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East Qinling Mo Belt - Jinduicheng, Shijiawan, Huanglongpu, Nannihug, Sandaozhuang, Majuan, Shangfanggou, Huoshenmiao, Shiyaogou, Yinjiagou, Yuchiling, Donggou, Leimengou, Qiushuwan, Yechangping, Huangshuian, Dahu, Zhifang, Yindonggou, Tumen, Zhaiwa
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The Qinling Molybdenum Belt is located in Shaanxi and Henan Provinces, central China, within the Qinling-Dabie Orogenic Belt separating the southern margin of the North China Craton/Paraplatform (also known as the Sino-Korean Craton) from the Yangtze Craton of the South China Block. It is one of the largest molybdenum provinces in the world, with proved contained Mo metal reserves of >8.4 Mt (Mao et al., 2011). It comprises a number of clusters of deposits in two parts, the East Qinling and Dabie segments to the west and east respectively. Six 'giant' porphyry Mo deposits (i.e., >0.5 Mt of contained Mo) are located within the Qinling segment, Jinduicheng, Nannihu-Sandaozhuang, Shangfanggou, Yuchiling and Donggou, all of which are described below. In addition to the porphyry-style deposits, which are predominantly of Mid to Late Mesozoic age, significant non-porphyry related orogenic vein and carbonatite/vein Mo deposits and occurrences are found within the same belt, emplaced between the Palaeoproteroxoic and Triassic, similarly summarised below. Mesozoic porphyry-style copper mineralisation is also encountered within the belt, as are numerous gold deposits of the Xiaoqinling Gold District and the West Qinling Gold Province of the contiguous West Qinling Orogen.
A selection of representative deposits within the East Qinling segment are described below, including: Jinduicheng and Shijiawan; Huanglongpu; Nannihu, Sandaozhuang and Shangfanggou; Huoshenmiao; Shiyaogou; Yinjiagou; Yuchiling; Donggou; Leimengou; Qiushuwan Cu-Mo; Yechangping; Huangshui'an District Mo-Pb-REE; Dahu Au-Mo; Zhifang Mo; Yindonggou Ag-Au-Mo; Tumen Mo-F and Zhaiwa Mo-Cu.
Similarly, representative deposits within the Tongbai-Hong'an-Dabie (Dabie) segment are described in the separate Qinling-Dabie Mo Belt record.
The Qinling-Dabie Orogenic Belt is part of the greater ~4000 km long Central China/Kunlun Orogen that extends across China, the result of a long-lived series of multiple divergence and convergence events between several blocks. The Qinling-Dabie section of the orogen trends east-west and is >1000 km long. In its most recent form, it is the result of Triassic continental collision between the North China Craton and accreted exotic terranes (collectively the North China Block) and the Yangtze Craton of the South China Block. The latter is the product of the amalgamation of the Yangzte Craton (to the NW) and Cathaysia Terrane (to the SE) along a NE-SW trending fold belt/tectonic zone (Jiangshan-Shaoxing suture/fault), following NW-directed subduction from the Mesoproterozoic to Early Palaeozoic. It also includes other terranes accreted to its margins in the Phanerozoic. The Yangtze Craton is composed of Archaean (e.g. 3.3 to 2.9 Ga Kongling TTG rocks) to Palaeoproterozoic crystalline basement with Late Mesoproterozoic to Early Neoproterozoic fold belts defining its margins (Qiu et al., 2000, Jiao et al., 2009, Gao et al., 2011). Exposure of the older Archaean rocks is limited, masked by the overlying Palaeoproterozoic and younger sequences. The older basement rocks are unconformably overlain by Mid to Late Neoproterozoic (Sinian) to Mid Triassic marine cover sequences (Yan et al., 2003, Zhao and Cawood, 2012). Abundant Neoproterozoic magmatic rocks outcrop along the margins of the craton, with ages primarily clustered between 820 and 760 Ma (U-Pb zircon; e.g., Zhang et al., 2009, Zhao and Zhou, 2009, Dong et al., 2011, Dong et al., 2012).
The Qinling-Dabie Orogenic Belt is truncated and offset on its eastern margin by the major NNE-SSW trending Tan-Lu (or Tancheng-Lujiang) transform fault which separates it from the NE-SW aligned Cretaceous Middle to Lower Yangtze River Valley Metallogenic Belt to the east. This belt contains significant Lower Cretaceous Fe-Cu-Au-Mo and Fe-Cu skarn and Upper Cretaceous Fe skarn and magnetite-apatite deposits. To the SW, this metallogenic belt curves west to trend WNW-ESE parallel to the orogen, whilst on its northeastern extremity (beyond the eastern margin of the second image below) it curves to the east.
The Qinling Molybdenum Belt is divided into two, the ~300 km long East Qinling and ~200 km long Dabie segments to the west and east respectively, separated by an ~100 km wide section of the overlying Late Cretaceous to Cenozoic Nanyang rift basin. Most of the deposits of the East Qinling segment of the molybdenum belt to the west, are located on the southern margin of the North China Craton, in what is known as the Huaxiong Block. This block, which is the reactivated southern margin of the North China Craton, continues eastward into the Dabie segment of the orogen. It is defined as that part of the North China Block south of the Lingbao-Lushan-Wushan fault system (LWF) and north of the Luonan-Luanchuan fault (LLF). However, to the east, in the Dabie segment, younger sedimentary rocks mask much of the Huaxiong Block, encroaching onto the northern fringes of the orogen, and the Luonan-Luanchuan fault becomes the Gushan Fault (GF). Also, in contrast to the East Qinling segment, the Qinling Molybdenum Belt cuts across the orogen, with the western-most deposits hosted within the Huaxiong Block, while the easternmost are within the the Dabie Complex of the South Qinling Terrane, south of the final suture between the North and South China blocks.
The sequence within the Huaxiong Block comprises a reworked Archaean to Palaeoproterozoic basement, covered by Mesoproterozoic to Phanerozoic sedimentary rocks. These rocks were thrust northward over the low angle north-vergent Lingbao-Lushan-Wushan fault system to overlie the southern margin of the North China Craton during the Mesozoic. The North China Craton crystalline basement was affected by two Palaeoproterozoic tectonic events i). a 1.95 to1.92 Ga collision between the Yinshan and Ordos blocks in the north and south to form the Western Block, and ii). ~1.85 Ga convergence between the Western and the Eastern Blocks along the north-south Trans-North China Orogen (or Central Orogenic Belt; Zhao, 2001; Zhao et al., 2005, 2011; Zhao et al., 2010). Rocks of the Palaeoproterozoic Trans-North China Orogen form much of the northern margin of the Mesozoic Qinling-Dabie Orogen.
The North China Craton remained a stable block until the Mesozoic, when it underwent extensive thinning from a thickness estimated to have been ~200 km during the Middle Ordovician to 60 to 80 km in the Cenozoic (Fan and Menzies, 1992; Menzies et al., 1993). This thinning is attributed to delamination and detachment of lithospheric mantle (Xu et al., 2013) following tectonic thickening of the crust and lithospheric mantle during compressional events. Deformation associated with the crustal thickening led to delamination between the lighter crust and heavier lithospheric mantle, and to thermal metamorphism and density increase of the latter as it was pushed deeper into hotter mantle. This was followed by the detachment of the denser, delaminated lithospheric mantle which then foundered into the underlying convecting mantle. Subsequent uplift of the lighter more buoyant crust, unecumbered by the dense lithospheric mantle, and upwelling of adiabatically melted asthenospheric mantle to replace the detached lithospheric mantle and underplate the crust. Episodic widespread, post-orogenic magma underplating of the ancient lower crust during the Phanerozoic has been identified throughout the North China Craton from Early Paleozoic to Cenozoic time, specifically from 490 to 410 Ma in the Early Palaeozoic (Caledonian), ~315 Ma in the Late Palaeozoic (Hercynian), ~220 Ma in the Triassic (Indosinian) and ~120 Ma in the Yanshanian (Zhang et al., 2013). This underplating was mainly along the northern and southern margins of the North China Craton with the Central Asian and Qinling-Dabie orogens respectively, except in the Cretaceous when it was more intense and widespread and affected most of the craton. The latter is interpreted to have been initiated by widespread crustal thickening related to almost coincident collision on three sides, namely: i). collision and accretion between the Sibumasu and Indochina plates during the early Triassic (between 258±6 and 243±5 Ma; Carter et al., 2001), which, in turn, caused compression, collision and uplift within the South China Block and the ~250 to 230 Ma collision between the South China and North China blocks to form the Qinling-Dabie and Central China orogens across China from east to west; ii). closure of the Central Asian Orogenic Belt from the north during the Triassic and Jurassic, and iii). approach of the Pacific Plate from the east in the Lower Jurassic onward. Asthenospheric upwelling to replace the detached lithospheric mantle and underplate the crust with Mesozoic meta-gabbros and pyroxenites (Chen et al., 2009b; Liu et al., 2008; Mao et al., 2011; Pirajno, 2013; Sun et al., 2012; Wu et al., 2008; Yang et al., 2008; Zhu et al., 2011). This asthenospheric upwelling accompanied a period of post-orogenic lithospheric extension culminated in partial melting of the underplate to produce the large-scale magmatic, metallogenic and structural Yanshanian events between ~180 and 170 Ma (Early Yanshanian), ~150 to 139 Ma (Mid-Yanshanian) and ~125 to 98 Ma (Late-Yanshanian) throughout central and southern China.
The basement rocks of the southern North China Craton that are exposed within the northern margin of the Qinling-Dabie Orogen, are largely part of the Palaeoproterozoic Trans-North China Orogen, and comprise the Neoarchaean to Palaeoproterozoic Taihua Supergroup and the Palaeo- to Mesoproterozoic Xiong'er Group.
The Taihua Supergroup comprises, from the base (after Li et al., 2011):
• Beizi Group - mainly amphibolites and gneisses, metamorphosed from amphibolite to granulite facies, occurring as enclaves, commonly <100 m in length, within the widely distributed TTG (tonalite-trondhjemite-granodiorite) rocks. The amphibolite has been dated at 2.77±0.03 Ga (whole rock Sm-Nd
isochron; Xue et al., 1995), while the TTG yielded and age 2.84 to 2.80 Ga (single-grain zircon age; Kröner et al., 1988).
• Dangzehe Group - amphibolites and biotite gneisses, interpreted to be emplaced between 2.5 and 2.3 Ga (Xu et al., 2009).
• Shuidigou Group - unconformably overlying the Dangzehe Group, is a khondalite series dominated by sillimanite-garnet-quartz gneiss, graphitic gneiss, marble, banded iron formation and minor amphibolites (Chen and Zhao, 1997; Wan et al., 2006). Detrital zircon grains from a graphite-garnet-sillimanite gneiss yielded ages ranging from 2.31 to 2.26 Ga (SHRIMP U-Pb), while metamorphic zircon grains were dated at 1.84±0.07 Ga (U-Pb; Wan et al., 2006).
Locally, the Taihua Supergroup and Xiong'er Group are separated across unconformities by the Tietonggou Formation, which is up to ~2850 m thick and is predominantly composed of quartzites, the protoliths of which were mature terrigenous clastic rocks. The age of this formation is constrained to between 1.91 and 1.80 Ga (Diwu et al., 2013).
These are unconformably overlain by the areally extensive Palaeoproterozoic volcanic-dominated rocks of the Xiong'er Group, which have a maximum thickness of 7 to 8 km (Zhang et al., 2011) and are divided, from the base, into (after He et al., 2008, 2010; Zhao et al., 2002, 2009):
• Dagushi Formation which only has sparse local outcrops, mainly conglomerate, sandstone and mudstone;
• Xushan Formation - porphyritic basaltic andesite and andesite with minor dacite and rhyolite. Dating of rocks from four different units within this formation returned ages of 1778±8 Ma, 1783±13 Ma, 1767±47 Ma and 1783±20 Ma, respectively (zircon U-Pb; He et al., 2009).;
• Jidanping Formation - mainly dacites, rhyodacites and rhyolites, interbedded with lesser basaltic andesites and andesites. Two rhyolite samples yielded ages of 1778±5.5 Ma and 1751±14 Ma, respectively (zircon U-Pb; He et al., 2009).
• Majiahe Formation - composed primarily of basaltic andesite and andesite with increasing interlayers of purple shale and mudstone.
Volcanic rocks within the Xiong'er Group have been dated at ~1.78 Ga (U-Pb zircon), with minor younger intrusions in the Jidanping Formation (He et al., 2009; Zhao et al., 2009 and references therein).
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These older basement rocks are unconformably overlain by a cover sequence of Meso-, Neoproterozoic and Lower Palaeozoic rocks that include:
• Mesoproterozoic Ruyang, Luoyu, Guandaokou and Luanchuan groups - The Ruyang Group, which is up to 13 000 m thick, overlies volcanic rocks of the Xiong'er Group in the north of the Huaxiong Block, to the south of the Lingbao-Lushan-Wushan fault system where it is the dominant cover sequence. It is composed of fluvial-littoral-shallow marine clastic and carbonate rocks with volcanic intercalations. The sequence generally comprises a lower sandstone, followed by an upper sequence of marine sandstone, stromatolitic dolomite and limestone. A tuff band in the Luoyu Group, which overlies the Ruyang Group in the north, has been dated at 1611±8 Ma (LA-MC-ICPMS zircon U-Pb; Su et al., 2012). This group is up to 600 m thick and is composed of shales, silty shales, quartz sandstone and dolomitic limestone which has intercalated mud banks and cherts. The Luanchuan Group, which predominates in the central eastern section of the Huaxiong Block, is ~2.5 km thick and contains volcanic rocks dated at 1.6 Ga. It is largely composed of metamorphosed limestone and dolomite, phyllites meta-grits and sandstones, with a ~600 m thick unit of trachyte, trachytic andesite, agglomerate and marble in the upper-central part of the package (Yang et al., 2016). It is interpreted to continue eastward into the northwestern part of the Dabie segment of the Orogen (e.g., Yang et al., 2017). The Guandaokou Group which occupies the bulk of the exposed Mesoproterozoic cover in the southern half of the interval between the Lingbao-Lushan-Wushan and Machaoying fault systems, comprises a 2000 to 5500 m thick carbonate-shale-chert sequence interpreted to be late Mesoproterozoic in age. Like the Ruyang Group, it also unconformably overlies the Xiong'er Group, but in the south;
• Late Neoproterozoic (Sinian) to Ordovician Taowan Group - is exposed in the south, immediately to the north of the Luonan-Luanchuan Fault. It comprises an ~1 km thick succession containing rocks interpreted to be metamorphosed diamictites, as well as biotite-muscovite-quartz schists, hematite-rich schist units and marble, deposited in an environment that ranges from fluvial to marine. This sequence reflects deposition on an extensional passive margin during the breakup of Rodinia. Further to the north, rocks of the Ruyang-Luoyu-Guandaokou groups are unconformably overlain by Cambrian carbonate rocks that are in part equivalent to the Taowan Group.
A depositional hiatus developed from the Middle Ordovician to Early Carboniferous. From the Middle Carboniferous to Triassic, sedimentary sequences deposited were mainly terrigenous mudstone, clastic and carbonate rocks, with intercalated coal beds in localised basins. Jurassic to Cenozoic sedimentary rocks are relatively rare and confined to intra-continental basins.
The North China Craton units of the Huaxiong Block, as described above, are intruded by extensive collisional and post-collisional Jurassic to Cretaceous batholithic granitoids of the Qinling Orogen, e.g., the Huashan complex, a multistage granitic pluton, which includes the 200 km2, 130.7±1.4 Ma Haoping, the 60 km2, 156.8±1.2 Ma Wuzhangshan, and the 132.0±1.6 Ma Huashan plutons (Li, 2005). Mineralised porphyries, diatremes and breccia pipes coeval with this large scale magmatism are associated with the molybdenum deposits of the Qinling Molybdenum Belt.
The east-west Machaoying fault zone is an important ~200 km long structure near the southern margin of the North China Block, within the Huaxiong Block. It has an inferred depth extent of 34 to 38 km, has been interpreted as a major north-dipping thrust zone formed during Mesozoic continental collision (Chen and Fu, 1992; Zhang et al., 1998; Chen et al., 2004). It has regularly spaced subsidiary fault splays, including NE- to NNE-trending compressive shears which control the location of orogenic Au and Ag deposits and shallow intrusions related to porphyry intrusion (Qi et al., 2005; Chen et al., 2009). Structural analysis indicate the NE-trending structures have undergone early compression, followed by a tensional regime and late extensional shearing (Chen et al., 2008). The main Machaoying fault has been subjected to multistage deformation (Chen and Fu, 1992; Chen et al., 2008b; Liu et al., 1998). 40Ar/39Ar dating of syn-tectonic biotite from the fault gave an Early Cambrian plateau age of 524.9±1.9 Ma, indicating southward thrusting in the Early Palaeozoic (Han et al., 2009). Further multistage activation of the fault zone is indicated (Chen et al., 2008).
