West Qinling Gold Province - Yangshan, Liba Goldfield (Jinshan, Maquan, Zhaogou, Wawugou, Magou), Maanqiao, Liziyuan, Shuangwang, Baguamiao, Pangjiahe, Daqiao, Huachanggou, Ludousou, Zaozigou, Gangcha
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The West Qinling Gold Province lies within the West Qinling Orogen in the Qinling Mountains of Shaanxi and Gansu Provinces, central China. Its smaller eastern extension, the Xiaoqinling Gold District overlaps into the Qinling-Dabie Orogen. The province includes over 50 gold deposits (excluding placer accumulations) varying from the large Yangshan, with a resource of 308 t Au, to small occurrences with resources of <1 t Au (Chen et al., 2004). It also includes the:
Liba Goldfield, which includes the Jinshan, Maquan, Zhaogou, Wawugou and Magou deposits,
Maanqiao, (Ma'naoke), Liziyuan, Shuangwang and Zaozigou orogenic deposits and goldfields which together contain >500 tonnes of Au (Zeng et al., 2012). Resources are listed at the end of the record.
The gold mineralisation of the West Qinling Gold Province is confined to a WNW trending belt of highly-deformed and regionally metamorphosed, mainly Devonian and Carboniferous flysch, over a length of 600 km and width of ~150 km within the West Qinling Orogen. The laterally contiguous Qinling-Dabie, West Qinling and East Kunlun orogens are the eastern, central and western segments respectively that collectively make up the greater Central China Orogen (see the Tectonic Setting map below). Together they extend for >3000 km, east-west, across central China, from the the major, north-south Tan-Lu Fault in the east, to the ENE-WSW trending Altyn-Tagh Fault Complex to the west. The Qinling-Dabie Orogen in the east separates the North China and Yangtze cratons. The East Kunlun Orogen to the west, separates the Qaidam Block, a cratonic fragment to the north, from the Bayan-Har Terrane of the Songpan-Ganzi Complex/Terrane to the south. The West Qinling Orogen grades laterally into the Qinling-Dabie and is faulted against the East Kunlun orogens on its two extremities. It lies to the east of the Qaidam Block, and is bounded to the north by the Qilian Block, another cratonic fragment that separates the Quidam Block and North China Craton in the west. To the south, the West Qinling orogen is bordered by the Songpan-Ganzi Complex, mainly composed of Mesozoic turbidites and volcanic rocks overlying Proterozoic basement.
The West Qinling Orogen has undergone a complex tectonic history that involved the closure of the Proto- and Palaeo-Tethys oceans between the North and South China composite blocks during the Palaeozoic and early Mesozoic respectively. These closures were accompanied by north- to north-eastward subduction of the ocean basin below the North China Craton in the eastern extremity of the orogen, and the Qilian cratonic terrane over the rest of its length (e.g., Zhang et al., 2001). The narrow, elongate, eastward tapering Qilian Terrane had collided with and been accreted to the southern margin of North China Craton during the early Palaeozoic to become part of the composite North China Block. Within the adjacent Qinling-Dabie Orogen to the east, it is only a narrow ~20 to 50 km wide sliver that obliquely crosses into the orogen, where it is known as the North Qinling Terrane. The northward subduction related to the closure of the Proto-Tethys ocean during the Palaeozoic, progressed to continental collision between the North and Yangtze cratons (and intervening North Qinling Terrane) in the east, and the Qilian terrane and basement to the Songpan-Ganzi Complex/Terrane elsewhere. The Yangtze Craton and basement to the Songpan-Ganzi Terrane constitute the composite South China Block.
The West Qinling Orogen is tectonically bounded by i). the North China Craton and Qilian Terrane to the north, marked by the Lingbao-Lushan-Wuyang Fault in the east, which becomes the Linxia-Wushan-Tianshui Fault to the west (see the Structural and geological setting map below); and ii). the Songpan-Ganzi Terrane and South China Craton to the south, where it is juxtaposed across a broad, up to 20 km wide tectonic and ophiolitic mélange known as the Mianlue Suture in the east. Further west, after an intervening gap along the same structure, the mélange is redeveloped as the A'nyemaqen Suture (Yang et al., 2015; Duan et al., 2016; Qiu et al., 2018). Within the orogen, the Kuanping, Shangdan and Mianlue-A'nyemaqen suture zones separate a number of individual terranes. The Kuanping Suture is only found in the Qinling-Dabie Orogen and eastern extremities of the West Qinling Orogen. It separates the Huaxiong Block in the north from the North Qinling/Qilian terranes before being truncated by the Lingbao-Lushan-Wuyang Fault. The Huaxiong Block is a west tapering sliver of remobilised North China Craton Archaean to Proterozoic rocks that is well developed in the Qinling-Dabie Orogen to the east, and is separated from the North China Craton by the Lingbao-Lushan-Wuyang Fault. The Kuanping Suture on its southern margin is marked by the Neoproterozoic Kuanping ophiolite unit, mainly comprising greenschist and amphibolite facies volcanic rocks (Qiu et al., 2019). The Palaeozoic Shangdan Suture separates the North Qinling/Qilian and South Qinling terranes within the Qinling-Dabie Orogen, but in the eastern West Qinling Orogen it approaches and merges with the Lingbao-Lushan-Wuyang Fault. It represents the closure of the Proto-Tethyan Ocean during the early to mid Palaeozoic and is defined by outcrops of ophiolitic assemblages, and subduction-related volcanic and sedimentary rocks (Dong et al., 2016). The Mianlue-A'nyemaqen Suture marks the South Qinling Terrane to South China Block boundary (e.g, Dong et al., 2016; Qiu et al., 2016). It is reflected by discontinuously exposed ophiolite sequences, ocean-island basalt, and island-arc volcanic rock units, and marks the Early Triassic closure of the Palaeo-Tethys Ocean (Dong et al., 2011; Deng et al., 2014; Li et al., 2018). All of these structures continue to the east into the Qinling-Dabie Orogen - see the Regional Setting section of the Qinling Molybdenum Belt record for a detailed description of the Qinling-Dabie Orogen. The equivalent continuation of the structural framework to the west in the East Kunlun Orogen is outlined in the Regional Setting section of the Xiarihamu record. For details of the Qaidam and Qilian Blocks and their interactions, see the Qilian and Qaidam Blocks and Qilian and North Qaidam Orogenic Belts paragraphs of the Regional Setting section of the Jinchuan record.
