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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 Chinab Institute of Geology, Chinese Academy of Geological Science, Beijing 100037, PR Chinac Snowden Mining Industry Consultants Ltd., Weybridge, Surrey, KT13 0TT, UKd School of Science & Engineering, University of Ballarat, Ballarat, Victoria 3353, Australiae Institute of Mineral Resources, Chinese Academy of Geological Science, Beijing 100037, PR China

⁎ Corresponding author.E-mail address: [email protected] (Y. Xie).

0169-1368/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.oregeorev.2008.10.006

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The Maoniuping REE deposi

Received 11 January 2008Received in revised form 28 October 2008Accepted 28 October 2008Available online 13 November 2008

Keywords:Maoniuping REE depositCarbonatiteMelt–fluid inclusionsMulticomponent fluidREE mineralization process

t is a world-class deposit with 1.2 Mt of REO grading on average 2.89 wt.% REO. Itis the largest in the 270-km long Mianning–Dechang REE belt and is associated with Himalayan carbonatite–alkalic complexes in the eastern Indo-Asian collisional zone, Western Sichuan Province, China. The deposit ishosted by nordmarkite stocks and carbonatite sills that display a radiometric age of 40 to 24 Ma and whichintruded a Yanshanian granite pluton. The 40.3 to 27.8 Ma REE mineralization occurs as vein systems hostedin nordmarkite and carbonatite with minor altered granite and rhyolite. Four ore types are recognized basedon ore texture and mineral assemblage: (1) disseminated; (2) pegmatitic; (3) brecciated; and 4) stockwork(stringer) types. Five mineralizing stages are confirmed according to vein crosscutting relationships, mineralassemblage and microthermometric results, these are: 1) carbonatite stage, 2) pegmatite stage, 3) barite–bastnaesite stage, 4) later calcite stage and 5) epigenetic oxidation stage. Varied inclusion assemblages arefound in fluorite, quartz, bastnaesite, barite and calcite from stages 1 through to stage 4. The dominantinclusion types include: melt, melt–fluid, CO2-rich fluid and aqueous-rich fluid inclusions. Fluid, melt–fluidand melt inclusion studies indicate that the ore-forming fluid resulted from the unmixing of carbonatite meltand 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 multi-component composition (e.g. K, Na, Ca, Ba, Sr and REE). The dominant anion is not Cl, but SO4. The evolutionof the ore-forming fluid is from a melt–fluid at high temperature, through a CO2-rich fluid at high to mediumtemperature to aqueous-rich fluid at low temperature. REE precipitation occurred from a high to mediumtemperature CO2-rich fluid. The main mechanism for REE precipitation was phase separation of CO2 andaqueous fluids resulting in a decrease of temperature and pressure.

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1. Introduction

Deposits of rare earth elements (REE) usually occur in a continentalrift environment, such as observed in the East African Rift (Mitchell andGarson,1981) or the Proterozoic Langshan–Bayan Obo Rift, North China(Wang and Li, 1992). They generally have a close genetic relation-ship with mantle-derived carbonatites in rift environments. Such REEdeposits have been studied in detail and their genesis iswell understood(e.g., Wang and Li, 1992; Buhn and Rankin, 1999; Buhn et al., 2002;Williams-Jones and Palmer, 2002). Recent studies have demonstratedthat REE deposits may also occur in continental collisional zones (cf.Hou et al., 2006a). The Maoniuping REE deposit, a giant deposit in the270 km-long Himalayan Mianning–Dechang (MD) REE belt in westernSichuan, China, is regarded to be the largest and most typical in a

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collisional zone (Hou et al., 2009-this issue). It is closely associatedwitha Cenozoic carbonatite–alkalic complex (Shi, 1993; Yuan et al., 1995),controlled by a Cenozoic strike–slip faulting system, and occurs in theEastern India-Asian Collision Zone of the eastern margin of the TibetanPlateau (Hou et al., 2006a; Hou and Cook, 2009-this issue). The depositis characterized by a large ore tonnage (1.2 Mt REO), young mineraliza-tion age (27.8 to 40.3 Ma; Shi, 1993) and a well-developed mineralizedvein system. The system provides a unique opportunity to establish thegenetic link between REE metallogeny and collisional orogeny, and tofurther understand the evolution of a REE-bearing ore-forming fluidsystem in a continental collisional zone.

Many researchers have carried out investigations into the geology,geochemistry and genesis of the Maoniuping REE deposit (Niu andLin, 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;Liu et al., 2004). There is still debate on its genesis, especially on thecharacteristics of ore-forming fluids, the evolution of ore-forming

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91Y. Xie et al. / Ore Geology Reviews 36 (2009) 90–105

fluids and REE precipitation mechanisms. Previous studies and newdata presented in this contribution indicate that the ore-formingsystem at Maoniuping is complex, as indicated by numerous types ofinclusions trapped in both REE minerals (bastnaesite) and gangueminerals (e.g., calcite, fluorite, quartz, and barite) (Niu et al., 1996a;Yang et al., 2001; Xie et al., 2005, 2007). However, these inclusions,varying from melt- and melt–fluid inclusions to fluid inclusions,probably record a continuous carbonatitic melt–fluid evolutionwithinthe REE-mineralization system (Xie et al., 2007). Systematic studieson inclusions enable us to characterize primary ore-forming fluids,establish their affiliation to the collisional magma systems, and assistunderstanding REE mineralization processes.

This paper reports new observations and data on variousinclusions from Maoniuping and provides an alternative interpreta-tion of previous inclusion data presented by the present authors andothers. Based on studies of melt/fluid inclusions integrated with

Fig. 1. Sketch geological map of the Maoniuping REE deposit, with locations of samples

geological and geochemical data, we discuss the origin and evolutionof ore-forming fluids and REE ore-forming processes. A model forMaoniuping mineralization is accordingly proposed.

2. Geology

The Himalayan Mianning–Dechang (MD) REE belt is located in thewestern margin of the Yangtze craton, consisting of an Archaean–Proterozoic metamorphic crystalline basement and an overlyingsequence of Phanerozoic clastic and carbonate rocks (Cong, 1988;Luo et al., 1998). The western margin of the craton underwentPermian rifting to form a N–S-trending paleorift zone and wasinvolved in the eastern Indo-Asian collisional zone which formedsince the Paleocene (Yin and Harrison, 2000; Zhong et al., 2001; Moet al., 2003). Accompanying Cenozoic collisional orogeny, a series ofCenozoic strike–slip faults and a set of NE-, NNE- and NNW-trending

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 (Luoet al., 1998; Hou et al., 2006a; 2009-this issue). More than fivecarbonatite–alkalic complexes occur in the MD belt, all controlled byCenozoic strike–slip faults. Available data define a relatively shortduration for magmatic activity (24 to 40 Ma, peaking at 34 Ma; Zhangand Xie, 1997; Wang et al., 2001; Chung et al., 1998; Hou et al., 2003;Guo et al., 2005).

The geology of the Maoniuping district is described in detail byHou et al. (2009-this issue). The Maoniuping complex and associatedREE mineralization is controlled by a series of NE–NNE trending faults(Fig. 1), and intruded a NS-striking Yanshanian granite pluton with a

Fig. 2. Photographs of field outcrops and polished samples from the Maoniuping REE depos(b) nordmarkite breccia in carbonatite; (c) quartz (Q)–barite (Bar)–fluorite (Fl) vein in nor(e) Barite (Bar)–bastnaesite (Bast)–acmite–augite (Acm) vein crosscutting fine-grained quainterval of euhedral crystal quartz(Q).

U–Pb age of 146 Ma (Yuan et al., 1995). The complex comprises anordmarkite stock with minor carbonatite sills (Fig. 2a; Hou et al.,2009-this issue). The carbonatite bodies are relatively small, butextend continuously downwards. Sometimes nordmarkite brecciaclasts can be observed in the carbonatite body (Fig. 2b). The REEorebodies are hosted principally in the nordmarkite stock, carbonatitesills, and to a lesser extent in the undated rhyolite and alteredYanshanian granite; they are overlain by a Triassic coal-bearingsequence.