To the south of the Huaxiong Block, the Qinling-Dabie Orogenic Belt is characterised by multi-stage orogenic processes and the development of voluminous magmatic intrusions. It separates the North China and Yangtze cratons. To the west of these cratons, it continues for >1500 km as the Kunlun/Central China Orogen, following the Kunlun Shan along the southern margin of the Qaidam Block, separating it from the Songpan-Garze accretionary complex to the south, and is terminated by the NE-SW Altyn Tagh Fault (Cowgill et al., 2003; Kusky et al., 2007).
Sutures, Terranes and Tectonic Framework
Three closely spaced sutures divide the Qinling-Dabie section of the orogen into a number of narrow terranes (Ratschbacher et al. 2006):
i). The Luonan-Luanchuan Fault/Suture represents the northernmost of the trio, separating the Huaxiong Block from the narrow (~10 to 50 km wide) Ordovician Erlangping intraoceanic magmatic arc, which is intruded by Silurian-Devonian arc plutons, and is most likely a continuation of one of the multiple Qilian Orogen sutures found to the west of the North China Craton. The northern part of the orogen, mainly to the south of the Luonan-Luanchuan Fault, but north of the Erlangping Arc, is occupied by the Late Neoproterozoic to Early Palaeozoic Kuanping Group. This group comprises greenschist to amphibolite facies mafic volcanic and meta-clastic rocks, including marble and two-mica quartz schists (Lu et al., 2003), but also includes an ophiolitic suite related to the Luanchuan suture. The N-MORB geochemical signatures of meta-basalts suggest a back-arc basin setting in the Neoproterozoic, i.e., a back-arc basin north of the Erlangping Arc (Diwu et al., 2010). The associated metaclastic rocks probably formed in the Early Palaeozoic and may have been deposited on a passive margin to the North China Craton, preserved in a Palaeozoic accretionary wedge (Ratschbacher et al., 2003).
The principal unit within the Erlangping Arc is the Early Palaeozoic Erlangping Group, which comprises a low-grade metamorphosed metavolcanic and metasedimentary complex represented by abundant upper greenschist- to amphibolite-facies rocks. Protoliths are interpreted to be mafic to intermediate volcanic rocks and hypabyssal dykes, fine-grained clastic rocks and cherts with Cambrian-Silurian fossils (Zhang et al., 2001), the youngest being Ludlovian-Wenlockian, 428 to 419 Ma radiolaria (Wang, 1989). Pillow basalts in the Erlangping Group possess calc-alkaline geochemical signatures, suggesting an intra-oceanic or adjacent back-arc setting (Lu et al., 2003). There are no exposed occurrences of old basement rocks below the Erlangping Arc. This arc is interpreted to have been the result of northward subduction of an ocean approaching from the south, ahead of the North Qinling Terrane (Micro-continent; see below). Numerous tonalite, trondhjemite, gabbro and rare pyroxenite intrusions cut the volcanic sequences, dated at 488 to 470 Ma (207Pb/206Pb; single-grain zircon; Lerch et al., 1995; Xue et al., 1996). Numerous volcanic and intrusive rocks subsequently intruded this unit at ~440 to 400 Ma (Zhai et al., 1998; Jiang et al., 2009) implying more than one episode of magmatism in the arc. Dating of garnet bearing rocks yields metamorphic ages of 394±5 to 440±3 Ma (Zircon U-Pb; Liu et al., 2011), interpreted as the age of amphibolite facies metamorphism.
ii). The Zhuyangguan-Xiaguan Fault is the second suture zone, and is of similar age, separates the Erlangping arc from a long narrow (~10 to 50 km wide) sliver (or Micro-continent) of Proterozoic rock, the North Qinling Terrane, that has been deformed by a ~1.0 Ga tectonic event, and been involved in both 0.8 to 0.7 Ga Neoproterozoic and Silurian to Devonian rifting. It is overlain by Devonian to Permian arc rocks and subduction complexes, all of which have been metamorphosed to granulite facies.
The North Qinling Terrane has a common Precambrian history with the Yangtze Craton, but differing Phanerozoic character. It is interpreted to have split from Gondwana at ~750 Ma, at the end of the Neoproterozoic, during the break-up of the Rodinia supercontinent and drifted north, forming the Shangdan Ocean in its wake and its overlying island arc magmatism. The principal unit exposed is the North Qinling Group/Unit, which has been subdivided into two structurally discordant parts (Zhang et al., 1988; You et al., 1993; Xue et al., 1996). The lower mainly consists of Palaeoproterozoic amphibolite facies metamorphic rocks, comprising complexly deformed biotite-plagioclase gneiss, granulite, amphibolite and thin layers of graphitic marbles and marble-calc silicates, cut by granitic intrusions. The gneisses have protolith ages of 1.0 to 0.9 Ga (Chen et al., 1992; Zhang et al., 2004; Lu et al., 2005; Pei et al., 2007). The upper part of the group is a thick pile of gently folded marbles (Xue et al., 1996). Early Palaeozoic (450 to 400 Ma; Su et al., 2004) intrusions (Chen et al., 1991; Lu et al., 2006; You et al., 1993) that are interpreted to represent an Early Palaeozoic island-arc terrane (Dong et al., 2011) intrude the Qinling Group. Extensive high-temperature granulite-facies metamorphism is apparent in the North Qinling unit (Kröner et al., 1993; Zhai et al., 1998; Ratschbacher et al., 2003; Liu et al., 2011; Wang et al., 2011; Xiang et al., 2012) with peak metamorphic conditions of 840 to 760°C and 980 to 950 MPa leading to partial melting, migmatisation and magmatism.
In addition to the high temperature granulite metamorphism, high pressure (HP) and ultra high pressure (UHP) eclogite-facies metamorphic zones which extend for intervals of >10 km, are found on the northern, central and southern North Qinling Group/Unit. Eclogite and retrograde metmorphosed eclogite occur as lenses, blocks and layers within garnet pyroxenites, amphibolites, garnet-bearing quartz and phengitic schists (Hu et al., 1995) and coesite and microdiamond in zircon (Yang et al., 2003, 2005). UHP metamorphism during the Palaeozoic is predominantly confined to the North Qinling Group/North Qinling Terrane and has been dated to imply three pulses from 490 to 480; 430 to 420 and ~310 Ma (Wu et al., 2013). No Palaeozoic HP or UHP metamorphism took place to the south of the North Qinling Terrane. These observations suggest the North Qinling Terrane was thrust below the Erlangping arc to mantle depths of at least 80 to 120 km following their collision during the Early to Mid Palaeozoic (Wu et al., 2013).
The Erlangping arc was accreted to the Qinling Terrane in the Ordovician to Silurian, and then both were accreted to the North China Craton by ~400 Ma.
iii). The Shangdan Suture is of Triassic age, and is the southernmost of the three sutures. It is related to upper Palaeozoic magmatism and marks the southern limit of Devonian deformation (Hacker et al., 2007), separating the North Qinling Terrane from the northern margin of the South Qinling Terrane to the south. In the East Qinling segment of the orogen, a dismembered suite of ultramafic rocks, gabbros, basalts, diabasic dikes, pillow lavas and radiolarian cherts known as the Shangdan Ophiolite Unit crops out between the North Qinling Group/Unit in the north and the Shangdan Fault in the south (Meng and Zhang, 2000). Mafic rocks include N-MORB, E-MORB and island arc basalts (IAB; Meng and Zhang, 2000; Dong et al., 2011), typical of arc to backarc basin basalts (e.g., Lawton and McMillan, 1999; Taylor and Martinez, 2003; Pearce and Stern, 2006). This implies the Shangdan Ophiolite Unit is associated with backarc basin closure and arc-continent collision. Gabbros in the suite are dated at 520 to 400 Ma (Zircon U-Pb and Sm-Nd mineral isochron dating), indicating subduction of the Shangdan oceanic crust from the Cambrian to the Silurian prior to arc-continent collision, consistent with radiolarian ages in the cherts (Cui et al., 1996). The formation of the ophiolitic suite has also been well constrained as 534±9 to 517.8±2.8 Ma (Dong et al., 2011, Li et al., 2015, Liu et al., 2016). Island arc rocks within the microcontinent have been dated at 507±3 to 499.8±4.0 Ma (Pei et al., 2005; Lu et al., 2009), indicating continued subduction of the Shangdan oceanic lithosphere to ~500 Ma. However, the isotopic composition of granites from this period are consistent with continent-continent collision. If this is the case, the South Qinling Terrane was rifted from, and advanced ahead of the craton, to collide with the North Qinling Terrane by ~450 Ma (e.g., Wu et al., 2103).
To the east, in the Dabie segment of the orogen, the Shangdan Suture equivalent, the Guishan-Meishan fault, which parallels and is ~35 km south of the Gushan Fault, is interpreted to define the southern margin of the North Qinling Terrane and final suturing between the North China and Yangtze blocks in this section of the orogen. The sequence north of this fault, which is similar to that described above in the East Qinling segment, is progressively concealed to the east by younger cover encroaching from the north.
The Dabie segment of the orogen is divided into three stepped blocks, separated by NE-SW trending faults, all of which are subparallel to the Tan-Lu Fault which offsets the Orogen in the east. From west to east, they are (after Wu , 2013 and sources quoted therein) the: a). Tongbai; b). Hong'an; and c). Dabie blocks. These blocks are respectively separated by the Dawa (DWF) and Shang-Ma faults (SMF). The absence in the easternmost, the Dabie Block, of Early Palaeozoic and Carboniferous HP metamorphism, as well as Palaeozoic arc-type magmatism, implies no intervening arcs or microcontinents were accreted to the North China Craton, and that collision between the Yangtze and North China cratons was direct during the Triassic.
The Liuling Flysch Unit occurs immediately to the south of the Shangdan Fault, in both the Qinling and Dabie segments of the orogen, and comprises an ~5 km thick sequence of greenschist facies, grey-green sandstone and mudstone with minor conglomerate and rare limestone (Mattauer et al., 1985; You et al., 1993; Ratschbacher et al., 2003, 2006) carrying Devonian to Carboniferous brachiopods and corals (Yu and Meng, 1995; Faure et al., 2008). These rocks are metamorphosed to greenschist and epidote amphibolite-facies (Ratschbacher et al., 2006; Liu et al., 2011). Ar/Ar dating of amphibole, muscovite and biotite varies spans the interval from 401 to 264 Ma (Wu et al., 2013 and references cited therein). Clasts containing gabbros, peridotites, amphibolites, granites, with garnet, quartz and minor hornblende and pyroxene in the north (Mattauer et al., 1985), may have been derived from erosion of the North Qinling Unit, Shangdan Ophiolite Unit, South Qinling Terrane and the underlying basement rocks of the South China Block, which suggest a marginal sea setting (Yu and Meng, 1995; Meng and Zhang, 2000; Dong et al., 2011a; Yan et al., 2012).
The equivalent of the Liuling Flysch Unit in the Dabie segment of the orogen is the Devonian to Triassic Xinyang Group, interpreted to be an accretionary complex, containing flysch, ophiolitic slices and Precambrian fragments (Liu et al., 2013). It is subdivided into the Guishan and Nanwan formations in ascending order, dominated by mafic to felsic volcanic and sedimentary rocks respectively (Henan Bureau of Geology and Mineral Resources, 1989). The Guishan Formation is intercalated with ophiolitic slices of volcanic rocks from the North Qinling Group, and is dominated by fore-arc clastic sediments with minor marble. The Nanwan Formation is a greenschist facies assemblage of bedded quartzites and pelites (Liu et al., 2013). The Xinyang Group is a structural wedge to the north of the exhumed UHP/HP rocks;
Dating of sedimentary zircons, geochemical data and regional geology of the Liuling Flysch Unit (Liao et al., 2017) indicate Early Palaeozoic granitoids and HP-UHP metamorphic rocks in the North Qinling Unit were already exhumed to the surface prior to the Middle Devonian, and were then eroded and flysch deposited in a post-orogenic extensional basin, rather than a subduction-related fore-arc basin or a foreland basin formed during or after continental collision.
The Douling Unit is found on the southern margin of the Liuling Flysch Unit. It occurs as lensoid, <10 to >100 km long developments of micaceous granitic gneiss and amphibolite, with subordinate marbles, graphite schists and quartzites, and includes metamorphosed quartz keratophyre, pyroclastic and massive to pillowed Ti-rich basalt (Dong and Santosh, 2016; Zhang et al., 2004; Zhao et al., 1995). These rocks have undergone amphibolite facies metamorphism and subsequent overprinting greenschist facies. Schists and gneisses have been dated as Palaeoproterozoic, while voluminous intrusion of Neoproterozoic magmatic rocks, mainly diorites and granitoids, have dismembered the succession. Isotopic data indicate none of these rocks underwent high-grade metamorphism at temperatures of >350°C during Phanerozoic tectonism. As such they most likely represent South Qinling Terrane rocks structurally interleaved during late collision. In the Dabie segment of the orogen, the major lens of Douling Unit equivalent, the Sujiahe Group, is separated by the Huwan Shear Zone from rocks to the south that underwent Triassic high pressure (HP) and ultra high pressure (UHP) metamorphism, as described below. The shear zone comprises a 5 to 10 km width that contains partially mylonitised, elongated blocks of eclogite, metagabbro, (epidote-)amphibolite marble, and quartzite in an argillic matrix that includes augen gneiss, quartzofeldspathic schist and graphitic schist (Ye et al., 1994; Li et al., 2001; Liu et al., 2004c; Ratschbacher et al., 2006).
The Sujiahe Group, has been dated at 0.85 to 0.43 Ga, although the protolith of the granitic gneiss in the Douling Complex has yielded ages of ~2.51 to 2.47 Ga (Hu et al., 2013). The Sujiahe Group comprises a Caledonian accretionary complex of highly strained granite, metagabbro, mica schist, quartzofeldspathic gneiss, amphibolite and marble but also includes structurally bounded enclaves of eclogite (Liu et al., 2008). This group occurs as a structural wedge to the north of the exhumed UHP/HP rocks.
Other poorly deformed rocks found to the north of the belt of UHP and HP metamorphosed rocks in the southern half of the Dabie segment of the orogen include:
• Carboniferous Meishan Group - a coal bearing sedimentary cover sequence deposited to the north of the Xinyang Group over the North Qinling Terrane and Erlangping Arc, which by then were accreted ot the North China Craton;
• Late Jurassic to Cretaceous volcanic rocks and interbedded sedimentary sequences are preserved as inliers surrounded by younger cover and as remnants overlying the North and South China blocks to the north, east and SW of the Darbie Block. These comprise a sequence of up to 5 km of calc-alkaline, crust-derived intermediate volcanic rocks, volcanogenic sandstone and some lava (Hacker et al., 2004). These volcanic rocks are broadly extrusive equivalents of the intrusive event that is coeval with the molybdenum porphyry Mo deposits of the Qinling Mo Belt.
The South Qinling Terrane has been thrust south over the northern margin of the Yangtze Craton. To the west of the Nanyang Basin, it comprises Palaeoproterozoic crystalline rocks similar to that of the Yangtze Craton, overlain by an intercalated Neoproterozoic bimodal suite of mafic and felsic volcanics, quartzite, sandstone, siliciclastic and carbonate rocks deposited in a rift setting (Wang et al., 2013; Ling et al., 2008; Hubei, 1990). These rocks are overlain by a 12 km thick sequence of Mid to Late Neoproterozoic (Sinian) to Ordovician age, mainly composed of platformal carbonate, shale and sandstone. They are succeeded by up to 10 km of Silurian to Devonian metagreywacke, slate, phyllite, and carbonate rocks (Gao et al., 1995), indicating large-scale subsidence and rapid sedimentation from the Early Silurian. The overlying Carboniferous to Triassic sequence is markedly condensed, and localised to very small basins, occurring as shallow-marine limestone with sparse continental clastic sedimentary rocks and coal measures (Gao et al., 1995). All of these strata are strongly folded (Mattauer et al., 1985). The sequence reflects deposition on the rifted and then passive shelf margin of the Yangtze Craton during the Neoproterozoic to Lower Palaeozoic, and following the Early Palaeozoic orogenic phase, Silurian and Devonian deposition in the opening Palaeotethyan Ocean. Palaeomagnetic data (Zhao and Coe, 1987; Lyn and Fuller, 1990; Enkin et al., 1992) suggest the North and South China blocks had reached their furthest separation during the Permian, before commencing to approach, with the South China Block undergoing ~60° of relative clockwise rotation by the Mid Jurassic.
