Evidence from Inclusions in the Maoniuping REE Deposit, Western Sichuan, China

Ore Geology Reviews 36 (2009) 90–105

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Continuous carbonatitic melt–ﬂuid evolution of a REE mineralization system: Evidence from inclusions in the Maoniuping REE Deposit, Western Sichuan, China

Yuling Xie a,⁎, Zengqian Hou b, Shuping Yin b, Simon C. Dominy c,d, Jiuhua Xu a, Shihong Tian e, Wenyi Xu e a School of Civil and Environmental Engineering, University of Science and Technology Beijing, Beijing 100083, PR China b Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, PR China c Snowden Mining Industry Consultants Ltd., Weybridge, Surrey, KT13 0TT, UK d School of Science & Engineering, University of Ballarat, Ballarat, Victoria 3353, Australia e Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100037, PR China article info abstract

Article history: The Maoniuping REE deposit is a world-class deposit with 1.2 Mt of REO grading on average 2.89 wt.% REO. It Received 11 January 2008 is the largest in the 270-km long Mianning–Dechang REE belt and is associated with Himalayan carbonatite– Received in revised form 28 October 2008 alkalic complexes in the eastern Indo-Asian collisional zone, Western Sichuan Province, China. The deposit is Accepted 28 October 2008 hosted by nordmarkite stocks and carbonatite sills that display a radiometric age of 40 to 24 Ma and which Available online 13 November 2008 intruded a Yanshanian granite pluton. The 40.3 to 27.8 Ma REE mineralization occurs as vein systems hosted in nordmarkite and carbonatite with minor altered granite and rhyolite. Four ore types are recognized based Keywords: Maoniuping REE deposit on ore texture and mineral assemblage: (1) disseminated; (2) pegmatitic; (3) brecciated; and 4) stockwork Carbonatite (stringer) types. Five mineralizing stages are confirmed according to vein crosscutting relationships, mineral Melt–fluid inclusions assemblage and microthermometric results, these are: 1) carbonatite stage, 2) pegmatite stage, 3) barite– Multicomponent fluid bastnaesite stage, 4) later calcite stage and 5) epigenetic oxidation stage. Varied inclusion assemblages are REE mineralization process found in fluorite, quartz, bastnaesite, barite and calcite from stages 1 through to stage 4. The dominant

inclusion types include: melt, melt–fluid, CO2-rich fluid and aqueous-rich fluid inclusions. Fluid, melt–fluid and melt inclusion studies indicate that the ore-forming fluid resulted from the unmixing of carbonatite melt and carbonatitic fluid. Initial ore-forming fluids were high-temperature (600 to 850 °C), high-pressure

(N350 MPa) and high-density supercritical orthomagmatic ﬂuids, characterized by SO4-rich and multicomponent composition (e.g. K, Na, Ca, Ba, Sr and REE). The dominant anion is not Cl, but SO4. The evolution

of the ore-forming fluid is from a melt–fluid at high temperature, through a CO2-rich fluid at high to medium temperature to aqueous-rich fluid at low temperature. REE precipitation occurred from a high to medium

temperature CO2-rich ﬂuid. The main mechanism for REE precipitation was phase separation of CO2 and aqueous ﬂuids resulting in a decrease of temperature and pressure. © 2008 Elsevier B.V. All rights reserved.

1. Introduction collisional zone (Hou et al., 2009-this issue). It is closely associated with a Cenozoic carbonatite–alkalic complex (Shi, 1993; Yuan et al., 1995), Deposits of rare earth elements (REE) usually occur in a continental controlled by a Cenozoic strike–slip faulting system, and occurs in the rift environment, such as observed in the East African Rift (Mitchell and Eastern India-Asian Collision Zone of the eastern margin of the Tibetan Garson, 1981) or the Proterozoic Langshan–Bayan Obo Rift, North China Plateau (Hou et al., 2006a; Hou and Cook, 2009-this issue). The deposit (Wang and Li, 1992). They generally have a close genetic relation- is characterized by a large ore tonnage (1.2 Mt REO), young mineraliza- ship with mantle-derived carbonatites in rift environments. Such REE tion age (27.8 to 40.3 Ma; Shi, 1993) and a well-developed mineralized deposits have been studied in detail and their genesis is well understood vein system. The system provides a unique opportunity to establish the (e.g., Wang and Li, 1992; Buhn and Rankin, 1999; Buhn et al., 2002; genetic link between REE metallogeny and collisional orogeny, and to Williams-Jones and Palmer, 2002). Recent studies have demonstrated further understand the evolution of a REE-bearing ore-forming ﬂuid that REE deposits may also occur in continental collisional zones (cf. system in a continental collisional zone. Hou et al., 2006a). The Maoniuping REE deposit, a giant deposit in the Many researchers have carried out investigations into the geology, 270 km-long Himalayan Mianning–Dechang (MD) REE belt in western geochemistry and genesis of the Maoniuping REE deposit (Niu and Sichuan, China, is regarded to be the largest and most typical in a Lin, 1994, 1995a,b; Niu et al., 1996a,b, 1997, 2003; Yuan et al., 1995; Yang et al., 2000; Xu et al., 2001a,b; 2003a,b, 2004; Wang et al., 2002; ⁎ Corresponding author. Liu et al., 2004). There is still debate on its genesis, especially on the E-mail address: [email protected] (Y. Xie). characteristics of ore-forming ﬂuids, the evolution of ore-forming

fluids and REE precipitation mechanisms. Previous studies and new geological and geochemical data, we discuss the origin and evolution data presented in this contribution indicate that the ore-forming of ore-forming fluids and REE ore-forming processes. A model for system at Maoniuping is complex, as indicated by numerous types of Maoniuping mineralization is accordingly proposed. inclusions trapped in both REE minerals (bastnaesite) and gangue minerals (e.g., calcite, fluorite, quartz, and barite) (Niu et al., 1996a; 2. Geology Yang et al., 2001; Xie et al., 2005, 2007). However, these inclusions, varying from melt- and melt–fluid inclusions to fluid inclusions, The Himalayan Mianning–Dechang (MD) REE belt is located in the probably record a continuous carbonatitic melt–fluid evolution within western margin of the Yangtze craton, consisting of an Archaean– the REE-mineralization system (Xie et al., 2007). Systematic studies Proterozoic metamorphic crystalline basement and an overlying on inclusions enable us to characterize primary ore-forming fluids, sequence of Phanerozoic clastic and carbonate rocks (Cong, 1988; establish their affiliation to the collisional magma systems, and assist Luo et al., 1998). The western margin of the craton underwent understanding REE mineralization processes. Permian rifting to form a N–S-trending paleorift zone and was This paper reports new observations and data on various involved in the eastern Indo-Asian collisional zone which formed inclusions from Maoniuping and provides an alternative interpreta- since the Paleocene (Yin and Harrison, 2000; Zhong et al., 2001; Mo tion of previous inclusion data presented by the present authors and et al., 2003). Accompanying Cenozoic collisional orogeny, a series of others. Based on studies of melt/fluid inclusions integrated with Cenozoic strike–slip faults and a set of NE-, NNE- and NNW-trending

Fig. 1. Sketch geological map of the Maoniuping REE deposit, with locations of samples for the inclusion study marked (modified from Yuan et al., 1995). Q — quaternary. 92 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 folds were developed in the eastern Indo-Asian collision zone (Luo U–Pb age of 146 Ma (Yuan et al., 1995). The complex comprises a et al., 1998; Hou et al., 2006a; 2009-this issue). More than five nordmarkite stock with minor carbonatite sills (Fig. 2a; Hou et al., carbonatite–alkalic complexes occur in the MD belt, all controlled by 2009-this issue). The carbonatite bodies are relatively small, but Cenozoic strike–slip faults. Available data define a relatively short extend continuously downwards. Sometimes nordmarkite breccia duration for magmatic activity (24 to 40 Ma, peaking at 34 Ma; Zhang clasts can be observed in the carbonatite body (Fig. 2b). The REE and Xie, 1997; Wang et al., 2001; Chung et al., 1998; Hou et al., 2003; orebodies are hosted principally in the nordmarkite stock, carbonatite Guo et al., 2005). sills, and to a lesser extent in the undated rhyolite and altered The geology of the Maoniuping district is described in detail by Yanshanian granite; they are overlain by a Triassic coal-bearing Hou et al. (2009-this issue). The Maoniuping complex and associated sequence. REE mineralization is controlled by a series of NE–NNE trending faults The carbonatite rocks are generally pink or fawn in colour, due to the (Fig. 1), and intruded a NS-striking Yanshanian granite pluton with a presence of purple fluorite and yellow–brown bastnaesite. They have

