Search Results (8)

The trapiche garnet, a gemstone of unparalleled beauty, boasts a rare structure comprising one core, six radiating arms, and a main body. The occurrence of garnet within the trapiche structure elevates it beyond the species, granting it significant scientific and gemological value. In this study, we conducted the first systematic investigation of trapiche garnets from the Chun’an area, Zhejiang Province, China. These samples were proven grossular through the analysis of spectroscopy and major elements. The trace element features are consistent with the distribution patterns of garnet in hydrothermal metasomatic skarn. Microscopic observation and Raman spectroscopy revealed that dark inclusions within the core and arms consist predominantly of amorphous carbon. The in situ U-Pb dating of the trapiche garnets revealed a crystallization age of 120.7 ± 4.7 Ma, corresponding to the late Yanshanian movement. It is speculated that the contact metasomatism between magma enriched in Al and surrounding rock led to the formation of calcareous skarn. This study provides insights into gemological, geochemical, and chronological characteristics, broadening the research on trapiche structures, and enhancing the understanding of gemstone mineralization timing and local tectonic activity. Full article

The Dashigou deposit is one of the most representative carbonatite-type Mo-REE deposits in the East Qinling metallogenic belt of China, with a molybdenum resource of more than 180 kt and a rare earth resource of 37.8 kt. Recent exploration has revealed a considerable scale of uranium mineralization within this deposit. Therefore, this study conducted detailed mineralogical and EPMA U-Th-Pb chemical dating on the uranium mineralization in the Dashigou deposit. The results indicate that the U-ore body in the Dashigou deposit mainly consists in carbonatite veins, and principally as anhedral, mesh-like uraninite. The mineral assemblage is characterized by uraninite + rutile + bastnasite + parisite or brannerite. The uraninite displays geochemical compositions of high Y and Ce and low Si, Ti, and Mg. The EPMA U-Th-Pb chemical dating is 144 ± 3.1 Ma, representing the Yanshanian uranium mineralization age in the region. The newly discovered uranium mineralization age indicates that the deposit experienced a uranium remobilization event during the Cretaceous and was formed in an intracontinental orogenic and extensional environment post-collision orogeny. Full article

Studying the activation, migration and precipitation processes of ore-forming elements is essential for understanding the genesis and mechanisms of skarn deposits. A typical skarn profile formed by the intrusion of Yanshanian granodiorite into the Carboniferous carbonate strata was studied. The profile is highly consistent with the classic skarn profile, ranging from the intrusion, weak alteration belt, skarn belt (inner and outer skarn belt) and mineralization belt (mainly characterized by Cu mineralization) to the surrounding marble without being affected by late-stage low-temperature or supergene weathering alteration. Whole-rock data show that the major and trace elements exhibit relatively small changes in the granodiorite and inner skarn, but huge variation in the boundary between the inner and outer skarn; Na, Al, Ti and Sr show significant decreases, while Fe, Mg, Zn, V and Ni show significant increases. The elemental content in the outer skarn is 10–100 times or more higher than that in the marble, but the elements such as Ca, Sr and Cs diluted from the marble. During the migration process from the inner skarn to the outer skarn, some elements (such as K, Rb and Ba) were depleted in the inner, but not enriched in the outer, indicating that they may migrate to farther locations. Grossularite developed in the inner skarn, with light rare earth element (LREE) depletion and heavy REE enrichment, as well as positive and negative anomalies of Eu (δEu = 0.42–3.95). Andradite developed in the outer skarn, with zonation development, light REE enrichment, and heavy REE depletion and a positive Eu anomaly (δEu = 0.36–46.83). Some negative Eu anomalies appear at the edges of garnets in the outer skarn, indicating fluctuations in fO₂ during the late skarn process. A positive correlation between Fe³⁺ and REE³⁺ in the garnets from the inner skarn, as well as between Al³⁺ and REE³⁺ from the outer skarn indicated that there are different YAG substitution mechanisms of REE between the inner and outer skarn. Low garnet REE contents and highly variable Y/Ho ratios in outer skarn suggest that the significant fluctuations in REEs may be primarily controlled by water-rock interactions. Considering the whole-rock major and trace element contents, as well as the trace element features of garnet, we found that whole-rock Na, Al, Ti and Sr elements, garnet Ti, Zr and Nb elements exhibit significant differences between the inner and outer skarn. These characteristics can be used to distinguish the boundary between the rock body and carbonate during the skarnification process. Full article

