Yidun / Zhongdian Arc - Pulang, Xuejiping, Hongshan, Lannitang, Chundu, Qiansui, Langdu, Donglufang, Bengge,
Yangtze Craton Margin - Suoluogou, Danba, Yanzigou |
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Yunnan, China |
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Cu Au Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Late Triassic and Late Cretaceous Yidun or Zhongdian Magmatic Arc hosts a number of both Triassic and Cretaceous porphyry and porphyry-skarn copper deposits, including Pulang, Xuejiping, Hongshan and Donglufang in northern Yunnan, southwestern China. Other smaller deposits and occurrences include Lannitang, Chundu, Qiansui, Osaila, Yaza and Langdu. The Cretaceous age, intrusion related, Bengge gold deposit, hosted by Triassic syenites, is located in the southwestern corner of the Yidun Terrane, 45 km SSW of Pulang.
The southern half of the Yidun or Zhongdian Magmatic Arc represented a compressive regime during the Mesozoic and is characterised by porphyry copper and gold deposits, whilst the northern segment was extensional and contains volcanic hosted massive sulphide deposits such as Gacun (Yu et al., 2020; Li et al., 2011).
The Yidun/Zhongdian Magmatic Arc lies ~100 km east of the younger Jinshajiang-Ailaoshan-Red River Metallogenic Belt (which incorporates the Yulong Porphyry and Ailaoshan-Red River) belts, within the broader East Tethyan Orogenic Belt of central to southeast Asia, which, in turn, is the eastern segment of the greater Tethyan Orogenic Belt that extends from Indochina to the Atlas Mountains of Morocco.
Porphyry style mineralisation is largely concentrated in the southern half of the Yidun Arc in two short, parallel, NNE-SSW trending arc remnants. Deposits are related to both Late Triassic subduction, and to post-collisional Cretaceous magmatic activity.
A number of orogenic gold deposits e.g., Suoluogou, Danba and Yanzigou form an important tectonic and metallogenic belt that straddles the northwestern margin of the Yangtse Craton and the Songpan-Garze oceanic plate/accretionary complex in the north, and the Yidun Terrane in the south, apparently unrelated to magmatic sources, but to Mesozoic tectonic activity.
Other significant porphyry clusters within the East Tethyan Orogenic Belt include the Duolong Cluster and enclosing Bangong Co-Nujiang Mineral Belt, Gangdese Belt, Yulong Porphyry Belt, and the Ailaoshan-Red River Belt.
Regional Setting
For detail of the regional setting and context, see the Regional Setting section of the separate Ailaoshan-Red River Belt (not yet complete) and Yulong Porphyry Belt records.
The Yidun Terrane, which is wedge-shaped, NNW tapering, ~550 km long and NNW-SSE aligned, is located to the east of the Qiangtang Terrane (see diagram below). It is bounded to the west by the Jinshajiang Suture and to the east by the Ganzi-Litang Suture ophiolite mélange zone which separates it from the Songpan-Garze Terrane in the north and east and the Yangtze Craton to the southeast. The Ganzi-Litang Suture converges and merges with the Jinshajiang Suture to the NNE and south, truncating the Yidun Terrane and juxtaposing the Qiangtang and Songpan-Garze terranes. Triassic magmatism in the Yidun Terrane is interpreted to be the product of westward subduction of section of the Palaeozoic Garze-Litang oceanic slab (Hou, 1993; Hou et al., 1995, 2001). From west to east, the Yidun Terrane includes the i). Zhongza Block (or Western Yidun Arc) composed of Palaeozoic rocks that were deformed and metamorphosed during the Early Triassic collision between the Qiangtang and Yidun Terranes across the Jinshajiang Suture Zone (Reid et al., 2007). This block is interpreted to have been rifted from the Yangtze Craton during opening of the Garze-Litang Ocean in the Mid to Late Palaeozoic, possibly associated with Permian regional extension (Zhang et al., 1998). It is separated by the Xiangcheng-Geza fault, from the ii). Eastern Yidun Terrane (or Arc), which is almost exclusively composed of Middle and Upper Triassic volcanosedimentary flysch and mafic to felsic volcanic successions of the intra-oceanic Yidun arc (Hou et al., 1993, 2003). These volcanic rocks include andesite, dacite, porphyritic rhyolite, and minor basalt, which were erupted at ~220 Ma and occur within a sequence of Triassic fine-grained clastic, bioclastic and mudstone rocks (Leng et al., 2014). These volcaniclastic rocks are intruded by voluminous granitoid intrusions, many of which are dioritic. The Yidun arc is apparently underlain in part by a block (or blocks) of Precambrian basement with a Cathaysian/Yangtze Block affinity (e.g., Reid et al., 2007) and covered by Palaeozoic warm-water sedimentary rocks and faunas (e.g., Metcalfe, 2013). This basement is interpreted to have been split from the Yangtze Craton in the mid- to late Palaeozoic. This arc volcanism apparently developed in response to westward subduction along what is now the Ganzi-Litang suture of the Songpan-Garze oceanic plate that separated it from the approaching Yangtze Craton to the SE.
Hf-Nd isotopic analyses suggest the Eastern Yidun Terrane is, in turn, divided into two sub-terranes. Volcanic and intrusive magmas in the northern subterrane likely derived from a Palaeoproterozoic or older mafic to intermediate lower crust with a variably, but minor addition of Triassic juvenile mantle melts. These rocks comprise 235 to 230 Ma basalt, andesite, dacite and rhyolite, and a 225 to 215 Ma granite batholith. In contrast, magmatic rocks in the southern part of the terrane include 228 to 215 Ma adakite-like andesite, as well as diorite to monzonite porphyry, which were dominated by arc juvenile mantle wedge melts with subordinate input of older crustal materials (Gao et al., 2018). This southern sub-terrane suite belongs to the high K calc-alkaline series with arc magmatic geochemical characteristics such as enrichment in Rb, Ba, Th and U, and depletion in HFSE. The two subterranes are also characterised by differing mineralisation styles, with volcanic massive sulphide Ag-Cu-Pb-Zn (e.g., Gacun) and epithermal Ag-Hg mineralisation hosted in the ~230 Ma rhyolites of the subterrane to the north; while in the southern subterrane, porphyry-skarn Cu-Mo-Fe deposits predominate, genetically related to the ~216 Ma dioritic to monzonitic porphyries e.g., Pulang and Xuejiping (Gao et al., 2018). The Yidun Terrane was sutured to the Qiangtang Terrane at 245 Ma in the Mid Triassic, as constrained by the age of post tectonic plutons (Reid et al., 2007) prior to formation of the Triassic porphyry deposits.
The Garze-Litang basin and underlying oceanic plate, which had been subducting below the eastern Yidun Terrane since the Mid Triassic, finally closed at ~80 Ma in the Late Cretaceous, and the Yangtze Craton collided with the Yidun Arc, and to the south of the Yidun Terrane with the Changdu-Simao Block of the eastern Qiangtang Terrane (Reid et al., 2005, 2007; Xu et al., 2015).
Subsequently during Late Cretaceous post-collisional transtension further porphyries were intruded, dated at 73.4 ±0.7 Ma (SHRIMP U-Pb zircon; Zu et al., 2016). The NNW-trending sinistral faults and shear zones described above, facilitated the emplacement of these plutons which are elongated parallel to sub-parallel to these structures (Yang et al., 2016; 2018). In contrast to the Triassic magmas, these Late Cretaceous quartz monzonite porphyries have shoshonitic I-type geochemical characteristics, with high SiO2, K2O, LILE, low HREE, Y and Yb contents, and high LREE/HREE and La/Yb ratios. These geochemical characteristics, together with the Sr-Nd-Pb isotopic compositions [average (86Sr/87Sr)i = 0.7085; εNd(t) = − 6.0; 206Pb/204Pb = 19.064, 207Pb/204Pb = 15.738, 208Pb/204Pb = 39.733]. These data are interpreted to suggest that the quartz monzonite porphyries originated from partial melting of ancient lower crust which included an underplating of mafic magma in the lower mantle lithosphere that had been metasomatised by earlier (Late Triassic) subduction to produce a fertilised hydrous cumulate zone containing large amounts of sulphur and metals (Zu et al., 2016). These quartz monzonite porphyries are apparently genetically related to at least four Mo-Cu (-W) deposits (Xiuwacu, Relin, Hongshan and Tongchanggou; Yang et al., 2018).
