and Hf. The admissible range of fertile PCD is matched by the cross plots of critical element ratios, demonstrating high calculated Ti in zircon temperature (up to 828 °C).

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Intrusion-hosted gold deposits within the Birimian terrane in Ghana have become attractive targets for exploitation. However, similarities between intrusion-related gold deposits (IRGDs) and orogenic gold deposits (OGDs) generally result in misclassification of these deposits. This paper reviews the literature on gold deposits hosted in intrusions of the Birimian terrane of Ghana, and in addition applies multivariate statistics of geochemical data, to ascertain their classification. The host intrusions can be linked to the two different magmatic pulses (2179 ± 2 to 2136 ± 10 Ma) and (2125 ± 2 to 2088 ± 1 Ma) that intruded the volcanic belts and sedimentary basins, respectively. Intrusion emplacement and gold deposition in the Birimian of Ghana are products of large-scale compressional to transpressional activity, the Eburnean tectonothermal event (ca. 2.1 Ga). Mineralization within sedimentary basin-type intrusions is concomitant with intrusion emplacement, while volcanic belt-type intrusions significantly predate mineralization. Low salinity (< 10 wt % NaCl equivalent), reduced fluids with H₂O–CO₂-rich and minor methane (CH₄) and nitrogen (N₂) compositions are responsible for gold emplacement at crustal depths of 4–6 km. The trapping temperature and pressure for hydrothermal fluids usually range between 180 and 350 °C and between 1 and 3 kbar, respectively. Trace element geochemistry has revealed anomalous concentrations of granophiles in some of the deposits and consistent low base metal concentrations associated with gold. The tectonic setting of the deposits, among other factors, aligns the intrusion deposits to the OGDs. Nonetheless, after analysing the distinguishing attributes of the deposits, it cannot be disregarded that magmatic fluids play a role in the overall hydrothermal fluid budget.

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We present a case study of high-resolution screening and integration of palynological and organic geochemical methodologies in the TSW-21 well in the Abu Gharadig Basin, north Western Desert of Egypt. A total of 84 cores and cuttings samples were analyzed for TOC wt%, Rock–Eval pyrolysis, molecular geochemistry, vitrinite reflectance (R_o%), and palynology. The categorization of dispersed organic matter in palynological macerals shows that it differs not only in palynofacies properties but also in distinct palynomorph characteristics (mainly abundance, diversity, marine versus non-marine, and preservation). The work highlights the value of integrating optical and geochemical methods in screening hydrocarbon source rock potential, and the implications for kerogen examination and the reconstruction of depositional environments. In the present study, data indicate a marine, near-shore depositional environment of the middle Cretaceous Bahariya Formation, and more open-marine conditions for the overlying strata of the Upper Cretaceous Abu Roash Formation. A variety of kerogen types (II/III, II, III, and IV) is predominant in the Bahariya Formation and is mainly gas-prone (highly phytoclastic and rich in amorphous organic matter [AOM]). In contrast, the Abu Raoash Formation yielded highly oil-prone Type II > I kerogen (AOM-rich). Thermal maturity proxies including Rock–Eval pyrolysis (T_max), R_o%, and Spore Coloration Index (SCI) reveal that the Bahariya and Abu Raoash formations are immature.

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