Fig. 2. Photographs of field outcrops and polished samples from the Maoniuping REE deposit. (a) Fluorite, quartz, aegirine–augite bearing carbonatite (Carb) vein in nordmarkite; (b) nordmarkite breccia in carbonatite; (c) quartz (Q)–barite (Bar)–fluorite (Fl) vein in nordmarkite (Nord); (d) quartz (Q)–barite (Bar)–fluorite (Fl) vein in nordmarkite (Nord); (e) Barite (Bar)–bastnaesite (Bast)–acmite–augite (Acm) vein crosscutting fine-grained quartz (Q) vein; (f) barite (Bar)–bastnaesite (Bast)–acmite–augite (Acm) occurring in the interval of euhedral crystal quartz(Q). Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 93

low SiO2 (b10.22 wt.%), FeO (b1.20 wt.%) and MgO (b0.73 wt.%), and a calcite, fluorite, barite, aegirine–augite, arfvedsonite, microcline and wide range of CaO content (40.7 wt.% to 55.4 wt.%), distinguishing them biotite, with minor galena, pyrite, pyrrhotite, chalcopyrite, hematite, from magnesiocarbonatites (Yuan et al., 1995). They are extremely magnetite, rutile and apatite (Hou et al., 2009-this issue). enriched in LILE (Sr, Ba) and light REE, but relatively depleted in high- Based on the mineral assemblages, ore textures and structures, field strength elements (Nb, Ta, P, Zr, Hf and Ti), suggesting a vein cross-cutting relationships, petrography and inclusion data, five 18 metasomatized mantle source (Hou et al., 2006b). Their δ OV-SMOW mineralization stages are recognized. These comprise: (6.4 to 10.5‰)andδ13C (−3.9 to −8.5‰) values (Tian et al., 2003; V-PDB 1) Carbonatite stage: This stage occurs as pink–white carbonatite dyke Xu et al., 2003a, Niu et al., 1996b, 2003) are also similar to those of intruding nordmarkite (Fig. 2a) in the middle segment or as fine pink primary mantle-derived carbonatites. Associated alkalic rocks are to light purple fluorite-bearing carbonatite cement of breccia in the mainly nordmarkites with high SiO (63.41 to 74.75 wt.%), alumina 2 southern segment of the deposit. The carbonatite dyke and (N13.3%) and alkali (K O+Na ON9.1%) contents, and a wide range of 2 2 carbonatite cement of the breccia comprise a calcite–fluorite– K O (3.63 to 5.94%) content and K O/Na O ratios (0.41 to 1.45), 2 2 2 acmite–augite–arfvedsonite–microcline–pyrite–chalcopyrite–pyr- distinguishing them from pre-Cenozoic granitoid plutons with K O/ 2 rhotite mineral assemblage. In this stage, minor disseminated REE Na O ratios of 1.25 to 1.83 (Yuan et al.,1995). Overlapping emplacement 2 mineralization occurs in carbonatite. ages, Sr–Nd isotopic compositions, and similar mantle-normalized trace 2) Pegmatite stage: Coarse-pegmatite aegirine–augite–fluorite–barite– element patterns to spatially-associated carbonatite, suggest a liquid bastnaesite, fluorite–barite–calcite–bastnaesite, aegirine–augite– immiscible origin for carbonatite and nordmarkite (Hou et al., 2006b). quartz–microcline, and narrow fluorite–quartz–bastnaesite veins formed during this stage. These veins occur in the carbonatite body 3. Mineralization or crosscutting nordmarkite (Fig. 2c, d). The mineral assemblage varies significantly between the vein types, but no crosscutting The Maoniuping REE deposit consists of various mineralized veins relationships have been observed. The mineral assemblage in this and stringer/stockwork zones. The deposit consists of approximately stage consists of calcite, fluorite, microcline, phlogopite, biotite, 71 orebodies (Pu, 1993), occurring as irregularly shaped veins, lenses quartz, aegirine–augite, magnesio–arfvedonite, bastnaesite, chevki- and pockets. Individual orebodies range from 10 m to 1,168 m in nite, pyrite, chalcopyrite and pyrrhotite. length, and between 1 m to 30 m in thickness (Yuan et al., 1995). In 3) Barite–bastnaesite stage: This stage occurs as a mineral assem- plan view, they are distributed in an approximately en-echelon pattern blage of barite–bastnaesite in narrow fluorite–quartz–bastnaesite (Fig. 1). The gross strike direction for the en-echelon system is N–S, veins or crystal barite and barite–bastnaesite in druses. The where individual orebodies strike mainly NNE and dip NW at 65–80° bastnaesite–barite veins crosscut the quartz veins (Fig. 2e). (Fig. 1). The vein system extends NNE, along strike, for 2,650 m and Under the microscope, small-scale barite–bastnaesite veins can shows a “S” shape in plan, indicating a strike–slip fault control. The be found crosscutting quartz and fluorite crystals. The mineral ore-bearing veinlets are usually greater than 30 cm thick, and occur as assemblage of this stage is barite, bastnaesite, calcite, celestine, a swarm in the centre of the complex. The mineralized stringers range magnetite, hematite, rutile, apatite, galena and unusual Cu-bearing in thickness between 1 and 30 cm, and commonly comprise parallel alloys. Two such phases were determined by EPMA and SEM/EDS vein zones enveloping the central ore zone. The stockwork zone to be Sn-bearing copper (Cu) and zinccopperite (Cu Zn) (Xie et al., usually occurs at the margins of the veinlets and vein zones. 2 2006). This stage, together with stage 2, formed the main REE body Four main primary ore types have been recognized: 1) dissemi- and is the most important REE mineralization stage. nated; 2) pegmatitic; 3) breccia; and 4) stringer (stockwork) ore. 4) Late calcite stage: This stage occurs as white–gray calcite veins, Their spatial distribution and major features are summarized in with a mineral assemblage of calcite and strontianite, filling Table 1. Disseminated ore is common in the fenitized wall rock, fractures in both wall rock and REE orebodies. The calcite veins whereas pegmatitic ore occurs as thick ore veinlets or pockets in the crosscut all previous vein generations; no REE mineralization has middle of Maoniuping segment. Breccia ore mainly occurs in the been observed in them. Baozicun (southern) segment of the deposit, and stockwork ore 5) Epigenic oxidation stage: This stage is dominated by REE-bearing Fe– occurs as a NNE-trending halo around the pegmatitic ore. The Mn mineralization, which formed Fe–Mn oxides and an associated stockwork ore occurs in the Sanchahe segment, the northern cerussite+witherite+wulfenite assemblage (Yuan et al., 1995). Maoniuping segment, the southern Maoniuping segment, and in the stockwork zone of the middle Maoniuping segment between the In general, stage 2 is characterized by the appearance of pegmatitic pegmatite ore and altered wall rocks. mineralization, whereas stage 3 is characterized by the appearance of More than 80 minerals have been identified at Maoniuping porous barite and bastnaesite. Mineralization of stage 2 and stage 3 (Pu, 1993; Yuan et al., 1995; Xie et al., 2006). Bastnaesite is the forms the majority of the economic REE deposit. No crosscutting dominant REE mineral in all four ore types. Bunsite, chevkinite and relationships between the pegmatite and narrow REE veins were cerianite have also been identified. The dominant gangue minerals are observed in the field. Observation of ore textures and micro-vein

Table 1 Characteristics of the four ore type in the Maoniuping REE deposit.

Ore type Ore texture Ore structure Mineral assemblage Occurrence Disseminated ore REE–minerals occur as euhedral– Disseminations, stringers Calcite, fluorite, acmite augite, quartz, Altered wall rock (granite and hypautomorphic crystals in carbonatite, biotite, Muscovite, microcline, acmite augite rhyolite) and carbonatite rhyolite and nordmarkite Pegmatitic ore Pegmatite, euhedral–hypautomorphic Massive, miarolitic Calcite, fluorite, quartz, barite, microcline, Only in the middle of Maoniuping crystals, metasomatic, poikilitic texture (cluster), dissemination acmite augite, biotite, arfvedsonite, galena, segment pyrite, chalcopyrite Breccia ore Cataclastic, euhedral–hypautomorphic Breccia; Dissemination Calcite, fluorite, quartz, barite, microcline, Baozicun segment crystals, interstitial, poikilitic mica, biotite, acmite augite, magnetite, hematite Stockwork ore euhedral–hypautomorphic–allotriomorphic Stringer and stockwork Fluorite, quartz, bastnaesite, barite, celestite, Maily in north and south of crystals, metasomatic, poikilitic, interstitial, calcite, biotite, acmite augite, alloys, magnetite, Maoniuping segment, Sanchahe poikilitic hematite, rutile segment 94 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 crosscutting relationships shows that barite and bastnaesite precipi- temperature and phase change characteristics during the heating tated later than most quartz and fluorite, and often filled in the process, inclusions were grouped into three principal types and eight interspaces of euhedral quartz and fluorite (Fig. 2f). sub-types. They are as follows:

