Inner Mongolia, China
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The Wunugetu (also known as Wunugetushan) porphyry Cu-Mo deposit is located 22 km south of Manchuri City, Inner Mongolia, China, within 70 km of the border with northeastern Mongolia and 30 km south of the Russian frontier (#Location: 53° 0' 34"N, 121° 58' 8"E).
The Wunugetu deposit is situated within the Argun massif, which is characterised by widespread Jurassic to Early Cretaceous volcanic rocks, Mesozoic granitoids, and rare exposures of deformed and metamorphosed pre-Mesozoic basement that varies from gneisses to very low-grade metamorphosed volcanic rocks and sedimentary sequences of Archaean to Permian age. These terrains represent a complex pre-Mesozoic tectonic evolution involving Palaeozoic subduction-related accretion, Early Mesozoic collision, and Late Mesozoic-Cenozoic post-collision tectonics associated with subduction of the Pacific plate (Chen et al., 2007).
The widespread Mesozoic calc-alkaline volcanic rocks of the Selanga-Gobi-Khanka magmatic arc are divided into three formations (Wang et al., 2006), namely the:
• Tamulan Formation composed mainly of basalts and basaltic andesitic rocks, with 39Ar/40Ar ages clustering at 163 to 160 Ma and 147 to 140 Ma;
• Shangkuli Formation, mainly basalt, andesite, trachyte and rhyolite lavas, with minor tuff and tuffaceous sandstone intercalations. 39Ar/40Ar isotope ages range between 120 and 125 Ma; and
• Yiliekede Formation composed mainly of basalt and andesite basalt, with 39Ar/40Ar ages of 116 to 113 Ma. The volcanic rocks are unconformably overlain by the Cretaceous Damoguaihe Formation, mainly composed of sandstones intercalated with coal beds, tuffaceous units, and mudstones.
The immediate wall rocks to the deposit are biotite monzogranite of a 120 km diameter Indo-Sinian batholith dated at between 177 and 201.6 Ma (K-Ar), and 212.85 ±6.76 Ma (Rb-Sr). The monzogranite batholith is intruded by a monzogranite porphyry stock (188.3 ±0.6 Ma by U-Pb in zircon) which is associated with the mineralisation.
These magmatic rocks are as follows (after Li et al., 2012):
• Biotite granite - The Wunugetu porphyry Cu-Mo system lies within a deformed, medium to coarse grained biotite granite batholith, which is uniform in mineralogical composition. Quartz and plagioclase, in roughly equal amounts, comprise 45 to 50% of the rock, whilst microcline constitutes 40 to 45% by volume. Biotite and accessory minerals such as apatite, zircon, magnetite and ilmenite make up the remaining 10 to 15%. Relicts of the Lower Cambrian Argun Formation can be observed within the batholith. Based on major, trace elements and REE geochemical features, Wang and Qin (1988) concluded that the magma was derived from an upper crustal source by partial melting. Previous geochronological studies (Qin et al., 1998) reported K-Ar ages of 176.9 to 201.6 Ma and a Rb-Sr isochron age of 212 Ma. Single zircon SIMS U-Pb analysis by Chen (2010) revealed a more precise age of 200±2 Ma.
• Quartz monzonite porphyry and rhyolite breccia are the most conspicuous intrusive rocks in the Wunugetu Igneous Complex, with the largest quartz monzonite porphyry body occurring in the northern half of the deposit.
The rhyolite breccia is interpreted to be a shallower phase of the largest quartz monzonite porphyry, with no clear boundary identified between the two lithologies. In the breccia, mineral shards which include quartz, plagioclase, K-feldspar and biotite, with diameters of 1 to 2 mm, are cemented by tuff or melt. Feldspars in the breccia are intensively altered to sericite, hydromuscovite, and illite. Its age has been constrained its formation age to 187±11 Ma (single-grain zircon LA-ICP-MS U–Pb analysis; Chen, 2010).
