Mineralogical characteristic of ductile–brittle shear zone in the Baguamiao gold deposit, Shaanxi Province: Implication for gold mineralization

The Baguamiao gold deposit in Fengxian County of Shaanxi Province is located in Fengtai polymetallic ore concentration area in the orogenic belt of West Qinling Mountain. It is a typical gold deposit under the control of brittle–ductile shear zone. The progressive deformation of brittle–ductile shear zone has been found to be closely associated with gold enrichment: the gold-bearing sulphides pyrrhotite and pyrite are distributed along blastobedding S0 in the undeformed area, they are developed along phyllitic foliation S1 (//S0) in the slightly deformed area, along mylonitic foliation (S2#S0), or S–C foliation in the highly deformed area, and they are enriched along rheological quartz vein (S2–L) and joint quartz vein (S3) in the highly deformed and the tectonic fluid area. Researches based on electron microprobe analysis and mineral–structure–geochemistry indicate that the characteristic metamorphic mineral assemblage of the brittle–ductile shear zone is albite–biotite–sericite–chlorite–quartz–ankerite, with diagenetic and metallogenic temperatures and pressures spanning from 350 to 450 °C and from 0.12 to 0.31 GPa, respectively. In addition, greenschist facies in intermediate temperature and intermediate and low-pressure environment is the metamorphic facies of shear zone in main period. A reconstruction of the palaeotemperature history, based on the biotite and chlorite compositional geothermometer, revealed an intermediate-temperature phase, a high-temperature phase, and a high-temperature critical phase. Biotitization tends to develop towards an oxidation phase, while chloritization is characterized by development towards a reduction phase, indicating large-scale gold enrichment and mineralization were accompanied by strong redox reactions in the ore-forming fluid.

A review of the Early Mesozoic granitoids in the Qinling Orogen: Implication for gold metallogeny

The Qinling Orogen is located between the North China and South China blocks, and considered to form by the multiple accretionary processes, being also characterized by development of voluminous magmatic intrusions as well as abundant mineral resources. Here we compile the geological, geochemical and geochronological data of the Early Mesozoic granitoids from the Qingling Orogen and attempt to constrain the Early Mesozoic tectonic evolution and mechanism of gold mineralization in this region. The Early Mesozoic granitoids are most widespread in the Qinling Orogen, especially in western part, and can be classified into S-, I-, I-A-types, with the I-type constituting the major variety. The isotopic ages of the three types granitoids are 200–222 Ma, 185–248 Ma, 210–227 Ma, respectively, indicating overlapping ages with a wide span of I-type granitoids. The S-, I-, I-A-type granitoids show obvious zonation characteristics, that is the S-type granitoids only occurred in southern South Qinling Belt, the I-type granitoids are most widespread in the western Qinling Orogen, and the I-A-type granitoids are mainly exposed in the North Qinling Belt. The S-type granitoids crystallized at relatively low temperature (800–850 °C) and deep-crustal level (ca. 8 kb) and derived mainly from partial melting of a clay-poor psammitic source. The I-type granitoids formed at high temperature (>925 °C) and pressures above the garnet-in phase boundary (>1.2 GPa) and derived from greywackes and partially igneous source. Moreover, the I-A-type granitoids probably derived from distinct sources with sufficient interaction, or common origin but underwent different degrees of crustal contamination, and generated at high temperatures (ca. 950 °C) and low pressures (1.0–0.2 GPa) conditions. These features indicate the Qinling Orogen experienced subduction, syn-collisional, and post-collisional during the Early Mesozoic, and also suggest the S-, I-, I-A-types granitoids underwent an episodic growth documenting the tectonic regime switchover at this stage. The gold deposits in the western Qinling Orogen can be classified as orogenic, Carlin-type, and Carlin-like gold deposits with the formation age of Late Triassic. The orogenic and Carlin-type gold deposits have no genetic relation to granitic magmatism, but the Carlin-like gold deposits is related to the synchronous magmatism. Combined with regional geology and metallogenic systems, we suggest that the multi-stage Qinling Orogenesis resulted in the formation of the Early Mesozoic S-, I-, I-A-type granitoids and different types of gold deposits with some characteristic differences.

