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Stable Isotopic Geochemical and Geochronological Constraints on the Formation of the Shihu Gold Deposit: The Intracontinental Metallogeny of the Taihang Tectonic Belt, Eastern China
Abstract
The Early Cretaceous Shihu gold deposit is located in the northern segment of the Taihang Tectonic belt, which extends across the central part of the North China Craton. The deposit is hosted predominantly by the Archean metamorphic crystalline units, and is spatially and temporally related to quartz diorite porphyry present extensively throughout the gold deposit. We studied the geology, geochronology and stable isotopic geochemistry. Zircon U–Pb LA–ICP–MS ages of the quartz diorite porphyry at deposit range from 134 ± 1 to 131 ± 2 Ma, which are coeval and probably genetically related to the mineralization. The majority of the sulfides of the gold deposit have δ34S values ranging from −1 to 2‰, which suggest an homogeneous magmatic source. In addition, the isotopic compositions of δ18Ofluid and δ18Dfluid vary from 2.1 to 7.0‰ and −93 to −65‰, respectively, suggesting that the magmatic fluids mingled with meteoric water. The Pb isotopic analyses reveal that both the ore-forming materials and the quartz diorite porphyry originated from the lower crust and may have been mixed with mantle material. The 87Sr/86Sri and 143Nd/144Nd (143Nd/144Nd)i ratios for the quartz diorite porphyry demonstrate that there was mixing of two end-member (crust and the mantle) isotopic compositions. These results suggest that the ore-forming fluids and materials were derived from lower-crustal melting induced by mantle processes. Processes associated with the formation of the Shihu gold deposit differ significantly from those that characterize orogenic gold deposits, and instead are representative of formation in an intracontinental tectonic environment.
... Some of the larger and better-studied deposits in the belt include Shihu (Wang, S.Z., et al., 2015), Xishimen (Li, Q., et al., 2013), and Liyuan . The deposits are dominated by auriferous quartz veins, many with abundant pyrite and including well-developed K-feldspar halos described as part of the alteration assemblage. ...
... The significant mineralization is mainly hosted by the Neoarchean tonalitic gneiss, although a few of the smaller deposits, if orogenic gold, may be hosted in the Cretaceous plutons (Zhu et al., 2001); ca. 134 to 131 Ma porphyritic dikes are also mineralized at Shihu (Wang, S.Z., et al., 2015). Reliable geochronology is lacking for most of these deposits, but spatial association with the ca. ...
... Hydrothermal sericite at the Liyuan deposit has an argon age of 133.3 ± 1.2 Ma . The location of the gold belt within the interior of the North China block has led to the argument that these deposits differ from typical orogenic gold deposits and should be instead classified as "anorogenic gold deposits" (Wang, S.Z., et al., 2015), although deposit features themselves show little distinction from typical orogenic gold deposits. ...
... This makes the deposit a focus of geoscientific research in recent years (e.g. Niu et al., 2009;Cao et al., 2010Cao et al., , 2011Cao et al., , 2012aWang et al., 2010;Wang et al., 2015). Various geochemical research such as geochronology (Cao et al., 2010(Cao et al., , 2011(Cao et al., , 2012bWang et al., 2015), fluid inclusion (Liu et al., 2007;Cao et al., 2012a), stable isotopes (Cao et al., 2012a;Wang et al., 2015) were carried out to interpret the source, fluid migration and gold precipitation mechanism in the Shihu Gold Deposit. ...
... Niu et al., 2009;Cao et al., 2010Cao et al., , 2011Cao et al., , 2012aWang et al., 2010;Wang et al., 2015). Various geochemical research such as geochronology (Cao et al., 2010(Cao et al., , 2011(Cao et al., , 2012bWang et al., 2015), fluid inclusion (Liu et al., 2007;Cao et al., 2012a), stable isotopes (Cao et al., 2012a;Wang et al., 2015) were carried out to interpret the source, fluid migration and gold precipitation mechanism in the Shihu Gold Deposit. However, the poor understanding of the oreforming process prevents us from alleviating the urgent resource demand effectively. ...
... Niu et al., 2009;Cao et al., 2010Cao et al., , 2011Cao et al., , 2012aWang et al., 2010;Wang et al., 2015). Various geochemical research such as geochronology (Cao et al., 2010(Cao et al., , 2011(Cao et al., , 2012bWang et al., 2015), fluid inclusion (Liu et al., 2007;Cao et al., 2012a), stable isotopes (Cao et al., 2012a;Wang et al., 2015) were carried out to interpret the source, fluid migration and gold precipitation mechanism in the Shihu Gold Deposit. However, the poor understanding of the oreforming process prevents us from alleviating the urgent resource demand effectively. ...
Most hydrothermal ore deposits have experienced multiple stages of hydrothermal activities and the overprinting makes the footprints of ore fluid pathway difficult to identify, which prevents us from targeting the mineral resource effectively. In this paper, the altered wall-rocks from all accessible underground tunnels of No.101 lode in the Shihu Gold Deposit, North China were investigated by the combined methods of petrographic microscopy and short wave infrared spectroscopy (SWIR). The results show that there are three types of alteration in the Shihu Gold Deposit, i.e., the proximal illitic alteration, the transitional chloritic alteration and the distal propylitic alteration. The wavelength (wvl) of the 2200 nm absorption features (2200wvl) of the illitic alteration rocks around ore bodies are shorter than those from the barren areas, suggestive of the compositional variation of illites derived from the multiple stages of hydrothermal fluids. The domains with higher 1400D, 1900D and 2200D are spatially coincident with the economic ore bodies, which is well in accordance with the field truth that the gold mineralization in the Shihu mining district is intimately related to illitic alteration. The band ratio of 2200D/1900D, which is regarded as the proxy to illite crystallinity, is positively related to the temperature of fluid inclusions. The coincidence between low crystallinity and gold mineralization reflects the fact that fluid temperature during the main mineralization stage was relatively low compared to those of previous stages. Detailed investigation indicated that the economic ore bodies mainly occurred in the structural dilation domains, i.e., the widened portions of the ore-controlling structure were more suitable for the fluids migration and probably acted as the fluid up-flow zones. Therefore, SWIR spectroscopy of illite is a powerful tool to trace the ore fluid pathway, which is crucial to the understanding and exploration of gold mineralization.
Based on the geological significance of the SWIR signatures and the investigation of the ore-forming conditions, it can be concluded that there are two fluid up-flow zones in No.101 lode, one of which lies on the north of No.13 cross section and centered at No.7 cross section, whereas the other on the south of No.17 cross section and centered at No.27 cross section. As a result, two steeply dipped ore bodies were formed along the up-flow zones. According to this ore-forming analysis, the location centered at No.47 cross section with elevation below 150 m would be an ideal site for the formation of a third ore body. This has recently been verified by underground drillings in the Shihu Gold Deposit (Yougang Zhang, pers. commun. 2020).
... Some of the larger and better-studied deposits in the belt include Shihu (Wang, S.Z., et al., 2015), Xishimen (Li, Q., et al., 2013), and Liyuan . The deposits are dominated by auriferous quartz veins, many with abundant pyrite and including well-developed K-feldspar halos described as part of the alteration assemblage. ...
... The significant mineralization is mainly hosted by the Neoarchean tonalitic gneiss, although a few of the smaller deposits, if orogenic gold, may be hosted in the Cretaceous plutons (Zhu et al., 2001); ca. 134 to 131 Ma porphyritic dikes are also mineralized at Shihu (Wang, S.Z., et al., 2015). Reliable geochronology is lacking for most of these deposits, but spatial association with the ca. ...
... Hydrothermal sericite at the Liyuan deposit has an argon age of 133.3 ± 1.2 Ma . The location of the gold belt within the interior of the North China block has led to the argument that these deposits differ from typical orogenic gold deposits and should be instead classified as "anorogenic gold deposits" (Wang, S.Z., et al., 2015), although deposit features themselves show little distinction from typical orogenic gold deposits. ...
China produces about 450 t Au per year and has government stated in-ground reserves of approximately 12,000 t Au. Orogenic gold, or gold deposits in metamorphic rocks, and associated placer deposits compose about 65 to 75% of this endowment, with lodes existing as structurally hosted vein and/or disseminated ore- bodies. The abundance of orogenic gold deposits reflects Paleozoic to Triassic closure of Paleo-Tethyan ocean basins between Precambrian blocks derived from Rodinia and Gondwana as well as late Mesozoic-Cenozoic cir- cum-Pacific events and Cenozoic Himalayan orogeny. The deposits range in age from middle Paleozoic to Pleis- tocene. The Jiaodong Peninsula contains about one-third of China’s overall endowment, and large resources also characterize East Qinling, West Qinling, and the Youjiang basin. Although gold ores in Jiaodong postdate formation and metamorphism of Precambrian host rocks by billions of years, they are nevertheless classified here as orogenic gold ores rather than as a unique Jiaodong-type or decratonic-type of gold deposit. Similarly, although many workers classify the gold lodes in the Youjiang basin and much of West Qinling as Carlin-type gold, they show significant differences from gold ores in Nevada, United States, and are better defined as epi- zonal orogenic gold deposits.