Three main deformation episodes have been distinguished within the orogen, related to the convergence of the South and North China blocks, reflecting the terranes and tectonic features described above: i). an early to middle Palaeozoic accretionary event along the southern margin of the North China Block, reflected by the Shangdan Suture, representing the closure of the Proto-Tethyan Ocean; ii). a late Palaeozoic to Triassic collisional event along the Mianlue suture zone, resulting in the closure of the Palaeo-Tethyan Ocean and the amalgamation of the South China block with earlier accreted terranes to its north. Peak metamorphism and deformation along the Mianlue-A'nyemaqen Suture Zone is best estimated at ~ 220 Ma (Qiu et al., 2019); iii). Jurassic to Cretaceous intracontinental tectonism related to post orogenic collapse and extension (Dong et al., 2016; Li et al., 2017; Qiu et al., 2017).
Between the Shangdan and Mianlue sutures, the South Qinling Terrane comprises two regional anticlinal structures that expose Palaeozoic sequences, separated by a syncline cored by a succession of less intensely faulted and folded, mainly Triassic meta-sandstone, slate and marble. The boundary between these two sequences on the limbs of the regional fold structures is defined by major, regional, SSW vergent thrusts. To the south, the Palaeozoic succession of the southern of the two anticlines overthrusts another sedimentary succession of Triassic age that separates it from the Mianlue Suture. The latter is reflected by a wide tectonic and ophiolitic mélange zone. All three regional folds appear to plunge to the WNW, with the Triassic sequence becoming dominant, overlying the Palaeozoic succession (e.g., Qiu et al., 2019).
The stratigraphy of the Orogen from the Proterozoic to the Late Triassic is summarised as follows (after Li et al., 2019):
• Proterozoic volcanic and sedimentary rocks, confined to the North Qinling Terrane, between the Kuanping and Shangdan sutures in the eastern extremities of the Orogen, as well as reworked Neoarchaean to Neoproterozoic volcanic and shelf facies sedimentary sequences of the Huaxiong Block. Proterozoic rocks are also exposed in the Bikou Block that separates the Mianlue Suture from the South China Craton to the SE and may be both an offset section of the South China Craton and an exposed segment of the basement to the eastern Songpan-Ganzi Complex;
• Cambrian to Ordovician slate and carbonaceous meta-sedimentary rocks, found to the north, in the eastern section of the North Qinling Terrane, overlying Proterozoic basement;
• Cambrian to Silurian carbonaceous and argillaceous slate that occurs in the core of the southern of the two major anticlines described above, and extending to the east into the Qinling-Dabie Orogen;
• Devonian phyllite, marble and metasandstone that constitute the bulk of the northern regional anticline and section of the southern, and occur within the Minalue Suture mélange zone.
• Carbonaceous to Permian marble and meta-sandstone that occurs to the south and west in both anticlines. Locally, pillow basalts are intercalated with Permian reef limestone. These rocks were strongly folded during the Triassic;
Most of the Palaeozoic sedimentary rocks in the South Qinling Terrane are interpreted to represent shallow fore-arc successions (Jin et al., 2017), whereas the late Permian to early Triassic rocks were formed in an extensional setting (Li et al., 2014).
• Triassic bathyal to continental slope facies siliciclastic meta-sandstone, slate and marble that dominates in the regional syncline, south of the southern anticline and to the NW. Triassic volcanic rocks, mainly rhyolite with minor trachyte and dacite are found closely associated, surrounding and intruded by the Triassic granitoids in the northwestern part of the orogen. Some of these have been dated at ~229 Ma.
• Jurassic to Neogene - Following the Triassic collisional orogenesis, post-orogenic extensional collapse occurred through the Early and Middle Jurassic within the East and West Qinling orogenic belts, producing a series of rift basins. Subsequently, in the Late Jurassic and Early Cretaceous, a NE-SW compressive stress field produced large scale sinistral strike slip faulting, intense uplift and intracontinental escape tectonics at the junction of the East and West Qinling orogens, including SW lateral escape of the Bikou Terrane. At the same time strike-slip related Early Cretaceous intracontinental sedimentary basins formed with a dextral en echelon pattern within sinistral shear zones along a major, near east-west fault. This was followed by compressive uplift in the Late Cretaceous (e.g., Li et al.< 2013). The Early to Middle Jurassic rift basins contain sequences that are several hundred to 1500 metres in thickness, comprising thick basal brown, grey and purple conglomerates with sub-angular clasts, passing upward into grey pebbly sandstones, coarse sandstones with shale and carbonaceous shale interbeds and coal seams. This sequence represents a braided river to lacustrine regime. The succeeding Cretaceous sequences unconformably overlie earlier rocks and commence with subangular to angular, poorly sorted, matrix-supported purple massive breccia and platy gravels of mixed sizes, cemented by a muddy matrix. These conglomerates may be up to 1000 m in thickness. These are overlain by bedded pebble conglomerates, pebbly coarse sandstones, sandstones interbedded with thinly bedded pebbly siltstones and mudstones. The third member of this sequence is composed of conformably overlying black-grey and grey-green mudstone, muddy siltstone with sandstone interbeds, conglomerate, carbonaceous shale, coal seams and limestone (e.g., Li et al.< 2013).
