Enrichment of iron ores by folding in the BIF‐hosted deposit: A case study from the Archean Qian’an iron deposit, North China Craton

As part of the Archean greenstone belt, the Qian’an iron deposit is a typical banded iron formation (BIF)‐hosted iron deposit in the North China Craton. The BIF iron orebodies, together with Archean host rocks, have experienced multiphase metamorphism and plastic deformation. Folds, especially large‐scale synclines, played the most important role in the enrichment and preservation of the Qian’an BIF iron deposit. Exploration and mining reveal that orebodies in the deposit are located in the cores of folds. Structural analysis of the BIF ores suggests that rocks and minerals show obvious plasticity and tend to form types II and III folds of the Ramsay fold classification in the early stage of high temperature deformations (granulite facies), whereas type Ib parallel folds and sometimes type Ic folds are widely developed in the late stage of relatively low temperature deformations (e.g., amphibolite facies). Hand specimens of different types of folded iron ores are investigated with respect to their thickening coefficients, dimensional fabrics and lattice‐preferred orientation fabrics, to find out the characteristics and variation of folded layers, and the importance of iron enrichment during formation of different types of folds. Our studies prove that folding in Qian’an ore deposit occurred under different deformation conditions and thus led to various degrees of enrichment of magnetite. Among them, the type III folds have the most important significance to the enrichment of magnetite. Thickening coefficient analysis shows that the thicknesses of the quartz layers remain relatively constant, and those of the magnetite layers vary evidently during folding. It is concluded that the quartz‐rich layers dominate the deformation as competent layers, and magnetite‐rich layers as relatively weak and passive layers. In addition, the EBSD (Electron Back‐Scattered Diffraction) technique is applied to analyze the dimensional and crystallographic fabrics of both quartz and magnetite grains in folded BIF ores. The quartz and magnetite fabrics of samples from the cores and limbs of the different types of folds show complex orientation patterns related to plastic deformation superposition. It is shown that the EBSD analysis is a very effective technique analyzing dimensional and crystallographic fabrics of deformed iron ores.

Evolution of a Pyrenean molassic basin in the Western Mediterranean area: The Eocene–Oligocene Cixerri Formation in Southern Sardinia (Italy)

The sedimentary features of the Cixerri Formation (Eocene–Oligocene) and its evolution in space and time are described through mapping, investigation of all the significant outcrops, and also by analysing the depositional architecture of well‐exposed and laterally extensive outcrops. The Cixerri Formation marks the middle Eocene Pyrenean unconformity and blankets a former structural ridge‐and‐swale morphology. This started by covering the flanks of the ridges with small alluvial fans and ultimately filled the swales with fluvial deposits in braided to meandering stream environments. In this latter sedimentation phase, the depositional environment of the formation evolved both upwards, and from west to east, to gradually lower energy environments, suggesting a transgressive‐type evolution. The Eocene–Oligocene Cixerri Formation was deposited by a fluvial network with a regular flow regime under a humid–warm climate: This network was formed by a main W‐E‐directed trunk and a NW‐SE‐directed minor branch that ultimately joined together. The Cixerri Formation, which was fed by the erosion of a sector of the Pyrenean Chain, passes upwards to coarser deposits that indicate the transition from a quiet tectonic environment to an active transtensional tectonic phase (Apennine phase). A comparison with similar coeval formations in Northern Sardinia, Corsica, and the Western Mediterranean area (Balearic Islands, France) is attempted, before the significance of the formation in the Pyrenean orogeny is then discussed.

