Xiaoqinling Gold Province - Tongyu, Yanzhihe, Wenyu, Dongchuang, Dahu, Qiangma, Sifangou, Yangzhaiyu, Hongtuling, Qiyugou
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The Xiaoqinling (or Xiao Qinling) Gold Province located between Tongguan in eastern Shaanxi Province and Lingbao in western Henan Province, central China, contains a series of more than 100 orogenic gold deposits of varying sizes, with ores occurring in a number of distinct belts. This province is to the east of the Qinling Gold Province. A number of the key representative deposits are descibed below. Ore occurs both in quartz veins and as disseminations in altered metamorphic rocks (#Location: Tonggu - 34° 26' 50'N, 110° 12' 03"E; Wenyu - 34° 25' 24"N, 110° 24' 20"E; Qiangma - 34° 23' 58"N, 110° 28' 9"E).
The gold deposits generally lie about 30 to 50 km inland of the southern margin of the North China craton and are concentrated in the Xiaoqinling, and in the adjacent Xiaoshan and Xiong'ershan areas progressively to the east.
For a more detailed description of the Qinling Orogen see the Regional Setting section of the Qinling Molybdenum Belt record.
These deposits lie within the eastern, EW-trending Qinling Orogen of the greater Central China Orogenic Belt that was developed during the Mesozoic collision between the North China and the Yangtze Cratonic blocks (Chen and Santosh, 2014). The orogen incorporates four distinct tectonic units from north to south: i). the Huaxiong Block representing the reactivated southern margin of the North China Craton, ii). the Northern Qinling accretionary belt, iii). the Southern Qinling Orogenic belt, and iv). the Songpan or Mianlue foreland fold-thrust belt along the northern margin of the Yangtze Craton, respectively separated by the regional San-Bao, Luanchuan, Shang-Dan, Mian-Lue and Longmenshan fault zoness (Chen et al., 2009, 2014; Li et al., 2013).
The Xiaoqinling gold field is within the northernmost Qinling Orogen, within the western half of the exposed Huaxiong Block, bounded to the north by the Taiyao Fault (part of the San-Bao fault belt) and to the south by the Xiaohe Fault. It is ~60 km long, with deposits concentrated in the 2 to 5 km wide, WNW-ESE trending Guanyintang shear zone.
The main hosts to mineralisation are Neoarchean to Palaeoproterozoic graphite gneisses, marbles, quartzites, banded iron formations, biotite/amphibole gneisses, migmatite and amphibolites of the Taihua Supergroup. The Taihua Supergroup is separated from the Mesoproterozoic Xiong'er Group to the south by the Xiaohe Fault. The Xiong'er Group is a weakly deformed and metamorphosed volcanic succession comprising basaltic andesite, andesite, dacite and rhyolites, which are well preserved on the southern North China Craton (Chen et al., 2014), but are only found on the southern margin of the Xiaoqinling gold field.
Taihua Supergroup was metamorphosed to amphibolite to granulite facies between 1.95 and 1.82 Ga, during assembly of the Nuna-Columbia supercontinent (He et al., 2009; Santosh, 2010; Deng et al., 2013). This sequence has been subdivided into the:
i). 3.0 to 2.55 Ga Beizi Group, composed of an intensely migmatitised high-grade greenstone assemblage containing abundant ultramafic rocks;
ii). 2.5 to 2.3 Ga Dangzehe Group, a less migmatitised greenstone assemblage without, or with only minor, ultramafic rocks; and
iii). 2.3 to 2.1 Ga Shuidigou Group, a metamorphosed sedimentary succession which is widespread in the North China Craton (Chen and Zhao, 1997).
The easliest intrusive event in the Xiaoqinling gold field is represented by 2.1 to 1.85 Ga pegmatite dykes (Zhao et al., 2009; Li et al., 2011), followed by the Paleoproterozoic (1748 Ma) Guijiayu granodiorite (Li et al., 1996), the Mesoproterozoic (1463 Ma) Xiaohe biotite granite (Li et al., 1996), Indosinian (213 to 202 Ma) alkalic porphyries and dykes and the Yanshanian (146 Ma) Huashan, Wenyu (138 to 131 Ma) and Niangniangshan (134 to 142 Ma) biotite granites (Mao et al., 2010; Li et al., 2011; Zhao et al., 2012).