A range of granitoid intrusions are recognised within and marginal to the orogen. These include Neoproterozoic plagio-granite and quartz diorite within the northern protuberance of the South China Craton that forms the neck separating the Qinling-Dabie Orogen to the east, from the West Qinling Orogen to the west. The second composite intrusive event reflects a Late Ordovician to Devonian and Carboniferous arc composed of a range of intrusions, including monzogranite to alkali granite dated at 455 ±2 Ma, with a Carboniferous outlier of 307 ±5 Ma; 438 ±3 Ma granodiorite; 414 ±13 Ma plagiogranite, 414 ±2 Ma granite to monzogranite within both the Qilian Block and North Qinling Terrane in the east, and tapering to the west to a few small, scattered stocks. These represent an arc related to the early to middle Palaeozoic accretionary event.
The remainder of the intrusive rocks of the Orogen are Mesozoic granitic intrusions, scattered throughout the West Qinling Orogen, mostly as small stocks or dykes, although some larger intrusion up to 100 km long batholiths are evident. The composition of the intrusions ranges from diorite to granite, belonging to the high-K calc-alkaline series, with in situ zircon U-Pb ages of 249 to 200 Ma (Qin et al., 2009; Dong and Santosh, 2016; Sui et al., 2018). Yang et al. (2015) subdivided these into the following: i). Early Triassic ~245 Ma granodiorite and ~238 Ma alkali granite to monzogranite, both found in the north-central to northwestern segment of the orogen; ii). Late Triassic granodiorite within the Bikou Block dated at ~226 to 223 to 215 Ma; iii). Late Triassic alkali granite to monzogranite dated at two locations as ~223 and ~213 Ma, confined to the eastern half of the South Qinling Terrane; iv). Late Triassic, ~221 Ma, granodiorite that overlaps the South Qinling Terrane and the Mianlue mélange zone.
Whilst the extensive Central China Orogen stretches across central China, the individual constituent segments, the Qinling-Dabie, West Qinling and East Kunlun orogens, each represent a differing tectonic regime. The Qinling-Dabie Orogen represents the part of the orogen that is immediately sandwiched between the two cratons and following collision, have undergone subduction of continental crust, and its consequent ultra-high pressure (UHP) metamorphism, followed by relaxation and exhumation. The East Kunlun Orogen is bounded to the north over its entire length by the cratonic Qaidam Block, which, in turn, is separated from the Qilian cratonic sliver to the north by the North Qaidam Belt. The Qilian sliver is separated from the Yinshan Block, the western extension of the North China Craton, by the Qilian Orogen. The latter is a complex of oceanic and continental sutures, and has been overthrust onto the Yinshan Block. During the Lower to Middle Palaeozoic, crustal shortening was accommodated by subduction within the Qilian, North Qaidam and East Kunlun orogenic belts that advanced to continental crust subduction and UHP metamorphism in all three. In contrast, the West Qinling Orogen appears to lack intervening microcontinents between the Qilian Block to the north and the continental basement of the Songpan-Ganzi Complex to the south. Crustal shortening appears to have been accommodated by subduction of oceanic crust below the Qilian Block representing a southeasterly continuation of the North Qaidam Belt, but lacked continental collision, except in the transition zone to the east with the Qinling-Dabie Orogen where more intense magmatic activity is evident. As a consequence, deposition within the West Qinling Orogen during much of the Lower to Middle Palaeozoic is composed of shelf facies sandstones to argillites and carbonates deposited in a broad fore-arc basin, grading north into an accretionary wedge. In all three orogenic segments, the Palaeozoic compressive regime was succeeded by orogenic collapse and extension, followed, in turn, by Triassic compression and subduction represented by sutures along the South Kunkun Fault in the East Kunlun Orogen, which is continuous with the A'nyemaqen-Mianlue Suture in the West Kunlun Orogen. This suture is offset across the Late Jurassic to Early Cretaceous transcurrent Longmenshan Fault before entering the Qinling Dabie Orogen.
The West Qinling orogenic belt is endowed with tens of significant gold deposits and >50 smaller occurrences that are mainly hosted in variably metamorphosed Palaeozoic to early Mesozoic marine sedimentary rocks, with lesser deposits occurring within Triassic intermediate to felsic dykes (Chen and Santosh, 2014; Jin et al., 2017; Sui et al., 2018). Most have been classified as orogenic, Carlin-style, or Carlin-like gold deposits based on the mineralisation style, alteration assemblages and geochemical characteristics (Chen et al., 2004; Liu et al., 2019; Li et al., 2019).
A selection of the important gold deposits of the Orogen are described below.
The Yangshan gold deposit, Wenxian county, Gansu province, contains 308 t Au with average grade of 4.74 g/t Au. It is a syn-collisional Carlin-like gold deposit, with orebodies controlled by an east-trending shear-zone and hosted in Devonian carbonaceous carbonate-phyllite-slate sequence or granite-porphyry dykes intruding into the Devonian strata. Isotopic ratios from fluid inclusions within quartz separates, suggest that the ore-fluids have been mainly sourced, through metamorphism and/or reworking, from the Devonian strata or/and similar lithologies which comprise carbonaceous phyllite, slate, chert and carbonate, with a significant input of meteoric water. In general, the ore-forming fluid-system varies from early, deep, metamorphic fluid to late, shallow, meteoric water (Chen et al., 2008). For more detail see the separate Yangshan record.
