Fluid inclusion constraints on the hydrothermal evolution of the Dalucao Carbonatite-related REE deposit, Sichuan Province, China

Accepted Manuscript Fluid Inclusion Constraints on the Hydrothermal Evolution of the Dalucao Carbonatite-related REE deposit, Sichuan Province, China Xiaochao Shu, Yan Liu PII: DOI: Reference:

S0169-1368(18)30199-9 https://doi.org/10.1016/j.oregeorev.2019.02.014 OREGEO 2829

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Ore Geology Reviews

Received Date: Revised Date: Accepted Date:

14 March 2018 7 December 2018 13 February 2019

Please cite this article as: X. Shu, Y. Liu, Fluid Inclusion Constraints on the Hydrothermal Evolution of the Dalucao Carbonatite-related REE deposit, Sichuan Province, China, Ore Geology Reviews (2019), doi: https://doi.org/ 10.1016/j.oregeorev.2019.02.014

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Fluid Inclusion Constraints on the Hydrothermal Evolution of the Dalucao Carbonatite-related REE deposit, Sichuan Province, China

Xiaochao Shu1, 2, Yan Liu2, *

1. State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing 100083, People’s Republic of China 2. Key Laboratory of Deep-Earth Dynamics, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, People’s Republic of China

*Corresponding author: Yan Liu E-mail address: [email protected]

Submitted to Ore Geology Reviews

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Abstract Carbonatite-related rare-earth element (REE) deposits are the most important source of the world’s REE resources. Hydrothermal fluids have been proposed to play a significant role in the transport and precipitation of REEs, but fluid inclusion data on the hydrothermal processes in carbonatitic settings are relatively sparse. The Dalucao deposit, located in the Mianning–Dechang (MD) REE belt, Sichuan, China, is a Cenozoic carbonatite-related REE deposit (c. 12 Ma) that offers an excellent opportunity to investigate the evolution of ore-forming fluids. Brecciated and weathered ores are common in this deposit. The former are characterized by mineral assemblages comprising fluorite + barite + celestite + calcite + quartz + bastnäsite (No. 1 orebody) or fluorite + celestite + pyrite + muscovite +calcite + quartz + bastnäsite (No. 3 orebody), whereas the latter contain REE minerals, clay minerals, and minor gangue minerals. We present a comprehensive study of fluid inclusions from the Dalucao deposit to constrain its hydrothermal evolution. Magmatic, pegmatitic, hydrothermal, and supergene stages have been recognized. During the pegmatitic stage, the main minerals that formed were coarse-grained fluorite, barite, celestite, calcite, and quartz, which host melt inclusions, melt–fluid inclusions, and minor high-salinity fluid inclusions. The presence of melt and melt–fluid inclusions suggests a magmatic origin for the ore-forming fluids. Hydrothermal processes included at least two stages, characterized by hydrothermal veins that are developed in fractures within the carbonatite–syenite complex: (1) Fluid inclusions during the formation of the fluorite–quartz–barite veins in the pre-REE stage were trapped under immiscible conditions, as evidenced by the presence of CO2-bearing inclusions coexisting with aqueous ones. These immiscible CO2-bearing inclusions recorded a range of pressures from 1050 to 1600 bar. All of fluid inclusions in this stage exhibited homogenization temperatures varying from 278

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to 442 °C, with salinities ranging from 3.2 to 45.1 wt% NaCl equivalent (equiv.). (2) The REE-stage fluids were represented by abundant aqueous inclusions, characterized by homogenization temperatures ranging from 147 to 323 °C and salinities between 1.1 and 9.5 wt% NaCl equiv. These data suggest that the ore-forming fluids forming the Dalucao deposit evolved from high-temperature, high-pressure, high-salinity, CO2-rich to low-temperature, low-pressure, low-salinity, CO2-poor. Gasand ion-chromatographic analyses combined with mineralogical features indicate that the initial fluids were rich in REEs, (SO4)2−, Cl−, F−, Na+, K+, Ca2+, and volatile components (e.g., H2O, CO2, N2, CH4, Ar, and C2H6). H–O isotope analyses of quartz suggest that the hydrothermal fluids had a dominantly magmatic signature and were gradually diluted by meteoric waters. Hydrothermal REE transport was probably controlled by F−, (SO4)2−, and Cl− as complexing ligands. We propose that fluid cooling and mixing rather than immiscibility led to the precipitation of bastnäsite during the waning stage of hydrothermal activity. Taken together, the inclusion data and observations of alteration, paragenesis and mineralization have provided insights into the development of REE mineralization and the further exploration of carbonatite-related REE resources.

Keywords: Fluid inclusions; Hydrothermal evolution; REE precipitation; Carbonatite; Dalucao deposit; Mianning–Dechang REE belt

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1 Introduction Carbonatites are rare mantle-derived rocks produced by extremely low-degree partial melting of carbonated mantle (Sweeney, 1994; Thomsen and Schmidt, 2008; Stoppa and Schiazza, 2013). Carbonatite intrusions are mostly situated in anorogenic settings related to intracontinental rifting (Woolley and Kjarsgaard, 2008) and frequently occur in ring-complexes associated with alkaline silicate rocks (Stoppa et al., 2005; Andersson et al., 2016; Song et al., 2016a). One of the notable features of carbonatites is a strong enrichment in incompatible lithophile elements such as rare-earth elements (REEs), Ba, Sr, and Nb (Nelson et al. 1988; Chakhmouradian, 2006; Xu et al., 2015; Song et al., 2016b). Rare-earth deposits associated with carbonatite intrusions are the world’s most significant source of these elements (Chakhmouradian and Wall, 2012; Weng et al., 2015; Liu et al., 2018). REEs are critical materials because of their increasing importance in renewable energy and high-technology applications (Xu et al., 2017). China supplies more than 85% of the World’s REE resources (Goodenough et al., 2016) and firmly established this country as a leader in the global REE market (Chakhmouradian and Wall, 2012; Kynický et al., 2012, 2018; Pirajno, 2016), as represented by the largest world-class Bayan Obo deposit (Smith et al., 2000; Smith and Henderson, 2000; Fan et al., 2006, 2016; Yang et al., 2009, 2016) and Cenozoic Mianning-Dechang (MD) REE belt (Xu et al., 2008, 2012; Hou et al., 2006, 2009, 2015; Liu et al., 2015a-c, 2018; Liu and Hou, 2017). Many carbonatite-related REE deposits are thought to have experienced a continuous evolution from a magmatic setting to a hydrothermal setting (e.g., Xie et al., 2009, 2015), in which the hydrothermal fluids played a significant role in the transport and precipitation of REEs (Samson and Wood, 2005; Sheard, 2012; Pandur et al., 2014). As summarized by Rankin (2005) and Williams-Jones et al. (2012), REE precipitation in carbonatitic systems is hybrid, with an early