Geochemical and geological characteristics of Permian Lucaogou Formation shale of the well Ji174, Jimusar Sag, Junggar Basin, China: Implications for shale oil exploration

Abundant shale oil resources were recently found in the organic-rich Permian Lucaogou (P2l) Formation shale, Jimusar Sag, Junggar Basin. Although some scholars have conducted some researches on the basic geochemical characteristics of the Lucaogou Formation shale, it is not enough to effectively guide the shale oil exploration in the area. This is because during the exploration process of shale oil, besides the basic geochemical characteristics of shale, its mineral composition, physical properties, and oil-bearing characteristics are all very important evaluation parameters. To systematically evaluate the favourable shale oil exploration sections in the Lucaogou Formation, 265 pieces of shale cores were sampled from well Ji174 in this study. Besides the basic geochemical characteristics of the 265 pieces of cores, the mineralogical, petrophysical, and oil-bearing characteristics were also analysed. Results show that the shale has abundant organic matter (average total organic carbon 3.51%, average petroleum generation potential 15.67 mg/g), dominated by Type II1 and Type II2 kerogen, and has entered the early-mature to mature stage. The Upper Member of the Lucaogou Formation has relatively higher organic matter and organic matter type is relatively more oil prone compared to the Lower Member; however, organic matter thermal maturity is lower. The P2l Formation shale is characterized by high carbonate minerals and low clay minerals, and when considering mineralogical brittleness, both the Upper and Lower members are ideally fracturable in shale oil development. The P2l Formation shale has a wide porosity range (1.1–13.9%), and the Lower Member’s porosity is slightly greater. The inversion of shale porosity in the P2l Formation is controlled by the differentiated mineral composition. The oil saturation index values of the Lower Member shale are significantly greater than the Upper Member shale. From the above, the Lower Member of the P2l Formation shale is ideally suited and has greater shale oil potential in the Jimusar Sag, Junggar Basin, especially in the depth intervals of 3,255–3,272, 3,282–3,298, and 3,317–3,334 m in the well Ji174.

Geochronology and geochemistry of early-middle Silurian intrusive rocks in the Lanzhou–Baiyin regions, eastern part of Qilian Block, NW China: Source and tectonic implications

The Qilian Block is one of many continental blocks involved in the early Paleozoic orogenesis of the Central China Orogen, which experienced complex interaction with adjacent blocks and orogens. In this study, we present zircon U–Pb–Hf isotopic, whole-rock elemental, and Sr–Nd isotopic data of the early-middle Silurian intrusive rocks in the Lanzhou–Baiyin regions, eastern part of the Qilian Block, to constrain their source and tectonic settings. The Houchangchuan granodiorite and Shichuan monzogranite were emplaced at ca. 437 Ma and ca. 428 Ma, respectively. These granitoids are peraluminous (A/CNK = 1.1–1.3), enriched in LREEs and LILEs (Rb, Th, U, Pb), depleted in HFSEs (Nb, Ta) with negative Eu and Sr anomalies, and have relatively high whole-rock εNd(t), −4.3 to −2.1, and zircon εHf(t), −10.9 to 1.1, values. The Shichuan monzonite is characterized by intermediate SiO2, metaluminous (A/CNK = ~0.8), and high LREEs fractionation with LILEs (Th, U, Pb) enrichment and HFSEs (Nb, Ta) depletion. We suggest the Houchangchuan granodiorite and Shichuan monzogranite were derived from the metasedimentary (metapelitic) source, which could be the Mesoproterozoic basement of the Qilian Block, and the mantle-derived materials injected into their source region. The Shichuan monzonite could have a mantle-derived origin. Integrated with data from previous studies, the early-middle Silurian (437–428 Ma) intrusive rocks in the eastern part of the Qilian Block were formed during the transition period from a syn-collisional to postcollisional setting, relating to the collisional process after the consumption of North Qilian Ocean (Proto-Tethys Ocean) in the northern margin of the Gondwana continent.