4. Sampling and analytical methods (1) Melt inclusions (M type). These often occur in calcite from carbonatite and gangue fluorite from pegmatite and veinlet The minerals sampled for the fluid inclusion study include calcite, (stringer) ore. There are also minor melt inclusions in quartz fluorite, quartz, barite and bastnaesite from mineralization stages 1 and arfvedsonite from pegmatitic and veinlet (stringer) ore. to 4. Samples covered all four ore types summarized in Table 1. The The inclusions generally comprise melt–glass and daughter sample locations are shown in Fig. 1. Microthermometry (for 23 minerals (Fig. 3b); with some containing a minor volatile phase samples), LRM (Laser Raman Microprobe) (for 43 inclusions in 6 (Fig. 3a). Most of the melt inclusions in calcite and fluorite sections) and SEM/EDS (for 5 samples) analysis were performed appear to be trapped minerals at room temperature under the after careful microscopic observation of 35 sections and 55 doubly- microscope, but have a relatively dark, thick borderline and polished sections. often contain a daughter mineral or/and volatile phase. On Microthermometry was undertaken using a Linkam THS600 heating, the phase change of melt inclusions differs from the heating-freezing stage, with a measurable temperature range of trapped minerals. The melt inclusions melt inhomogeneously between −196 and +600 °C (precision of freezing data and at more than 500 °C, in contrast to the trapped minerals that homogenization temperature of ±0.1 °C and ±1 °C, respectively) melt homogeneously at their respective melting points. These and a Linkam THS1500 heating stage with the measurable tempera- melt inclusions were trapped during the late magmatic ture range from room temperature to +1500 °C (precision of carbonatite stage and survived a later period of hydrothermal homogenization temperature of ±1 °C). Microthermometry was alteration. Thus they provide an important method for the undertaken in the Department of Resources Engineering of USTB study of carbonatites (Aldous, 1980; Andersen, 1986) and (University of Science and Technology Beijing) and the Geology and carbonatitic fluids. Geophysics Institute of CAS (China Academy of Science). SEM/EDS (2) Melt–fluid inclusions (ML type). ML type inclusions are (Scanning Electron Microscope/Energy Dispersive Spectrometry) abundant in gangue fluorite and relatively rare in quartz. analysis were performed at the State Key Laboratory for Advanced Almost all of them have negative crystal or polygonal shapes. Metals and Materials at USTB, using a Cambridge S250 SEM equipped The inclusions show variable size and liquid/vapor/solids with a Link Systems 860 energy dispersive spectrometer operating at a ratios. Their petrographic features are similar to the solid-rich 20 kV accelerating voltage. The detailed sample preparation method L–V–S type of inclusions reported in fluorite–REE deposits in for SEM/EDS used is reported in Xie et al. (2000). The mineral the Gallinas Mountains, New Mexico (Williams-Jones et al., fragments were first crushed into small pieces (usually b5mmin 2000). Most ML inclusions contain several solid phases and diameter) to open the inclusions; a number were selected and then have been termed multi-solid inclusions by Niu and Lin stuck onto a piece of glass with their flat side up. The samples were (1995b). Based on microthermometric work, the melt phase kept dry and clean, and quickly carbon coated. LRM was carried out at appears at some point during the heating process and this kind the Mineral Resource Institute, CAGS (Chinese Academy of Geological of inclusions is thus termed “melt–fluid type” in this paper. Science) using a Renishaw 2000 Laser Raman Spectroscope with a Based on microscopic measurement, it has been determined blazing power of 20 mw and blazing wave-length of 514.5 nm. that the solid phases occupy about 40 to 80 vol.% of the inclusions (Fig. 4). Raman spectroscopy and SEM/EDS results 5. Inclusion results (see below) indicate that the solid phases were dominated by sulfate minerals. ML inclusions with different phase ratios often 5.1. Petrography and classification of inclusions occur together in clusters, and are sometimes accompanied by

melt inclusions or CO2-rich fluid inclusions. Their close spatial The samples used for the inclusion study were doubly polished association with melt inclusions and CO2-rich fluid inclusions sections of fluorite, quartz, calcite, barite and bastnaesite from suggests that these inclusions were trapped in the transition mineralization stages 1 to 4. A number of inclusion types were stage from melt to fluid. Meanwhile, the melt phase coexisted identified. These include negative crystals and elongate round, together with an aqueous phase. The continuous series from polygonal or irregular shapes with a size range from b1 µm to several low- to high-solid volume percentage melt–fluid inclusion tens of µm. Based on their petrographic characteristics at room records an immiscible process.

Fig. 3. Photomicrographs of melt inclusions from the Maoniuping REE deposit. (a) Melt inclusion bearing aqueous and daughter mineral in fluorite; (b). daughter mineral bearing melt inclusion in fluorite; D—daughter mineral; V — vapor phase; M — melt glass; LH2O — aqueous phase; Fl — fluorite (host mineral). Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 95

Fig. 4. Photomicrographs of melt–fluid inclusions from the Maoniuping REE deposit. (a) Melt–fluid inclusion with net-like sulfate and daughter minerals in it (ML-1) and ADC sub-type fluid inclusions in fluorite; (b) melt–fluid inclusion with net-like sulfate and daughter minerals (ML-1) in fluorite; (c) melt–fluid inclusion with glass phase and daughter minerals

(ML-2) in fluorite; (d) melt–fluid inclusion with multi-daughter minerals (ML-2) in fluorite; D—daughter mineral; V—vapor phase; M—melt glass; LH2O—aqueous phase; Fl— fluorite (host mineral); N—net-like sulfate.

Based on the petrographic character of melt–ﬂuid inclusions accompanied by melt–ﬂuid inclusions in a single cluster. Five at room temperature, two sub-types can be distinguished as sub-types (Table 2) were distinguished on the basis of phase follows: character at room temperature and characteristics during a. ML-1 sub-type: Containing multi-solid phases, net-like sulfate heating.

(mirabilite), aqueous and a bubble (Fig. 4a, b). The solid phases (a) AC sub-type: Aqueous-liquid CO2 ﬂuid inclusions (Fig. 5a, occupy ca. 75 vol.% of the inclusion volume. At room b, d). These occur in calcite, quartz, barite, bastnaesite and

temperature, they appear dark brown in color because the fluorite. They comprise aqueous and liquid CO2, some with net-like sulfate occupies most of the inclusion volume. All ML- vapor CO2 at room temperature. During heating, most of 1 inclusions include several solid phases. The solid phases them homogenized to a CO2–H2O critical phase (AC-2); a show rod-like, globular, and rounded-polyhedron shapes. few homogenizing to an aqueous phase (AC-1). The CO2 b. ML-2 sub-type: Similar to ML-1, but without net-like sulfate bubble represents N20 vol.% of the inclusion volume. in the inclusions (Fig. 4c, d). At room temperature, they look (b) AV sub-type: Aqueous-vapor fluid inclusions (Fig. 6b). This clearer than ML-1 inclusions. They often contain several fluid inclusion sub-type generally occurs in fluorite, quartz solid phases with rod-like, globular or rounded-polyhedron shapes. The solid phase accounts for 40 to 80 vol.% of the fi inclusions. Most solids were con rmed by LRM and EDS as Table 2 sulfate (see below). Some could not be identified, though Characteristics of different sub-types of fluid inclusions. LRM results identify them to be a carbonate and sulfate Sub-type Phase composition Distribution and occurrence complex (melt glass). of fluid (3) Fluid inclusions (L type). Fluid inclusions are the most inclusions abundant type of inclusions in quartz, barite and REE minerals, AC Aqueous and liquid Very abundant inclusion type in bastnaesite,

and include primary and secondary fluid inclusions. In fluorite CO2 or aqueous, liquid barite and quartz occurring as isolated inclusions from stage 2, minor primary fluid inclusions are usually and vapor CO2 or clusters. Some was found in fluorite or calcite. accompanied by melt–fluid inclusions. Most secondary fluid ADC Aqueous, liquid CO2, Same as AC sub-type inclusions are distributed as clusters or trails along the healed one or more daughter fissures. In quartz from stage 2, L-type fluid inclusions are the minerals dominant type; some occur as isolated inclusions or clusters C Liquid CO2 or liquid Same as AC sub-type with negative crystal shapes, showing the features of pseudo- and vapor CO2 AV Aqueous and vapor Occurring as isolated inclusions or clusters in secondary inclusions. Others are distributed as trails or lines, calcite from stage 4. Most AV fluid inclusions are showing secondary features. All primary and pseudosecondary found as lines or trails in quartz and fluorite. fluid inclusions contain a liquid CO2 bubble (see below), ADV Aqueous, vapor, one Same as AV sub-type whereas secondary fluid inclusions contain a vapor phase or two daughter minerals bubble. Primary CO2-rich fluid inclusions are sometimes 96 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105