Although there are widespread exposures of Precambrian rocks in China, there are no significant Precam- brian gold deposits. If large ancient orogenic gold deposits formed in Archean and Paleoproterozoic rocks, then they have been eroded, because these deep crustal rocks that are now exposed in China’s cratonic blocks have been uplifted from levels too deep for orogenic gold formation. The oldest large gold deposits in China are perhaps those of the Qilian Shan that were formed in association with Silurian tectonism along the present-day southwestern margin of the North China block. Closure of ocean basins in the outer parts of the Central Asian orogenic belt led to late Carboniferous to Middle Triassic orogenic gold formation in the Tian Shan, Altay Shan, Beishan, and northwestern North China block. Deformation associated with amalgamation of the North China block, northern Tibet terranes, South China block, and Indochina, as well as initial Paleo-Pacific sub- duction, can be related to Late Triassic orogenic gold formation in West Qinling, East Kunlun, Youjiang basin, West Jiangnan (Xuefengshan belt), Hainan Island, and Yunkaidashan gold provinces. In the middle Mesozoic, continued subduction along the Paleo-Pacific margin was associated with gold ores forming in East and Cen- tral Jiangnan, whereas early to middle Mesozoic deformation along the northern North China block formed important orogenic lodes in Precambrian basement (e.g., Jiapigou, Zhangjiakou, and Yanshan districts). Contin- ued Yanshanian orogeny in the eastern half of the North China block led to extensive orogenic gold formation during the main period of decratonization and regional extension at ca. 135 to 120 Ma (e.g., Jiaodong, Liaodong, Chifeng-Chaoyang, Zhangbaling, Taihangshan, and East Qinling). At the same time, strike-slip events in central Transbaikal were associated with orogenic gold formation in both Russia and adjacent northeastern China and likely are the source for China’s most productive gold placers in the upper Heilongjiang basin. China’s youngest orogenic gold deposits formed in the Ailaoshan, Lanping basin, Ganzi-Litang belt, Daduhe district, and areas south of the Lhasa terrane in Tibet during the middle Cenozoic, as well as in the northern half of the Central Range of Taiwan during the Pliocene-Pleistocene.
... The enrichment of Te, Bi, and Hg in many of the lode gold deposits also remain enigmatic (e.g., Gao et al., 2017;Li, Li, Santosh, Zhu, & Suo, 2018), including the source of these elements and their role in gold precipitation. Some theories on the origin of the gold lode, such as the mantle-branch tectonic model (Niu et al., 2009;Yu, Ren, & Zhang, 1996), "decratonic" model (e.g., Cao, 2012;Wang, Hu, Song, Cai, & Wang, 2015;Zeng et al., 2019;Zhu et al., 2015), and the orogenic model (e.g., Goldfarb & Santosh, 2014), illustrate the diverse opinion with regard to the origin of the elements associated with ore. Previous researchers suggested that structural defects in pyrite caused by isomorphic replacement of anion such as As can lead to gold precipitation (Simon, Huang, Penner-Hahn, Kesler, & Kao, 1999), whereas some others disputed about gold scavenging elements (Tooth, Brugger, Ciobanu, & Liu, 2008;Tooth, Ciobanu, Green, O'Neill, & Brugger, 2011). ...
... The Shihu gold deposit is located at the eastern periphery of the Trans-North China Orogen, which marks the collision zone between the Western and Eastern Blocks of the North China Craton (Tang et al., 2017). The basement rocks surrounding the orebody are represented dominantly by tonalite-trondhjemite-granodiorite (TTG) gneisses intercalated with amphibolite, dolomite, and marble Tang et al., 2017), overprinted by multiple folding and faulting associated with the Neoarchean tectonothermal events in the central NCC (He, Santosh, & Yang, 2016;Niu et al., 2009;Wang et al., 2015). These rocks are intruded by the Mapeng batholith and intermediate dykes, representing the Mesozoic magmatic events ( Figure 1; Li, Santosh, Zhang, et al., 2013;Li, Santosh, Li, & Guo, 2014;Men et al., 2014). ...
... Au-bearing minerals recognized in previous studies include electrum, küstelite, native silver, acanthite, and hessite (Cao, 2012;Park et al., 2013). The corresponding mineral sequence is shown in Figure 4. Fluid inclusion studies suggest that the fluid is rich in CO 2 and H 2 O, with lower salinity (Wang et al., 2015). The homogenization temperature of fluid inclusions in quartz ranges from 200°C to 350°C, with an average of 300°C (Liu, Dai, Fu, Sun, & Hu, 2007). ...
... ite, secondly in pyrrhotite, bornite, argentite, etc. The gangue minerals are mainly quartz, secondary magnetite, and haematite, as the hydrothermal alteration associated with the ore formation has generated sericite (fine-grained muscovite), orthoclase, chlorite, and carbonate minerals such as calcite, ankerite, and siderite (C. Chen et al., 2017;S. Z. Wang, Hu, Song, Cai, & Wang, 2015). ...
... Previous stable isotopic studies including S, H-O, and Pb, suggest primary magmatic fluids mixed with minor amounts of meteoric water and that the ore-forming materials were derived from the lower crust with additional input of mantle material (S. Z.Wang et al., 2015). Magmatic-hydrothermal activity was proposed as the main dynamics of the Shihu gold deposit (S. ...
... including S, H-O, and Pb, suggest primary magmatic fluids mixed with minor amounts of meteoric water and that the ore-forming materials were derived from the lower crust with additional input of mantle material (S. Z.Wang et al., 2015). Magmatic-hydrothermal activity was proposed as the main dynamics of the Shihu gold deposit (S. R.Li et al., 2013;S. Z. Wang et al., 2015). The initial 87 Sr/ 86 Sr ratio is one of the important proxies for judging the material sources. The initial 87 Sr/ 86 Sr ratio for upper mantle is estimated as 0.707 ± 0.002, and the mean value of continental crust mean value is 0.719(Faure, 1986). And the initial 87 Sr/ 86 Sr ratio of the mantle beneath the Taihang Mountain in the Me ...
... Some researchers proposed that the deposits in the region were metamorphic-hydrothermal in origin (e.g., Niu et al., 1994Niu et al., , 1997Tian et al., 1998). Others (e.g., Li et al., 2013;Wang et al., 2015;Zhang et al., 2017) favored that the gold deposits were formed in magmatichydrothermal processes. The Liyuan gold deposit is a newly discovered gold deposit in the TM region. ...
... 130 Ma He and Santosh, 2014;Zhang et al., 2015). Previous studies suggested that the Mapeng pluton and associated intermediate-felsic dikes were generated from the reworking of the ancient continental crust (He and Santosh, 2014;Wang et al., 2015). In the western part of the Mapeng pluton is one of the largest gold prospects in the central TM region with the Shihu gold deposit (Fig. 2, Li et al., 2013). ...
... The Pb evolution curves for major geological units are from Zartman and Doe (1981). Data of the Shihu, Xishimen, Yixingzhai and Xinzhuang deposits are from Li et al. (2013), Zhao (2013), , and Wang et al. (2015). Pb isotope data for early Cretaceous intrusions are quoted from Zhao (2013), , Wang et al. (2015). ...
The Liyuan lode gold deposit is located in the Shanxi province, central North China Craton. Gold orebodies are mainly hosted in Archean metamorphic rocks and structurally controlled by the NNE-trending faults. Gold occurs in disseminated and auriferous quartz-sulfide veins and veinlets within hydrothermally altered rocks. Mineralization can be divided into three intervals: (1) quartz-pyrite stage, (2) quartz-polymetallic sulfides stage, and (3) quartz-carbonate stage. Gold formed mainly in the middle stage. Sericite sample associated with the middle stage pyrite from phyllic alteration zones yields an well-defined ⁴⁰Ar/³⁹Ar plateau age of 133.3 ± 1.2 Ma, which is remarkably consistent with zircon U-Pb age (133.4 ± 1.1 Ma) of the quartz porphyry dikes in the Liyuan mine, indicating a close relationship between gold mineralization and granitic magmatism in the area. The sulfur isotopic compositions of pyrite (Py1-2) from the early stage have a narrow range from −0.3 to 4.1‰, indicating a deep-seated magmatic source. However, the sulfides (Py3-5, sphalerite, galena) from the middle stage have lower δ³⁴SV-CDT values of −7.2 to 3.0‰, which is thought to be a result of fluid oxidation during gold mineralization. The Pb isotopic compositions of sulfides from Liyuan ores have ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb, and ²⁰⁸Pb/²⁰⁴Pb ratios of 16.671–16.860, 15.236–15.255, and 41.452–44.159, respectively, indicting a high-thorium lower continental crust source of ore-forming materials. The δ¹³CVPDB and δ¹⁸OVSMOW values of late stage calcite ranging from −7.1 to −5.8‰ and 12.2 to 13.7‰, respectively, illustrating a deep-seated magmatic source of carbon-bearing portion. Given the absence of granite bodies at the present mining levels, we speculate that the ore-forming fluids may have been exsolved from a concealed granite pluton at greater depth. Based on the regional tectono-magmatic events, ore geology, geochronologic and isotopic data obtained in this study together with the previous published fluid inclusion data, we suggest that the Liyuan deposit is an orogenic gold deposit that is probably related to the early Cretaceous granite magmatism in the interior of the North China Craton.
