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immiscibility as an ore-forming process in the giant REE–Nb–Fe deposit, Inner Mongolian, China: Evidence from fluid inclusions
Journal of Geochemical Exploration 89 (2006) 104 – 107 www.elsevier.com/locate/jgeoexp
Fluid unmixing/immiscibility as an ore-forming process in the giant REE–Nb–Fe deposit, Inner Mongolian, China: Evidence from fluid inclusions Hong-Rui Fan ⁎, Fang-Fang Hu, Kui-Feng Yang, Kai-Yi Wang Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Received 29 August 2005; accepted 4 November 2005 Available online 9 March 2006
Abstract The giant Bayan Obo REE–Nb–Fe deposit consists of replacement bodies hosted in dolomite marble made up of magnetite, REE fluorocarbonates, fluorite, aegirine, amphibole, calcite and barite. Two or three phase CO2-rich, three phase hypersaline liquid–vapor–solid, and two phase liquid-rich inclusions have been recognized in mineralized fluorite and quartz samples. Microthermometry measurements indicate that the carbonic phase in CO2-rich inclusions is nearly pure CO2. Fluids involving in REE–Nb–Fe mineralization at Bayan Obo might be mainly of H2O–CO2–NaCl–(F–REE) system. Coexistences of brine inclusions and CO2-rich inclusions with similar homogenization temperatures give evidence that immiscibility happened during REE mineralization. An unmixing of an original H2O–CO2–NaCl fluid probably derived from carbonatitic magma. The presence of REE-carbonates as an abundant solid in fluid inclusions shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo. © 2006 Elsevier B.V. All rights reserved. Keywords: Fluid inclusion; Fluid unmixing/immiscibility; REE mineralization; Bayan Obo
1. Introduction The Bayan Obo REE–Nb–Fe deposit is located approximately 50 miles south of the China and Mongolia border, at 109°57′13″E and 41°46′22″N. The deposit hosts the world's largest known rare earth element (REE) resource, as well as being a major Nb and Fe producer in China. REE reserves in the deposit, which represents 70% of the world's REE resources, are 57.4 Mt with an ⁎ Corresponding author. Tel.: +86 10 62008088; fax: +86 10 62010846. E-mail address: [email protected] (H.-R. Fan). 0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.11.039
average grade of 5.17 to 6.19 wt.% REE2O3, and Nb reserves are estimated at 2.2 Mt with an average grade of 0.126 to 0.141 wt.% Nb2O5 (Chao et al., 1997; Hao et al., 2002). Smith and Henderson (2000) have characterised fluid inclusions at Bayan Obo, and REE daughter minerals (might be cebaite and bastnaesite) are identified in fluid inclusions (Fan et al., 2004). The present study focuses on the nature of the fluids involved in REE–Nb–Fe mineralization in the deposit, and on the possibility that fluid unmixing/immiscibility of an original H2O–CO2– NaCl–REE fluid may have contributed to ore genesis for this giant REE–Nb–Fe deposit.
H.-R. Fan et al. / Journal of Geochemical Exploration 89 (2006) 104–107
2. Geological setting The Bayan Obo deposit is hosted within the sediments of the Middle Proterozoic Bayan Obo Group, which consist predominantly of Hb1 to Hb7 and Hb9 units of slate, sandstone and quartzite, and Hb8 unit of dolomite (Fig. 1). The origin of the Hb8 dolomite hosting the majority of the REE–Nb–Fe ores is still disputed; it has been proposed to be either sedimentary (Meng, 1982; Chao et al., 1997), or the result of carbonatite magmatism (Yuan et al., 1992; Le Bas et al., 1997; Hao et al., 2002; Yang et al., 2003). The paragenesis of the deposit is extremely complex with at least 11 stages from deposition, through metamorphism and mineralization, to the intrusion of Hercynian granitoids to the south of the deposit (Chao et al., 1997). The Th–Pb dates of REE mineralization from monazite range from 555 to ∼400 Ma (Wang et al., 1994). At the simplest level, the main-stage banded mineralization (∼430 to 420 Ma) shows a generalized paragenetic sequence of strongly banded REE and Fe ores showing alteration to aegirine, fluorite and minor alkali amphibole. These rocks are cut by aegirine-rich veins containing apatite, bastnaesite, fluorite and quartz, and later aegirine-rich veins and pods associated with barite, bastnaesite and Ba–REE–fluorocarbonates. The final stage in REE mineralization is associated with the formation of Nb-bearing minerals such as columbite, ilmenorutile, aeschynite and fergusonite. Several possible formation modes have been proposed for the deposit, including syngenetic sedimentary
105
deposition (Meng, 1982), metasomatism associated with granitic magmatism (Wang and Li, 1973), and deposition from exhalative, possibly carbonatite related, hydrothermal fluids (Yuan et al., 1992). Most recently the works of Chao et al. (1997) have demonstrated an epigenetic origin for the deposit via multistage hydrothermal metasomatism. On this basis models involving metasomatism by fluids derived from either subduction (Wang et al., 1994), or carbonatite, or alkaline, magmatism (Yang et al., 2003) have been proposed. The interpretation of the carbonatite source for the metasomatic fluids is supported by the presence of carbonatite dykes cutting the sediments to the north and southeast of the deposit and the apparent carbonatitic affinities of the host dolomite (Le Bas et al., 1997). 3. Samples and analytical techniques 42 fluorite and quartz samples examined in this study were taken from orebodies of the East Pit and the Main Pit at Bayan Obo (Fig. 1). Doubly polished sections were prepared for fluid inclusion optical examination and microthermometric analysis. Heating and cooling experiments were carried out using a Linkam THMS 600 programmable heating–freezing stage at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The stage was calibrated with pure CO2 (− 56.6 °C) and pure H2O (0 and 374.1 °C) synthetic fluid inclusion standards supplied by FLUID INC. Carbonic phase melting (Tm,CO2) and clathrate melting
Fig. 1. Geological sketch map of the Bayan Obo area (modified after Fan et al., 2004).