Fig. 5. Photomicrographs of CO2-rich ﬂuid inclusions in the Maoniuping REE deposit. (a) Aqueous-liquid CO2 (and vapor CO2) inclusion (AC); (b) aqueous-liquid CO2 inclusion

(AC) and daughter minerals-bearing CO2-aqueous(ADC) ﬂuid inclusions; (c) liquid CO2 inclusion (C); (d) aqueous-liquid CO2 inclusion (AC) and liquid CO2 inclusion; D—daughter mineral; LH2O—aqueous phase; LCO2—liquid CO2;VCO2—vapor CO2.

and calcite as secondary inclusions observed as trails along solid phase accounts for a much lower vol.% (b15%) than healed fractures or in calcite from stage 4. This sub-type ML inclusions. They are usually accompanied by AV fluid comprises aqueous and vapor phase. The vapor bubble inclusions. These inclusions often occur as secondary fluid accounts for a lower percentage (b20 vol.%, mostly 5– inclusions in quartz, fluorite, bastnaesite and barite from 10 vol.%) than AC and ADC sub-types. When heated, they stages 2 and 3.

homogenize to an aqueous phase at low temperature, (e) C sub-type: Pure CO2 fluid inclusion (Fig. 5c). They comprise mostly below 200 °C. a liquid CO2 or liquid and vapor CO2.PureCO2 fluid inclusions (c) ADC sub-type: Aqueous daughter minerals–liquid CO2 fluid often occur in quartz, bastnaesite and barite, and are inclusions (Fig. 5b). Each comprises one or more daughter commonly accompanied by AC and ADC fluid inclusions.

minerals, aqueous and liquid CO2, usually accompanied by AC and C sub-type inclusions. Inclusions of this kind often occur 5.2. Inclusion assemblages in barite, bastnaesite and quartz as primary fluid inclusions, or in fluorite and igneous calcite as secondary fluid inclusions. Inclusion assemblages vary with host minerals and ore-forming (d) ADV sub-type: Aqueous-daughter minerals–vapor fluid stages in the Maoniuping deposit. From stages 1 to 4, the dominant inclusions (Fig. 6a). Inclusions of this kind comprise one inclusions vary from melt inclusions (M type), melt–fluid inclusions

or two daughter minerals, aqueous and vapor phase. The (ML type), high-medium temperature CO2-rich ﬂuid inclusions (AC+

Fig. 6. Photomicrographs of aqueous rich fluid inclusions in the Maoniuping REE deposit; (a). Daughter minerals-bearing aqueous vapor fluid inclusions (ADV) in fluorite;

(b) Aqueous-vapor ﬂuid inclusion (AV) in quartz; D—daughter mineral; LH2O—aqueous phase; V—vapor phase.; Q — quartz; Fl — ﬂuorite. Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 97

Table 3 crystal shapes. The ML-2 inclusions have a varied solid volume Microthermometric results of melt and melt–fluid inclusions in fluorite and quartz in percentage. Under the microscope, this assemblage forms a the Maoniuping REE deposit. continuous sold/aqueous/vapor series. Sample Ore type Mineral Type of Th (°C) CO2-rich fluid inclusion assemblage: Includes high-medium tem- inclusion Range Average perature aqueous-CO2 fluid inclusions (AC sub-type), aqueous- – MNP-13 Pegmatite Quartz M 790 791(3) 791 fl MNP-16 Stringer Fluorite M N737(1)a / daughter mineral-bearing uid inclusions (ADC sub-type) and a MNP-125 Pegmatite Fluorite M N778(1) / pure CO2 inclusions (C sub-type). They are abundant in barite, MNP-17 Stringer Fluorite M 795–850(2) 823 bastnaesite and hydrothermal calcite from stage 3 or as secondary – MNP-11 Pegmatite Fluorite ML-1 650 850 (3) 705 fl fl MNP-12 Pegmatite Quartz ML-1 600–750(4) 658 inclusions in uorite and quartz from stage 2. Most AC uid MNP-13 Pegmatite Fluorite ML-2 687(1) 687 inclusions (AC-2) have a critical homogenization behavior; some

a Unsuccessful acquisition of fluid inclusion data. At this temperature, the inclusion (AC-1) homogenize to a liquid phase. comprises a melt phase and a bubble, but the inclusions cannot homogenize because of Aqueous-rich fluid inclusions assemblage: Includes aqueous-vapor leakage. fluid inclusions (AV sub-type) and aqueous-daughter mineral- fl ADC+C sub-type) to low-temperature aqueous rich fluid inclusions bearing uid inclusions (ADV sub-type). They occur in calcite from (AV+ADV sub-type). Four inclusion assemblages were identified in stage 4 or as secondary inclusions in fluorite, quartz, barite and minerals from the different ore-forming stages. They are as follows: bastnaesite from stages 2 and 3. Compared to CO2-rich fluid inclusions, the aqueous-rich fluid inclusion assemblage has a Melt inclusion assemblage: Occurring in calcite and fluorite from smaller bubble, lower CO2 content and lower homogenization the carbonatite stage (stage 1), it comprises melt inclusions temperature. accompanied by trapped minerals. Because of the birefringence effect, the melt inclusions in calcite are not clear under the 5.3. Microthermometric results microscope. Most melt inclusions have no volatile component at room temperature, but some have an irregular bubble or/and a 5.3.1. Melt inclusions daughter mineral. They are observed as isolated inclusions or along Microthermometric results are shown in Table 3.Atroom the growth zone in calcite and fluorite from carbonatite dykes. temperature, the aqueous phase was absent in most of the melt Melt–fluid inclusion assemblage: This comprises melt inclusions inclusions, but on heating at 500 to 700 °C, the glass phase began to (M type), melt–fluid inclusions (ML type) and minor high- melt inhomogeneously. Firstly there are a few small dark spots or dark cracks appearing in the inclusions and with the increasing tempera- temperature daughter mineral-bearing fluid inclusions (ADC ture, the small bubbles agglomerate into one bubble. Continuing sub-type). They mainly occur in fluorite from stage 2. The melt– heating, the bubble gets smaller and smaller, and finally homogenizes fl fl uid inclusion is the dominant type in uorite from stage 2, and is to a melt phase at 790 to 850 °C. Some of these inclusions decrepitated often accompanied by melt inclusions and daughter mineral- or leaked prior to homogenization. Some would not homogenize even fl –fl bearing CO2-rich uid inclusions. The melt uid inclusions have up to 1100 °C (the host mineral–fluorite–melts at this temperature), varied solid volume percentages. ML-1 inclusions have a relative which is considered to be due to volume increase and inclusion stable solid volume percentage and are isolated and with negative expansion. Cooling from homogenization to room temperature,

Fig. 7. Phase-change in melt–fluid inclusion in fluorite; (a) melt–fluid inclusion at room temperature; (b) the bubble phase disappears; (c) New bubble appears; (d) net-like sulfate disappears; (e) melt phase (sulfate melt) appears; (f) new solid phases occur. 98 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 inclusions of this kind cannot return to their original state, but became were distinguished (AC, ADC, AV, ADV and C sub-type) in quartz, aqueous-bearing multi-solid inclusions (with multi-solid phase, fluorite, calcite, barite and bastnaesite. Microthermometric results for little aqueous phase and a bubble) with a very high (N80%) solid these different sub-types are shown in Table 4 and Fig. 8. percentage. During heating, the AC-1 sub-type homogenized to an aqueous phase, and the AC-2 sub-type homogenized to the critical phase. All C

