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Maoduan

Zhejiang, China

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The Maoduan lead-zinc-molybdenum deposit is located in southwestern Zhejiang province in eastern China, ~25 km SW of Longquan, ~170 km NW of Nanping and 160 km west of Wenzhou.
(#Location; 27° 57'N, 118° 58'E).

The Maoduan Pb-Zn-Mo deposit comprises a hydrothermal vein system, made up of a pyrrhotite stage followed by a molybdenite and then a base metal stage. It is a representative member of a group of similar deposits in the Longquan district, e.g., Wu'ao, Xiaomei, Nannong and Yinchang. The Longquan area is located in the foreland thrust-fold belt of the Cathaysia Block, which is separated from the Yangtze craton by the Jiangshan-Shaoxing fault 80 km to the NW, and from the Southeast China Fold Belt by the Zhenghe-Dapu fault 50 km to the SE.

Regional Setting

The Yangzte Craton (to the north) and Cathaysia were amalgamated, along a NE-SW trending fold belt/tectonic zone (Jiangshan-Shaoxing suture/fault), following north-directed subduction between the Mesoproterozoic and Early Palaeozoic. Together they form the South China Block, separated from the North China Craton to the north by the WNW to east-west Qinling-Dabie orogenic belt, a major Late Triassic collision zone that resulted from the closure of a Palaeo-Tethys oceanic arm.
    The Yangtze Craton is composed of a thick basement sequence of Archaean (hornblende-plagioclase gneiss, biotite-plagioclase gneiss, amphibolite, granulites, marbles and banded iron formation) and Palaeoproterozoic (quartzite, mica schist, slates and metavolcanic rocks and biotite-plagioclase gneiss, amphibolite, quartz schist, marble), overlain by the vast Yangtze Platform succession of shallow-water carbonates of south China that spanned the Neoproterozoic to Middle Triassic, accumulating as much as 4000 m during the Early and Middle Triassic, followed by an additional 2500 m of mostly siliciclastic rocks which spread across the platform in the Late Triassic (Pirajno, 2013; Enos et al., 2006; Rui and Mei, 2012).
    The Cathaysia Block, to the south of the NE-SW trending Jiangshan-Shaoxing suture/fault that separates it from the Yangzte Craton, consists of sparsely exposed Palaeo- and Mesoproterozoic continental crust basement rocks, represented by 1.9 to 1.8 Ga sedimentary rocks and 1.8 to 1.4 Ga granites and volcanic rocks (Yu et al., 2005). During the Neoproterozoic the Cathaysia Block underwent a rifting stage in the period of 857 to 837 Ma, corresponding to the break-up of the Rodinia supercontinent. Due to Early Palaeozoic (Caledonian) orogenic activity, Late Ordovician to Middle Devonian strata are absent, although crustal material of Palaeo- to Neoproterozoic age, with some Archaean components, are all extensively reworked at various stages during Caledonian (450 Ma), Indosinian (240 Ma) and early Yanshanian (160 Ma) thermal events, resulting in widespread granitic intrusions (Yu et al., 2005). The Indosinian (Triassic) magmatism produced voluminous granitic plutons in south China (Chen and Jahn 1998), with a wide range of ages, from ~260 to 200 Ma (Wang et al., 2005, 2007; Zhou et al., 2006; Chen et al., 2011). The Indosinian magmatism is interpreted to be due to NW-directed flat subduction of the Pacific plate beneath south China, leading to NW migration of shortening and broadly zonal distribution of plutons (Cui and Li 1983; Li and Li 2007). The Indosinian deformation and magmatism were also influenced by the collision between the Indochina Craton from the west with the South China Block in response to the closure of the Palaeotethys Ocean (Wang et al., 2005, 2007; Zhou et al., 2006; Lepvrier et al., 2008; Chen et al., 2011; Rui and Mei, 2012).
    The giant Yanshanian igneous province subsequently swept across South China, forming a swath >1000 km wide across the whole Cathaysian Block and the eastern part of the Yangtze Craton (Li and Li 2007). The granitic rocks in this province range in age from Jurassic to Cretaceous, notably the Early (180 to 125 Ma) and Late (110 to 85 Ma) Yanshanian periods interpreted to be related to further westward subduction of the Pacific oceanic lithosphere beneath the South China Block.
    The Southeast China Fold Belt is the 100 to 150 km wide NE-SW trending coastal part of the Cathaysia Block, which was directly influenced by subduction-related Yanshanian calc-alkaline magmatism in the Jurassic to Cretaceous when oceanic crust of the Palaeo-Pacific was subducted westward beneath the Cathaysia Block. During this time mineral deposits of Sn, W, Mo, Pb-Zn, Cu, Sb, hydrothermal vein-type, porphyry Cu-Au, skarns, and precious metal epithermal were formed (Pirajno, 2013).