The southern sub-terrane arc rocks of the Yidun Terrane are divided the into two NNW-SSE trending belts of Triassic magmatic rocks, both of which are <70 km long and 5 to 10 km wide. These two arc remnants contain the East and West Porphyry Belts, and are separated by the 5 to 15 km wide Shudu Ophiolite Zone. The volcanic rocks hosting both porphyry belts and the ophiolite zone are truncated by the generally north-south Xiangcheng-Geza Fault and related structures which juxtapose them with the Zhongza Block metamorphosed Palaeozoic rocks to the west. The two arc segments and ophiolite zone are structurally bounded on all sides. To the south, the longitudinal structures bounding the ophiolite zone diverge and truncate the two arc remnants. The Shudu Ophiolite Zone is composed of basalts and thinly bedded cherts and limestones with discontinuously exposed ultramafic rocks, interpreted to represent an ophiolite mélange zone (Li et al., 2011).
The volcanic rocks of the West Porphyry Belt are dominantly andesitic volcaniclastic rocks and lavas, with coeval hypabyssal intrusions of diorite, quartz diorite, etc. that were emplaced at ~249 Ma; (Zeng et al., 2003, 2004) in the Early Triassic. They were formed earlier and are
more intensely foliated than rocks of the East Porphyry Belt. This belt hosts the Xuejiping, Lannitang and Chundu deposits.
The East Porphyry Belt lies to the east of the Heishuitang Fault which separates the Shudu Ophiolite Zone from the eastern magmatic arc and hosts the Late Triassic Pulang and Qiansui deposits to the north and south respectively, and the Late Cretaceous Hongshan porphyry and Donglufang porphyry skarn deposits to the north and south respectively. Thick, massive lavas are located adjacent to the Heishuitang fault, part of the sequence in the belt that is dominantly andesitic, with cleavage much less developed than in the West Porphyry Belt. Hypabyssal quartz monzonite porphyry near Hongshan has an age of 214 Ma (Rb/Sr isochrons on whole-rock samples; Tan et al., 1985), whilst biotite separates from the Langdu quartz monzonite porphyry were dated at 216.93 ±4.34 Ma (40Ar/39Ar isochron age; Zeng et al., 2000, 2003). The Pulang intrusion is located at the south end of the East Porphyry Belt.
The Mesozoic tectonics within the Yidun Terrane are dominated by the NNW-SSE to north-south trending regional faults that formed during the westward subduction of the Garze-Litang and Jinshajiang oceanic plate below the terrane, followed by the arc-continental collision between the Yidun Terrane, Qiangtang Terrane and Yangtze Craton (Reid et al., 2005). A series of NW-SE oriented sinistral strike-slip faults also cut across the Yidun Terrane (Hou et al., 2003, 2004). Mesozoic intrusions are elongated NW-SE, parallel to the fault systems, and are commonly bounded by ductile to brittle deformation along their margins, although they are relatively undeformed at their centres suggesting intrusion within active structures. Magmatic lineations and flow structures in the plutons and the mylonitic textures and shear zones in the host rocks are generally subparallel to each other (Yang et al., 2016).
LATE TRIASSIC PORPHYRY AND PORPHYRY SKARN DEPOSITS
Extensive Late Triassic andesitic volcanic and intermediate to felsic porphyritic intrusive rocks are found within the southern Yidun Terrane. The Late Triassic (peak at 215 Ma) porphyritic intrusions occur as NW elongated stocks and dykes, and are characterised by low Y and HREE contents with high Sr/Y and La/Yb ratios, resembling adakitic signatures (Wang et al., 2011). Many of these intrusions have associated porphyry and/or porphyry-skarn Cu-polymetallic mineralisation. Representative deposits include the porphyry-type Pulang, Xuejiping, Songnguo and Chundu Cu- polymetallic deposits, and the Langdu skarn Cu-polymetallic deposit (Li et al., 2011, 2011; Wang et al., 2011; Leng et al., 2012).
Pulang Cu-Au (#Location - Pulang: 28° 2' 10"N, 99° 58' 19"E)
The Pulang porphyry copper deposit is located ~470 km NW to NNW of Kunming and ~460 km SW of Chengdu on the Tibetan Plateau, in Shangri-La county, Yunnan province, southwestern China at an altitude of 3520 to 4570 m.a.s.l. It lies within the East Porphyry Belt of the southern subterrane of the Yidun Arc in the Yidun Terrane.
History - Although small copper showings had long been known in the Pulang, the main deposit was only discovered in the late 20th centurt during exploration undertaken by a joint venture between Billiton Plc. and the Yunnan Bureau of Exploration and Development of Geology and Mineral Resources, during follow-up of a mineralised target generated by the Bureau. The Bureau and Billiton formed the Gaoshan Mining Company in 1999 to conduct exploration in the Pulang area. Stream sediment sampling, an aeromagnetic survey and surface geologic mapping resulted in Cu mineralised outcrops and hydrothermally altered porphyry being located that were coincident with an aeromagnetic anomaly. Three initial diamond drill holes were completed, the first of which was mineralised throughout, including 103 m from surface averaging >1% Cu. The other two holes did not intersect significant mineralisation and in early 2001, Billiton withdrew from the joint venture. In 2002, the Yunnan Geological Survey, supported by the China Geological Survey, used a special government fund to begin systematic exploration of the area. By the end of 2005, the Yunnan Geological Survey had completed 25 000 m of drilling, 400 m of engineering pits, and extensive trenching, as well as ground magnetic surveys and remote sensing investigations. The defined Cu resources at Pulang is 4.18 Mt of contained Cu metal (Li et al., 2011).
Geology - The country rock in the Pulang district is dominated by metasandstone and slate with interbedded andesite of the Late Triassic Tumugou Formation. The Pulang complex which intruded these rocks comprises five mineralised intrusive bodies that were emplaced into the core of the Pulang dome following NW-SE and NE-SW trending faults to form a horizontal ‘Y’ shaped cluster. The individual intrusions are numbered from I to V inclusive and have outcrop areas that are respectively 6.53, 1.19, 0.26, <0.3 and 0.64 km2. These Late Triassic intrusions can be divided into three stages, as follows (after Li et al., 2011):
• Quartz diorite porphyry - which is grey and has a porphyritic texture, with phenocrysts of 22% plagioclase and <5% hornblende set in a matrix of 65% plagioclase, 7% quartz, 1% hornblende and minor K feldspar with a fine to microgranular texture and massive structure. It includes accessory zircon and apatite with alteration dominated by quartz, sericite and propylitic assemblages with local deutiric conversion of pyroxene to uralite. Quartz diorite porphyry was the earliest stage, and it was crosscut by each of the two following intrusive stages.
• Quartz monzonite porphyry - which is grey, microgranular and massive; with phenocrysts of 15% plagioclase, 13% K feldspar, 2% biotite and minor quartz, set in a matrix of 25% oligoclase, 28% K feldspar, 15% quartz and 2% biotite, with accessory magnetite, zircon and apatite. Alteration dominated by quartz-sericite and K feldspar-quartz, with local chlorite and hydrothermal biotite. This stage cuts the quartz diorite, but is cut by the following granodiorite porphyry.
• Granodiorite porphyry - which occurs in dykes and is grey, porphyritic, microgranular, crystalloblastic and massive. It comprises phenocrysts of 20% plagioclase (oligoclase-andesine), 5% K feldspar, 5% biotite, 3% quartz and 10% hornblende set in a matrix of 39% plagioclase-quartz, 15% hornblende and 5% biotite, with accessory magnetite, zircon and apatite. Alteration is dominated by sericite-quartz and K feldspar-quartz, with local chlorite and hydrothermal biotite. This stage intrudes each of the two preceding pulses.