5.3.2. Melt–fluid inclusions sub-type homogenized to a liquid CO2 phase. Heating tests for AC and Microthermometric results are listed in Table 3.MostML ADC sub-type inclusions were difficult, due to decrepitation; the inclusions were unable to homogenize because they decrepitate majority would not homogenize. Microthermometry showed that the before reaching the homogenization temperature. Some did not AC-1 sub-type had a Th (homogenization temperature) of between homogenize below 1000 °C because of obvious volume-increase and 194 and 371 °C, AC-2 has a Th of 227 to 453 °C, and a CO2 density expansion of the inclusion; only a few homogenization temperature between 0.636 and 0.982 g/cm3 according to the partial homogeniza- data were thus obtained. The homogenizing process of the ML tion temperature of liquid CO2 and vapor CO2 (Angus et al., 1976; cf. inclusion is complex during heating (Fig. 7). The dominant phase- Roedder, 1984 and Lu et al., 1994). For ADC sub-type inclusions, while changes for the ML-1 inclusion are reported as: (1) at 115 to 180 °C the heating, the daughter minerals melted first at 223–386 °C, then the bubble disappeared. This temperature range is similar to the inclusion homogenized to a liquid phase at 224 to 477 °C. The density 3 homogenization temperature of AV fluid inclusions; (2) at 240 to of the CO2 in fluid inclusions was estimated as being 0.644 g/cm (only 250 °C the net-like sulfate dissolved and the aqueous phase occupied a one data point was acquired). AV sub-type fluid inclusions have the greater volume percentage (potentially caused by dehydration of lowest Th of 101 to 226 °C and a simple homogenizing behavior. The mirabilite). The inclusions become clearer after melting of the net-like bubbles in most of these inclusions disappear below 200 °C. Cooling sulfate; (3) at 350–500 °C, a melt phase appears from a solid phase- measurements on pure CO2 inclusions show that the melting point of melting, and then gets larger with more solid melting; (4) at about CO2 is between −56.5 and −58.5 °C. This is equal to or a little lower 450 to 500 °C some new solid minerals grew along the inclusion wall than that of the pure CO2 system (Angus et al., 1976; cf. Roedder, 1984 (the authors believe this may be caused by deformation and volume and Lu et al., 1994) and suggests a dominant CO2 system, as well as the increase of the inclusion); (5) at 600 to 700 °C much of the solid phase possibility of minor other volatile components such as CH4,H2 or N2 dissolved or melted; and (6) at 600 to 850 °C the inclusion present. The C sub-type inclusions have homogenization tempera- homogenized by critical behavior. The homogenization temperatures tures of −7.7–14.7 °C. According to the pure CO2 system phase 3 of ML inclusions were between 650 and 850 °C in fluorite, and 600 and diagram, the density of CO2 was estimated as 0.829–0.972 g/cm . 750 °C in quartz. Most ML inclusions decrepitated at a lower Results of SEM/EDS and LRM studies (see below) indicate that most temperature than their homogenization temperature. daughter minerals in both ADC and ADV sub-type inclusions are sulfates including barite, mirabilite, gypsum, aphthitalite and arcanite. 2− 5.3.3. Fluid inclusions SO4 in aqueous phase was also detected in all fluid inclusions by LRM Based on their phase composition at room temperature and (see below). Based on LRM and SEM/EDS results, most daughter 2− homogenizing characteristics during heating, five inclusion sub-types minerals in ADC and ADV inclusions are sulfate, and SO4 exists

Table 4 Microthermometric results of ﬂuid inclusions in the Maoniuping REE deposit.

Sample Ore type Mineral Type of Th (°C) Density of CO2 inclusion Range Average Range Average MNP-6 Carbonatite Q AC 238 (1) 238 0.647–0.868(2) 0.758 MNP-6 Carbonatite Q AV 133–151(5) 142 / / MNP-12 Pegmatite Q AV 124–219(12) 172 / / MNP-12 Pegmatite Q AC 247–371 (7) 296 / / MNP-12 Pegmatite Q ADC 237–477(13) 362 / / MNP-12 Pegmatite Q C / / 0.923–0.958(4) 0.94 MNP-12 Pegmatite Q AC / / 0.636(1) 0.636 MNP-13 Pegmatite Q AC / / 0.692–0.807(3) 0.759 MNP-13 Pegmatite Q C / / 0.944–0.964(4) 0.955 MNP-13 Pegmatite Q ADC 328–459(3) 411 / / MNP-33 Pegmatite Q AC / 0.982(1) 0.982 MNP-33 Pegmatite Q AV 166–185(3) 177 / / MNP-33 Pegmatite Q AC 217–357(5) 278 / / MNP-33 Pegmatite Q C / / 0.938–0.972(6) 0.957 MNP-33 Pegmatite Q ADC / / 0.644(1) 0.644 MNP-132 Pegmatite FL ADC 249–285 (5) 272 / / MNP-132 Pegmatite FL AV 181(1) 181 / / MNP-161 Stringer ore FL ADC 240–386(7) 304 / / MNP-161 Stringer ore Q ADC 224–296 (2) 260 / / MNP-161 Stringer ore FL AV 101–153(16) 122 / / MNP-161 Stringer ore Q AV 113–226(12) 152 / / MNP-153 Stringer ore Q AV 108–159(8) 129 / / MNP-153 Stringer ore Q AC 254–324 2) 289 / / MNP-157 Stringer ore Cal AC 218–242 (3) 230 / / MNP-157 Stringer ore Cal AV 158–159 (2) 159 / / MNP-157 Stringer ore Cal C / / 0.829–0.839(2) 0.834 MNP-157 Stringer ore Bar AC 194–253(4) 219 / / MNP-28 Stringer ore Bar AV 186–199(4) 193 / / MNP-28 Stringer ore Cal AV 141–167(9) 158 / / MNP-28 Stringer ore FL AV 137–143(2) 140 / / MNP-28 Stringer ore FL AC 254–279(2) 266 / / MNP-313b Stringer ore Bast AC 211–255(6) 244 / /

Note: “/” means that no useful data or could not be measured; Fl—ﬂuorite; Q—quartz; Bast—bastnaesite; Cal—calcite; Bar—barite. Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 99

Fig. 8. Histogram of homogenization temperature for ﬂuid inclusions in the Maoniuping REE deposit.

widely in aqueous phases of all kinds of fluid inclusions. It is density of pure CO2 inclusions, the pressure for stage 4 should be concluded that the fluid in all fluid inclusion sub-types is not a typical lower than 100 MPa.

NaCl (KCl)–H2O–CO2 system, so the salinity of the ore-forming ﬂuid cannot be determined by measurements of freezing point or point of 5.4. Laser Raman Microprobe (LRM) analysis of inclusions

CO2 hydrate decomposition. The CO2-rich fluid inclusion assemblage (AC, ADC and C fluid inclusions) shows a continuous CO2/aqueous LRM was performed on the CO2, aqueous and solid phases in fluid ratio in the same cluster, implying the immiscible trapping character. inclusions and melt–fluid inclusions hosted in quartz and fluorite. Th of AC-2 inclusions (227 to 453 °C) should be the best candidate for Selections of spectrograms are shown in Figs. 9 and 10.ForML −1 the trapping temperature, and the pure CO2 inclusions were also inclusions, there is strong peak around 3500 cm which indicates an −1 trapped at almost the same temperature (or a little lower). According aqueous phase dominated by H2O. A peak at 981 to 984 cm was also 2− to the pure CO2 phase diagram (cf. Roedder, 1984; Lu et al., 1994), the clearly seen, indicating the presence of SO4 in the aqueous phase. pressure can be estimated using the isochore method. The capture Interference fluorescence from fluorite masks the peaks which indicates −1 pressure was estimated as being between 150 and 350 MPa for the the presence of CO2 (1386 cm ) in the bubble phase. The spectrograms CO2-rich fluid inclusion assemblage. This pressure should reflect the of solid-phase in melt–fluid inclusions suggest that the solid phases unmixing condition between CO2 and aqueous fluid. For stage 2, the were dominated by a sulfate assemblage, including mirabilite (Fig. 9a), fluid temperature is higher than that of stage 3, and so stage 2 should gypsum, barite, celestine (Fig. 9b), and nahcolite. In some ML inclusions, be at a higher pressure than stage 3. For stage 4, the fluid temperature a solid phase has peaks at 1005,1069 and 988 cm−1; these may indicate is lower than stage 3, and no liquid CO2, but the vapor can be sulfate–carbonate complexes. determined in AV and ADV fluid inclusions. It is concluded that the In CO2-rich fluid inclusions (AC, ADC), the aqueous phase was also − 1 capture pressure for stage 4 should be much lower than stage 3. dominated by H2O. The peaks at 981 to 984 cm (Fig. 10b, c, f) 2− According to the Th of AV and ADV fluid inclusions and the lowest CO2 suggest that SO4 was also the dominant anion in the aqueous phase.

Fig. 9. LRM spectrum of melt–fluid inclusions (ML) in fluorite from the Maoniuping REE deposit. (a) Mirabilite daughter mineral in melt–fluid inclusion of fluorite; (b) celestite daughter mineral in melt–fluid inclusion of fluorite. 100 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105

2− Fig. 10. LRM spectrum of fluid inclusions in quartz from the Maoniuping REE deposit; (a) CO2 in fluid inclusion in quartz(AC sub-type); (b) SO4 in aqueous phase of fluid inclusion in 2− quartz (AC sub-type); (c) H2O and SO4 in aqueous of fluid inclusion (AC sub-type); (d) CO2 in bubble phase of fluid inclusion (ADC sub-type); (e) Nahcolite daughter mineral in 2− fluid inclusion (ADC sub-type); (f) SO4 in aqueous phase of fluid inclusion (ADC sub-type).