Yanshanian deformation in Western Shandong, eastern North China Craton: Response to a transition from paleo-Pacific to Pacific Plate subduction

The eastern part of the North China Craton has experienced cratonic destruction during the Mesozoic–Cenozoic period. However, the mechanism and geodynamic process of cratonic destruction is still not clear. The Shandong Province is a typical region that suffered the Yanshanian (Jurassic to Cretaceous) tectono-thermal event relating to the cratonic destruction. In this paper, we focus on the Yanshanian deformation in Western Shandong and integrate the previous studies of contemporaneous magmatism. There are two-stage compressive structures developed in Western Shandong, NE- or NNE- trending open folds, which subsequently evolved the same trend as the thrust faults. A series of NW-trending normal faults converted from the pre-existing Indosinian (Triassic) thrusts controlled the formation of Mesozoic basins. These deformations are a tectonic response to the subduction of the Izanagi Plate (paleo-Pacific Plate) under the North China Craton during the Middle Jurassic and the Early Cretaceous. The Cretaceous magmatic rocks in Western Shandong mainly occurred between 130–125 Ma and have multi-magmatic sources, including enriched lithospheric mantle, delaminated lithosphere, and subducted oceanic slab. We propose that the subduction of the Izanagi Plate converting to the Pacific Plate occurred during the middle Early Cretaceous induced a delamination of the lower crust linked to the destruction of the eastern North China Craton.

An integrated approach for extraction of lithology information using the SPOT 6 imagery in a heavily Quaternary-covered region—North Baoji District of China

We studied the applicability of SPOT (Systeme Probatoire d’Observation de la Terre) 6 satellite imagery for red sandstone interpretation in the heavily loess-covered northern Baoji District, China, which is always a crucial but challenging issue in remote sensing geological mapping. First, the non- or sparsely vegetated subareas were outlined in order to narrow the interpretation target area; second, band-math and the fractal “DN-N” (digital number-frequency) algorithm was conducted to extract the sandstone-associated anomalies; third, a series of spatial analysis, for example, overlaying and clumping, was employed for elimination of the false anomalies; finally, we took advantage of matched filtering to discern the non- (red) sandstone outcrops in local areas. Generally, the interpretation accuracy based on this novel technical proposal is acceptable—74% or slightly less (taking the geological sketch as reference criteria)—and can be well proven by field investigation. This study may have contributed a useful case study for remote-sensing lithological mapping in the Quaternary-covered areas.

Geochronology and geochemistry of the Late Jurassic bimodal volcanic rocks from Hailisen area, central-southern Great Xing’an Range, Northeast China

Zircon U–Pb dating and whole-rock geochemical analysis have been studied on the Late Jurassic volcanic rocks in the Hailisen area, Northeastern China, with the aim of constraining the tectonic evolution of the central-southern Great Xing’an Range during the Late Jurassic. The volcanic rocks mainly consist of andesite from the Tamulangou Formation, and rhyolite and minor dacite from the Manketouebo Formation. The results of inductively coupled plasma-mass spectrometry Zircon U–Pb dating for two andesites and one dacite indicate that they formed in the Late Jurassic (161–150 Ma). The mafic rocks are characteristic of low TiO2 (1.01–1.04 wt.%) and P2O5 (0.23–0.31 wt.%) contents, and high Al2O3 (17.19–20.18 wt.%) and CaO (6.69–7.45 wt.%) contents, and belong to the low-K tholeiitic series. These mafic rocks are also characterized by moderately enriched light rare earth element (LREE) patterns and high abundances of Th, U, Zr, and Hf but negative Nb, Ta, and Ti anomalies. The felsic rocks are enriched in alkalis, Th, U, and LREEs; depleted in Ba, Sr, Nb, Ta, and Ti; and exhibit moderately LREE-enriched patterns. These features indicate that the mafic volcanic rocks were likely formed by the partial melting of a lithospheric mantle that was metasomatized by subduction-derived components, but the felsic rocks could derived by partial melting of a crustal source. The Tamulangou and Manketouebo formations have compositions of typical bimodal volcanism, an extensional environment, similar to a post-orogenic setting, which might be related to the closure of Mongol–Okhotsk Ocean.

Early Paleozoic arc magmatism and metamorphism in the northern Qilian Block, western China: Petrological and geochronological constraints