Widespread dolerite, gabbro and lamprophyre dykes were emplaced in the Taihua Supergroup metamorphic rocks. Most are E-W striking, although some trend NE and NW. Most were emplaced at ~1.85 to 1.80 Ga, with minor dolerite dykes intruded from 128.6±4.7 to 126.9±4.8 Ma (Wang et al., 2008; Bi et al., 2011).
The Guijiayu granodiorite and Xiaohe biotite granite were emplaced along the Xiaohe Fault, which also subsequently displaced them. The Huashan, Wenyu and Niangniangshan granites occur in the western and eastern parts of the Xiaoqinling gold field. The Wenyu granite intrudes the central zone of the gold-rich area, (just west of Tongguan) where it is exposed over an area of about 20 sq. km, although deposits are generally hosted in the Precambrian basement rocks hundreds of metres to as much as 10 km from the intrusions and their related hornfelsed aureoles.
The principal structures within the Xiaoqinling gold field are near east-west striking faults and folds. The folds are predominantly Precambrian in age, while the faults largely evolved from the Triassic to Jurassic by south-directed thrusting, and Cretaceous north-directed normal faulting (Zhang et al., 1998; Mao et al., 2002; Li et al., 2011; Zhao et al., 2011, 2012). Some faults possibly branch from reactivated Precambrian structures. The parallel Huanchiyu (to the north) and Guanyintang (~5 km to the south) shear zones, which extend over strike lengths of more than 20 km, are the core structures in the orefield. The latter is the western half of the regional east-west trending, north dipping, >60 km long Maxundao deep fault zone (that extends from Tongguan in Shaanxi Province to Lingbao in Henan), originally a compressional structure, which shows evidence for late extension.
Subsidiary faults branching from the major structures mainly trend E-W, NW or NE, and range from <1 to a few kilometers in length, with widths of several to tens of metres. These structures were developed locally, during Jurassic to Cretaceous time (Mao et al., 2002), and are characterised by an early stage of ductile deformation that was overprinted by a late brittle phase, possibly related to the Mesozoic uplift of the Neoarchean to Paleoproterozoic basement in an extensional regime (Liu et al., 1998; Zhang et al., 2000). These secondary structures are the most favourable sites for localising orebodies and granite porphyry dykes. Zhang et al. (1998) suggested the shear zones, which were intruded by the granites, are coeval with syn-collisional crustal thickening before 127 Ma, whilst the normal faults and shear zones, which crosscut the granites (e.g., the Wenyu pluton), were developed synchronously with post-collisional crustal thinning after 127 Ma.
A string of significant gold deposits, with total resources of 300 to 450 t of contained Au, occur at intersections of second order WNW to east-west striking faults with NE and NW striking faults to the north of the first-order Maxundao fault zone.
There are more than 50 gold deposits in the Xiaoqinling gold field. Li et al. (2002) constrained the mineralisation of the Dongchuang deposit to have been emplaced between 143 and 128 Ma. They also obtained 40Ar/39Ar plateau ages of 142.9±2.9 Ma, 132.2±2.6 Ma and 128.3±6.2 Ma for hydrothermal quartz separates from stages I, II and III mineralised veins, respectively, and a 40Ar/39Ar plateau age of 132.6±2.7 Ma for sericite separates from a mineralised stage II veinlet. Li et al. (2012) have reported 40Ar/39Ar plateau ages of 124.07±1.27 and 125.4±0.4 Ma for mineralisation related sericite from the Qiangma deposit.
This suggests that the gold mineralisation in the Xiaoqinling orefield was emplaced in the Early Cretaceous, and was intimately involved in the Mesozoic tectonic-metallogenic event caused by the Yangtze-North China continental collision that commenced at the close of the Triassic, culminated in the Jurassic, and waned by the Early Cretaceous (Chen et al., 2009; Jiang et al., 2009, 2010; Xu et al., 2010; Li et al., 2011; Ni et al., 2012, 2014). The tectonic regime within the Qinling Orogen changed from Jurassic compression, through Late Jurassic transpressive compression, to Early Cretaceous extension. During this period, large-scale fluid circulation, granitic
magmatism and metallogenesis took place within the Orogen, although Zhou et al. (2014) note that most gold deposits show no direct spatial relationship with
Yanshanian intrusions. The gold deposits are interpreted to have formed during the period of relaxation of compressional stresses, following the main collisional phase. Hydrothermal and magmatic events occurred locally where extension-related Precambrian basement uplift took place along the suture between the Yangtze and North China cratonic blocks.