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magmatic component that is overprinted by a later hydrothermal stage, the latter being critical in producing economic concentrations. The potential importance of REE mobility under hydrothermal conditions is supported by recent experimental researches (e.g., Migdisov et al., 2009; Trofanenko et al., 2016). These studies now accommodate the fact that REEs can be mobilized and transported by hydrothermal fluids in the form of complexing ligands, such as fluoride, sulfate, chloride, or carbonate, depending on temperature, pressure, and the composition and pH value of the fluids (Smith and Henderson, 2000; Williams-Jones et al., 2012; Migdisov et al., 2014). Fluid inclusions have long been considered as reliable recorders of primary fluids, which can provide crucial constraints on hydrothermal processes and ore precipitation (Roedder, 1984, 1997). Thus, systematic fluid inclusion studies based on detailed petrographic observations are essential for defining the mechanism of hydrothermal REE mineralization. The Dalucao deposit in the MD REE belt offers an excellent opportunity for studying the genesis of carbonatite-related REE deposits owing to its young age (c. 12 Ma; Liu et al., 2015a) and well-preserved evidence for the magmatic–hydrothermal processes responsible for its formation (Liu and Hou, 2017). Previous studies of the Dalucao deposit have focused on geochronology (Liu et al., 2015a; Ling et al., 2016), fluorite trace-element chemistry (Xu et al., 2012), ores (Liu et al., 2015b, c), and isotopes (Liu and Hou, 2017). Some studies have noted the complexity of the hydrothermal system related to the Dalucao deposit, as evidenced by the numerous types of inclusions that are trapped within both REE (bastnäsite) and gangue minerals (e.g., fluorite and quartz) (Yang et al., 1998; Hou et al., 2009). However, their conclusions were based simply on limited microthermometric data and lacked fluid-composition analyses and pressure estimations. Importantly, field relations reported in these literatures provide a dearth of information on the mineral assemblages, even though

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these assemblages host the bulk of the REE mineralization. Thus far, the hydrothermal evolution affecting REE transport and precipitation remain poorly understood. In this contribution, we propose a detailed description of the geology, alteration, paragenesis and mineralization in the Dalucao deposit based on field investigations and petrographic observations. We further present a comprehensive study that incorporates fluid inclusion petrography, microthermometry, laser Raman spectroscopy, gas- and ion-chromatographic analyses, and stable isotope measurements (H, O) of quartz. This new dataset allows us to better constrain the source(s) and evolution of hydrothermal fluids and to further advance our understanding of REE precipitation in this deposit. Finally, the implications of studying these fluid inclusions for the exploration of economic REE deposits in carbonatitic settings are explored.

2 Geological setting The Cenozoic MD belt is located on the western margin of the Yangtze Craton and measures about 270 km in length and 15 km in width (Fig. 1; Hou et al., 2009). The belt contains a total estimated resource in excess of 3 Mt of light rare-earth oxides (REOs), hosting one giant (Maoniuping), one large (Dalucao), and two small–medium-sized deposits (Muluozhai and Lizhuang), together with numerous other mineralized carbonatite-syenite complexes (Hou et al., 2009; Liu and Hou, 2017). The basement rocks of the Yangtze Craton are dominated by high-grade Archean metamorphic rocks, Proterozoic metasedimentary rocks, and overlying Phanerozoic clastic and carbonate sequences (Cong, 1988; Luo et al., 1998). The Emeishan mantle plume produced mid-Permian flood basalts, with minor picrites and picritic basalts, which cover the western part of the Yangtze Craton, forming a large igneous province over an area of ~500,000 km2 (Lu, 1996; Xu et

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al., 2001). In eastern Tibet, late-collisional metallogenesis (c. 40–26 Ma) during the Indo-Asian collision developed within a transform structure dominated by Cenozoic strike-slip faulting, shearing, and thrusting, thereby producing one of the most economically significant metallogenic provinces in China (Hou and Cook, 2009).This area hosts porphyry-type Cu–Mo–Au systems controlled by Cenozoic strike-slip faults, orogenic Au systems related to sinistral ductile shearing, Pb–Zn–Ag–Cu systems controlled by Cenozoic thrusting and subsequent strike-slip faulting (Wang et al., 2014a, b, 2016; Deng et al., 2014a, b, 2017a–c; Deng and Wang, 2016), and REE mineralization associated with carbonatite–alkaline complexes (Hou et al., 2006, 2009, 2015; Liu et al., 2015a–c, 2018; Liu and Hou, 2017). The complexes intrude a Proterozoic crystalline basement and an overlying Paleozoic–Mesozoic volcano-sedimentary sequence, and constituted a narrow, NS-tending REE-bearing belt (Fig. 1; Hou et al., 2009). Recent geochronological studies have yielded concordant ages of 12.13 ± 0.19 and 11.32 ± 0.23 Ma for the Dalucao deposit, 22.81 ± 0.31 and 21.3 ± 0.4 Ma for the Maoniuping deposit, 26.77 ± 0.32 Ma for the Muluozhai deposit, and 27.41 ± 0.35 Ma for the Lizhuang deposit (Liu et al., 2015a). These data indicate that the 27–22 Ma ages for syenites in the northern part of the MD belt (Maoniuping, Lizhuang, and Muluozhai) are coeval with strike-slip shearing along the Ailao Shan–Red River shear zone. However, the ages for syenites within the Dalucao deposit in the southern part of the MD belt indicate that shearing along strike-slip faults in that area continued until at least 12 Ma, and possibly longer (Liu et al., 2015a). Strike-slip faulting along the MD belt in eastern Tibet is only one of several structural responses to the Indo–Asia collision that started at 60–55 Ma (Hou and Cook, 2009).

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3 Ore deposit geology 3. 1 General geology The four REE deposits in the MD belt formed under similar geological conditions but are characterized by different ore grades and tonnages and vary widely in the style of mineralization (Liu and Hou, 2017). The Dalucao REE deposit, which is controlled by the Dalucao strike-slip fault, is the second largest in this belt (Fig. 2), and contains about 0.76 Mt REO, grading 5.0% REO on average (Shi and Li, 1996). Due to regional uplift, a Proterozoic quartz diorite pluton with a surface exposure of 70 km2 was exhumed. The Cenozoic syenites intrude mainly the quartz diorite pluton, and associated carbonatite sills intrude along fissures that formed by strike-slip faulting (Fig. 2). Two breccia pipes associated with REE mineralization are developed in the syenites, constituting the majority of the No. 1 (Fig. 3a, c) and No. 3 orebodies (Fig. 3b). The diameters of the long axes of these pipes vary from 200 to 400 m, and those of the short-axes between 180 and 200 m; the pipes extend downwards for some 450 m (Hou et al., 2009). Clastic rocks within the breccia-pipes consist predominately of magmatic detritus and ore fragments hosted within a calcite-rich matrix with subordinate quartz and REE minerals. Sensitive high-resolution ion microprobe U-Pb dating results for zircon from syenites (Liu et al., 2015a) and SIMS (secondary ion massspectrometry) data for bastnäsite (Ling et al., 2016) have yielded a systematic and precise age of c. 12 Ma.

3. 2 Alteration and mineralization Fenitization is regarded as reflecting multiple pulses of fluids expelled from cooling and crystallizing carbonatitic or alkaline melt (Morogan, 1994; Le Bas, 2008) and is the most significant style of alteration in the Dalucao deposit. Syenites in much of the deposit were subjected to some degree of fenitization. However, the most intense alteration occurs in and around the breccias,

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resulting in the formation of a fenitization halo. The breccias are typically clast-supported, in which the clasts are 4 to 17 cm in diameter and consist largely of angular to rounded fragments (Fig. 4f). Field observations show that the fenites are typically cut by calcite veins containing REE mineralization (Fig. 4g), indicating that REE mineralization formed after fenitization. Fenitization is marked by the replacement of albite by K-feldspar, the replacement of primary aegirine–augite by biotite, and the development of fine-grained assemblages of albite and biotite overprinting the syenites (Fig. 5g, h). Late-stage alteration is dominated by intense carbonatization, which overprinted the early-formed fenitization halo. Despite multiple phases of brecciation (four events are recorded in each of the pipes), mineralized veins are contained wholly within altered syenite within the Dalucao deposit (Fig. 2; Liu et al., 2015c). Tectonism and brecciation facilitated the circulation of ore-forming fluids and provided the space for REE precipitation. The Dalucao deposit consists of two large ore lenses and comprises distinct two mineralization systems (Fig. 2; Hou et al., 2009). The first mineralization system occurs within the two breccia pipes. Ore veinlets are usually transitional to mineralized breccia and brecciated ores, and are commonly enveloped by fine-veins and stringer zones with comparable mineral assemblages. The second mineralization system occurs between the two breccia pipes, and it has been displaced by the Mofanggou strike-slip fault (Fig. 2). This system consists of swarms of ore-bearing veins within syenite stocks and an underlying semi-layered mineralized zone developed above the syenites (Hou et al., 2009). The ores are weathered, brecciated, and dominated by fine-grained REE minerals (Liu et al., 2015b, c). Brecciated ore is characterized by mineral assemblages comprising fluorite + barite + celestite + calcite + quartz + bastnäsite (No. 1 orebody; Fig. 4a, b) or fluorite + celestite + pyrite +

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muscovite +calcite + quartz + bastnäsite (No. 3 orebody; Fig. 4c), and it occurs as small lenses or veinlets. The second significant type of mineralization is weathered ore, which is found predominantly in fissures within the No. 1 orebody but is rarely present in other deposits of the MD belt (Fig. 4h). Weathered ore contains REE minerals (including bastnäsite and monazite; Fig. 5e, f) together with minor gangue minerals (e.g., fluorite; Fig. 5e) and abundant clay minerals (Liu et al., 2015c).

