Solid Earth Sciences最新文献

Jiaodong is the largest gold deposit concentration area in China. In recent years, great breakthroughs have been made in deep prospecting, and it has become the third largest concentration area of gold deposits in the world. A series of prospecting discoveries in the Jiaodong area provided the basic conditions for summarizing the regional metallogenic law, constructing the deposit model, carrying out research on the genesis of the deposit and innovating metallogenic theory. The Jiaodong gold deposits are mainly distributed in Northwest Jiaodong, Qi-Peng-Fu and Mu-Ru metallogenic regions, and mainly occur in Precambrian metamorphic rock series, Jurassic Linglong-type granite, Early Cretaceous Guojialing-type granite and Laiyang group. The orebodies are controlled by Sanshandao, Jiaojia, Zhaoping, Xilin-Douya, Jinniushan and other major ore-controlling fault zones. The main mineralization types include altered rock-type, quartz vein-type, stockwork type, sulfide quartz vein-type, interlayer detachment zone-type, altered conglomerate-type, basin margin fault breccia-type and pyrite carbonate vein-type and so on. The important progress of deep prospecting is summarized: two super giant deposits have been evaluated in the Sanshandao and Jiaojia areas; the amount of altered rock-type gold resources identified by deep prospecting has exceeded that of quartz vein-type gold deposits in the Linglong gold deposit field; the Hushan large scale gold deposit has been discovered in Qixia gold deposit field; and the Liaoshang large-size gold deposit is a new discovery of deep prospecting in Pengjiakuang gold deposit field. The ore-controlling faults of gold deposits have several changing steps with dip angle varying from steep to gentle along the dip angle. The gold ore bodies are mainly enriched in sections along the steep, gentle turning points and relatively gentle parts of the fault dip angle, forming a stepped distribution pattern. In the Early Cretaceous, the subduction and rollback of the Paleo-Pacific plate induced crust mantle interaction, resulting in large-scale magmatic and fluid activities. The crust tension and magma uplift form the dome extension structure of granite, which provides a channel for the migration of ore-forming fluid and also provides favorable space for the deposition of ore-forming fluid.

The Wilson cycle predicted that the passive continental margin finally collapses and forms a new subduction zone, because the transition between oceanic and continental lithosphere provides as a natural boundary with great density and rheology contrasts. However, this process is hardly constrained, due to the lack of obvious Cenozoic examples in the natural Earth. The rather stable passive margin of Atlantic Ocean, without any clear sign for subduction initiation (SI), also challenges this hypothesis. In this study, we have reviewed the main ideas and models for the SI at passive margin, which are classified into two categories, i.e. driven by either local forces or external forces. The local forces, mainly due to the local gravitational instability, are generally insufficient to break the lithosphere and lead to SI on the present Earth, although they are possible driving mechanisms for the SI in early Earth with higher potential temperature of the mantle. The external forces may come from the push of mid-ocean ridge and/or mantle plume, the lateral drag of neighboring sinking slab, as well as the basal drag of large-scale mantle convection. They are feasible to drive SI for young oceanic basins with thin and weak lithosphere on the present Earth. But the old and stable Atlantic type margin with high rheological strength is difficult for SI, which requires pre-existing weakness and/or significant weakening mechanism. Previous numerical studies have generally focused on the geodynamic conditions of SI. However, the SI for either present or early Earth is still not well constrained, especially in the aspects of geological responses and possible petrological records, which thus requires further systematic comparative studies between numerical models and geological records.

Clay is one of the most important minerals having both geological and industrial significance. Clay lenses in the Dupi Tila Formation of the Bengal Basin have been investigated to characterize and determine their origin in terms of geological environment. Clay samples have been collected from the fresh cropped surface from the areas of Dupigaon (Jaintiapur Clay), Birisiri (Bijoypur Clay), and Bandarban (Bandarban Clay) in Bangladesh. Characterizations of the clay deposits have been carried out by field investigation, scanning electron microscope (SEM), X-ray diffraction (XRD), laser particle size analysis techniques, and X-ray fluorescence (XRF). Field investigation shows that clays are white, yellowish to variegated in color with a thickness of 15 cm to about 4 m, and moderately soft. The X-ray diffraction analysis reveals that kaolinite is the predominant clay mineral in all samples with minor amounts of quartz and feldspar. SEM analysis indicates that abundant tiny disc and irregular shape of kaolinite crystals are mixed with sparse gathers of K-feldspar and detrital quartz grains. The presence of silt in all the clay samples has been verified by the gran size ranges from laser particle size analysis. The grittiness is the result of the deposition of silt considering the highest percentage in Bandarban and Bijoypur and lowers in Jaintiapur. The XRF analysis shows that SiO₂ (68–75%) is the most abundant among major oxides, Al₂O₃ and Fe₂O₃ come next, along with the lower percentage of MnO, MgO, CaO, Na₂O, K₂O, TiO₂, P₂O₅, and SO₃. The average concentration of different trace elements has been ranging from 3 to 430 ppm. Several geochemical indices suggest that the sediments might be derived from the Shillong plateau and the Himalayas in the north and the Naga-Lushai-China Hill-Arakan Yoma mobile belt in the northeast, which are highly weathered sedimentary terrains. Provenance studies and elemental ratio supports the evidence of the felsic origin of the studied sample. The Chemical Index of Alteration (CIA) value of 93% indicates the occurrence of significant weathering in different sediment recycling phases. From the results of the aforementioned analyses, it is clear that clay lenses in the Dupi Tila Formation are in-situ in origin and parent materials are detrital rather than authigenic nature confirmed by the presence of detrital quartz and reviewing the depositional environment, which gives us the insights of the geological condition of that time as well as the possible mode of sediment transportation.

