As the important strategic mineral resource, molybdenum is widely utilised by various industries of national economy. Most of the Chinese Mo deposits are classified as porphyry deposits, such as the Nannihu porphyry Mo-W deposit that is one of the biggest Mo deposits in China. Previous studies mostly focused on the geochemistry, geochronology, and isotopic geochemistry of the Nannihu deposit, but its element immigration and occurrence as well as the ore-forming source remain unclear, which limits the development of ore prospecting. Here we document in-situ major and trace element geochemistry of different stages of molybdenite and rutile from the Nannihu deposit, with major aims to determine ore-forming element immigration and occurrence and to reveal ore-forming processes. Based on mineral assemblages and micro-texture, three types of rutile were identified in the Nannihu deposit. The type-1 rutile has stable Y/Ho ratios 16.74–20.82 and similar upper crust rare earth elements (REEs) patterns, which suggest magmatic and crustal origin. The type-2 and 3 rutiles show wide variation Y/Ho ratios (10.69–49.55 for type-2 and 17.85–43.64 for type-3) and belong to hydrothermal rutiles. In addition, two types of molybdenite are also presented, with the Mo1-type for the major ore-forming stage and the Mo2-type for the late ore-forming stage. The fluctuation of elements in the time resolved LA-ICP-MS spectra curve diagram of the Mo1- and Mo2-type molybdenites indicate that the Te, Bi, Pb, Fe and Mn elements are hosted as mineral inclusions in molybdenite lattices. However, the Re and Se elements in these molybdenites display flat signatures and have negative correlations with Mo and S elements, which imply that the Mo and S elements are substituted and occur as isomorphous constituents in molybdenite lattices. The low Re contents (1.60–12.90 ppm) of molybdenites also suggest crustal-mantle mixing metallic sources for the Nannihu deposit. The increasing Fe (4245 to 23943 ppm) and W (1631 to 5922 ppm) contents in hydrothermal type-2 and type-3 rutiles as well as the increasing W contents (113 to 243 ppm) in Mo1- and Mo2-type molybdenites both indicate that the Nannihu ore-forming fluids were under the temperature and oxygen fugacity (fO2) of reduction conditions. The Mo1-type molybdenite is enriched in light REEs and depleted in high field strength elements (HFSEs), and coupled with the late ore-forming stage of F-rich fluorite, which indicate the ore-forming fluid evolution from Cl- to F-rich. Integrating with previous studies, the mineralising fluids of the Nannihu deposit are suggested to be featured by high Cl−, CO2 and metallic contents. The Mo and W ions in mineralising fluids immigrate as Cl-complexes at the early stage of mineralisation, the Mo4+ is then replaced by W4+ in molybdenite lattices at the later stage, and the high temperature and fO2 of the early ore-forming fluids promote the migration of Mo and W elements during ore-forming processes. Eventually, the fluids boiling, and the reduction conditions in temperature and fO2 jointly result in the final precipitation of Mo and W to generate the giant Nannihu deposit under extensional tectonics.