... (b) Tectonic settings of the North China Craton, also showing the major gold districts in the North China Craton (after [29]). (c) Regional geological map of the Liyuan gold deposit, modified after Wang et al. [21]. "Red-circle": Au deposit, "orange-circle": Cu-Mo deposit, and "sky blue-circle": Pb-Zn deposit. ...
... Most of the gold deposits in the northern TM region are distributed within the Precambrian metamorphic rocks and are strictly controlled by the brittle-ductile fractures, which share many similar features (i.e., mineralization style, mineral compositions of ores, hydrothermal alterations, and characteristics of ore-forming fluids) with those of orogenic gold deposits worldwide [15][16][17][18][19]. Niu et al. [16,17], Chen et al. [18], and Tian et al. [19] proposed that the deposits in the region belong to the metamorphic-hydrothermal or orogenic type gold deposits, whereas others (e.g., [20,21]) considered that the gold deposits were formed in magmatichydrothermal processes. The Shihu deposit is the largest gold deposit in the region with the gold resources of over 50 t (an average grade of 5.9 g/t), and some studies have been carried out on the Shihu gold deposit (e.g., [20][21][22][23][24]). ...
... Most of the gold deposits in the northern TM region are distributed within the Precambrian metamorphic rocks and are strictly controlled by the brittle-ductile fractures, which share many similar features (i.e., mineralization style, mineral compositions of ores, hydrothermal alterations, and characteristics of ore-forming fluids) with those of orogenic gold deposits worldwide [15][16][17][18][19]. Niu et al. [16,17], Chen et al. [18], and Tian et al. [19] proposed that the deposits in the region belong to the metamorphic-hydrothermal or orogenic type gold deposits, whereas others (e.g., [20,21]) considered that the gold deposits were formed in magmatichydrothermal processes. The Shihu deposit is the largest gold deposit in the region with the gold resources of over 50 t (an average grade of 5.9 g/t), and some studies have been carried out on the Shihu gold deposit (e.g., [20][21][22][23][24]). It is suggested that the Shihu gold deposit was formed at ca.141 Ma (Quartz 40 Ar- 39 Ar age), and the ore-forming fluids in this deposit were characterized by low salinity, mesothermal, and CO 2 -H 2 O-NaCl system and were mainly derived from the lower crust with minor mantle component [20][21][22]. ...
The Liyuan gold deposit is hosted within Archean basement metamorphic rocks and controlled by the NNE-trending faults in the central North China Craton. The ore-forming processes can be divided into three stages (early, middle, and late). Three types of primary fluid inclusions (FIs) are identified in the Liyuan, including pure carbonic, carbonic-aqueous, and aqueous inclusions. The primary FIs of three stages are mainly homogenized at temperatures of 318–408 ∘ C, 201–329 ∘ C, and 136–229 ∘ C, with salinities of 2.1–8.9, 0.5–12.4, and 0.4–6.3 wt.% NaCl equivalent, respectively. The main Au mineralization is related to the middle stage, and water-rock interaction caused rapid precipitation of gold in this stage. The initial ore-forming fluids were likely magmatic water or metamorphic fluid and mixed with meteoric water at later stages. Due to the lack of granite body at the present mining levels, we speculate that it was magmatic water that might have been exsolved from a concealed granite body at greater depth or it was metamorphic fluid that was directly transported from depth via deep faults. Based on all the available geological and geochemical evidence, we suggest that the Liyuan deposit belongs to orogenic gold deposit that located in the interior North China Craton.
... Abbreviations: YXZ-the Yixingzhai gold deposit, XZ-the Xinzhuang gold deposit, LD-the Lingdi gold deposit, SH-the Shihu gold deposit, SXM-the Xishimen gold deposit, BQ-the Boqiang Mo deposit, TS-the Tanshang Mo deposit, YD-the Yindong Mo deposit, YJG-the Yanjiagou Mo deposit, QSL-the Qiushulin Mo deposit, BYXG-the Beiyingxigou Ag-Pb-Zn deposit, QBD-the Qiubudong Ag deposit. significant contributions of mantle and crust materials related to the formation of Au and Ag, as well as Mo deposits in the HWF Complex and these deposits herein are genetically linked to the destruction of the NCC (e.g., Sun et al., 2015;Wang et al., 2015;Li and Santosh, 2017). ...
... Rainier, Washington (John et al., 2008), Pierina, Peru (Fifarek and Rye, 2005). Published Mesozoic Au deposits in the CTM (Li and Li, 1997;Wang et al., 2010;Wang et al., 2014;Wang et al., 2015). Oxygen isotope fractionations of pyrophyllite-water with equation 10 3 lnα pyrophyllitewater = 4.4 × 10 6 /T 2 −5.62 × 10 3 /T + 1.87 (Zheng, 1991) and Hydrogen with equation of 10 3 lnα muscovite/pyrophyllite-water = -20 ± 5 (Marumo et al., 1980) are calculated. ...
... The largest Au deposit (Yixingzhai; ~ 94 t Au; ~ 140 Ma) is temporally bracketed by pre-ore and post-ore diorite dikes of similar isotopic ages. Auriferous quartz veins of the second largest Au deposit (Shihu; ~ 30 t Au; ~ 130 Ma) are spatially associated with numerous coeval quartz-diorite dikes, both being controlled by NNEto NNW-trending faults (Wang et al. 2015). The auriferous quartz veins in the Xiaolinggen Au deposit and many pre-ore or syn-ore lamprophyre and diorite dikes consistently strike NW to NNW (Ding et al. 1992). ...
Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.
... The largest Au deposit (Yixingzhai; ~ 94 t Au; ~ 140 Ma) is temporally bracketed by pre-ore and post-ore diorite dikes of similar isotopic ages. Auriferous quartz veins of the second largest Au deposit (Shihu; ~ 30 t Au; ~ 130 Ma) are spatially associated with numerous coeval quartz-diorite dikes, both being controlled by NNEto NNW-trending faults (Wang et al. 2015). The auriferous quartz veins in the Xiaolinggen Au deposit and many pre-ore or syn-ore lamprophyre and diorite dikes consistently strike NW to NNW (Ding et al. 1992). ...
Magmatic-hydrothermal gold–copper deposits in post-subduction settings represent essential targets for mineral exploration, but controls on their formation remain controversial. The early Cretaceous lode Au districts that formed during lithosphere destruction of the North China Craton provide an ideal opportunity to better understand the key tectono-magmatic factors responsible for the genesis of Au-rich deposits in post-subduction settings. Here, we present a LA-ICP-MS study of silicate melt inclusions and sulfide inclusions from ore-related mafic to intermediate rocks in the central Taihangshan Au district in the interior of the North China Craton to constrain the content and evolution of magmatic ore metals ± volatiles. The results, combined with numerical modeling, suggest that the ore-related magmas contained only a few ng/g Au, which is similar to the Au content of non-mineralization-related mafic to intermediate magmas worldwide. The low Au content of the lode Au-related magmas suggest that large volumes of magmas had to accumulate in the middle to lower crust through trans-lithospheric fault systems to produce the lode Au deposits. It is further suggested that the lode Au-related magmas were alkali-rich, hydrous, oxidized and relatively rich in sulfur and chlorine (mafic melt inclusions contain 0.14‒0.24 wt% S and 0.1‒0.2 wt% Cl). These properties are considered critical for the generation of auriferous ore fluids. By comparing the tectono-magmatic setting of the giant Jiaodong Au province (~ 4000 t Au) with the central Taihangshan district (~ 150 t Au), we propose that the much larger total Au tonnage of the Jiaodong district results from the accumulation of a much larger volume of ore-forming magmas at deep crustal levels, induced by a stronger degree of lithosphere modification. In addition, given that the composition of lode Au-related magmas is similar to that of porphyry Cu–Au-related magmas, the lack of giant, early Cretaceous porphyry Cu–Au deposits in the North China Craton suggests that strong extensional settings favor the formation of lode Au deposits instead of porphyry Cu–Au deposits. The present study, therefore, has general implications for the genesis of Au-rich deposits in strongly extensional settings.
... , Men (2011), Chen (2013), Li et al. (2012), Qu (2012), Qu et al. (2014), Zhang et al. (2016), Shen et al. (2015), Sheng (2016), Huang (2014), Wu (2014),Wang, Hu, Song, Cai, and Wang (2015),Chen, Chen, and Tian (2007),Dong et al. (2013),,Song et al. (2014),Gao et al. (2012Gao et al. ( , 2013Gao et al. ( , 2011 and references therein [Colour figure can be viewed at wileyonlinelibrary.com] ...