The constituents of each of the mineralised porphyry intrusions at Pulang are as follows: Intrusion I - Quartz monzonite and granodiorite porphyries surrounded by quartz diorite porphyry. This is the largest of the mineralised intrusions and contains the bulk of the Pulang resource; Intrusions II, III and V are composed of quartz-diorite porphyry; whilst intrusions IV is a quartz monzonite porphyry. The orebodies are mainly located within the potassium silicate and phyllic alteration zones, within quartz monzonite porphyry emplaced within quartz-diorite porphyry (Li et al., 2011).
Alteration - Intense alteration and mineralisation appear to be predominantly related to the syn-mineral quartz-monzonite porphyry stage, while the late granodiorite porphyry is generally limited in extent and usually only contains weak mineralisation and is probably late-mineral. The early quartz-diorite appears to be pre- to early-mineral. Where surrounded by hornfelsed country rock, the porphyry intrusions is zoned from a core of quartz passing to K silicate (biotite and K feldspar) alteration to an outer propylitic (chlorite, epidote) periphery with a later overprinting phyllic shell straddling the transition between the latter two early phases in the upper parts of the deposit. The system passes downward into an albite zone, and upwards to an irregular cap of clay alteration. The components of this alteration pattern are as follows (after Li et al., 2011):
• Silicic zone - which occupies the core of the porphyry body, characterised by densely disseminated quartz or stockworks of quartz with lesser sulphides (predominantly pyrite), broadly corresponding to a low-grade (0.20 to 0.40% Cu) core to the deposit.
• Potassium silicate zone - which is predominantly associated with Mo-bearing Cu orebodies (with 0.40 to 1.56% Cu) and forms a shell over and laterally surrounding the intense silicic core. This zone is characterised by a substantial increase in the content of K feldspar, quartz and minor biotite veinlets, with quartz and quartz-sulphide veinlets. Hornblende in the protolith was replaced by hydrothermal biotite with associated apatite and zircon.
• Quartz-sericite / phyllic zone - sericite associated with quartz progressively replaces potassium silicate minerals in this zone, as well as Plagioclase and K feldspar in the protolith with the porphyritic texture preserved. Magmatic quartz exhibits hydrothermal overgrowths, while biotite is crosscut by sulphide veinlets and stockwork veining. Molybdenite-bearing quartz veinlets are typically found in this zone on the margin of the main Cu mineralisation, which also may in part extend into the phyllic zone which generally only contains 0.05 to 0.22% Cu. There is no clear boundary between the potassium silicate and phyllic zones.
• Propylitic zone - which mainly occurs outside of the phyllic zone, and is characterised by an assemblage of chlorite, zoisite and pyrite and carbonate veins. Pyrite is widespread, with some weak Cu mineralisation and locally Au.
• Hornfels zone - forming an extensive halo, typically two to three times the diameter of the associated intrusion. It is characterised by an assemblage that may include quartz, sericite, chlorite, carbonate and extensive pyrite, with minor sulphide veins. Extensive limonite staining at surface is an indicator of the hornfels and sulphides. Pb-Zn veining occurs within the hornfels as a halo to the Cu-Au and Mo mineralisation.
Mineralisation - Six orebodies have been delineated in the initial 3.65 x 4 km mining area. The main orebody, KT1, is located in the core of the Pulang porphyry cluster. It outcrops at an elevation of 3870 to 4030 m, and is >1600 m long where is was unconstrained to the north (in 2011). Its width changes form 360 to 600 m in the south, to 120 to 300 m in the north. The vertical extent of the ore zone varies from 52 to 370 m, to a maximum thickness of 700 m. Drill hole intersections of the ore zone varies from 17 to 700 m with grades of from 3.74% to 0.20, averaging 0.52% Cu. The central part of the orebody is very thick with high Cu grades that gradually decrease outward, accompanied by 0.87 to 0.06, averaging 0.18 g/t Au; 3.93 to 0.34 g/t, averaging 1.27 g/t Ag; 0.030 to 0.001, averaging 0.010% Mo; 1.90 to 0.94, averaging 1.34 ppb Pt; and 14.00 to 2.20, averaging 4.95 ppb Pd (Li et al., 2011).
Mineralisation occurs in the following three styles:
• Veinlet and disseminated mineralisation - Cu mineralisation of this type is most intense in the K silicate (biotite and K feldspar) zone. Chalcopyrite replaces hornblende and early biotite as circular patches or is distributed as veinlets and disseminations in association with quartz in yellowish-pink K feldspar-biotite altered quartz-monzonite porphyry. Cu grades are generally >1%, but decline outward where chalcopyrite veinlets and disseminations occur in quartz-sericite altered quartz monzonite porphyry.
• Crypto-explosive, breccia-related mineralisation - which occur as east-west oriented breccia dykes on the western margin of the quartz-diorite porphyry, defining five ore breccia veins (KT4-KT6) with varying lengths that cut early, quartz-monzonite porphyry. This ore type has grade of 1 to 10% Cu and is found at the top of the mineralised intrusion, above the veinlet-disseminated zone, suggesting little of the deposit has been eroded.
• Pb-Zn veining - occurring as a few galena-sphalerite-quartz veins that fill tensional fractures north of the mine area.
The ore is dominated by sulphides, with local oxidised and mixed mineralisation. The oxidised zone is generally only to depths of 10 to 40 m. Malachite is rarely observed in the oxidised zone, whilst tenorite is found on joint planes, the result of high altitude, and relatively low atmospheric CO2. This tenorite one of the main prospecting indicators for porphyry copper mineralisation in wet, high altitude zones. The Metallic minerals in the sulphide ore zone are dominated by chalcopyrite, bornite, pyrite, pyrrhotite, magnetite and molybdenite.
Li et al. (2011) determined ages of hydrothermal biotite and K feldspar from several intrusive phases, using K-Ar and Ar/Ar methods, and the age of molybdenite in orebody KT1 using the Re/Os method. The potassium silicates returned ages in the range of 235.4 ±2.4 to 221.5 ±2.0 Ma, whilst the molybdenite was estimated to be 213 ±3.8 Ma.
Mineral Resources are quoted as follows:
Initial orebody KT1 mining block - 300 Mt @ 0.52% Cu, 0.18 g/t Au, 0.01% Mo for 1.57 Mt Cu and 54 t Au (Li et al., 2011).
Total resource - 1.23 Gt @ 0.34% Cu, 0.01% Mo, 0.18 g/t Au, 1.27 g/t Ag (USGS Mineral Resource Database, viewed Jan., 2019).
Total resource - 803.85 Mt @ 0.52% Cu, 0.18 g/t Au (He et al., 2018).
Xuejiping Cu (#Location: 28° 0' 38"N, 99° 48' 41"E)
The Xuejiping porphyry copper deposit is located ~440 km NW to NNW of Kunming and ~475 km SW of Chengdu on the Tibetan Plateau, in the Sanjiang district of northwestern Yunnan province, southwestern China. It is ~15 km WSW of the Pulang deposit and lies within the West Porphyry Belt of the southern subterrane of the Yidun Arc in the Yidun Terrane.
The main country rock sequence exposed in the deposit area is the Upper Triassic Tumugou Formation, composed of sandy slates, argillaceous limestone and andesite. The Xuejiping intrusive complex comprises diorite-, quartz diorite- and quartz monzonite-porphyries, intruded into argillaceous limestone of the Tumugou Formation. Its location is controlled by structures related to the NNW-trending Xiangcheng-Geza Fault and it is moderatley foliated. The complex is exposed over an area of 0.98 km2 with dimensions of 2200 x 200 to 870 m (Zhong, 1982).