The carbonatite rocks are generally pink or fawn in colour, due to thepresence of purple fluorite and yellow–brown bastnaesite. They have

it. (a) Fluorite, quartz, aegirine–augite bearing carbonatite (Carb) vein in nordmarkite;dmarkite (Nord); (d) quartz (Q)–barite (Bar)–fluorite (Fl) vein in nordmarkite (Nord);rtz (Q) vein; (f) barite (Bar)–bastnaesite (Bast)–acmite–augite (Acm) occurring in the


low SiO2 (b10.22 wt.%), FeO (b1.20 wt.%) and MgO (b0.73 wt.%), and awide range of CaO content (40.7wt.% to 55.4wt.%), distinguishing themfrom magnesiocarbonatites (Yuan et al., 1995). They are extremelyenriched in LILE (Sr, Ba) and light REE, but relatively depleted in high-field strength elements (Nb, Ta, P, Zr, Hf and Ti), suggesting ametasomatized mantle source (Hou et al., 2006b). Their δ18OV-SMOW(6.4 to 10.5‰) and δ13CV-PDB (−3.9 to−8.5‰) values (Tian et al., 2003;Xu et al., 2003a, Niu et al., 1996b, 2003) are also similar to those ofprimary mantle-derived carbonatites. Associated alkalic rocks aremainly nordmarkites with high SiO2 (63.41 to 74.75 wt.%), alumina(N13.3%) and alkali (K2O+Na2ON9.1%) contents, and a wide range ofK2O (3.63 to 5.94%) content and K2O/Na2O ratios (0.41 to 1.45),distinguishing them from pre-Cenozoic granitoid plutons with K2O/Na2O ratios of 1.25 to 1.83 (Yuan et al., 1995). Overlapping emplacementages, Sr–Nd isotopic compositions, and similarmantle-normalized traceelement patterns to spatially-associated carbonatite, suggest a liquidimmiscible origin for carbonatite and nordmarkite (Hou et al., 2006b).

3. Mineralization

The Maoniuping REE deposit consists of various mineralized veinsand stringer/stockwork zones. The deposit consists of approximately71 orebodies (Pu, 1993), occurring as irregularly shaped veins, lensesand pockets. Individual orebodies range from 10 m to 1,168 m inlength, and between 1 m to 30 m in thickness (Yuan et al., 1995). Inplan view, they are distributed in an approximately en-echelon pattern(Fig. 1). The gross strike direction for the en-echelon system is N–S,where individual orebodies strike mainly NNE and dip NW at 65–80°(Fig. 1). The vein system extends NNE, along strike, for 2,650 m andshows a “S” shape in plan, indicating a strike–slip fault control. Theore-bearing veinlets are usually greater than 30 cm thick, and occur asa swarm in the centre of the complex. The mineralized stringers rangein thickness between 1 and 30 cm, and commonly comprise parallelvein zones enveloping the central ore zone. The stockwork zoneusually occurs at the margins of the veinlets and vein zones.

Four main primary ore types have been recognized: 1) dissemi-nated; 2) pegmatitic; 3) breccia; and 4) stringer (stockwork) ore.Their spatial distribution and major features are summarized inTable 1. Disseminated ore is common in the fenitized wall rock,whereas pegmatitic ore occurs as thick ore veinlets or pockets in themiddle of Maoniuping segment. Breccia ore mainly occurs in theBaozicun (southern) segment of the deposit, and stockwork oreoccurs as a NNE-trending halo around the pegmatitic ore. Thestockwork ore occurs in the Sanchahe segment, the northernMaoniuping segment, the southern Maoniuping segment, and in thestockwork zone of the middle Maoniuping segment between thepegmatite ore and altered wall rocks.

More than 80 minerals have been identified at Maoniuping(Pu, 1993; Yuan et al., 1995; Xie et al., 2006). Bastnaesite is thedominant REE mineral in all four ore types. Bunsite, chevkinite andcerianite have also been identified. The dominant gangueminerals are

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

Ore type Ore texture Ore structure

Disseminated ore REE–minerals occur as euhedral–hypautomorphic crystals in carbonatite,rhyolite and nordmarkite

Disseminations, stringers

Pegmatitic ore Pegmatite, euhedral–hypautomorphiccrystals, metasomatic, poikilitic texture

Massive, miarolitic(cluster), dissemination

Breccia ore Cataclastic, euhedral–hypautomorphiccrystals, interstitial, poikilitic

Breccia; Dissemination

Stockwork ore euhedral–hypautomorphic–allotriomorphiccrystals, metasomatic, poikilitic, interstitial,poikilitic

Stringer and stockwork

calcite, fluorite, barite, aegirine–augite, arfvedsonite, microcline andbiotite, with minor galena, pyrite, pyrrhotite, chalcopyrite, hematite,magnetite, rutile and apatite (Hou et al., 2009-this issue).

Based on the mineral assemblages, ore textures and structures,vein cross-cutting relationships, petrography and inclusion data, fivemineralization stages are recognized. These comprise:

1) Carbonatite stage: This stage occurs as pink–white carbonatite dykeintruding nordmarkite (Fig. 2a) in themiddle segment or as fine pinkto light purple fluorite-bearing carbonatite cement of breccia in thesouthern segment of the deposit. The carbonatite dyke andcarbonatite cement of the breccia comprise a calcite–fluorite–acmite–augite–arfvedsonite–microcline–pyrite–chalcopyrite–pyr-rhotite mineral assemblage. In this stage, minor disseminated REEmineralization occurs in carbonatite.

2) Pegmatite stage: Coarse-pegmatite aegirine–augite–fluorite–barite–bastnaesite, fluorite–barite–calcite–bastnaesite, aegirine–augite–quartz–microcline, and narrow fluorite–quartz–bastnaesite veinsformed during this stage. These veins occur in the carbonatite bodyor crosscutting nordmarkite (Fig. 2c, d). The mineral assemblagevaries significantly between the vein types, but no crosscuttingrelationships have been observed. The mineral assemblage in thisstage consists of calcite, fluorite, microcline, phlogopite, biotite,quartz, aegirine–augite, magnesio–arfvedonite, bastnaesite, chevki-nite, pyrite, chalcopyrite and pyrrhotite.

3) Barite–bastnaesite stage: This stage occurs as a mineral assem-blage of barite–bastnaesite in narrow fluorite–quartz–bastnaesiteveins or crystal barite and barite–bastnaesite in druses. Thebastnaesite–barite veins crosscut the quartz veins (Fig. 2e).Under the microscope, small-scale barite–bastnaesite veins canbe found crosscutting quartz and fluorite crystals. The mineralassemblage of this stage is barite, bastnaesite, calcite, celestine,magnetite, hematite, rutile, apatite, galena and unusual Cu-bearingalloys. Two such phases were determined by EPMA and SEM/EDSto be Sn-bearing copper (Cu) and zinccopperite (Cu2Zn) (Xie et al.,2006). This stage, together with stage 2, formed themain REE bodyand is the most important REE mineralization stage.

4) Late calcite stage: This stage occurs as white–gray calcite veins,with a mineral assemblage of calcite and strontianite, fillingfractures in both wall rock and REE orebodies. The calcite veinscrosscut all previous vein generations; no REE mineralization hasbeen observed in them.

5) Epigenic oxidation stage: This stage is dominatedbyREE-bearing Fe–Mn mineralization, which formed Fe–Mn oxides and an associatedcerussite+witherite+wulfenite assemblage (Yuan et al., 1995).