The I-type Pan-African Misajé granites, situated in the Western domain of the Central African Fold Belt are among the late-to post tectonic granites that intruded the ante-Pan-African country rocks. In this study, magnetic susceptibility and geochemistry data are integrated to evaluate the productive and non-productive metal potential of these I-type plutonic granites, which comprise Leucocratic granite, Biotite granite, Biotite-hornblende granite, and granodiorites. The magnetic susceptibility of the representative rock samples of the Misajé pluton ranges from 0.001 × 10⁻³ SI to 33.2 × 10⁻³ SI. These values indicate the coexistence of both magnetite and ilmenite-series. The magnetite-series (>3 × 10⁻³ SI; oxidized type) constitute about 32 vol.% while the ilmenite-series (<3 × 10⁻³ SI; reduced type) represent 68 vol.%. These magnetite-series vary from small MD (Ms ≤ 1) to coarse PSD (Ms > 1) grains sizes. The Misajé plutonics are high potentially productive in Zn, Th, ilmenite, and magnetite and low productive in Sn, W and Cu though their extension remains limited, probably due to the remobilization of the ancient Paleoproterozoic crust that would have contributed to the dispersing of pre-Pan-African metal deposits.

While performing various steps for exploration and exploitation of geothermal resources, the applications of geochemical methods are pivotal for reservoir characteristics evaluation. It is used concurrently with geological and hydro-geological appraisals to supplement available information regarding the prospectively of the region of investigation at the relatively low cost compared to geophysical methods or drilling. Temperature range of 40 °C to 47 °C is demonstrated by the study area comprising of several thermal water manifestations. Studies from existing wells and surveys have suggested the presence of a sizable low enthalpy geothermal resource in the field. One of the most prominent uses of geochemical sampling is the determination of subsurface temperatures using Geothermometry. In this study, commonly accepted concepts of cationic and Silica Geothermometry have been applied to understand conditions in the reservoir. Interpretation of the data has given us some input on the reservoir characteristics like reservoir temperature, mixing process and multiple fluid origins. In Dholera geothermal field for determination of variation in hydrochemical facies and understanding of the advancement of the hydrochemical forms, the current research additionally imagines the significance of graphical portrayals like Piper chart, Scholler and so forth individually. The seawater intrudes the composition of water shows the reservoir. The physio-chemical properties of the water also influence exploitation strategies.

The Naojiao Fe deposit (Lingxiang district of Daye orefield) in Eastern China is closely associated with the Early Cretaceous Lingxiang diorite intrusion. However, the metallogenic origin of the Fe deposit and its possible magmatic link remains controversial. Here, we present microstructural and trace element features of magnetite grains from the ore-related diorite (type 1), orebodies (type 2), and the orebody-wallrock contact (type 3) at Naojiao. Type 1 magnetite grains have primary automorphic texture, and high P, V, Cr, Ni, and Ga contents but low Mg and Al contents. However, both type 2 and 3 magnetite grains display alteration zoning and oscillatory zoning, and have high Mg, Al, Zn, Zn/V, Co/Ni, Ti/V, and Ni/Cr, but low V, Cr, Ni, and Ga contents. Based the multi-element variation plots, and (Ca+Al+Mn) vs. (Ti+V), Ti vs. Ni/Cr, and (Ti+V) vs. (Al+Mn) discrimination plots, type 1 magnetite could be magmatic and underwent metasomatism, and was formed under relatively high temperature (>500 °C). Type 2 and 3 magnetite grains may have been hydrothermal and closely associated with relatively oxidizing fluids, and formed under relatively low temperature (300–500 °C). Type 3 has wider range of trace element contents, and may have undergone late-stage alteration. Moreover, the Naojiao Fe deposit has similar magnetite microstructures and trace element contents, pyrite δ³⁴S values, and mineralization ages to other skarn deposits in the Daye orefield, and further suggests that it is a skarn deposit.