Much of the eastern half of the South Qinling Terrane in the East Qinling segment of the orogen is occupied by the ~120 km diameter Wudang Shan metamorphic core complex, the northern half of which is composed of HP blueschist rocks dated at 237 to 232 Ma (Ar/Ar; phengite and glaukamphibole/riebeckite; Mattauer et al., 1985; Ratschbacher et al., 2003; Hacker et al., 2004; Faure et al., 2008). Blueschist typically comprise an assemblage that includes glaucophane-riebeckite amphibole + lawsonite/epidote + chlorite + albite + quartz ± sodic clinopyroxene (jadeite). These blueschists underwent a moderately rapid cooling event of ~9°C/m.y. corresponding to the development of the Wudang metamorphic core complex (Wudang Dome) which ended at ~220 Ma (Hu et al., 2006). The basement protoliths of the Wudang Dome are composed of a lower sequence of mafic and felsic volcanic rocks of probable Palaeo- to Mesoproterozoic age, overlain by clastic rocks containing detrital zircons older than 1 Ga (Hieu et al., 2009). These rocks have a geochemical character consistent with a Gondwana/Yangtze Craton affinity (Hieu et al., 2009). The southern margin of the South Qinling Terrane is defined by the major, south vergent Mianlue Fault in the East Qinling segment of the orogen, interpreted to continue as the Xiangfan-Guangji Fault to the east of the Nanyang Basin, in the Dabie segment.
The Mianlue and Xiangfan-Guangji faults (also known as the 'Mianlue-Bashan-Xiangguang Fault') may either represent a reactivated Proterozoic to Palaeozoic rift margin fault, or a suture reflecting a subducted Mianlue Ocean that opened when a microcontinent, represented by the South Qinling Terrane, was rifted from the northern margin of Gondwana in the Mid Devonian. In the latter case, the Mianlue Ocean opened between the craton and microcontinent during the late Palaeozoic, but was subducted and the two blocks were reunited across the Mianlue Suture in the Mesozoic. This structure is now a low angle south vergent thrust that places South Qinling Terrane rocks over the northern margin of the Yangtze Craton. Sections of this structure enclose rocks interpreted to be ophiolites.
To the east of the Nanyang Basin, in the Darbie segment of the orogen, South Qinling Terrane rocks occupy a wedge shaped area that is ~350 km long, elongated WNW-ESE, with a width that expands eastward from <100 to >150 km south from the North Qinling Terrane. It is predominantly composed of sequences of metamorphosed basement rocks that have an affinity with Gondwana/Yangtze Craton, overlain by Neoproterozoic to Early Mesozoic cover sequences. To the east, these rocks and the orogen are structurally terminated against an eastern arm of the Yangtze Craton by the NNE-SSW Tan-Lu Fault with dextral offset of the orogens eastern continuation by some 500 km to the NNE as the Sulu Orogen on the Shandong Peninsular. Adjacent to the Tan-Lu Fault, the Yangtze Craton is overlain by the 50 to 100 km wide, deformed Middle to Lower Yangtze River Valley Metallogenic Belt that is characterised by a string of Mesozoic intrusions and volcanosedimentary basins and is the subject of a separate record.
In the Darbie segment of the orogen, the wedge-shaped block of South Qinling Terrane rocks are predominantly composed of sequences that were subjected to Triassic HP and UHP metamorphism and have subsequently been exhumed. Formation of these metamorphic suites, and the HP rocks of the Wudang Dome, followed mid-Carboniferous to Late Permian subduction and closure of the Shangdan/Palaeotethys Ocean. In the Late Permian to Early Triassic, the leading edge of the South Qinling Terrane began to be subducted/thrust below the continental North Qinling Terrane (represented by the North Qinling Group on the image above) which by then, was amalgamated with the Erlangping Arc and North China Craton to form the North China Block. Concurrently, the Mianlue Ocean was being subducted northward below the South Qinling Terrane followed by the collision of the Yangtze Craton (South China Block) which had approached from the south. Resultant ongoing compressive stress caused the South Qinling Terrane crust to continue subducting, to be underthrust and imbricated below the North China Block and carried to mantle depths of >120 km, as is evident from the occurrence of microdiamonds in eclogites (Xu et al., 1992), producing the HP and UHP metamorphism. Exsolution minerals in eclogitic garnet in the UHP metamorphosed rocks indicate depths of formation of >200 km (Ye et al., 2000). As part of this continental subduction event, the Liuling/Xinyang and Douling/Sujiahe units/groups were 'scraped' from the top of the sedimentary pile on the South Qinling Terrane to form imbricated thrust bounded accretionary prisms at shallower depths, while the thus thinned lower section of the terrane was subducted. Subsequent exhumation occurred during subhorizontal post-collisional relaxation, crustal extension and rise of the buoyant UHP rocks from the Triassic to present. Extension within the Dabie segment of the orogen appears to have occurred below a major detachment that follows the Huwan Shear Zone, marking the boundary between UHP metamorphosed rocks to the south and relatively un-metamorphosed rocks of the Liuling Flysch Unit/Xinyang Group and Duolong Unit/Sujiahe Group to the north.
Subduction of the Shangdan/Palaeotethys Ocean, followed by closure and continental collision is interpreted to have started at ~252 Ma in the Late Permian, with the peak UHP metamorphism at ~240 to 220 Ma in the Early-Middle Triassic (Sm-Nd mineral isochron and U-Pb zircon; Ames et al., 1993; Li et al., 1993; Chavagnac and Jahn, 1996; Hacker et al., 1998; Zheng et al., 2002, 2003; Liu et al., 2004; Wu et al., 2006; Zheng et al., 2009; Liu and Liou, 2011).
It is likely that the extreme underthrusting of the South Qinling Terrane resulted first in the detachment of the oceanic slab during the Triassic, then in delamination and detachment of the underlying Sub-Crustal Lithospheric Mantle (SCLM) of the structurally thickened lithosphere. This resulted in a thinned, lower density lithosphere which led to buoyant uplift/updoming and exhumation. This uplift, which exposed the UHP metamorphosed rocks, also promoted partial melting in the underlying mantle, asthenospheric upwelling, mafic magma underplating and crust-mantle interaction, but also a transformation in tectonic setting from compression to extension (e.g., Li et al. 2002). This north-south extension was influenced by a coeval east-west back-arc extension triggered by the distal westward Palaeo-Pacific Plate subduction below Japan, and enhancing the magmatism in the Qinling-Dabie Orogen. The asthenosphere is expected to have been fertilised during the previous multiple subduction events within the orogen. The crust-mantle interaction led to an evolving magmatism from I- to A-type as the tectonic regime progressed from compressional to extensional - see the Magmatism section below.
As an alternate, Zheng et al. (2019) suggest slabs/sheets of lighter continental lithosphere from the upper surface of the subducting South China Block were detached at different times and depths and buoyantly rose on the underside of the overriding North China Block, and as the latter was eroded, were sequentially exhumed at the surface. This gave rise to a sequence of HP-UHP metamorphosed slices with different metamorphic grades juxtaposed along the margin of the suture between the two plates.
Zircon SHRIMP U-Pb dating of Early Cretaceous granitoids of the Dabie Massif indicate they were the product of two episodes of migmatisation (Wu et al., 2007). Despite the systematic differences between the two episodes of magmatism and migmatisation (see the Magmatism section below), geochemical studies indicate that they all result from partial melting of the deeply subducted continental crust of the South China Block (Zhao et al., 2017).
During exhumation, UHP metamorphics underwent retrograde recrystallisation to HP quartz eclogite facies at ~600 to 750°C and 2.4 to 1.2 GPa, and to amphibolite-facies at ~450 to 600°C and 0.6 to 1.0 GPa through a process of nearly isothermal decompression (Zheng et al., 2009 and references cited therein). The retrograde transition from UHP to HP facies is dated at ~225 to 215 Ma, while retrogression to amphibolite facies is interpreted to have been at 215 to 205 Ma. These ages are based on dating of zircons associated with the respective metamorphic facies (references cited by Wu et al., 2013). Based on these dates, exhumation rates of ~0.3 and 0.6 cm/yr have been calculated (Wu et al., 2013 and references cited therein).
Exhumation has been to a greater degree than to the west of the Nanyang Basin, where only the upper fringes of blueschist HP metamorphism are exposed in the northern half of the Wudang Shan metamorphic core complex. From immediately south of the Liuling and Douling structural wedges in the NNE, the degree of this metamorphism in mafic protoliths progressively decreases to the SSW, from coesite eclogite → quartz eclogite → eclogite → amphibolite → blueschist → greenschist. In felsic protoliths, principally the trondhjemite-tonalite-granodiorite suite of the Dabie/Tongbai Complex (see below) an analogous progression of orthogneiss is developed. Coesite is a polymorph of silica, formed when very high pressure (2 to 3 GPa) and moderate to high temperature (700 to 1300°C), are applied to quartz. The presence of coesite and diamond in eclogites from the Dabie segment of the orogen indicates peak metamorphic pressure of >2.8 GPa (Okay et al., 1989; Xu et al., 1992) accompanied by peak metamorphic temperatures calculated at 630 to 850°C, mostly 700±50°C (Cong et al., 1995; Wang et al., 1995; Carswell et al., 1997; Li et al., 2004; Shi and Wang, 2006; Zhang et al., 2009; Gao et al., 2011; Zheng et al., 2011).
The exposed UHP rocks in the Dabie segment of the Qinling-Dabie Orogen form a wedge, widest in the east where it is truncated by the NNE-SSW trending, Early Cretaceous to Cenozoic, Tan-Lu Fault, and tapers westward. The sequence in this section of the orogen is similar to that of the East Qinling segment, but has been subdivided as follows:
• Dabie/Tongbai Complex, the protoliths of which are dated at >0.75 Ga. It is the most extensively developed unit in the eastern South Qinling Terrane between the Yangtze Craton and the North Qinling Terrane, and includes HP and UHP eclogite and orthogneiss suites. Protoliths include Archaean to Proterozoic supracrustal rocks metamorphosed to granulite, amphibolite, biotite-plagioclase gneiss and marble (Liu and Liou, 2011). It also includes high-grade, ~195 Ma (40Ar/39Ar; Wang et al., 2002) upper amphibolite to granulite facies (HP/UHP) gneisses after Neoproterozoic tonalite-trondhjemite-granodiorite (TTG) and subordinate migmatites (Jahn et al., 1999; Hacker et al., 2000; Zheng et al., 2006). The migmatites formed between 131 and 111 Ma which preceded their exhumation (zircon U-Pb; Wang et al., 2002). The UHP metamorphosed sections of the complex have been subdivided into eclogite and orthogneiss facies influenced by the respective mafic or felsic protoliths, separated in part by the NE-SW trending Xishui-Tongcheng fault (Zhang et al., 2002; Zhao et al., 2008, 2011). In addition to Neoproterozoic TTG granitoids, the Dabie/Tongbai Complex has been intruded by Palaeozoic plutonic rocks, and extensive Cretaceous intrusions and porphyry dykes.
• Neoproterozoic Hong'an Group, that comprises quartzofeldspathic schist and muscovite-albite and two-mica gneisses, with minor eclogite, amphibolite, marble, metaphosphorite layers, and graphite schist, and has only been metamorphosed to UHP eclogite to HP amphibolite facies (Liu et al., 2004; Liu et al., 2008).
• Neoproterozoic Suixian Group - a low to moderate metamorphic grade Neoproterozoic volcanic-sedimentary sequence of schist and phyllite with intercalated rhyolitic, dacitic and meta-basic rocks, containing voluminous blueschist, exposed towards the SSW margin of the UHP/HP wedge (Liu et al., 2004; X.C. Liu et al., 2008). This unit is largely within the HP blueschist facies section of the exhumed UHP/HP block.
Zircon U-Pb dating indicates granitoid magmatism in the Qinling-Dabie Orogenic Belt predominantly occurred in four distinct pulses (after Wang et al., 2013):
• Neoproterozoic from 979 to 711 Ma, occurring as:
i). strongly deformed S-type granites emplaced from 979 to 911 Ma, largely within the Dabie/Tongbai Complex
ii). weakly deformed I-type granites from 894 to 815 Ma, and
iii). A-type granites from 759 to 711 Ma.
Widespread mafic to ultramafic intrusions occur throughout the South Qinling Terrane and sections of the northern Yangtze Craton, variously dated at between 850 and 630 Ma (e.g., Wang et al., 2013).
These have been interpreted to be the products of syn-collisional and extensional settings respectively, in response to the assembly, initiation of break-up and extension related to the final break-up of the Rodinia supercontinent.
• Palaeozoic from 507 to 400 Ma, i.e., Mid Cambrian to Early Silurian, in three stages:
i). 507 to 470 Ma magmatism, dominated by I-type, and lesser S-type granitoids. The former are granodiorite, tonalite, plagiogranite and monzogranite, which are high-K calc-alkaline and metaluminous. The latter have monzogranitic compositions and are high-K calc-alkaline and shoshonitic. These granitoids are contemporaneous with eclogites and HP granulites in the North Qinling Group (Liu et al., 1996; Yang et al., 2002; Chen et al., 2004), and thus the I-type intrusions can be inferred to be related to the subduction system. They are relatively small intrusions interpreted to have been spatially and temporally related to the northward subduction and closure of the intervening ocean ahead of the approaching North Qinling Terrane. These granites are similar in age to volcanic rocks of the Erlangping Arc. The few S-type granites may be the result of partial melting of continental crust, corresponding to the rapid exhumation of the UHP rocks metamorphosed in the early-stage subduction (Wang et al., 2009b).
ii). 460 to 422 Ma magmatism, extensively development in the North Qinling Belt, extending into the Erlangping and
Kuanping groups. They are dominated by I-type granitoids, including biotite granodiorite, tonalite, quartz diorite and monzogranite. These intrusives are calc-alkaline to high-K calc-alkaline, and metaluminous to peraluminous and are high Sr/Y granitoids which have TDM older than 1000 Ma. They are interpreted as originating from the lower crust with involvement of mantle derived magma in a collisional setting, rather than as a result of arc magmatism. Evidence includes the isotopic characteristics of the granitoids which suggest that they were derived from partial melting of a thickened lower crust (Wang et al., 2013 and references cited therein). This magmatism overlaps, and may have contributed to the late stages of the Erlangping Arc, and is interpreted to have been related to the collision of the Yangtze Craton, or a microcontinent rifted from the craton, and the North Qinling Terrane which was by then accreted to the North China Craton (e.g., Wu et al., 2013).
iii). 415 to 400 Ma, which are only minor and restricted to the middle part of the North Qinling Belt, dominated by I-type granitoid intrusions, including diorite, quartz diorite, granodiorite and monzogranite, probably forming in the late-stage of the same collisional setting. They are metaluminous to peraluminous, and high-K calc-alkaline to shoshonitic and relatively young TDM of 1360 to 910 Ma (Wang et al., 2013).
• Early Mesozoic from 252 to 185 Ma in the Early Triassic to Early Jurassic, which is predominantly within the western part of the South Qinling Terrane, and represent two stages of intrusion:
i). Initial, low volume I-type quartz diorites and plagiogranites of a 250 to 240 Ma (Lower to Middle Triassic) magmatism which have low SiO2 but high MgO and CaO, and other characteristics of a metaluminous to peraluminous, and high-K calc-alkaline series (Jin et al., 2005; Zhang et al., 2006). These granitoids are LREE-enriched and HREE-depleted, and show slightly negative Eu anomalies, while some have high Sr/Y ratios (Jin et al., 2005; Zhang et al., 2006). They have evolved Sr-Nd isotopic compositions (Zhang et al., 2006), and may have been generated by partial melting of a thickened Early to Middle Neoproterozoic crust in a continental arc setting related to subduction of the Mianlue Ocean below the South Qinling Terrane (Wang et al., 2013).
ii). Voluminous late-stage (225-185 Ma; Upper Triassic to Lower Jurassic) magmatism evolved from early I-type to later I-A-type granitoids associated with contemporaneous lamprophyres, and comprise granitoids which are quartz diorites, quartz monzonites, granodiorites and monzogranites with abundant enclaves of 219±2 Ma (Ar-Ar; Wang et al., 2007) microgranular mafic rocks. They have high Na2O and K2O contents and are metaluminous to slightly peraluminous, and belong to high-K calcalkaline or shoshonitic series. They have moderate LREE enrichment and slight HREE-depletion, with negligible to slightly negative Eu anomalies, and are enriched in LILE and HFSE, but depleted in Nb, Ta, P and Ti, with evolved Sr-Nd isotopic compositions. The majority yield Mesoproterozoic
to Neoproterozoic TDM (e.g., Qin et al., 2010; Wang et al., 2011; Dong et al., 2011), although some towards the southern margin of the North China block have Archean to Paleoproterozoic TDM (Ding et al., 2011; Qi et al., 2012). However, zircon Hf isotopic data and other geochemical characterisitcs suggest the involvement of a juvenile component in their sources (Wang et al., 2011; Zhang et al., 2008; Qin et al., 2010). These granitoids do not have deformed fabrics and are located along the Shangdan Suture, mostly intruding the Proterozoic Qinling Complex and Paleozoic Liuling Group. The characteristics of these granitoids support an origin from partial melting of Archaean to Palaeoproterozoic (North China Block) and Mesoproterozoic to Neoproterozoic (South China Block) crustal materials with subsequent mixing or mingling of mantle-derived mafic magma (Zhang et al., 2008; Qin et al., 2009, 2010). These granitoids may reflect a transition from syn- to post-collisional setting in response to the collision between the North and the South China blocks (Wang et al., 2013).