3. 3 Paragenetic sequence The paragenetic sequence of REE and gangue minerals in the Dalucao deposit is similar to that in the Maoniuping deposit, although the assemblages are somewhat simpler. Based on field observations, petrographic examinations of typical ore specimens, and previous studies (e.g., Liu et al., 2015b), magmatic, pegmatitic, hydrothermal, and supergene stages have been recognized (Fig. 6). The magmatic stage was marked by the formation of the carbonatite–syenite complex. Many rock-forming minerals such as K-feldspar and albite, along with minor accessory minerals such as zircon and apatite, formed in the carbonatite and associated syenite. The pegmatitic stage is developed mainly within the No. 1 orebody. During this stage, coarse-grained fluorite, barite, celestite, calcite, and quartz were the main minerals that formed (Fig. 4d). Hydrothermal processes include at least two stages, characterized by the formation of hydrothermal veins in fractures within the carbonatite–syenite complex (Fig. 4e): (1) The pre-REE stage was associated with the formation of fine-grained gangue minerals. Typical gangue minerals are fluorite, quartz, calcite, barite, and celestite. Only where fluorite occurs, as opposed to the other gangue minerals (e.g., barite, calcite), can bastnäsite be found. (2) The REE stage was characterized by the crystallization of bastnäsite. Bastnäsite forms fractured or subhedral to euhedral prismatic crystals that commonly overprint early-formed gangue minerals (e.g., fluorite, calcite, quartz, and barite) or fill the cavities between

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these minerals (Fig. 5a, b, d–f, i), indicating that REE mineralization occurred during the late-stage of hydrothermal activity. The supergene stage was marked by the occurrence of abundant clay minerals (e.g., illite, kaolinite) (Fig. 4i).

4 Samples and analytical methods 4. 1 Samples The samples used in this study are all taken from representative hydrothermal minerals within ore veins, and include: (1) the pegmatite-stage fluorite; (2) the pre-REE-stage fluorite, quartz, and barite; (3) the REE-stage bastnäsite, together with calcite and fluorite associated with bastnäsite in a paragenesis. The locations of selected samples are schematically shown in Fig. 2. More than 40 doubly polished thin sections were prepared for optical examination, from which 20 representative ones were chosen for subsequent microthermometric measurements and laser Raman spectroscopic analyses. In addition, three samples from the same ore veins were prepared for gas- and ion-chromatographic analyses to determine fluid compositions, and three quartz samples were taken for stable isotope (H, O) analyses to constrain the source(s) of the fluids.

4. 2 Methods 4. 2. 1 Microthermometry Microthermometric measurements of fluid inclusions were performed on a Linkam THMSG 600 heating–freezing stage attached to a Leitz microscope at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, China. The temperature calibration of the heating–freezing stage was carried out by analyzing standard samples of synthetic fluid inclusions supplied by Fluid Inc. (USA). The temperature accuracy of the heating–freezing stage

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from −120 to −70 °C is ± 0.5 °C, from −70 to 100 °C is ± 0.2 °C, and from 100 to 500 °C is ± 0.2 °C. The rate of heating–freezing was generally 0.2 to 5.0 °C/min but was reduced to 0.2 °C/min close to the phase transition temperature for CO2-bearing inclusions (such as CO2 clathrate melting temperature) and to 0.2–0.5 °C/min close to the freezing point and homogenization temperature for aqueous inclusions, in order to accurately record the phase transition temperatures. Heating cycles of about 5 °C were used to determine the homogenization temperature of vapor and solid phases in solid-bearing fluid inclusions. Heating cycles also constrained the melting temperature of clathrate to within ± 0.2 °C in most CO2-bearing inclusions. The salinities of aqueous fluid inclusions were approximately calculated using the data of Bodnar and Vityk (1994) for the NaCl–H2O system. Salinities of solid-bearing inclusions were estimated according to the formula of Bodnar and Vityk (1994). For inclusions that homogenized by solid phase dissolution, this method might underestimate salinity by up to 3 wt% NaCl equiv. (Bodnar and Vityk, 1994). The salinities of CO2-bearing inclusions were determined using the equations of Collins (1979). The densities of fluid inclusions were calculated using the Flincor program (Brown, 1989), according to the calculation method of Brown and Lamb (1989).

4. 2. 2 Laser Raman spectroscopy The compositions of individual inclusions from different minerals were analyzed at room temperature using a Renishaw System-2000 Raman microspectrometer according to the method of Burke (2001) at the Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing, China. The excitation wavelength was the 514.53 nm line of an Ar-ion laser operating at 20 mW. The spectra were recorded with counting time of 30 s and spot size of 1μm, and ranged from 10 to 4000 cm−1, 1 accumulation, and the spectral resolution was 1–2 cm–1.

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4. 2. 3 Gas- and ion-chromatographic analyses The gas- and ion-chromatographic analyses of fluid inclusions were performed at the Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China. The gaseous compositions of the fluids were determined using a Prisma TM QMS200 Quadrupole Mass Spectrometer using the method of Zhu and Wang (2000). The dried washed sample weighing 2 g was put in a clean quartz tube and heated to 100 °C. Then, the valve was turned on, and the gas pipe was vacuumed. When the pressure in the quartz tube was less than 6 × 10−6 Pa, the burst stove was heated to 550 °C at a rate of 0.333 °C/s. The vacuum valve was turned on to determine gas composition. A Shimadzu HIC-SP instrument was used to analyze the ionic composition of the fluid inclusions. After cleaning and drying, crushed samples were decrepitated at 500 °C in a vacuum and then leached repeatedly using millipure water in an ultrasonic cleaning bath at room temperature until the electrical conductivity of the leachate was the same as the millipure water. All the collected leachates were volumetrically fixed to 30 ml and then analyzed.

4. 2. 4 Hydrogen and oxygen isotopes Hydrogen and oxygen isotopic analyses were conducted at Beijing Createch Testing Technology Co., Ltd, Beijing, China. Hydrogen isotopic ratios of decrepitated quartz-hosted fluid inclusions were measured by mechanically crushing quartz grains according to the method of Gong et al. (2007). Samples were first degassed by heating under a vacuum at 120 °C for 3 h. The water was then released from the fluid inclusions by heating the samples to ~1000 °C in an induction furnace. The subsequently released water was trapped, reduced to H2 by zinc at 410 °C (Friedman, 1953), then analyzed using a MAT-253 mass spectrometer, with a precision for δD of ± 1‰. Oxygen was liberated from quartz by reaction with BrF5 (Clayton and Mayeda, 1963) and converted to CO2 on a platinum-coated carbon rod.