Copper mineralisation and associated alteration are centred on the main quartz diorite porphyry intrusive phase. There is a zonation of alteration outward from the core of the deposit, grading from strong silicification → quartz-sericite → clay-quartz-sericite → propylitic alteration on the peripheries. Cu mineralisation occurs over an area of ~1100 x 300 m, mainly within the zone pf strong silicification and the quartz-sericite alteration zone of the quartz diorite porphyry intrusive (Zhong, 1982).
In 2009, mining activity is predominantly concentrated on four main orebodies extracting mineralisation that occurred as disseminations, stockwork veining and coarse veining, with the major ore mineral being chalcopyrite, accompanied by minor chalcocite, galena, sphalerite and molybdenite. The principal gangue minerals are quartz, calcite, plagioclase, sericite and biotite, with minor chlorite, barite and kaolinite.
The hydrothermal mineral paragenesis and veining intersection relationships suggest, four stages of Cu mineralisation and alteration (Leng et al., 2008):
• Stage I - disseminated mineralisation associated with potassic alteration. Pyrite and chalcopyrite occurs as 'spots' that are generally disseminated in weakly potassic altered porphyry with few quartz veins.
• Stage II - 'patchy' and stockwork veining associated with strong silicification. Sulphides are variably distributed in the form of veining, stockwork veining or 'patches' in coarse quartz veins, or disseminated patches.
• Stage III - occurs as stockwork veined mineralisation, associated with clay alteration, and overprints early stockwork veining.
• Stage IV - carbonate-quartz-sulphide veining type.
Rui et al. (2005) quote an age of 225 Ma for the host intrusions and mineralisation at Xuejiping.
Production in 2003 was reported to be from 30 to 80 m thick ore zones with grades of 0.63% Cu and 0.06 g/t Au (China Gold International, 2003).
Mineral Resources are quoted as follows:
Mineral Resource - 60 Mt @ 0.5% Cu, 0.06 g/t Au, 1.4 g/t Ag (Kirkham and Dunne, 2000)
Mineral Resource - 54.15 Mt @ 0.53% Cu, 0.06 g/t Au (He et al., 2018)
Resource undifferentiated - 240 Mt @ 0.5% Cu (Li et al., 2011),
Langdu Skarn Cu
The small, but high grade Langdu skarn-porphyry copper deposit is located ~490 km NW to NNW of Kunming, ~450 km SW of Chengdu and 50 km NE of Xianggelila City on the Tibetan Plateau, in the northwestern Yunnan province, southwestern China. It is ~18 km north of the Pulang deposit and lies within the East Porphyry Belt of the southern subterrane of the Yidun Arc in the Yidun Terrane.
This summary is included to demonstrate one of the styles of mineralisation in the district, rather then the economic significance of the deposit.
The deposit was discovered in 1964 by Team No. 1 of the Regional Geological Survey, Bureau of Geology and Mineral Resources, Yunnan Province, China. Resources have been extended through the discovery by the Yunnan Huaxi Mining Co., Ltd, mainly during 2006-7, of several additional copper ore bodies in the contact zone of porphyry intrusions and marbles to the north and west of the original Langdu skarn deposit.
Exposed country rock predominantly belong to the the Upper Triassic Qugasi and Tumugou Formations, while a small section of the deposit is masked by up to 30 m of Cenozoic alluvium. The Tumugou Formation is largely found to the SW and comprises ~1430 m of slate, micaceous slate, meta-sandstone and limestone. The underlying Qugasi Formation predominates in the deposit area where it occupies the core part of the Langdu anticline. It consists of bedded limestone, micaceous slate and fine meta-sandstone, and has a total thickness varying from 487 to 2683 m. These sedimentary rocks have been metamorphosed by porphyry intrusions, with the clastic rocks converted to felsic and brecciated hornfels, while the limestone have been metasomatised to skarn or converted to marble in the contact zone. The main intrusive phase is a monzonite porphyry that has been dated (Zeng et al., 2004; Ar-Ar biotite separate)at ~216.93 Ma.
The structural framework of the deposit area comprises the NW-trending Langdu anticline and crosscutting NW and NE trending faults. The NW-trending fractures are mainly bedding parallel fractures along the strike of sedimentary rocks on the flanks of the Langdu anticline. Locally the NE-trending Bidu fault cuts through the core of the Langdu anticline. Copper mineralisation is mainly localised at the intersection between the NW and NE trending faults on the SW limb of the Langdu anticline.
The known copper ore bodies are mainly localised along the contact zone between the porphyry intrusions and Upper Triassic Qugasi Formation limestone, although one ore body occurs within the monzonite porphyry. In 2015, 6 ore blocks and 21 ore bodies have been identified in the deposits.
The Langdu deposit is a multistage skarn with complex mineralisation, including:
• Ca-Fe anhydrous silicate minerals (andradite and hedenbergite) dominate, formed during the prograde skarn stage. Andradite varies from Ad57 to Ad94, while the pyroxene is Hd78-92 Di3-11 Jo5-11.
• Hydrous silicate minerals, including actinolite and grunerite, formed during the retrograde stage, with late deposition of minor Cu-Fe sulphide, quartz and calcite.
• Chalcopyrite and pyrrhotite are the dominant ore-forming sulphides, occurring as veins, veinlets or patches in the coarse veins of quartz or calcite, with an average grade of 6.49 wt.% Cu (and >10 wt.% in some localities).
• A late ore-stage involves chalcopyrite occurring in veins or veinlets that fill fissures in marble, or as patches in Fe-dolomite coarse veins.
• Post-ore hydrothermal minerals include prominent calcite, quartz and chlorite.
Mineral Resources are quoted as - 1.67 Mt @ 6% Cu (He et al., 2018).
Other resources
Contained copper resources in other Triassic deposits/prospects in the southern sub-terrane of the Yidun Arc include :
• East Porphyry Belt: Qiansui - 0.5 Mt; Yaza - 0.2 Mt;
• West Porphyry Belt: Lannitang - 1 Mt; Osaila - 0.5 Mt; Chundu - 0.8 Mt.
LATE CRETACEOUS PORPHYRY AND PORPHYRY SKARN DEPOSITS
Two porphyry skarn deposits of post-collisional Late Cretaceous age are described below as examples of the mineralisation of that age in the arc. Both are in the East Porphyry Belt. Late Cretaceous mineralisation ages have also been determined in the Xiuwacu W-Mo (83±1 Ma; Li et al., 2007), Relin W-Mo (81.2±2.3 Ma; Li et al., 2007) and Tongchanggou Mo-Cu (85±10 Ma; Li et al., 2012) deposits in the East Porphyry Belt of the southern sub-terrane of the Yidun Arc. The Bengge Intrusion-related Au is also described below. Although it is hosted within a Late Triassic alkaline igneous complex in the West Porphyry Belt, the phyllic alteration related to the mineralisation has been dated at ~126 Ma in the Cretaceous.
Hongshan Cu-(Mo-Pb-Zn)
The Hongshan skarn-porphyry copper-base metal deposit is located ~490 km NW to NNW of Kunming, ~445 km SW of Chengdu and 30 km NE of Shangri-la on the Tibetan Plateau, in northwestern Yunnan province, southwestern China. It is ~16 km NNW of the Pulang deposit and lies in the western margin of the East Porphyry Belt of the southern subterrane of the Yidun Arc in the Yidun Terrane, immediately adjacent to the Shudu Ophiolite Zone.
Geology - Exposed country rock in the Hongshan area includes the second and third members of the Upper Triassic Qugasi Formation which is overlain across a faulted contact by the second member of the Upper Triassic Tumugou Formation. These units dip at 60 to 80°WSW and dominantly comprise dark grey slate, meta-sandstone, andesite, rhyolite and volcaniclastic rocks with locally interbedded limestone and siliceous rocks (Hou et al., 2003; Wang et al., 2008). They have undergone contact thermal metamorphism and metasomatic replacement to produce extensive hornfels, marble and skarn throughout the deposit and surrounding areas, suggesting the presence of concealed intrusions. The ore-bearing skarns are predominantly restricted to the transition between slate and marble lenses in the second and third members of the Qugasi Formation (Huang et al., 2001; Wang et al., 2008).