Abstract The margin of NE China, a part of the West Pacific metallogenic belt, contains innumerable low-sulphidation mineral deposits. Gold deposits in this region can be classified into three distinct types based on geology and ore mineral paragenesis: (1) low-sulphidation epithermal silver–gold deposits, (2) low-sulphidation tellurium–gold deposits, and (3) low-sulphidation epithermal tellurium–gold deposits. Ores formed during the late Early Cretaceous and the early Late Cretaceous reflect three distinct metallogenic periods: the Fuxin Stage at 115.98 ± 0.89 Ma, the Quantou Stage at 107.2 ± 0.6 Ma or <103 Ma, and the Qingshankou or Yaojiajie Stage at < 97 Ma and 88.2 ± 1.4 Ma. The Fuxin Stage is dominated by trachyandesitic magmatism, with magmas emplaced at hypabyssal depths. In comparison, the Quantou Stage is characterized by high-K calc-alkaline, calc-alkaline, and sodic andesitic, dacitic, and rhyolitic magmatism of three different suites. The first of these is a high-K calc-alkaline andesitic magmatic suite that was accompanied by the emplacement of a calc-alkaline sodic dacite during the formation of the Ciweigou and Wufeng ore deposits. The second suite is dominated by calc-alkaline sodic rhyolite and high-K calc-alkaline sodic dacite magmatism associated with the formation of the Sipingshan ore deposit. The third suite is typified by high-K calc-alkaline andesitic magmatism associated with the emplacement of calc-alkaline hypabyssal granitoid complexes accompanying the formation of the Dong'an and Tuanjiegou ore deposits. The Qingshankou or Yaojia Stage is characterized by calc-alkaline sodic dacite magmatism associated with the formation of the Wuxing ore deposit. Metallogenesis during the Fuxin Stage characterized by trachytic magmatism is closely related to the formation of a deep-seated fault within a magmatic arc or the back-arc region of an immature continental margin and is associated with the Early Cretaceous subduction of the Pacific plate beneath Eurasia. Ore deposits that formed during the Fuxin Stage were generally related to magmato-hydrothermal fluids associated with mantle-derived magmas. In contrast, metallogenesis during the Quantou and Qingshankou or Yaojiajie stages was closely related to the formation of a mature high-K calc-alkaline magmatic arc within a continental margin setting again associated with the westward subduction of the Pacific plate. This metallogenic event was a product of magmato-hydrothermal systems derived from crust–mantle interaction and mixing of magmas derived from partial melting of different sections of the continental crust. Keywords: mineral deposit typeslow-sulphidation epithermal gold depositore-forming processescontinental margin of NE ChinaWest Pacific metallogenic belt Acknowledgements This research work was jointly supported by the National Natural Science Foundation of China (grant nos. 40772052 and 41172072), Project for Scientific and Technology Development (grant no. 20100450), Chinese Geological Survey Programme (grant no. [2010]26-06), and Geological Survey of Heilongjiang Province (grant no. 3R1101604422). We sincerely thank the staff of the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, for analyses of U–Pb dating of zircon.
The Jiapigou mining district (JMD) in the northeastern margin of the North China Craton mainly contains quartz vein- and altered rock-type gold deposits that have been prospected and mined for over 200 years. However, the ore-formation mechanism and geodynamic setting are still limited understood. Here we present new geological, mineralogical, gochronological, geochemical and Hf isotopic evidences of the ore-related intrusions from the deposits in the JMD to reveal the tectonic setting and genetic relationship between the Au mineralizitions and synchronic magmatism. The results show the following: (1) zircon U–Pb dating of the ore-related intrusions, such as basic dikes (gabbro and porphyritic gabbro), granites, and acid dikes (granite aplite dike and rhyolite porphyry) bracket their emplacement in the range of 177–171 Ma; (2) The basic dikes are characterized by enrichment in large ion lithophile elements (LILEs; i.e., Rb, Ba, and U), moderately depletion of high field strength elements (HFSEs; i.e., Nb and Ta), and high εHf (t) values (–13 to +12.8), whereas granites are characterized by high SiO2 (73.24–75.12 wt%) and total alkali (K2O + Na2O = 5.74–8.58 wt%) concentrations, low TFe2O3 (0.78–1.43 wt%) and CaO (0.98–1.36 wt%) concentrations, enrichment of LILEs (i.e., Rb, Ba, Th, and K) and depletion of HFSEs (i.e., Nb, Ta, Ti, and P), and low high εHf (t) values (–13.6 to –11.0), which features as well as those of the acid dikes. These results indicate that the ore-related intrusions, such as basic dikes, granites, and acid dikes were emplaced during the Middle Jurassic (177–171 Ma), constrained that the giant Au mineraliziton in the JMD occurred in the Middle Jurassic. And the basic dikes were likely derived from an enriched lithospheric mantle source that has been influenced by fluids expelled from a subducted slab, whereas granites and acid dikes were likely derived from partial melting of Archean crustal materials. Combined with regional geological observations, we conclude that the giant Au mineralization and ore-related magmatism occurred in an extensional setting associated with the subduction of the Paleo-Pacific Plate. The basic dikes and granites can be regarded as a precursor for the Au mineraliziton in the JMD. And mantle- and crustal-fluids and metals substantially contributed to the formation of the giant Au mineralization in the JMD.