The quartz monzonite porphyry contains ~25% phenocrysts of plagioclase and quartz with some orthoclase in a microcrystalline matrix of quartz and plagioclase. These porphyries have been interpreted to be a calc-alkaline I-type granite based on major element studies (Chen 2010) which show they have high contents of SiO2 (69.20 to 71.70%), Al2O3 (14.65 to 16.05%), and Na2O (4.26 to 4.98%) and low contents of CaO (0.92 to 1.58%), MgO (0.10 to 0.57%), and FeOTotal (0.84 to 1.58%). REE and trace elements data are characterised by inconspicuous Eu anomalies, LILE enrichment and HFSE depletion, especially for a significantly negative Nb-Ta and Sr-Yb anomaly. Determinations of their intrusive ages include 188.3±0.6 Ma (Qin et al., 1998; U-Pb age by single-grain zircon evaporation method) and 183.9±1.0 Ma (whole-rock Rb-Sr isochron age); 179±2 Ma (SIMS U-Pb zircon age); 179.0±1.9 Ma (39Ar/40Ar plateau age of for the matrix), and 179.0±1.9 Ma (whole-rock Rb-Sr age; Chen, 2010), which represent the formation age of the quartz monzonite porphyry.
• Dacite breccia and rhyolite dyke, which occurs as a 1.26 km3 dumbbell-shaped dacite breccia unit in the southeastern part of the district. The breccia is mainly composed of plagioclase, orthoclase and quartz, plus lithic shards of biotite granite, dacite ignimbrite, quartz monzonite and ores. These shards generally range from 0.2 to 40 mm in diameter and are cemented by tuff or melt which underwent weak carbonate, illite or hydromuscovite alteration. The breccia unit covered, intruded, and is in contact with quartz monzonite porphyries along faults, indicating it postdated the quartz monzonite porphyries and Cu-Mo mineralisation.
• Dacitic to rhyolitic dykes, numerous of which occur along the NE-trending or circular fault systems, and intrude the biotite granite, quartz monzonite porphyry, rhyolite breccia and Cu–Mo orebodies, although none have been recorded cutting into the dacite breccia, indicates they are possibly coeval with the dacite breccia. In contrast, diorite or syenite dykes postdate the dacite breccia as they intruded into the dacite breccia.
The Wunugetu deposit is located to the NW of the major NE-trending, crustal-scale Derbugan fault (Wu et al., 2002) and lies on the southern limb of the Mongolian Orocline. Subsidiary NW- and NWW-trending faults controlled the occurrence of volcanic edifices and mineralisation within the district (Zhu et al., 2001; Qi et al., 2005). The deposit is hosted within the Wunugetu Igneous Complex, a ~1600 x 900 m breccia pipe-like body composed of intermediate to felsic volcanic to subvolcanic rocks formed mainly by two magmatic events: i). rhyolite breccias and ignimbrite, followed by a quartz monzonite porphyry and the later ii). dacite breccias followed by diorite and syenite dykes.