Oxygen isotope composition (δ¹⁸O) in Porites coral skeletons at the micrometer scale has been proposed to be uninfluenced by temperature in previous studies. Considering that temperature is the main controlling factor of coral δ¹⁸O at the macro scale, the effect of temperature on the δ¹⁸O variations at the micrometer scale should be evaluated carefully. To better understand the climatic and biological significance of high-resolution δ¹⁸O in coral skeletons, in situ δ¹⁸O values were analyzed in three modern Porites corals from the South China Sea and the Great Barrier Reef by a Cameca IMS 1280-HR secondary ion mass spectrometry (SIMS). A twice vacuum impregnation procedure for sample mount preparation and a subsection calibration method based on multiple controlling points for coral matrix effect correction were used for SIMS analysis. The derived SIMS coral δ¹⁸O time series exhibit consistent long-term variations with the monthly δ¹⁸O (measured by conventional gas isotope ratios mass spectrometry), monthly Sr/Ca, and daily sea surface temperature, first highlighting the dominant temperature control on micrometer-scale coral δ¹⁸O. The 1.5‰ fluctuation amplitude of SIMS coral δ¹⁸O at the scales of 400–800 μm can be explained by the combined SST effects of ~10-day variation and day–night fluctuation, whereas the effects of seawater δ¹⁸O and growth rate on SIMS coral δ¹⁸O seem not dominant. Some abnormally negative δ¹⁸O values possibly resulted from the primary ion bombardment on indistinguishable centers of calcification.

The electrical conductivity of mineral aggregates depends both on the properties of the constitutive minerals and the ways those minerals are assembled. Mixing, or average models combine the conductivity of single phases to give bulk conductivity of rocks, thereby linking experimental measurements to geophysical observations. In order to compare these mixing models and allow an informed choice, several popular approaches, including bounds and average models, have been used to estimate the conductivity of a typical dry upper mantle and transition zone with a pyrolite composition. All the estimations calculated using the various average models lie between the rigorous constraint that is given by the HS bounds. The average models in this study are found to give similar bulk conductivities with the difference of less than 0.5 orders of magnitude, except the geometric mean, implying that the choice of the average models is insignificant. The effective electrical conductivity of pyrolite mantle has been derived from the conductivity of dry mantle minerals using the effective medium theory, and was found consistent with observed conductivity values for some subsurface regions of the Earth which we expect to be relatively dry. This provides us with baseline conductivity for a dry mantle, which is helpful to understand the water distribution in the deep earth.

In the western Kunlun Mountain region, due to the convergence of the Paleotethys Ocean in the Late Paleozoic, through northward subduction towards the West Kunlun Block, forming the Kangxiwa–Waqia arc magmatism and back-arc extensional basins, including the Tamu–Kalangu and Oytag–Kurliang basins. This study explores the relationship between basin evolution and mineralization. First, the Late Paleozoic back-arc and far-field continental back-arc basins were distinguished by analyzing sediment constructions in the basins. The former was continuously deposited in the Middle Devonian–Late Permian above the Precambrian basement due to the existence of the depressions. The latter were intermittently deposited in the Middle Devonian–Late Permian due to rifting above the Caledonian orogenic belt. Although the two settings were separated by the Tiekelike fault, their formations were subject to mantle upwelling caused by the subduction of the Kangxiwa oceanic crust. We also divided the Hercynian and Indosinian magmatic activities related to the basin; the former of which included the formation of bimodal magmatic rocks (339–291 Ma) as the basin expanded, which resulted in hydrothermal sediments and hydrothermal–magmatic mineralization, mainly including volcanic massive copper sulfide, Cu and Ni sulfide, and hydrothermal deposits. During the Indosinian orogeny, intermediate acidic magmatic rocks (265–206 Ma) associated with the closure of the basin was formed, leading to the development of hydrothermal Cu–polymetallic and porphyritic copper deposits. The relationship between basin evolution and mineralization was determined. Based on the findings, we concluded that the basin extended between the Middle Devonian and Early Permian. Syngenetic sedimentary deposits formed in the closed basins and anoxic environments of local depressions and mainly included sedimentary rock-hosted stratiform Cu deposits, exhalation-sedimentary Pb–Zn deposits, and sedimentary Mn deposits. Between the Middle Permian and Early Triassic, the basin began to close and the transformation from a basin to a mountain range finally occurred. The sedimentary basement was transformed by folding and faulting, thus forming tectonic deposits, including hydrothermal vein-type Pb–Zn deposits and Cu–Pb deposits within tectonically altered rocks. The results of this work show that the Late Paleozoic extensional tectonic environment formed both the back-arc basin and the far-field continental basin simultaneously. Periods of magmatism related to rifting and convergence could be distinguished by the corresponding mineralization. The metallogenic types related to the basin could also be divided into sedimentary, magmatic hydrothermal and tectonic hydrothermal deposits.