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Maoniuping is the most significant of a cluster of REE deposits located within the Mianning-Dechang REE Belt, and is located ~180 km SW of Chengdu in Sichuan Province, on the eastern margin of the Tibetan Plateau in SW China (#Location: 28° 26' 57"N, 101° 58' 47"E).

The north-south elongated, 270 x 15 km Mianning-Dechang REE Belt has total resources of more than 5 million tonnes of rare-earth oxide (REO) within the giant Maoniuping, large Dalucao, 110 km to the south, and medium-sized Muluozhai and Lizhuang deposits that are 20 km SW and 30 km SSW of Maoniuping respectively. Maoniuping, which is towards the northern extremity of the belt, has an estimated Resource of >3 Mt of REO in ore with a grade that ranges from ~2.7 to 3.9, but averages ~3% REE2O3 (109 Geological Brigade of Sichuan Bureau of Geology and Mineral Resources, 2010), which would equate to an ore tonnage of ~100 Mt. Weng et al. (2022) quote reserves of 3.17 Mt of REO with an average grade of 2.95 wt.% REE2O3. Grades of secondary/oxide ore ranges from 10 to 13.6% REE2O3, while the deposit includes at least 0.33 Mt of Pb, 174 t of Ag, 3.78 Mt of barite and 2.40 Mt of fluorite (109 Geological Brigade of Sichuan Bureau of Geology and Mineral Resources, 1999; 1995; quoted by Xu et al. 2004). The deposit was discovered in 1984, with open pit production beginning in the following year in the shallow oxide section of the deposit where the ore minerals are easily separated (Wang et al., 2001)

The Mianning-Dechang REE Belt lies within the western margin of the Yangtze Craton, close to the structural boundary with the Songpan-Ganze flysch belt that comprises Palaeozoic and Mesozoic sedimentary rocks floored by oceanic crust (see figure below). Much of the contact between the craton and the Sangpan-Ganze complex/flysch belt is formed by the broad, NNE to NE trending, SE-vergent Longmenshan Thrust belt. The latter is offset by the sinistral NW-SE Xianshuihe Fault which, to the NW of the thrust belt, trends NW-SE, parallel to the suture between the Yidun terrane and Songpan-Ganze flysch belt. To the SE of the thrust belt, this fault bifurcates, with the main structure, which becomes the Anninghe Fault, rotating to trend north-south, following the core of the Mianning-Dechang REE Belt. The second strand is the Xiaojlang or Daliangshan Fault, which trends NNW-SSE into the Yangtze Craton, immediately east of the REE Belt.

In the Mianning-Dechang REE Belt area, the Qinghe-Chenghai fault, part of the Longmenshan Thrust belt, juxtaposes early Mesozoic Songpan-Ganze sedimentary rocks southeastward over Neoproterozoic metamorphic basement, which is, in turn, faulted against a Palaeozoic sequence to the east. A little further to the east, the craton is covered by the extensive intracratonic Palaeozoic to Mesozoic Sichuan Basin (Wang et al., 2001).

Setting of Maoniuping

In the immediate Maoniuping deposit area, the bedrock comprises five major lithological units (after Weng et al., 2021):
i). Neoproterozoic, crystalline 711.8 ±9.4 Ma rhyolite (Zhang et al., 2021);
ii). Devonian to Permian metamorphic platformal sequence to the east, composed of clastic rocks, limestones and flood basalts intruded by Mesozoic granite;
iii). Triassic metaturbidite sedimentary sequence that overlies the Palaeozoic metamorphic rocks;
iv). Mesozoic granite to the west of the Neoproterozoic rhyolite, dated at ~146 Ma (mean zircon U-Pb age; Zhang et al., 1988), and v). Cenozoic REE-mineralised syenite-carbonatite complex.

Maoniuping is the central of three syenite-carbonatite complexes distributed over a 10 km x 200 to 600 m interval along the the dextral Haha strike-slip fault, a second order structure with respect to the large Anninghe strike-slip fault. Sanchahe to the north, has only weak REE mineralisation, occurring as a limited number of fluorite-quartz-barite-orthoclase-REE veins filling joint fissures within alkali-feldspar granites. However, late stage molybdenite is very common in sulphide-quartz veins. Baozicun, immediately to the south of Maoniuping, also only contains weak in REE mineralisation, lacks any carbonatite and is rarely altered. In contrast, the central Maoniuping zone is enriched in REE mineralisation (Wang et al., 2001).