The Datong–Menyuan Complex, located in the northern margin of the Qilian Block, is composed dominantly of high-grade metamorphic rocks covered by a Paleozoic–Mesozoic sedimentary sequence. In this study, we present petrographic observation, conventional thermobarometry and P–T pseudosection modelling, whole-rock major and trace geochemistry, zircons U–Pb dating and Hf isotopic data together, to reveal the relationship between the magmatism and metamorphism along the northern margin of the Qilian Block. The anticlockwise P–T paths of gneiss and amphibolites are obtained using the garnet isopleths thermobaromety combined with phase equilibria modelling. The P–T pseudosection also shows that the gneiss and amphibolite underwent similar retrograde metamorphism with slightly different peak metamorphic conditions at ~720 °C and ~6.4 kbar for the former and ~700 °C and ~7.1 kbar for the latter. This indicates the Datong–Menyuan Complex recorded an amphibolite–granulite-facies metamorphism. The zircon U–Pb analyses of 3 intrusive rocks including granitic dike, diorite, and granite are dated at ca. 499.8 ± 4.3, 495.9 ± 3.3, and 505.5 ± 3.2 Ma, and 3 representative metamorphic rocks underwent contemporaneous metamorphism at ca. 498.9 ± 4.1, 504.4 ± 3.9, and 499.3 ± 2.9 Ma. Zircon Hf isotopic analyses show that 505-Ma zircons from the granitoid have positive εHf(t) values ranging from +8.5 to +12.8, indicating a depleted mantle source. Based on the penecontemporaneous magmatic and metamorphic event, we suggest that the southward subduction of the north Qilian Ocean triggered the activity of mantle-derived magma and coeval metamorphic event. A subsequent compression is attributed to crustal thickening, indicating counter clockwise P–T paths. Combining these data with previous studies, it suggests paired metamorphic belts: the penecontemporaneous high-temperature metamorphic belt related to an arc along the northern Qilian Block and high-pressure/low-temperature metamorphic belt existed in the north Qilian suture.

Early Mesozoic intracontinental deformation in the eastern North China Block: Implication for an indentation model of North China to South China blocks

The Qinling-Dabie-Sulu Orogen was formed by the collision of the North and South China blocks during the Indosinian Period. The intracontinental deformation was subsequently developed during the Late Triassic to Early Jurassic in the northeastern, eastern, and southeastern parts of eastern North China Block with different structural patterns. In this paper, we present structural analysis of the Indosinian deformation in the western Shandong and southern Liaoning provinces and synthesized the previous studies of the Triassic-Early Jurassic deformation in different areas of the eastern North China Block, from the northeastern, central to southeastern parts. Integrating the previous studies of petrology, geochronology, and geophysics, we suggested that (1) a series of top-to-the-northwest thrust-nappes were formed in the Sulu Orogen, and the southeastward antithetic faults developed in the South China Block. In the eastern North China Block, the Indosinian intracontinental deformation is intensely developed near the orogenic belt as thrust-nappes and away from the belt developed open folds. (2) The present-day structural lines in the northeastern, central, and southeastern parts of eastern North China Block show the S-N-, E-W-, and NE-trending lines, respectively. It could be the structural response to the dynamics of an orocline or indentation induced by Triassic collision of the North and South China blocks; (3) the eastern margin of the North China Block subducted southeastward beneath the South China Block along the Wulian-Qingdao-Yantai Fault during the Indosinian Period.

Genetic analysis of bentonite—Permian Wangpo shale in Liangshan, Hanzhong, South Shaanxi Province

Fine pyroclastic materials form bentonites through subsidence, hydrolysis, and argillation in alkaline marine environments, which can further transform to K-bentonite. Bentonites or K-bentonites are quite popular with geologists because their formation often reflects geologic events. However, studies mainly focus on K-bentonite, and research on ordinary bentonites in China is almost nonexistent. Recent research shows that the Wangpo shale consists of ordinary bentonites. Through thorough field investigations, indoor experimentation and analysis, and comparison of petrologic and geochemical characteristics of the Wangpo shale with K-bentonites, this paper explores the nature and genesis of source rocks and provides a basis for the study of ordinary bentonites. The petrological study shows that the Wangpo shale is a montmorillonite vitric tuff, which is mainly composed of montmorillonite and small amounts of quartz, opal, and heulandite. These indicate that it is a typical bentonite. In addition, the rock contains an abundance of fossils reflecting a coastal–shallow sea depositional environment. We chose the elements with stable chemical characteristics for obtaining source rock characteristics, and the results showed that the Wangpo shale has a genetic relationship with neutral alkaline magma. As a boundary between the Upper Permian and the Middle Permian, the Wangpo shale has an internal connection with eruption of the Emeishan basalt in Southwest China. In combination with previous work, it can be inferred that studying the volcanic activity represented by the Wangpo shale plays an important role in studying Emeishan large igneous provinces and the end-Guadalupian mass extinction.