The geophysical identification, characteristics, and petroliferous significance of sublacustrine fan deposits in the second member of Dongying Formation in Liaozhong Depression, Bohai Bay Basin

Large sublacustrine fan deposits have been identified within the lacustrine successions of the second member of the Palaeogene Dongying Formation in the Liaozhong Depression. In this study, by using comprehensive and detailed analyses of the borehole lithology, limited cores, wireline logs, and seismic facies, 4 types of sublacustrine fan deposits were identified and characterized. Then, an integrated depositional model was established based on the above investigations. The result indicates that the geophysical characteristics of the different sublacustrine fan deposits differed from each other, in terms of their internal configuration and external geometry of seismic reflections, and stacking patterns of wireline logs, and stratigraphic position in the vertical successions. Subsequently, the model was established according to the comprehensive analysis of the geophysical characteristics and stratigraphic position. The discussion of developing conditions and hydrocarbon discoveries within different types of sublacustrine fan deposits provided robust insights regarding how to locate and evaluate reservoirs in such deposits. Furthermore, the results of this study may potentially assist in achieving a new understanding of how to identify sublacustrine fan deposits in similar lacustrine basins and may also assist in making further decisions regarding the terms of hydrocarbon migration and accumulation.

Diagenesis of tight oil sand reservoirs: Upper Triassic tight sandstones of Yanchang Formation in Ordos Basin, China

Upper Triassic sandstones in the Ordos Basin, northern‐central China, comprise tight oil reservoirs. Using a combination of thin sections, SEM, BSE, EDS, XRD, and fluid inclusion analyses, 24 core samples from 13 wells were collected to study the petrology, paragenesis, and diagenetic processes and implications for reservoir quality. Quartz cement usually occurs as overgrowths or euhedral quartz. Extensive dissolution and albitization of K‐feldspar can be observed. Five types of carbonate cements, ferrocalcite, ankerite, dolomite, calcite, and siderite, occur during different diagenetic stages. Two main types of illite and 5 main habits of chlorite are observed in this study. Kaolinite mainly occurs as booklets and vermicular aggregates. Diagenetic illite, chlorite, biotite, mixed‐layer illite/smectite (I/S), and other minor minerals are also observed. The diagenetic processes include compaction, alteration of volcanic materials and mica, clay mineral transformation, cementation (silica, aluminosilicate, and carbonate), and dissolution of feldspars and rock fragments. Compaction was a significant porosity‐reducing agent, and the presence of carbonate cement exerts a dominant impact on the reduction of porosity. Quartz cement and authigenic clays are less important; however, it is worth mentioning that pore‐lining clays are conducive to porosity preservation. In this study, most of the porosity variation is caused by a combination of compaction, carbonate cements, quartz cement, and authigenic clays. This study gives insights into diagenetic alterations within tight sandstones and has implications for reservoir quality prediction in similar settings.

Mineralogical compositions and elemental enrichment of shales in lacustrine rift basin: a case study in the Cenozoic Lunpola Basin, central Tibet

The Lunpola Basin in central Tibet is a Cenozoic lacustrine rift basin with widespread organic‐rich shale and oil shale depositions. Forty‐nine samples were collected from the Lunpola section to evaluate the controlling factors of trace‐element enrichment and mineralogical composition in the lacustrine rift basin. Minerals identified in the Lunpola section include abundant quartz, calcite, and clay minerals; minor quantities of feldspar, dolomite, siderite, and mirabilite; and trace amounts of aragonite, magnesite, salt, pyrite, hematite, zeolite, barite, amphibole, gypsum, anhydrite, anatase, galena, sphalerite, apatite, chromite, zircon, and monazite. The Lunpola shale and oil shale are enriched in trace elements B, Cr, Ni, Mo, and U, in comparison to the upper continental crust. Three processes were probably responsible for the geochemical anomalies found in the Lunpola Basin, including the detrital material input, lake water, and hydrothermal activities. Mineralogical and geochemical data show that the lake water is the dominant influences on the elevated trace element concentrations in the shale and oil shale. The depletion or enrichment of trace elements in sediments from the lacustrine rift basin is governed by general processes (e.g. anoxic environment). Additionally, hydrothermal fluids also cause remobilization of some trace elements in the Lunpola lacustrine rift basin. Copyright © 2017 John Wiley & Sons, Ltd.