The Maoniuping syenite-carbonatite complex has surface dimensions of ~1400 x 260 to 350 m trending NNE, and dips at ~70°WNW (Hou et al., 2009; Liu and Hou, 2017). As mentioned above it is developed within the broad Haha strike-slip fault zone, and also parallel to the general trend of the Qinghe-Chenghai thrust fault. The complex predominantly intrudes Neoproterozoic rhyolite and Mesozoic granite (Liu and Hou, 2017). It mainly comprises syenite, carbonatite, pegmatite (composed of aegirine-augite, barite, fluorite and bastnäsite), and various hydrothermal ore veins (Niu and Lin, 1994; Yuan et al., 1995; Weng et al., 2021). Syenite is the dominant lithology in the complex, and is the major host rock for REE mineralisation (Yuan et al., 1995; Hou et al., 2009; Liu and Hou, 2017; Liu et al., 2019). Maoniuping Geology

Carbonatite intrudes the centre of the complex as dykes, that may be up to 90 to 200 m thick (Xu et al., 2003), the bulk of which are located in the northern part of the deposit, the Guangtoushan section. Two distinct groups of these carbonatite dykes have been differentiated, based on their colour and mineral assemblages. Both are mainly composed of coarse-grained, up to 3 cm across, calcite crystals (i.e., sövite) with a pegmatoid texture, but show distinctive differences in mineralogy and geochemistry. Fine grained calcite carbonatite (i.e., alvikite), ankeritic carbonatite, beforsite and other kinds of carbonatites are rare in the Maoniuping district. The two main groups are (after Wang et al., 2001):
i). yellowish pink sövite, which contains 50 to 90% calcite, with associated microcline, aegirine-augite, biotite, riebeckite, arfvedsonite, xenotime, pyrochlore and other minerals common to carbonatites. The sövite assemblage includes significant REE minerals, dominantly bastnäsite, with lesser parisite, chevkinite, orthite, xenotime and pyrochlore, as well as galena, molybdenite, chalcopyrite, pyrite and sphalerite;
ii). white sövite which has a calcite content as high as >95%, with subordinate barite, fluorite, aegirine-augite, arfvedsonite and K feldspar. Both groups are cut by a variety of veins, which also contain fluorite and bastnäsite (Wang et al., 2001; Hou et al., 2006, 2009; 2015; Xie et al., 2009, 2015).

The central part of the deposit, the Dagudao section, only contains a relatively minor proportion of carbonatite (Liu and Hou, 2017; Zheng and Liu, 2019; Jia and Liu, 2020; Zhang et al., 2021), and is predominantly composed of syenite, which hosts a stockwork grading into multiple branching veins of variable thickness (Liu and Hou, 2017; Liu et al., 2019; Zheng and Liu, 2019).

Some four alkaline granite dykes of varying size have been outlined in the deposit area, mainly in the northeastern Guangtoushan and central Dagudao sections of the deposit (Pu, 1988, 1993; Yuan et al., 1995; Liu et al., 2004). In the Guangtoushan section, these alkaline granite dykes are generally continuous and intrude syenite stocks. They are 5 to 15 m thick, with a NE strike length of ~300 m and dip at 65 to 75°NW. The contacts between the alkaline granite dykes and the syenite-carbonatite complex are well defined. In the Dagudao section, the alkaline granite dykes are discontinuous and mainly intrude Mesozoic granites, syenite stocks, and related ore-bearing veins. They are 1.5 to 12 m thick, strike NE or north-south over lengths of ~200 m, and dip near ~90°NW or W. The cross-cutting relationships between these dykes and ore-bearing rocks indicate that they post-date the Maoniuping alkaline complex. They have been dated at 25.5 ±0.4 Ma (Zircon U-Pb; Weng et al., 2022).

Most of the mineralisation of the deposit occurs as disseminated, veined, brecciated and massive forms (Hou et al., 2009; Liu and Hou, 2017; Zhang et al., 2021). Bastnäsite is the dominant economic mineral (e.g., Weng et al., 2022), accounting for ~97% of the REE resource (Wang et al., 2001). It occurs in four distinct paragenetic types in the syenite-carbonatite complex (after Weng et al., 2022): i). primary euhedral (type-A) in syenite; ii). macro-crystalline tabular euhedral (type-B) in pegmatitic dykes;iii). fine-grained, anhedral veinlet-disseminated (type-C) in syenite; andiv). coarse-grained anhedral (type-D) in carbonatite dykes, occurring as veinlets or interstitial to calcite, fluorite and barite. Paragenetic and compositional variations, are interpreted to infer that type-A bastnäsite is of primary magmatic origin, whilst the other three types have characteristics of hydrothermal origin. This type is not of economic significance, except where overprinted by the other types (Weng et al., 2022). The host syenite has has been dated at 29 to 26 Ma (Zircon U-Pb; Ling et al., 2016; Liu et al., 2019), whilst the bastnäsite is constrained to 28 to 25 Ma (U-Th-Pb ages; Ling et al., 2016; Yang et al., 2019; Weng et al., 2021).