The Dacaoping and Sadaigoumen Mo deposits were recently discovered along the northern margin of the North China Craton. The newly determined Re–Os isochronous age of molybdenite from the Dacaoping Mo deposit is ca. 147 Ma, and the Re–Os model age of molybdenite from the Sadaigoumen Mo deposit is ca. 248 Ma, which represents two of their respective mineralization stages. Combined with previous data, we suggest four phases of mineralization in the Dacaoping and Sabagaogoumen Mo deposits; these phases are 248, 236, 147, and 140 Ma. The Early Triassic monzogranite and Early Cretaceous granodiorite in the Dacaoping Mo deposit and the Middle Triassic monzogranite in the Sabagaogoumen Mo deposit are I‐type granite with similar geochemical characteristics. The Triassic granites formed in a collisional–post‐collisional setting between the North China Plate and the Siberian Plate, and the Cretaceous granodiorite formed from the rapid thinning of the lithosphere. The partial melting of the lower crust is the main reason for their formation, and small amounts of mantle‐derived components were added. Compared to other Mo deposits in the region, we can further divide these Mo deposits into three phases, namely, (a) Triassic (ca. 248–223 Ma), (b) Early–Middle Jurassic (ca. 187–165 Ma), and (c) Late Jurassic–Early Cretaceous (ca. 155–130 Ma), which are consistent with the times of magmatic activity. These activities occurred during the collision and post‐collision between the North China Plate and the Siberian Plate after the Paleo‐Asian Ocean's closure, intracontinental orogeny, and rapid thinning of the lithosphere, respectively.
... , Men (2011), Chen (2013), Li et al. (2012), Qu (2012), Qu et al. (2014), Zhang et al. (2016), Shen et al. (2015), Sheng (2016), Huang (2014), Wu (2014),Wang, Hu, Song, Cai, and Wang (2015),Chen, Chen, and Tian (2007),Dong et al. (2013),,Song et al. (2014),Gao et al. (2012Gao et al. ( , 2013Gao et al. ( , 2011 and references therein [Colour figure can be viewed at wileyonlinelibrary.com] ...
The North Taihang Mountain region located within the Trans‐North China Orogen is one of the major Mesozoic metallogenic belts in the North China Craton and carries varied polymetallic mineralization associated with multiple magmatic pulses. Here, we investigate the Zhijiazhuang skarn iron deposit from this region through petrological, geochemical, zircon U–Pb geochronological and Lu–Hf isotopic studies of the associated granitoids to constrain the ages, petrogenesis, and process of mineralization. Zircon U–Pb dating shows multiple pulses of magmatism with three major peaks at 134, 129, and 125 Ma as well as some scattered spot ages of 121, 118, and 112 Ma consistent with regional multistage magmatic events during the Early Cretaceous. Zircon Hf data display negative εHf (t) values from −21.7 to −7.8 and two‐stage crustal model ages (TDMC) in the range of 1,680–2,560 Ma, indicating magma derivation from reworking of late Neoarchean to early Paleoproterozoic crustal materials possibly with minor lithospheric mantle input. Geochemical data indicate I‐type adakitic signature for the intrusion, suggesting the involvement of subduction‐related components. The adakitic rocks identified in this study originated from a mixture of crustal and lithospheric mantle components followed by upper crustal fractional crystallization. Collectively, the data suggest that the granitoids were formed by interaction of crust–mantle materials through partial melting of a thickened or delaminated lower continental crust and lithospheric mantle corresponding to the peak event of lithospheric thinning and craton destruction of the North China Craton. The tectonics is correlated with the westward subduction of the Paleo‐Pacific Plate. Combined with information from previous studies associated with porphyry–skarn‐type deposits in the whole Taihang Mountain region, we correlate the multiple phases of magmatism and mineralization of the deposit to the peak event of lithospheric thinning and destruction of the North China Craton, and the Zhijiazhuang skarn iron deposit was likely generated by the interaction of magmas and fluids with the dolomitic wall rocks.
China produces about 450 t Au per year and has government stated in-ground reserves of approximately 12,000 t Au. Orogenic gold, or gold deposits in metamorphic rocks, and associated placer deposits compose about 65 to 75% of this endowment, with lodes existing as structurally hosted vein and/or disseminated orebodies. The abundance of orogenic gold deposits reflects Paleozoic to Triassic closure of Paleo-Tethyan ocean basins between Precambrian blocks derived from Rodinia and Gondwana as well as late Mesozoic-Cenozoic circum-Pacific events and Cenozoic Himalayan orogeny. The deposits range in age from middle Paleozoic to Pleistocene. The Jiaodong Peninsula contains about one-third of China’s overall endowment, and large resources also characterize East Qinling, West Qinling, and the Youjiang basin. Although gold ores in Jiaodong postdate formation and metamorphism of Precambrian host rocks by billions of years, they are nevertheless classified here as orogenic gold ores rather than as a unique Jiaodong-type or decratonic-type of gold deposit. Similarly, although many workers classify the gold lodes in the Youjiang basin and much of West Qinling as Carlin-type gold, they show significant differences from gold ores in Nevada, United States, and are better defined as epizonal orogenic gold deposits.
Although there are widespread exposures of Precambrian rocks in China, there are no significant Precambrian gold deposits. If large ancient orogenic gold deposits formed in Archean and Paleoproterozoic rocks, then they have been eroded, because these deep crustal rocks that are now exposed in China’s cratonic blocks have been uplifted from levels too deep for orogenic gold formation. The oldest large gold deposits in China are perhaps those of the Qilian Shan that were formed in association with Silurian tectonism along the present-day southwestern margin of the North China block. Closure of ocean basins in the outer parts of the Central Asian orogenic belt led to late Carboniferous to Middle Triassic orogenic gold formation in the Tian Shan, Altay Shan, Beishan, and northwestern North China block. Deformation associated with amalgamation of the North China block, northern Tibet terranes, South China block, and Indochina, as well as initial Paleo-Pacific subduction, can be related to Late Triassic orogenic gold formation in West Qinling, East Kunlun, Youjiang basin, West Jiangnan (Xuefengshan belt), Hainan Island, and Yunkaidashan gold provinces. In the middle Mesozoic, continued subduction along the Paleo-Pacific margin was associated with gold ores forming in East and Central Jiangnan, whereas early to middle Mesozoic deformation along the northern North China block formed important orogenic lodes in Precambrian basement (e.g., Jiapigou, Zhangjiakou, and Yanshan districts). Continued Yanshanian orogeny in the eastern half of the North China block led to extensive orogenic gold formation during the main period of decratonization and regional extension at ca. 135 to 120 Ma (e.g., Jiaodong, Liaodong, Chifeng-Chaoyang, Zhangbaling, Taihangshan, and East Qinling). At the same time, strike-slip events in central Transbaikal were associated with orogenic gold formation in both Russia and adjacent northeastern China and likely are the source for China’s most productive gold placers in the upper Heilongjiang basin. China’s youngest orogenic gold deposits formed in the Ailaoshan, Lanping basin, Ganzi-Litang belt, Daduhe district, and areas south of the Lhasa terrane in Tibet during the middle Cenozoic, as well as in the northern half of the Central Range of Taiwan during the Pliocene-Pleistocene.
This volume of Ore Geology Reviews is dedicated to Dr Feng-Jun Nie and credits his significant contribution to understanding the ore deposits of China. In recent years, he has dedicated his life to the study of mineralization in the North China Craton and Central Asian Orogen to the north, and that is the focus of this volume.
Plumbotectonics is an attempt to model the geochemical behaviour of U, Th and Pb, among major terrestrial reservoirs in agreement with observational data. By recycling rock through the orogenic environment, a dynamically communicating upper crust, lower crust, and mantle can produce the required patterns of lead-isotope evolution.
Garnetiferous amphibolites preserving prograde inclusion assemblages (M1) within garnet porphyroblasts, metamorphic peak assemblages (M2) and post-peak symplectitic assemblages (M3) have been found in the Lushan terrane, Henan Province, Taihua Metamorphic Complex, southernmost section of the NS-striking Palaeoproterozoic Trans-North China Orogen (TNCO). Geothermobarometric computation shows that the M1, M2 and M3 assemblages were formed under the P–T conditions of 650–730 °C/4.9–8.1 kbar, 740–810 °C/9–10.8 kbar and 750–760 °C/5.0–7.7 kbar, respectively, suggesting that these amphibolites record clockwise P–T paths including nearly isothermal decompression (ITD) segments similar to the west-Alpine type P–T paths, indicative of being resulted from an orogenic process. High resolution SIMS U–Pb dating of metamorphic zircons suggests that the metamorphism took place at ca. 1.95–1.75 Ga. These data imply that the Palaeoproterozoic collision between the Eastern and Western Blocks of the North China Craton along the southernmost terminal of the TNCO not only started earlier, but also continued longer, than the middle and north sections of the TNCO.
The growth of the continental crust is generally believed to have been essentially completed in the Precambrian, and the amount of juvenile crust produced in the Phanerozoic is considered insignificant. Such idea of negligible growth in the Phanerozoic is now challenged by the revelation of very large volume of juvenile crust produced in the period of 500 to 100 Ma in several orogenic belts. While appreciable volumes of juvenile terranes in North America (Canadian Cordillera, Sierra Nevada and Peninsular Range, Appalachians) have been documented based on Nd isotopic data, the mass of new crust formed in the East-Central Asian Orogenic Belt (ECAOB), eastern part of the Altaid Tectonic Collage, appears to be much greater than the above terranes combined. New and published Nd-Sr isotope data indicate that the Phanerozoic granitoids from the southern belt of the ECAOB (Xinjiang-West Mongolia-Inner Mongolia-NE China) as well as from Mongolia and Transbaikalia were generated from sources dominated by a depleted mantle component. These granitoids represent a significant growth of juvenile crust in the Phanerozoic.