In general, stage 2 is characterized by the appearance of pegmatiticmineralization, whereas stage 3 is characterized by the appearance ofporous barite and bastnaesite. Mineralization of stage 2 and stage 3forms the majority of the economic REE deposit. No crosscuttingrelationships between the pegmatite and narrow REE veins wereobserved in the field. Observation of ore textures and micro-vein

Mineral assemblage Occurrence

Calcite, fluorite, acmite augite, quartz,biotite, Muscovite, microcline, acmite augite

Altered wall rock (granite andrhyolite) and carbonatite

Calcite, fluorite, quartz, barite, microcline,acmite augite, biotite, arfvedsonite, galena,pyrite, chalcopyrite

Only in the middle of Maoniupingsegment

Calcite, fluorite, quartz, barite, microcline,mica, biotite, acmite augite, magnetite, hematite

Baozicun segment

Fluorite, quartz, bastnaesite, barite, celestite,calcite, biotite, acmite augite, alloys, magnetite,hematite, rutile

Maily in north and south ofMaoniuping segment, Sanchahesegment


crosscutting relationships shows that barite and bastnaesite precipi-tated later than most quartz and fluorite, and often filled in theinterspaces of euhedral quartz and fluorite (Fig. 2f).

4. Sampling and analytical methods

The minerals sampled for the fluid inclusion study include calcite,fluorite, quartz, barite and bastnaesite from mineralization stages 1to 4. Samples covered all four ore types summarized in Table 1. Thesample locations are shown in Fig. 1. Microthermometry (for 23samples), LRM (Laser Raman Microprobe) (for 43 inclusions in 6sections) and SEM/EDS (for 5 samples) analysis were performedafter careful microscopic observation of 35 sections and 55 doubly-polished sections.

Microthermometry was undertaken using a Linkam THS600heating-freezing stage, with a measurable temperature range ofbetween −196 and +600 °C (precision of freezing data andhomogenization temperature of ±0.1 °C and ±1 °C, respectively)and a Linkam THS1500 heating stage with the measurable tempera-ture range from room temperature to +1500 °C (precision ofhomogenization temperature of ±1 °C). Microthermometry wasundertaken in the Department of Resources Engineering of USTB(University of Science and Technology Beijing) and the Geology andGeophysics Institute of CAS (China Academy of Science). SEM/EDS(Scanning Electron Microscope/Energy Dispersive Spectrometry)analysis were performed at the State Key Laboratory for AdvancedMetals and Materials at USTB, using a Cambridge S250 SEM equippedwith a Link Systems 860 energy dispersive spectrometer operating at a20 kV accelerating voltage. The detailed sample preparation methodfor SEM/EDS used is reported in Xie et al. (2000). The mineralfragments were first crushed into small pieces (usually b5 mm indiameter) to open the inclusions; a number were selected and thenstuck onto a piece of glass with their flat side up. The samples werekept dry and clean, and quickly carbon coated. LRM was carried out atthe Mineral Resource Institute, CAGS (Chinese Academy of GeologicalScience) using a Renishaw 2000 Laser Raman Spectroscope with ablazing power of 20 mw and blazing wave-length of 514.5 nm.

5. Inclusion results

5.1. Petrography and classification of inclusions

The samples used for the inclusion study were doubly polishedsections of fluorite, quartz, calcite, barite and bastnaesite frommineralization stages 1 to 4. A number of inclusion types wereidentified. These include negative crystals and elongate round,polygonal or irregular shapes with a size range from b1 µm to severaltens of µm. Based on their petrographic characteristics at room

Fig. 3. Photomicrographs of melt inclusions from the Maoniuping REE deposit. (a) Melt inclmelt inclusion in fluorite; D—daughter mineral; V — vapor phase; M — melt glass; LH2O — a

temperature and phase change characteristics during the heatingprocess, inclusions were grouped into three principal types and eightsub-types. They are as follows:

(1) Melt inclusions (M type). These often occur in calcite fromcarbonatite and gangue fluorite from pegmatite and veinlet(stringer) ore. There are also minor melt inclusions in quartzand arfvedsonite from pegmatitic and veinlet (stringer) ore.The inclusions generally comprise melt–glass and daughterminerals (Fig. 3b); with some containing a minor volatile phase(Fig. 3a). Most of the melt inclusions in calcite and fluoriteappear to be trapped minerals at room temperature under themicroscope, but have a relatively dark, thick borderline andoften contain a daughter mineral or/and volatile phase. Onheating, the phase change of melt inclusions differs from thetrapped minerals. The melt inclusions melt inhomogeneouslyat more than 500 °C, in contrast to the trapped minerals thatmelt homogeneously at their respective melting points. Thesemelt inclusions were trapped during the late magmaticcarbonatite stage and survived a later period of hydrothermalalteration. Thus they provide an important method for thestudy of carbonatites (Aldous, 1980; Andersen, 1986) andcarbonatitic fluids.

(2) Melt–fluid inclusions (ML type). ML type inclusions areabundant in gangue fluorite and relatively rare in quartz.Almost all of them have negative crystal or polygonal shapes.The inclusions show variable size and liquid/vapor/solidsratios. Their petrographic features are similar to the solid-richL–V–S type of inclusions reported in fluorite–REE deposits inthe Gallinas Mountains, New Mexico (Williams-Jones et al.,2000). Most ML inclusions contain several solid phases andhave been termed multi-solid inclusions by Niu and Lin(1995b). Based on microthermometric work, the melt phaseappears at some point during the heating process and this kindof inclusions is thus termed “melt–fluid type” in this paper.Based on microscopic measurement, it has been determinedthat the solid phases occupy about 40 to 80 vol.% of theinclusions (Fig. 4). Raman spectroscopy and SEM/EDS results(see below) indicate that the solid phases were dominated bysulfateminerals. ML inclusions with different phase ratios oftenoccur together in clusters, and are sometimes accompanied bymelt inclusions or CO2-rich fluid inclusions. Their close spatialassociation with melt inclusions and CO2-rich fluid inclusionssuggests that these inclusions were trapped in the transitionstage from melt to fluid. Meanwhile, the melt phase coexistedtogether with an aqueous phase. The continuous series fromlow- to high-solid volume percentage melt–fluid inclusionrecords an immiscible process.

usion bearing aqueous and daughter mineral in fluorite; (b). daughter mineral bearingqueous phase; Fl — fluorite (host mineral).

Table 2Characteristics of different sub-types of fluid inclusions.

Sub-typeof fluidinclusions

Phase composition Distribution and occurrence

AC Aqueous and liquidCO2 or aqueous, liquidand vapor CO2

Very abundant inclusion type in bastnaesite,barite and quartz occurring as isolated inclusionsor clusters. Some wasfound in fluorite or calcite.

ADC Aqueous, liquid CO2,one or more daughterminerals

Same as AC sub-type

C Liquid CO2 or liquidand vapor CO2

Same as AC sub-type

AV Aqueous and vapor Occurring as isolated inclusions or clusters incalcite from stage 4. Most AV fluid inclusions arefound as lines or trails in quartz and fluorite.

ADV Aqueous, vapor, oneor two daughterminerals

Same as AV sub-type

Fig. 4. Photomicrographs ofmelt–fluid inclusions from theMaoniupingREE deposit. (a)Melt–fluid inclusionwith net-like sulfate and daughterminerals in it (ML-1) andADC sub-typefluid 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 inclusionwith 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 inclusionsat room temperature, two sub-types can be distinguished asfollows:a. ML-1 sub-type: Containingmulti-solid phases, net-like sulfate

(mirabilite), aqueous andabubble (Fig. 4a, b). The solidphasesoccupy ca. 75 vol.% of the inclusion volume. At roomtemperature, they appear dark brown in color because thenet-like sulfate occupiesmost of the inclusion volume. All ML-1 inclusions include several solid phases. The solid phasesshow rod-like, globular, and rounded-polyhedron shapes.

b. ML-2 sub-type: Similar to ML-1, but without net-like sulfatein the inclusions (Fig. 4c, d). At room temperature, they lookclearer than ML-1 inclusions. They often contain severalsolid phases with rod-like, globular or rounded-polyhedronshapes. The solid phase accounts for 40 to 80 vol.% of theinclusions. Most solids were confirmed by LRM and EDS assulfate (see below). Some could not be identified, thoughLRM results identify them to be a carbonate and sulfatecomplex (melt glass).