• Late Mesozoic from 158 to 100 Ma, from the Late Jurassic to Upper Cretaceous, occurring as either voluminous batholiths free of deformation, or as small porphyritic intrusion. Emplacement of these intrusions can be divided into two stages, from 158 to 130 Ma and 120 to 100 Ma. Those of the first stage intrude the Archaean Taihua, Proterozoic Xiong’er and the Kuanping Group along the southern margin of the North China Block, mainly in the Qinling segment of the orogen. In contrast, the later stage granitoids spread south to intrude the Devonian Liuling Group and equivalents, and the Triassic HP and UHP metamorphics of the Dabie Complex in the South China Block. The smaller porphyritic intrusions in both segments are frequently associated with Mo, W, and Au mineralisation.
i). Early stage, variously bracketed between 158 to 130 Ma (Wang et al., 2013) or 145 to 130 Ma (Zheng et al., 2019 and sources cited therein) granitoids comprise quartz diorite, granodiorite, monzogranite and granitic porphyry which have I-type, calc-alkaline to shoshonitic compositions, and are metaluminous. All the granitoids are LREE enriched but have flat HREE patterns with moderate to slightly negative Eu anomalies, and are depleted in Ti, P and Ba.
ii). Late stage, 120 to 100 Ma (Wang et al., 2013) or 130 to 120 Ma (Zheng et al., 2019 and sources cited therein) granitoids are granodiorite, monzogranite and granite porphyry, classified as I- to A-type or A-type. They are alkaline, and slightly peraluminous, LREE enriched and slightly HREE depleted, with moderately negative Eu anomalies and significant Ba, Sr, P and Ti depletion (Wang et al., 2011).
The isotopic signatures of both stages are suggestive of derivation from ancient crustal material although certain data, combined with the presence of mafic microgranular enclaves in the granitoid plutons, imply the involvement of a juvenile component. In addition, the Ti-depleted geochemistry of the
early-stage granitoids, compared with slight enrichments in the late-stage intrusions, is interpreted to indicate the formation of the latter had a higher contributed proportion of juvenile material, probably from lithosphere metasomatised by slab-derived fluids from Triassic or earlier subduction. However, geochemical data closely reflect the basement of the terranes or blocks in which the granitoids are located.
The Late Mesozoic granitoids define a transition from I-type through I-A-type to A-type magmatism from the early to late stage, and are associated with contemporaneous mafic to intermediate trachyandesitic volcanic rocks at 117±2 Ma (Xie et al., 2007), suggesting emplacement during an extensional setting. The final collisional event of the Qinling-Dabie Orogen was completed in the Triassic, and the extensional setting occurred thereafter in a post-orogenic or intra-plate regime.
All of the main terranes/units that make up the Qinling-Dabie Orogen, are intruded by Cretaceous granitoids related to both molybdenum and lesser copper porphyry style mineralisation, which were emplaced into a post-collisional extensional regime; i.e., i). the Huaxiong Block on the southern margin of the North China Block, the ii). Lower Palaeozoic Erlangping magmatic arc and iii). the North Qinling Terrane, as well as iv). the UHP metamorphosed northern margin of the South Qinling Terrane structurally overling the northern margin of the Yangtze Craton.
Distribution of Mineralisation
While the bulk of the deposits in the Dabie segment of the orogen are Late Jurassic to Early Cretaceous porphyry style mineralisation, the string of mineralised centres that define the belt in the East Qinling segment include Triassic and a few Early Jurassic porphyry-style deposits, but also a selection of orogenic vein type mineralisation (e.g., Dahu) not apparently related to mineralised porphyry centres and some hosted by carbonatite veins (e.g., Huanglongpu and Huangshui'an). These include deposits that are of Triassic, Silurian, Neoproterozoic and Palaeoproterozoic in age. However, all economically significant deposits were formed during the Mesozoic. Re/Os dating of molybdenite from Mesozoic deposits defines three episodes of Mo mineralisation, i.e., Late Triassic (233 to 221 Ma), Late Jurassic to Early Cretaceous (148 to 138 Ma) and Early to middle Cretaceous (131 to 112 Ma).
Those of Late Triassic age include orogenic molybdenite-quartz veins, carbonatite vein and porphyry styles. The porphyry Mo systems of this temporal group are concentrated in the western and middle part of the East Qinling Orogen, whereas carbonatite veins and orogenic Mo systems are located in the northeastern part. The porphyry Mo systems, including porphyry, porphyry-skarn and intrusion-related veins, are spatially associated with granitic porphyries typical exhibiting porphyry type alteration systems. The carbonatite veins have stable isotope systematics for C, O and S and high contents of Re and Sr indicating they are mantle-derived, and Mo mineralisation is associated with magmatic fluids (Li and Pirajno, 2017; Mao et al., 2011). In contrast, the Late Triassic orogenic Mo systems occur as fault-controlled quartz lodes, and ore-forming fluids are characteristic of medium to high temperature, CO2-rich metamorphic fluids. The orogenic Mo deposits were the earliest, mainly between 250 and 220 Ma, during the late orogenic stage related to subduction of the Mainlue ocean and continental crust to generate the Triassic UHP and HP metamorphic rocks further east. These were followed by carbonatite veins, mainly between 225 and 205 Ma and porphyry Mo systems, predominantly between 205 and 190 Ma. The latter two ore styles accompanied the voluminous I and A type magmatism that accompanied post orogenic relaxation and extension as the UHP/HP metamorphic suites were exhumed further to the east in the Dabie Domain. With some exceptions, there is generally a trend of decreasing deposit ages toward the western part of the orogen (Li and Pirajno, 2017). None of these Late Triassic deposits are of major economic significance.
In contrast, the Late Jurassic to Early Cretaceous and Early to middle Cretaceous Mo mineralisation, collectively emplaced between 156 and 110 Ma (Chen et al., 2017), predominantly occur as porphyry and porphyry-skarn Mo mineral deposits, generally associated with high-K calcalkaline metaluminous-peraluminous Yanshanian granitic intrusions. Both appear to be related to a period of extension. Mao et al. (2011) have suggested these intrusions are linked to a tectonic regime of lithospheric thinning, asthenospheric upwelling and partial melting of the crust, induced by a change in Izanagi/Pacific Plate motion parallel to the eastern Eurasian continent margin.
Granite porphyries associated with Mo deposits that are of Late Jurassic to Early Cretaceous age generally have limited surface expressions and are situated in the East Qinling domain, east of the Late Triassic mineralisation, but still well to the west of China's continental margin. Intrusions during this period are generally quartz diorite, granodiorite, monzogranite and granitic porphyry which have I-type, calc-alkaline to shoshonitic compositions, and are metaluminous as described above in the Magmatism section, under Early Mesozoic, i). early stage.
Early to Middle Cretaceous batholiths and granite porphyry stocks with associated Mo deposits are dominantly found in the Dabieshan area and eastern sections of the East Qinling domain, closer to the eastern Eurasian continental margin at the time of deposition, but still substantially west of the west subducting Izanagi/Pacific plate. This continues the eastward progression of the locus of mineralisation along a trend normal to the eastern margin of the Eurasian continent margin. Intrusion during this period is described above in the Magmatism section, under Early Mesozoic, ii). late stage and are alkaline, and slightly peraluminous, I- to A-type or A-type granitoids.
Porphyry and/or porphyry-skarn Mo (W) deposits in the East Qinling-Dabie belt are commonly surrounded by related vein type Pb-Zn-Ag deposits, forming well defined ore clusters, possibly reflecting an outward temperature decrease from highly fractionated granite plutons (Mao et al., 2011).
The range of Mo mineralisation and deposits in this well defined belt might imply the subcrustal lithospheric mantle (SCLM) below the East Quinling and Dabie segments of the Qinling-Dabie Orogen was fertilised during the Palaeoproterozoic and repeated partial melting of that lower lithosphere has produced magmas and fluids that promote Mo mineralisation. As detailed below, and illustrated in the 'regional setting' image below, this segment of the east-west trending Qinling-Dabie Orogen corresponds to reworked rocks of the North China Craton that coincide with the cratonised north-south trending Trans-North China Orogen. The latter contains Palaeoproterozoic porphyry mineralisation to the north, as at Tongkuangyu in the Zhongtiaoshan District, supporting the possibility of fertilisation of the SCLM during that period.
A representative selection of the significant deposits of the East Qinling segment of the Qinling Molybdenum Belt are described below, while a similar set from the Dabie segment are described in the separate Qinling-Dabie Mo Belt record.
EAST QINLING MO DEPOSITS
Jinduicheng (Jinduichen or Jin Dui Cheng) (#Location: 34° 19' 51"N, 109° 57' 17"E) and Shijiawan - Cretaceous porphyry Mo deposits.
These deposits, which are ~100 km east of Xian, are 5 km apart. They are classic porphyry molybdenum ores, comparable to Climax in Colorado, USA. At the Jinduicheng deposit, mineralisation is associated with a small 1.8 x 0.4 km, Late Triassic, 124±6 Ma granite porphyry stock, located just beyond an apophysis on the south-eastern margin of the 130±5 Ma, >50 x 15 km Laoneushan monzogranite batholith.
The host stratigraphy in the Jinduicheng district has been locally subdivided into the following, from the base (after Nie, 1994):
Archaean to Palaeoproterozoic Taihua Group - dated between 2.6 and 2.0 Ga (Hu et al., 1986), this ~4000 m thick sequence comprises granulite, amphibolite, amphibole-plagioclase gneiss, biotite-plagioclase gneiss, banded and streaked ptygmatic migmatite and phlogopite-diopside marble, and is only exposed 7 to 13 km north of the Mo deposits.
Palaeoproterozoic Tietonggou Formation - an ~2850 m thick sequence, which unconformably overlies the Taihua Group, but underlies the Xiong'er Group. It is predominantly composed of micaceous quartz schist, schist, schistose quartzite and quartz conglomerate, and is exposed in the SW of the district. The depositional ages of the protoliths of the Formation were well constrained to 1.91 to 1.80 Ga (Diwu et al., 2013).
Palaeoproterozoic Xiong'er Group - a ~3900 m thick sequence, which is made up of metaspilite, porphyritic basalt, tuffaceous slate, sericite-phyllite, biotite-quartz schist, with marble lenses.
Neoproterozoic Gaoshanhe Formation - which unconformably overlies the Xiong'er Group and is composed of quartzite, muddy-sandy slate, conglomeratic quartz sandstone, varicoloured sericite-slate and lenticular dolomite, with lesser intermediate to mafic lavas, sills and dykes.
Neoproterozoic Longjiayuan and Xiongjianshi Formations - comprising ~1500 m of sandy slate, banded chert, tuffaceous slate, banded siliceous dolomite and limestone.
The Jinduicheng and Shijiawan granite porphyry stock intrudes rocks of the Xiong'er Group, close to the boundary with the overlying arenaceous slates of the Gaoshanhe Formation. The Huanglongpu vein deposit (see below) is developed within the Gaoshanhe Formation arenaceous slates (Nie, 1994).
The district has undergone multiple deformation and metamorphism since the Mesoproterozoic. The major fold in the district is the east-west oriented Huanglongpu anticline, which hosts the Laoneushan monzogranite batholith in its axis. All three deposits lie on the southern limb of the anticline, within 2 to 5 km of the batholith margin. Faults and fractures trend east-west, ENE to NE, and NW.
The ore deposits comprises disseminated and stockwork Mo mineralisation, hosted by quartz ±K feldspar ±pyrite ±fluorite veins and veinlets, with accessory disseminated magnetite, chalcopyrite, sphalerite, galena, cassiterite, bismuthinite, sericite, biotite, beryl, apatite, topaz and calcite. Both the porphyry stock and enveloping Proterozoic host rocks have been intensely altered to produce ore related silicification and K feldspathisation, passing out into phyllic and propylitic zones. The ore at Jinduicheng, is 25% within the granite porphyry stock and 75% in the country rock. The Jinduicheng deposit had a total production + resource of 910 Mt @ 0.10% Mo, 0.03% Cu (Mutschler, 2000).
Huanglongpu (#Location: 34° 20' 52"N, 110° 01' 31"E) - Cretaceous Mo-Pb bearing carbonatite veining.
The Huanglongpu deposit which is ~7 km ENE of Jinduicheng is characterised by Mo-Pb bearing carbonatite veining developed within the Gaoshanhe Formation arenaceous slates (Nie, 1994). At Huanglongpu ore occurs as four separate vein systems Yuantou, Dashigou, Shijiawan and Taoyuan. These are hosted by highly deformed and metamorphosed Archaean to Palaeoproterozoic andesites and tuffaceous shales, and is overlain by a Mesoproterozoic sedimentary sequence formed in a continental-margin basin setting and a succeeding Neoproterozoic to Middle Ordovician package deposited in a passive continental-margin setting. Veins are generally up to 1 m wide and may reach 500 m in length and consist of 50 to 70% calcite, 30 to 50% quartz and ~5% microcline, 4.5% barytocelestine and 1% biotite with significant molybdenite, galena and pyrite and subordinate magnetite, hematite, chalcopyrite and sphalerite as well as REE-fluorocarbonates, monazite and brannerite. Molybdenite occurs as disseminated aggregates, as bands or crusts paralleling vein walls and as infillings along calcite and galena cleavage planes. Hydrothermal alteration is limited to the margins of the veins. Molybdenum mineralisation has been dated at ~221 Ma (Re-Os; Huang et al., 1994; Stein et al., 1997).
In contrast to the porphyry-type deposits, the Huanglongpu alteration envelopes to the veins consist of biotite, epidote, pyrite and anhydrite, which are only developed as selvages to the veins (Mao et al., 2008).
Total reserves are estimated to be >0.18 Mt of contained Mo in ore with an average grade of 0.06% Mo (Xu et al. 2010).
Nannihu, Sandaozhuang, Majuan and Shangfanggou, Luanchuan County, Henan Province (#Location: 33° 55' 13"N, 111° 29' 45"E) - Cretaceous porphyry and porphyry-skarn Mo deposits.
The Nannihu porphyry molybdenum (-tungsten), Sandaozhuang skarn molybdenum (-tungsten) and Shangfanggou porphyry-skarn molybdenum deposits, which are some 80 km ESE of the Jinduicheng and Huanglongpu deposits, and 12 km NW of Luanchuan, have total reserves of ~1.9829 Mt of contained Mo at grades of from 0.06% to 0.24%, with an associated 0.7379 Mt of W grading from 0.091% to 0.129% (Li et al., 2004).
The sequence within the Nannihu district includes the Mesoproterozoic Guangdaokou Group banded chert-bearing carbonate rocks, overlain by a thick sequence of medium- and low-grade Neoproterozoic metamorphic shelf clastic and carbonate rocks, comprising the Luanchuan Group, carbonates rocks and trachyte, Taowan Group terrigenous clastic-carbonate rocks, Guandaokou Group marble, and Kuanping Group, marble and basic volcanic rocks.
The major regional faults curve from near east-west, through WNW to NW directions (from west to east), and were formed in the Late Triassic and Early Jurassic (Ren et al., 1998), with a less well developed NE-trend, formed during the Mid to Late Jurassic transition to the tectonic regime of eastern China (Mao et al., 2003). These fault sets vary considerably, but occur in swarms and are concentrated in defined zones. The Nannihu-Sandaozhuang-Shangfang district is sandwiched between the major Luanchuan and Machaoying faults to the south and north respectively.
The major magmatic rocks in the district are Caledonian metagabbro and syenite porphyry, and mid to late Yanshanian intermediate to acid intrusions at Nannihu, Shangfang, Shibaogou, Yuku and Majuan. The host sequences have been extensively metamorphosed into various hornfels and skarn rocks by the Yanshanian intrusions, in particular the marbles of the Sanchuan Formation (altered to diopside-garnet skarns) and the Nannihu Formation sandstones (altered to various hornfels). These intrusive rocks were derived from partial remelting of the lower crust producing I-type granitoids (Wang et al., 1986).
Molybdenum and tungsten mineralisation is associated with small, hypabyssal and near surface complex stocks of late Jurassic Yanshanian calc-alkaline granite porphyries (Xu et al., 2000).
Mineralisation occurs as stockwork quartz-molybdenite veins within the granite porphyry and disseminated molybdenite within the skarn altered zone. Alteration stages include skarn alteration (described above), potassic (K feldspar-quartz alteration with associated 0.2 to 1 cm veins of dark quartz and molybdenite, and lesser pyrite and fluorite), silicification (quartz-sulphide alteration with 0.5 to 2 cm veins of molybdenite with milky quartz with lesser magnetite and fluorite), phyllic (quartz-sericite-pyrite), zeolite-carbonate (zeolite, quartz and carbonate, with 0.5 to 5 cm veins of molybdenite, clear quartz and lesser calcite and pyrite) and propylitisation (Liu et al., 1988).