Although most plutons in this huge orogenic belt belong to the calc-alkaline series, the ECAOB is also characterized by the emplacement of voluminous A-type granites. The origin of these rocks is probably multiple and is still widely debated. However, the isotopic data (Sr-Nd-O) and trace element abundance patterns of A-type granites from the ECAOB clearly indicate their mantle origin.
The evolution of the ECAOB and the entire Altaid Collage is most likely related to successive accretion of arc complexes. However, the emplacement of a large volume of post-tectonic A-type granites requires another mechanism—probably through a series of processes including underplating of massive basaltic magma, partial melting of these basic rocks to produce granitic liquids, followed by extensive fractional crystallization. The proportion of juvenile to recycled, as well as that of arc-related to plume-generated, continental crust remains to be evaluated by more systematic dating and isotope tracer studies.
The NNE-trending Taihang Mountain belt, North China, is an intracontinental anomalous zone with a significant magmatic and gravity signature. From structural investigations, U-Pb dating, and 40Ar/39Ar chronological analyses, a three-stage tectonic evolution sequence can be reconstructed for the northern part of the Taihang Mountain belt during the middle to late Mesozoic period. This sequence comprises a WNW-ESE-directed compression that occurred at 175-150 Ma, followed by magmatic intrusions (146-142 Ma) and ending with the emplacement of alkaline granitic intrusions and their exhumation at 142-120 Ma. Normal faulting during late Cretaceous-Cenozoic time exposed the Taihang Mountains. The earlier compressional deformation and the following volcanic and magmatic episodes were probably controlled by some far-field stress release associated with the subduction of the paleo-Pacific plate beneath the eastern Asian continent. After the orogeny, the structural evolution, sedimentation, and tectonic settings on the western side differ from those on the eastern side of the Taihang Mountain belt.
We report here the results of a study to develop natural zircon geochemical standards for calibrating the U-(Th)-Pb geochronometer and Hf isotopic analyses. Additional data were also collected for the major, minor and trace element contents of the three selected sample sets. A total of five large zircon grains (masses between 0.5 and 238 g) were selected for this study, representing three different suites of zircons with ages of 1065 Ma, 2.5 Ma and 0.9 Ma. Geochemical laboratories can obtain these materials by contacting Geostandards Newsletter.
A technique has been developed in which bromine pentafluoride is used as a reagent for quantitative liberation of oxygen from oxides and silicates. For all of the rocks and minerals analysed, the oxygen yields are 100 ± 2 per cent of the theoretical amount. The advantage over techniques involving reduction with carbon lies in the consistently better oxygen yields, with consequent decrease in systematic errors in isotopic composition. Bromine pentafluoride has advantages over fluorine in being easier and safer to handle in the laboratory, in being readily purified, and in reacting with some minerals which do not react completely with fluorine. The results of isotopic analyses are compared with measurements made in other laboratories by other procedures.
Gold Mineralization of the Shihu deposit can be divided into 4 stages, pyrite-quartz stage, quartz-pyrite stage, polymetallic sulfide stage and quartz-carbonate stage. Cell parameters of pyrite and quartz from 4 stages were analyzed in this paper. The analytical results of pyrite indicated that the value of &Q and uo is minimum in UJ stage; the values of a- and u0 decrease as mineralization stage proceeds from I to IV and as the depth of ore-body increases from 560 to 180 meters. The analytical results of quartz indicated that the values of co and the Co/ao, ratios decrease as mineralization stage changes; the values of ao increase as the depth of ore-body increases; the values of Co and the C o/ao, ratios change rhythmically as the depth of ore-body increases. Results of cell parameters suggested that pyrites have high contents of Co, Ni and As, and have not undergone S depletion. Moreover, Al and Fe are the major replacement impurities in isomorphism of quartz. Gold-bearing pyrites are characterized by small ao , while gold-bearing quartz are characterized by big ao, small Co, and C o/ao ratio. The values of ao, of pyrite and the values of Co and Co/ao ratios of quartz are mostly larger than their ideal values. Therefore, denudation of the gold ore-body is slight, and potential value of this deposit is considerable.
This paper introduces the application of the pyrite thermoelectric property measurements in gold exploration of the Shihu gold deposit. Pyrite is the main gold-carrying mineral. Four gold mineralization stages may be distinguished. The thermoelectric type assemblage of pyrite is P<N⇒P≥N⇒P>N⇒P≤N from mineralization stages I to IV. In the mineralization area, from the top downward when (P and P% decrease, (N and N% increase, which in the main coincides with the results of mineralogical mapping by using the distribution of pyrite type (P%) and the mean of the thermoelectric coefficient (P. Some typomorphic information such as formation temperatures of pyrite, relative burial depths of orebodies, characteristics of orebodies and prospects of the deep part may be obtained according to thermoelectric properties of pyrite.
The Shihu deposit, which is a well known quartz vein type gold deposit in the northern part of the Taihang orogen of the North China craton (NCC), is hosted by ductile-brittle faults within Archean metamorphic core complex of the Fuping Group. Lead, sulfur, carbon, oxygen, hydrogen and strontium isotope geochemistry is used to help understand the sources of the ore fluid and materials. Pb isotopic compositions from sulfides range from 16.002-16.958 for (206)Pb/(204)Pb, 15.134-15.345 for (207)Pb/(204)Pb, and 36.825-37.8194 for (208)Pb/(204)Pb. The data straddle the lower crust and upper mantle growth curve. These values are similar to those of country rocks, and indicate that there are mixed sources for the Pb, with end members equating to mantle and crustal reservoirs. Most delta(34)S values of the sulfides range from +1.5 to +2.5 parts per thousand and display a pronounced tower effect, and suggest that magmatic sulfur dominates in sulfides, mixed with minor, isotopically light sulfur. Sulfur isotope temperatures calculated by mineral pairs are between 160 degrees C and 315 degrees C. Gangue calcite in three samples from the veins have similar isotope compositions, with delta(13)C values in the range of -5.93 to -6.92 parts per thousand with respect to PDB and delta(18)O values in the range of 8.81 to 12.36 parts per thousand with respect to SMOW. delta D values of fluid inclusions in the seven samples of auriferous quartz have a narrow range of -101 to -91 parts per thousand and the delta(18)O(H2O) values calculated from delta(18)O values of quartz range from 2.21 to 7.05 parts per thousand. These data suggest that the ore fluid is likely a mixture of meteoric water and magmatic water. The initial (87)Sr/(86)Sr values range from 0.714057 to 0.825903 for quartz, from 0.71345 to 0.723227 for pyrite, which are lower than that of the host rock ranging from 0.77569 to 1.06413. Those results suggest that the ore-forming elements were leached from the Precambrian basement that reacted with deep-source fluids.
Most of the quartzofeldspathic rocks in the Fuping Complex of the Trans-North China Orogen bear a large quantity of leucocratic veins, dykes and granitic intrusions. Magmatism or melt activity was responsible for the ubiquitous migmatization of the complex. Migmatization can be manifested with prominent melt injections in the felsic gneisses and faint dispersed melt crystallization in the competent quartzite of the Wanzi Series metasedimentary rocks. Melt infiltration migmatization can be deduced through euhedral crystallization of feldspar and quartz, transition of biotite to amphibole, albite rims and rare occurrence of partial melting reactions. In the process of melt infiltration migmatization, the rock composition was changed and the system was open in chemistry. Due to crystallization of the infiltrated melt, some zircons of magmatic feature may be formed and can be easily mistaken as detrital zircons in metasedimentary rocks.
The main purpose of this paper is two-fold. The first is to summarize the specific types of information on ore-forming processes (e. g. , conditions and mechanisms of ore formation, sources of ore constituents) that can be obtained from a stable isotope study of an ore deposit, and the principles behind the methodologies. The uncertainties, limitations, and problems that are associated with the methodologies are discussed. The second purpose is to summarize some of the important concepts and models of ore-forming processes that have been developed from stable isotope investigations of ore deposits.
Dyke swarms are products of typhonic magma which may provide important information about the crust-mantle evolution. This work studies several dyke swarms in the north of North China, including the Early Mesozoic Datong carbonatite-lamprophyre dyke swarms, Early Mesozoic Chifeng dioritic porphyrite-diabasic dyke-swarms, Mesozoic Linxi diabasic dyke-swarms, and North-Beijing Late Mesozoic bimodal dyke swarms. It is revealed that there were two successive processes of underplating and accompanying extension in the Mesozoic in North China. Comparison of the Early Mesozoic, Late Mesozoic, and Protozoic dyke swarms in North in certain extent reflects revolution of the mantle composition in North China.