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The δ18O determinations were made using a MAT-253 mass spectrometer, with a precision for δ18O of ± 0.2‰. The isotope data are expressed in the delta (δ) notation as the per mil (‰) deviation relative to the Vienna Standard Mean Ocean Water (V-SMOW).

5 Results 5. 1 Inclusion petrography Based on phase associations at room temperature and phase transitions upon heating, the inclusions from the Dalucao deposit were grouped into melt inclusions, melt–fluid inclusions, and fluid inclusions. The fluid inclusions were further classified as aqueous (H-type), pure CO2 (PC-type), CO2-bearing (HC-type), solid-bearing (HS-type), and solid-bearing inclusions containing CO2 (CS-type). In this classification system, the letters H, C, and S denote liquid phase, carbonic phase, and solid phase, respectively. Inclusion petrography and classification strictly concentrated on fluid inclusion assemblages (Goldstein and Reynolds, 1994). Melt inclusions were trapped in fluorite during the pegmatite stage. At room temperature, these inclusions are typically composed of a molten glass phase, in some cases containing a small number of bubbles (Fig. 7a). On cooling, the melt inclusions convert into melt–fluid inclusions containing CO2, an aqueous solution, and a melt phase. Melt–fluid inclusions with different phase ratios are commonly found in clusters and in some cases are associated with melt inclusions or CO2-bearing fluid inclusions. Most of these inclusions contain several solids, liquid CO2, aqueous solutions, and in some cases glass phase (Fig. 7b), in which the solids occupy most of the inclusion volume. Most H-type inclusions show two visible phases at room temperature, namely, liquid H2O and

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vapor H2O (LH2O + VH2O), and occur as elliptical or negative shapes 4 to 27 μm in size (Fig. 7c). In general, the vapors are relatively small, with filling degrees ranging from 10 to 40 vol.%. In REE-stage bastnäsite, most of them are densely scattered or grouped in clusters, and thus, are considered to be primary (Fig. 7i). The PC-type inclusions are pure CO2 inclusions and contain two phases (LCO2 + VCO2) (Fig. 7d) or one phase (LCO2) (Fig. 7j) at room temperature. The latter are transformed into two-phase inclusions (LCO2 + VCO2) during freezing, indicating that the PC-type inclusions are liquid at room temperature (Roedder, 1984). These inclusions are small (2–15 µm in diameter) and generally occur in pre-REE-stage quartz and barite. The HC-type inclusions have been widely observed in gangue minerals of the pre-REE stage. These inclusions contain three phases (LH2O + LCO2 + VCO2) (Fig. 7e) or two phases (LH2O + LCO2) (Fig. 7j) at room temperature. For the latter type, the liquid CO2 may separate CO2 bubbles (LCO2 → LCO2 + VCO2) during freezing. The inclusions are 4–26 µm in diameter and the volume fraction of the CO2 phase varies from 20 to 80 vol.% at room temperature. The HS-type inclusions comprise one or several solids, aqueous and vapor phases (LH2O + VH2O + S) (Fig. 7f, h), in which the volume fraction of the solid phase varies from 10 to 40 vol.%. These inclusions are usually present in the pre-REE stage and are generally 6–25 µm in diameter. Sulfate with irregular crystal shapes is the dominant daughter mineral for most of these inclusions (Fig. 9a, b), although minor halite crystals are also observed in fluorite and quartz as regular prismatic or cubic shapes. The CS-type inclusions contain solid, aqueous, and CO2 phases (LH2O + LCO2 + S or LH2O + LCO2 + VCO2 + S) (Fig. 7g). These inclusions range in size from <10 to 25 μm and typically show elliptical

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shapes, although some irregular inclusions with intermediate-sized bubbles (10–50 vol.% at room temperature) are also recognized. They are generally found in quartz and fluorite from the pre-REE stage and are closely spatially associated with HC-type inclusions.

5. 2 Microthermometry Microthermometric studies were conducted on H-, PC-, HC-, and HS-type inclusions from the pre-REE stage and on H-type inclusions from the REE stage. Microthermometric measurements were performed based on the concept of fluid inclusion assemblages, that is, closely associated groups of inclusions with visually identical phase ratios and similar shapes (Goldstein and Reynolds, 1994). The microthermometric data and calculated parameters for fluid inclusions are summarized in Table 1 and graphically illustrated in Fig. 8. Pre-REE stage: A total of 131 fluid inclusions in fluorite, quartz, and barite yielded homogenization temperatures varying from 278 to 442 °C. The PC- and HC-type inclusions exhibited melting temperatures of solid CO2 ranging from −61.4 to −60.0 °C. The temperatures are lower than the triple point for pure CO2 (−56.6 °C), indicating the presence of a subordinate amount of other gases. This is consistent with ion-chromatographic analyses indicating minor amounts of other gases (e.g., N2, CH4) occur in fluid inclusions (see below). Two different modes of partial homogenization of CO2 phase, viz. to liquid and to vapor, can be observed within the fluid inclusion assemblages. More than 60% of the analyzed LC-type inclusions showed partial homogenization to liquid, whereas the others to vapor. All of these inclusions displayed partial homogenization temperatures ranging from 12.1 to 29.6 °C, with total homogenization temperatures exhibiting a wide range from 300 to 442 °C. The melting of CO2 clathrate in the presence of CO2 liquid occurred between −9.6 and 8.4 °C, with the calculated salinities ranging from 3.2 to 23.4 wt% NaCl equiv. These

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measurements for H-type inclusions constrained a salinity range of 6.7 to 13.1 wt.% NaCl equiv. based on freezing points of −9.2 to −4.2 °C. Total homogenization was observed at temperatures between 252 and 370 °C, with two modes of homogenization to liquid or vapor phases. In all LVS inclusions, the liquid–vapor homogenization occurred at temperatures from 286 to 440 °C, and the solid dissolution occurred at temperatures from 202 to 378 °C, corresponding to salinities from 30.1 to 45.1 wt.% NaCl equiv. REE stage: H-type inclusions are abundant and constitute primary inclusion assemblages. All calculated H-type inclusions in the REE-stage minerals showed consistent microthermometric mode, i.e., homogenization to a liquid phase, with total homogenization temperatures ranging from 147 to 323 °C. These inclusions had ice-melting temperatures ranging from −6.2 to −0.6 °C, corresponding to salinities ranging from 1.1 to 9.5 wt% NaCl equiv.

5. 3 Laser Raman spectroscopy Laser Raman spectrograms of selected fluid inclusions are shown in Fig. 9. The spectrograms show that the daughter minerals in HS-type inclusions are dominated by sulfates (e.g., anhydrite, Fig. 9a; celestite; Fig. 9b). Attempts to analyze the compositions of the carbonic phase within HC-type inclusions in fluorite were unsuccessful because of the strong fluorescence of fluorite that obscured the CO2 peaks. Thus, the HC-type inclusions in quartz were chosen with the purpose of analyzing the compositions of the vapor bubbles (Fig. 9c, d). In contrast, CO2 was absent in the vapor phase of H-type inclusions hosted by bastnäsite (Fig. 9e, f). In addition, the peak at 982 cm−1 suggests that (SO4)2− is the dominant anion in the liquid phase (Fig. 9e).

5. 4 Gas- and ion-chromatographic analyses The gas-chromatographic analyses (Table 2) reveal the common presence of H2O, CO2, N2, CH4,

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Ar, and C2H6 in the fluids, in which H2O and CO2 are the dominant gas components. The ion-chromatographic analyses (Table 3) show that the ions consist of (SO4)2−, Cl−, F−, Na+, K+, and Ca2+. Ions of (CO3)2− were not analyzed, as an Na2CO3 solution was used as the flow phase during analysis. Charge imbalance between anions and cations in these data is because some cations, in particular REE3+, were not analyzed.