We report the finding of the Wolitu Pb-Zn deposit in Inner Mongolia, China, through a series of geochemical surveys. The Wolitu area, located in the loess-cover area in the Hure Banner, Tongliao City, Inner Mongolia, and neighboring the Horqin Sandy Land to the north, had no previous history of Pb-Zn mining or record of Pb-Zn mineralization. Our study identified a large Pb-Zn anomaly with potential zones of mineralization by stream sediment survey. Random rock sampling reveals limonitization at sporadic outcrops in the gullies. The high concentrations of Pb in the residual debris provided guidelines to fix the position for exploratory trench. Oxidized concealed orebodies were identified by trenching. Blind orebodies in veins hosted within the structural zone between slates and marbles of the upper Carboniferous Shizuizi Formation and the Permian granite were discovered by drilling. It is computed that the ore reserve may reach up to 540,000 tones with Pb grade of 1.27% and Zn of 1.9%. This case study is an excellent example for identifying potential polymetallic deposits in loess covered terrains using geochemical exploration.
The Mushgai Khudag complex consists of numerous silicate volcanic-plutonic rocks including melanephelinites, theralites, trachytes, shonkinites, and syenites and also hosts numerous dykes and stocks of magnetite-apatite-enriched rocks and carbonatites. It hosts the second largest REE–Fe–P–F–Sr–Ba deposit in Mongolia, with REE mineralization associated with magnetite-apatite-enriched rocks and carbonatites. The bulk rock REE content of these two rock types varies from 21,929 to 70,852 ppm, which is much higher than that of syenites (716 ± 241 ppm). Among these, the altered magnetite-apatite-enriched rocks are characterized by the greatest level of REE enrichment (58,036 ± 13,313 ppm). Magmatic apatite from magnetite-apatite-enriched rocks is commonly euhedral with purple luminescence, and altered apatite displays variable purple to blue luminescence and shows fissures and hollows with deposition of fine-grained monazite aggregates. Most magmatic apatite within syenite is prismatic and displays oscillatory zoning with variable purple to yellow luminescence. Both magmatic and altered apatite from magnetite-apatite-enriched rocks were dated using in situ U–Pb dating and found to have ages of 139.7 ± 2.6 and 138.0 ± 1.3 Ma, respectively, which supports the presence of late Mesozoic alkaline magmatism. In situ 87Sr/86Sr ratios obtained for all types of apatite and calcite within carbonatite show limited variation (0.70572–0.70648), which indicates derivation from a common mantle source. All apatite displays steeply fractionated chondrite-normalized REE trends with significant LREE enrichment (46,066 ± 71,391 ppm) and high (La/Yb)N ratios ranging from 72.7 to 256. REE contents and (La/Yb)N values are highly variable among different apatite groups, even within the same apatite grains. The variable REE contents and patterns recorded by magmatic apatite from the core to the rim can be explained by the occurrence of melt differentiation and accompanying fractional crystallization. The Y/Ho ratios of altered apatite deviate from the chondritic values, which reflects alteration by hydrothermal fluids. Altered apatite contains a high level of REE (63,912 ± 31,785 ppm), which are coupled with increased sulfur and/or silica contents, suggesting that sulfate contributes to the mobility and incorporation of REEs into apatite during alteration. Moreover, altered apatite is characterized by higher Zr/Hf, Nb/Ta, and (La/Yb)N ratios (179 ± 48, 19.4 ± 10.3, 241 ± 40, respectively) and a lack of negative Eu anomalies compared with magmatic apatite. The distinct chemical features combined with consistent Sr isotopes and ages for magmatic and altered apatite suggest that pervasive hydrothermal alterations at Mushgai Khudag are most probably being induced by carbonatite-evolved fluids almost simultaneously after the alkaline magmatism.
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