The Cenozoic volcanicity of eastern China is entirely basaltic and occurred as relatively small eruptions widely dispersed in space and time, closely associated with graben basins and their regional bounding faults. Samples (157) from over 30 sites in eastern China have been studied. They are predominantly alkaline basalts, but vary in composition from olivine nephelinites and leucitites to quartz tholeiites. The majority are aphyric but some contain olivine and clinopyroxene phenocrysts. Whole-rock analyses (X-ray fluorescence) of all samples for the major and 13 trace elements are used, as are the compositions of all the major mineral phases determined by electron microprobe.It is argued that the most primitive basanites, alkali olivine basalts, and olivine tholeiites represent primary or near-primary magmas which were formed by different degrees of partial melting of the upper mantle at different depths. The olivine tholeiites represent larger degrees of partial melting (8-9%) of a spinel peridotite at depths of <66 km. The alkalic basalts carry xenoliths of spinel and garnet peridotite and appear to have been derived by 1-7% partial melting of a garnet lherzolite (50% ol, 25% opx, 15% cpx, 10% garnet) at depths > 79 km. The olivine nephelinite may have formed by even smaller degrees of partial melting.Most flows are not primary; the variations in their compositions are consistent with fractional crystallization from the spectrum of primary parents created by varying degrees of partial melting of a mineralogically heterogeneous source. The tholeiites have fractionated by the removal of clinopyroxene and some olivine; the alkali basalts by the removal of clinopyroxene with a smaller proportion of olivine. The incompatible behavior of Sr implies the absence of plagioclase from any of the fractionating assemblages and, together with the high Al content of the pyroxene phenocrysts, suggests that much of the fractionation occurred at mantle depths and pressures.The Cenozoic magmatism of eastern China is seen as a typical example of volcanism associated with continental extension. That is, small volumes of predominantly alkalic basalts and olivine tholeiites erupted over a prolonged period and associated with extensional basins and their bounding faults. As such, the province is distinct from continental flood basalt provinces.
The well-known Shihu quartz vein-type Au deposit in the Taihang Mountains in the North China Craton (NCC) is hosted by ductile-brittle faults within the Neo-archean-Palaeo-proterozoic Fuping Group metamorphic complex. The deposit exhibit four stages of quartz veins: (1) quartz-K-feldspar-sericite; (2) quartz-pyrite; (3) quartz-polymetallic sulphide; and (4) quartz-carbonate +/- pyrite. Three types of fluid inclusions in quartz are recognized in this study. Solid-bearing high-salinity (Type I) fluid inclusions have homogenization temperatures of up to 390 degrees C and high salinities of 33-47 wt% NaCl equivalent. Two-phase H2O-rich (Type II) fluid inclusions homogenized between 129-396 degrees C and have salinities of 1.7-12.5 wt% NaCl equivalent. Two/three-phase CO2-rich (Type III) fluid inclusions, which are abundant in auriferous quartz veins, have homogenization temperatures of 205-358 degrees C and salinities of 1.63-7.64 wt% NaCl equivalent. Laser Raman spectroscopy and quadrupole mass spectrometry confirmed that the vapour phase in the fluid inclusions is dominated by CO2, H2S, CH4, C2H4, N-2 and Ar besides H2O, whereas the liquid phase is composed of Cl-, SO42-, Na+, and K+ with minor Mg2+ and Ca2+. Fluid inclusions in pyrite associated with quartz ore-stages (2) and (3) have Sigma REE values of 0.61-342.17 ppm with negative Ce, Eu and Y anomalies and LREE enrichment relative to HREE, generally indicating a crustal source of ore fluids. Helium isotope studies of fluid inclusions in pyrites associated with quartz ore-stages (2) and (3) yielded He-3/He-4 ratios of 0.12-0.93 Ra (Ra= 1.4x10(-6) for air) and Ar-40/Ar-36 ratios of 3690-23678. The noble gas data suggest c. 10-20% mantle-derived ore fluids, reflecting an increased interaction of ore fluids with surrounding crustal rocks contributing additional He-4 to the fluids. The present data and various information from published works suggest that the Shihu Au deposit was formed during lithospheric thinning or decratonization beneath the Taihang Mountains. The fluids derived from the lower crust, formed due to partial melting and dehydration of the lithospheric mantle, were mixed with magmatic and meteoric waters, and finally precipitated Au and associated metals in the Shihu deposit.
Identification of a new type mineralization often leads to discovery of a great number of ore deposits and ore provinces. Predicting and identifying new type mineralizations is an important aim of ore deposit study. To date the majority of orogenic-type gold deposits has been well shown by worldwide studies. However, the other commodities of orogenic-type are rarely discussed. This paper addresses the concept of orogenic-type deposits which formed by fluid systems mainly sourced from metamorphic devolatilization. The paper also develops genetic models for orogenic deposits at various scales, including deposit -, orefield/terrain - and province/orogen -scales. The genetic models are linked to a three -stage tectonic evolution of convergent orogens. According to these models, the transition from compression to extension of thickened accretionary or/and collisional orogens is conducive to mineralizations; and the syn -orogenic ore -systems must be characteristically lagged behind compressional orogenesis. As case studies, orogenic -type silver, lead -zinc, molybdenum and copper deposits are reported in the paper. This implies that China has great potential for orogenic-type deposits. The logics and validity of the metallogenic models are evidenced by introduction of several successful ore deposit prediction.
The Junggar orogen, Xinjiang, China, is an important part of the Ural-Mongolian orogen as well as one of the typical collisional orgenic areas. The collisional orogenesis in the area occurred in the Carboniferous and Permian, and shows the character of the early compression and late extension. Mineralizations of gold and other metals in the Junggar orogen occurred mainly in the Permian and in a few cases in the Late Carboniferous. Hence gold mineralization took place contemporaneously with collisional orogenesis. Gold deposits are mainly distributed in areas where collisional orogensis was intense, which indicates a spatial consistence between collisional orogenesis and gold mineralization. The mineralization geodynamic setting occurred in the transition stage from collisional compression to extension. The matching of gold mienralization with collisional orogenesis in time, space and geodynamic setting supports that the metallogenic model for collisional orogenesis should be the metallogenic model of gold deposits in the Junggar orogen. And a great deal of facts evidenced that the distribution of gold deposits and other mineral deposits of the Junggar orogen abides by the metallogenic model for collisional orogenesis, or in other words, is controlled by the metallogenic model.
Central Fujian Rift is another new and important volcanogenic massive sulfide Pb-Zn polymetallic metallogenetic belt. In order to find out the material genesis and mineralization period of Meixian-type Pb-Zn-Ag deposits, S and Pb isotope analysis and isotope geochronology of ores and wall rocks for five major deposits are discussed. It is concluded that the composition of sulfur isotope from sulfide ore vary slightly in different deposits and the mean value is close to zero with the δ34S ranging from −3.5%‰ to +5.6%‰ averaging at +2.0‰, which indicates that the sulfur might originate from magma or possibly erupted directly from volcano or was leached from ore-hosted volcanic rock. The lead from ores in most deposits displays radioactive genesis character (206Pb/204Pb>18.140, 207Pb/204Pb>15.584, 208Pb/204Pb>38.569) and lead isotope values of ores are higher than those of wall rocks, which indicates that the lead was likely leached from the ore-hosted volcanic rocks. Based on isotope data, two significant Pb-Zn metallogenesis are delineated, which are Mid- and Late-Proterozoic sedimentary exhalative metallogenesis (The single zircon U-Pb, Sm-Nd isochronal and Ar-Ar dating ages of ore-hosted wall rocks are calculated to be among 933–1788 Ma.) and Yanshanian magmatic hydrothermal superimposed and alternated metallogenesis (intrusive SHRIMP zircon U-Pb and Rb-Sr isochronal ages between 127–154 Ma).
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The Dawan Mo–Zn–Fe deposit located in the Northern Taihang Mountains in the middle of the North China Craton (NCC) contains large Mo-dominant deposits. The mineralization of the Dawan Mo–Zn–Fe deposit is associated with the Mesozoic Wanganzhen granitoid complex and is mainly hosted within Archean metamorphic rocks and Proterozoic–Paleozoic dolomites. Rhyolite porphyry and quartz monzonite both occur in the ore field and potassic alteration, strong silicic–phyllic alteration, and propylitic alteration occur from the center of the rhyolite porphyry outward. The Mo mineralization is spacially related to silicic and potassic alteration. The Fe orebody is mainly found in serpentinized skarn in the external contact zone between the quartz monzonite and dolomite. Six samples of molybdenite were collected for Re–Os dating. Results show that the Re–Os model ages range from 136.2 Ma to 138.1 Ma with an isochron age of 138 ± 2 Ma (MSWD = 1.2). U–Pb zircon ages determined by laser ablation inductively coupled plasma mass spectrometry yield crystallization ages of 141.2 ± 0.7 (MSWD = 0.38) and 130.7 ± 0.6 Ma (MSWD = 0.73) for the rhyolite porphyry and quartz monzonite, respectively. The ore-bearing rhyolite porphyry shows higher K2O/Na2O ratios, ranging from 58.0 to 68.7 (wt%), than those of quartz monzonite. All of the rock samples are classified in the shoshonitic series and characterized by enrichment in large ion lithophile elements; depletion in Mg, Fe, Ta, Ni, P, and Y; enrichment in light rare earth elements with high (La/Yb)n ratios. Geochronology results indicate that skarn-type Fe mineralization associated with quartz monzonite (130.7 ± 0.6 Ma) formed eight million years later than Mo and Zn mineralization (138 ± 2 Ma) in the Dawan deposit. From Re concentrations in molybdenite and previously presented Pb and S isotope data, we conclude that the ore-forming material of the deposit was derived from a crust-mantle mixed source. The porphyry-skarn type Cu–Mo–Zn mineralization around the Wanganzhen complex is related to the primary magmatic activity, and the skarn-type Fe mineralization is formed at the late period magmatism. The Dawan Mo–Zn–Fe porphyry-skarn ores are related to the magmatism that was associated with lithospheric thinning in the NCC.