Isotope and fluid inclusion geochemistry studies of the Wenyu and Qiangma deposits (described below) by Zhou et al., (2014 and in press in 2014) show i). a decrease in pressure and temperature from early to late stages of development of the deposit, probably related to uplift and mountain-building; ii). ore fabrics changed from early-stage compressive shearing to late-stage open space filling, with quartz and pyrite changing from structurally deformed early-stage anhedral grains to unstrained late-stage euhedral crystals; and iii). ore-forming fluids (in fluid inclusions) changed from pure carbonic metamorphic to mixed carbonic-aqueous to aqueous water-dominated meteoric fluids. The latter, supported by inclusion geochemistry, shows these deposits were associated with the mixing of mesothermal, CO2-rich fluids that originated from metamorphic devolatilisation of the host sequence, and descending meteoric waters, in a regime that fluctuated from lithostatic/supralithostatic to hydrostatic fluid pressure, and occurred at depths of 10 to 14 km, straddling the brittle-ductile transition. Whilst these deposits are beyond the metamorphic aureole of the Yanshanian granites (e.g., the anatectic Wenyu Granite), the same authors imply other deposits within the zone of influence of the intrusions may be intrusive-related and epithermal in nature. The same orogenic processes would have driven both metamorphic devolatilisation and granite formation.
From west to east in the Xiaoqinling Gold Province, gold deposits hosted in rocks of the Taihua Supergroup are concentrated in three goldfields within a 60 x 15 km corridor, 2 to 15 km north of the Maxundao fault. These are the (after Mao et al., 2002):
i). Tongyu Goldfield, including the Tongyu (32 t Au @ 8 to 20 g/t Au) and Yanzhihe deposits;
ii). Wenyu Goldfield, including the Wenyu (50 t Au @ 6.5 g/t Au), Dongchuang (55 t Au @ 7 g/t Au), Sifangou (37 t Au @ 10 g/t Au), Yangzhaiyu (50 t Au @ 11 g/t Au) and Qiangma (>50 t Au @ 8.9 g/t Au) deposits; and
iii). Dahu Goldfield, including the Dahu (63 t Au @ 6 g/t Au) and Linghu deposits.
The larger deposits occur as 4 to 20 m wide and >4 km long quartz veins which lie within second order faults, while lesser amounts of gold occur in altered rocks along ductile brittle shear zones and in breccia bodies.
The Xiaoqinling belt has a proven reserve of more than 630 t of contained gold (Wu, 2012; Liu, 2013)
More than 1200 auriferous quartz veins are known in the Xiaoqinling Gold Province. Ores contain pyrite, galena, sphalerite and minor magnetite, scheelite, wolframite, molybdenite, stibnite, pyrrhotite and gold. The gangue assemblage includes quartz, calcite, ankerite, minor rutile, barite, siderite and fluorite. The alteration halos sandwiching quartz veins or shear zones comprise mainly quartz, sulphide minerals, white mica and carbonate minerals, with lesser chlorite, epidote, and biotite.
The Yangzhaiyu gold deposit (Li et al., 2012). Gold mineralisation is hosted in Neoarchaean to early Palaeoproterozoic amphibolite facies metamorphic rocks, occurring as both auriferous quartz veins and subordinate disseminated ores in the alteration zone proximal to the veins. Ore-related hydrothermal alteration is dominantly sericite + quartz + sulphide assemblages close to gold veins, and biotite + quartz + pyrite ± chlorite ± epidote distal from mineralisation. The dominant sulphide mineral is pyrite, locally coexisting with minor amounts of chalcopyrite, sphalerite and galena. Gold occurs mostly as free gold, enclosed within, or filling, microfractures of pyrite and quartz, and is also present in equilibrium with Au-bearing tellurides, mainly petzite and calaverite coexisting with hessite, tellurobismuthite and altaite.
The Wenyu gold deposit, which comprises ~40 auriferous quartz veins, is located in the southern part of the Xiaoqinling terrane, and hosted by metamorphic rocks of the Taihua Supergroup within the Guanyintang shear zone. The large gold bearing veins at Wenyu are usually hosted in ductile–brittle shear faults that trend the east-west, whilst the relatively small quartz veins occupy NNE and NNW trending structures (Xu and Fan, 2003). These veins commonly dip at 40 to 60° to the south and where the strike and/or dip change tend to swell. The largest vein (S505), has a strike length of 4.2 km, and varies from 0.2 to 8.2 m in thickness. The second largest vein (S512), has a strike length of up to 4 km, with a thickness of 0.1 to 2 m (Jiang, 2000). Other veins are ~100 to 2000 m in length, and 0.5 to 1.5 m thick, but may extend for as much as 1 km down-dip. Ore grades range from 2.3 to 248 g/t, although most are from 5 to 17 g/t Au (Mao et al., 2002; Wang, 2009).