The sedimentary- and granite porphyry-hosted Pangjiahe gold deposit is located in the north belt of the South Qinling Orogen, with a proven reserve of 38 t of contained Au at an average grade of 6.3 g/t. The deposit is located in the NW corner of the Fengxian-Taibai graben basin. Gold mineralisation is predominantly
hosted in the Upper Devonian Dacaotan Formation, which locally comprises euhedral pyrite-bearing phyllite and sandstone. Euhedral, granular pyrite occurs in both altered and unaltered phyllite and sandstones of Dacaotan formation. The host sedimentary rocks were weakly metamorphosed to sub-greenschist facies
during the Triassic orogeny, when a number of approximately east oriented faults with localized shearing and the Pangjiahe anticline were developed. The Pangjiahe anticline (axial plane striking 75 to 85° and dipping <70 to 75°) is the principal structural feature in the mine. Five subsidiary
east-striking strike-slip faults are mapped in the mining area, hosting five gold ore bodies outcropping at surface and a sixth that is not exposed. The mineralised faults predominantly dip to the south with high angle (<65°to near vertical to the north). They range from between 170 and 360 m in length, and are 0.1 to 12.5 m thick. All six ore bodies are hosted by discrete zones of strongly foliated phyllite rocks, suggesting that mineralisation occurred either during or after deformation. Gold mineralisation predominantly occurs in veins of zoned auriferous pyrite and arsenopyrite that are parallel with the phyllite foliation. Pyrite and arsenopyrite within these veins do not show any evidence of deformation (Ma et al., 2018).
The Liushagou granite stock is situated 500 m west of the deposit, with two other intrusive phases recognised in the main deposit area, namely granite porphyry and dolerite dykes. The granite porphyry dykes are predominately in the western part of the deposit. They strike east-west and extend for between 20 and 300 m along strike, with widths of 1 to 10 m, and are normally parallel to the sediment hosted lodes. The granite porphyry dykes have sharp contact with the host rocks and are locally mineralised with up to 4 g/t Au, although this is only a minor component of the ore. The dolerite dykes are predominantly found in the eastern part of the deposit, and also strike east-west with lengths of between 5m and >100 m and widths of <<1 to 3 m. They cross the ore lodes and have sharp contacts and chilled margins (Ma et al., 2018).
Baguamiao, which contains >80 t Au @ 3.5 to 6 g/t Au, is one of the largest orogenic gold deposit in the Qinling Gold Province occurring as a main ore zone of about 1.7 km along strike, 50 to 160 m wide, and a 500 m down dip extent. The major ore minerals, comprising about 5% of the hydrothermal phases, are pyrrhotite, pyrite and secondary marcasite with lesser sphalerite, galena, tellurobismuth and gold. Alteration halos that surround the orebodies are characterised by a broad bleaching zone of white mica and ankerite, and a proximal zone of silicification with abundant sulphide minerals. Biotite, albite and tourmaline are locally developed.
The Liba goldfield in Gansu province, contains a gold resource of 87 tonnes (2.8 Moz), and is located to the NE of the Zhongchuan Granite of the West Qinling Orogen. The structurally controlled gold mineralisation is located ~2 km NE of the Zhongchuan Granite, and is hosted by Devonian metasedimentary rocks, comprising metamorphosed siltstone, sandstone, mudstone and shale, assigned to the Shujiaba Formation, which have been intensely deformed to phyllitic rocks that commonly trend ESE (Cheng and Zhang 2001). Others deposits in the region include the Jinshan and Maquan gold deposits that are located 2 to 5 km south of the 30 km diameter Zhongchuan Granite, and are hosted by the Devonian Xihanshui and Carboniferous Xiajialing groups respectively. The Shujiaba Group, referred to as the Liba Group by Li (1999), comprise a ~5000 m thick succession of clastic-dominant flysch deposits, including siltstone, slate, micrite, sandstone and quartzite (Zhang et al., 2004). By contrast, the Xihanshui Group, regarded as a lateral equivalent of the Liba Group, is a 5800 m thick succession of carbonate with interbedded clastic units, interpreted as a shallow continental shelf deposit (Jin and Li 1996).
The Devonian rocks are overlain by metamorphosed Carboniferous shale and siltstone to the NW and SE of the Zhongchuan Granite, and metamorphosed Cretaceous sandstone and conglomerate to the NE, SE and SW of the granite. The Jurassic and Carboniferous rocks in the SE are in fault contact. The Cenozoic successions form isolated outcrops in the east and SW where they comprise reddish terrigenous clastic units that total 2000 m in thickness. Up to 50 m thick Quaternary eluvial and alluvial sediments contain small placer gold deposits locally. Sedimentary rocks in the region have been regionally metamorphosed to greenschist-facies.
Mineralisation in the Liba Goldfiled is associated with silica-sericite-chlorite-carbonate alteration, which have also affected most of the dykes, suggesting that most of the dykes are pre- to syn-mineralisation.
Individual deposits include the: Zhaogou orebody is the northernmost ore zone delineated in the Liba goldfield, has 50 t of gold having been delineated at grade of 1.5 g/t Au (Dragon Mountain Gold, ASX Announcement 2009); Wawugou orebody located in the Wawugou Fault, enveloped by breccia. WWG-1 has a northwest-trending lenticular shape that is over 900 m long and dips ~60°SW, with a thickness of 1 to 12 m, and average grade of 3.78 g/t Au (Zhang 2003). WWG-II is subparallel to WWG-I and located 60 m to the NE. The mineralised zone is steeply dipping, strikes 290°, and is over 400 m long and ~1 m wide; At the Magou orebody, 36 t of contained gold has been delineated at 1.8 g/t Au (Dragon Mountain Gold ASX Announcement, 2009) in two zones, the largest of which is >930 m long, averaging 5.78 g/t Au with an average thickness of 8 m (Shi 2001).