(3) Fluid inclusions (L type). Fluid inclusions are the mostabundant type of inclusions in quartz, barite and REE minerals,and include primary and secondary fluid inclusions. In fluoritefrom stage 2, minor primary fluid inclusions are usuallyaccompanied by melt–fluid inclusions. Most secondary fluidinclusions are distributed as clusters or trails along the healedfissures. In quartz from stage 2, L-type fluid inclusions are thedominant type; some occur as isolated inclusions or clusterswith negative crystal shapes, showing the features of pseudo-secondary inclusions. Others are distributed as trails or lines,showing secondary features. All primary and pseudosecondaryfluid inclusions contain a liquid CO2 bubble (see below),whereas secondary fluid inclusions contain a vapor phasebubble. Primary CO2-rich fluid inclusions are sometimes

accompanied by melt–fluid inclusions in a single cluster. Fivesub-types (Table 2) were distinguished on the basis of phasecharacter at room temperature and characteristics duringheating.(a) AC sub-type: Aqueous-liquid CO2 fluid inclusions (Fig. 5a,

b, d). These occur in calcite, quartz, barite, bastnaesite andfluorite. They comprise aqueous and liquid CO2, some withvapor CO2 at room temperature. During heating, most ofthem homogenized to a CO2–H2O critical phase (AC-2); afew homogenizing to an aqueous phase (AC-1). The CO2bubble represents N20 vol.% of the inclusion volume.

(b) AV sub-type: Aqueous-vapor fluid inclusions (Fig. 6b). Thisfluid inclusion sub-type generally occurs in fluorite, quartz

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—daughtermineral; LH2O—aqueous phase; LCO2—liquid CO2; VCO2—vapor CO2.


and calcite as secondary inclusions observed as trails alonghealed fractures or in calcite from stage 4. This sub-typecomprises aqueous and vapor phase. The vapor bubbleaccounts for a lower percentage (b20 vol.%, mostly 5–10 vol.%) than AC and ADC sub-types. When heated, theyhomogenize to an aqueous phase at low temperature,mostly below 200 °C.

(c) ADC sub-type: Aqueous daughter minerals–liquid CO2 fluidinclusions (Fig. 5b). Each comprises one or more daughterminerals, aqueous and liquid CO2, usually accompanied by ACand C sub-type inclusions. Inclusions of this kind often occurin barite, bastnaesite and quartz as primaryfluid inclusions, orin fluorite and igneous calcite as secondary fluid inclusions.

(d) ADV sub-type: Aqueous-daughter minerals–vapor fluidinclusions (Fig. 6a). Inclusions of this kind comprise oneor two daughter minerals, aqueous and vapor phase. The

Fig. 6. Photomicrographs of aqueous rich fluid inclusions in the Maoniuping REE depos(b) Aqueous-vapor fluid inclusion (AV) in quartz; D—daughter mineral; LH2O—aqueous pha

solid phase accounts for a much lower vol.% (b15%) thanML inclusions. They are usually accompanied by AV fluidinclusions. These inclusions often occur as secondary fluidinclusions in quartz, fluorite, bastnaesite and barite fromstages 2 and 3.

(e) C sub-type: Pure CO2 fluid inclusion (Fig. 5c). They comprisea liquid CO2 or liquid and vapor CO2. Pure CO2fluid inclusionsoften occur in quartz, bastnaesite and barite, and arecommonly accompanied by AC and ADC fluid inclusions.

5.2. Inclusion assemblages

Inclusion assemblages vary with host minerals and ore-formingstages in the Maoniuping deposit. From stages 1 to 4, the dominantinclusions vary from melt inclusions (M type), melt–fluid inclusions(ML type), high-medium temperature CO2-rich fluid inclusions (AC+

it; (a). Daughter minerals-bearing aqueous vapor fluid inclusions (ADV) in fluorite;se; V—vapor phase.; Q — quartz; Fl — fluorite.

Table 3Microthermometric results of melt and melt–fluid inclusions in fluorite and quartz inthe Maoniuping REE deposit.

Sample Ore type Mineral Type ofinclusion

Th (°C)

Range Average

MNP-13 Pegmatite Quartz M 790–791(3) 791MNP-16 Stringer Fluorite M N737(1)a /MNP-125 Pegmatite Fluorite M N778(1)a /MNP-17 Stringer Fluorite M 795–850(2) 823MNP-11 Pegmatite Fluorite ML-1 650–850 (3) 705MNP-12 Pegmatite Quartz ML-1 600–750(4) 658MNP-13 Pegmatite Fluorite ML-2 687(1) 687

a Unsuccessful acquisition of fluid inclusion data. At this temperature, the inclusioncomprises a melt phase and a bubble, but the inclusions cannot homogenize because ofleakage.


ADC+C sub-type) to low-temperature aqueous rich fluid inclusions(AV+ADV sub-type). Four inclusion assemblages were identified inminerals from the different ore-forming stages. They are as follows:

Melt inclusion assemblage: Occurring in calcite and fluorite fromthe carbonatite stage (stage 1), it comprises melt inclusionsaccompanied by trapped minerals. Because of the birefringenceeffect, the melt inclusions in calcite are not clear under themicroscope. Most melt inclusions have no volatile component atroom temperature, but some have an irregular bubble or/and adaughtermineral. They are observed as isolated inclusions or alongthe growth zone in calcite and fluorite from carbonatite dykes.Melt–fluid inclusion assemblage: This comprises melt inclusions(M type), melt–fluid inclusions (ML type) and minor high-temperature daughter mineral-bearing fluid inclusions (ADCsub-type). They mainly occur in fluorite from stage 2. The melt–fluid inclusion is the dominant type in fluorite from stage 2, and isoften accompanied by melt inclusions and daughter mineral-bearing CO2-rich fluid inclusions. The melt–fluid inclusions havevaried solid volume percentages. ML-1 inclusions have a relativestable solid volume percentage and are isolated and with negative

Fig. 7. Phase-change in melt–fluid inclusion in fluorite; (a) melt–fluid inclusion at room temdisappears; (e) melt phase (sulfate melt) appears; (f) new solid phases occur.

crystal shapes. The ML-2 inclusions have a varied solid volumepercentage. Under the microscope, this assemblage forms acontinuous sold/aqueous/vapor series.CO2-rich fluid inclusion assemblage: Includes high-medium tem-perature aqueous-CO2 fluid inclusions (AC sub-type), aqueous-daughter mineral-bearing fluid inclusions (ADC sub-type) andpure CO2 inclusions (C sub-type). They are abundant in barite,bastnaesite and hydrothermal calcite from stage 3 or as secondaryinclusions in fluorite and quartz from stage 2. Most AC fluidinclusions (AC-2) have a critical homogenization behavior; some(AC-1) homogenize to a liquid phase.Aqueous-rich fluid inclusions assemblage: Includes aqueous-vaporfluid inclusions (AV sub-type) and aqueous-daughter mineral-bearing fluid inclusions (ADV sub-type). They occur in calcite fromstage 4 or as secondary inclusions in fluorite, quartz, barite andbastnaesite from stages 2 and 3. Compared to CO2-rich fluidinclusions, the aqueous-rich fluid inclusion assemblage has asmaller bubble, lower CO2 content and lower homogenizationtemperature.

5.3. Microthermometric results

5.3.1. Melt inclusionsMicrothermometric results are shown in Table 3. At room

temperature, the aqueous phase was absent in most of the meltinclusions, but on heating at 500 to 700 °C, the glass phase began tomelt inhomogeneously. Firstly there are a few small dark spots or darkcracks appearing in the inclusions and with the increasing tempera-ture, the small bubbles agglomerate into one bubble. Continuingheating, the bubble gets smaller and smaller, and finally homogenizesto a melt phase at 790 to 850 °C. Some of these inclusions decrepitatedor leaked prior to homogenization. Somewould not homogenize evenup to 1100 °C (the host mineral–fluorite–melts at this temperature),which is considered to be due to volume increase and inclusionexpansion. Cooling from homogenization to room temperature,

perature; (b) the bubble phase disappears; (c) New bubble appears; (d) net-like sulfate


inclusions of this kind cannot return to their original state, but becameaqueous-bearing multi-solid inclusions (with multi-solid phase,little aqueous phase and a bubble) with a very high (N80%) solidpercentage.