The Nannihu and Shanfanggou granite porphyries have been dated at 157.1±2.9 Ma and 157.6±2.7 Ma respectively (SHRIMP zircon U-Pb ages; Mao et al., 2005), while the Nannihu-Sandaozhuang and Sanfanggou deposits have a Re-Os isochron age of 141.5±7.8 Ma (Li et al., 2004), the nearby Lengshuibeigou Ag-Pb-Zn deposit has an 40Ar/39Ar plateau age of 137.87±0.39 Ma. These ages show that the granitic porphyries are slightly older than the deposits, and that the porphyries and deposits exhibit different evolution stages of the same tectonic-magmatic-ore fluids system in the same geodynamic setting.
Geochemical studies indicate anomalous halos surrounding granite porphyries, e.g., the Nannihu granite porphyry is accompanied by an anomalous zone that is 40 km long, by 20 km wide with a well-defined outward zonal pattern, from a core of high temperature porphyry-skarn type molybdenum (-tungsten), to an outer zone of medium to low temperature hydrothermal vein type lead-zinc-silver mineralisation. The porphyry-skarn type molybdenum (-tungsten) deposits occur within the core or contact zones of granite porphyries, surrounded by copper-lead-zinc skarn altered and stockwork mineralisation.
The Nannihu porphyry molybdenum (-tungsten) system at Nannihu is hosted by the Nannihu and Sanchuan Formations of the Luanchuan Group. The associated porphyry intrusion has an area of ~0.12 km2 and comprises shallow porphyritic monzogranite passing down to porphyritic biotite granodiorite at depth. The porphyritic monzogranite contains 50% phenocrysts, which include of 10% plagioclase, 20% K feldspar and 20% quartz in a matrix of fine grained K -feldspar, plagioclase, quartz and minor biotite, and accessory aspidelite, zircon, apatite and scheelite. Hornfels were formed by contact metamorphism caused by intrusion of the Nannihu porphyritic monzogranite, and include biotitic felsic hornfels.
The porphyritic monzogranite and immediate surrounding hornfelsed country rocks host 22 ore blocks. The largest pf these is 2700 m long and between 1000 and 1500 m wide, dipping gently to the SW. The porphyry intrusive, skarn and altered hornfels host the three main ore types.
The principal ore minerals are molybdenite, scheelite, pyrite, galena, sphalerite and chalcopyrite in a gangue that includes quartz, feldspar, actinolite, garnet, biotite, sericite, epidote, fluorite and calcite. Ore occurs as disseminations, veinlets and stockworks, including flaky, metasomatic embayment and idiomorphic to hypidiomorphic replacement of grains and phenocrysts.
Types of hydrothermal alteration include: i). potassic, predominantly hydrothermal biotite and feldspar; ii). silicification, which is widespread in the porphyry and wallrocks, particularly in association with the quartz-sulphide stockworks or veinlets; iii). sericitic, typically as replacement of feldspar and biotite, accompanied by disseminated pyrite and quartz-sericite veinlets; and iv). carbonate, typically replacement of mafic minerals by carbonates. Hydrothermal alteration and mineralisation can be divided into four stages:
Stage 1, characterised by the assemblage K feldspar, quartz, biotite, with minor pyrite and molybdenite. Sulphides are mostly disseminated, whilst pyrite forms idiomorphic to hypidiomorphic cubes. This suite reflects coeval potassic, silicic and weak sericitic alteration.
Stage 2 comprises quartz-molybdenite stockworks, containing minor amounts of other sulphides. Molybdenite is flaky, occurring as disseminations in stockwork veining, or as fine-grained films coating fractures, whilst pyrite and chalcopyrite are found as xenomorphic to idiomorphic grains.
Stage 3 is distinguished by the extensive developed quartz-polymetallic sulphide veinlets, dominated by pyrite, chalcopyrite, molybdenite, sphalerite, and galena. Silicification and phyllic alteration are most conspicuous in stages 2 and 3.
Stage 4 veins comprise one to three phases of quartz, carbonate and fluorite, with little or no accompanying sulphide, crosscutting the earlier veins, stockworks and altered porphyry blocks.
Dating on the Nannihu porphyritic monzogranite yielded ages of 141 to 157 Ma (Rb-Sr and zircon U-Pb respectively; Bao et al., 2009; Hu et al., 1988; Li et al., 1993; Mao et al., 2010), whilst molybdenite separates from the Nannihu porphyry Mo-W mineralisation gave an age of ~140 Ma (Re-Os; Huang et al., 1995; Li et al., 2003; Mao et al., 2008), a little younger than the porphyry complex. These data indicates the porphyritic monzogranite and the ore-system formed during the Late Jurassic to Early Cretaceous transition from collisional compression to extensional tectonics between the North China and Yangtze Cratons (Chen et al., 2000; Li, 1998; Li et al., 2007), during the regional large-scale magmatic-metallogenic event recognised throughout the Qinling Molybdenum Belt (e.g. Chen and Fu, 1992; Chen et al., 2007, 2009; Fan et al., 2011; Li et al., 2007; Mao et al., 2008, 2010; Zhang et al., 1998). The porphyritic monzogranite is characterised by high-Si, high-K, alkali-rich and negative εNd(t) values of -12.7 to -15.5, with Nd model ages of 1.48 to 1.71 Ga (Bao et al., 2009).
Proved reserves in the 22 ore blocks at Nannihu contain 1.24 Mt of Mo metal, with an average grade of 0.076% - which equates to 1/6 Gt of ore. It also contains 0.5 Mt of W, with an average grade of 0.103% WO3 (proven reserve; Li et al., 2007).
The Sandaozhuang skarn molybdenum (-tungsten) deposit, mainly hosted in skarn and marble, contains 0.6725 Mt of Mo grading from 0.093% to 0.149% W, and 0.6025 Mt of W grading 0.091% to 0.129% W (from Wang et al., 2008).
The Shangfang porphyry-skarn molybdenum deposit occurs in the contact zones of the Shangfang porphyry and dolomitic marble of the Meiyaogou Formation of the Neoproterozoic Luanchuan Group. It has a total metal reserve of 0.7158 Mt grading 0.11 to 0.24% Mo, and by-product reserves of 2.5 Mt grading 1.49%S and 81.3 t grading 0.0042% Re (Wang et al., 2008, after Luo et al., 1991).
Huoshenmiao, Henan Province - Cretaceous porphyry-skarn Mo.
The Huoshenmiao Mo deposit is located ~9 km west of the Nannihu, Sandaozhuang, Shangfanggou cluster of deposits. The country rock sequence is composed of clastic, carbonate and alkaline volcanic rocks of the Luanchuan Group, which comprises, from the base, the Baishugou, Sanchuan, Nannihu, Meiyaogou and Dahongkou formations. Seven faults within two main fracture trends, namely NWW and NE are developed in the deposit area. The most prominent of these trends NNW and dips at 60 to 80°NE. It is flanked by four NE-trending secondary faults, which dip at 50 to 70°NWW, one of which controls the southeastern boundary of the mineralised skarn.
Intrusive rocks in the deposit area include the Huoshenmiao intrusion and a series of gabbro dykes. The latter dykes were coeval with the eruption of the volcanic rocks of the Dahongkou Formation at ~830 Ma (Wang et al., 2011), and are oriented in between east-west to NE-SW. They intrude along the contacts between stratigraphic units. The augen-shaped Huoshenmiao intrusion is a stock that covers an area of 0.2 km2), intruding marble of the east-west striking Sanchuan. It has a core of monzogranite that grades progressively outward into granite porphyry and peripheral quartz diorite, and is spatially and temporally related to the Mo mineralisation (Xu and Ren, 1988; Xu et al., 1995; Yang et al., 1997; Xin, 2010; He and Chen, 2013). The ages of the monzogranite, granite porphyry and quartz diorite are 146.0±0.6 Ma, 145.1± 0.5 Ma and 150.3±0.6 Ma, respectively (zircon U-Pb; Wang et al., 2016).
The principal hosts to the deposit are the Huoshenmiao porphyry and the Sanchuan marble. Mineralisation within the latter is mainly (>95%) associated with calcic skarn, with <4% in non-skarn altered Sanchuan marble and <1% in the granite porphyry (Xin, 2010). The calcic skarn mainly occurs along the contact between the granite porphyry and the Sanchuan marble, characterised by an assemblage that is dominated by pyroxene, garnet, amphibole, epidote, biotite, actinolite and chlorite (Wang, 2016). It has a lenticular shape, and is ~500 m long, 50 to 150 m in vertical thickness, with a gentle south dip.
Molybdenum is the primary commodity (Xin, 2010), with high-grade ore the result of stockwork veining overprinting the calcic skarn. These veins are mainly composed of K feldspar, quartz-K feldspar, quartz-molybdenite and quartz-pyrite. The principal metalliferous minerals are molybdenite and pyrite, with minor pyrrhotite, ilmenite and chalcopyrite. Molybdenite mostly exhibits platy, clotted or veinlet forms in quartz and K feldspar veins filling open spaces within fractures, and as fine-grained crystals and aggregate flakes along the margin of quartz veins and fractured surfaces in skarn. Gangue minerals are predominantly pyroxene, garnet, amphibole, quartz and K feldspar, with minor plagioclase, epidote, biotite, chlorite, sericite and calcite.
Based on mineralogy, crosscutting relationships and hydrothermal alteration, six stages of mineralisation have been recognised, divided into skarn and quartz-sulphide groupings, with each stage partially replacing its predecessors:
• Skarn alteration
Stage I - Prograde skarn development, to produce a massive anhydrous silicate mineral assemblage. Light to dark green, 0.1 to 2 mm pyroxene, dominantly hedenbergite, is the major new mineral; accompanied by minor red to brown, 1 to 10 mm, andradite to predominantly grossularite garnet; plagioclase and K feldspar, but no metallic mineralisation.
Stage II - Retrograde skarn alteration, which produced considerable quantities of hydrous silicate minerals, including light green, tremolitic amphibole, epidote, biotite, actinolite and chlorite, to replace the stage I assemblage. These were accompanied by small amount of quartz, K feldspar, plagioclase, pyrite, ilmenite and molybdenite.
• Quartz-sulphide mineralisation
Stage III - Quartz-K feldspar - principally represented by barren quartz, K feldspar and quartz-K feldspar veins, with silicification and potassic alteration of the wall rocks. The barren quartz veins only contain quartz with very fine euhedral pyrite, whilst, K feldspar and quartz-K feldspar veins carry considerable molybdenite, commonly intergrown with the K feldspar. Late aplite veins crosscut the preceding veining, implying that they formed prior to final solidification of the granite porphyry.
Stage IV - Quartz-molybdenite, which produced quartz-molybdenite veins that are the dominant mineralisation of the deposit. A significant amount of platy molybdenite is found in fractures within veins, and as fine-grained crystals occurring as flake aggregates distributed along veinlet walls. The principal hydrothermal alteration is phyllic, represented by pervasive quartz and sericite. Molybdenite is generally intergrown with sericite. Crosscutting relationships suggest these quartz molybdenite veins postdate the aplite veins, but predate quartz-pyrite veining.
Stage V - Quartz-pyrite, characterised by extensive quartz-pyrite veining and phyllic alteration. The veining contains abundant pyrite with minor molybdenite, pyrrhotite and chalcopyrite. Two quartz-pyrite vein subtypes are recognised: i). star-like, characterised by fine-grained subhedral to anhedral pyrite; and ii). disseminated, composed of coarse-grained pyrite with a small amount of fine-grained chalcopyrite. Both are cut by calcite veins.
Stage VI - Quartz-calcite, characterised by quartz-calcite and calcite veins, with only very minor fine-grained pyrite without molybdenite. These veins cut all of the previously listed veins.
Reserves comprise ~40 Mt @ 0.12% Mo for ~50 000 t of contained Mo (Xin, 2010).
Shiyaogou, Henan Province (#Location: 34° 11' 50"N, 110° 47' 30"E) - Cretaceous porphyry Mo.
The Shiyaogou low-F molybdenum deposit is located in the southern Xiong'er Mountains on the southern margin of the North China Craton, in the eastern Qinling metallogenic belt. It is ~15 km NNE of the Nannihu, Sandaozhuang and Shangfanggou cluster of deposits, and is surrounded, within 2 to 3 km to the NW, NE and SE respectively by the Yuanling, Nanping and Hongzhuang gold deposits. Both the Mo and Au deposits are controlled by three brittle splays of the regional, east-west trending Machaoying fault zone. These brittle faults have both sinistral strike-slip and tensional characteristics and are interpreted to have been activated during the Late Jurassic to Early Cretaceous (Han et al., 2009; Zhang et al., 2001).
The Shiyaogou area is underlain by Palaeo- to Mesoproterozoic volcanic rocks of the Xiong'er Group, of which only the upper three formations are exposed. These are the Xushan, Jidanping and Majiahe formations. The Xushan Formation is composed of dark-greenish porphyritic andesite and basaltic andesite. The Jidanping Formation is mainly reddish-grey rhyolitic dacite and minor andesite. The Majiahe Formation is largely composed of andesite, basaltic andesite and trachyte with minor rhyolite and rhyolitic porphyry at its top. The sequence has been subjected to low grade greenschist facies metamorphism.
No igneous intrusions are exposed within a radius of 5 km surrounding the deposit. However, drilling has intersected deep-seated ore-bearing granitic porphyries, all of which are vertically below the Shiyaogou Mo deposit. These porphyries are:
• Grey medium- to fine grained biotite monzogranite with biotite, plagioclase, K feldspar, quartz (size <3 mm) and minor K feldspar phenocrysts (<10 vol.%)
• Pink coarse-grained porphyritic monzogranite with ~25 vol.% phenocrysts, including nearly equal amounts of euhedral K feldspar (0.5 to 1.5 cm in diameter) and quartz (~7 mm across). The matrix comprises K feldspar, plagioclase, quartz and sparse biotite. Cooling margins reveal these porphyries intruded the previous described.
• Pink fine-grained K feldspar granitic veins/dykes with equigranular texture, composed of K feldspar, plagioclase and quartz, without any mafic minerals.
Molybdenite bearing ore bodies at the Shiyaogou deposit are within or adjacent to the Tieling-Baitu-Xiayankan fault which dips at 50 to 80°N and extends for ~800 m with a width of 80 to 300 m. Intense Mo mineralisation occurs at depths of >100 m with the density of ore-bearing veinlets increasing towards the concealed porphyries. The deposit is characterised by abundant stockworks of quartz-molybdenite veins which are commonly 1 to 2 cm thick and are generally banded and symmetric with alternating quartz and molybdenite interlayers. Quartz-molybdenite veinlets are common at a shallower depth and mainly hosted in altered volcanic rocks. Disseminated fine-grained molybdenite occurs in volcanic rocks and porphyries. Vein molybdenite is usually as curved flakes between quartz grains. The principal ore minerals are molybdenite, pyrite, magnetite and chalcopyrite with minor pyrrhotite, galena, bornite and hematite. The main gangue minerals are quartz, K feldspar, calcite, sericite and chlorite.
The alteration pattern is complex, but has been shown to comprise an outward zonation from the porphyries of potassic, silicic, phyllic, argillic and propylitic zones. Molybdenum mineralisation is spatially associated with potassic and phyllic alteration and silicification. Potassic and phyllic alteration zones are usually near the top of the intrusion and extend for ~600 m above it. Potassic alteration occurs as secondary K feldspar or greenish
biotite-bearing veins and becomes more intense toward the intrusion. The phyllic alteration comprises sericite, quartz, chlorite, pyrite and chalcopyrite. Silicification is common and accompanies the potassic and phyllic assemblages, characterised by the formation of new quartz or opal in quartz-bearing veins and wall rocks. Argillic alteration is mainly found in the upper part of the deposit and is characterised by montmorillonite and kaolinite. Propylitic assemblages occur in the peripheries and is represented by albite, epidote, chlorite and carbonate that replace plagioclase and mafic minerals.
The veining sequence is as follows: quartz-K feldspar → quartz-molybdenite → quartz-pyrite → barren quartz → carbonate. Quartz and K feldspar veins formed prior to quartz-molybdenite veining, which are, in turn, normally crosscut by quartz-pyrite veins. Late-stage barren and clean quartz veins commonly cut quartz-pyrite and quartz-molybdenite veins and are subsequently crosscut by latest-stage carbonate veining.
Mineralisation has been dated at 132.3±2.8 Ma (Re-Os; molybdenite; Han et al., 2013), whilst the ore related porphyries are 135 to 132 Ma (LA-ICP-MS U-Pb; zircon; Han et al., 2013).
The Shiyaogou deposit has a resources of at least 0.152 Mt of contained Mo metal.
This summary is paraphrased from Han et al. (2013).
Yinjiagou Mo-Cu, Henan Province - Cretaceous porphyry Mo deposit.
The Yinjiagou Mo-Cu-pyrite deposit is located ~40 km south of Lingbao, in Henan Province, and was discovered in 1958. The country rocks in the mine area belong to the Mesoproterozoic Guandaokou Group and comprise dolostone, locally interbedded with sandstone and shale. These rocks are intruded by the Yinjiagou intrusion, which is subcircular and covers an area of ~0.6 km2. It is principally composed of monzogranite porphyry, K feldspar granitic porphyry and quartz diorite porphyry, with minor biotite quartz monzonite porphyry and diorite dykes.