Two main styles of mineralisation are known on the goldfield: i). disseminated sediment-hosted and ii). quartz vein hosted types. Pyrite, arsenopyrite and arsenian pyrite are major gold carriers, while native gold grains and electrum are spatially associated with the sulphides. Numerous felsic/intermediate dykes have a similar structural distribution as the mineralisation, and their contacts with host rocks are considered to be favourable zones for mineralisation.
Three phases of deformation have been recognised in the area. The first deformation (D1) event had a broadly north-south orientation and was compressional. The D2 event was also compressional and orientated in a NE-SW direction, while D3 was post-mineralisation, associated with the emplacement of barren calcite and anhydrite veins. Compression related to D2 controlled the distribution of igneous dykes and gold mineralisation in the Liba goldfield. Both igneous and hydrothermal fluids preferentially focused along dilational jogs under local transtension, which took place during the late stage of D2. Precise dating with high-resolution ion microprobe (SHRIMP) U-Pb on zircon and 40Ar/39Ar on muscovite, biotite, hornblende and plagioclase of crosscutting pre-mineralisation granitic porphyry and diorite dykes have constrained the mineralisation age to after ~227 Ma. 40Ar/39Ar analysis of minerals formed in hydrothermal alteration zones associated with gold mineralisation indicates that there was a widespread ~216 Ma hydrothermal event that affected almost all lithologies in the area (Zhou et al., 2012).
The Daqiao epizonal orogenic gold deposit is located ~60 km SE of Liba and ~160 km west of Baguamiao. It is hosted in organic-rich Triassic pumpellyite-actinolite facies metamorphosed turbidites. It is characterised by high grade hydraulic breccias that overprint an earlier tectonic breccia.
The deposit is structurally controlled by the Zhouqu-Chengxian-Huixian fault, hosted in the Middle Triassic Huashiguan Formation turbidite sequences of siltstone, siliceous, calcareous, and pelitic slates, with lesser pyritic carbonaceous slates and thin- to thick-bedded limestone. These rocks are in fault contact with Carboniferous limestone to the SE. Structurally, the deposit occurs on the northwestern flank of an inferred anticline (You and Zhang, 2009), the core of which is occupied by Carboniferous limestone, with Triassic clastic rocks in both limbs. Gold mineralisation is found on the northern limb. A number of NE and less commonly east-west or north-south striking reverse faults cut the deposit. The Carboniferous limestones and Triassic turbidites are separated by the secondary Yaoshang-Shixia (or F9) fault, part of the regional Zhouqu-Chengxian-Huixian fault system. Both the anticline and reverse faults are regarded to be due to Mesozoic deformation (Dong et al., 2011; Dong and Santosh, 2016).
A number of intermediate to silicic composition dykes intrude the Carboniferous and Triassic sedimentary rocks in the deposit area. Within the Daqiao deposit, granodiorite dykes have a close spatial relationship to gold orebodies and to strongly silicified zones. NE or NW striking, 2 to 10 m thick dykes extend for several tens to a few hundred metres along strike, are weakly deformed and have been variably altered by silica, sericite, sulphides and carbonates with occasionally weak gold mineralisation. These dykes were emplaced between 215 and 212 Ma (LA-ICP-MS zircon U-Pb; Gansu Geological Survey, unpub. report, 2017).
Host rocks of most gold mineralisation are siliciclastic breccias that are mainly localised between the Triassic slate and Carboniferous limestone units, as well as interlayered structures within the Triassic slates, and regarded to be tectonic breccias of the Huashiguan Formation (Gansu Geological Survey, unpub. report, 2011). The hydrothermally altered and mineralised tectonic breccias are developed along NE-striking reverse faults in the southern part of the deposit, whilst NW-striking faults are more important in the north. The F6-F8 faults, which strike NE and dip 55 to 70°SE, are the major structures in the deposit area and host the largest gold orebody. Other gold orebodies are cut by these faults, indicating fault reactivated during and after gold mineralisation (Gansu Geological Survey, unpub. report, 2011).
The Daqiao gold deposit comprises >100 'orebodies', with additional resources indicated at depth and surrounding areas (Gansu Geological Survey, unpub. report, 2017). Gold mineralisation is mainly hosted in the highly competent and permeable Triassic silicified breccias and, less commonly, silicified slates (Figs. 2–4, 5C-H). The upper limits of gold orebodies are mostly concealed at depths of 50 to 120 m below the current surface, although the uppermost parts of some orebodies in the southern section of the deposit are exposed and intensely oxidised to form supergene ores. The hypogene sulphide ores have an average grade of 3 to 4 g/t Au, although high-grades of up to 30 g/t are not uncommon in the extensively silicified fault gouge (Gansu Geological Survey, unpub. report, 2011). The supergene ores are generally 20 to 50 m thick and together contain ~9 t of Au at 7 to 9 g/t Au. The ores also contain 2 to 50 g/t Ag, locally with maximum grades of 370 g/t Ag. There is mostly a positive correlation between Au and Ag display (You and Zhang, 2009). Based on milling and flotation of the past to 2018, about 70% of gold in the hypogene ores can be recovered.
