Abstract: The Lüliang Mountains, located in the North China Craton, is a relatively stable block, but it has experienced uplift and denudation since the late Mesozoic. We hence aim to explore its time and rate of the exhumation by the fission‐track method. The results show that, no matter what type rocks are, the pooled ages of zircon and apatite fission‐track range from 60.0 to 93.7 Ma and 28.6 to 43.3 Ma, respectively; all of the apatite fission‐track length distributions are unimodal and yield a mean length of ∼13 μm; and the thermal history modeling results based on apatite fission‐track data indicate that the time‐temperature paths exhibit similar patterns and the cooling has been accelerated for each sample since the Pliocene (c.5 Ma). Therefore, we can conclude that a successive cooling, probably involving two slow (during c.75–35 Ma and 35–5 Ma) and one rapid (during c.5 Ma‐0 Ma) cooling, has occurred through the exhumation of the Lüliang Mountains since the late Cretaceous. The maximum exhumation is more than 5 km under a steady‐state geothermal gradient of 35°C/km. Combined with the tectonic setting, this exhumation may be the resultant effect from the surrounding plate interactions, and it has been accelerated since c.5 Ma predominantly due to the India‐Eurasia collision.
Multi-period mineralizations commonly form superimposed deposits in the orogenic belt, resulting in some disputes about the ore geneses and metallogenic background. Here, we present a comprehensive geological and geochronological study on the Yindongpo super-large gold deposit in the Tongbai composite orogen, central China, to reveal the ages of multi-period gold mineralizations and their links to the complex tectonic evolution framework of the Tongbai orogen. The Yindongpo gold deposit is located in the Weishancheng ore field in the northern segment of the Tongbai orogen and is mainly hosted in the carbonaceous schist of the Waitoushan Formation. Zircon U-Pb dating of leptynite (445 ± 3 Ma) from the Waitoushan Formation shows that it was deposited in the Ordovician, in an island arc or back-arc basin setting during the accretionary orogeny. Orebodies at Yindongpo are strictly controlled by the Zhuzhuang anticline and related fault systems, and are mainly distributed in the turning end, the collapsed part of the dipping end, and the conjugate thrust shear zone of the limbs. Based on field and petrographic observations, two periods of mineralization and four paragenetic stages (stage I to IV) are recognized: the early period consists of the laminated quartz-molybdenite-pyrite stage (I) and laminated-disseminated pyrite stage (II), whereas the late period consists of the quartz-pyrite vein stage (III) and the quartz-polymetallic sulfide vein stage (IV). Molybdenite from stage I has a Re-Os model age of 409 ± 6 Ma, which broadly matches the U-Pb age of intergrown rutile grains (396 ± 4 Ma). These two ages, in combination with the mineralization characteristics, indicate that the early-period gold mineralization was generated by the metamorphism of the accreted terranes during the Early Devonian northward subduction of the oceanic plate. The timing of the late-period mineralization is constrained by the U-Pb age of monazite (127 ± 2 Ma) intergrown with gold-bearing pyrite from stage III and the 40Ar/39Ar ages of ore-related sericite (131 ± 1 Ma) from stage IV and altered sericite (133 ± 1 Ma) from the carbonaceous schist. Combined with the extensive development of coeval granitoids and porphyry Mo deposits in the region, the late-period gold mineralization at Yindongpo belongs to a unified large-scale metallogenic event related to the Early Cretaceous intensive granitic magmatism in an extensional tectonic setting. The results presented here thus reveal two discrete mineralization processes at Yindongpo related to the Tongbai composite orogeny: the early mineralization of Devonian accretionary orogeny and the later mineralization of Cretaceous intracontinental orogeny. Our study contributes to the knowledge of superimposed metallogenesis in composite orogen, and also highlights the decisive role of multi-mineral isotopic dating in revealing the metallogenic epoch of superimposed deposits.