The peak at 1066 to 1069 cm− 1, compared with the spectrum of host according to the strong peaks at 1282 and 1386 cm− 1 (Fig. 10a, d). The 2− quartz, suggests the presence of CO3 in the aqueous phase. The daughter minerals are also dominated by sulfate minerals. Compared 2− 2− occurrence of SO4 and CO3 is consistent with the presence of sul- with the host mineral, the spectrogram of the daughter minerals in fate and bicarbonate daughter minerals in the inclusions. The bubbles quartz and fluorite have additional peaks at 1007–1005, 998, 988 − 1 in AC and ADC sub-type fluid inclusions are dominated by CO2, and 992 cm . These indicate gypsum, celestite, barite and mirabilite Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 101 daughter minerals, respectively. Nahcolite was also determined via the or more daughter minerals. According to the solid percentage and strong peak at 1046 cm− 1 (Fig. 10e). The determination of daughter comparison with the petrographic features of the different inclusion minerals is based on the inorganic material LRM spectrum database types, the inclusions observed under SEM were defined as ML, ADC of Renishaw, and reference to Samson et al. (1995) and Rankin (2003). and ADV types. EDS analysis showed that most of the daughter In aqueous-fluid inclusions (AV, ADV), the aqueous phase is minerals in all inclusion types contain S, K and/or Na. Integrating the − 1 also dominated by H2O. The peaks at 981 to 984 cm also suggest results of LRM and SEM/EDS analysis, it appears that the daughter 2− that SO4 is the dominant anion in the aqueous phase. The bubbles in phases in melt–fluid and fluid inclusions are dominated by sulfates AV and ADV sub-type fluid inclusions are dominated by H2O, with no such as arcanite (K2SO4; Fig. 11b, c), aphthitalite (NaK3[SO4]2; Fig. 11a, − 1 peaks at 1282 or 1386 cm . Sulfate minerals were also confirmed as c), mirabilite (Na2SO4 10H2O) and gypsum (CaSO4 2H2O). Arcanite the dominant daughter phases. and aphthitalite are the most common types seen. No typical LRM spectra for arcanite and aphthitalite were available for comparison, so 5.5. SEM/EDS analysis of daughter minerals in inclusions some of the mirabilite determined by LRM could be arcanite or aphthitalite. Carbonate or bicarbonate daughter minerals was also Numerous open inclusions in the prepared samples (fluorite and determined by EDS. Barite, strontianite and REE minerals have also quartz) were observed under the SEM; most inclusions contained one been reported as daughter phases in inclusions by Xu et al. (2004).

Fig. 11. Secondary electron images and EDS spectrum of daughter minerals in the Maoniuping REE deposit; (a) aphtalose (Aph) daughter mineral in quartz; (b) arkanite(Ark) and unidentified (X) daughter mineral in fluorite; (c) aphtalose (Aph) and arkanite (Ark) daughter mineral in fluorite; Fl—fluorite; Q—quartz. 102 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105

6. Discussion 6.2. T–P–X properties of carbonatitic ﬂuid

Most carbonatites worldwide are believed to be genetically linked The characteristics of carbonatite fluids in a continental rift environ- to silicate alkaline magmatism. As a result of their enrichment in REE ment have been studied by numerous researchers (cf. Morogan, 1994; minerals, many carbonatite bodies form economic REE deposits Samson et al., 1995; Palmer and Williams-Jones, 1996; Smith et al., 2000; (cf. Rankin, 2003). Significant evidence from phase equilibrium Smith and Henderson, 2000; Buhn et al., 2002; Williams-Jones and experiments has shown that carbonatites can be generated by primary Palmer, 2002; Rankin, 2003). Although it is not known if these results mantle melting and by the differentiation of carbonated silicate melts, areapplicabletoanenvironmentofthetypediscussedhere,their i.e., liquid immiscibility and crystal fractionation (cf. Bell et al., 1999). consideration is still helpful in any attempt to understand carbonatitic The genesis of carbonatite–alkaline complex at Maoniuping fluids and their associated mineralization in a continental collision has been extensively researched, including aspects of petrology and environment. C, O, Pb, Sr and Nd isotopes (Yuan et al., 1995; Niu et al., 1996b; Rankin (2003) summarized the composition of ore-forming fluids Wang et al., 2002; Xu et al., 2003a, 2004; Tian et al., 2003;). in carbonatite-associated rare metal deposits. Rankin suggests that

These results imply an igneous carbonatite and mantle origin. CO2-rich, L-V (aqueous-vapor), L-V-S (aqueous-vapor-solid) and L-V- The carbonatite in the Maoniuping area is rich in incompatible MS (aqueous-vapor-multisolid) inclusions are common in carbona- elements such as Sr, Ba and REE, with a mantle-source C and O tite-related deposits. CO2-rich fluid inclusions are noted in most isotope composition, high 87Sr/86Sr and low ξNd implying a meta- carbonatite related deposits, however not, for example, in the Oka somatically enriched mantle source (Xu et al., 2003a). The carbonatite (Samson et al., 1995). The orthomagmatic fluids evolved unmixing of a CO2-rich alkaline silicate magma related to the from carbonatite magmas are Na–K–chloride–carbonate/bicarbonate partial melting of metasomatic mantle is likely to be the source of brines and have variable salt content and rather consistent K/Na ratios. the carbonatite and nordmarkite. Sulfate and fluoride contents may also be high due to the occurrence of Carbonatite melt is low density, aqueous-rich (Wyllie, 1966) sulphate and fluoride-bearing daughter minerals (Rankin, 2003). Cl− 2− 2− and much lower viscosity than that of silicate melt (Wolff, 1994). and SO4 are the dominant anions present, with minor CO3 and − + + 2+ Alkaline-rich fluid in carbonatite melts can decrease the melt density HCO3 .K ,Na and Ca are the dominant cation with levels of Fe, and viscosity. Low-density, low-viscosity carbonatite melts are able Sr, Ba, REE, etc. Nahcolite is the most common daughter phase and to be squeezed into shallow crust (Rankin, 1977; Samson et al., 1995; some other sulfate daughter phases are arcanite, mirabilite, celestine Buhn and Rankin, 1999) and often result in strong alteration and REE and barite with minor carbonate and fluoride minerals (Rankin, 2003). mineralization. Carbonatite fluids play an important role in miner- Similar characteristics are confirmed for the Maoniuping deposit. alization and alteration in REE deposits globally. Experimental and Due to the high capture pressure and highly developed cleavage in field studies have shown that carbonatite magma is rich in volatile the host minerals (fluorite, calcite, barite and bastnaesite), micro- components (Roedder, 1973; Palmer and Williams-Jones, 1996; thermometric studies are very difficult to undertake on various Andersen, 1987; Nesbitt and Kelly 1977; Buhn and Rankin, 1999; inclusion types, particularly for melt–fluid (multi-solid) inclusions in Buhn et al., 2002; Williams-Jones and Palmer, 2002; Rankin, 2003). the deposit. This explains why very few microthermometric data have However, how the carbonatite fluid is derived from the carbonatite been published for the deposit. For example, Yuan et al. (1995) only magma is still open to debate. Besides crystal fractionation, im- give the microthermometric data for low temperature fluid inclusions miscible processes are believed to cause separation between car- (AV sub-type) and few for daughter mineral bearing fluid inclusions. bonatite melt and carbonatitic fluid (Roedder, 1984; Brooker and Niu and Lin (1995b), Niu et al. (1996a) performed microthermometric