Large-scale Mesozoic magmatism and associated ore formation in the Cathaysia Block and Southeast China Fold Belt of south China, peaked during three episodes (Mao et al., 2008; Wang et al., 2011):
240 to 210 Ma, Indosinian magmatism, characterised by skarn, greisens and vein type W-Sn-Nb-Ta mineralisation and ore-related peraluminous granites (Mao et al., 2008).
170 to 150, early Yanshanian magmatism, which includes an initial pulse of 170 to 160 Ma with mostly porphyry and skarn type Cu-Au and granite-related vein type Pb-Zn-Ag, and a subsequent 160 to 150 Ma pulse characterised by granite-related polymetallic W-Sn mineralisation, mainly in the Nanling region of the western Cathaysia Block.
100 to 90 Ma late Yanshanian event, which consists of epithermal Au-Ag-Cu and granite-related polymetallic W-Sn mineralisation (Mao et al., 2008; Wang et al., 2011).

Maoduan deposit

The Longquan area, which includes the Maoduan deposit has a Proterozoic basement (that underwent a 252 to 233 Ma amphibolite-facies metamorphism) unconformably overlain by Jurassic volcano-sedimentary rocks (Chen et al., 1998; Xiang et al., 2008; Yu et al., 2009). The lowermost stratigraphic member of the basement is the up to 3600 m, 1850 to 1766 Ma (U-Pb), Badu group, composed of biotite-plagioclase gneiss, biotite-K feldspar gneiss, granulite, biotite schist and amphibolite (Li 1997; Xiang et al., 2008) which can be divided into five formations from the base, namely, the Tangyuan, Qiantou, Zhangyan, Siyuan and Dayanshan Formations. These are overlain by the 825 to 710 Ma (U-Pb) Neoproterozoic Longquan (or Mamianshan) group, dominated by meta-volcanic rocks, granulite, mica-schist and minor amounts of quartzite and marble (Li et al., 2005; Wan et al., 2007; Xiang 2008; Wu et al., 2010). The Longquan group is the host rock of the Xiaomei, Yinchang, and Nannong Pb-Zn(-Mo) deposits (Li and Lü 2006).
    The Jurassic sequences of the Longquan area are divided into the:
The Fengping Formation, which is ~600 m thick, and consists of pebbly quartz sandstone, quartz sandstone, feldspar quartz sandstone with thin-bedded siltstone, carbonaceous shale and coal.
The Maonong Formation, that is up to 800 m thick, and consists of 180±4 Ma (U-Pb) dacitic tuff, yellow pebbled sandstone, grey siltstone, and shale with thin-bedded coal (ZJBGM 1985; Chen et al., 2007).

Following the 1040 to 940 Ma orogenic activity between the Yangtze craton and the Cathaysia block (Li et al., 2007), the Jiangshan-Shaoxing fault was formed on the northern margin of Cathaysia Block. During the Indo-Sinian orogenic movement in South China, large-scale deformation produced a series of NE and NNE trending faults, e.g., the Lishui-Yuyao fault and Longquan-Suichang ductile shear zone, which controlled the Mesozoic magmatism and mineralisation.

Palaeoproterozoic and Mesozoic magmatic activity were both intense in the Longquan area. The former is represented by some gneiss, gneissic granites and granodiorites, including S- and A-type granites with U-Pb ages of 1888 to 1832 Ma (Li and Li 2007; Liu et al., 2009; Yu et al., 2009). The Mesozoic igneous rocks are numerous, small NE-trending intermediate to felsic granites, including the Linggen and Maoduan stocks and some intermediate-basic dykes.