5.3.2. Melt–fluid inclusionsMicrothermometric results are listed in Table 3. Most ML

inclusions were unable to homogenize because they decrepitatebefore reaching the homogenization temperature. Some did nothomogenize below 1000 °C because of obvious volume-increase andexpansion of the inclusion; only a few homogenization temperaturedata were thus obtained. The homogenizing process of the MLinclusion is complex during heating (Fig. 7). The dominant phase-changes for theML-1 inclusion are reported as: (1) at 115 to 180 °C thebubble disappeared. This temperature range is similar to thehomogenization temperature of AV fluid inclusions; (2) at 240 to250 °C the net-like sulfate dissolved and the aqueous phase occupied agreater volume percentage (potentially caused by dehydration ofmirabilite). The inclusions become clearer after melting of the net-likesulfate; (3) at 350–500 °C, a melt phase appears from a solid phase-melting, and then gets larger with more solid melting; (4) at about450 to 500 °C some new solid minerals grew along the inclusion wall(the authors believe this may be caused by deformation and volumeincrease of the inclusion); (5) at 600 to 700 °Cmuch of the solid phasedissolved or melted; and (6) at 600 to 850 °C the inclusionhomogenized by critical behavior. The homogenization temperaturesof ML inclusions were between 650 and 850 °C in fluorite, and 600 and750 °C in quartz. Most ML inclusions decrepitated at a lowertemperature than their homogenization temperature.

5.3.3. Fluid inclusionsBased on their phase composition at room temperature and

homogenizing characteristics during heating, five inclusion sub-types

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

Sample Ore type Mineral Type ofinclusion

MNP-6 Carbonatite Q ACMNP-6 Carbonatite Q AVMNP-12 Pegmatite Q AVMNP-12 Pegmatite Q ACMNP-12 Pegmatite Q ADCMNP-12 Pegmatite Q CMNP-12 Pegmatite Q ACMNP-13 Pegmatite Q ACMNP-13 Pegmatite Q CMNP-13 Pegmatite Q ADCMNP-33 Pegmatite Q ACMNP-33 Pegmatite Q AVMNP-33 Pegmatite Q ACMNP-33 Pegmatite Q CMNP-33 Pegmatite Q ADCMNP-132 Pegmatite FL ADCMNP-132 Pegmatite FL AVMNP-161 Stringer ore FL ADCMNP-161 Stringer ore Q ADCMNP-161 Stringer ore FL AVMNP-161 Stringer ore Q AVMNP-153 Stringer ore Q AVMNP-153 Stringer ore Q ACMNP-157 Stringer ore Cal ACMNP-157 Stringer ore Cal AVMNP-157 Stringer ore Cal CMNP-157 Stringer ore Bar ACMNP-28 Stringer ore Bar AVMNP-28 Stringer ore Cal AVMNP-28 Stringer ore FL AVMNP-28 Stringer ore FL ACMNP-313b Stringer ore Bast AC

Note: “/” means that no useful data or could not be measured; Fl—fluorite; Q—quartz; Bast—

were distinguished (AC, ADC, AV, ADV and C sub-type) in quartz,fluorite, calcite, barite and bastnaesite. Microthermometric results forthese different sub-types are shown in Table 4 and Fig. 8.

During heating, the AC-1 sub-type homogenized to an aqueousphase, and the AC-2 sub-type homogenized to the critical phase. All Csub-type homogenized to a liquid CO2 phase. Heating tests for AC andADC sub-type inclusions were difficult, due to decrepitation; themajority would not homogenize. Microthermometry showed that theAC-1 sub-type had a Th (homogenization temperature) of between194 and 371 °C, AC-2 has a Th of 227 to 453 °C, and a CO2 densitybetween 0.636 and 0.982 g/cm3 according to the partial homogeniza-tion temperature of liquid CO2 and vapor CO2 (Angus et al., 1976; cf.Roedder, 1984 and Lu et al., 1994). For ADC sub-type inclusions, whileheating, the daughter minerals melted first at 223–386 °C, then theinclusion homogenized to a liquid phase at 224 to 477 °C. The densityof the CO2 in fluid inclusionswas estimated as being 0.644 g/cm3 (onlyone data point was acquired). AV sub-type fluid inclusions have thelowest Th of 101 to 226 °C and a simple homogenizing behavior. Thebubbles in most of these inclusions disappear below 200 °C. Coolingmeasurements on pure CO2 inclusions show that the melting point ofCO2 is between −56.5 and −58.5 °C. This is equal to or a little lowerthan that of the pure CO2 system (Angus et al., 1976; cf. Roedder, 1984and Lu et al., 1994) and suggests a dominant CO2 system, as well as thepossibility of minor other volatile components such as CH4, H2 or N2present. The C sub-type inclusions have homogenization tempera-tures of −7.7–14.7 °C. According to the pure CO2 system phasediagram, the density of CO2 was estimated as 0.829–0.972 g/cm3.Results of SEM/EDS and LRM studies (see below) indicate that mostdaughter minerals in both ADC and ADV sub-type inclusions aresulfates including barite, mirabilite, gypsum, aphthitalite and arcanite.SO42− in aqueous phasewas also detected in all fluid inclusions by LRM(see below). Based on LRM and SEM/EDS results, most daughterminerals in ADC and ADV inclusions are sulfate, and SO42− exists

Th (°C) Density of CO2

Range Average Range Average

238 (1) 238 0.647–0.868(2) 0.758133–151(5) 142 / /124–219(12) 172 / /247–371 (7) 296 / /237–477(13) 362 / // / 0.923–0.958(4) 0.94/ / 0.636(1) 0.636/ / 0.692–0.807(3) 0.759/ / 0.944–0.964(4) 0.955328–459(3) 411 / // 0.982(1) 0.982166–185(3) 177 / /217–357(5) 278 / // / 0.938–0.972(6) 0.957/ / 0.644(1) 0.644249–285 (5) 272 / /181(1) 181 / /240–386(7) 304 / /224–296 (2) 260 / /101–153(16) 122 / /113–226(12) 152 / /108–159(8) 129 / /254–324 2) 289 / /218–242 (3) 230 / /158–159 (2) 159 / // / 0.829–0.839(2) 0.834194–253(4) 219 / /186–199(4) 193 / /141–167(9) 158 / /137–143(2) 140 / /254–279(2) 266 / /211–255(6) 244 / /

bastnaesite; Cal—calcite; Bar—barite.

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 isconcluded that the fluid in all fluid inclusion sub-types is not a typicalNaCl (KCl)–H2O–CO2 system, so the salinity of the ore-forming fluidcannot be determined by measurements of freezing point or point ofCO2 hydrate decomposition. The CO2-rich fluid inclusion assemblage(AC, ADC and C fluid inclusions) shows a continuous CO2/aqueousratio in the same cluster, implying the immiscible trapping character.Th of AC-2 inclusions (227 to 453 °C) should be the best candidate forthe trapping temperature, and the pure CO2 inclusions were alsotrapped at almost the same temperature (or a little lower). Accordingto the pure CO2 phase diagram (cf. Roedder, 1984; Lu et al., 1994), thepressure can be estimated using the isochore method. The capturepressure was estimated as being between 150 and 350 MPa for theCO2-rich fluid inclusion assemblage. This pressure should reflect theunmixing condition between CO2 and aqueous fluid. For stage 2, thefluid temperature is higher than that of stage 3, and so stage 2 shouldbe at a higher pressure than stage 3. For stage 4, the fluid temperatureis lower than stage 3, and no liquid CO2, but the vapor can bedetermined in AV and ADV fluid inclusions. It is concluded that thecapture pressure for stage 4 should be much lower than stage 3.According to the Th of AV and ADV fluid inclusions and the lowest CO2

Fig. 9. LRM spectrum of melt–fluid inclusions (ML) in fluorite from the Maoniuping REE ddaughter mineral in melt–fluid inclusion of fluorite.

density of pure CO2 inclusions, the pressure for stage 4 should belower than 100 MPa.