Hamilton, 1990; Mitchell, 1997). The genesis and evolution of work on aqueous-rich fluid inclusions (AV sub-type), CO2-bearing carbonatite fluids has become a key topic of current geological fluid inclusions and melt–fluid inclusions. However, they reported research (Fan et al., 2001). that all melt–fluid inclusions had a vapor phase homogenization behavior. These results were not duplicated in the present study. In 6.1. Origin of carbonatite fluid our observations, all vapor phase homogenization of melt–fluid inclusions was caused by inclusion destruction (e.g., leakage or Microthermometric studies of melt and melt–fluid inclusions explosion). Yang et al. (2001) reported similar microthermometric provide good evidence for volatile rich melt and an immiscible results from CO2-rich fluid inclusions in quartz and bastnaesite to process between carbonatite magma and carbonatitic fluid. The those reported in the present study. Microthermometric results for coexistence of different solid/aqueous ratio melt–fluid inclusions in melt inclusions, melt–fluid inclusions and pure CO2 inclusions are still the same cluster, particularly the occurrence of the melt phase in unavailable and no clear corresponding relationship was confirmed melt–fluid inclusions while heating up to 350 to 500 °C, indicates between different types of inclusions and mineralization stages in the inhomogeneity during trapping. The low melting temperatures in published papers. melt–fluid inclusions indicate that the residue-melt trapped in melt– According to the petrographic observations and microthermo- fluid inclusions is sulfate melt. Based on microthermometry results, metric results, it is concluded that melt inclusions characterize the the unmixing occurred between 790 and 850 °C. The melt, melt–fluid carbonatite stage (stage 1); melt–fluid inclusions sponsor the and fluid inclusion assemblage also implies a carbonatite magma pegmatite stage (stage 2); CO2-rich fluid inclusions dominate the origin of the ore-forming fluid and continuous evolution from melt to barite–bastnaesite stage (stage 3), but only AC-2 inclusions should be ore-forming fluid. the best candidate for stage 3. The AC-1 and C sub-type inclusions Carbon, oxygen and hydrogen isotopic results (Tian et al., 2003; Xu were trapped after phase separation. Aqueous rich fluid inclusions et al., 2003b) show that δ13D has a range from −52×10− 3 to relate to the late calcite stage (stage 4). − 3 13 − 3 − 3 −86×10 , and δ OSMOW has a range from 7.8×10 to 13.3×10 , A key question to consider is which type of inclusions should be 13 and δ CPDB for CO2 in fluid inclusions has a range from −3to the best samples for earliest orthomagmatic fluids evolved from −5.6×10− 3, which is similar to that of carbonatite. The results imply carbonatite magma? Inclusion petrography shows that there are the character of the carbonatite-related orthomagmatic fluid and their volatiles in the melt inclusion from both fluorite and calcite of stages 1 close relationship to a mantle origin. It is concluded that the ore- and 2. Melt inclusions can transform into the melt and volatile phase forming process is closely related to carbonatite–alkaline magmatism, during heating, which indicates the volatile-rich characteristic of the and that the ore-forming fluid results from the unmixing of a volatile- carbonatite melt. ML inclusions are dominant in fluorite from the early rich carbonatite melt. mineralization stage that follows the carbonatite stage. The ML-2 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 103 inclusions have varied solid/aqueous/vapor ratios, implying inhomo- According to microthermometric results, the T–P evolution path from genous entrapment. However the ML-1 type inclusions have a stable carbonatite melt to ore-forming fluid is illustrated in Fig. 12.The solid/aqueous/vapor ratio, and show primary inclusion characteristics evolution of carbonatite fluid is from high-temperature melt–fluids, with negative crystal shape. During heating, most of ML-1 type high-moderate temperature CO2-rich fluids to low-temperature aqu- inclusions explode and only a few homogenized to the critical phase at eous rich fluids. Unmixing of melt and fluid occurs at between 790 and a high temperature. It is thus likely that the ML-1 inclusions represent 850 °C according to the homogenization temperature of melt inclusions. the best sample of the earliest orthomagmatic fluid derived from A large quantity of critical CO2–H2O fluid inclusions (AC-2) in carbonatite magma. High homogenization temperatures, high solid- bastnaesite and barite indicate that the ore-forming fluid for stage 3 is percentages in inclusions and critical-homogenization behavior imply akindofH2O–CO2 supercritical fluid. Aqueous-CO2 phase separation that the initial ore-forming fluids were high-temperature (N600 to occurs at 227 to 453 °C. Secondary fluid inclusions in fluorite and quartz 850 °C), high-pressure (N350 Mpa), very high-concentration and trapped during stage 4 show lower homogenization temperatures and supercritical. LRM, SEM/EDS results and daughter mineral assem- lower CO2 concentrations. However, the LRM results indicate that the + + 2+ 2+ 2+ blages show that K ,Na ,Ca ,togetherwithBa ,Sr and REE, are later fluid is also dominated by the K–Na–H2O–SO4 system. It can be 2− the dominant cations. The dominant anions are SO4 , with minor concluded that from stages 3 to 4, the ore-forming fluid progresses from − 2− amounts of HCO3 and CO3 .CO2 in the bubble phase of ML-1 inclusions a high to moderate temperature CO2-rich fluid to a low temperature is usually difficult to confirm by LRM. It is possible that the peaks at aqueous-rich fluid, but K–Na–H2O–SO4 still dominated the fluid system. 1282 cm−1 and 1386 cm−1 are masked by the fluorescence of fluorite The mineral assemblage provides some useful information about or the absence of CO2 in inclusions of this kind. No ML-1 inclusions have the evolution of the ore-forming fluid. The mineral assemblage in been observed in quartz, but the LRM spectra for the bubbles in ML-2 Maoniuping is similar to that of the Kizilcaoren fluorite–barite–REE inclusions of quartz have peaks at 1282 cm−1 and 1386 cm−1. It is likely deposit, Turkey (Gultekin et al., 2003). However, in contrast to other that the bubble in ML-1 inclusions from fluorite comprise CO2. deposits, Maoniuping also contains some alloys (as determined by CO2-rich fluid inclusion assemblages (AC, ADC and C-type) SEM/EDS and EPMA). Metallic minerals at Maoniuping are relatively represent the next step of carbonatitic fluid evolution, since these rare and are dominated by Fe-oxides. Sulfide is mainly present as inclusions are sometimes associated with melt–fluid inclusions and galena and pyrite, with minor chalcopyrite and pyrrhotite. From stages occur principally in barite and bastnaesite from stage 3. The 1 to 3, the metallic mineral assemblage passes from sulfide-dominated microthermetric and LRM results for CO2-rich fluid inclusions suggest to oxide-dominated. In stages 1 and 2, the dominant minerals are that the ore-forming fluid in stage 3 has a high CO2 concentration, pyrite, chalcopyrite, pyrrhotite and magnetite. In stage 3, the moderate to high temperature and high pressure. LRM and SEM/EDS dominant metallic minerals are magnetite, hematite and rutile, also proved that K+,Na+ and Ca2+ are the dominant cations, together together with native copper, a Cu–Zn alloy and minor galena. The 2+ 2+ 2− with minor Ba ,Sr , REE, and the dominant anions are SO4 . The native metal, oxide and lead–sulfide mineral assemblage is special and bubble phase is dominated by liquid CO2. The similar chemical likely implies unique physico-chemical conditions. From stages 1 to 3, composition between ML inclusions and CO2-rich inclusions, and the variation of the metallic mineral assemblages indicates that the close petrographic relationship, shows their affiliation to the evolution ore-forming environment ranged from reductive to oxidative. No of an ore-forming fluid. metallic minerals were identified in stage 4. Aqueous-rich fluid inclusions have low temperatures and low captive pressures (b100 MPa), indicating late-stage fluid character- 6.4. REE mineralization istics. The LRM and SEM/EDS results of AV and ADV inclusions indicate + + 2− that K ,Na ,H2O and SO4 dominated the system. The white calcite REE mineral precipitation mechanism is an important aspect of veins of stage 4 cross-cuts all styles of REE veins and wallrocks, and carbonatitic fluid research. Smith et al. (2000) and Smith and often occurs as geodes or fracture fillings. That means the calcite of Henderson (2000) proposed that bastnaesite precipitated at a stage 4 precipitated in an extensional environment. temperature of between 250 and 280 °C from CO2-rich fluids, and + + 2− In summary, CO2,K ,Na and SO4 rich carbonatite fluids are that phase separation provided a mechanism for REE concentration likely to be a common characteristic of both the continental rift and and deposition at the Bayan Obo REE deposit. Palmer and Williams- collision orogenic environment. But for the temperature of ore- Jones (1996) and Williams-Jones and Palmer (2002) argue that in the forming fluids, there is a large range reported for different deposits, Amba Dongar fluorite deposit, fluorite precipitated by the mixing of even for a deposit like Maoniuping. The reason for this is that most microthermometric work on high capture pressure inclusions is unsuccessful. The sulfate daughter minerals rich in K+ and Na+ might be a characteristic of inclusions related to carbonatite in both continental rift and continental collision orogenic environments.