The Pandian deposit is a newly discovered contact metasomatic skarn magnetite deposit found in the Cainozoic super-thick overburden on the northwest margin of Luxi Uplift (LXU). Presently, the horizontal scale of the deposit delineated by the potential field (gravity and magnetic method) has shown giant potential for ore deposits, and mapping the ore-controlling structures in the vertical scale becomes a primary task for metallogenic prediction. In our study, the wide-field electromagnetic method (WFEM), with a strong anti-noise ability in recording electromagnetic signals on the surface at multiple frequencies, is applied to characterize the deep conductivity distribution of the Pandian area. Based on the inversion results from two parallel WFEM profiles, which consist of 105 sites and previous geological and geophysical results, the 2D geoelectric models are established. The low-resistivity regions (with a typical range of 25~32 Ω·m) in the electrical models are proven to be ore bodies of Pandian deposit, which are developed along the contact zone between Yanshanian intrusive rocks and Paleozoic Ordovician strata. The scattered bodies (typically >32 Ω·m) in Ordovician limestone strata are probably caused by intrusive diorite pluton closely related to magnetite mineralization. Due to contact metasomatism, bedded limestone near magnetite was metamorphosed into marble and accompanied by low-resistivity skarn alteration, with resistivity much different from its high-resistivity protolith. The inverted geoelectrical models visually reflect the spatial distribution features of intrusive rocks and lithologic alteration/fracture zones. Full article

The Hadamengou deposit is the largest gold deposit in Inner Mongolia. However, given that the sources of ore-forming alkaline magmatic hydrothermal solutions and ore-controlling structures are still controversial, the theories behind the genesis of the deposit have been controversial. In this study, four controlled-source audio magnetotellurics (CSAMT) and spectral induced polarization (SIP) profiles in the mining area were used to obtain the underground resistivity model and the pseudo section map of the apparent frequency dispersivity based on fine inversion. In the resistivity model, there are two high-resistivity blocks with resistivity greater than 3000 Ω m and three low-resistivity channels with resistivity less than 50 Ω m. Combined with the regional geological and drilling data, it is inferred that the high-resistance bodies, R4 and R5, may be alkaline magmatic intrusions related to multiple stages of magmatic hydrothermal activities, ranging from the Precambrian to Yanshanian periods. The highly conductive channels, C3, C5, and C4, may represent the Baotou-Hohhot fault, secondary faults, and ductile shear zone, respectively, which were formed in the Precambrian era and underwent multiple activations during the Hercynian to Yanshanian period. According to the spatial relationship, it is inferred that the ductile shear zone is an important ore-controlling and ore-hosting structure. However, the Baotou–Hohhot fault may be a pre-metallogenic fault rather than an ore-controlling fault. By comparing the resistivity model with the pseudo section of the apparent frequency dispersivity, it was found that all the known gold veins are located in the superimposed area of low resistivity and high-frequency dispersivity. It is speculated that the ductile shear zone outside the alkaline magmatic rock with the superimposed characteristics of low resistivity and high-frequency dispersivity is the favorable area for mineralization. Full article

The Dahutang tungsten deposit is one of the largest deposits in the Jiangnan tungsten belt. The Jiningian pluton is widely distributed in the orefield, which is considered an ore-bearing wall rock and Ca source for scheelite mineralization. The Jiningian granodiorite samples near ore have high W contents (average 93 ppm). Moreover, their SiO₂ and P₂O₅ contents are positively correlated in Harker diagrams, and the A/CNK values vary between 1.18–1.71, suggesting that the Jiningian granodiorite is high fractionated S-type granites and has the potential for W mineralization. The zircon U-Pb ages of the Jiningian granodiorite samples (17SWD-1, 17SWD-2) are 845 ± 21 Ma (MSWD = 1.7) and 828.7 ± 7.5 Ma (MSWD = 1.0), respectively, representing the formation ages of the Jiningian pluton. The U-Pb age of hydrothermal zircons (~140 Ma) in the Jiningian granodiorite samples is consistent with the mineralization age (150–139 Ma), indicating the strong superimposed modification of the Yanshanian mineralizing fluids. The positive correlation between Ca and W molarity in the Jiningian granodiorite samples demonstrates that they provide considerable Ca and W during Yanshanian mineralization. The W activation migration due to sodium alteration can be inferred from the inverse correlation between Na and W molarity. The study tries to provide a new perspective on the origin of mineralized material in the world-class Dahutang tungsten deposit. Full article