Orebodies are mainly hosted by ductile–brittle faults with reverse or normal displacement of several to tens metres. High-grades preferentially within structural jogs, strike or dip changes, bifurcations and splays (Li et al., 2012; Wang, 2009). The gold ore is predominantly hosted by quartz veins, with lesser mineralisation in the altered wallrock selvedges.
Hydrothermal alteration associated with gold mineralisation includes silicification, potassic alteration, pyrite, sericite and carbonates. Major ore minerals are pyrite and native gold and electrum, with subordinate galena, sphalerite, chalcopyrite, and locally molybdenite and scheelite (Chen and Fu, 1992). The dominant gangue mineralogy includes quartz (~90%), feldspars, sericite, chlorite and calcite. Both native gold and electrum are predominantly present as veinlets or inclusions in pyrite coexisting with galena and chalcopyrite, with lesser disseminations in quartz. Minor Au-tellurides (e.g., calaverite) are also found as inclusions in pyrite. The morphology of the gold is variable, ranging from veinlet, tear-drop-like to irregularly-shaped and dendritic forms (Zhao et al., 2011, 2012; Zhou et al., 2011).
The mineralisation has been divided into:
i). Early stage, characterised by an assemblage of quartz + pyrite. The quartz is white or milky, and comprise ~95 vol.% of the vein, whilst pyrite is ~5 vol.%, occurring as coarse-grained, euhedral and cubic forms.
ii). Main stage, which contains the bulk of the gold mineralisation, and is characterised by quartz–sulphide veins. Locally this phase may occur along both margins of early-stage veins. Quartz is smoky grey and fine-grained, and is characteristically coherent with subhedral to anhedral fine-grained pyrite, as well as chalcopyrite, galena, sphalerite, native gold and tellurides. The sulphides usually occur as bands, thin veinlets and crumb forms within the quartz.
iii). Late stage, which is characterised by carbonate + quartz veins, with trace pyrite containing no gold, commonly occurring as veinlets cross-cutting the earlier formed quartz. (This description os paraphrased after Zhou et al., 2014).
Published reserves and resources are (Zhou et al., 2014, after Cun, 1992):
proven reserve - 75.144 t Au @ 7.65 g/t Au, plus
indicated resource of 34.3 t Au.
The Qiangma gold deposit located in the southern part of the Xiaoqinling gold field. It comprises ~13 auriferous quartz veins, hosted by Taihua Supergroup metamorphic rocks within the Guanyintang shear zone. The host sequence is intruded by numerous dykes in the mine area, including granite, pegmatite and dolerite. Some of the latter, which are dated at 1829.5±7.6 Ma (in-situ zircon U-Pb, Wang et al., 2008) and are strongly deformed and mylonitised, are cut by gold-bearing veins.
The auriferous veins of the Qiangma deposit are mainly controlled by near E-W trending subsidiary faults that dip to the south at variable angles of from 25 to 70°, and by minor NNW-trending structures, dipping NEE at high angles, to vertical (Yang et al., 2010). The faults hosting mineralised veins are ductile-brittle structures, with reverse or normal displacement of several to a few tens metres. High-grade sections of the veins preferentially occur in structural jogs, changes in strike and dip, bifurcations, and splays (Wang et al., 1993; Li et al., 2012). Some veins show laminated textures.
Gold occurs both within the quartz veins, and to a lesser degree in adjacent altered wallrocks. The veins have been subdivided into two groups, based on their orientations, as indicated above. The first group strikes at 275 to 290°, and dips south at angles of 25 to 60°. The second group strikes at 330 to 360º, dipping 70 to 90° to the NEE and includes the largest veins in the deposit, e.g., the largest vein of the deposit (No. 410), with a length of 1680 m, and thickness of 0.54 to 1.08 m, and the second largest (No. 101), developed over a strike length of 1250 m, that is 0.48 to 0.83 m thick (Wang et al., 1993; Yang et al., 2010). The other veins are generally 100 to 1000 m long, and 0.1 to 4.15 m thick. Ore grades vary from 0.05 to 386.7 g/t, with the majority in the range 6 to15 g/t (Wang et al., 1993).
The principal ore minerals are gold and pyrite, with subordinate chalcopyrite, galena, sphalerite and magnetite. Gangue minerals are predominately quartz, feldspars, sericite, chlorite, ankerite, and calcite.