The Beiminghe (BMH) iron ore in the southern part of the Taihang
Mountain (TM), Hebei province, is one of the largest skarn iron deposits
in China. Here we report phlogopite
40Ar-39Ar and zircon U-Pb age data, as
well as sulfur, lead, and He-Ar isotope geochemistry of pyrite
from the ores and skarnitized rocks in the deposit in an attempt to
constrain the timing and mechanism of formation of the mineralization.
The phlogopite 40Ar-39Ar and LA-ICP-MS
zircon U-Pb data show markedly consistent ages constraining the
timing of ore formation as 136-137 Ma. The presence of several
inherited zircons with late Archean or Paleoproterozoic ages indicates
the participation of the basement rocks during the ore-forming process.
The δ34S values of pyrite from the ores range from 12.2
to 16.5‰, with 206Pb/204Pb =
17.84-18.79, 207Pb/204Pb =
15.46-15.62, and 208Pb/204Pb =
37.93-39.75, suggesting that continental crust is the major
contributor. This is further confirmed by the He-Ar isotope data
(3He/4He = 0.0648-0.1886 Ra, mean 0.1237Ra;
40Ar/36Ar = 311.7-22909.4; and
40Ar*/4He = 0.036-0.421). The
Mesozoic magmatism and metallogeny in the BMH correlate well with the
peak event of lithospheric thinning and destruction of the North China
Craton during this process, the early Precambrian lower crustal rocks in
the region were re-melted through underplating of mantle magmas, leading
to the formation of the Beiminghe monzodioritic pluton. Minor mantle
input occurred during the evolution of the monzodiorite magma, which
scavenged the ore-forming materials from the lower crust. Interaction of
the magmas and fluids with the surrounding rocks resulted in the
formation of the Beiminghe skarn iron deposits. The magmatism and
metallogeny in the Taihang Mountain are signatures of the extensive
craton destruction and lithospheric thinning in the eastern part of the
North China Craton during Mesozoic, probably associated with Pacific
slab subduction.
The Katherina Volcanics of Gabal Ma’ain in the Sinai comprise an Ediacaran (580–590 Ma) approximately 450 m thick succession dominated by porphyritic rhyolite lava flows with subordinate related pyroclastics. These volcanics unconformably overlie the calc-alkaline Younger Granites (≥590 Ma) and are intruded by alkaline granitoids (578 ± 8 Ma). The rhyolites have a potassic alkaline affinity and peraluminous to slightly metaluminous character. They exhibit many of the classic features of A-type magmas, including enrichment of incompatible elements, such as Zr, Nb, Y, Ga, Zn and Ce and total REE, as well as high FeO*/(FeO* + MgO) and 10,000*Ga/Al2O3 ratios. The A-type rhyolites have LREE-enriched patterns with pronounced negative Eu anomalies that are comparable with typical REE profiles for “hot-dry-reduced rhyolites”. Saturation thermometry has yielded zircon and apatite crystallization temperatures ranging between 913 and 925 °C and 669 and 931 °C, respectively. The investigated trace element patterns indicate that the Katherina A-type rhyolites were very likely to have evolved through simple fractional crystallization of a parental magma derived from an enriched (most probably asthenospheric) mantle source, supplemented by a crustal component inherited from pre-collision subduction events, or a ‘recycled component’ in the source. Katherina A-type rhyolites were likely erupted in a within-plate setting. The eruption of these rhyolites marks the onset of the anorogenic period during which the rigid massif (Arabian–Nubian Shield) was subjected to post-collisional tensional stresses and intra-plate rifting.
The Dahu Au–Mo deposit is a structure-controlled lode system occurring in the northern Xiaoqinling terrane, Huaxiong Block, Qinling Orogen. This paper reports a new Sr–Nd–Pb isotope dataset obtained for ore sulfides and the hostrocks within the Taihua Supergroup, in an attempt to constrain the source of the ore-forming fluids from a new dimension. 16 sulfide samples yield ISr ratios of 0.70470–0.71312, with an average of 0.70854; εNd(t) values between − 13.5 and − 18.1, with average of − 15.1; and (206Pb/204Pb)i, (207Pb/204Pb)i and (208Pb/204Pb)i ratios of 17.033–17.285, 15.358–15.438, and 37.307–37.582, with averages of 17.162, 15.405, and 37.440, respectively. 5 gneiss samples from the Taihua Supergroup yield ISr ratios of 0.70947–0.73201, averaging 0.72294; εNd(t) values of − 20.0 to − 31.1, averaging − 25.1; and (206Pb/204Pb)i, (207Pb/204Pb)i, (208Pb/204Pb)i ratios of 17.127–18.392, 15.416–15.604 and 37.498–37.814, with averages of 17.547, 15.470 and 37.616, respectively. These data show that the ore sulfides have less radiogenic Sr–Nd–Pb isotope systematics than the hostrocks, and suggest that the ore-forming fluids, which interacted with the wallrocks to form ores, must be sourced from a depleted mantle or a depleted, subducted oceanic slab. In combination with the spatial scenario and geochemical signatures of the Triassic magmatites and mineral systems, we argue that in the Triassic the Mianlue Ocean was not completely closed, and that the northward oceanic plate subduction still survived along the Mian-Lue suture, which caused the Late Triassic magmatism and associated mineralization in Qinling Orogen, including the Au–Mo Dahu deposit.
The large scale Mesozoic magmatism and related metallogeny in the Taihang Mountains (TM) provide im-portant clues for the lithospheric thinning of the North China Craton (NCC). Among the ore deposits, the vein gold mineralization of Shihu in the Fuping region and the skarn ore deposit of Xishimen in the Wu'an region represent typical Mesozoic metallogeny in the TM. In the Shihu gold mine, the Mapeng batholith is dominantly composed of monzogranite and granodiorite, whereas, the Wu'an pluton in the Xishimen iron mine mainly comprises monzonite and diorite. Here we present zircon LA–ICP-MS U–Pb data from 8 samples which reveal the timing of magmatism in the TM as ca. 130 Ma, which is contemporaneous with the large-scale metallogeny in the margins of the NCC. The δ 34 S values recorded in the sulfide minerals from the Shihu gold deposit and the Xishimen skarn iron deposit show a range of 2.2‰–5.0‰, and 11.6‰–18.7‰, re-spectively. Helium isotopic compositions of fluid inclusions in pyrite from the Shihu gold deposit vary from 0.12 to 1.98 Ra (where Ra is the 3 He/ 4 He ratio of air = 1.39 × 10 − 6), with calculated mantle helium values of 1.4%–25%, whereas, those of the Xishimen skarn iron deposit range from 0.06 to 0.19 Ra, with calculated mantle helium of 0.7%–2.2%. The S–He–Ar isotopic data suggest a lower crustal origin for the ore-forming components, with variable inputs of mantle source. The large population of inherited zircons in our samples, with 207 Pb/ 206 Pb ages ranging between 2500 Ma and 1800 Ma, also supports crustal participation. Our data reveal that the Shihu gold deposit witnessed greater mantle input than the Xishimen skarn iron deposit, sug-gesting that the continental lithosphere is markedly thinner under the Fuping region than that under the Wu'an region. Our interpretation is also supported by published data from two ultra-broadband high-precision magnetotelluric sounding profiles across the TM region showing a variation in the lithosphere thickness from 155 km to 70 km while moving from the south (Wu'an region) to the north (Fuping region). Our study suggests that inhomogeneous lithospheric thinning in the central NCC occurred at least as early as ca. 130 Ma ago.
Equilibrium constants for oxygen isotope exchange between quartz and
water have been measured from 195°C (1000 ln α = 12.0) to
750°C (1000 ln α = 0.4). Over limited temperature ranges the
behavior of fractionation with temperature can be approximated by 1000
ln α = 3.38 (106 T-2) - 3.40 for
200°-500°C and by 1000 ln α = 2.51 (106
T-2) - 1.96 for 500°-750°C. The results of
measurements in the quartz-water system can be combined with analogous
results from other mineral systems to make mineral-pair isotopic
thermometers for application to problems of petrogenesis.
The eastern North China craton contains the largest association of gold deposits in China. Hundreds of gold deposits of different scales, commonly in groups or belts, constitute seven subclusters. In contrast, there are no industrial gold deposits in the neighboring areas of the UHP Sulu region and the northern Yangtze craton. The host rocks of the gold deposits are chiefly Precambrian high-grade metamorphic rocks and granites (anatectic melts derived from Precambrian metamorphic rocks). Traditionally the gold deposits were considered to be of the Archean greenstone type. However, recent geochronology of granitoid bodies and veins related to gold mineralization and gold-bearing minerals in ore lodes reveals that the main metallogenic episode was 110-130 Ma in all seven subclusters. Geochemical data also indicate that the metallogenic materials mainly came from the Precambrian basement of the North China craton and its underlying mantle. Sr-Nb-Pb and S-O-H isotopic data of the ore lodes show crust-mantle mixing characteristics, corresponding to the metamorphic host rocks and basic-alkaline dikes. The main structures controlling the mineralization are Mesozoic faults that trend roughly NE-ENE and NW. Therefore, Precambrian rocks and Mesozoic magma-tectonic events provide fundamental controls on the mineralization.Problematic relationships between the Mesozoic tectonic events (Yanshanian movements) and large-scale metallogenic activity have attracted substantial attention from Chinese and foreign geologists. Mesozoic tectonic inversion in eastern China mainly involved a thick lithosphere that thinned at depth and a reconstructed basin-and-range structure at the surface, which resulted from a catastrophic lithosphere-asthenosphere event. This tectonic event led to a large-scale mantle upwelling, which induced disturbance and adjustment of thermal, density, and chemical phenomena, resulting in large-scale crustal remelting (especially the lower crust), material exchange and mixing between mantle and crust, movement and circulation of fluids, and finally a new magma-fluid-mineralization system.