5.4. Laser Raman Microprobe (LRM) analysis of inclusions

LRM was performed on the CO2, aqueous and solid phases in fluidinclusions and melt–fluid inclusions hosted in quartz and fluorite.Selections of spectrograms are shown in Figs. 9 and 10. For MLinclusions, there is strong peak around 3500 cm−1 which indicates anaqueous phase dominated by H2O. A peak at 981 to 984 cm−1 was alsoclearly seen, indicating the presence of SO42− in the aqueous phase.Interference fluorescence from fluoritemasks the peakswhich indicatesthe presence of CO2 (1386 cm−1) in thebubble phase. The spectrogramsof solid-phase in melt–fluid inclusions suggest that the solid phaseswere dominated by a sulfate assemblage, including mirabilite (Fig. 9a),gypsum, barite, celestine (Fig. 9b), and nahcolite. In someML inclusions,a solid phase has peaks at 1005,1069 and 988 cm−1; thesemay indicatesulfate–carbonate complexes.

In CO2-rich fluid inclusions (AC, ADC), the aqueous phase was alsodominated by H2O. The peaks at 981 to 984 cm−1 (Fig. 10b, c, f)suggest that SO42− was also the dominant anion in the aqueous phase.

eposit. (a) Mirabilite daughter mineral in melt–fluid inclusion of fluorite; (b) celestite

Fig.10. LRM spectrum of fluid inclusions in quartz from theMaoniuping REE deposit; (a) CO2 in fluid inclusion in quartz(AC sub-type); (b) SO42− in aqueous phase of fluid inclusion inquartz (AC sub-type); (c) H2O and SO42− in aqueous of fluid inclusion (AC sub-type); (d) CO2 in bubble phase of fluid inclusion (ADC sub-type); (e) Nahcolite daughter mineral influid inclusion (ADC sub-type); (f) SO42− in aqueous phase of fluid inclusion (ADC sub-type).


The peak at 1066 to 1069 cm−1, compared with the spectrum of hostquartz, suggests the presence of CO32− in the aqueous phase. Theoccurrence of SO42− and CO32− is consistent with the presence of sul-fate and bicarbonate daughter minerals in the inclusions. The bubblesin AC and ADC sub-type fluid inclusions are dominated by CO2,

according to the strong peaks at 1282 and 1386 cm−1 (Fig. 10a, d). Thedaughter minerals are also dominated by sulfate minerals. Comparedwith the host mineral, the spectrogram of the daughter minerals inquartz and fluorite have additional peaks at 1007–1005, 998, 988and 992 cm−1. These indicate gypsum, celestite, barite and mirabilite


daughterminerals, respectively. Nahcolitewas also determined via thestrong peak at 1046 cm−1 (Fig. 10e). The determination of daughterminerals is based on the inorganic material LRM spectrum databaseof Renishaw, and reference to Samson et al. (1995) and Rankin (2003).

In aqueous-fluid inclusions (AV, ADV), the aqueous phase isalso dominated by H2O. The peaks at 981 to 984 cm−1 also suggestthat SO42− is the dominant anion in the aqueous phase. The bubbles inAV and ADV sub-type fluid inclusions are dominated by H2O, with nopeaks at 1282 or 1386 cm−1. Sulfate minerals were also confirmed asthe dominant daughter phases.

5.5. SEM/EDS analysis of daughter minerals in inclusions

Numerous open inclusions in the prepared samples (fluorite andquartz) were observed under the SEM; most inclusions contained one

Fig. 11. Secondary electron images and EDS spectrum of daughter minerals in the Maoniupiunidentified (X) daughter mineral in fluorite; (c) aphtalose (Aph) and arkanite (Ark) daug

or more daughter minerals. According to the solid percentage andcomparison with the petrographic features of the different inclusiontypes, the inclusions observed under SEM were defined as ML, ADCand ADV types. EDS analysis showed that most of the daughterminerals in all inclusion types contain S, K and/or Na. Integrating theresults of LRM and SEM/EDS analysis, it appears that the daughterphases in melt–fluid and fluid inclusions are dominated by sulfatessuch as arcanite (K2SO4; Fig. 11b, c), aphthitalite (NaK3[SO4]2; Fig. 11a,c), mirabilite (Na2SO4 10H2O) and gypsum (CaSO4 2H2O). Arcaniteand aphthitalite are the most common types seen. No typical LRMspectra for arcanite and aphthitalite were available for comparison, sosome of the mirabilite determined by LRM could be arcanite oraphthitalite. Carbonate or bicarbonate daughter minerals was alsodetermined by EDS. Barite, strontianite and REE minerals have alsobeen reported as daughter phases in inclusions by Xu et al. (2004).

ng REE deposit; (a) aphtalose (Aph) daughter mineral in quartz; (b) arkanite(Ark) andhter mineral in fluorite; Fl—fluorite; Q—quartz.


6. Discussion

Most carbonatites worldwide are believed to be genetically linkedto silicate alkaline magmatism. As a result of their enrichment in REEminerals, many carbonatite bodies form economic REE deposits(cf. Rankin, 2003). Significant evidence from phase equilibriumexperiments has shown that carbonatites can be generated by primarymantle melting and by the differentiation of carbonated silicate melts,i.e., liquid immiscibility and crystal fractionation (cf. Bell et al., 1999).

The genesis of carbonatite–alkaline complex at Maoniupinghas been extensively researched, including aspects of petrology andC, O, Pb, Sr and Nd isotopes (Yuan et al., 1995; Niu et al., 1996b;Wang et al., 2002; Xu et al., 2003a, 2004; Tian et al., 2003;).These results imply an igneous carbonatite and mantle origin.The carbonatite in the Maoniuping area is rich in incompatibleelements such as Sr, Ba and REE, with a mantle-source C and Oisotope composition, high 87Sr/86Sr and low ξNd implying a meta-somatically enriched mantle source (Xu et al., 2003a). Theunmixing of a CO2-rich alkaline silicate magma related to thepartial melting of metasomatic mantle is likely to be the source ofthe carbonatite and nordmarkite.

Carbonatite melt is low density, aqueous-rich (Wyllie, 1966)and much lower viscosity than that of silicate melt (Wolff, 1994).Alkaline-rich fluid in carbonatite melts can decrease the melt densityand viscosity. Low-density, low-viscosity carbonatite melts are ableto be squeezed into shallow crust (Rankin, 1977; Samson et al., 1995;Buhn and Rankin, 1999) and often result in strong alteration and REEmineralization. Carbonatite fluids play an important role in miner-alization and alteration in REE deposits globally. Experimental andfield studies have shown that carbonatite magma is rich in volatilecomponents (Roedder, 1973; Palmer and Williams-Jones, 1996;Andersen, 1987; Nesbitt and Kelly 1977; Buhn and Rankin, 1999;Buhn et al., 2002; Williams-Jones and Palmer, 2002; Rankin, 2003).However, how the carbonatite fluid is derived from the carbonatitemagma is still open to debate. Besides crystal fractionation, im-miscible processes are believed to cause separation between car-bonatite melt and carbonatitic fluid (Roedder, 1984; Brooker andHamilton, 1990; Mitchell, 1997). The genesis and evolution ofcarbonatite fluids has become a key topic of current geologicalresearch (Fan et al., 2001).

6.1. Origin of carbonatite fluid

Microthermometric studies of melt and melt–fluid inclusionsprovide good evidence for volatile rich melt and an immiscibleprocess between carbonatite magma and carbonatitic fluid. Thecoexistence of different solid/aqueous ratio melt–fluid inclusions inthe same cluster, particularly the occurrence of the melt phase inmelt–fluid inclusions while heating up to 350 to 500 °C, indicatesinhomogeneity during trapping. The low melting temperatures inmelt–fluid inclusions indicate that the residue-melt trapped in melt–fluid inclusions is sulfate melt. Based on microthermometry results,the unmixing occurred between 790 and 850 °C. The melt, melt–fluidand fluid inclusion assemblage also implies a carbonatite magmaorigin of the ore-forming fluid and continuous evolution frommelt toore-forming fluid.