Three types of pyrite are recognised, based on morphology, texture and paragenesis, namely:
i). Type 1 pyrite, which occurs as coarse (generally >2 mm), euhedral to subhedral grains, which have a cubic or pyritohedral form. These appear as isolated or aggregate masses in milky quartz veins.
ii). Type 2 pyrite is dominantly subhedral to anhedral, and fine- to medium-grained (0.02 to 1 mm). Many grains are broken or brecciated, and show foam textures, micro-fissures, and are brecciated. These porous or fractured pyrites often contain higher Au concentrations and usually coexist with chalcopyrite, galena and sphalerite in smoky grey quartz veins.
iii). Type 3 pyrite is fine-grained (0.01 to 0.5 mm) and euhedral, occasionally observed in quartz+carbonate veins.
The ore is disseminated, brecciated, stockwork, cloddy, veined or veinlets. Ore minerals are euhedral to subhedral, whilst metasomatic textures and fragmentation are commonly observed, indicating the deposit was formed as a result of hydrothermal replacement, exemplified by the replacement of, chalcopyrite by galena. Hydrothermal alteration is widespread within the deposit, with quartz, sericite, K feldspar, chlorite, carbonates and sulphides being main wallrock alteration phases. Gold mineralisation is closely associated with silica, pyrite, K feldspar and sericite (Yang et al., 2010).
Three hydrothermal stages can be identified, based on the mineralogical assemblages and crosscutting relationships, namely:
i). An early stage, represented by the assemblage of quartz+pyrite. The quartz is white or milky, characterised by minor coarse-grained, euhedral, cubic pyrite.
ii). A middle stage, which is characterised by quartz-polymetallic sulphide veinlets, representing the main gold-introducing event. Quartz is smoky grey and fine-grained, generally coherent with subhedral to anhedral, fine to medium grained pyrite, as well as chalcopyrite, galena, sphalerite and native gold. The sulphides usually occur as veinlets and disseminations in the middle-stage quartz.
i). A late stage is typified by a quartz-carbonate assemblage, with a trace amounts of pyrite but no gold, commonly occurring as veinlets crosscutting the earlier formed quartz veins and altered wallrocks.
The deposit contains an estimated reserve of >50 t Au @ 8.9 g/t (Mao et al., 2002; Zhao et al., 2011)
The Hongtuling deposit is developed within a host sequence composed of Neoarchaean to Palaeoproterozoic metamorphic rocks of the Taihua
group, consisting of biotite plagioclase gneiss, amphibolite and migmatite. The structures in the deposit area are dominated by
WNW- to east-west trending faults that have been interpreted to have been produced during the Triassic orogeny (Li et al., 2012), with minor NE-trending structures, both of which host molybdenum and gold veins. Granite pegmatite, monzogranite, and numerous mafic dykes intrude the metamorphic rocks. Whole rock K-Ar results indicate emplacement of mafic dykes in the Early Cretaceous (Pang et al., 2005). There is no spatial relationship between these intrusions and mineralisation veins.
The Hongtuling deposit comprises ten gold-bearing quartz-sulphide veins. With exception of vein S1002 that is localized in a NE-trending fault, all other gold veins are hosted in WNW- to east-west trending faults dipping at 16 to 60°S. The largest vein in the mine, S8201 accounts for 47% of the known resource (Ye et al., 2003). This vein is 722 m long, 0.15 to 7.55 m thick and extends vertically from 1672 to 936 m above sea level (Qi, 2010). The longest vein, S875, is ∼3.7 km in length, 1.0 to 3.5 m in width and ∼1500m in vertical extent (Wang et al., 2002; Lingbao Hongxin Mining Ltd, 2015).
Pyrite is the dominant sulphide mineral in gold veining. It is associated with minor to trace amounts of chalcopyrite, galena, sphalerite, melonite,
calaverite and tellurobismuthite. The gangue minerals include quartz, sericite and biotite with accessory monazite. Gold mainly occurs as native gold interstitial to pyrite, quartz and galena or included in those minerals (Ye et al., 2003) and is associated with hydrothermal alteration including pyrite, silica and sericite.
