Inner Mongolia, China

Main commodities: Mo
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The Caosiyao porphyry Mo deposit is located 4 km SE of Xinghe County, and ~170 km east of Hohot in Inner Mongolia, China.

Caosiyao, which was discovered in 2011, is the largest deposit in the ~600 x 150 km, east-west trending Yinshan-Yanshan-Liaoning molybdenum belt which lies lies along 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. As of 2017, the belt was known to contain one giant (>0.5 Mt of Mo) at Caosiyao; 7 x large (0.1 to 0.5 Mt of Mo) at Dasuji, Jiajiaying, Sadaigoumen, Xiaojiayingzi, Yangjiazhangzi, Lanjiagou and Gangtun); 11 x medium and numerous small Mo deposits. The bulk of these are in the eastern half of the belt, although the large Caosiyao and Dasuji (100 km to the west) are at the western end.

The Caosiyao deposit lies within the Khondalite Belt section of the Western Block of the North China Craton. For a detailed description of the North China Craton, the Western Block and the Khondalite Belt, see the Regional Setting - North China Craton section of the Jinchuan record.

The main sequences exposed in the Caosiyao deposit area include the Archaean to Palaeoproterozoic metamorphic rocks of the Xinghe and Jining groups, a Jurassic terrestrial volcano-sedimentary sequence and Paleogene to Neogene sedimentary rocks as follows (after Wu et al., 2017):
• The Xinghe Group, considered to be Neoarchaean (Dong et al., 2012, Jian et al., 2012, Ma et al., 2012) has also been interpreted as Palaeoarchaean (Inner Mongolian Autonomous Region Bureau of Geology and Min, Xu et al., 2008). It is found in the southeastern part of the Caosiyao deposit and is predominantly composed of pyroxene granulite, pyroxene plagiogneiss, and amphibolite locally interlayered with diopside-magnetite quartzite.
• The Jining Group, which predominates at Caosiyao, has been considered to be Mesoarchaean (Inner Mongolian Autonomous Region Bureau of Geology and Min, Xu et al., 2008) or Palaeoproterozoic (Xia et al., 2006, et al., 2009, Dong et al., 2012). It is mainly composed of sillimanite-garnet-feldspar gneiss, and fine grained quartzo-feldspathic granulite and leucogranulite, locally interlayered with graphite-bearing gneiss and diopside marble.
Jurassic rocks that include Middle Jurassic clastic and Upper Jurassic andesite and tuff.
Yanshanian granite porphyries of the Caosiyao Intrusive Complex, which comprise Late Jurassic to Early Cretaceous syenogranite porphyry and Early Cretaceous monzogranite porphyry.
Paleogene to Neogene sedimentary rocks are also found in the Caosiyao deposit and its periphery, and include Neogene iddingsite basalts that are mainly exposed in the southwestern section of the deposit.

The two main fault systems at Caosiyao are roughly NE- and NW-trending, with the former being larger in scale. Mesozoic intrusions were predominantly emplaced at the intersections of these two fault systems.

The Caosiyao Intrusive Complex comprises (Wu et al., 2017):
Late Jurassic syenogranite porphyry has a close temporal and spatial relationship with the Caosiyao Mo deposit. It is exposed as six separate intrusions and contains 15 to 25% phenocrysts that include mainly quartz and K feldspar, minor plagioclase and biotite, and trace zircon, titanite and apatite set in a groundmass dominated by cryptocrystalline quartz and feldspar. It is mostly moderately to strongly silicified and sericitised, with local potassic (K feldspar and biotite) and muscovite alteration with associated molybdenite mineralisation. s such it is syn-mineral.
Early Cretaceous monzogranite porphyry is mainly blind, occurring at a depth of ~750 m as apophyses that intruded the Late Jurrasic syenogranite porphyry stock. It contains 30 to 40% phenocrysts, mainly plagioclase, K feldspar, quartz and biotite, set in a microcrystalline quartz and feldspar groundmass with accessory zircon, titanite and apatite. The rocks are largely unaltered, implying they are post-mineral. • Early Cretaceous syenogranite porphyry, which is present as three separate intrusions, contains 30 to 40% phenocrysts, mainly quartz and K feldspar with minor biotite, set in a cryptocrystalline quartz and feldspar groundmass. The main accessory minerals are zircon, titanite and apatite. It is only slightly sericitised with no molybdenite, indicating it is also post-mineral.