Hydrothermal alteration at Wunugetu is highly variable, with alteration mineral assemblages of different generations and settings, occurring either separately or as overlapping alteration stages (Qin et al. 1993), indicating a complex and extended history of fluid activity (Li et al., 2012). Alteration is typically zoned from an inner quartz-K feldspar zone, outward to quartz-sericite halo, with later illite-hydromuscovite-carbonate overprinting the former assemblages. Li et al. (2012) describe these zones as follows:
• Quartz-K feldspar (potassic) zone - this stage includes both veinlet infilling and alteration, and is widespread throughout the central part of the Wunugetu system, predominantly affecting the biotite granite and quartz monzonite porphyry. An assemblage of K feldspar, biotite, quartz, sericite and some anhydrite, plus late-stage hydromuscovite, illite and carbonate, define the alteration zone, which is ~2 km long and 0.8 km wide. K feldspar comprises ~5 to 10% of the alteration minerals in this zone, and is characterised by pale pink to cream K feldspar. The alteration within the biotite granite occurs as an irregular replacement of the igneous matrix, whilst in more intensely altered samples, it has also replaced the phenocrysts and obliterated the original texture of the rock. In the quartz monzonite porphyry, it is mainly represented by various veins, which are proximal to Mo mineralisation. Secondary quartz comprises 10 to 15% of the total alteration assemblage, but locally can be up to 60 to 75%. It usually occurs as various quartz-dominated or quartz-only veins and as anhedral grains. In the innermost concentric zone, barren quartz veins with minor molybdenite and thin K feldspar alteration haloes define a “barren quartz core.” Numerous centimetre-sized, discontinuous, irregular aggregates and pods of quartz are also found in the biotite granite. Locally, biotite occurs as micro-scale crystals adjacent to quartz-K feldspar veins, comprising ~1 to 2 vol.%.
• Quartz-Sericite (phyllic) zone, which is found upward and outward from, but also overlapping the Quartz-K feldspar zone, and contains 6 to 20% quartz, 5 to 35% sericite, 6% to 30% hydromuscovite, and minor illite and carbonate. This zone is mainly found in biotite granite, rhyolite breccia and quartz monzonite porphyry, where all the original mafic minerals and most feldspars have been converted to sericite or hydromuscovite, giving the rock a dull brownish colour. Quartz-sericite-sulphide veins are characteristically not straight and have wide alteration envelopes, and are mainly associated with Cu and minor Mo mineralisation. The sericite alteration associated with mineralisation has been dated at 182.3 to 184.7 Ma (Qin et al., 1997)
• Illite-hydromuscovite zone, which overprinting both of the other zones. It is characterised by 2 to 10% illite, 15 to 25% hydromuscovite, 1 to 10% sericite and minor carbonate, and occurs in the biotite granite and rhyolite breccia. The original textures of these rocks are partly or completely replaced by a lepidoblastic texture. Quartz veins appear monomineralic in hand specimen, commonly without visible alteration haloes. In contrast to the contorted veins of the quartz-sericite zone, these veins are parallel, and are accompanied by minor Cu, Pb and Zn mineralisation.
There is a pronounced mineralisation zoning at the Wunugetu deposit, passing outward from the quartz monzonite porphyry, through Mo, Mo-Cu, Cu-Pb-Zn to Pb-Zn dominated zones. The Mo mineralisation is mainly found in the endo- and exo-contact of the quartz monzonite porphyry, accompanied by quartz and K feldspar, and occurring as either disseminations in the altered rock or as quartz-molybdenite veins. The molybdenite veins cut veins of quartz-K-feldspar-cubic pyrite, but, in turn, are cut by chalcopyrite-bearing veins, suggesting molybdenite is earlier than chalcopyrite, but later than the formation of euhedral pyrite.
Chalcopyrite is the dominant copper mineral and, with minor bornite, chalcocite, tennantite and tetrahedrite, appears to be paragenetically later than molybdenite. The Cu-bearing assemblage is associated with phyllic quartz-sericite alteration, and occurs as quartz-sulphide veins, whilst additional late covellite occurs in cracks cutting earlier formed minerals. The ores that are characterised by distributed veinlets generally have moderate copper grade of 0.2 to 0.6% Cu, with the highest up to 2.08%. Minor chalcopyrite with uneconomic galena and sphalerite occurs as infilling of earlier fractures in the illite-hydromuscovite zone, where sphalerite and galena occur as unevenly distributed anhedral 0.01 to3.0 mm crystals. Chalcopyrite is occasionally distributed in sphalerite due to unmixing of the higher temperature solid solution while some pyrite occurs as 1 to 3 mm wide veins. Sphalerite, galena and minor chalcopyrite commonly selectively replace earlier pyrite and chalcopyrite along fractures.