Four main fracture/fault trends are recognised, namely EW, NNE, NW, and NNW.
The sulphide mineralisation of the Yinjiagou deposit is concealed, occurring at a depth of ~100 m below the surface, except for a small outcrops of Pb-Zn veining and gossans. In cross section, the Mo mineralisation forms ovoid bodies hosted in the core of a >650 x 500 m, pyritic K feldspar granitic porphyry. Molybdenite occurs as veinlets and disseminations. Individual ore blocks are 4.6 to 91 m thick, with lateral extents of from 70 to 350 m.
Pyrite mineralisation, with associated copper and gold, forms an east-west elongated ring that is up to ~1000 x 800 m, mostly hosted in the outer intrusion and contact with the country rocks, and in adjacent fault zones. The pyrite zone grades into pyrite-magnetite. Both are disseminated to densely disseminated and brecciated. Individual blocks are typically 500 to 600 m long and 0.7 to 164 m thick, extending over a vertical interval of 415 to 620 m. Pb-Zn mineralisation forms a patchy outer ring hosted by dolostone peripheral to the intrusion, either near the contact zone or in wall rock fissure veins. Gold occurs in the pyrite bodies, and in gossans. Iron oxides occurs mainly as primary magnetite associated with pyrite mineralisation zones or supergene limonite in the gossans.
Seven x pyrite, 40 x Cu, 31 x Mo, 28 x Au and 17 x Pb-Zn(-Ag) ore blocks have been delineated (Chen and Guo, 1993; Yan et al., 2007; and Zhang et al., 2008).
The hypogene ore minerals are pyrite, magnetite, chalcopyrite, bornite, molybdenite, galena and sphalerite, with supergene limonite, hematite, chalcocite, malachite, covellite, azurite and cerussite in the oxide zone that is ~100 m thick. The main gangue minerals associated with the molybdenite mineralisation are quartz, sericite and kaolinite, while the pyrite mineralisation is accompanied by quartz, sericite and dolomite and has a euhedral to subhedral granular, stressed and sandy crystal fragmental textures. The pyrite-magnetite has a gangue of serpentine, talc, dolomite, chondrodite and phlogopite and euhedral to subhedral granular and anhedral granular textures. In the oxide zone, limonite occurs with opal, chalcedony and calcite with microcrystalline to aphanitic textures.
The dominant alteration types are potassic (K feldspar and biotite), quartz-sericite and argillic (kaolinite and hydromica) followed by magnesite, carbonate (siderite, Mn dolomite and Fe-Mn carbonate) and lesser skarns. The skarn is mainly composed of forsterite and chondrodite with minor tremolite and actinolite, traces of garnet and diopside, and includes associated phlogopite, serpentine and talc. The alteration type is influenced by the wall rock. Potassic, silicic, sericitic and argillic alteration is extensively developed within the intrusion; magnesite, carbonate and silica are mainly found in the dolostone of the Guandaokou Group; whilst skarn alteration is mainly confined the contact zone between the intrusion and dolostone. Consequently there is a zonation of alteration from a central potassic core within the intrusive, accompanied by quartz-sericite and argillic assemblages with associated molybdenite and peripheral magnetite mineralisation, passing out into a contact zone with skarn, quartz-sericite with associated pyrite, magnetite, Cu, Zn and Au mineralisation; whilst the outer country rock exhibits carbonate minerals with Pb, Zn, Ag and Mn mineralisation (Chen and Fu, 1992; Chen et al., 2007).
Skarn alteration and associated mineralisation has been divided into three stages: i). An early prograde assemblage of forsterite, chondrodite, garnet, and diopside; ii). An overprinting retrograde assemblage of actinolite, tremolite, epidote, phlogopite, and serpentine; and iii). A late retrograde stage with the deposition of large quantities of magnetite.
Similarly, the sulphide mineralisation has been split into three phases: i). Quartz-molybdenite, occurring as stockwork quartz, veinlet and disseminated molybdenite within the intrusion, representing the main stage of molybdenum introduction. Minor molybdenite also occurs near the contact in the skarn zone; ii). Quartz-calcite-pyrite-chalcopyrite-bornite-sphalerite with gold-bearing minerals, developed as stockwork veining, with quartz, calcite, chlorite and sericite in the outer intrusion and in adjacent skarn altered wall rock; and iii). calcite-galena-sphalerite with Fe-Mn carbonates occurring as veins in dolostone.
Supergene overprinting in the upper 100 m of the deposit oxidised pyrite to form limonite and jarosite with Cu, Pb and Zn oxides, accompanied by chalcedony.
Five molybdenite samples from the Yinjiagou Mo-Cu-pyrite deposit yielded ages that range from 142.9±2.1 to 143.7±2.3 Ma, with a model age weighted average of 143.4±0.9 Ma (Re-Os; Wu et al., 2014). Dating of alteration sericite gave a plateau age of 143.6 Ma and an isochron age of 143.0 Ma
(40Ar/39Ar; Wu et al., 2014).
The Yinjiagou deposit has resources of ~5.5 Mt @ 0.096% Mo for 5363 t of contained Mo metal; ~240 Mt @ 20.32% Pyrite for 48.81 Mt of pyrite; and ~5.7 Mt @ 32.28% Magnetite for 1.84 Mt of magnetite; 4. 9 t of Au at hypogene grades of 0.67 g/t Au and oxides at up to 8 g/t Au in the gossan. Cu, Pb and Zn totals are 122, 14 and 59 Kt of contained metal respectively at grades of 0.46% Cu, 0.59% Pb and 1.94% Zn, whilst the deposit contains 33.5 t of Ag at a grade of 12.7 g/t.
Yuchiling Mo, Henan Province - Cretaceous porphyry Mo deposit.
The Yuchiling porphyry Mo deposit is associated with a phase of the Heyu granite batholith which intrudes amphibolite to granulite facies metamorphic rocks of the Archaean to Palaeoproterozoic Taihua Supergroup and basaltic andesite, andesite and dacite, with minor rhyolite of the Palaeoproterozoic Xiong'er Group.
At the present level of erosion, the Heyu batholith is a concentrically zoned suite consisting of four nested, visually distinguishable phases, exposed over an area of 784 km2, namely:
• Phase 1 is a medium grained biotite monzonite dated at 143.0±1.6 Ma (zircon U-Pb; Li et al., 2011).
• Phase 2 is also a medium grained biotite monzonite, but with larger K feldspar phenocrysts that forms a narrow ring surrounding Phase 1, and has been dated at 138.4±1.5 Ma (zircon U-Pb; Li et al., 2011).
• Phase 3 is the most extensive, and is dominantly a reddish mega-porphyritic biotite monzonite aged at 132 to 135 Ma (zircon U-Pb and biotite 40Ar/39Ar; Han et al., 2007; Guo et al., 2009).
• Phase 4 is the 133.6±1.3 Ma Mo-mineralised Yuchiling porphyry (zircon U-Pb; Li et al., 2011), which intruded Phase 1 and has NW-SE elongated dimensions of ~1500 x 500 to 100 m. It is a medium-grained, reddish biotite monzonite, containing phenocrysts of 2 to 3% quartz and 3 to 5% K feldspar set in a groundmass of 25 to 35% plagioclase, 30 to 40% K feldspar, 20 to 25% quartz and 3 to 5% biotite, with accessory apatite, titanite, zircon and magnetite.
The Yuchiling porphyry is intensely altered and mineralised. Potassic alteration produced an assemblage of K feldspar, quartz and/or magnetite, imparting a pink to reddish colour to the porphyry. A range of pre-, syn- and post-mineralisation veining is hosted within the potassic altered porphyry. Phyllic alteration, which involved replacement of plagioclase, biotite and K feldspar by quartz and sericite, is locally developed and generally overprints the potassic alteration. Intergrowth of molybdenite and sericite is observed in both Mo-bearing quartz veins and their alteration halos. Propylitic alteration surrounds most of the ore-related porphyries, but is weak and only locally evident at Yuchiling.
Molybdenum is the only metal of economic value at Yuchiling. It occurs as moderately to steeply dipping veinlets and minor disseminations in the altered porphyry. The following vein stages have been distinguished, from early to late, based their crosscutting relationships, vein mineralogy and morphology, and wallrock alteration:
• Early barren veins, which are generally 0.3 to 8 cm thick, composed of quartz only or quartz-potassic feldspar, and hosted by potassic altered porphyry. Some veins and their wallrocks are overprinted by phyllic alteration. Vein relationships suggest these veins are pre-mineralisation.
• Quartz-pyrite veins, that are 0.5 to 2.5 cm thick, and include two mineral assemblages, i). quartz-pyrite and ii). quartz-pyrite-K feldspar. These veins occasionally contain minor molybdenite, sericite and fluorite. Pyrite is characteristically present as 0.6 to 5 mm euhedral to sub-euhedral crystals distributed along the vein walls or as isolated crystals. These veins are generally surrounded by potassic altered rocks, and can be cut by quartz-molybdenite veins, also indicating they are pre-mineralisation.
• Quartz-molybdenite veins which are 0.5 to 6 cm thick and are composed of quartz and molybdenite, with or without minor pyrite, as well as sericite and fluorite. They account for more than 90% of the Mo metal resource. They tend to have planar walls, and are associated with potassic±phyllic alteration. Molybdenite occurs as fine-grained, <3 mm, crystals forming discontinuous streaks, or aggregate flakes with a rosette texture along veinlet walls, or as disseminations throughout the vein quartz. Pyrite, where present, is generally disseminated in the veins, and is surrounded by molybdenite. Sericite typically occurs as 0.1 to 0.5 mm diameter aggregate clusters and as intergrowths with molybdenite. Cross-cutting relationships indicate these veins invariably post-date the early barren and quartz-pyrite stages but predate quartz-polymetallic sulphide and late barren veins.
• Quartz-polymetallic sulphide veins, which are characterised by abundant polymetallic sulphides, including pyrite, molybdenite, chalcopyrite, digenite, galena and sphalerite, with a gangue that includes quartz, sericite, fluorite and calcite. Pyrite is the earliest of these sulphides, occurring as sub-euhedral crystals, usually replaced by molybdenite, chalcopyrite, digenite and sphalerite along borders or fissures. Intergrowth of molybdenite and sericite can be observed along vein walls. Such veins are generally hosted by potassic altered zones overprinted by sericite.
• Late barren veins that include assemblages of i). quartz (chalcedony) only; ii). quartz-calcite; and iii). quartz-fluorite, surrounded by potassic or phyllic alteration. They cut all of the previously described veins and are 0.2 to 3 cm thick, and typically show comb or drusy structure. Sometimes, differently coloured fluorite is zoned in an individual vein, with green crystals on the wall, and the purple and brown in the center of the vein. A selvage of sericite, chlorite and occasional pyrite can be recognised.
Mineralisation appears to be largely confined to the Phase 4 porphyry stock, extending marginally into the surrounding Phase 1 intrusion.
Reserves are stated at ~ Mt @ 0.057% Mo for 0.54 Mt of contained Mo metal at a 0.03% Mo cut-off (Ni et al., 2012).
Donggou Mo, Henan Province - Cretaceous porphyry Mo deposit.
The Donggou porphyry Mo deposit lies within the Waifangshan Mo-Pb-Zn ore field which is also characterised by numerous surrounding Pb-Zn deposits and occurrences. The country rocks principally belong to the Palaeoproterozoic Xiong'er Group, a well-preserved, unmetamorphosed volcanic sequence, dominated by basaltic-andesite, andesite and dacite, with minor rhyolite. Age dating suggest it was deposited in the interval from 1.78 to 1.75 Ma (LA-ICP-MS zircon U-Pb; Zhao et al., 2004, 2009). This group, is subdivided, from the oldest to the youngest, into the:
• Dagushi Formation, a suite of basal conglomerate, sandstone and mudstone;
• Xushan Formation, which consists of basaltic-andesite, andesite and minor dacitic-rhyolitic lava, without intercalation of sedimentary rocks;
• Jidanping Formation, mainly composed of basaltic-andesite to rhyolite lava, intercalated with minor pyroclastic and sedimentary rocks;
• Majiahe Formation, that includes basaltic-andesite, andesite and pyroclastic and sedimentary rocks.
Structures in the district are dominated by NW- and east-west trending faults that are subsidiaries and splays of the Machaoying fault, and are crosscut by north-south trending faults controlling the distribution of the Mo and Pb-Zn mineralisation.
The Donggou Mo deposit is associated with the Donggou granite porphyry (or Bamudi porphyry; Chen and Fu, 1992), and occurs as an east-west elongated, ~205 x 50 m stock that is exposed over an area of ~0.01 km2. It lies <3 km NW of the 290 km2 Early Cretaceous Taishanmiao batholith which has a similar age and geochemical characteristics.
The Taishanmiao batholith is composed of three phases, from early to late, of coarse grained granite, fine grained granite and granite porphyry (Ye et al., 2008). All three phases have similar rock-forming minerals, namely perthite, albite, quartz and biotite, with accessory monazite, zircon, apatite, magnetite, ilmenite and fluorite. The coarse grained (3 to 7 mm), pink granite mainly occurs in the southern part of the batholith; while the fine grained (1 to 3.5 mm) phase is grey to pink and is found in the centre of the intrusion. The granite porphyry occurs in the northern part of the batholith, and has a distinctive porphyritic texture characterised by phenocrysts of quartz, perthite and minor albite. The coarse grained granite has a weighted average age of 115±2 Ma (SHRIMP zircon U-Pb; Ye et al., 2008), and is characterized by high SiO2 and K2O, and low contents of Fe, Mg and Ca, with A/CNK values of 1.09 to 1.02. It has negative Eu, Sr and Ba anomalies, positive Nb, Ta, Ce and Y anomalism, and a high ratio of LREE/HREE. It is therefore interpreted to be an aluminous A-type granite (Ye et al., 2008).
The Donggou granite porphyry contains about 15% phenocrysts, mainly plagioclase, K feldspar and quartz, set in a matrix of fine-grained K feldspar, plagioclase, quartz and minor biotite, with accessory titanite, zircon and magnetite. It has been dated at 112±1 Ma (SHRIMP zircon U-Pb; Ye et al., 2006) and 114±1 to 117±1 Ma (LA-ICP-MS zircon U-Pb; Dai et al., 2009). Geochemically, it has high SiO2 and K2O, low Fe, Mg and Ca, negative Eu, Sr, Ba and P anomalies, enrichment of Rb, U, Th, Zr, Hf, Nb and Ta, and a high ratio of LREE/HREE (Dai et al., 2009; Ye et al., 2006). Its εNd(t) and εHf(t) values range from -17.3 to -14.3 and from -10.1 to -18.7 respectively, with TDM2(Nd) and TDM2(Hf) values of 1.5 to 1.8 Ga and 1.3 to 1.7 Ga, respectively, suggesting that the magma was derived from ancient lower crust (Dai et al., 2009). The porphyry was also argued to be an aluminous A-type granite (Dai et al., 2009; Ye et al., 2006), with TDM2(Nd) and TDM2(Hf) values very similar to those of the Xiong'er Group they intrude.
Molybdenite separates from the Donggou porphyry Mo deposit yielded ages of 114 to 116 Ma (Re-Os; Yang et al., 2015), comparable to the age of the granite porphyry. Re contents of molybdenite range from 4.04 to 4.26 ppm (Mao et al., 2008; Ye et al., 2006), interpreted to suggest the Mo was mainly sourced from the continental crust (Chen et al., 2014; Li et al., 2007; Mao et al., 2008).
The Donggou granite porphyry appears to expand with depth. Minor mineralisation occurs in the outer contact zones of the altered intrusion, while the bulk of the resource is in the adjacent altered country rocks. As a consequence, the shape of the mineralised zone is controlled by the porphyry contact, which dips outward. This produces a peripheral, outward thickening, and an inner and upward tapering wedge shaped mineralised zone, with thicknesses ranging from 47 to 254 m, and a 'barren' core.
Mineralisation occurs as disseminations, stockwork veining, breccia fill and veinlets. Various kinds of veinlets can be recognised, namely, K feldspar-quartz; K feldspar-molybdenite; quartz-molybdenite; quartz-beryl-molybdenite; molybdenite only; fluorite-calcite; and calcite. Ore minerals are predominantly molybdenite, followed by pyrite, chalcopyrite, galena and sphalerite. The main gangue minerals are quartz, feldspar, epidote, beryl, biotite, sericite, chlorite, fluorite and calcite, representing the alteration assemblage.
Alteration at the Donggou deposit includes (after Yang et al., 2015):
• Potassic, resulting in the replacement of plagioclase by K feldspar, and the formation of secondary biotite and K feldspar±sulphide veinlets;
• Silicification associated with quartz±sulphide stockworks or veinlets;
• Sericitic, typified by replacement of feldspar and biotite by sericite, accompanied by disseminated pyrite and quartz-sericite veinlets;
• Propylitic, characterised by dominant hydrothermal epidote, chlorite and calcite;
• Carbonate, mainly resulting in carbonate veinlets; and
• Fluorite, reflected by disseminated purple fluorite veinlets.