The largest orebody, I-1, has a strike length of ~2 km, with an average thickness of 12 m, and extends over a maximum vertical interval of 525 m from 1127 to 1652 m above sea level. The other more significant orebodies are 60 to 300 m long, 2 to 30 m thick and persist for 40 to 355 m downdip (Gansu Geological Survey, unpub. report, 2011). These orebodies are effectively stacked over a thickness of ~400 m within the Triassic turbidite sequence, enveloped by thicker and more extensive stratabound zones of silicification.
These orebodies are characterised by well-developed auriferous breccias. Breccia ores have been divided into tectonic and hydraulic types, with the best mineralisation associated with the latter, carrying grades of between 1 and 12 g/t Au, although tectonic breccias with relatively low gold grades of <4 g/t Au, also hosts significant amounts of gold due to their larger mass and volume. The tectonic breccias roughly occurs along the contact zone of the Triassic and Carboniferous sedimentary rocks and are composed of in situ siltstone, slate and limestone clasts, which are cemented by fault gouge, hydrothermal microcrystalline quartz and very fine grained sulphides. These breccias were subsequently further brecciated and cemented by chalcedony and sulphides to form intensely silicified hydraulic breccia, known as 'breccia B'. 'Breccia C', a second hydraulic breccia classification, resulted from repeated hydraulic fracturing of breccia B in proximity to the contact zone between the Triassic and Carboniferous strata, containing hydrothermal cements dominated by calcite, sulphides and minor chalcedony.
The dominant ore minerals of the hypogene ores are arsenian pyrite and marcasite, with associated minor to trace amounts of stibnite, chalcopyrite, sphalerite, galena, arsenopyrite, pyrrhotite, unidentified uranium oxides and platinum group element minerals. The gangue consist of quartz, calcite, sericite, kaolinite and carbonaceous material, with accessory apatite, rutile, and monazite. Gold is predominantly invisible, occurring in arsenian pyrite and marcasite, although free gold grains are abundant in the oxidised ores, ranging from 1 to 70 µm in width (You and Zhang, 2009), found as irregular, tabular or dendritic inclusions in limonite or as stringers filling fractures in quartz.
The sulphide minerals occur as disseminations or thin veinlets within the alteration zones, with a variety of crosscutting relationships and textures. Four sulphides stages are recognised:
S1. Layered or nodular pyrite aggregates, which are attributed to the pre-ore stage and occur in the black shale or carbonaceous layers within the Triassic turbidites;
S2. The early ore stage, which mainly consists of fine-grained euhedral pyrite cubes, pyritohedrons, columnar marcasite and irregular sulphide aggregates with abundant inclusions of apatite, chalcopyrite, galena, carbonaceous material and silicates. These sulphides are mainly hosted by the tectonic breccias and, less commonly, by minor quartz-pyrite veinlets typically parallel to the bedding of altered calcareous slate;
S3. The main ore stage sulphides are dominated by fine-grained pyrite aggregates that occur in milky quartz in breccia B and medium- to coarse-grained colloform pyrite or marcasite veinlets in breccia C;
S4. The late ore stage, which is characterised by widespread coarse-grained carbonate and marcasite veinlets in deformed pelitic slates close to the orebodies. The marcasite, which is characterised by a strong anisotropy with yellowish-brown to greyish-blue polarization colours, is only distinguished from the surrounding pyrite using optical microscopy. Detailed microscopic, scanning electron microscopy (SEM), and EBSD observations, show that marcasite accounts for approximately 0%, 20 to 35%, 15 to 25%, and 70 to 85% of total sulphides in the pre-, early, main, and late ore stages, respectively.
Carbonaceous material is widespread in the silicified black tectonic breccia ores at Daqiao, commonly occurring as sparse to dense disseminations that are 10 to 200 µm wide. Some carbonaceous material is enveloped by S2 stage irregular porous pyrite aggregates, with boundaries occupied by irregular or curvilinear interfaces. Fine-grained carbonaceous material also occurs as intergrowths with aggregates of porous pyrite and euhedral marcasite. No carbonaceous veinlets are observed in the hydrothermal cements of auriferous breccia types B and C.
The principal alteration assemblages at Daqiao include silica, sulphides, sericite and carbonates with no clear zonation. Multistage silicification is the most pervasive alteration type, controlled by the porosity and permeability of the immediate host rocks. It mainly occurs within the calcareous siltstone, siliceous slates and complex breccias that are characterised by high permeability.
Early penetrative silicification formed narrow selvages of a few µm of quartz in the granodiorite dykes, as well as quartz-sulphide veinlets, and more widespread microcrystalline quartz in the altered slate through replacement of pre-existing wall rock plagioclase and carbonate. Silicification is also broadly coeval with the formation of disseminated S2 sulphides in the tectonic breccias. The second stage of silicification mainly produces milky to chalcedonic microcrystalline quartz, precipitated with coeval irregular aggregates of fine-grained pyrite in the cements of hydraulic breccia B. Late, relatively weak silicification occurs as minor chalcedony, intergrown with calcite, and pyrite or marcasite aggregates and is associated with the breccia C hydraulic fracturing event.
The introduction of sulphides was in the four main stages detailed above, and was associated with multistage silicification. Sulphide veinlets occur in the black silicified silty slate, whilst fine-grained disseminated or irregular sulphide aggregates occur in the tectonic breccia as well as in the cements of hydraulic breccias B and C. Sericite is widespread as an alteration product of plagioclase and K feldspar, and is commonly intergrown with disseminated sulphides, occurring both in the altered granodiorite dykes and breccia-type ores. Carbonate minerals are most extensive in the main and late ore stages, mainly occurring as cements in hydraulic breccia C, or in veins of coarsely crystalline calcite associated with marcasite in the weakly silicified slates.