Most of the ore grade REE mineralisation is composed of pegmatitic bastnäsite, pegmatitic calcite veins and stockworks, all of which have a gangue that comprises fluorite and varying amounts of barite, calcite, quartz, mica and aegirine-augite. These mineralisation styles have varying characteristics, as follows (after Wang et al., 2001):
• The pegmatitic bastnäsite mineralisation fills tensile fractures and is enriched at the intersection of fissures, forming barite-aegirine-augite-bastnäsite veins. These veins have a zonation, from the core to outer sections of the veins, of quartz-fluorite-bastnäsite with minor barite and aegirine-augite → barite-fluorite-bastnäsite containing barite and aegirine-augite which increases gradually → barite-aegirine-augite with decreasing bastnäsite and fluorite, and without quartz. The minerals within these lodes are coarse-grained, and have an idiomorphic texture. The largest grain of bastnäsite are of the order of 50 × 15 × 10 cm.
• Barite-aegirine-augite ore veins which fill compression-shear fissures and have a zonation pattern of, aegirine + augite → barite → barite + aegirine + augite + bastnäsite, from the core to periphery. The minerals are relatively fine, with hypidiomorphic to anhedral textures.
• The syenite hosted veins are also zoned with small quartz nests in the core, progressing outward from microcline → barite + aegirine + augite + microcline → microcline + barite + aegirine + augite to barite + aegirine + augite → aegirine + augite (Wang et al., 2001).

In the central Dagudao section of the deposit where mining has been concentrated, REE mineralisation is dated at 26.4 ±1.2 Ma in the Late Oligocene (
40Ar/39Ar dating of fluorphlogopite associated with bastnäsite) within a hydrothermal vein system developed in the coeval syenite intrusion. The lower part of the deposit contains low-grade stockworks of multiple veinlets and breccias which grade upwards into progressively thicker veins of up to 12 m in width. These veins are typically zoned and contain ferromagnesian micas (biotite to fluorphlogopite), sodium clinopyroxenes (aegirine to aegirine-augite), sodium amphiboles (magnesio-arfvedsonite to fluororichterite), K feldspar, fluorite, barite, calcite and bastnäsite. The latter four minerals are most common in the uppermost 80 m of the Dagudao section and represent the apex of hydrothermal activity (Liu et al., 2019).

The boundary between 'ore' grade and sub-economic mineralised host rock cannot be discriminated by eye, with the disseminated REE contents in the host rocks, both carbonatite and syenite, usually >2000 ppm. Wang et al. (2001) noted that >100 REE 'orebodies' have been delineated at the time of publication. The size of individual 'orebodies' range from 30 to more than 1000 m in length, 4 to 90 m in thickness, and extending to 60 to 350 m below the surface. In addition to REE, Pb, Mo, Bi, Ag, Nb, U, Th, barite and strontiobarite, bariocelestine and fluorite are by-products. Bastnäsite accounts for 97% of the REE resources, followed by parisite and chevkinite. Lesser minerals include cerapaite, xenotime, orthite, monazite, pyrochlore, aeschynite, fergusonite, pyromorphite and betafites. Associated minerals include galena, molybdenite, bismuthinite, argentite, thorite and uranthorite. The REE elements are predominantly cerium, lanthanum, praseodymium and neodymium which are light REE (Weng et al., 2001).

Fenitisation (alkali metasomatism), is the dominant alteration type at Maoniuping, affecting rock at the margins of carbonatites which is replaced by arfvedsonite. This alteration is particularly significant at Dagudao where it occurs as sodic clinopyroxene and amphibole (Xie et al., 2009, 2015; Liu and Hou, 2017; Zheng and Liu, 2019; Zheng et al., 2021).