5. 5 Hydrogen and oxygen isotopes Oxygen and hydrogen isotopic results are listed in Table 4, which includes the data of Liu et al. (2015b). The δ18OV-SMOW values of six quartz samples range from 3.0‰ to 8.5‰. The δ18Ofluid values were calculated using the quartz–water equilibrium function (Clayton et al., 1972): 1000lnαquartz-water = 3.38 × 106 × T−2 − 3.40, with the average homogenization temperature of all fluid inclusions. The measured δDV-SMOW values for the pre-REE and REE stages are −80.2‰ to −71.0‰ and −105.1‰ to −88.4‰, respectively. The calculated δ18Ofluid values for the pre-REE and REE stages are 1.3‰ to 1.6‰ and −7.5‰ to −5.0‰, respectively.

6 Discussion 6. 1 Source of the ore-forming fluids The Sr–Nd–Pb isotopes for the Dalucao deposit, published by Hou et al. (2015) and Liu and Hou (2017), require a shared origin for both host rocks and hydrothermal minerals. The H–O isotopic data provide further constraints on the source of the ore-forming fluids. The early-formed quartz samples in fluorite–quartz–barite hydrothermal veins from the pre-REE stage show relatively high δ18Ofluid (1.3‰ to 1.6‰) and δDV-SMOW (−80.2‰ to −71.0‰) values, consistent with those reported from other carbonatite-related REE deposits within the MD belt (e.g., Maoniuping, Hou et al., 2009; Liu and Hou,

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2017). These data are plotted in the region between the primary magmatic water box and the meteoric water line but still dominated by magmatic water (Fig. 10; Taylor, 1974). The magmatic signature is also supported by the petrographic features of the inclusions. These inclusions, varying from melt inclusions, melt–fluid inclusions, and fluid inclusions, suggest a magmatic origin for the ore-forming fluids and record a continuous evolution from a magmatic setting to a hydrothermal setting. Combined with the close spatial and temporal association between the carbonatite–syenite complex and REE mineralization, it is assumed that the ore-forming fluids in the Dalucao deposit were derived from the carbonatite–syenite magmas. In comparison, the quartz samples from the REE stage yield lower δ18Ofluid (−7.5‰ to −5.0‰) and δDV-SMOW (−105.1‰ to −88.4‰) values than those from the pre-REE stage, strongly implying the involvement of meteoric waters (Fig. 10), consistent with the low measured homogenization temperatures (147 to 323 °C) and salinities (1.1 to 9.5 wt% NaCl equiv.) of fluid inclusions. These inclusions are interpreted to have resulted from the mixing of primary fluids with meteoric waters during the late-stage circulation of low-temperature and low-salinity fluids. A similar scenario is inferred for the mineralization at Bayan Obo (Smith and Henderson, 2000) and in the Gallinas Mountains (Williams-Jones et al., 2000). In conclusion, all H–O isotopic values in quartz gradually decrease from the pre-REE stage to the REE stage, indicating that the ore-forming fluids in the Dalucao deposit have a dominantly magmatic signature and were gradually diluted by meteoric waters during hydrothermal processes.

6. 2 Estimation of the minimum trapping pressure Pressures can be approximately estimated according to microthermometric data of fluid inclusions (Driesner and Heinrich, 2007). The homogenization temperature determined by fluid

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inclusions on immiscibility or boiling traits will represent absolutely fluid trapping temperature (Roedder and Bodnar, 1980; Brown and Hagemann, 1995). In contrast, fluid inclusions trapped without immiscibility or boiling traits will give the minimum homogenization temperature, yielding the minimum trapping pressure (Roedder and Bodnar, 1980; Rusk et al., 2008). The estimation of P–T conditions in (SO4)2−-related fluid systems is extremely difficult but can be attempted if the fluids are treated as similar to those in NaCl-related systems. Petrographic observations and microthermometry of fluid inclusions indicate that the fluids in the pre-REE stage experienced intense immiscibility caused by aqueous–carbonic phase separation, as evidenced by: (1) HC-type inclusions with variable CO2 volume fractions coexist with PC-type inclusions (Fig. 7j); (2) H- and HC-type inclusions have intimate spatial relationship, and exhibit similar homogenization temperature ranges but contrasting salinities (Fig. 7k) (Fig. 7k); and (3) HC-type inclusions identified in pre-REE-stage quartz are closely spatially associated with CS-type inclusions (Fig. 7l). Fournier (1992, 1999) argued that when lithostatic conditions translated into to hydrostatic conditions due to the change from ductile to brittle behavior of the hydrothermal system, the fluid pressure would drop rapidly, resulting in such immiscibility. Not all homogenization temperatures of HC-type inclusions can be used to constrain the conditions of fluid immiscibility, as the different sizes of inclusions may influence homogenization temperatures. Inclusions with greater volume and larger surface area may deform during heating such that the measured homogenization temperatures are unreliable. Similar difficulties arise from the fact that a phase may not be seen owing to its shape and small volume (Mao et al., 2015). In view of these factors, immiscible HC-type inclusions with moderate sizes, CO2 volume fractions of 20% to 80%, homogenization temperatures of 310 to 350 °C, and densities of 0.75 to 0.92 g/cm3 were used to

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evaluate the pressures of immiscibility. The trapping pressures are estimated to range from 1050 to 1600 bar, with the isochores calculated by the Flincor program (Brown, 1989) and the formula of Bowers and Helgeson (1983) (Fig. 11a). If the fluid system is assumed to be a simple water–salt system, the formula given by Driesner and Heinrich (2007) can be used to estimate the minimum trapping pressure during the REE stage. Figure 11b gives the trapping pressures of <100 bar in homogenization temperature versus salinity. The low pressure reflects a shallow and open mineralizing environment in which little CO2 was present. The well-developed tensile fractures and the tectonism that resulted in the formation of fissures and/or breccias throughout the hydrothermal processes appear to have been the main causes of decreasing pressure.

6. 3 Fluid evolution The petrographic and microthermometric data from the fluid inclusions indicate that different types of fluids were present during different stages in the Dalucao deposit. The evolution of hydrothermal fluids in terms of salinity, temperature, and pressure has been reconstructed on the basis of fluid inclusion microthermometry, and used to establish a model for fluid evolution in such carbonatite-related REE deposits. It is assumed that the carbonatite–syenite magmas would release highly oxidized, high-flux, and REE-rich fluids during stress relaxation (Hou et al.,2015). The gas- and ion-chromatographic analyses combined with the observed mineralogical features indicate that the initial fluids were rich in REEs, (SO4)2−, Cl−, F−, Na+, K+, Ca2+, and volatile components (e.g., H2O, CO2, N2, CH4, Ar, and C2H6), similar to those in other carbonatite-related REE deposits, such as Maoniuping (Xie et al., 2009, 2015) and Hoidas Lake (Pandur et al., 2014). As the initial fluids were enriched in Na+ and K+, fenitization occurred as metasomatism in this mining area. At depth, where the country rocks are hottest, little