This description of the Daqiao deposit is drawn from Wu et al. (2018).
The smaller Huachanggou gold deposit is located in Lueyang County, ~300 km SW of Xi'an in Shaanxi Province, China. It has a contained gold reserve of ~10 tonnes with a grade of 2 to 10 g/t Au (Li et al., 2014). The deposit is controlled by a 15 km long by ~2 km wide WNW-striking ductile-brittle shear zone within the Mianlue suture zone. The deposit is developed in spilite, limestone and phyllite in the Mid to Lower Devonian Sanhekou Group. Three ore zones, denoted as I, II and III make up the Huachanggou deposit. These zones are subparallel to the strike of the host sequence, only partially overlapping laterally and are found within different units at three levels within 200 m of the stratigraphic sequence. Altered spilite hosts gold mineralisation in ore zone I where five NW-striking orebodies dip at 55 to 65°N, with grades ranging from 2 to 6 g/t and locally as much as 36.8 g/t Au. Ore zone II is in the NE part of the district, extending over an interval of ~1.8 km, hosted by bedded crystalline limestone and silty sericite phyllite. Gold orebodies dip at 60 to 80°N with grades varying from 2 to 10 g/t, up to 38.2 g/t Au. Ore zone III is in the SE of the district, developed over a strike length of ~2 km. Mineralisation is hosted within a sequence of microcrystalline limestone and silty sericite phyllite as WNW-trending orebodies which dip at 40 to 75°N. Grades vary from 2 to 8 g/t, locally up to 30.7 g/t Au.
Ores occur as either altered spilite-type and carbonate-quartz vein type. The former contain an assemblage of pyrite, chalcopyrite and native gold in a gangue of quartz, albite, calcite and ankerite. Pyrite occurs as disseminations. The carbonate-quartz vein type ores in zones II and III have higher gold grades, and are characterised by banded quartz veins. The bulk of metallic minerals are pyrite, chalcopyrite and native gold, with minor galena and sphalerite. Gangue minerals are quartz, albite, calcite and ankerite. Pyrite occurs as disseminations.
Pyrite is the main gold-bearing mineral, and the native gold is either visible or microscopic. Visible gold is mainly in quartz veins and varies from ~3 mm. Microscopic gold is mainly interstitial, as minor inclusions and as fissure fill.
Gold mineralisation is accompanied by intense hydrothermal alteration, which includes silica, carbonate, sericite, chlorite and epidote. Silicification is widespread and has a close relationship with mineralisation in the form of quartz veinlets infilling schistosity and fissures in the wall rocks. Carbonates developed after silicification and generally occur as pyrite-quartz-carbonate veining. Gold is best developed where silicification is most intense and pyrite is disseminated in the silicified haloes.
This description of the Huachanggou deposit is drawn from Liu et al. (2016).
Ludousou (#Location: 35° 5' 30"N, 102° 59' 30"E)
The Ludousou deposit is located ~18 km NE of Hezuo City in Gansu Province, China. It is dominantly hosted by metasedimentary rocks of the Lower Permian Daguanshan Formation, comprising a SW-dipping greenschist facies sequence mainly composed of metalimestone, metasiltstone, metasandstone and metaconglomerate of greenschist facies. This sedimentary sequence was covered by rhyolitic tuff eruptions of the Gari volcanic rocks, and intruded by the calc-alkaline to high-K calc-alkaline Dewulu intrusive complex stock (Qiu and Deng, 2017; Sui et al., 2017). Hornfels are developed along the contact between the stock and the metasedimentary rocks (Chen et al., 2007). In the deposit area, the Gari volcanic sequence contains glassy fragments, quartz and fine-grained scoriae with plagioclase and biotite. The quartz grains commonly have crescent, sickle, biconcave or rounded shapes. The Dewulu intrusive complex contains quartz diorite porphyry, quartz diorite and numerous mafic microgranular enclaves hosted by the quartz diorite. Phenocrysts within the grey-white coloured quartz diorite porphyry are mostly 0.5 to 2mm in diameter, comprising 30 to 50 vol.% plagioclase, 30 to 50 vol.% biotite and 5 to 10 vol.% amphibole set in a groundmass that is mainly composed of quartz and plagioclase. Accessory minerals include zircon and apatite. The quartz diorite is grey and is mainly composed of 40 to 50 vol.% plagioclase, 15 to 30 vol.% amphibole, 10 to 15 vol.% quartz and ~5% biotite with accessory apatite and zircon. More than 80% of the mafic microgranular enclaves hosted within the quartz diorite range from 10 to 30 cm in diameter, with ellipsoidal to spherical shapes and are dioritic with a porphyritic texture. The contacts between enclaves and host rocks are sharp and some are chilled. Phenocrysts within these enclaves comprise 10 to 15 vol.% and 0.1 to 0.7 mm diameter amphibole, 5 to 10 vol.% and 0.3 to 2.0 mm diameter plagioclase, 5 vol.% and 0.5 to 1.3 mm diameter quartz and 3 vol.% and 0.1 to 0.8 mm diameter biotite. The matrix comprises 45 vol.% plagioclase, 20 vol.% biotite and 10 vol.% amphibole. Quartz megacrysts surrounded by amphibole are commonly found in these enclaves. Accessory minerals are zircon and acicular apatite with length to width ratio of about 30:1.
Most of the Ludousou deposit resources are restricted to the two largest orebodies, referred to as the tourmaline-rich Au-Cu orebody and the Au3 fault and breccia related Au-Sb orebody, which represent two different styles of ore. Most of the Au-Cu orebodies are hosted in the upper part of the intrusive complex (i.e., the 3250 to 3220m level (Yang and Qian, 2017). In contrast, the Au-Sb orebodies are mainly hosted in disseminated breccias within reverse fault zones, within laminated quartz-sulphide lodes.