A Mo-dominated vein (M-1) has been encountered at the deepest extremity of vein of S875 in the northern part of the deposit area. This vein is 380 m long and 10.6 to 37.0 m thick, and comprises K-feldspar, quartz, calcite, molybdenite, and other sulphide minerals (Lingbao Hongxin Mining Ltd, 2015)lies within the same fault as the S875 gold vein, but it is locally crosscut by the gold veins. The molybdenum vein is dominated by molybdenite, with lesser pyrite, galena and scheelite, which account for ∼4 vol.% of the vein. Gangue minerals are calcite, quartz, K feldspar, and celestite with accessory monazite, titanite, rutile, xenotime, apatite and aeschynite. Hydrothermal alteration associated with the Mo mineralisation is well developed, forming calcite, quartz and K feldspar as selvages on the molybdenum veins.
Proved reserves are ∼42 t of contained Au in ore with a grade of 12.87 g/t (Qi, 2010; Lingbao Hongxin Mining Ltd, 2015 as quoted by Zhao et al., 2019) equating to 3.26 Mt of ore.
This description is drawn from Zhao et al. (2019).
The quartz vein-style Dahu Au-Mo deposit has proved reserves of ~ 6.6 Mt @ 4.7 g/t Au, for 31 t of contained gold and ore averaging 0.13% Mo containing 30 000 t molybdenum (Feng et al., 2011), with an annual gold production of ~0.30 t Au and ~900 t Mo.
The deposit is located on the northern side of the Wulicun anticline, hosted by biotite plagiogneiss, amphibole plagiogneiss and amphibolite of the Archaean Taihua Supergroup, half way between the Wenyu granite, 7 km to the west, and the Niangniangshan granite, a similar distance to the east. The Taihua Supergroup was intruded by more than 50 mafic dykes (1816±14 Ma by U-Pb zircon; Bi et al., 2011), that dominantly strike NW and dip NE are known in the deposit area, with varying strike lengths and widths from 10 to 1100 m and 0.5 to 50 m, respectively ( Yang et al., 1995). Several granite porphyry dykes are also present, striking NW, and dipping NE, with strike lengths and widths from 10 to 1100 m and 3 to 6 m, respectively (Yang et al., 1995). These rocks are locally cut by mineralised quartz veins, of undetermined age. The major structures at the Dahu deposit are E-W, NW-SE, and NE-SW striking faults. The NW-SE and NE-SW faults are predominantly filled by mafic and granite porphyry dykes, whilst the E-W structures control the mineralised quartz veins, designated from north to south, F1, F8, F7, F35, F5, and F6. The curvilinear 8 km long by 10 to 150 m wide F5 fault zone is the largest ore-bearing structure. It varies in strike from WNW-ESE to ENE-WSW, but is generally north dipping.
Mineralised quartz veins dip at 23 to 52°NW, with individual veins being parallel, or slightly oblique to, each other and the host structures. Gold and molybdenum ore blocks are based on a cutoff grade of 1 g/t Au and 0.03% Mo, respectively. The two metals occur in different veins or in different parts of the same vein and locally overlap. Molybdenite is widespread in gold ores and gold and molybdenum ore blocks usually contain sub-economic concentrations of the other metal. Within individual faults, gold orebodies are more developed in the higher levels, while molybdenum orebodies are more developed at deeper levels.
Twenty-five gold orebodies extend down dip over a ~1200 m vertical interval from surface. The No. 19 orebody, which dips a 33°NW, and is from 0.5 to 17.4 m thick (average 2.0 m) is the largest gold lode, containing 14 t of Au at an average grade of 6.6 g/t and an average molybdenum grade of 0.04% (Yang et al., 1995). It has a total vertical extent of 940 m, and a strike length of about 1490 m, with pinch and swell domains and branches.
Ten molybdenum orebodies, which are all blind,extend from ~260 m to ~800 m below the surface. The largest molybdenum orebody 'F5-up2', has a molybdenum reserve of 12 000 tonnes at a grade of 0.12% Mo. It dips at 29°NW and varies in thickness from 0.7 to 38.2 m, averaging 8.9 m, with a strike length of 1660 m. The Mo(%):Au (g/t) grade is ~5:1.
The mineralised quartz veins are mainly composed of quartz (>80%) and sulphides (<10%), with widths typically varying from tens of centimetres to several metres. The dominant sulphides are pyrite, molybdenite, chalcopyrite and galena. Gold and molybdenite often appear closely together, although a direct contact of gold and molybdenite is rare. Other commonly observed vein minerals are K feldspar, covellite, bornite, anhydrite, celestine, barite, calcite, ankerite, monazite and biotite.