Carbon, oxygen and hydrogen isotopic results (Tian et al., 2003; Xuet al., 2003b) show that δ13D has a range from −52×10−3 to−86×10−3, and δ13OSMOW has a range from 7.8×10−3 to 13.3×10−3,and δ13CPDB for CO2 in fluid inclusions has a range from −3 to−5.6×10−3, which is similar to that of carbonatite. The results implythe character of the carbonatite-related orthomagmatic fluid and theirclose relationship to a mantle origin. It is concluded that the ore-forming process is closely related to carbonatite–alkaline magmatism,and that the ore-forming fluid results from the unmixing of a volatile-rich carbonatite melt.

6.2. T–P–X properties of carbonatitic fluid

The characteristics of carbonatite fluids in a continental rift environ-ment have been studied by numerous researchers (cf. Morogan, 1994;Samson et al., 1995; Palmer andWilliams-Jones, 1996; Smith et al., 2000;Smith and Henderson, 2000; Buhn et al., 2002; Williams-Jones andPalmer, 2002; Rankin, 2003). Although it is not known if these resultsare applicable to an environment of the type discussed here, theirconsideration is still helpful in any attempt to understand carbonatiticfluids and their associated mineralization in a continental collisionenvironment.

Rankin (2003) summarized the composition of ore-forming fluidsin carbonatite-associated rare metal deposits. Rankin suggests thatCO2-rich, L-V (aqueous-vapor), L-V-S (aqueous-vapor-solid) and L-V-MS (aqueous-vapor-multisolid) inclusions are common in carbona-tite-related deposits. CO2-rich fluid inclusions are noted in mostcarbonatite related deposits, however not, for example, in the Okacarbonatite (Samson et al., 1995). The orthomagmatic fluids evolvedfrom carbonatite magmas are Na–K–chloride–carbonate/bicarbonatebrines and have variable salt content and rather consistent K/Na ratios.Sulfate and fluoride contentsmay also be high due to the occurrence ofsulphate and fluoride-bearing daughter minerals (Rankin, 2003). Cl−

and SO42− are the dominant anions present, with minor CO32− andHCO3−. K+, Na+ and Ca2+ are the dominant cation with levels of Fe,Sr, Ba, REE, etc. Nahcolite is the most common daughter phase andsome other sulfate daughter phases are arcanite, mirabilite, celestineand baritewithminor carbonate and fluorideminerals (Rankin, 2003).Similar characteristics are confirmed for the Maoniuping deposit.

Due to the high capture pressure and highly developed cleavage inthe host minerals (fluorite, calcite, barite and bastnaesite), micro-thermometric studies are very difficult to undertake on variousinclusion types, particularly for melt–fluid (multi-solid) inclusions inthe deposit. This explains why very fewmicrothermometric data havebeen published for the deposit. For example, Yuan et al. (1995) onlygive the microthermometric data for low temperature fluid inclusions(AV sub-type) and few for daughter mineral bearing fluid inclusions.Niu and Lin (1995b), Niu et al. (1996a) performedmicrothermometricwork on aqueous-rich fluid inclusions (AV sub-type), CO2-bearingfluid inclusions and melt–fluid inclusions. However, they reportedthat all melt–fluid inclusions had a vapor phase homogenizationbehavior. These results were not duplicated in the present study. Inour observations, all vapor phase homogenization of melt–fluidinclusions was caused by inclusion destruction (e.g., leakage orexplosion). Yang et al. (2001) reported similar microthermometricresults from CO2-rich fluid inclusions in quartz and bastnaesite tothose reported in the present study. Microthermometric results formelt inclusions, melt–fluid inclusions and pure CO2 inclusions are stillunavailable and no clear corresponding relationship was confirmedbetween different types of inclusions and mineralization stages in thepublished papers.

According to the petrographic observations and microthermo-metric results, it is concluded that melt inclusions characterize thecarbonatite stage (stage 1); melt–fluid inclusions sponsor thepegmatite stage (stage 2); CO2-rich fluid inclusions dominate thebarite–bastnaesite stage (stage 3), but only AC-2 inclusions should bethe best candidate for stage 3. The AC-1 and C sub-type inclusionswere trapped after phase separation. Aqueous rich fluid inclusionsrelate to the late calcite stage (stage 4).

A key question to consider is which type of inclusions should bethe best samples for earliest orthomagmatic fluids evolved fromcarbonatite magma? Inclusion petrography shows that there arevolatiles in the melt inclusion from both fluorite and calcite of stages 1and 2. Melt inclusions can transform into the melt and volatile phaseduring heating, which indicates the volatile-rich characteristic of thecarbonatitemelt. ML inclusions are dominant in fluorite from the earlymineralization stage that follows the carbonatite stage. The ML-2

Fig. 12. Schematic T–P path for evolution of the ore-forming fluid in the MaoniupingREE deposit. I—carbonatite stage, melt inclusion assemblage; II—pegmatite stage, meltfluid inclusion assemblage; III—barite–bastnaesite stage, CO2 rich fluid inclusionassemblage; IV—calcite stage, aqueous rich fluid inclusion assemblage. Shaded ellipserepresents T–P range based on microthermometric data, unshaded ellipse showsestimated range based on unhomogenized inclusion data for AC+ADC inclusions andML inclusions.


inclusions have varied solid/aqueous/vapor ratios, implying inhomo-genous entrapment. However the ML-1 type inclusions have a stablesolid/aqueous/vapor ratio, and show primary inclusion characteristicswith negative crystal shape. During heating, most of ML-1 typeinclusions explode and only a few homogenized to the critical phase ata high temperature. It is thus likely that the ML-1 inclusions representthe best sample of the earliest orthomagmatic fluid derived fromcarbonatite magma. High homogenization temperatures, high solid-percentages in inclusions and critical-homogenization behavior implythat the initial ore-forming fluids were high-temperature (N600 to850 °C), high-pressure (N350 Mpa), very high-concentration andsupercritical. LRM, SEM/EDS results and daughter mineral assem-blages show that K+, Na+, Ca2+, together with Ba2+, Sr2+ and REE, arethe dominant cations. The dominant anions are SO42−, with minoramounts of HCO3− and CO32−. CO2 in the bubble phase ofML-1 inclusionsis usually difficult to confirm by LRM. It is possible that the peaks at1282 cm−1 and 1386 cm−1 are masked by the fluorescence of fluoriteor the absence of CO2 in inclusions of this kind. NoML-1 inclusions havebeen observed in quartz, but the LRM spectra for the bubbles in ML-2inclusions of quartz have peaks at 1282 cm−1 and 1386 cm−1. It is likelythat the bubble in ML-1 inclusions from fluorite comprise CO2.

CO2-rich fluid inclusion assemblages (AC, ADC and C-type)represent the next step of carbonatitic fluid evolution, since theseinclusions are sometimes associated with melt–fluid inclusions andoccur principally in barite and bastnaesite from stage 3. Themicrothermetric and LRM results for CO2-rich fluid inclusions suggestthat the ore-forming fluid in stage 3 has a high CO2 concentration,moderate to high temperature and high pressure. LRM and SEM/EDSalso proved that K+, Na+ and Ca2+ are the dominant cations, togetherwith minor Ba2+, Sr2+, REE, and the dominant anions are SO42−. Thebubble phase is dominated by liquid CO2. The similar chemicalcomposition between ML inclusions and CO2-rich inclusions, andclose petrographic relationship, shows their affiliation to the evolutionof an ore-forming fluid.