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cooling of fluids occurred and the pressure approximated lithostatic pressure. Such fluids had an appreciable capacity to transport REEs owing to the high temperature of the hydrothermal system and, consequently, no major REE mineralization developed. The further evolution of the hydrothermal system was marked by the formation of ore veins, the widespread precipitation of gangue minerals, and the onset of bastnäsite crystallization. All of the fluid inclusion types identified in the pre-REE stage, including aqueous, CO2-bearing, and solid-bearing inclusions, record homogenization temperatures verying from 278 to 442 °C and salinities ranging from 3.2 to 45.1 wt% NaCl equiv., in an fluid system of NaCl-H2O-CO2 (Table 1, Fig. 8). The temperatures of the pre-REE-stage fluids are significantly lower than those of any plausible magmatic source and must have originated at depth and cooled thereafter. As mentioned above, CO2- and solid-bearing inclusion assemblages are common and, importantly, strong immiscibility caused by aqueous–carbonic phase separation took place during this stage. Immiscibility, which characterizes the pre-REE stage, resulted in a dramatic decrease in the CO2 content of the fluids, as indicated by the presence of CO2-bearing inclusions coexisting with aqueous ones. Along with immiscibility and the progressive cooling of ascending fluids, the fluids reached the limits of solubility for CaF2, BaSO4, and CaCO3, triggering the precipitation of fluorite, barite, and calcite in hydrothermal veins that had not yet been blocked by earlier-crystallized material. The REE stage was characterized by the crystallization of bastnäsite, representing the waning stage of hydrothermal activity. Abundant aqueous inclusions that developed during the REE stage constitute the primary inclusion assemblages and record homogenization temperatures ranging from 147 to 323 °C and salinities varying from 1.1 to 9.5 wt% NaCl equiv. in a fluid system approximating binary NaCl–H2O (Table 1, Fig. 8). The prevalence of aqueous inclusions in bastnäsite, as well as the

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scarcity of CO2-bearing inclusions, indicates that REEs are most likely to have been precipitated from low- to medium-saline aqueous fluids with minor amounts of CO2. Fluid mixing was the major event during this stage, as evidenced by the H–O isotopic data from quartz, and appears to be the most plausible explanation for the recorded decrease in temperature. With an inflow of more meteoric waters and the termination of immiscibility, the fluids gradually became predominantly meteoric, cooled and diluted, as reflected by the absence of CO2-bearing and high-salinity fluid inclusions in the minerals that grew during the REE stage.

6. 4 REE precipitation Generally, hydrothermal REE precipitation may be triggered by one or several of the following parameters: (1) a decrease in temperature (Trofanenko et al., 2016); (2) a change in the pH value (Williams-Jones et al., 2010; Chiaradia, 2014); (3) immiscibility or boiling of the ore-forming fluids (Smith and Henderson, 2000; Xie et al., 2009, 2015); and (4) the mixing of various fluids (Williams-Jones et al., 2000; Gultekin et al., 2003). Experimental studies have pointed to the importance of the presence of available ligands for high REE solubility (e.g., Wood, 1990; Haas et al., 1995). The initial fluids in the Dalucao deposit were rich in (SO4)2−, Cl−, and F−, as mentioned above, suggesting that REEs can form strongly associated complexes with complexing ligands including sulfate, chloride, and, in particular, fluoride, consistent with the case studies of REE deposits (e.g., Sin Quyen, Li et al., 2017; Li and Zhou, 2018) and experimental petrography work (e.g., Migdisov and Williams-Jones, 2009). The mineralogical signal in which fluorite, barite, calcite, and bastnäsite typically form stable mineral assemblages in the Dalucao deposit also supports this view. Trofanenko et al. (2016) noted that owing to the high content of CO2 in hydrothermal fluids, fluid–rock interaction does not cause the precipitation of REE-bearing complexes by increasing the

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activity of carbonic species and pH. Rather, precipitation of bastnäsite will occur spontaneously once the temperature falls below the limit of its thermal stability. Similarly, there is a clear trend of markedly decreasing temperatures from the pre-REE stage to the REE stage during hydrothermal activity in the Dalucao deposit (Table 1, Fig. 8). This suggests that a simple cooling process might have played a key role in REE precipitation. The possible eff ects of fluid mixing on REE precipitation also appear to be reasonable since characteristics of fluid mixing were commonly observed in bastnäsite-host inclusions. One plausible explanation is that fluid mixing further lowered the temperature of hydrothermal system and promoted the mineralization process. In contrast, fluid immiscibility is unlikely to have been responsible for bastnäsite crystallization because of its occurrence during the pre-REE stage. To say the least, fluid immiscibility is not a direct factor or a dominant mechanism resulting in REE precipitation. Obviously, the schematic diagram shown in Fig. 12 does not reflect the three-dimensional character of ore veins or the apparent continuity of vein development. As discussed above, with changes in the physicochemical properties of the hydrothermal system, the fluids during the pre-REE stage experienced intense immiscibility, causing the escape of most of the CO2 (Fig. 12a). We propose that the precipitation of bastnäsite did not occur until the immiscibility ended, and there were massive inflows of external fluids. As fluids ascended to the near surface, they were diluted by the inflow of meteoric waters that further reduced the temperature of the hydrothermal system. These changes, followed by an increase in the pH value of the hydrothermal system, destabilized the REE-bearing complexes and triggered the precipitation of bastnäsite (Trofanenko et al., 2016). Such mechanism of hydrothermal REE precipitation has parallels with the suggestions of others regarding carbonatite-related REE deposits, such as the Gallinas Mountains fluorite–REE deposits in the USA

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(Williams-Jones et al., 2000) and the Kizilcaören fluorite–barite–REE deposit in Turkey (Gultekin et al., 2003). As a result, bastnäsite crystallized at low temperatures (mostly below 300 °C) and overprinted, or filled the cavities between, early-formed gangue minerals (Fig. 12b).

7 Implications for REE exploration Previous studies have shown that the formation of carbonatite-related REE deposits involves two processes of REE enrichment: REE enriched in the magmatic source of fertile carbonatites is the prerequisite, and the secondary process for REE precipitation under hydrothermal conditions is also vital (Hou et al., 2015; Liu and Hou, 2017). Thus, discovering REE resources related to carbonatite intrusions requires geochemical analyses of primary carbonatites and a meticulous study of fluid inclusions. More than 500 carbonatite localities have been documented worldwide (Woolley and Kjarsgaard, 2008), but only a few of these host significant REE deposits (≥ 100-kt REO). The fact that most large REE deposits are located along cratonic margins implies that areas of metamorphic basement that are bounded, or cut by, translithospheric faults along cratonic margins have significant potential for generating REE deposits. It is accepted that fertile carbonatites generally tend to be extremely enriched in Ba, Sr, and REEs compared with most barren carbonatites (Hou et al., 2015; Liu and Hou, 2017). Furthermore, Hou et al. (2015) noted that fertile Cenozoic carbonatites within the MD belt formed by the partial melting of subcontinental lithospheric mantle (SCLM) that had been metasomatized by REE- and CO2-rich fluids derived from subducted marine sediments. The metasomatism, a process that introduces abundant REEs into SCLM, is critical to REE enrichment in magmatic source of fertile carbonatites.

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Our fluid inclusion study on the basis of alteration, paragenesis, and mineralization in the Dalucao deposit has provided several indicators that may aid in REE exploration. First, the pegmatite containing melt and melt-fluid inclusions is prominent in the No. 1 orebody of the Dalucao deposit, which is also reported in the giant Maoniuping REE deposit, but does not occur within small–medium-sized Muluozhai and Lizhuang deposits (Liu and Hou, 2017). The occurrence of such pegmatite implies that the magmas exsolved high-flux fluids. Thus, pegmatite appears to be one of the signs for potential large REE deposits. Second, extensive fenitization occurs as metasomatism on wall-rocks in the Dalucao deposit, suggesting that the fluids derived from carbonatite-syenite magmas experienced multi-stage activities. Therefore, fenitization is potentially an exploration indicator of REE mineralized system, as in the giant Bayan Obo deposit (Le Bas, 2008). Third, field and petrographic observations both reveal that hydrothermal fluorite, barite, and calcite have a close genetic relationship with bastnäsite in the ore veins of the Dalucao deposit. This is consistent with ion-chromatographic analyses indicating that the fluids were rich in (SO4)2−, Cl− and F−, which are critical in forming complexes with REEs (Williams-Jones et al., 2012; Migdisov et al., 2014). In contrast, similar mineral assemblages are not found in low-grade REE prospects, such as Qieganbulake (Ye et al., 2013) and Miaoya (Xu et al., 2010), even although both areas have experienced REE enrichment in the magmatic source, similar to that inferred for the MD belt. Hence, a stable mineral assemblage comprising fluorite, barite, and calcite is an effective exploration guide. Finally, a large number of CO2-bearing and high-salinity fluid inclusion assemblages, the most characteristic inclusion types of carbonatitic fluids (Rankin, 2005), are widely observed in hydrothermal gangue minerals (e.g., fluorite, quartz) within the Dalucao deposit. Thus, ore veins that contain fluid inclusions that are characterized by bearing CO2 and high salinity are suggested to be a

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preferred prospecting target area.