Four mineralisation stages are recognised, from early to late, i). quartz-K-feldspar stage, ii). pyrite-molybdenite, iii). sulphide-telluride-sulphosalts-gold, iv). and carbonate-barite stage. Stage , i). and ii). are molybdenite deposition stages, while stage iii). is the gold deposition event.
Ore-related hydrothermal alteration assemblages includes K feldspar, biotite, chlorite, sericite, silica and carbonates, divided into an:
• Inner alteration zone - which surrounds all of the molybdenum-mineralised quartz veins and some gold-mineralised quartz veins, comprising a K feldspar rich halo with a characteristic reddish colouration. The width of this selvedge is less than the width of the vein, and varies from several mm to several metres. Wall-rock plagioclase and biotite have been >50% altered, and amphibole virtually obliterated. The alteration assemblage mainly comprises K feldspar, sericite, carbonates and quartz, with K feldspar, formed by replacement of plagioclase, dominating.
• Outer alteration zone - more distal to the veins, where the inner alteration zone gradually gives way to an alteration zone characterised by sericitisation of plagioclase, and chloritisation of biotite and amphibole. Plagioclase has been >30% altered to sericite. Amphibole has been >20% altered to biotite and chlorite.
Some large gold deposits hosted by Proterozoic basement are also known. These include Kangshan (22 t Au @ 4 to 8 g/t Au), Shanggong (>30 t Au @ 6.9 g/t Au) and Qiyugou, controlled by a group of NE striking faults and shear zones, which are the second-order structures to another major east-west striking fault zone. Shariggong and Kangshan are on the 33 km long, NE-trending Kangshan-Qiliping ductile-brittle shear. Mineralisation is hosted in Mesoproterozoic felsic to intermediate volcanic rocks. The steeply dipping orebodies are 250 to 750 m long and I to 2.8 m wide veins filling brittle structures, lenses in tension gashes, alteration bands along shear zones and brecciated country rock. The ores generally contain anomalous Ag, Te and Pb concentrations. Alteration halos around the orebodies are characterised by a I to 3 m wide proximal sulphide-ankerite-muscovite zone, a I to 20 m wide pyrite-ankerite-muscovite-chlorite transitional zone, and an outer 50 m wide distal chlorite-calcite zone.
Epithermal accumulation are also noted in the district (e.g., Qiyugou and Dianfang ) hosted in volcanic breccia pipes. Other deposits include: Laowan (24 t Au @ 5 g/t Au); Yindongpo (46 t Au @ 5 g/t Au).
The Qiyugou breccia pipes are spatially associated with the Leimengou porphyry Mo deposit which is ~2 km to the west (described in the Qinling Molybdenum Belt record which includes a geological map of the belt). At least seven auriferous breccia pipes are found within the Qiyugou district. No. 2 and No. 4 have been the main producers with total 'reserves' of ~40 t of contained gold (Mao et al., 2002). Ore grades range from 3 to 5 g/t and up to 7 g/t Au (Chen et al., 2009). Higher grades occur in zones of complex alteration and of greater clast population.
The auriferous breccia pipes are hosted by the Taihua Supergroup to the west of a Cenozoic basin. NW- and NE-trending faults are the principal hosting structures, formed by NE–SW and NW–SE compression during the Mesozoic (Gao et al.,1994). The breccia pipes have an elliptical shape in plan, with long axes ranging from <40 m to >1 km and have been traced vertically for more than 300 m. They contain clasts of Archaean basement rocks (migmatite, gneiss and amphibolite), Palaeoproterozoic Xiong'er Group volcanic rocks and Mesozoic granitic lithologies. Clasts range from a few cm to metres across and vary from angular 'jigsaw-fit' to rounded, suggesting multiple phases of volatile activity from hydraulic fracturing to features typical of fluidisation. The outer margins with unbrecciated wallrock are abrupt. Wallrocks proximal to the breccia pipes have been cracked, forming shatter rims several to tens of meters wide (Fan et al., 2000). Orebodies are located in the parts of the breccia pipes that are associated with faults, and the ores are associated with vein-like fine-grained metasomatic chert.