The host sedimentary sequences has been isoclinally folded and is commonly crosscut by well developed NNW trending strike-slip faults that are consistent with the regional structural trend. Displacement on these faults is interpreted to have produced local dilation zones that afforded channels for magmatic and hydrothermal fluids (Hou et al., 2003).
Intrusions take the form of abundant felsic dykes throughout the deposit area. Two petrologic types of intrusion have been identified:
• Diorite porphyry - which are the most widely distributed, mainly found in the southeastern and northern parts of the deposit area. They are mostly medium to dark grey in colour, with a typical porphyritic texture, composed of plagioclase and hornblende phenocrysts in a groundmass of plagioclase, quartz, hornblende, biotite and chlorite. The plagioclase phenocrysts and groundmass commonly have weak developments of sericite and chlorite. Geochronological determinations indicate these diorite porphyries were formed in the Triassic (Huang et al., 2012) with an age of 214 ±2 Ma (SHRIMP U-Pb zircon; Zu et al., 2016). However, no direct contact between these porphyries and the orebodies has been observed in the Hongshan deposit (Zu et al., 2016).
• Quartz monzonite porphyry - which outcrops as three small intrusions in the middle of the Hongshan deposit area, although drilling has revealed larger concealed sub-surface mineralised stocks. It is greenish-grey to dark grey in colour, with a typical porphyritic texture. The phenocrysts comprise 10 to 20% plagioclase, 15 to 20% quartz, 5 to 10% K feldspar, 5% biotite and 5% hornblende, with diameters varying from 3 to 20 mm. The plagioclase phenocrysts are generally smaller, mostly 1 to 3 mm euhedral crystals with well developed polysynthetic twin and rhythmic zonation. The groundmass is fine to micro-granular with visible plagioclase and quartz grains. Accessory minerals include magnetite, zircon, apatite, tourmaline, monazite, titanite and rutile (Zu et al., 2016). The bulk of the quartz monzonite porphyry has undergone weak sericite and chlorite alteration, whilst minor volumes are altered to endoskarn near contact zones with marble, where they carry minor disseminated pyrite, chalcopyrite and molybdenite. Geological evidence indicates that the quartz monzonite porphyries are temporal and spatially associated with the mineralisation in the Hongshan deposit (Zu et al., 2016). These porphyries have been dated at 73.4 0.7 Ma (SHRIMP U-Pb zircon; Zu et al., 2016), while age determinations of the ore minerals yielded ages of 77 to 80 Ma (Re-Os isochron ages of molybdenite and pyrrhotite; Zu et al., 2016).
Alteration and Mineralisation - Approximately 20 copper orebodies have been delineated at the Hongshan deposit, with lenticular, podiform and stratabound shapes reflecting the form of the host skarn altered bodies. Individual orebodies generally strike NNW, varying from 30 to 1223 m in strike length, 20 to 420 m down dip, and 4 to 20 m in thickness (Zu et al., 2016).
Copper orebodies are mainly hosted in skarn that is developed at the contact between marble and hornfels. The composition of the skarn grades from diopside-grossularite adjacent to the hornfels, to andradite±diopside at the transition to marble (Huang et al., 2001).
On the basis of ore textures and mineral assemblages, two types of Cu-Mo ores have been recognised (Zu et al., 2016), namely:
i). Massive to disseminated Cu-Mo skarn - This style accounts for the bulk of the Cu produced fo the Hongshan deposit (Zu et al., 2016). Prograde skarn minerals which include garnet, pyroxene and magnetite are overprinted by later retrograde actinolite, epidote, chlorite and sulphides (Peng et al., 2014). Garnet in the layered skarn is brownish red and subhedral, with well-developed oscillatory zoning. Chalcopyrite, pyrite, pyrrhotite and occasionally magnetite, molybdenite, galena and sphalerite are well developed in the Cu-Mo skarn ores.
ii). Veining, which is only minor (Zu et al., 2015). Veins vary from 0.5 to 200 cm in width, and are composed of assemblages of quartz-chalcopyrite-pyrrhotite, quartz-pyrrhotite-chalcopyrite-pyrite and quartz-pyrrhotite-chalcopyrite-molybdenite. The Cu content of veins decrease outward from the intrusion (Peng et al., 2014). The main vein gangue minerals are quartz, K feldspar, sericite and chlorite.
Two other types of ore are also present, although as of 2016, their economic viability had not been investigated,
iii). Hydrothermal Pb-Zn mineralisation that is developed in marble peripheral to the Cu-Mo deposit. This mineralisation is commonly banded, with parallel laminae of galena, sphalerite and locally pyrrhotite.
iv). Mo-Cu porphyry style mineralisation associated with concealed intrusions at depth and encountered in some drill holes (Li et al., 2011; Wang et al., 2014), although no tonnage and grade have not yet been defined (Zu et al., 2016). Mineralisation comprises of sparsely disseminated Mo-Cu sulphides within the intrusion.
On the basis of cross-cutting relationships both at an outcrop and microscopic scale, the mineralisation and alteration at Hongshan has been divided into three stages (Zu et al., 2016):
• Contact thermal metamorphism - related to emplacement of the Triassic and Cretaceous intrusions, forming an extensive hornfels aureole with an assemblage characterised by albite-quartz-sericite-andalusite.
• Prograde skarn - the result of metasomatism from interaction between magmatic hydrothermal fluids and marble country rock. This produced a diverse assemblages of calc-silicate minerals, including garnet, diopside, wollastonite and magnetite.
• Retrograde skarn - divided into two phases:
a). early retrograde, characterised by hydrosilicate minerals, including epidote, chlorite, actinolite and tremolite, and sulphide minerals include chalcopyrite, pyrite, pyrrhotite and molybdenite. This assemblage overprints the prograde calc-silicate minerals, to form the massive and disseminated skarn ores and resulted in the bulk of the Cu emplacement.
b). Quartz-sulphide stage, which featured a variety of quartz-sulphide veins overprinting skarns and hornfels.
Marble hosted hydrothermal Pb-Zn replacement mineralisation scattered around the periphery of the Cu-Mo skarn orebodies represents an outer mineralisation zone.
Mineral Resources are quoted as follows:
Mineral Resource - 23.76 Mt @ 1.01% Cu, 4.87% Pb, 4.86% Zn (Leng et al., 2012).
Donglufang Mo-Cu
The Donglufang skarn-porphyry copper-molybdenum deposit is located ~430 km NW to NNW of Kunming, ~430 km SW of Chengdu and 90 km SE of Shangri-la in northwestern Yunnan province, southwestern China. It is ~35 km SSE of the Pulang deposit, and occurs at the southern extremity of the East Porphyry Belt of the southern Yidun Arc (He et al., 2018). It lies on the northern margin of the Yangtze Craton, just to the south of the Garze-Litang Suture Zone and Yidun Terrane.
The geology of the deposit area is principally composed of Triassic limestone, marble and sandstone intruded by Late Cretaceous granodiorite porphyry and small lensoid dykes of diorite porphyry. The Triassic stratigraphic section is as follows, from the base (He et al., 2018):
• Qingtianbao Formation - arkose, greywacke and sandstone with intercalated basaltic volcaniclastic rocks;
• Beiya Formation - 210 to 590 m of white or light coloured limestones and dolomitic limestones, the main host unit at Donglufang;
• Songgui Formation - sandstones, marble and mudstones.
These are overlain by Quaternary lacustrine sedimentary rocks, mainly sandstone and conglomerate.
The granodiorite porphyry has been dated at from 87.4 to 84.2 Ma (LA-ICP-MS U-Pb zircon; He et al., 2018) with a weighted mean 206Pb/204Pb age of 85.1±0.5 Ma (He et al., 2018).
Regional structural trends are NNW, NW and east-west, including the NE-SW trending Baihuashan syncline, the east-west trending F5 fault; and the NW-SE trending F6 fault. The core of the Baihuashan syncline is occupied by the upper unit of the Beiya Formation, with rocks of the lower unit of the same formation and the underlying Qingtianbao Formation on the limbs (He et al., 2018).