Geochronological, geochemical, and Sr–Nd–Hf isotopic studies of the Aketas adakitic granites in Eastern Junggar: Petrogenesis and tectonic implications

The Aketas adakitic granites are located in the Dulate Arc, in the northern part of the eastern Junggar region, Xinjiang (NW China). In this study, we determined the zircon U–Pb isotopic compositions, bulk‐rock Sr–Nd–Hf isotopic compositions, and major and trace elements of Aketas granite samples. These samples show a narrow range in SiO2 content (65.05–67.19 wt.%) as well as high Al2O3 (14.95–18.09 wt.%; A/CNK = 0.96–1.08), Na2O + K2O (7.94–8.42 wt.%), and CaO (2.05–3.48 wt.%) content. Similar to modern adakites, the samples have high Na2O (4.55–5.61 wt.%) and Sr (554.00–915.00 ppm) content, high Sr/Y ratios (85.10–127.38), and low Y (6.51–7.12 ppm) and Yb (0.42–0.79 ppm) content. The low initial 87Sr/86Sr ratios (0.70298–0.70350) and positive εNd(t) (+5.1 to +7.0) and εHf(t) values (+10.4 to +14.9) suggest that the Aketas adakitic granites were derived from the partial melting of lower crustal material in a subduction setting. This is further evidenced by high Mg# values (42.00–65.67), high SiO2 and K2O content, and low Cr (<17.30 ppm), Co (<9.62 ppm), and Ni (<12.60 ppm) content. Bidirectional subduction occurred between eastern Junggar and the Altai Orogen during the Late Carboniferous, and in combination with other exposed Palaeozoic granites in eastern Junggar, the Aketas adakitic granites may represent the latest record of this subduction event. The emplacement of the Aketas adakitic granites implies that eastern Junggar was involved in southward subduction at 308 ± 2.3 Ma; therefore, the final collision between the Siberian and Kazakhstan blocks likely occurred between 310.3 and 307 Ma. Copyright © 2017 John Wiley & Sons, Ltd.

Carbonatite‐metasomatism signatures hidden in silicate‐metasomatized mantle xenoliths from NE China

Lithium isotopes measured in situ in minerals from typical silicate‐metasomatized mantle xenoliths from NE China show that olivine has higher Li abundances of 1.2–4.0 ppm and higher δ7Li values of −0.12–15.46‰ than those in orthopyroxene (Li = 0.56–3.0 ppm; δ7Li = −11.84–7.31‰) and clinopyroxene (Li = 0.21–2.1 ppm; δ7Li = −9.81–14.75‰). The Li distributions between these coexisting minerals show a prominent signature of carbonatite metasomatism. The correlation of δ7Li with Li abundance in orthopyroxene is in accordance with petrological expectation of preferential reaction of orthopyroxene with carbonatite melt. The carbonatite metasomatic agent inferred from the Li isotope systematics is compatible with petrological and geochemical features such as the presence of apatite, appearance of wehrlite, and presence of high‐Mg# minerals and low Ti/Eu in clinopyroxene. The complicated Li isotope composition of olivine implies that carbonatite metasomatism occurred after silicate metasomatism. These features indicate that the predominant silicate metasomatism is extensively recorded by petrology, mineralogy, and geochemistry, whereas Li isotope systematics is more sensitive to subordinate carbonatite metasomatism. The metasomatic melts are inferred to be relatively enriched in 6Li and thus suggests that they were probably derived from a highly dehydrated slab.

