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Suyunhe |
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Xinjiang, China |
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Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Suyunhe reduced porphyry Mo deposit is located in the West Junggar Terrane in Xinjiang, western China, ~250 km west of Karamay and ~430 km NNW of Urumqi. It lies within the West Junggar Terrane, which is, in turn, in the central section of the Palaeozoic Central Asian Orogenic Belt.
Region Setting
For background on the setting of the West Junggar Terrane see the separate Baogutu record.
Two significant porphyry Mo deposits Suyunhe and Hongyuan are known in the West Junggar Terrane. Suyunhe lies within the southwestern segment of the terrane, whilst Hongyuan is located ~250 km to the east in the southeastern segment, just to the north of Karamay and ~60 km NE of the Baogutu porphyry Cu deposit.
Geology
The dominant sequence covering >85% of the Suyunhe district, is the Middle Devonian Barluk Formation. This unit is composed of crystalvitric tuff, tuffaceous siltstone, sedimentary tuff and pebbly greywacke (Zheng et al., 2014; Cao et al., 2017). It is intruded by three small granitic complexes, namely stocks I, II and III, as follows:
• Stock I, which occurs in the west of the district and is mainly composed of granodiorite porphyry with minor granite and granite porphyry phases. Granodiorite porphyry from this intrusion has been dated at ~293 Ma (SIMS zircon U-Pb; Shen et al., 2017).
• Stock II is found within the middle of the district and comprises granite porphyry dated at ~296 Ma (SIMS zircon U-Pb; You, 2016).
• Stock III, located in the east of the district, comprises granodiorite porphyry and tonalite porphyry.
In addition to these outcropping complexes, medium- and fine-grained granite with minor granodiorite porphyry is widely intersected by drilling below much of the district. These intrusions have been dated at 298 to 293 Ma (SIMS zircon U-Pb; Shen et al., 2017), closely coincident with the ages of Mo mineralisation (296 to 293 Ma; Zhong, 2015). In addition, there are also numerous felsites, granite and diorite dykes distributed in the district.
The molybdenite ore-bodies are mainly hosted within the Barluk volcanic-sedimentary strata, with minor amounts in the intrusive rocks. Based on the space–time relationship between intrusive rocks and orebodies, Cao et al. (2020), propose that mineralisation is associated with the medium-grained and fine-grained granite, granite porphyry and granodiorite porphyry intrusions. In contrast, the tonalite porphyry of the district, also of Early Permain age, appears to be unrelated to Mo mineralisation, based on the lack of hydrothermal alternation and associated mineralisation.
Hydrothermal alteration associated with mineralisation includes well developed potassic, albite-chlorite-muscovite, sericite-chlorite and calcite assemblages. The potassic alteration is sub-divided into two types. That found within the intrusive rocks is often pink, and typically comprises massive K feldspar, and quartz with minor disseminated pyrite. The plagioclase in granite is often replaced by K feldspar, although subsequent hydrothermal activity commonly overprints this with extensive and intense phyllic alteration in the upper part of the intrusive body. The phyllic assemblage includes muscovite-chlorite which partially or completely replaces primary biotite and plagioclase as well as the K feldspar. Microscopic observations indicate Mo mineralisation to be rare in the potassic alteration hosted in granite.
In contrast, the potassic alteration hosted in the Barluk Formation wall rocks is usually present as quartz-K feldspar-molybdenite ±magnetite veining, and as such in these rocks is related to Mo mineralisation, that is dominated by disseminated molybdenite, pyrite and minor magnetite.
At depth, albite-chlorite-muscovite alteration assemblages commonly occur in both the intrusive rocks and Barluk Formation strata. In the volcano-sedimentary strata of the latter, albite-chlorite alteration assemblages usually occur as quartz-albite-chlorite-molybdenite-ilmenite ±titanite veins with minor pyrrhotine and chalcopyrite, which are usually crosscut by quartz-sericite veins.
Sericite-chlorite-quartz alteration assemblages are common in the shallow parts of wallrock strata with associated, but usually weak, disseminated or occasionally massive molybdenite, chalcopyrite, and pyrite. Calcic alteration is widely distributed in both the intrusive rock, and wallrock strata. In the former, plagioclase frequently is replaced by calcite. In the shallow wallrock strata, this alteration is manifested as quartz-calcite-pyrite veins or calcite veinlets, usually without molybdenite. A number of barren quartz veins occur in both the intrusions, but also in the wallrock strata.
Investigations reported by Cao et al. (2020) indicate that the primitive granitoid magma was oxidised with ƒO2 greater than the Ni-NiO oxygen buffer, but was subsequently reduced by contamination from the reduced host sequences. The decreasing magmatic ƒO2 does not apparently significantly diminish the enrichment of Mo metals during magmatic evolution. In contrast, they show that the evolved parental magma, which is characterised by high differentiation and is fluorite-rich, is responsible for improving the Mo concentration in melts. Most of reduced gases involved are apparently derived from decomposition of organic matter within the reduced volcano-sedimentary rocks of the Barluk Formation. These reduced gases (e.g. CH4) play a role by improving the efficiency of molybdenite precipitation.
The deposit is reported to have a reserve of 0.57 Mt of contained Mo @ an average grade of 0.05 to 0.09% Mo, equating to an ore tonnage of ~800 Mt (Cao et al., 2020).
The most recent source geological information used to prepare this decription was dated: 2020.
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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Cao, C., Shen, P., Pan, H., Zheng, L., Li, C. and Feng, H., 2020 - The formation mechanism of reduced porphyry Mo deposits in the West Junggar region, Xinjiang: The Suyunhe example: in Ore Geology Reviews v.117, 16p., doi.org/10.1016/j.oregeorev.2019.103286
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