Ten layered, lensoid, stratabound skarn hosted orebodies that are generally conformably with the host stratigraphy have been recognised, together forming the Donglufang deposit. The strike length of individual lenses varies from 15 to 450 m, with down dip extents of from 5 to 45 m. The thickness of these orebodies varies from 2.85 to 31.45 m with mineralisation grading from 0.44 to 1.76% Cu and 0.065 to 0.405% Mo (He et al., 2018).
The skarn hosted mineralisation generally occurs as layered, disseminated, massive and banded bodies that are mainly developed at the contact between marble and granodiorite porphyry with dominant Cu and minor Mo and Zn-Pb mineralization. The skarn is composed of 20 to 60% garnet, 5 to 25% pyroxene, 5 to 15% actinolite, 5% epidote and minor tremolite and wollastonite, with superimposed sulphide minerals that including 30 to 80% pyrrhotite, ~10% chalcopyrite and ~10% pyrite, with minor galena, sphalerite, scheelite and arsenopyrite (He et al., 2018).
In addition to the skarn hosted ores, porphyry-style mineralisation is also known, the bulk of which is hosted by the granodiorite porphyry, and only minor amounts of low grade copper in the diorite porphyry dykes. Mineralisation occurs as veins, veinlets and disseminations of pyrite, chalcopyrite and molybdenite, with minor pyrrhotite and arsenopyrite with associated quartz, K feldspar, sericite, epidote, chlorite, calcite and fluorite as gaunge minerals, selvages and/or pervasive alteration (He et al., 2018).
Molybdenite from the skarn ore yielded a weighted mean age of 84.9 ±1.0 Ma and an isochron age of 85.2 ±0.6 Ma (Re-Os; He et al., 2018). These dates are comparable to the age of the granodiorite porphyry.
The granodiorite porphyries are I-type, with SiO2 contents of 64.3 to 66.7 wt.%. They are typically metaluminous with high K2O/Na2O ratios, low MgO (1.32 to 1.56 wt.%), Cr (5.6 to 12.9 ppm), Ni (3.79 to 10.81 ppm) and high Sr (304 to 844 ppm), Sr/Y (21.2 to 50.8) and La/Yb ratios (37.0 to 60.1). They are enriched in light rare-earth elements (LREE) relative to heavy rare-earth elements (HREE), with slightly negative Eu anomalies, and are enriched in Th, U and large ion lithophile elements (LILE, e.g., K and Rb), and depleted in high field strength elements (HFSE, e.g., Nb, Ta, P and Ti). They also show negative zircon εHf(t) values (-6.7 to -2.3) and negative whole rock εNd(t) values (-5.2 to -4.3), as well as old Hf-Nd model ages, indicating the magmas were derived from a thickened ancient lower crust within the garnet-amphibolite facies (He et al., 2018).
Mineral Resources are quoted as follows:
Mineral Resource - 80 Mt @ 0.15 wt.% Mo, 0.48 wt.% Cu (He et al., 2018).
Bengge Intrusion-related Au
The Bengge deposit lies towards the southern extremity of the West Porphyry Belt of the Eastern Yidun Terrane, as described above. It is hosted by a Late Triassic alkaline igneous complex that comprises a cluster of intrusions cutting Upper Triassic sedimentary rocks, mainly mudstones, shales and metasandstones known as the Wangchika Formation. Six intrusive phases are differentiated in the Bengge alkaline intrusive complex (Jiang et al., 2020), namely:
i). biotite-pyroxene syenite - a greenish black, fine- to coarse-grained rock, which is the most widely exposed phase at Bengge, composed of subhedral alkali feldspar, pyroxene, biotite, plagioclase and <5 vol.% quartz;
ii). biotite syenite, dominated by K feldspar and biotite;
iii). nordmarkite, largely composed of K feldspar with minor alkali amphibole;
iv). quartz syenite found in the core in the main intrusion of the complex;
v). porphyritic syenite, characterised by phenocrysts of alkali feldspar and ferromagnesian minerals set in a micro-crystalline groundmass of alkali feldspar and minor quartz; and
vi). leucosyenite pegmatoid, which are characterised by a holo-crystalline texture and comprise 90 vol.% euhedral K feldspar and 10 vol.% intergranular quartz. These pegmatoids trend ENE-WSW and ESE–WNW as vertical 0.5 to 40 cm thick dykes that cut the biotite-pyroxene syenite and quartz syenite. They post-date both the biotite-pyroxene syenite and quartz syenite.
The biotite-syenite and biotite-pyroxene-syenite crystallisation age is 213.8 ±2.1 and 219.1 ±4.7 Ma respectively (zircon U-Pb LA ICP MS; Jiang et al., 2015).
Geochemically, these syenites have very high potassium- and alkaline-contents, and are members of the shoshonitic series. They are characterised by a high Mg#, Zr/Nd ratio, and Ni-, Co-contents, and are enriched in LREE and LILE and depleted in HFSE. In addition, the εHf (t) of biotite-pyroxene- and biotite-syenite mostly range from -2 to +2, interpreted to suggest they were derived from an enriched mantle source involving some depleted mantle materials and/or contamination by crustal material during magma evolution.
The syenites underwent phyllic alteration producing an assemblage of quartz-sericite-pyrite-carbonate. The main gold mineralisation occurs i). as gold-sulphide-quartz veins in NNE-SSW and ENE-WSW trending fault zones, ii). in altered syenite as telluride in veinlets and as disseminations, and, iii). as gold in fissure-filling and in disseminated pyrite. Available geochemical data indicates that Au in the Bengge deposit is associated with Bi, W, As, Mo, Te, Sb and low concentrations of other base metals (Jiang et al., 2015).
Four vein stages have been differentiated (Jiang et al., 2020), namely:
Stage I). sheeted K feldspar-quartz-magnetite veins/dykes with minor pyrite. K feldspar is dominant, with minor interstitial quartz, whilst magnetite, which is intergrown with barite, is locally replaced by pyrite, all of which are overprinted by selective phyllic alteration, suggesting a late hydrothermal pulse;
Stage II). comprising initial quartz-carbonate-K feldspar veins with sericite-quartz-pyrite selvages. Quartz in the veins is
typically granular, whilst K feldspar occurs locally along the vein edges and pyrite is rare. These veins are several centimetres wide with irregular margins, interpreted to suggest an extensional regime. The granular quartz and feldspar are cemented by late clear quartz veinlets;
Stage III). auriferous poly-metallic sulphide-telluride-sericite-quartz veins within zones of pervasive phyllic alteration (Jiang et al., 2020), also described to as 'base metal sulphide gold bearing quartz sericite carbonate veins' (Jiang et al., 2015). Gold occurs as Au-tellurides in this stage. These veins are composed of an assemblage that includes pyrite, rutile, arsenopyrite, bournonite, tetrahedrite, tellurides, galena, quartz and sericite, with rare chalcopyrite, and are associated with extensive phyllic alteration. They are generally <0.5 cm thick and cut thick early stage II quartz-carbonate ± K feldspar veins. Quartz grains in the latter show undulose extinction, implying shear failure provided the space for stage III mineralisation; and
Stage IV). quartz-carbonate-stibnite veins that were developed in the extensively altered syenite, distal to the hydrothermal
system.
The stages II and III veins are closely associated with gold mineralisation which is closely related to pyrite-sericite (Jiang et al., 2020). Within the Bengge pit, the sulphide content is generally ~5 vol.%, with pyrite being the most abundant, along with minor bournonite, sphalerite and galena. Two generations of pyrite have been recognised from the main gold mineralising stage. The first, Py-I, occurs in the stage II vein phyllic selvages, as coarse-grained crystals in moderately to strongly altered sericite-quartz syenites. It is accompanied by arsenopyrite, and is generally fractured and deformed by micro-shearing to the point of being crushed or brecciated. The fractures within Py-I are generally healed by a mineral assemblage of fine-grained pyrite (Py-II), sericite, carbonate, quartz, rutile and REE-bearing phosphate of stage III. Py-II occurs as fine-grained cubic grains in these stage III veins, which locally cross-cut the quartz–carbonate stage II veins. Locally gold-bearing tellurides and tetrahedrite are found in the matrix between the Py-II grains or as inclusions in Py-II.