The members of the intrusive complex have U-Pb ages that range from 148.5 ±1.2, → 146.4 ±1.0 → 144.6 ±1.0 → 142.8 ±3.2 Ma. Four Re-Os model ages of the molybdenite range from 145.3 ±2.1 to 143.1 ±2.4 Ma, which are consistent with the U-Pb ages for the igneous complex. The rocks of the Caosiyao Intrusive Complex have high contents of silica (SiO2 = 71.7 to 75.6 wt.%) and total alkalis (mostly K2O + Na2O >8 wt.%), low contents of CaO and MgO, and have peraluminous high‐K calc‐alkaline to shoshonitic affinities. They are enriched in large‐ion lithophile elements (LILLE, e.g., K, Rb, U, and Th) and light REEs (LREE) but are depleted in Eu, Ba, Sr, Nb, P and Ti. These geochemical characteristics are interpreted to indicate advanced fractional crystallisation and that the intrusives are strongly fractionated I‐type granites (Zhou et al., 2018). The negative Hf isotopic compositions of the granitic rocks, combined with old two‐stage Hf model ages, are interpreted to imply the magmas were derived from partial melting of the ancient lower crust of the North China Craton (Zhou et al., 2018). All the intrusive rocks in the complex have relatively high Ce4+/Ce3+ ratios and oxygen fugacity, which are interpreted to have facilitated Mo extraction into the melts owing to the much higher solubility of S as sulphate (SO42−) than sulphide (S2−). The magma crystallisation temperatures decreased from 882 to 770°C as the magma evolved (Zhou et al., 2018). The multistage magmatic‐hydrothermal processes, and the prolonged fractional crystallization, combined with high magma oxygen fugacity are interpreted to be the key factors to form the Caosiyao giant Mo deposit (Zhou et al., 2018). The tectonic setting of deposit formation was one of extension during the Late Jurassic to Early Cretaceous (Zhou et al., 2018).

The principal metallic minerals include molybdenite and pyrite, with minor magnetite, chalcopyrite and pyrrhotite. The gangue is primarily composed of quartz and sericite, with minor K feldspar, biotite, muscovite, fluorite, epidote and scapolite. Molybdenite is the only economic ore mineral in the Caosiyao deposit. It is mainly of leaf-like with anhedral granular textures. Pyrite is predominantly anhedral granular, euhedral to subhedral granular and poikilitic. Magnetite mainly has an anhedral granular texture; chalcopyrite is primarily anhedral granular; pyrrhotite is chiefly of anhedral granular and usually replaces molybdenite, pyrite and chalcopyrite, developing metasomatic dissolution textures. The ore is primarily veinlet, disseminated and veinlet disseminated, with a small amount of densely disseminated sulphides (Wu et al., 2017; Wang et al., 2017).

Hydrothermal alteration is well developed, with major wall rock alteration that includes silica, sericite, K feldspar, biotite, muscovite, fluorite, argillic minerals and carbonates. Silicic and sericite are the dominant alteration minerals, and are closely related to the molybdenite mineralisation. Potassic alteration has only been seen in the Jurassic syenogranite porphyry. The silica-sericite alterations is usually overprinted by an argillic alteration assemblage characterised by kaolinite and montmorillonite, which occur peripheral of the potassic alteration zone. Minor chlorite, epidote, scapolite and carbonate alteration is present, but only in metamorphic country rocks of the Jining Group (Wang et al., 2017).

Wang et al. (2017) interpret the ore-forming process in the Caosiyao deposit to have occurred in three stages: i). an early quartz-molybdenite ±magnetite ±pyrite stage; ii). a middle quartz-molybdenite-pyrite ±pyrrhotite ±chalcopyrite stage; and iii). a late quartz-carbonate-pyrite ±fluorite ±pyrrhotite stage.

The Caosiyao deposit is estimated to contains reserves (No. 2 Geoexploration Party of Henan Bureau of Geoexploration, Wang et al., 2014) of:
  1.7556 Mt of contained molybdenum @ an average grade of 0.078% Mo, which would equate to an ore tonnage of 2.25 Gt.

In cross-sections, the Mo orebodies are of asymmetric dumbbell shape, with dimensions of ~2000 m long x 1600 m wide x 400 to 900 m thick (Li et al., 2012, Wang et al., 2014) and are mainly hosted by the Jining Group metamorphic rocks and to lesser extent by the Neoproterozoic dolerite dykes and Late Jurassic syenogranite porphyry.

The most recent source geological information used to prepare this summary was dated: 2017.    
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, Y.-J., Zhang, C., Wang, P., Pirajno, F. and Li, N.,  2017 - The Mo deposits of Northeast China: A powerful indicator of tectonic settings and associated evolutionary trends: in    Ore Geology Reviews   v.81, pp. 602-640.
Shu, Q. and Chiaradia, M.,  2021 - Mesozoic Mo Mineralization In Northeastern China Did Not Require Regional-Scale Pre-Enrichment: in    Econ. Geol.   v.116, pp. 1227-1237.
Wang, G., Wu, G., Xu, L., Li, X., Zhang, T., Quan, Z., Wu, H., Li, T., Liu, J. and Chen, Y.,  2017 - Molybdenite Re-Os age, H-O-C-S-Pb isotopes, and fluid inclusion study of the Caosiyao porphyry Mo deposit in Inner Mongolia, China: in    Ore Geology Reviews   v.81, pp. 728-744.
Wu, G., Li, X., Xu, L., Wang, G., Liu, J., Zhang, T., Quan, Z., Wu, H., Li, T., Zeng, Q. and Chen, Y.,  2017 - Age, geochemistry, and Sr-Nd-Hf-Pb isotopes of the Caosiyao porphyry Mo deposit in Inner Mongolia, China: in    Ore Geology Reviews   v.81, pp. 706-727.
Zhou, T., Zeng, Q., Chen, P., Li, X., Cen, C. and Yang, Y.,  2018 - The formation of the Caosiyao giant porphyry Mo deposit on the northern margin of the North China Craton: Constraints from U-Pb and Re-Os geochronology, whole-rock geochemistry, Hf isotopes, and oxygen fugacity of the magma: in    Geological Journal   v.54, pp. 2160-2184. doi.org/10.1002/gj.3289.

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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