Aqueous-rich fluid inclusions have low temperatures and lowcaptive pressures (b100 MPa), indicating late-stage fluid character-istics. The LRM and SEM/EDS results of AV and ADV inclusions indicatethat K+, Na+, H2O and SO42− dominated the system. The white calciteveins of stage 4 cross-cuts all styles of REE veins and wallrocks, andoften occurs as geodes or fracture fillings. That means the calcite ofstage 4 precipitated in an extensional environment.

In summary, CO2, K+, Na+ and SO42− rich carbonatite fluids arelikely to be a common characteristic of both the continental rift andcollision orogenic environment. But for the temperature of ore-forming fluids, there is a large range reported for different deposits,even for a deposit like Maoniuping. The reason for this is that mostmicrothermometric work on high capture pressure inclusions isunsuccessful. The sulfate daughter minerals rich in K+ and Na+ mightbe a characteristic of inclusions related to carbonatite in bothcontinental rift and continental collision orogenic environments.

6.3. Evolution of ore-forming fluid

The evolution of ore-forming fluids is one of the most importantsubjects of carbonatite research. Ting et al. (1994) suggested anevolutionary path varying from an early CO2-rich fluid, via medium-high salinity H2O-rich fluid, to a CH4-rich fluid using fluid inclusions inapatite 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-richtypes, based on studies on fluid inclusions in gangue and REEmineralsfrom the Bayan Obo Fe-REE-Nb deposit. Although, there is still debateon the genesis of the Bayan Obo deposit, results of inclusion studiesappear compatible with carbonatitic fluids (Rankin, 2003).

In the Maoniuping REE deposit, different inclusion types, rangingfrommelt to fluid inclusions, record the evolution of a carbonatitic fluid.

According to microthermometric results, the T–P evolution path fromcarbonatite melt to ore-forming fluid is illustrated in Fig. 12. Theevolution of carbonatite fluid is from high-temperature melt–fluids,high-moderate temperature CO2-rich fluids to low-temperature aqu-eous rich fluids. Unmixing of melt and fluid occurs at between 790 and850 °C according to the homogenization temperature ofmelt inclusions.A large quantity of critical CO2–H2O fluid inclusions (AC-2) inbastnaesite and barite indicate that the ore-forming fluid for stage 3 isa kind of H2O–CO2 supercritical fluid. Aqueous-CO2 phase separationoccurs at 227 to 453 °C. Secondary fluid inclusions in fluorite and quartztrapped during stage 4 show lower homogenization temperatures andlower CO2 concentrations. However, the LRM results indicate that thelater fluid is also dominated by the K–Na–H2O–SO4 system. It can beconcluded that from stages 3 to 4, the ore-forming fluid progresses froma high to moderate temperature CO2-rich fluid to a low temperatureaqueous-rich fluid, but K–Na–H2O–SO4 still dominated the fluid system.

The mineral assemblage provides some useful information aboutthe evolution of the ore-forming fluid. The mineral assemblage inMaoniuping is similar to that of the Kizilcaoren fluorite–barite–REEdeposit, Turkey (Gultekin et al., 2003). However, in contrast to otherdeposits, Maoniuping also contains some alloys (as determined bySEM/EDS and EPMA). Metallic minerals at Maoniuping are relativelyrare and are dominated by Fe-oxides. Sulfide is mainly present asgalena and pyrite, withminor chalcopyrite and pyrrhotite. From stages1 to 3, themetallic mineral assemblage passes from sulfide-dominatedto oxide-dominated. In stages 1 and 2, the dominant minerals arepyrite, chalcopyrite, pyrrhotite and magnetite. In stage 3, thedominant metallic minerals are magnetite, hematite and rutile,together with native copper, a Cu–Zn alloy and minor galena. Thenativemetal, oxide and lead–sulfidemineral assemblage is special andlikely implies unique physico-chemical conditions. From stages 1 to 3,the variation of the metallic mineral assemblages indicates that theore-forming environment ranged from reductive to oxidative. Nometallic minerals were identified in stage 4.

6.4. REE mineralization

REE mineral precipitation mechanism is an important aspect ofcarbonatitic fluid research. Smith et al. (2000) and Smith andHenderson (2000) proposed that bastnaesite precipitated at atemperature of between 250 and 280 °C from CO2-rich fluids, andthat phase separation provided a mechanism for REE concentrationand deposition at the Bayan Obo REE deposit. Palmer and Williams-Jones (1996) and Williams-Jones and Palmer (2002) argue that in theAmba Dongar fluorite deposit, fluorite precipitated by the mixing of


carbonatite orthomagmatic fluid and late-stage Ca-bearing meteoricwaters. Niu and Lin (1995b), Niu et al. (1997), Wang et al. (2002) andXu et al. (2002, 2003b) emphasize the role of carbonatite, andregarded REE mineralization to be related to a late magmatic stage.Yuan et al. (1995) believe that the magmatic fluid played the mostimportant role in mineralization and that REE deposition occurred at arelatively low temperature (b200 °C) in the Maoniuping deposit.

Mineral paragenetic and characteristics of inclusions in REEminerals, provides evidence for REE mineral precipitation sequenceand P–T conditions. Petrographic studies show that most REEmineral precipitation occurred after fluorite and quartz in stage 3.The fluid inclusions in bastnaesite formed a CO2-rich fluid inclusionassemblage (AC+ADC+C sub-types). This fluid inclusion assem-blage suggests that REE minerals precipitated in a CO2-rich fluid, andin the CO2-aqueous unmixing process. Integrating microthermo-metric, LRM and SEM/EDS results, the dominant ore-forming fluid isconsidered to be a supercritical CO2–H2O system enriched in K, Naand SO42−. The CO2-rich fluids may complex and transport REE (Buhnet al., 2002), phase separation of CO2 and aqueous is considered asthe dominant factor for REE precipitation.

CO2–H2O separation may have been caused by a fast decrease influid pressure. Breccia ore with carbonatite cement is found in thesouthern segment of the deposit, indicating the existence of a crypto-explosion process. A crypto-explosion process will result in thedecrease of fluid pressure, and hence phase separation between CO2and aqueous fluids, leading to REE precipitation.

7. Conclusions

The key conclusions of this study are:

(1) Mineral association, daughter mineral composition and LRMresults show that the composition of the ore-forming fluid is richin CO2, SO42−, K, Na, Ca, Ba, S and REE and formed a complexmulticomponent (K–Na–Ca–Ba–Sr–REE–SO4)–CO2–H2O system.

(2) The transition of melt, melt–fluid and fluid inclusions suggestcontinuous evolution from carbonatite magma to hydrothermalfluid, and also suggests a magmatic source and an unmixingprocess of carbonatite melt and ore-forming fluid. The initialcarbonatite orthomagmatic fluids were high-temperature, high-pressure and high-density supercritical hydrothermal fluids. Theevolution of ore-formingfluids ranged from the carbonatitemelt,melt fluids, high-medium temperature CO2-rich fluids to lowtemperature aqueous-rich fluids.

(3) The fluids related to REE mineralization are considered to be atype of supercritical CO2–H2O system. REE minerals predicatedin a CO2-rich fluid, and REE precipitation occurred at between227 to 453 °C and 150 to 350 MPa. Phase separation caused by adecrease in temperature and pressure is considered as thedominant mechanism in REE precipitation.

Acknowledgments

This study was supported financially by “National Nature ScienceFoundation of China” (No. 40573035 and No. 40425014) and by “theState Basic Research Program of China” (No.2002-CB-412600).

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Continuous carbonatitic melt–fluid evolution of a REE mineralization system: Evidence from incl.....IntroductionGeologyMineralizationSampling and analytical methodsInclusion resultsPetrography and classification of inclusionsInclusion assemblagesMicrothermometric resultsMelt inclusionsMelt–fluid inclusionsFluid inclusions

Laser Raman Microprobe (LRM) analysis of inclusionsSEM/EDS analysis of daughter minerals in inclusions

DiscussionOrigin of carbonatite fluidT–P–X properties of carbonatitic fluidEvolution of ore-forming fluidREE mineralization

ConclusionsAcknowledgmentsReferences

continuous carbonatitic melt–fluid evolution of a ree ... · further understand the evolution of...

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