Mineralisation styles include vein, disseminations and stockworks, with ore zones forming sub-parallel sheets that are near perpendicular to the pipe walls. The bulk of the gold is associated with sub-horizontal quartz veins that contain adularia and pyrite. The main ore minerals are pyrite, chalcopyrite, galena and native gold, with lesser sphalerite, electrum, molybdenite, chalcocite and magnetite, filling open spaces between the breccia clasts. Bi-sulphosalts and -sulphides have been reported (Shao and Li, 1989). Gold is predominantly within the fine-grained pyrite, both as inclusions and as fissure-fillings. The wallrocks have undergone potassic, silica, epidote, sericite, chlorite, carbonate, propylitic and pyritic alteration. Adularia, epidote and silica alteration are mainly confined to the breccia pipe, whereas propylitic alteration affects the wallrock. Within the breccia pipe, the matrix has been more altered and gold mineralised than in the breccia clasts. Potassic alteration (including K feldspar, adularia, biotite and sericite), silicification and abundant sulphides (particularly pyrite) are generally indicative of better gold grades (Chen and Fu, 1992). Two alteration events are recognised. Potassic alteration is the earlier and precedes gold mineralisation, mainly affecting gneisses and volcanic wall rocks and comprising of K feldspar and quartz. The second is restricted to the pipes and comprises two sub-stages: i). Pervasive, comprising chlorite, actinolite, green biotite, epidote, quartz, adularia, calcite and sericite. The paragenetic sequence proceeds from green biotite+actinolite to epidote+chlorite+pyrite. ii). Vein and open space filling quartz, adularia with calcite and minor sericite, which affects not only the breccia clasts, but also the matrix. Calcite is a late phase, and cuts quartz veins, clasts, and the cementing material. Adularia and calcite infill open spaces and/or fissures in pyrite, or form veins that cut altered clasts. Textural relationships suggest gold mineralisation is paragenetically associated with adularia-calcite and pyrite.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 2019.
Record last updated: 10/3/2020
This description is a summary from published sources, the chief of which are listed below.
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Chen, Y.-J., Pirajno, F., Li, N., Guo, D.-S. and Lai, Y., 2009 - Isotope systematics and fluid inclusion studies of the Qiyugou breccia pipe-hosted gold deposit, Qinling Orogen, Henan province, China: Implications for ore genesis: in Ore Geology Reviews v.35, pp. 245-261.|
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.|
Jiang, N., Xu, J. and Song M., 1999 - Fluid inclusion characteristics of mesothermal gold deposits in the Xiaoqinling district, Shaanxi and Henan Provinces, Peoples Republic of China: in Mineralium Deposita v.34, pp. 150-162.|
Li, J.-W., Li, Z.-K., Zhou, M.-F., Chen, L., Bi, S.-J., Deng, X.-D., Qiu, H.-N., Cohen, B., Selby, D. and Zhao, X.-F., 2012 - The Early Cretaceous Yangzhaiyu Lode Gold Deposit, North China Craton: A Link Between Craton Reactivation and Gold Veining: in Econ. Geol. v.107, pp. 43-79.|
Liu, J., Liu, C., Carranza, E.J.M., Mao, Z, Wang, J., Wang, Y., Zhang, J., Zhai, D., Zhang, H.,Shan, L., Zhu, L. and Liu, R., 2015 - Geological characteristics and ore-forming process of the gold deposits in the western Qinling region, China: in J. of Asian Earth Sciences v.103, pp. 40-69.|
Mao, J., Goldfarb, R.J., Zhang, Z., Xu, W., Qiu, Y. and Deng, J., 2002 - Gold deposits in the Xiaoqinling-Xiongershan region, Qinling Mountains, central China: in Mineralium Deposita v.37, pp. 306-325.|
Ni, Z.-Y., Chen, Y.-J. and Zhang, H., 2012 - Pb-Sr-Nd isotope constraints on the fluid source of the Dahu Au-Mo deposit in Qinling Orogen, central China, and implication for Triassic tectonic setting: in Ore Geology Reviews v.46, pp. 60-67|
Xu, J., Xie, Y., Jiang, N. and Bei, F., 1998 - Mineralogical, fluid inclusion and stable isotope study of Wenyu-Dongchuang gold deposits in the Xiaoqinling Mt. area, west Henan, China: in Exploration & Mining Geology, CIM v.7, pp. 321-332.|
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.|
Zhou, T., Goldfarb, R.J. and Phillips, G.N., 2002 - Tectonics and distribution of gold deposits in China - an overview: in Mineralium Deposita v.37, pp. 249-282.|
Zhou, Z.J., Chen, Y.J., Jiang, S.Y., Zhao, H.X., Qin, Y. and Hu, C.J., 2014 - Geology, geochemistry and ore genesis of the Wenyu gold deposit, Xiaoqinling gold field, Qinling Orogen, southern margin of North China Craton: in Ore Geology Reviews v.59, pp. 1-20.|
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