Mineralisation and associated hydrothermal alteration has been divided into 3 stages (after Yang et al., 2015):
• Stage 1, characterised by the assemblage K feldspar-quartz-biotite-molybdenite veining and alteration. K feldspar-quartz and K feldspar-molybdenite veinlets are at a millimetre-scale, discontinuous and irregular. Some epidote occurs in the vein walls, some fluorite intergrown with quartz, and molybdenite is mainly in vein centres or disseminated in the altered porphyritic wall-rocks. Coeval alteration, zoned outwards, includes potassic, silicic and propylitic assemblages.
• Stage 2 is the main Mo mineralising episode, characterized by the assemblage of quartz-molybdenite-beryl with minor pyrite ±minor chalcopyrite, with the development of quartz-beryl-molybdenite, quartz-molybdenite±fluorite and/or molybdenite-only veinlets. Molybdenite occurs as a flaky crystal intergrown with biotite, or as scaly aggregates disseminated in wall-rocks and in quartz-molybdenite stockworks. Minor fluorite coexists with quartz and molybdenite in quartz-molybdenite stockwork veinlets. Silicification and phyllic alteration are the most conspicuous in stages 2.
• Stage 3 veinlets are composed of quartz±carbonate±fluorite, with little or no sulphide. Carbonate and fluorite are also developed in stage 3
Reserves quoted are ~630 Mt @ 0.113% Mo for a total of 0.71 Mt of contained Mo metal (Li et al., 2007).
Leimengou Mo, Henan Province - Cretaceous porphyry Mo deposit.
The Leimengou porphyry Mo deposit is located ~50 km NE of the Nannihu deposit and 55 km NNE of Luanchuan in Henan Province. The exposed country rock in the deposit area is mainly composed of Archaean gneiss of the Taihua Group, including biotite-plagioclase, hornblende-plagioclase and biotite-hornblende-plagioclase gneiss. Mid Neoproterozoic and Mesozoic igneous rocks are also exposed in the deposit area. The former are predominantly mafic dykes, mainly gabbro dolerite and dacite-porphyry. The Mesozoic magmatism is represented by mainly intermediate to felsic dykes of syenite porphyry, quartz porphyry and monzonitic granite porphyry, with a more extensive intrusions of granite porphyry and crypto-explosive breccia. The monzonitic granite porphyry and quartz porphyry dykes have been dated at 124±0.6 Ma and 127±1 Ma, respectively (LA-ICP-MS zircon U-Pb; Chen et al., 2011), whilst the age of the granite porphyry is 136±2 Ma (SHRIMP zircon U-Pb; Li, et al., 2006). The syenite porphyry dykes are cut by the granite porphyry intrusive and by the quartz porphyries. The granite porphyry has produced crypto-explosive breccia both internally and within the surrounding country rock. This intrusion and the breccias are temporally and spatially closely related to the Mo mineralisation.
The Leimengou granite porphyry stock, which is east-west elongated with dimensions of ~2200 x 200 to 450 m, and is exposed over an area of ~0.77 km2, resembles a downward tapering cone in cross section. The granite porphyry is composed of 40 to 50% potassium feldspar, 3 to 40% quartz, 15 to 25% plagioclase and 5% biotite, with magnetite, ilmenite, rutile and zircon. The phenocrysts account for ~10 to 15% of the porphyry and include K feldspar, quartz, plagioclase and minor biotite. Phenocrysts of K feldspar vary from 1 to 4 mm; quartz crystals are mostly anhedral granular and have a grain size of 2 to 5 mm; whilst plagioclase varies greatly, mostly ranging from 2 to 6 mm. The granite porphyry grades outwards to monzonitic granite porphyry and take on a greyish-white colour, with a massive structure and more porphyritic texture. The phenocryst
content increases to 25 to 35%, including 10 to 25% K feldspar and 10 to 20% plagioclase, with minor quartz.
The crypto-explosive breccia is intermittently developed as a hollow cylinder around the edge of the intrusion and appears to in part be the focus of the mineralisation.
Folding is not well developed in the deposit area, although faulting is important, with four main sets, east-west, NNE, NE and NW trends, with the NNE set being the best developed, increasing in intensity to the east where they truncate the east-west faults. Most of the faults are filled by later dykes.
The Mo orebody straddles the contact between the Leimengou granitic stock and enclosing Taihua Group gneiss, and is concentrated within 0 to 600 m internal to, and 0 to 300 m external to the contact. The inner and outer limits of mineralisation are gradational. The ore occurs as a series of stacked annuli to horseshoe shaped bodies, open to the south. The annuli are elongated east-west, and dip gently to the south.
The main ore minerals are molybdenite and pyrite, with minor chalcopyrite, galenite and sphalerite,in a gangue of quartz, potassium feldspar, plagioclase, sericite and biotite. The molybdenite mainly occurs as disseminations, veinlets and stockworks. Alteration products includes K feldspar, quartz, sericite, fluorite, chlorite, carbonate and kaolinite. Potassic alteration predominates, occurring mainly in the granite porphyry in the internal contact zone. The silicification best developed close to the internal contact zone of the Leimengou granite stock, whilst the sericitisation is often superimposed above the potassic and siliceous zones.
The Leimengou Mo deposit is estimated to comprise ~500 Mt @ an average grade of 0.07% Mo for ~0.34 Mt of contained Mo (Chen et al., 2011).
Qiushuwan Cu-Mo - Late Jurassic porphyry Cu-Mo deposit.
The Qiushuwan Cu-Mo porphyry-skarn deposit was the first Mo bearing deposit discovered in the northern Qinling accretionary belt in the 1980s. It was formed at ~148 Ma (Re-Os; Molybdenite) in the Late Jurassic, associated with the Yanshanian orogeny. The principal metal in the deposit is Cu, unlike the other porphyry systems in the East Qinling-Dabie belt (Mao et al., 2011), which are all Mo-rich. It is located ~120 km SE of the Nannihu, Sandaozhuang and Shangfanggao cluster of deposits.
The deposit is hosted by the Yanlinggou and Guozhuang formations of the Palaeoproterozoic Qinling Group, which are composed of biotite gneissic rocks, biotite quartz schist, plagioclase amphibolites and marble (Mao et al., 2011). It is located on the eastern margin of the northern Qinling accretionary belt, and lies to the north of Zhu-Xia fault (Guo, 2006; Guo et al., 2006), and is crosscut by several NE-trending faults, similar to the Mo deposits along the southern margin of the North China Craton. The mineralisation is hosted by a number of Mesozoic biotite granodiorite stocks that were emplaced along the intersection of these NE-trending faults and WNW-trending thrusts. These fault intersections are responsible for the typically irregular shapes of the mineralised stocks which have areal extents of about 0.06 km2 (generally ~300 x 200 m, elongated east-west; Guo et al., 2006). The Cu-Mo mineralisation of the deposit occurs as two distinct types:
• a Mo-dominant skarn type along the southern contact of the Qiushuwan granodiorite stock; and
• a Cu-dominant style within a 1000 m long, NE-striking, breccia pipe (Guo, 2006; Mao et al., 2008, 2011; Qin et al., 2012).
Cu-Mo mineralisation is associated with siliceous, K feldspar, sericitic, calc-silicate (skarn) and propylitic alteration assemblages. Mo mineralisation is associated with K feldspar alteration, whilst the Cu is related to sericitic and skarn assemblages. The principal ore minerals are chalcopyrite, molybdenite, pyrite, sphalerite, galena and pyrrhotite, whilst the main gangue minerals are quartz, epidote, calcite and diopside (Guo et al., 2006; Mao et al., 2011).
On the basis of mineral assemblages, ore fabrics, alteration assemblages and cross cutting relationships, mineralisation at Qiushuwan can be divided into (Qin et al., 2012):
• Early stage - which has been subdivided into: i). prograde skarn-K feldspar-quartz; ii). crypto-explosive breccias; iii). retrograde hydrous skarn; and iv). magnetite sub-stages;
• Middle stage - which includes a porphyry Cu(-Mo) mineralisation and quartz-sulphide phase;
• Late stage - characterised by calcite, barite and quartz.
Homogenisation temperatures of fluid inclusions from these stages, are: Early stage - 222 to 406°C; Middle stage - 152 to 315°C; and Late stage - 119 to 189°C. Salinities of these same stages are 4.2 to 36.5%; 3.3 to 34.8%; and 4.2 to 11.9%, respectively (Qin et al., 2012).
The same authors concluded that the early stage fluids responsible for the Qiushuwan deposit were high temperature, high-salinity, CO2-bearing H2O-NaCl-CO2 magmatic fluids, followed by lower temperature precipitation of metal sulphides in the middle and late stages, degrading into late low-salinity, low-temperature, CO2-poor fluid system.
The Qiushuwan granites related to Cu-Mo mineralisation are geochemically similar to other highly fractionated Jurassic I-type granites within the Qinling-Dabie orogen (Fu, 2003; Guo, 2006; Mao et al., 2008, 2011; Qin et al., 2011). High Ba-Sr characteristics of this granite were reported (Qin et al., 2011; Zhang et al., 2002), combined stable H and O isotope compositions suggesting a mantle source, implies the metallogenic components were derived from the lower crust and partly from the upper mantle (Guo, 2006; Guo et al., 2006; Qin et al., 2011, 2012; Zhang et al., 2002, 2011).
The Qiushuwan deposit has has a proved reserve of 0.5 Mt of contained Cu in ore averaging 0.8% Cu and 0.1 Mt of contained Mo in ore averaging 0.12% Mo (Guo et al., 2006; Mao et al., 2008).
Yechangping - Jurassic skarn Mo-W, Henan Province.
Yechangping is a skarn type Mo-W deposit hosted by the Longjiayuan and Xunjiansi Formations of the Mesoproterozoic Guandaokou Group. Potassic feldspar (syenite) porphyry dykes, which are up to 170 m in length and 10 to 20 m wide intrude chert banded dolostone of the Longjiayuan Formation. The porphyry dykes, which have been dated at 169 Ma, follow east-west striking faults. Wall-rock metasomatic alteration is zoned from the inner contact, outwards from skarn → barite-dolomite → ferromagnesian carbonate → silicification. Two types of mineralisation are recognised, namely an upper zone that is 150 to 230 m thick, dominated by molybdenite; and a lower group that is ~ 180 m thick and mainly contains molybdenite together with chalcopyrite and pyrite. Mineralisation occurs as veinlet, disseminated or has a granular texture. Gangue minerals include: gypsum, calcite, serpentinite, K feldspar and biotite
Reserves comprise ~100 Mt @ 0.13 to 0.15% Mo, averaging 0.133% Mo for 0.137 Mt of contained Mo, accompanied by an average 0.102% WO3 .
Huangshui'an Mo and REE district, Henan Province - Two stage Triassic and Jurassic Vein system.
The Huangshui'an Mo district is located in Songxian County, Henan Province. Like the Leimengou deposit, <10 km to the SW, the country rocks of the district are dominated by Neoarchaean to Palaeoproterozoic tonalitic-trondhjemitic-granodioritic (TTG) gneisses, hornblende plagioclase gneiss and biotite plagioclase gneiss with amphibolite and minor migmatite of the Neoarchean to Paleoproterozoic Taihua Group (Cao et al., 2014). A dense swarm of WNW-ESE to NW-SE trending, moderately to steeply NNE dipping carbonatite dykes are controlled by faults and fractures over a strike length of ~2.25 km and width of up to 750 m. The dominant fault sets are NE-, north-south and NW-trending with the latter being the most important. Dykes are common throughout the district, including (Cao et al., 2014):
• Granite porphyry, which intrude along early stage fractures are the most common;
• Quartz porphyry, are generally tens of to hundreds of metres long and crosscut and/or overprint the Mo orebodies;
• Diorite, which are mainly east-west trending, 100 to 160m long, and are overprinted by later breccia pipes;
• Carbonatite, which are locally brecciated and mainly composed of calcite and quartz, with minor K feldspar, and are controlled by NW-trending fractures.
Three, irregular, 300 to 500 m diameter, ovoid breccia pipes, centred on the denser development of a more extensive swarm of WNW-ESE to NW-SE trending Si-rich carbonatite dykes, are found in two areas in the district, each with a different suite of clasts (Cao et al., 2014). In the Pangxiegou breccia has 50 to 70% of clasts in a matrix of quartz and fluorite. The clasts are 70 to 90% calcite and quartz, with 10 to 15% K feldspar. The Mutougounao breccia, which is ~750 m to the NW along the carbonatite dyke trend, contains almost equal amounts of gneiss and quartz-calcite clasts, constituting 50 to 70% by volume and are cemented by rock flour, quartz, K feldspar and fluorite. The clasts in the third pipe, which is ~500 m to the ENE of Pangxiegou, are 70 to 80% gneiss, with 20 to 30% quartz-calcite and very rare leptite in a matrix dominated by rock flour, with minor quartz and fluorite. This latter breccia zone appears to lack significant associated carbonatite dykes.
The Huangshui'an district hosts two deposits:
• Angou deposit
Molybdenum mineralisation at Angou is hosted by swarms of metre-scale carbonate (carbonatite?) veins, clustered to form seven, irregular, parallel Mo ore zones, each with lengths of up to 700 m and widths from a few tens of metres to as much as 350 m. They generally strike west to NW and dip at 74 to 78°NE. Alteration associated with mineralisation comprise assemblages containing quartz, sericite, K feldspar, pyrite, calcite, barite and fluorite. The principal sulphide minerals are molybdenite, pyrite, galena, sphalerite and chalcopyrite (Ye et al., 2012). The eastern half of the vein system cuts through the Mutougounao area breccia.
• Dashimengou deposit
The Dashimengou deposit is composed of two main ore zones. The more significant is Pangxiegou (or D201; Li et al., 2008; Li, 2011). This ore zone encloses two breccia pipes and comprises a series of linear bodies over a length of 1400 m, with individual bodies varying from 50 to 230 m in width, distributed over a width of up to 700 m. This zone represents a continuation of the mineralised lenses in the Mutougounao area. The second ore zone, D25, is a concealed Mo orebody controlled by a fault zone. Molybdenite is the main ore mineral. It occurs as either disseminations, as thin coatings on fractures or as flake aggregates. Hydrothermal alteration includes potassic, silicification, carbonate and fluorite.
Carbonate from the vein system displays limited variations in C-O isotope values that are indicative of a mantle source (Huang et al., 2009; Cao et al., 2015). Calcite and sulphide (pyrite and galena) yield similar Pb isotope compositions that are consistent with an enriched mantle component (Huang et al., 2009). Cao et al. (2015) proposed a magmatic source for the ore-forming fluids, which are high temperature (clustering between 250 and 370°C), high salinity (up to 60 wt.% NaCl equiv.) and CO2-rich.
Dating of molybdenite samples yielded the following:
i). samples from carbonatite dykes yielded ages from 206.3±2.9 to 212.8±3.2 Ma, with a weighted mean age of 209.5±4.2 Ma (Re-Os model ages). Similarly, molybdenite samples, from quartz-calcite clasts in a breccia pipe, from carbonatite dykes and from a quartz-K feldspar vein, gave similar model ages between 207.9 Ma and 210.9 Ma, and an isochron age of 208.4±3.6 Ma (Cao et al., 2014).
ii). eight additional molybdenite samples from breccias yielded two populations, i.e. 217.1±8.5 Ma and 158.6±3.4 Ma (Li, 2014), interpreted to reflect two Mo mineralising events, one in the Triassic and the other in the Jurassic (Li and Pirajno, 2017).
The Huangshui'an Mo district is estimated to contain a resource ~230 Mt @ an average grade of 0.086% Mo for ~0.199 Mt of contained Mo (Cao et al., 2014). More recently Feng et al., 2022 quote the Huangshui'an deposit as having Mo reserves of 0.4 Mt of contained Mo with an average grade of 0.062 wt.% Mo, plus minor by products of Pb and REE (after Tang et al., 2021, Zhang et al., 2019).
Feng et al., 2022 also describe the Huangshui'an deposit from the perspective of its by-product Rare Earth Element (REE) and Pb content, as follows. Most of the Mo-Pb-REE mineralisation occurs over a strike length of ~1600 m within the carbonatite dykes with lesser amounts within breccias or the wall rock Taihua Group TTG gneisses. These mineralised zones strike at 280 to 310° and at elevations of between 890 to 320 m above sea level, dip at 75 to 82°NE in the northwest, and ∼80°SSW in the southeast (Tang et al., 2021).
Widespread silica and K feldspar alteration is associated with the deposit. Sulphide minerals include molybdenite, pyrite, galena and minor sphalerite. The REE-bearing minerals comprises monazite, bastnäsite, parisite, apatite and allanite, whilst the gangue assemblage is mainly quartz and calcite, with lesser K feldspar, fluorite and anhydrite. On the basis of crosscutting relationship and mineral assemblages, the mineralisation paragenesis can be split into two main magmatic-hydrothermal stages.