The Trans-North China Orogen separates the North China Craton into two small continental blocks: the Eastern and Western Blocks. As one of the largest exposure in the central part of the orogen, the Hengshan–Wutai–Fuping Complexes consist of four lithotectonic units: the Wutai, Hengshan and Fuping Complexes and the Hutuo Group. The Hengshan Complex contains high pressure mafic granulites and retrograded eclogites. Structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding (D1 to D3) and two stages of ductile thrust shearing (STZ1 between D1 and D2 and STZ2 after D3). The D1 deformation formed penetrative axial planar foliations (S1), mineral stretching lineations (L1), and rarely-preserved small isoclinal folds (F1) in the Hengshan and Fuping Complexes. In the Wutai Complex, however, large-scale F1 recumbent folds with SW-vergence are displayed by sedimentary compositional layers. Penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones (STZ1). The kinematic indicators of STZ1 in the Hengshan and Wutai Complexes show top-to-the-S230°W thrusting likely related to northeastward, oblique pre-collisional subduction. D1 resulted in crustal thickening with resultant prograde peak metamorphism. The Hutuo Group did not undergo the D1 deformation, either because sedimentation was coeval with the D1 deformation or because it was at a high structural level and was not influenced directly by the early deformation. The D2 deformation produced NW-verging asymmetric and recumbent folds. The D2 deformation is interpreted to have resulted from collision between the Eastern and Western Blocks of the North China Craton. In the Hutuo Group and the Fuping Complex, the development of ESE-verging asymmetric tight folds is associated with D2. The structural pattern resulting from superimposition of D1 and D2 is a composite synform in the Hengshan–Wutai–Fuping Complexes. All four lithotectonic units were superposed during the later D3 deformation. The D3 deformation developed NW-trending open upright folds. Ongoing collision led to development of transpressional ductile shearing (STZ2), forming the transpressional Zhujiafang dextral ductile shear zone between the northern Hengshan Complex and the southern Hengshan Complex, and generating the sinistral Longquanguan ductile shear zone between the Fuping Complex and the Wutai Complex, respectively. The STZ1 and D2 deformation were possibly responsible for fast syn-collisional exhumation of the high pressure mafic granulites and retrograded eclogites. The structural patterns and elucidation of the deformation history of the Hengshan–Wutai–Fuping Complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the Eastern and Western Blocks underwent northeastward-directed oblique subduction beneath the western margin of the Eastern Block, and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the North China Craton.
In this contribution we present a reconstruction of the overall lithotectonic architecture, from inner zones to external ones, of the Paleoproterozoic Trans-North China Orogen, within the North China Craton. Moreover, forward thermobarometrical modelling on a kyanite-bearing gneiss yields a reliable prograde P–T–t–D path. In addition, 40Ar/39Ar dating on rocks from distinct litho-tectonic units helps us to distinguish several tectono-metamorphic events during the orogenic development. Considering these results, we propose a geodynamic model involving three cratonic blocks, namely the Western, Fuping and Eastern Blocks, separated by two oceans, the Lüliang and Taihang Oceans. The opening of oceanic basins occurred around 2.2–2.3 Ga. After the westward subductions of oceanic lithosphere, the Trans-North China Orogen was built up through a polyphase tectonic evolution within the period 1900–1800 Ma. The first event (D1) corresponded to the emplacement of lower and upper nappes herein called the Orthogneiss-and-Volcanite Unit (OVU) and the Low-Grade-and-Mafic Unit (LGMU), respectively. The syn-metamorphic D1 deformation (1880 ± 10 Ma) is characterized by a NW–SE stretching and mineral lineation with a top-to-the SE sense of shear. During ongoing compression of the thickening orogenic crust, a second deformation event D2 (1850 ± 10 Ma) was responsible for (1) syn-anatectic lateral flow and exhumation of the orogenic root and (2) folding of the middle and upper parts of the orogenic wedge that consequently acquired a fan-type geometry. The late D3 (1830 ± 10 Ma) and D4 (1810 ± 10 Ma) events are related to late-orogenic normal and strike-slip shearing, respectively. In our present state of knowledge, the Paleoproterozoic Trans-North China Orogen might be regarded as the assemblage of two continent–continent collisional belts, both of which are characterized by nappe stacking accommodated by top-to-the E/SE ductile shearing. Continental subduction, crustal thickening, partial melting of overthickened crust, exhumation of HP rocks and deposition of syn-orogenic detrital basins are typical features of modern collisional-type orogens.Highlights► Final amalgamation of the North China Craton was completed after the welding of the Eastern, Fuping and Western Blocks. ► Two collisions responsible for the Trans-North China Orogeny occurred at 1.9–1.8 Ga. ► The Trans-North China Orogen exhibits the typical structural, metamorphic, magmatic and sedimentary features of a modern collisional-type orogen.
Abstract Dike swarms are generally ascribed to intrusion of mantle-source magma result from extension. Basic dike swarms around the Shanxi-Hebei-Inner Mogolia borders in the northern peripheral area of the North China Craton can be divided into five age groups according to isotopic dating: 1800–1700 Ma, 800–700 Ma, 230 Ma, 140–120 Ma, and 50–40 Ma. Geological, petrological and isotope geochemical features of the five groups is investigated in order to explore the variation of the mantle material composition in the concerned area with time. And the various extensional activities reflected by the five groups of dike swarms are compared with some important tectonic events within the North China Craton as well as around the world during the same period.
The Junggar orogen, Xinjiang, China, is an important part of the Ural–Mongolian orogen. The collisional orogenesis in this region occurred primarily in the Carboniferous and Permian with an evolutional process of early compression and late extension. Mineralization of gold and other metals in the Junggar orogen occurred mainly in the Permian and in a few cases in the Late Carboniferous. The deposits are largely distributed in areas where collisional orogenesis was intensive and formed in a transitional stage from compression to extension. Therefore, gold mineralization in the Junggar orogen is fully consistent with the collisional orogenesis in time, space and geodynamic setting. This indicates that the mineral deposit model of collisional orogenesis is applicable to prospecting and study of ore deposits in the Junggar orogen. Furthermore, the factual distribution of gold and other deposits in this region is just the same as the collisional orogenic model presents.
Most porphyry Cu deposits in the world occur in magmatic arc settings and are formed in association with calc-alkaline arc magmas related to subduction of oceanic lithosphere. This contribution reviews a number of significant porphyry Cu deposits in the eastern Tethyan metallogenic domain. They widely occur in a variety of non-arc settings, varying from post (late)-collisional transpressional and extensional environments to intracontinental extensional environments related to orogenic and anorogenic processes. Their spatial–temporal localization is controlled by strike–slip faults, orogen-transverse normal faults, lineaments and their intersections in these non-arc settings. These deposits are dominated by porphyry Cu–Mo deposits with minor porphyry Cu–Au and epithermal Au deposits, and exhibit a broad similarity with those in magmatic arcs. The associated magmas are generally hydrous, relatively high fO2, high-K calc-alkaline and shoshonitic, and show geochemical affinity with adakites. They are distinguished from arc magmas and/or oceanic-slab derived adakites, by their occurrence as isolated complexes, high K2O contents (1.2–8.5%), and much wider range of εNd(t) values(− 10 to + 3) and positive εHf(t) values (+ 4.6 to + 6.9). These potassic magmas are most likely formed by partial melting of thickened juvenile mafic lower-crust or delaminated lower crust, but also involving various amounts of asthenospheric mantle components. Key factors that generate hydrous fertile magmas are most likely crust/mantle interaction processes at the base of thickened lower-crust in non-arc settings, rather than oceanic-slab dehydration (as in arc settings). Breakdown of amphibole in thickened lower crust (e.g., amphibole eclogite and garnet amphibolite) during melting is considered to release fluids into the fertile magmas, leading to an elevated oxidation state and higher H2O content necessary for development of porphyry Cu–Mo–Au systems. Copper and Au in hydrous magmas are likely derived from mantle-derived components and/or melts, which either previously underplated and infiltrated at the base of the thickened lower crust, or were input into the primitive magmas by melt/mantle interaction. In contrast, Mo and (part of the) S in the fertile magmas are probably supplied by old crust during melting and subsequent ascent.Research Highlights► Propose a synthesis model of porphyry deposits in non-arc setting by detailed review. ► Crust/mantle interaction is reagrded to be key process to generate fertile adakitic porphyries. ► Cu, Au was derived from mantle-derived components, but Mo was provided by old crust.► Breakdown of amphibole in magma source is regared to have resulted in high fO2 and H2O content of magma.