The carbonatites from the Maoniuping REE deposit are enriched in the incompatible elements, e.g., sampling by Xu et al. (2002) that gave LREE contents of 1922 to 3508 ppm; Sr of 10 530 to 15 017 ppm; and Ba of 426 to 1629 ppm; interpreted to be consistent with the carbonatite magma being the product of low-degree partial melting of mantle rocks. The mantle source of the syenite-carbonatite complex is interpreted to have been metasomatized by REE-rich carbonatitic melts/fluids derived from subducted marine sediments and altered oceanic crust, as indicated by Sr-Nd-Pb-C-O-Li-B isotopic compositions (e.g., Hou et al., 2006, 2015; Tian et al., 2015; Liu and Hou, 2017; Weng et al., 2021).

There are systematic variations in the fluid inclusion data which indicate a continuous hydrothermal evolution from about 400 to 230°C in feldspar, clinopyroxene and amphibole → 240 to 140°C in bastnäsite, fluorite and calcite. Hydrothermal REE transport is regarded as probably being controlled by F
-, (SO4)2-, Cl-, and (CO3)2- as complexing ligands (Liu et al., 2019). The same authors suggest that in the Dagudao section, silicate magmas produced orthomagmatic fluids that exploited and expanded a fissure system generated by strike-slip faulting. They suggest, the fluids had initially had an appreciable capacity to transport REE and, consequently, no major mineralisation was developed. The earliest minerals to precipitate were alkali- and Fe-rich silicates containing low levels of F, which caused progressive enrichment of the fluid in Ca, Mg, F, Cl, REE, SO4)2-, and (CO3)2-, gradually leading to the crystallisation of aegirine-augite, fluororichterite, fluorphlogopite, fluorite, barite, calcite and bastnäsite. Barite, fluorite, calcite and bastnäsite are the most common minerals in typical ores, and bastnäsite generally postdates the gangue minerals. Thus, Liu et al. (2019), conclude that it is very probable that fluid cooling and formation of large amount of fluorite, barite and calcite triggered bastnäsite precipitation in the waning stage of hydrothermal activity.

The most recent source geological information used to prepare this decription was dated: 2022.    
This description is a summary from published sources, the chief of which are listed below.
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  References & Additional Information
   Selected References:
Liu, Y., Chakhmouradian, A.R., Hou, Z., Wenlei, S. and Kynicky, J.,  2019 - Development of REE mineralization in the giant Maoniuping deposit (Sichuan, China): insights from mineralogy, fluid inclusions, and trace-element geochemistry: in    Mineralium Deposita   v.54, pp. 701-718.
Wang, D., Yang, J., Yan, S., Xu, J., Chen, Y., Pu, G. and Luo, Y.,  2001 - A Special Orogenic-type Rare Earth Element Deposit in Maoniuping, Sichuan, China: Geology and Geochemistry: in    Resource Geology,   v.51, pp. 177-188.
Wang, Z.-Y., Fan, H.-R., Zhou, L., Yang, K.-F. and She, H.-D.,  2020 - Carbonatite-Related REE Deposits: An Overview: in    Minerals (MDPI)   v.10, 26p. doi:10.3390/min10110965.
Weng, Q., Niu, H.-C., Qu, P., Li, N.-B., Shan, Q. and Yang, W,-B.,  2022 - Controlling factors of prolonged REE mineralization in the Maoniuping REE deposit: Constraints from alkaline granite in the syenite-carbonatite complex: in    Ore Geology Reviews   v.142, 14p. doi.org/10.1016/j.oregeorev.2022.104705.
Weng, Q., Yang, W,-B., Niu, H.-C., Li, N.-B., Mitchell, R.H., Zurevinski, S. and Wu, D.,  2022 - Formation of the Maoniuping giant REE deposit: Constraints from mineralogy and in situ bastnasite U-Pb geochronology: in    American Mineralogist   v.107, pp. 282-293.
Weng, Q., Yang, W,-B., Niu, H.-C., Li, N.-B., Shan, Q., Fan, G.-Q. and Jiang, Z.-Y..,  2021 - Two discrete stages of fenitization in the Lizhuang REE deposit, SW China: Implications for REE mineralization: in    Ore Geology Reviews   v.133, 20p. doi.org/10.1016/j.oregeorev.2021.104090.
Xu, C., Huang, Z., Liu, C., Qi, L., Li, W. and Guan, T.,  2003 - PGE geochemistry of carbonatites in Maoniuping REE deposit, Sichuan Province, China: Preliminary study: in    Geochemical Journal,   v.37, pp. 391-399
Xu, C., Zhang, H., Huang, Z., Liu, C., Qi, L., W. and Guan, T.,  2004 - Genesis of the carbonatite-syenite complex and REE deposit at Maoniuping, Sichuan Province, China: Evidence from Pb isotope geochemistry: in    Geochemical Journal,   v.38, pp. 67-76.

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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