Locally, Au-Sb mineralisation crosscuts the tourmaline-rich ores and cement the tourmaline breccia. The tourmaline-rich Au-Cu ores contribute ~30% of the gold endowment (Yang and Qian, 2017) and are characterised by ~30% tourmaline, occurring as massive, vein, euhedral disseminated and nodular tourmaline that postdate the emplacement of quartz diorite porphyry. The disseminated and nodular tourmaline commonly occurs as tourmaline-enriched domains in host quartz diorite porphyry. Vein tourmaline commonly occurs along joints in the host rocks, whilst massive tourmaline usually cements granite breccia with strong silicification, sericitisation and sulphidation. The sulphides are pyrite, arsenopyrite and chalcopyrite. Pyrite and arsenopyrite generally occur as coarse-grained subhedral or euhedral crystals.
The fault and breccia related Au-Sb ores contribue ~70% of the gold endowment. Structures hosting the orebodies are irregular, varying from 0.2 to 3 m in width. These faults dip at 15 to 30°S, SE and SW. Mineralisation-related alteration patterns are structurally controlled by faults with the same strike and dip, and including sulphidation (pyrite and arsenopyrite), sericitisation, silicification and carbonatisation. The extensive carbonate halos are ~30 cm thick. Pyrite, arsenopyrite and stibnite are the dominant sulphide minerals with minor to trace amounts of chalcopyrite, sphalerite and galena. Invisible gold occurs in solid solution and/or as nanoparticles within pyrite and arsenopyrite.
This description of the Ludousou deposit is drawn from Yu et al. (2020).
The Gangcha epithermal gold deposit is located in southern Gansu Province, ~20 km NE of Hezuo City. The sequences within the deposit area include the Permian Daguanshan Formation, the Early and Middle Triassic Longwuhe Formation and the Tertiary Gansu Group. The deposit area is intruded by the eastern segment of the Mid-Triassic Dewulu pluton which is mainly composed of quartz diorite and quartz dioritic porphyry dated at ~242 Ma (Li et al., 2016). The deposit comprises five bodies of gold mineralisation, all of which are hosted in the Gangcha volcanic to sub-volcanic suite of the Longwuhe Formation, which are composed of andesites, dacites and quartz diorite porphyry, with minor volcanic breccia, volcanic agglomerate and tuff. Mineralisation occurs as auriferous quartz-sulphide veins or veinlets and disseminated sulphides that are principally controlled by a series of nearly north-south or NNW trending faults. The mineralised bodies predominantly strike NNW and dip at 50 to 70°W. Gold grades range from 1.00 to 24.15 g/t, averaging 4.19 g/t. The highest grade Au3-1 vein has a length of at least 857 m and a thickness of 0.83 to 8.04 m.
Hydrothermal alteration related to gold mineralisation includes the development of sericite, silica, pyrite and carbonate. The most abundant sulphides are pyrite and arsenopyrite with minor sphalerite, chalcopyrite and galena. Gangue minerals include sericite, quartz, calcite, plagioclase, epidote and chlorite. The mineralogical and textural characteristics of ore minerals and the crosscutting relationships of the mineralised veins, suggest five mineralising stages:
i). sericite-quartz-pyrite, characterised by minor disseminated, granular coarse-grained euhedral to subhedral pyrite in early pyrite-sericite-quartz alteration;
ii). pyrite-quartz, occurring as pyrite veins;
iii). quartz-pyrite, characterised by numerous massive milky quartz veinlets containing minor fine-grained euhedral to subhedral pyrite that crosscut the second stage pyrite veins;
iv). quartz-polymetallic sulphides, occurring as abundant quartz-sulphide veins that consist of a number of other sulphide minerals, including arsenopyrite, galena, sphalerite and chalcopyrite, along with pyrite. These sulphides occur as fine grained anhedral aggregates, with the quartz-sulphide veins normally crosscutting the third stage quartz-pyrite veins; and
v). calcite-pyrite, consisting of calcite-pyrite veinlets that contain a few coarse pyrite grains and crosscut all the earlier formed veins. The third and fourth stages are considered to represent the major period of gold metallogenesis, with mineralisation of these two stages containing grades of 0.5 to 11.68 g/t Au, compared to the other stages which generally carry <0.5 g/t Au.
This description of the Gangcha deposit is drawn from Nie et al. (2017).
The resources contained within other deposits in the Qinling Gold Province (Zeng et al., 2012) were: Yangshan (>300 t Au @ 4.74 g/t Au); Ma'anqiao (20 t Au @ 5 g/t Au); Shuangwang (60 t Au @ 3 g/t Au); Pangjiahe (>40 t Au @ 6 g/t Au); Daqiao (>105 t Au @ 3 to 4 g/t Au); Baguamiao (>106 t Au; Liu et al., 2015); Zaozigou (142 t Au; Sui et al., 2017); Huachanggou (>10 t Au @ 2 to 10 g/t Au; Liu et al., 2016); Ludousou (>8 t Au @ 3.8 to 5.5 g/t Au; Yu et al., 2020); Gangcha (>19 t Au; Nie et al., 2017).
The most recent source geological information used to prepare this summary was dated: 2021.
Record last updated: 11/8/2020
This description is a summary from published sources, the chief of which are listed below.
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Deng, J. and Wang, Q., 2016 - Gold mineralization in China: Metallogenic provinces, deposit types and tectonic framework: in Gondwana Research v.36, pp. 219-274.|
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