Nine ore veins, comprising lodes, lenses and stringer zones in NE-trending fault zones have been tested in the Maoduan district. The country rocks are dominated by the Siyuan and Dayanshan Formations of the Palaeoproterozoic Badu group, with minor exposures of the Early Jurassic Fengping Formation. The 1500 m thick Siyuan Formation is dominantly composed of biotite-plagioclase gneiss, biotite-plagioclase granulite, biotite-monzogneiss and biotite-K feldspar gneiss, that has undergone regional migmatisation and subsequent chloritisation and sericitisation. The Dayanshan Formation is pedominantly a weakly sericitised two-mica-schist. The Fengping Formation consists of quartz and arkosic sandstones.
    The dominant structures are NE- and NW-trending faults, with subordinate north-south and nearly east-west striking fractures. The NE-trending faults are the primary ore-bearing structures and dip steeply to south at 60 to 85°. The faults all cut the ore veins, indicating they ore post-mineralisation.
    The principal intrusions are the:
Linggen granite porphyry, which is elongated NE-SW over an area of about 4 km2, controlled by NE-trending faults, has been dated at 152.2±2.2 Ma (LA-ICP-MS zircon 206Pb/238U; Yan et al., 2012). It is generally light grey and porphyritic, with phenocrysts of quartz (8 to 10 vol.%), plagioclase (10 to 15 vol.%), K feldspar (5 to 8 vol.%), hornblende (2 to 3 vol.%) and biotite (1 to 2 vol.%). Anhedral quartz phenocrysts ranges from 0.5 to 5 mm, some of which are corroded by groundmass. Subhedral plagioclase and K feldspar phenocrysts are up to 3 mm while groundmass feldspars are <0.1 mm in size. The groundmass is dominantly quartz (25 to 30 vol.%), plagioclase (25 to 30 vol.%) and K feldspar (15 to 20 vol.%). Accessory minerals are zircon, apatite and magnetite.
Maoduan monzogranite, which is not exposed at the surface, has been dated at 140.0±1.6 Ma (LA-ICP-MS zircon
206Pb/238U; Yan et al., 2012). It is a yellowish-red, fine-grained and has granophyric textures, is composed of quartz (30 to 35 vol.%), K feldspar (40 to 45 vol.%), plagioclase (20 to 25 vol.%) with minor hornblende (2 to 3 vol.%) and biotite (1 to 2 vol.%). Anhedral quartz ranges from 0.4 to 2 mm, while subhedral K feldspar phenocrysts are 0.5 to 3 mm and weakly sericitised, and subhedral plagioclase ranges from 0.4 to 2.5 mm. Prismatic hornblende and platy biotite are slightly chloritised. The groundmass is dominantly quartz, plagioclase and K feldspar. Accessory minerals are zircon, apatite and magnetite.
Small intrusions of granite and diorite porphyry dykes that cut across the ore veins, indicating post-ore magmatism.

The Maoduan deposit is composed of pyrrhotite-sphalerite-galena, quartz-molybdenite, and quartz-galena veinlets. Three stages of hydrothermal alteration and hypogene mineralisation are recognised (Yan et al., 2012):
Pyrrhotite stage - the earliest hydrothermal phase, occurring as disseminated pyrrhotite, variably altered to marcasite, in actinolite-epidote-altered country rocks. Homogenisation temperatures of fluid inclusions in the pyrrhotite are from 389 to 265°C (n = 40), peaking at 370 to 330°C.
Molybdenite stage - characterised by molybdenite, commonly with fine-grained quartz and rare sericite as strictly fault controlled lodes. Flaky 0.5 to 3 mm molybdenite is intergrown with quartz. Fluid inclusions have homogenisation temperatures of 380 to 220°C (n = 72), peaking at 340 to 260°C. The Re-Os model ages of five molybdenite samples range from 138.6±2.0 to 140.0±1.9 Ma, with an isochron age of 137.7±2.7 Ma (Yan et al., 2012).
Polymetallic stage - characterised by an assemblages of sphalerite-galena-pyrite-chalcopyrite, with less abundant pyrite-galena-quartz. The sulphides were emplaced in the following sequence (from oldest to youngest): pyrite, sphalerite, chalcopyrite and galena. The polymetallic sulphides occur in cusp- or vein-style replacing pyrrhotite, marcasite and molybdenite. The paragenetic position of molybdenite is not clear although, it has been interpreted to precede the molybdenite stage. Fluid inclusions in quartz from the polymetallic stage have homogenisation temperatures of 315 to 160°C with two peak values of 190 to 180 and 270 to 240°C. Isotopic and geochemical data presented by Yan et al. (2012) suggest the intrusion of the Pb-Zn-Mo mineralisation is contemporaneous with emplacement of the Maoduan monzogranite.

The Maoduan deposit contains (Yan et al., 2012):
    ~20 Mt of ore @ average grades of 1.96% Pb, 3.48% Zn, with 7 to 75 g/t Ag, 0.01 to 0.14% Cu.

The most recent source geological information used to prepare this summary was dated: 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.


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