The Shulan area in Jilin Province is a part of the Lesser Xing’an–Zhangguangcai Range polymetallic ore belt, which is an important Cu–Mo ore region of northeast China. The discovery of three large Mo ore deposits (Fu’anbu, Chang’anbu, and Jidetun) highlights its potential for porphyry Mo ore deposits. Here we investigated the tectonic setting and mineralization of Mo ore deposits in the Shulan area, based on comparative study of the Fu’anbu, Chang’anbu, and Jidetun deposits. The ore-controlling structures are NE–SW- and NW–SE-trending faults. The main ore mineral in all three deposits is molybdenite. The ore bodies are all hosted in granites, have a stratiform or lenticular shape, and have strongly altered wall rocks. These observations indicate the Mo deposits in the Shulan area are typical porphyry Mo deposits. All were formed during the early Yanshanian (199.6–133.9 Ma). Biotite adamellites from the Chang’anbu deposit yield a U–Pb age of 182.10 ± 1.20 Ma. Molybdenites from the Fu’anbu and Jidetun deposits have Re–Os isochron ages of 166.9 ± 6.7 and 169.1 ± 1.8 Ma, respectively. Quartz and ore minerals were analysed for H–O and S–Pb isotopes, respectively. The results suggest the ore-forming materials were predominantly of upper-mantle origin, with secondary contributions from the lower crust. The ore-hosting granites have high concentrations of SiO₂ (66.67–75.43 wt.%) and Al₂O₃ (12.91–16.44 wt.%), low concentrations of MgO (0.09–1.54 wt.%), and Ritman index (σ = K₂O + Na₂O)²/(SiO₂ − 43)) ratios of 2.09–2.57. The granites are enriched in large-ion lithophile elements and depleted in high-field-strength elements, and have negative Eu anomalies. The ore-hosting rocks are geochemically similar to granites in northeastern China that were generated in a collisional orogeny. We conclude that early Yanshanian (199.6–133.9 Ma) mantle–crust-derived magmatism caused by the subduction of the Palaeo-Pacific Plate was the main source of Mo deposits in the Shulan area. Full article

The Xiajinbao gold deposit is a medium-sized gold deposit in the Jidong gold province. Ore bodies mainly occur within the Xiajinbao granite porphyry and along the contact zone between the intrusion and Archean plagioclase hornblende gneiss. The zircon LA-ICP-MS age of the Xiajinbao granite porphyry yields 157.8 ± 3.4 Ma, which reflects the metallogenic age of the gold mineralization. Its petrographic features, major and trace element contents, zircon Hf isotopic model ages and compositional features all demonstrate that the Xiajinbao granitic magma is derived from partial melting of the Changcheng unit. The results of H–O isotopic analyses of auriferous quartz veins indicate that the ore-forming fluids are derived from magmatic waters that gradually mixed with meteoric waters during the evolution of the ore-forming fluids. S–Pb isotopic data indicate that the ore-forming fluids were mainly provided by the magma and by plagioclase hornblende gneisses. The gold metallogeny of the Xiajinbao gold deposit is temporally, spatially, and genetically associated with the high-K calc-alkaline-shoshonitic granitic magma emplaced during the Yanshanian orogeny and intruding the Archean plagioclase hornblende gneisses. These magmatic events mainly occurred during the period of 223–153 Ma and comprise three peak periods in the late Triassic (225–205 Ma), the early Jurassic (200–185 Ma) and the middle–late Jurassic (175–160 Ma), respectively. The metallogenic events in this area mainly occurred during the period of 223–155 Ma with the peak periods during the late Triassic (223–210 Ma) and the middle–late Jurassic (175–155 Ma), respectively. Both mineralization and magmatism occurred in a post-collisional tectonic setting related to the collision between the Mongolian plate and the North China plate at the end of the Permian. The magmatism of the early Jurassic occurred during the collision between the Siberian plate and the Mongolian plate, which caused the thickening and melting of the northern margin of the North China plate. The middle and late Jurassic magmatism and metallogenic activities are products of crustal thickening and partial melting during the Yanshanian intra-continental orogeny at the northern margin of the North China plate. Full article