8 Conclusions 1.

The presence of melt and melt–fluid inclusions in pegmatite stage suggests a magmatic

origin for the ore-forming fluids, which were derived from carbonatite–syenite magmas. Gas- and ion-chromatographic analyses indicate that the initial fluids formed in an SO4–Cl–F–Na–K–Ca–REE hydrothermal system in which H2O and CO2 were the dominant volatile components. 2.

As indicated by microthermometric data and the estimated minimum trapping pressures of the

fluid inclusions, as well as H–O isotope systematics, the ore-forming fluids evolved from high-temperature, high-pressure, high-salinity and CO2-bearing to low-temperature, low-pressure, low-salinity and CO2-poor via immiscibility, cooing and mixing. Cooling of the fluids and the inflow of meteoric waters were the key factors resulting in hydrothermal REE precipitation. 3.

From an exploration viewpoint, primary carbonatites located along cratonic margins that are

enriched in Sr, Ba, and REEs are a prerequisite for regional REE-searching. In addition, several indicators reflecting hydrothermal processes, including the occurrence of pegmatite and fenitization, and the development of stable mineral assemblages comprising fluorite–barite–calcite and containing CO2-bearing and high-salinity inclusion assemblages, appear to be excellent exploration guides for REE mineralized systems.

Acknowledgments The present study was funded by the Major State Basic Research Program of China

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(2015CB452600), the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDA20070304), the Natural Sciences Foundation of China (Grant No. 41772044), Chinese State Key Research and Development Program (2016YFE0203000), Fundamental Research Funds for the Chinese Academy of Geological Sciences (YYWF201705), and China Geological Survey (1212011020000150011-03).

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Figure captions Fig. 1. (a) Tectonic framework showing the spatial distribution of REE deposits in China (modified after Hou et al., 2015). (b) Sketch geological map showing the Cenozoic carbonatite–syenite complexes with zircon U-Pb ages (Liu et al., 2015a) in the MD REE belt, which is controlled by Cenozoic strike-slip faults (modified after Yuan et al., 1995). The Jinpingshan orogen was formed by subduction of the downgoing Proto-Tethyan oceanic lithosphere in the Proterozoic (Cong et al., 1988; Luo et al., 1998). The Proterozoic metamorphic basement was locally covered by Paleozoic-Mesozoic sedimentary strata. Abbreviations: NCC = North China Craton, TM = Tarim Block, YC = Yangtze Craton, CC = Cathaysia Craton, CAO = Central Asian Orogen, CCO = Central China Orogen, AHO = Alps-Himalayan Orogen, SGO = Songpan-Ganzi Orogen.

Fig. 2. (a) Schematic geological map showing the key features of the carbonatite–syenite complex and associated REE orebodies in the Dalucao REE deposit. (b-d). Geological cross-sections along exploration lines 10 and 14 in the No. 1 orebody and line 30 in the No. 3 orebody (modified after Yang et al., 1998).

Fig. 3. (a) Field photograph of the No. 1 orebody. (b) Field photograph of the No. 3 orebody. (c) The cross section selected exhibiting the alteration, ore vein, carbonatite and associated syenite in the

40

Dalucao deposit (modified after Liu et al., 2015b).

Fig. 4. Photographs showing various geologic features in the Dalucao deposit. (a) Representative ore sample from the No. 1 orebody. (b) Altered brecciated ore from the No. 1 orebody (from Liu et al., 2015b). (c) Representative ore sample from the No. 3 orebody. (d) Coarse-grained mineral dikes in syenite (from Liu et al., 2015b). (e) Hydrothermal ore vein cutting syenite. (f) Representative breccia in the No. 3 orebody. (g) Fenite containing calcite vein. (h) Silty gray-purple weathered sample with a fluffy texture in altered syenite. (i) Supergene minerals from the No. 1 orebody. Abbreviations: Fl = fluorite, Ms = muscovite, Brt = barite, Cal = calcite, Qtz = quartz, Py = pyrite.

Fig. 5. (a) Needle-like bastnäsite aggregates overprinting fluorite, barite, celestite and phlogopite. (b) Bastnäsite filling in the spaces of celestite. (c) Typical magmatic minerals (e.g., zircon). (d) Typical brecciated ore in the No. 3 orebody, with bastnäsite overprinting fluorite, celestite and phlogopite. (e) Weathered ore in the No. 1 orebody, with bastnäsite overprinting fluorite. (f) Monazite and bastnäsite overprinting early-formed gangue minerals. (g) The fenitization in the No. 3 orebody, with the occurrence of K-feldspar, albite and muscovite. (h) Celestite-bearing calcite vein cutting altered syenite. (i) Sulfide filling in the spaces of celestite. Abbreviations: Fl = fluorite, Bsn = bastnäsite, Ms = muscovite, Cls = Celestite, Ap = apatite, Kfs = K-feldspar, Ab = albite, Phl = phlogopite, Bt = biotite, Cal = calcite, Py = pyrite, Zrn = zircon, Gn = galena, Mnz = monazite.

Fig. 6. Paragenetic sequence of REE and gangue minerals in the Dalucao deposit (modified after Liu et al., 2015b).

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Fig. 7. Photomicrographs of individual fluid inclusions (a-h) and fluid inclusion assemblages (i-l) from the Dalucao deposit. (a) Melt inclusion in fluorite. (b) Melt–fluid inclusion in fluorite. (c) H-type inclusion in fluorite. (d) PC-type inclusion in fluorite. (e) HC-type inclusion in fluorite. (f) HS-type inclusion in fluorite. (g) CS-type inclusion in fluorite. (h) HS-type inclusion comprising multi-solids in fluorite. (i) Abundant H-type inclusions with constant bubble fractions in bastnäsite. (j) Coexistence of PC- and HC-type inclusions in quartz. (k) Coexistence of HC- and H-type inclusions in fluorite. (l) HC inclusions are closely spatially associated with CS inclusions in quartz. Abbreviations: Fl = fluorite, Bsn = bastnäsite, Qtz = quartz, V = vapor phase, L= liquid phase, S = solid phase.

Fig. 8. Histograms of homogenization temperature and salinity for fluid inclusions from the Dalucao deposit.

Fig. 9. Laser Raman spectroscopy spectrograms for individual inclusions in the Dalucao deposit. (a) Anhydrite spectrum in the solid phase of a solid-bearing inclusion. (b) Celestite spectrum in the solid phase of a solid-bearing inclusion. (c) CO2 spectrum in the vapor phase of a CO2-bearing inclusion. (d) CO2 and H2O spectrums of a CO2-bearing inclusion. (e) (SO4)2− spectrum in the liquid phase of an aqueous inclusion. (f) H2O spectrum in the liquid phase of an aqueous inclusion. Abbreviations: Fl = fluorite, Qtz = quartz, Bsn = bastnäsite.

Fig. 10. Plot of δDV-SMOW versus δ18Ofluid of the fluids in the Daluaco deposit. Metamorphic water field, magmatic water field, and meteoric water line are from Taylor (1974); SMOW = standard

42

mean ocean water.