Stage I is characterised by the intrusion of carbonatite dykes and coeval potassic alteration and silicification of the surrounding rocks. This is subdivided into stage Ia that comprises pink calcite + quartz + molybdenite veins, and the later stage Ib, characterised by the pinkish potassic altered wall rock. The carbonatite is predominantly composed of coarse-grained, pinkish calcite and quartz aggregates, with minor occurrence of flaky molybdenite, galena and REE minerals.
Stage II is characterised by fluorite + quartz + white calcite alteration, with fluorite + quartz veins cross-cutting the carbonatite dykes and wall rocks. The appearance of white calcite, vuggy quartz, as well as purple, green and colourless fluorite is the main feature of this stage.
In situ bastnäsite U-Th-Pb dating results yield mean ages of 211.7 ±3.1 and 209.6 ±2.1 Ma, representing the timing of Mo-Pb-REE mineralization in the Huangshui'an deposit (Feng et al., 2022), consistent with the Triassic age detailed above.
Li and Pirajno (2017) suggest the deposits of the Huangshui'an Mo district are similar to those at Huanglongpu.
Dahu Au-Mo - Triassic to Jurassic vein deposit.
The Dahu Au-Mo deposit is located on the northern margin of the Xiaoqinling gold belt and of the Huaxiong Block, and is hosted by migmatite and biotite-plagioclase gneisses of the Taihua Supergroup. Gold and molybdenum mineralisation are associated with a series of quartz veins controlled by NEE- and NWW-trending and north-dipping faults and does not represent porphyry style mineralisation. Structural analysis indicates these faults evolved from Triassic south-directed thrusting to Cretaceous north-directed normal faulting (Zhang et al., 1998, Zhang et al., 2009). Consequently, the quartz veins were deformed, brecciated and/or mylonitised. According to Li et al. (2011), the molybdenite at Dahua occurs as disseminated aggregates in quartz veins and altered wallrocks, spotty flakes within breccias from deformed quartz vein, and thin films on or fine veinlets within breccias or on blocks of broken quartz vein. Spatial and textural relationships suggest that coarse-grained molybdenite in deformed orebodies or breccias formed in an earlier mineralisation event, whereas fine-grained molybdenite films or veinlets were deposited in a subsequent remobilisation phase, resulting in the precipitation of the fine-grained molybdenite. Gold mainly occurs as native gold inclusions in pyrite (Zhao et al., 2011), with minor inclusions in quartz, or as native gold + pyrite + galena + quartz veinlets filling fissures in cubic pyrite. Minerals coeval with native gold, which include pyrite and galena, show no evidence of structural deformation.
Dating of hydrothermal monazite intergrown with early-stage molybdenite yielded an initial precipitation age of 216±5 Ma ( SHRIMP U-Th-Pb; Li et al., 2011), which is very close to the molybdenite Re-Os isochron age of 218±41 Ma (Li et al., 2008), suggesting the Dahu deposit was first formed in the Late Triassic, Indosinian Orogeny. The monazite also provided SHRIMP U-Th-Pb ages clustering at 185 to 135 Ma, with minimum value of 125 Ma, suggesting the Dahu mineral system was reworked by a Yanshanian tectono-thermal event that resulted in large-scale gold mineralisation in the Xiaoqinling terrane (Li et al., 2011).
The deposit apparently contains ~0.10 Mt of contained Mo at an average grade of 0.24% Mo. This summary and resource figure is drawn from Ni et al. (2012) and Li and Pirajno (2017). See also the more detailed description in the Xiaoqinling Gold Province record.
Zhifang - Triassic orogenic vein.
The Zhifang Mo deposit is located in the Huaxiong Block of the northern Qinling Orogen, hosted by the Jidanping Formation of the Xiong'er Group that comprises dacites and rhyolites interlayered with basaltic andesites and andesites (Wen et al., 2008). Molybdenum mineralisation is associated with a series of quartz veins controlled by NW-, NE-, and minor north-south and east-west trending faults that are interpreted to be splays of the regional Machaoying fault system. Minor intrusive dykes are also exposed, including the Jingningian diorite, Hercynian syenitoids and Yanshanian granitoids (Deng et al., 2016).
Six mineralised bodies have been delineateded at Zhifang, all of which are structurally controlled and/or stratabound veins. Individual vein systems vary from 100 to 2800 m in length and 1 to 8.2 m in thickness, extending down dip to >550 m below the surface.
The mineralised bodies have associated silicic and potassic alteration, and display a complex mineral assemblage. The principal ore minerals are molybdenite and pyrite, with minor galena, chalcopyrite and sphalerite. Gangue minerals are principally quartz and K feldspar with minor calcite, fluorite, barite, rutile, apatite, sericite and chlorite. Molybdenite occurs as clotty and/or banded quartz veins filling fractures, or as a film coating fractures and quartz crystal. Mineralisation occurs in three stages based on crosscutting relationships and mineral association (Deng et al., 2014): i). Early stage, characterised by fractured or deformed quartz-veins with minor idiomorphic granular pyrite and allotriomorphic barite; ii). Middle stage, with formation of quartz + molybdenite + pyrite + chalcopyrite + galena + sphalerite, controlled by the early stage fractures; and iii). Late stage, represented by quartz, carbonate and minor pyrite.
Five molybdenite separates from the mineralised veins yielded individual ages from 241.2±1.6 to 247.4±2.5 Ma, with a weighted mean Triassic age of 243.8±2.8 Ma (Re-Os isotope model; Deng et al., 2016). Deng et al. (2016) propose the Zhifang Mo deposit is an orogenic-type vein system.
The Zhifang Mo deposit is estimated to contain between 0.1 and 0.5 Mt of Mo at grades that range from 0.07 to 0.16%, averaging 0.12% Mo (Deng et al., 2016), in between >80 Mt of ore.
Yindonggou Ag-Au-Mo - Silurian Mo bearing Ag-Au-base metal vein system.
The Yindonggou Ag-Au-Mo fault-controlled vein deposit is located in Neixiang County, Henan Province, within the Xiaguan Ag-dominant poly-metallic ore-field. It is hosted in the volcanosedimentary Erlangping Group of the Erlangping Terrane. The Erlangping Group contains, from the base, the:
• Xiaozhai Formation - mainly composed of sericite-quartz schist, biotite-(muscovite)-quartz schist and biotite-(sericite)-plagioclase schist, interpreted to represent metamorphosed flysch protoliths with carbonate interbeds (Wang et al., 2003).
• Huoshenmiao and Damiao formations - spilitic-keratophyre and gabbroic rocks (Zhang et al., 2016).
These lithostratigraphic units were intruded by multiphase granitoids, including the 482±30 Ma Muhuding granite (Rb-Sr isochron; Wei et al., 2003), 459.5±0.7 Ma Manziying granite (zircon U-Pb; Guo et al., 2010), 446±7 Ma Lujiaping granite (zircon U-Pb; Li et al., 2012), the concealed ~190 to 140 Ma Songduo granite (Wang et al., 2003) and the ~140 Ma Erlangping biotite monzonitic granite (Rb-Sr isochron; Chen et al., 1996).
The Xiaguan district contains 53 Ag-Pb-Zn mineralised zones, 39 of which are regarded as economic. These vein systems are oriented NE-, north-south, east-west and NW and controlled by splays from the Zhu-Xia fault (Wang et al., 2003). North- to NE-trending veins dominate. Three main vein systems appear to be developed at Yindonggou; Y1 which is the largest, is 1800 m long, 0.5 to 1.7 m thick and 390 m deep, containing 80 to 446 g/t Ag, 0.37 to 10.04 g/t Au, with average Pb and Zn grades are 1.65 and 1.45% respectively. It is oriented east-west to NW-SE and dips at 16 to 50°, and is mainly within the schists of the Xiaozhai Formation. Y2 and Y3 have similar orientations and are hosted by spilites, with lengths and thicknesses of 1200 x 0.3 to 1.5 m and 1600 x 0.8 to 1.4 m respectively. Five molybdenite samples from the deposit yield ages of 423.4 ±4.4 to 432. 2±3.4 Ma, with an average of 429. 3±3. 9 Ma (Re-Os isotope model; Li et al., 2009).
Tumen Mo-F - Neoproterozoic vein system.
The Tumen Mo-F deposit comprises 1 to 10 m thick veins clustered in a number of localities the Tumen area. These veins are hosted by dolostone and schist assigned to the Neoproterozoic Luanchuan Group, and can be traced for several kilometers along a NW-trending fault zone. The veins carry 60 to 94% fluorite and were initially mined to depths of <250 m, with a minimum reserve of 2 Mt of fluorite (Deng et al., 2014). The molybdenite content of the fluorite veins increases with depth with grades approaching 10% Mo.
At least four mineable Mo veins and vein clusters have been identified in the Tumen district. The mineralised bodies are lenticular to tabular in shape, and are spatially controlled by NW-trending faults. They are associated with syenite dykes which intrude the transitional contact zone between quartz-sericite schist and dolostone. The most significant of these is 1300 m long by 60 to 70 m thick, and dips at 50 to 70° NE. The average or background Mo grades are 6.03 ppm in quartz sericite schist, 3.75 ppm in dolostone and 16.20 ppm in the syenite dykes (Ye et al., 2004).
The ore minerals are predominantly molybdenite and pyrite, with lesser sphalerite, galena and chalcopyrite. The gangue includes fluorite, calcite and quartz, with minor biotite, muscovite and sericite. Hydrothermal alteration is characterised by silica, fluorite, sericite, carbonate and kaoline, concentrated along faults, without an obvious lateral zonation, although the presence of silicate increases at depth. Based on mineral assemblages and crosscutting relationships, the mineralisation has been divided into four stages by Deng et al. (2013).
• Stage 1 veins are composed of brecciated white to green fluorite;
• Stage 2 veins contain purple fluorite-molybdenite-pyrite(-galena), and include two varieties, Stage 2a with purple, anhedral, 0.05 to 8 mm wide fluorite-rich veins without sulphides, Stage 2b purple veins containing pyrite and molybdenite. There are no crosscutting relationships between these two varieties making it impossible to determine the paragenesis.
• Stage 3 veins are characterised by the assemblage of calcite-galena-pyrite-sphalerite (-chalcopyrite),
• Stage 4 veins are barren carbonate.
The Mo-F veins have been dated at 844 ±2 Ma (Nd/Nd; Bao et al., 2008), whilst the associated syenite dyke returned and age of 847±7 Ma (Deng et al., 2013).
The economic reserves of molybdenite are at a depth >150 m with grades averaging from 0.05 to 3% Mo,for an inferred minimum resource of 0.01 Mt of contained Mo (Ye et al., 2004).
Zhaiwa Mo-Cu - Palaeoproterozoic porphyry/intrusion-related deposit.
The Zhaiwa Mo-Cu deposit is hosted by the Taihua Supergroup within the Huaxiong block. The deposit was first discovered as high grade Cu-bearing veins, locally containing >10% Cu regarded as possibly representing a larger porphyry-type Cu(-Ag-Au) deposit (Yang et al., 2003). Subsequent prospecting has outlined an Mo-dominated poly-metallic system, with an average grade of 0.064% Mo and a total inferred resource up to 0.1 Mt of contained Mo metal (Deng et al., 2012, Li et al., 2009). Mineralisation largely occurs as quartz veins developed following faults surrounding a concealed granitic stock interpreted from aeromagnetic data (Wang et al., 1997). The 58 to 80° dipping veins are hosted by amphibolites and gneisses, and are several hundred metres long by 2 to 10 m thick. These veins are commonly coarse-grained, have massive, brecciated, banded and disseminated structures, and form stockworks or disseminated veinlets (Deng et al., 2012). Alteration associated with the Mo-Cu mineralisation includes assemblages of silica, K feldspar, epidote, sericite, chlorite, carbonate and fluorite (Deng et al., 2012).
Molybdenite occurs as films or scaly euhedral crystals, with accompanying pyrite, sparse chalcopyrite, sphalerite, pyrrhotite, native bismuth, bismuthine and galena. Gangue minerals include quartz, with minor K feldspar, epidote, fluorite, muscovite, calcite and chlorite. Three types of veinlets are identified from early to late stages, characterised by i). quartz-molybdenite; ii). quartz-polymetallic sulphides and iii). quartz-carbonate (Deng et al., 2012). Homogenisation temperatures of fluid inclusions range from >500 to 370°C, 360 to 180°C, and 180 to 120°C respectively for these three stages (Deng et al., 2012). Other fluid inclusion data indicate the fluids initially had high salinity and high temperature, suggesting they originated from magmas (Chen et al., 2007, Mernagh et al., 2007). The Mo-Cu veining is interpreted to represent the upper or outer zones of a porphyry system (Deng et al., 2012, Yang et al., 2003).
Six molybdenite separates from samples collected from the deposit yielded individual ages from 1734.5±22.3 to 1787.9±30.1 Ma, with a weighted mean age of 1757±24 Ma (Re-Os isotope; Deng et al., 2009).
This mineralisation is developed in the tectonically reworked Trans North China Orogen where it is overlapped by the northern margin of the Qinling Orogen and may represent a much earlier mineralising event fortuitously incorporated in the Qinling Molybdenum Belt, as apparently also seems to be the case for Tumen as described above. Other, but earlier Palaeoproterozoic porphyry Cu-Mo mineralisation has been identified well to the north of the Qinling Orogen within the Trans North China Orogen, dated at 2108±32 Ma (molybdenite Re-Os isotopic isochron) and 2122±10 Ma (U-Pb; zircon) in the Zhongtiaoshan District.
The Zhaiwa Mo-Cu deposit is estimated to comprise ~160 Mt @ 0.064% Mo with an inferred resource of 0.1 Mt of contained Mo (Deng et al., 2012; H.M.
Li et al., 2009; J. Li et al., 2009).
NOTE; Reserves/resources quoted in the descriptions above are Chinese estimates quoted in academic papers and may not be compliant to JORC, NI 43-101 or equivalent conventions. Resources quoted in the original sources are as tonnes of contained metal and average grade. Ore tonnages quoted above have been back calculated from these figures assuming no recovery criteria are included in estimation of the contained metal reserves.
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2019.
This description is a summary from published sources, the chief of which are listed below.
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Jinduicheng and Shijiawan
Nannihu, Sandaozhuang and Shangfang
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Wu, Y.-B. and Zheng, Y.-F., 2013 - Tectonic evolution of a composite collision orogen: An overview on the Qinling-Tongbai-Hongan-Dabie-Sulu orogenic belt in central China: in Gondwana Research v.23, pp. 1402-1428.|
Xu, C., Kynicky, J., Chakhmouradian, A.R., Qi, L. and Song, W., 2010 - A unique Mo deposit associated with carbonatites in the Qinling orogenic belt, central China: in Lithos v.118, pp. 50-60.|
Yang, Y., Liu, Z.-J. and Deng, X.-H., 2017 - Mineralization mechanisms in the Shangfanggou giant porphyry-skarn Mo-Fe deposit of the east Qinling, China: Constraints from H-O-C-S-Pb isotopes: in Ore Geology Reviews v.81, pp. 535-547.|
Yang, Y.-F., Chen, Y.-J., Piranjo, F. and Li, N., 2015 - Evolution of ore fluids in the Donggou giant porphyry Mo system, East Qinling, China, a new type of porphyry Mo deposit: Evidence from fluid inclusion and H-O isotope systematics: in Ore Geology Reviews v.65, pp. 148-164.|
Yang, Y.-F., Li, N. and Chen, Y.-J., 2012 - Fluid inclusion study of the Nannihu giant porphyry Mo-W deposit, Henan Province, China: Implications for the nature of porphyry ore-fluid systems formed in a continental collision setting: in Ore Geology Reviews v.46, pp. 83-94.|
Zhang, W., Chen, T., Gao, J.-F., Chen, H.-K. and Li, J.-H., 2019 - Two episodes of REE mineralization in the Qinling Orogenic Belt, Central China: in-situ U-Th-Pb dating of bastnasite and monazite: in Mineralium Deposita v.54, pp. 1265-1280.|
Zhang, Y., Zhang, S., Xu, M., Jiang, X., Li, J., Wang, S., Li, D., Cao, H., Zou, H. and Fang, U., 2015 - Geochronology, geochemistry, and Hf isotopes of the Jiudinggou molybdenum deposit, Central China, and their geological significance: in Geochemical Journal, v.49, pp. 321-342.|
Zhang, Z., Yang, X., Dong, Y., Zhu, B. and Chen, D., 2011 - Molybdenum deposits in the eastern Qinling, central China: Constraints on the geodynamics: in International Geology Review v.53, pp. 261-290|
Zhao, S.-R., Li, J.-W., Lentz, D., Bi, S.-J., Zhao, X.-F. and Tang, K.-F., 2019 - Discrete mineralization events at the Hongtuling Au-(Mo) vein deposit in the Xiaoqinling district, southern North China Craton: Evidence from monazite U-Pb and molybdenite Re-Os dating: in Ore Geology Reviews v.109, pp. 413-425.|
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