Evidence of warm climate during late Sakmarian–early Artinskian in Posht‐e Badam Block, Central Iran and its paleogeographic implication

Examination of the geochemical data, faunal associations, and bauxite/laterite horizons in the Khan Formation, in Kalmard area, Posht‐e Badam Block, Central Iran are presented. The data provide insights on the paleogeographic position of Central Iran as part of the Cimmerian during the late Sakmarian–early Artinskian time. Bauxite/laterite as valuable warm and humid paleoclimate proxies are observed in all studied sections of the mixed siliciclastic‐carbonate Khan Formation and are meters to tens of meters thick. The warm and humid climate is also verified by the previous studies of Khan Formation sandstones yielding high chemical weathering and highly mature quartz grains. Plant fossils recovered from these sandstones are further evidence of the warm and wet climate. The results of the geochemical studies of the Khan Formation carbonates are in robust agreement with the inferred paleoclimate condition from bauxite/laterite horizons and sandstone studies of the Khan Formation. The bivariate plot of major and minor elements shows that aragonite as an indicator of warm water conditions was the major carbonate mineralogy of the Khan Formation limestones. Temperature calculation based on the heaviest oxygen isotope value of the least‐altered sample indicates that the temperature was around 36 and 29 °C for dolomites and limestones of the Khan Formation, respectively. The skeletal grains of the Khan Formation limestones are representative of a photozoan association with fusulinids, smaller foraminifers, calcareous algae, and common crinoids, brachiopods and bryozoans and also ooids and peloids as non‐skeletal grains. Therefore, the results of this study are not in agreement with previous fusulinid‐based paleogeographic interpretations, which suggested a high‐latitude paleoposition for the Posht‐e Badam Block and its separation from other Central Iran terrains during the late Sakmarian–early Artinskian. These data indicate that Posht‐e Badam Block was part of Central Iran during the late Sakmarian–early Artinskian, and its paleoposition was at a low paleolatitude.

Detachment fold vs. ramp anticline: a gravity survey in the southern Pyrenean front (External Sierras)

The Santo Domingo Anticline (External Sierras, Southern Pyrenees), which separates the Jaca piggyback basin from the Ebro foreland basin, is a key structure of the Pyrenees. Its geometry has been interpreted both as a detachment fold and as a hangingwall anticline associated with an underlying thrust. In this paper, we present the results from a gravity survey and 2.5D gravity modelling carried out around the Santo Domingo Anticline. Density measurements indicate a sharp density contrast between the Triassic evaporites‐mudstones in the core of the anticline and the sedimentary sequence (limestones and sandstones) at its limbs. Gravity anomalies together with 2.5D gravity models allow to discern the along‐strike structural changes. From east to west, we document a change from the ramp‐associated fold to the detachment anticline. The capabilities and limitations of the gravimetric method for the determination of fold geometry are also discussed. Copyright © 2017 John Wiley & Sons, Ltd.

Geochemistry of Oligocene Huagang Formation clastic rocks, Xihu Sag, the East China Sea Shelf Basin: Provenance, source weathering, and tectonic setting

The East China Sea Shelf Basin (ECSSB) is an important tectonic unit of the western Pacific active continental margin. It is divided further into a series of sags and basement ridges. The Xihu Sag on the east margin of the ECSSB is an important hydrocarbon exploration area and contains significant petroleum resources. The Huagang Formation is the major reservoir in Xihu Sag, and its provenance has been the focus of debate at all times. The major and trace elements (including rare earth elements) compositions were analyzed for the Oligocene Huagang Formation sandstone and mudstone samples in the Xihu Sag. The provenance and tectonic setting of these clastic rocks are discussed. The Chemical Index of Alteration value and A–CN–K diagrams suggest low to moderate weathering in the source areas of the Huagang Formation. The mudstones have lower index of compositional variation values (<1), indicating that the mudstone samples are compositionally mature, whereas the index of compositional variation values for the sandstone samples (0.59 to 1.87) indicate that the sandstones are compositionally immature and mature. The major and trace element composition and ratios of the clastic rock samples indicate that mudstones are mainly from Late Mesozoic igneous rocks in SE China, which is dominated by the acidic volcanic rock. The provenance of sandstones is from two different directions, the Central Uplift Group of ECSSB and the Taiwan–Sinzi Fold Belt with more mafic volcanic rock. Based on the geochemical analysis of the clastic rock samples, the tectonic setting of Xihu Sag was Japanese‐type continental margin during the Huagang Formation deposition time (Oligocene).