Inner Mongolia, China

Main commodities: Mo
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The Dasuji porphyry molybdenum deposit is located 26  km SE of Zhuozi County in Inner Mongolia (Nie Mongolia), northern China, ~125 km west of the major Caosiyao Mo deposit (#Location: 49° 58' 59"N, 119° 8' 18"E).

The Dasuji Mo deposit is located in the western part of the east-west trending Yinshan-Yanshan-Liaoning molybdenum belt on the northern margin of the North China Craton and comprises predominantly porphyry and porphyry-skarn, with lesser skarn and vein type deposits. This belt extends from Hohot (inner Mongolia) in the west, to the head of the Bohai Sea (Liaoning) in the east. For more background and regional geology see the Caosiyao record.

The deposit and occurs within the garnet biotite plagioclase gneiss of the Mesoarchean Jining Group as described in the Caosiyao linked above.

Mesozoic igneous rocks intruded Mesoarchean granitoids and in the deposit area. These include Triassic granitoids, as well as Jurassic granite porphyries and minor dykes of dolerite and diorite porphyry. The Triassic granitoids particularly syenogranite, syenogranite, granite porphyry and quartz porphyry, are closely related to the Mo mineralisation. The Triassic syenogranite is composed of 40 to 55  vol.% K feldspar, 20 to 25  vol.% plagioclase, 30  vol.% quartz and 5  vol.% biotite. The Triassic granite porphyry has a porphyritic texture and massive structure, with phenocrysts that are mainly quartz and K feldspar in roughly equal amounts of up to 30 to 40  vol.% each, and 5 to 10  vol.% plagioclase. The Triassic quartz porphyry contains 30  vol.% phenocrysts, >80  vol.% of which are quartz with minor K feldspar and plagioclase. A concealed, post-ore, weakly altered Jurassic granite porphyry is also recognised. It contains 15 to 20  vol.% phenocrysts of K feldspar and quartz in equal amounts, as well as ~5  vol.% plagioclase. The NW-SE trending dykes of dolerite and diorite porphyry are mineralisation (Chen et al., 2019).

The Mo orebodies are concealed and have lenticular shapes in cross-section. The mineralisation is characterised by veinlets and stockworks with lesser brecciated, disseminated and banded ores. The ore minerals are principally molybdenite and pyrite accompanied by subordinate galena and sphalerite, and more rarely chalcopyrite, magnetite and hematite. Gangue minerals include quartz, plagioclase and K feldspar, as well as mica, kaolinite, fluorite, calcite, epidote and chlorite. Molybdenite is widely distributed in a variety of hydrothermal veins in leaf-like and scale-like forms, whilst a minor amount also occurs disseminated in the host rock. Pyrite occurs in veins as anhedral or euhedral-subhedral granular aggregates. Galena, sphalerite and chalcopyrite are mostly found in veins that represent the late stages of hydrothermal evolution, often occurring together in euhedral-subhedral granular aggregates (Chen et al., 2019).

All of the pre-mineral rocks at Dasuji area are altered to some degree, predominantly silica, sericite, chlorite, K feldspar, kaolinite, carbonate, and fluorite alteration. The following zones of alteration are recognized: i). a strong silica + intense potassic alteration zone at depth; ii). a quartz- sericite (phyllic) zone at mid to shallow depths, which is closely related to the Mo mineralisation and iii). a carbonate- fluorite (propylitic) zone near the surface and peripheral to the deposit (Chen et al., 2019).