Whilst the gold mineralisation at Bengge has been considered to be related to the syenite intrusives, Jiang et al. (2020) report 40Ar/39Ar dating of hydrothermal sericite intergrown with gold-bearing pyrite and telluride from the phyllic alteration zone, yielded an age of ~126 Ma. This is taken to indicate the mineralising event most likely occurred during the Cretaceous, when the area was undergoing extension.
Raman spectroscopy and microthermometry of fluid inclusions in stage II quartz revealed heterogeneous trapping of low-salinity (0.14 to 6.63 wt.% NaCl Equiv. and mesothermal (300 to 270°C) CO2-H2O-NaCl ore-forming hydrothermal fluids. Jiang et al. (2020) calculated the fluid-trapping pressure to mainly range between 850 and 750 bar. They also reported δ18O values of quartz from Stage II gold mineralisation to vary between 15.21 and 19.35‰. The δ18O values for waters in equilibrium with quartz were shown to vary from 6.96 to 10.73‰, consistent with magmatic waters. They therefore proposed that the auriferous fluids were derived from an underlying intrusion during the Cretaceous. They conclude the geological characteristics, low salinity, mesothermal nature and the magmatic source of the ore-forming fluids, all support the hypothesis that the Bengge deposit is an intrusion-related gold deposit.
The Bengge gold deposit was systematically explored by Sinogold Goldfields Alliance in 2008, which defined an Inferred Resource of 10 tonnes of contained of gold at grades that varied from 1 to 20 g/t (Zeng et al., 2000; Sinogold Goldfields Alliance, 2008; Jiang et al., 2020).
MESOZOIC OROGENIC GOLD DEPOSITS OF THE YANGTSE CRATON MARGIN
A number of orogenic gold deposits e.g., Suoluogou, Danba and Yanzigou form an important tectonic and metallogenic belt that straddles the northwestern margin of the Yangtse Craton and the Songpan-Garze oceanic plate/accretionary complex in the north, and the Yidun Terrane in the south. This NE-SW trending belt, while parallel to the craton margin is almost normal to the NW-SE trends of the other two terranes/tectonic entities.
The NW margin of the Yangtze Craton is characterised by a large-scale Mesozoic extensional metamorphic core complex domain where extensive Neoproterozoic crystalline basement is exposed in domal cores along the domain (Chen and Wilson, 1996; Wallis et al., 2003). The evolution of this margin involved four main events: i). Late Neoproterozoic oceanic crust subduction (Zhao and Zhou, 2008; Zhou et al., 2002, 2008; Deng and Wang, 2016); ii). Late Triassic compression, which resulted in the collision between the northwestern margin of the Yangtze Craton and the NNW-SSE Garze-Litang suture zone. The latter separates the Songpan-Garze and Yidun terranes; iii). Late Triassic to Early Jurassic domal extension; and iv). Cenozoic thrusting related to the India-Asia collision (Hou and Cook, 2009). During this process, the 'typical' orogenic Danba and Yanzigou gold deposits were formed along the margin of domes (Zhao et al., 2018; Zhao, 2019). The Garze-Litang suture zone, to the west of the Yangtze Craton and on the east of the Yidun arc belt (Burchfiel and Chen, 2012; Hou et al., 2003; Reid et al., 2005; Yang et al., 2012, 2014), is predominantly characterised by ophiolite assemblages (Deng et al., 2014; Mo et al., 1993; Zhong, 1998). This suture zone has undergone a complex tectonic evolution from oceanic consumption and collision in the Late Triassic, post-orogenic extension during the Jurassic to Cretaceous, and Cenozoic shearing (Hou, 1993; Hou et al., 2004; Huan et al., 2013; Leng et al., 2014; Song et al., 2004; Wang et al., 2013; Zhang et al., 2015).
Suoluogou Au
The Suoluogou deposit is located ~435 km SW of Chengdu, within the southeastern margin of the Yidun Terrane. It lies in the southwestern part of the Tangyang dome, on the northwestern side of the metamorphic core complex dome belt along the northwestern margin of the Yangtze Craton as detailed above. The core of the dome mainly comprises Ordovician quartz schist, sericitic slate and granitic mylonite (Zeng et al., 2019). This core is mostly overlain by Triassic slate and marble, characterised by sub-greenschist to greenschist facies assemblages (Yan et al., 1997; Zeng et al., 2019).
Geology - The immediate district surrounding the Suoluogou deposit is occupied by the Late Triassic Qugasi Formation, which is composed of limestone, quartz sandstone and carbonaceous slate, as well as mafic tuff and basalt. The sequence is cut by east-west, NW, north-south and NE-trending fault sets, with orebodies spatially related to the near east-west trending, dextral Fa fault. This structure is ~3500 m long and generally strikes at 240 to 290° and dips at 60 to 80°. It is mainly developed along the contact between carbonaceous slate and mafic tuff which are, as a consequence, strongly fractured, forming 10 to 20 m wide structural breccia zones. Mineralisation is predominantly hosted by the structural breccias and within altered mafic tuff. Fault breccias, fault gouge and fault cataclastic rocks are well developed within the fault zone, indicating brittle deformation (Yang et al., 2014). The location of orebodies within the Fa fault is controlled by gentle 'bending' of the fault plane as well as changes in the local stress field and by the intersection with faults of the other orientations listed above.
No significant igneous intrusion are found within the Suoluogou deposit, apart from several dolerite and lamprophyre dykes. The former are mainly composed of plagioclase and augite, and commonly intrude carbonaceous slate and mafic volcanic rock. Alteration related to mineralisation, which include chlorite and sericite, have been seen in the dolerite. Lamprophyre dykes are generally intruded into the Fa fault and cut the ore bodies. Phenocrysts within lamprophyre are predominantly biotite within a matrix dominated by plagioclase and augite. The lamprophyre dykes have been dated at 26.4 to 26.7 Ma (biotite K-Ar; Zhang et al., 2015). They have generated their own weak carbonate alteration, which is different to that associated with mineralisation. These observations indicate the lamprophyre is post-mineral.
Alteration and Mineralisation - The deposit is made up of eight orebodies, the largest and most representative of which is No. 15. The gold ores are dominated by disseminated sulphides in alteration zones, with lesser quartz-sulphide veins. Pyrite and arsenopyrite are the dominant sulphides, with trace chalcopyrite, sphalerite, tetrahedrite, galena and gersdorffite. Rutile, sericite, apatite and other trace minerals, e.g., barite, as well as carbonaceous material that is commonly observed in the carbonaceous slate, are included in, or coexist with pyrite. Gold is invisible, occurring in pyrite and arsenopyrite. The Au-bearing pyrite, typically accompanied by euhedral arsenopyrite, occurs as disseminated grains, and less commonly along quartz veins or shear foliations. Outward from the core of the Fa fault, the degree of mineralisation and the gold grades gradually decrease.
Hydrothermal alteration, which is well-developed, is also primarily controlled by the Fa fault. It involved a combination of silica, carbonate, sericite, chlorite and sulphide alteration. Silicification is widespread, manifested by quartz veins, stockworks, or aphanitic accumulations. Sericite alteration is characterised by disseminated or massive sericite within orebodies. Chlorite is generally distal to the orebodies, frequently associated with rutile. Carbonate was introduced at a late stage of the hydrothermal evolution, as indicated by carbonate veins which crosscut quartz veins and the other alteration styles. The alteration gradually passes outward from orebody veins to wallrock, from sericite, grading to chlorite. Gold enrichment is closely related to the quartz-sericite-pyrite alteration assemblage.