6.3. Evolution of ore-forming ﬂuid

The evolution of ore-forming fluids is one of the most important subjects of carbonatite research. Ting et al. (1994) suggested an evolutionary path varying from an early CO2-rich fluid, via medium- high salinity H2O-rich fluid, to a CH4-rich fluid using fluid inclusions in apatite from the Sukulu carbonatite (Uganda). Smith and Henderson (2000) propose that fluids varied from high-temperature, high- pressure CO2-rich, to low-temperature, low-pressure aqueous-rich types, based on studies on fluid inclusions in gangue and REE minerals Fig. 12. Schematic T–P path for evolution of the ore-forming fluid in the Maoniuping from the Bayan Obo Fe-REE-Nb deposit. Although, there is still debate REE deposit. I—carbonatite stage, melt inclusion assemblage; II—pegmatite stage, melt fl — – fl on the genesis of the Bayan Obo deposit, results of inclusion studies uid inclusion assemblage; III barite bastnaesite stage, CO2 rich uid inclusion assemblage; IV—calcite stage, aqueous rich fluid inclusion assemblage. Shaded ellipse appear compatible with carbonatitic fluids (Rankin, 2003). represents T–P range based on microthermometric data, unshaded ellipse shows In the Maoniuping REE deposit, different inclusion types, ranging estimated range based on unhomogenized inclusion data for AC+ADC inclusions and from melt to fluid inclusions, record the evolution of a carbonatitic fluid. ML inclusions. 104 Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105 carbonatite orthomagmatic fluid and late-stage Ca-bearing meteoric Angus, S., Armstrong, B., de Reuck, K.M., 1976. International thermodynamic tables of the fluid state. Carbon Dioxide, vol. 3. Pergamon Press, New York. 385 pp. waters. Niu and Lin (1995b), Niu et al. (1997), Wang et al. (2002) and Bell, K., Kjarsgaard, B.A., Simonetti, A., 1999. Carbonatites — into the twenty-first Xu et al. (2002, 2003b) emphasize the role of carbonatite, and Century. Journal of Petrology 39, 1839–1845. regarded REE mineralization to be related to a late magmatic stage. Brooker, R.A., Hamilton, D.L., 1990. Three liquid immiscibility and the origin of fl carbonatites. Nature 346, 459–462. Yuan et al. (1995) believe that the magmatic uid played the most Buhn, B., Rankin, A.H., 1999. Composition of natural, volatile-rich Na–Ca–REE–Sr important role in mineralization and that REE deposition occurred at a carbonatitic fluids trapped in fluid inclusions. Geochimica et Cosmochimica Acta relatively low temperature (b200 °C) in the Maoniuping deposit. 63, 3781–3797. Mineral paragenetic and characteristics of inclusions in REE Buhn, B., Rankin, A.H., Schneider, J., Dulski, P., 2002. The nature of orthomagmatic, carbonatitic fluids precipitating REE, Sr-rich fluorite: fluid-inclusion evidence from minerals, provides evidence for REE mineral precipitation sequence the Okorusu fluorite deposit, Namibia. Chemical Geology 186, 75–98. and P–T conditions. Petrographic studies show that most REE Chung, S.L., Lo, C.H., Lee, T.Y., Zhang, Y.Q., Xie, Y.W., Li, X.H., Wang, K.L., Wang, P.L., 1998. fl Dischronous uplift of the Tibetan plateau starting from 40 Ma ago. Nature 349, mineral precipitation occurred after uorite and quartz in stage 3. – fl fl 769 773. The uid inclusions in bastnaesite formed a CO2-rich uid inclusion Cong, B.L., 1988. Formation and Evolution of the Panxi Paleo-Rift. Science Press, Beijing. assemblage (AC+ADC+C sub-types). This fluid inclusion assem- 427 pp. (in Chinese with English abstract). Fan,H.R.,Xie,Y.H.,Wang,K.Y.,Yang,X.M.,2001.Carbonatiticfluid and REE blage suggests that REE minerals precipitated in a CO2-rich fluid, and mineralization. Earth Science Frontiers 8 (4), 289–295 (in Chinese with English in the CO2-aqueous unmixing process. Integrating microthermo- abstract). metric, LRM and SEM/EDS results, the dominant ore-forming fluid is Gultekin, A.H., Orgun, Y., Suner, F., 2003. Geology, mineralogy and fluid inclusion data of considered to be a supercritical CO –H O system enriched in K, Na the Kizilcaoren fluorite–barite–REE deposit, Eskisehir, Turkey. Journal of Asian 2 2 – 2− fl Earth Sciences 21, 365 376. and SO4 . The CO2-rich uids may complex and transport REE (Buhn Guo, Z.T., Hertogen, J., Liu, J.Q., Pasteels, P., Boven, A., Punzalan, L., He, H.Y., Luo, X.J., et al., 2002), phase separation of CO2 and aqueous is considered as Zhang, W.H., 2005. Potassic magmatism in western Sichuan and Yunnan provinces, the dominant factor for REE precipitation. SE Tibet, China: petrological and geochemical constraints on petrogenesis. Journal – CO –H O separation may have been caused by a fast decrease in of Petrology 46, 33 78. 2 2 Hou, Z., Cook, N.J., 2009. Metallogenesis of the Tibetan collisional orogen: A review and fluid pressure. Breccia ore with carbonatite cement is found in the introduction to the special issue. Ore Geology Reviews 36, 2–24 (this issue). southern segment of the deposit, indicating the existence of a crypto- Hou, Z.Q., Ma, H.W., Zaw, K., Zhang, Y.Q., Wang, M.J., Wang, Z., Pan, G.T., 2003. The – explosion process. A crypto-explosion process will result in the Himalayan Yulong porphyry copper belt: product by large-scale strike slip faulting in Eastern Tibet. Economic Geology 98, 125–145. fl decrease of uid pressure, and hence phase separation between CO2 Hou, Z.Q., Pan, G.T., Wang, A.J., Mo, X.X., Tian, S.H., Sun, X.M., Ding, L., Wang, E.Q., Gao, Y.F., and aqueous fluids, leading to REE precipitation. Xie, Y.L., Zeng, P.S., Qing, K.Z., Xu, J.F., Qu, X.M., Yang, Z.M., Yang, Z.S., Fei, H.C., Meng, X.J., Li, Z.Q., 2006a. Metallogenesis in the Tibetan collisional orogenic belt: II. Mineralization in the late-collisional transformation setting. Mineral Deposits 7. Conclusions 25, 521–533 (in Chinese with English abstract). Hou, Z.Q., Tian, S.H., Yuan, Z.X., Xie, Y.L., Yin, S.P., Yi, L.S., Fei, H.C., Yang, Z.M., 2006b. The The key conclusions of this study are: Himalayan collision zone carbonatites in western Sichuan, SW China: petrogenesis, mantle source and tectonic implication. Earth and Planetary Science Letters 244, (1) Mineral association, daughter mineral composition and LRM 234–250. fl Hou, Z., Tian, S.H., Xie, Y.L., Yang, Z.S., Yuan, Z.X., Yin, S.P., Yi, L.S., Fei, H.C., Zou, T.R., Bai, results show that the composition of the ore-forming uid is rich G., Li, X.Y., 2009. The Himalayan Mianning–Dechang REE belt associated with 2− in CO2,SO4 ,K,Na,Ca,Ba,SandREEandformedacomplex carbonatite–alkaline complexes, eastern Indo-Asian collision zone, SW China. Ore Geology Reviews 36, 65–89 (this issue). multicomponent (K–Na–Ca–Ba–Sr–REE–SO4)–CO2–H2Osystem. –fl fl Liu, C.Q., Huang, Z.L., Xu, C., Zhang, H.X., Su, G.L., Li, H.P., Qi, L., 2004. Geofluids in earth's (2) The transition of melt, melt uid and uid inclusions suggest mantle and its role in mineralization: a case study of the Mianning REE deposit, continuous evolution from carbonatite magma to hydrothermal Sichuan Province, China. Geological Publishing House, Beijing. 229 pp. (in Chinese fluid, and also suggests a magmatic source and an unmixing with English abstract). process of carbonatite melt and ore-forming fluid. The initial Lu, H.Z., Fan, H.R., Ni, P., Ou, G.X., Shen, K., Zhan, W.H., 1994. Fluid Inclusions. Science Press, Beijing. 487 pp. (in Chinese). carbonatite orthomagmatic fluids were high-temperature, high- Luo, Y.N., Yu, R.L., Hou, L.W., Lai, S.M., Fu, D.M., Chen, M.X., Fu, X.F., Rao, R.B., Zhou, S.S., pressure and high-density supercritical hydrothermal fluids. The 1998. Longmenshan–Jinpingshan Intracontinental Orogenic Belt, Sichuan. Science evolution of ore-forming fluids ranged from the carbonatite melt, and Technology Publishing House, Chengdu. 171 pp. (in Chinese with English abstract). fl fl melt uids, high-medium temperature CO2-rich uids to low Mitchell, R.H., 1997. Carbonate–carbonate immiscibility, neighborite and potassium iron temperature aqueous-rich fluids. sulphide in Oldoinyo Lengai natrocarbonatite. Mineralogical Magazine 61, 779–789. (3) The fluids related to REE mineralization are considered to be a Mitchell, A.H.G., Garson, M.S. (Eds.), 1981. Mineral Deposits and Global Tectonic – Settings. Academic Press, London. 405 pp. type of supercritical CO2 H2O system. 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