Fig. 11. Pressure-temperature conditions for fluid inclusions in different stages. (a) Pressure estimation for fluid immiscibility in pre-REE stage based on intersections (shaded area) between isochores constructed with the minimum and maximum densities of immiscible HC-type inclusions, and minimum and maximum homogenization temperatures. The isochores are calculated using the Flincor program (Brown, 1989) and the formula of Bowers and Helgeson (1983). (b) P-T-X phase diagram of H-type inclusions in NaCl-H2O system (modified after Driesner and Heinrich, 2007). The two-phase surface separates the single-phase field at high pressure from the two-phase fluid stability field at lower pressure and is contoured for composition and temperature with the critical curve on its crest. The halite-liquid-vapor surface is contoured for temperature. The liquid-halite coexistence surface is omitted for clarity.

Fig. 12. Schematic diagram depicting the evolution of the hydrothermal system in the Dalucao deposit. In this model, the carbonatite–syenite magmas exsolved high-flux and REE-rich fluids. (A) Pre-REE stage: The fluids experienced intense immiscibility and, consequently, caused most CO2 escape. Accompanied with the immiscibility and the progressively cooling of ascending fluids, they reached the limits of solubility for CaF2, BaSO4 and CaCO3, triggering the precipitation of fluorite, barite and calcite in fractures. (B) REE stage: With fluids ascending to near surface, massive meteoric waters mixed with the cooler hydrothermal fluids and, consequently, the isotherms retreated downward and hydrostatic pressure dominated in this hydrothermal system. Fluid cooling and mixing severely diminished the stability of REE-bearing complexes and led to the precipitation of

43

bastnäsite (Williams-Jones et al., 2000, 2010; Trofanenko et al., 2016).

44

Fig. 1 45

Fig. 2

46

Fig. 3

47

Fig. 4

48

Fig. 5

49

Fig. 6

50

Fig. 7

51

Fig. 8

52

Fig. 9

53

Fig. 10

54

Fig. 11

55

Fig. 12

56

Table 1 Microthermometric data of fluid inclusions from the pre-REE stage and the REE stage in the Dalucao deposit. Salinity CO2-bearing inclusions Host Inclusion N Tm, s (°C) Th, tot (°C) Tm, ice (°C) (wt% NaCl mineral type Tm, CO2 (°C) Tm, cla (°C) Th, CO2 (°C) equiv.) Pre-REE stage HS 20 220 to 378 331 to 440 32.9 to 45.1 Fl HC 29 −61.4 to −60.4 −4.8 to 8.2 22.2 to 27.0 305 to 402 3.5 to 19.6 H 37 278 to 364 −8.5 to −4.7 7.4 to 12.3 HS 5 202 to 330 286 to 360 32.0 to 40.6 PC 5 −60.4 to −60.3 13.3 to 27.7 Qtz HC 8 −60.4 to −60.0 −5.9 to 8.4 12.1 to 29.6 300 to 442 3.2 to 23.4 H 7 302 to 404 −9.2 to −6.7 10.1 to 13.1 HS 5 216 to 290 305 to 363 30.1 to 32.9 PC 4 −60.7 to −60.2 22.2 to 24.0 Brt HC 5 −60.7 to −60.2 −9.6 to 7.8 24.1 to 27.1 308 to 390 20.9 to 21.4 H 6 310 to 360 −6.5 to −4.2 6.7 to 9.9 REE stage Fl H 51 162 to 323 −5.8 to −0.6 1.1 to 8.9 Bsn H 30 163 to 301 −4.3 to −0.8 1.4 to 6.9 Cal H 8 147 to 248 −6.2 to −3.5 5.7 to 9.5 Notes: N = number of fluid inclusions analyzed, Tm, CO2 = melting temperature of solid CO2, Tm, ice = melting temperature of final ice, Tm, cla = melting temperature of CO2 clathrate, Th, CO2 = partial homogenization temperature of CO2 phase, Tm, s = melting temperature of solid phase, Th, tot = temperature of total homogenization; Mineral abbreviations: Fl = fluorite, Qtz = quartz, Brt = barite, Cal = calcite, Bsn = bastnäsite; Fluid inclusion type: H = Aqueous inclusion, PC = pure CO2 inclusion, HC = CO2-bearing inclusion, HS = solid-bearing inclusion.

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Table 2 Proportions of gases (mol%) in the gas-chromatographic analyses of fluid inclusions in hydrothermal minerals from the Dalucao deposit. Sample number1 Mineral2 Host inclusions3 H2O

CO2

N2

CH4

Ar

C2H6

H2S

DLC13-1-15(3) Fl

H, HS, HC, CS

44.3

50.0

3.35

1.24

0.76

0.33

0.00

DLC11-28(2)

H, HS, HC

87.0

11.6

0.76

0.38

0.17

0.03

0.00

H, PC, HC

81.3

16.9

0.89

0.49

0.32

0.09

Fl

DLC13-1-15(2) Brt 1

0.01 2

Notes: These samples for gas-chromatographic analyses were collected in the same ore vein; Mineral abbreviations: Fl = fluorite, Brt = barite; 3 Fluid inclusion type: H = Aqueous inclusion, HS = solid-bearing inclusion, PC = pure CO2 inclusion, HC = CO2-bearing inclusion, CS = solid-bearing inclusion contain CO2.

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Table 3 Ion compositions (μg/g) in the ion-chromatographic analyses of fluid inclusions in hydrothermal minerals from the Dalucao deposit. Sample number1 Mineral2 Host inclusions3

(SO4)2− Cl−

F−

DLC13-1-15(3) Fl

H, HS, HC, CS

15.9

0.19

DLC11-28(2)

H, HS, HC

3.90

H, PC, HC

2.53

Fl

DLC13-1-15(2) Brt Notes:

Na+

K+

Ca2+

N.O.4 0.28

N.O.

N.O.

0.38

N.O.

0.60

0.21

N.O.

1.18

3.90

0.70

0.27

1.39

Charge imbalance between anions and cations in these data is because some cations, in particular

3+

REE , were not analyzed; 1 Samples as in Table 2; 2 Mineral abbreviations as in Table 2; 3 Fluid inclusion type as in Table 2; 4 The detection result was not obtained.

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Table 4 Hydrogen and oxygen isotope data of selected quartz in the Dalucao deposit. Sample number Mineral

Th (°C)

18 δ18OV-SMOW δ Ofluid (‰) (‰)

δDV-SMOW

DLC185-01

Quartz

300

8.3

1.4

−79.6

This study

DLC185-02

Quartz

300

8.5

1.6

−80.2

This study

DLC185-04

Quartz

300

8.2

1.3

−71.0

This study

DL13-1-6Qz

Quartz

215

5.8

−5.0

−92.8

Liu et al. (2015b)

DL11-28Qz

Quartz

215

3.0

−7.5

−88.4

Liu et al. (2015b)

DL11-33Qz

Quartz

215

5.5

−5.3

−105.1

Liu et al. (2015b)

(‰)

Reference

Notes: Th is the approximate average homogenization temperature of all inclusions hosted in selected quartz; δ18Ofluid values are calculated according to the quartz-water equilibrium temperature formula provided by Clayton et al. (1972).

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Graphical Abstract

Schematic diagram depicting the fluld immiscibility in pre-REE stage (a), as well as fluid mixing in REE stage (b) during hydrothermal evolution in the Dalucao deposit

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Highlights 4. The initial fluids were rich in REEs, (SO4)2−, Cl−, F−, Na+, K+, Ca2+ and volatiles. 5. Pre-REE-stage fluids experienced intense immiscibility. 6. Fluid mixing was the major event in REE stage. 7. Fluid cooling and mixing were the key factors resulting in REE precipitation

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