The hydrothermal ore-forming process has been divided into four stages (after Chen et al., 2019):
i). Stage 1 characterised by K-feldspar-quartz and quartz veins, which only contain minor amounts of molybdenite. The quartz veins are only a few millimeters thick and occur in stockworks, and are dominated by quartz with minor molybdenite pyrite and rutile. Envelopes of potassic alteration are developed around the quartz veins.
ii). Stage 2, the main stage of molybdenum mineralisation and occur in the granite, granite porphyry and quartz porphyry. Four different types of veins are recognized: quartz-molybdenite, quartz-molybdenite stockworks, quartz-molybdenite-fluorite and quartz-molybdenite-pyrite. They are best developed at moderate to shallow depths, and are all accompanied by intense silica-sericite, and locally by weak potassic alteration. The veins are always bordered by selvages that include quartz, sericite, minor K-feldspar, and sometimes a little calcite and chlorite. Quartz-molybdenite veins have wider alteration envelopes than the stage 1 veins, and locally are >3 cm thick. The quartz-molybdenite veins are 5 to 20 mm thick, and are dominated by quartz and molybdenite with minor pyrite, magnetite, rutile and anhydrite, as well as occasional K feldspar. Quartz-molybdenite stockwork veins, which are the dominant vein type, are <5 mm wide. Quartz-molybdenite-fluorite veins are 5–10mm wide, contain quartz, molybdenite and fluorite with minor pyrite and rutile. Most are found in the shallow parts of the deposit. They have slightly weaker silica-sericite selvages, usually <3 cm thick compared to the other stage 2 veins. The quartz-molybdenite-pyrite veins are generally co-extensive with the quartz-molybdenite-fluorite veins. They are approximately 1 cm thick, and are composed of quartz, molybdenite, pyrite and minor muscovite and fluorite. The metallic minerals, including molybdenite and pyrite, tend to appear on one side of the veins, while pyrite may be either earlier or later than the molybdenite.
iii). Stage 3 lead-zinc mineralisation, developed mainly in the shallow sections of the deposit. They include quartz-carbonate-galena-sphalerite veins and fluorite veins, cutting all stage 2 veins. These veins are accompanied by a gradual decrease in sericite-silica and a gradual increase in carbonate. The Quartz-carbonate-galena-sphalerite veins are 20 to 30 mm thick and mainly occur at the periphery of the main molybdenum deposit. The fluorite veins are 5 to 50mm wide and are mainly found in the shallow parts of the deposit. They are accompanied by weak alteration zones and they lack Mo mineralisation. The fluorite veins are depleted in metallic minerals, and some contain minor amounts of quartz and carbonates.
iv). Stage 4 characterised by a lack mineralisation. They represent the final phase of hydrothermal evolution at the Dasuji Mo deposit, and are mainly composed of carbonates, quartz, and minor accessory minerals.

Re-Os of the molybdenum mineralisation yielded an age of 223 Ma, which suggests the deposit formed in a period of post-collision extensional setting (Zhang et al., 2009).

The deposit has reserves of 0.144 Mt of contained Mo with an average grade of 0.133% Mo, which would equate to a tonnage of 108 Mt of ore (Chen et al., 2019).

The most recent source geological information used to prepare this summary was dated: 2019.    
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 & Additional Information
   Selected References:
Chen, P., Zeng, Q., Wang, Y., Zhou, T., Yu, B. and Chen, J.,  2018 - Petrogenesis of the Dasuji porphyryMo deposit at the northernmargin of North China Craton: Constrains from geochronology, geochemistry and isotopes characteristics: in    Lithos   v.322, pp. 87-103
Chen, P., Zeng, Q., Zhou, T., Wang, Y., Yu, B. and Chen, J.,  2019 - Evolution of fluids in the Dasuji porphyry Mo deposit on the northern margin of the North China Craton: Constraints from Microthermometric and LA-ICP-MS analyses of fluid inclusions: in    Ore Geology Reviews   v.104, pp. 26-45.
Zhang, Y., Ma, Y.-B., Zhang, T., Zhang, Z.-J., Ding, J.-H. and Li. C.,  2018 - Geochronology and geochemistry of the Dasuji Mo deposit in the northern margin of the North China Block: Implications for ore genesis and tectonic setting: in    Ore Geology Reviews   v.104, pp. 101-113.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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