There are 33 Cu and 13 Mo bodies in the deposit, controlled by the porphyry body and the contact zone (Nokleberg, et al.., 2010). The centre of the porphyry system is cut by a WNW-trending fault that offsets the volcanic pipe by approximately 600 to 700 m, dislocating the deposit into north and south ore zones (Zhang 2006). In the northern zone, the 0.42 km2 quartz monzonite porphyry stock intrudes the biotite granite batholith. Five Cu and two Mo orebodies, with pod-like, banded or tabular shapes, host ~80% of total Cu and Mo reserves. In the southern zone, small quartz monzonite porphyries host numerous Cu and Mo occurrences. Individual orebodies are mainly elongated for hundreds of metres, with widths of tens to hundreds of metres and ranging in depth from 300 to 650 m.
Based on field observation, as well as petrographic and textural relationships, a four-stage paragenesis is proposed by Li et al. (2012):
• Early quartz + K feldspar ± anhydrite ± sulphides (pyrite-molybdenite) ± magnetite, accompanied by deformation and brecciation. Precipitation of quartz, K feldspar, anhydrite and magnetite commenced during the potassic alteration phase, with limited molybdenite and pyrite.
• The main episode of Mo-mineralisation, defined by the deposition of quartz-molybdenite and quartz-molybdenite-chalcopyrite veins, and disseminated molybdenite ± chalcopyrite ± pyrite with quartz + K feldspar of the potassic phase of alteration.
• The Cu-mineralisation stage, characterised by rapid precipitation of fine-grained Cu minerals in an assemblage of quartz + sericite + chalcopyrite + tennantite + bornite + tetrahedrite + chalcocite. Molybdenite decreases, whereas the Cu-bearing sulphides, increase significantly. These sulphides occur as variable quartz-sulphides veins in the quartz-sericite phyllic zone.
• Late stage mineralisation of the illite, hydromuscovite, carbonate alteration phase, represented by an assemblage of quartz + illite + hydromuscovite + carbonate ± sulphides, which include some sphalerite and galena. Comb quartz-carbonate veins that cut earlier veins occur in this stage. It is assumed that the formation of calcite is slightly later than quartz, although they are probably deposited by the same fluid system.
The deposit is quoted as containing:
495 Mt @ 0.45% Cu, 0.085% Mo (Mutschler et al., 2000).
127 Mt @ 0.46% Cu and 42 Mt @ 0.05% Mo (Qin et al., 1999).
2.2 Mt of contained Cu in ore with an average grade of 0.46% Cu = 480 Mt @ 0.46% Cu; and
0.26 Mt of contained Mo in ore with an average grade of 0.019% Mo = 14 Mt @ 0.019% Mo (Zhang et al., 2016)
The most recent source geological information used to prepare this summary was dated: 2012.
Record last updated: 30/11/2012
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.
References to this deposit in the PGC Literature Collection:
Li N, Chen Y-J, Ulrich T and Lai Y, 2012 - Fluid inclusion study of the Wunugetu Cu-Mo deposit, Inner Mongolia, China: in Mineralium Deposita v.47 pp. 467-482|
Zhang, F.F., Wanga, Y.H., Liu, J.J., Wang, J.P., Zhao, C.B. and Song, Z.W., 2016 - Origin of the Wunugetushan porphyry Cu-Mo deposit, Inner Mongolia, NE China: Constraints from geology, geochronology, geochemistry, and isotopic compositions: in J. of Asian Earth Sciences v.117, pp. 208-224.|
| References to this deposit in PGC Publications:||
Seltmann R and Porter T M, 2005 - The Porphyry Cu-Au/Mo Deposits of Central Eurasia: 1. Tectonic, Geologic & Metallogenic Setting and Significant Deposits, in Porter T M, (Ed), Super Porphyry Copper and Gold Deposits: A Global Perspective, v2 pp 467-512|
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