The mineral paragenesis and mineralisation occurred in four stages, as follows: i). the pre-mineral Stage Ⅰ, which includes pyrite with minor chalcopyrite and silica formed by either early hydrothermal or early diagenetic processes; ii). the main mineralisation Stage ⅠI, represented by an assemblage of quartz-pyrite-arsenopyrite-sericite-chlorite. Pyrite occurs as medium-sized, disseminated euhedral grains with overgrowth zoning wrapping around Stage I pyrite; iii). characterised by Stage IⅠI carbonate-quartz-sulphide veins, with minor gold mineralisation, cutting Stage Ⅱ quartz veins. Minerals formed during this stage include pyrite, arsenopyrite, quartz, carbonates and sericite with trace barite. Pyrite of this stage is partly found with the carbonate-quartz veins, but mainly occurs as euhedral and coarse grains as overgrowth rims surrounding early pyrite in structural breccias, and as fine subhedral to anhedral grains in altered mafic tuff; iv). the post-mineral Stage IV characterised by carbonate-pyrite veins, cutting the veins of stages Ⅱ and III.
Danba and Yanzigou are ~30 km NW and 30 km SW of the town of Danba which is ~315 km NNE of Suoluogou. Whilst Suoluogou is located on the margin of a metamorphic core complex piercing younger rocks of the Yidun Terrane, the other two deposits are on a NW arm of the main corridor of metamorphic core complexes that separates the Songpan-Garze and Yidun terranes from the Yangtze Craton. The structural arm that hosts these two deposits surrounds the smaller core complexes that penetrate up into the Songpan-Garze Terrane, ~125 km NW of the craton margin.
The Suoluogou gold deposit had a pre-mining resource of 57 t of contained Au (Sichuan Geology and Mineral Bureau Regional Geological Survey Team, 2010).
This Suoluogou summary is largely drawn from Sun et al. (2020).
The most recent source geological information used to prepare this decription was dated: 2020.
Record last updated: 5/6/2023
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Pulang Xuejiping
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Selected References:
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Cao, K., Yang, Z.-M., Mavrogenes, J., White, N.C., Xu, J.-F., Li, Y. and Li, W.-K., 2019 - Geology and Genesis of the Giant Pulang Porphyry Cu-Au District, Yunnan, Southwest China : in Econ. Geol. v.114, pp. 275-301.
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Cao, K., Yang, Z.-M., White, N.C. and Hou, Z.-Q., 2022 - Generation of the Giant Porphyry Cu-Au Deposit by Repeated Recharge of Mafic Magmas at Pulang in Eastern Tibet: in Econ. Geol. v.117, pp. 57-90.
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Deng, J., Wang, Q., Li, G., Li, C. and Wang, C., 2014 - Tethys tectonic evolution and its bearing on the distribution of important mineral deposits in the Sanjiang region, SWChina: in Gondwana Research v.26, pp. 419-437.
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Gao, X., Yang, L.-Q. and Orovan, E.A., 2018 - The lithospheric architecture of two subterranes in the eastern Yidun Terrane, East Tethys: Insights from Hf-Nd isotopic mapping: in Gondwana Research v.62, pp. 127-143.
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He, W, Xie, S., Liu, X., Gao, X. and Xing, Y., 2018 - Geochronology and geochemistry of the Donglufang porphyry-skarn Mo-Cu deposit in the southern Yidun Terrane and their geological significances: in Geoscience Frontiers v.9, pp. 1433-1450.
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Hou, Z., Zeng, P., Gao, Y, Du, A. and Fu, D., 2006 - Himalayan Cu-Mo-Au mineralization in the eastern Indo-Asian collision zone: constraints from Re-Os dating of molybdenite: in Mineralium Deposita v.41, pp. 33-45.
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Hou, Z.,Khin Zaw, Pan, G., Mo, X., Xu, Q., Hu, Y. and Li, X., 2007 - Sanjiang Tethyan metallogenesis in S.W. China: Tectonic setting, metallogenic epochs and deposit types: in Ore Geology Reviews v.31, pp. 48-87. doi.org/10.1016/j.oregeorev.2004.12.007.
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Jiang, L., Xue, C., Hou, Z. and Xiang, K., 2015 - Petrogenesis of the Bengge syenites, northwestern Yunnan:Geochemistry, geochronology and Hf isotopes evidence.: in Abstract in English, body in Chinese. Acta Petrologica Sinica v.31, pp. 3234-3246.
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Leng, C.-B., Cooke, D.R., Hou, Z.-Q., Evans, N.J., Zhang X.-C., Chen, W.T., Danisik, M., McInnes, B.I.A. Yang, J.-H., 2018 - Quantifying Exhumation at the Giant Pulang Porphyry Cu-Au Deposit Using U-Pb-He Dating: in Econ. Geol. v.113, pp. 1077-1092.
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Leng, C.-B., Wang, D.-Z., Yu, H.-J., Tian, F. and Zhang, X.C., 2024 - Mapping hydrothermal alteration zones with short wavelength infrared (SWIR) spectra and magnetic susceptibility at the Pulang porphyry Cu-Au deposit, Yunnan, SW China: in Mineralium Deposita v.59, pp. 699-716.
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Li, Y.-J., Wei, J,-H.,x Tan, J, Fu, L.-B., Li, H. and Ke, K.-J., 2020 - Albian-Cenomanian A-type granite-related Ag-Pb-Zn veins in the central Yidun Terrane, SW China: constraints from the Xiasai deposit: in Mineralium Deposita v.55, pp. 1047-1070.
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Ren, T., Zhang, X., Han, R. and Hou, B., 2015 - Carbon-oxygen isotopic covariations of calcite from Langdu skarn copper deposit, China: implications for sulfide precipitation: in Chinese Journal of Geochemistry v.34, pp. 21-27
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Sun, P., Wang, Q., Li, H., Dong, C. and Deng, J., 2020 - Geology and pyrite sulfur isotopes of the Suoluogou gold deposit: Implication for crustal continuum model of orogenic gold deposit in northwestern margin of Yangtze Craton, SW China: in Ore Geology Reviews v.122, 16p. doi.org/10.1016/j.oregeorev.2020.103487.
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Wang, D.-Z., Hu, R., Hollings, P., Bi, X.-W., Zhong, H., Pan, L.-C., Leng, C.-B., Huang, M.-L. and Zhu, J.-J., 2021 - Remelting of a Neoproterozoic arc root: origin of the Pulang and Songnuo porphyry Cu deposits, Southwest China: in Mineralium Deposita v.56. pp. 1043-1070.
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Yang, L.-Q., Gao, X. and Shu, Q.-H., 2017 - Multiple Mesozoic porphyry-skarn Cu (Mo-W) systems in Yidun Terrane, east Tethys: Constraints from zircon U-Pb and molybdenite Re-Os geochronology: in Ore Geology Reviews v.90, pp. 813-826. dx.doi.org/10.1016/j.oregeorev.2017.01.030.
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Yu, H.-J., Jiang, J.-W. and Li, W.-C., 2020 - Controls of variable crustal thicknesses on Late Triassic mineralization in the Yidun Arc, Eastern Tibet: in J. of Asian Earth Sciences v.195, 13p. doi.org/10.1016/j.jseaes.2020.104285
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Zhang, X.-C., Leng, C.-B., Qin, C.-J., Wang, S.-X., Ren, T. Ma, D.-Y., 2009 - Geologic and Fluid Inclusion Characteristics of the Xuejiping Porphyry Cu Deposit, Yunnan, China: in Williams et al., 2009 2009 Smart Science for Exploration and Mining Tenth Biennial SGA Meeting, Townsville, 2009 Proceedings pp. 548-550
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Zu, B., Xue, C., Chi, G., Zhao, X., Li, C., Zhao, Y., Yalikun, Y., Zhang, G. and Zhao, Y., 2016 - Geology geochronology and geochemistry of granitic intrusions and the related ores at the Hongshan Cu-polymetallic deposit: Insights into the Late Cretaceous post-collisional porphyry-related mineralization systems in the southern Yidun arc, SW China: in Ore Geology Reviews v.77, pp. 25-42.
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