The transformation of inherent minerals during pyrolysis is the key for coke quality and metallurgy. Five different coals were selected to investigate the mineral transformation in coking coal from Shanxi province, and the influences of the coal matrix on mineral transformation were investigated. Fourier transform infrared (FTIR) spectroscopy and scanning electron microscopy–energy-dispersive spectrometry (SEM–EDS) were also used for exploring the mineral transformation. X-ray diffraction (XRD) and Rietveld-based TOPAS 4.2 software package were applied to quantify mineral matters at different temperatures. The results show that the mineral matter in the Shanxi coking coal includes a significant content of kaolinite (67.56–96.13%), tobelite (9.86–19.18%), calcite (1.56–12.75%), and quartz (0.78–4.93%). The catalytic minerals are rare in the Shanxi coking coals. Most minerals undergo complex reactions during pyrolysis, and the major mineral transformation is decomposition of kaolinite and tobelite and formation of mullite. The maximum decomposition rate temperatures of kaolinite and tobelite are 510 and 625 °C, and the formation of mullite occurs between 970 and 1010 °C. The amorphous phase is the dominant phase in the char above 700 °C. It is also found that the organic carbon matrix inhibits the mineral transformation, including dehydroxylation of kaolinite and deamination of tobelite, during pyrolysis.
The southern North China Craton owns substantial Mo, W, Zn, Pb, Cu, and Ag resources, and most of them are hosted in hydrothermal deposits that may co-exist in the same ore field. However, the relationship between the different types of deposits is not well-constrained. To better understand this, here we present a comprehensive geological, geochronological, and pyrite geochemical study on the Nannihu ore field in the Luanchuan district. The Nannihu ore field is largely represented by the Nannihu Mo-W deposit, Luotuoshan Zn-Cu-W deposit, and Lengshuibeigou Pb-Zn-Ag deposit. Mo-W mineralization at Nannihu occurs in the contact zone between the Nannihu intrusion and the wall rocks, showing typical characteristics of the porphyry-type deposit. Three paragenetic stages are recognized at Nannihu, including quartz-potassium feldspar stage, quartz-sulfide stage, and quartz-carbonate stage. The Zn-Cu-W orebodies at Luotuoshan are mainly layered or lens-like, occurring within the skarn zones in the Luanchuan Group. Skarn stage, polymetallic sulfide stage, and carbonate stage are revealed at Luotuoshan. Pb-Zn-Ag mineralization at Lengshuibeigou is characterized by banded sulfide veins that are hosted in the NNE-trending fault zone. Three paragenetic stages are developed at Lengshuibeigou: quartz-pyrite stage, quartz-polymetallic sulfide stage, and quartz-carbonate stage. New geochronological data confirm that the Nannihu intrusion was formed by at least two phases of magmatism including the granite porphyry (zircon U-Pb age of 145 ± 2 Ma) and the porphyritic monzogranite (zircon U-Pb ages of 142 ± 1 Ma and 140 ± 1 Ma), which accompanied by two episodes of Mo mineralization at the Nannihu deposit identified by molybdenite Re-Os dating (144.9 ± 0.7 Ma and 142.8 ± 0.6 Ma). The Luotuoshan deposit and Lengshuibeigou deposit formed at 141 ± 1 Ma to 140 ± 1 Ma and 140 ± 3 Ma to 139 ± 3 Ma revealed by U-Pb dating of garnet and monazite, respectively, which are contemporaneous with the late episode of the Nannihu intrusion and Mo mineralization. Pyrite grains of the three deposits have Co/Ni ratios higher than 1, suggesting a unified magmatic-hydrothermal origin for them. Pyrite from Nannihu and Luotuoshan is enriched in Co, Ni, Se, Mo, Mn, and Bi, whereas pyrite from Lengshuibeigou contains high Zn, Pb, As, Cu, Ag, and Au. The regular elemental distribution of pyrite in the Nannihu ore field is consistent with that of typical porphyry-related systems worldwide. Collectively, the Early Cretaceous magmatic rocks and diverse deposits in the Nannihu ore field make up a giant magmatic-hydrothermal Mo-W-Cu-Zn-Pb-Ag metallogenic system, with the porphyry, skarn, and epithermal types of mineralization displaying a zonation from inside to outside. The multi-episode magmatism and mineralization could result in the remarkable enrichment of mineral resources. The close spatial relationship among different types of mineralization in the Nannihu ore field has a significant implication for porphyry-related mineral prospecting in the southern NCC.
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