106
H.-R. Fan et al. / Journal of Geochemical Exploration 89 (2006) 104–107
(Tm,clath) were determined with extreme care by temperature cycling, and heating rate near Tm,CO2 and Tm,clath was 0.1 to 0.2 °C/min. The reproducibility of measurements was ± 0.2 below + 30 °C and ± 2 above 100 °C, where the chips were centered in the specimen holder. Salinity is expressed as equivalent wt.% NaCl and was calculated from measurements of the ice melting temperature (Tm,ice) using the equations of Bodnar (1993) for aqueous inclusions. Salinities of carbonic and hypersaline inclusions were calculated from the clathrate melting and halite dissolution temperatures, respectively, using the MacFlincor program package (Brown and Hagemann, 1995). In order to confirm the suggested fluid inclusion volatile species, representative samples were analyzed using a Renishaw 2000 Laser Raman microspectrometer equipped with a CCD detector and an Ar ion laser with a wavelength of 514.5 nm at IGGCAS. 4. Fluid inclusions In the Bayan Obo REE–Nb–Fe district, abundant carbonic (CO2-rich) and aqueous inclusions have been reported in ores (Smith and Henderson, 2000; Fan et al., 2004). So far, at least three types of fluid inclusions have been recognized on the basis of their appearance at room temperature: two or three phase CO2-rich, three phase hypersaline liquid–vapor–solid with one or more daughter minerals (L–V–nS), and two phase aqueous liquid-rich (L–V) inclusions. 4.1. CO2 CO2 inclusions occur as isolated cavities or planar arrays in healed microfractures in fluorite and quartz. The inclusions are generally less than 20 μm in diameter and occur as two or three phases consisting of an aqueous liquid, a carbonic liquid and/or vapor with relatively constant phase ratios. Melting of the carbonic phase occurs either at the CO2 triple point of − 56.6 °C, or over a small interval with depressed melting temperatures between − 57.0 and − 56.7 °C. These measurements indicate that the carbonic phase in these inclusions is nearly pure CO2. Laser Raman analyses with only sharp around 1386 and 1285 cm− 1 peaks have revealed the presence of CO2 and almost no traces of other compositions like CH4 and/or N2. Melting of the CO2 clathrate in the presence of CO2 liquid occurs between 4.1 and 8.5 °C. Calculated salinities of the inclusions are in the range of 3.0 to 10.3 wt.% NaCl eq. Partial homogenization of CO2 liquid + CO2 vapor to liquid CO2 occurs between 20.2 and 29.5 °C. Upon heating, more
than one-half of the studied inclusions decrepitated prior to final homogenization, at temperatures from 200 to 280 °C. Total homogenization temperatures to liquid, obtained mainly from inclusions with lower CO2 contents and smaller diameters, range from 250 to 320 °C. 4.2. Hypersaline brines (L–V–nS) Hypersaline brine (L–V–nS) inclusions are commonly observed in samples, and are concentrated mainly in fluorite, but are also found in quartz cores and in REE fluorocarbonates. They are isolated and primary. During heating experiments some L–V–S inclusions decrepitated or stretched prior to total homogenization. Only a few of them survived, and the hexagonal or irregular shaped daughter minerals were nearly always the last phase left in the inclusions. With continuous heating and then cooling, the hexagonal shaped daughter minerals displayed the following behavior: (1) complete dissolution at temperatures of 420∼480 °C, these are also the total homogenization temperatures of the inclusions; (2) re-crystallization to one or more new hexagonal shaped daughter minerals at about 400–320 °C. Cubic halite, if present, melted from 224 to 352 °C during heating and formed again during cooling. Salinities calculated for these inclusions based on halite dissolution temperature range from 33 to 42 wt.% NaCl. Fan et al. (2004) had identified REEcarbonates, halite, sylvite, barite, calcite and pyroxene (?) on the basis of crystal habit (microscopic and SEM) and EDX analysis in the hypersaline brine inclusions. By comparison with Raman spectra of reference REEcarbonate mineral crystals, the hexagonal or irregular shaped daughter minerals in the L–V–S inclusions might be cebaite and bastnaesite (Fan et al., 2004). The presence of REE-carbonates as a solid in the L–V–nS inclusions from ore-forming veins shows that the original ore-forming fluids are very rich in REE. 4.3. Liquid-rich fluids (L–V) L–V inclusions occur in fluorite, quartz, barite and REE fluorocarbonates, and range up to 20 μm in diameter. Primary L–V inclusions are most abundant in the latest coarse-grained fluorite veins, which cut obviously banded REE and Fe ores. These primary L– V inclusions are related to the latest fluids. Most other L–V inclusions in quartz and REE fluorocarbonates occur along secondary planes. The inclusions do not nucleate a recognizable clathrate phase, indicating minor amounts (b2.7 wt.%) of CO2. L–V inclusions
H.-R. Fan et al. / Journal of Geochemical Exploration 89 (2006) 104–107
showed final homogenization to liquid at temperatures between 153 and 325 °C and melting of ice in the range of − 3.4 to − 11.2 °C, corresponding to salinities from 5.6 to 15.2 wt.% NaCl eq.
107
comments. This work was supported by The National Natural Science Foundation of China (Project No. 40472057). References
5. Discussion and conclusions The main REE ores at Bayan Obo are related with fluorite and magnetite/hematite mineralization. Combined with fluid inclusion results above, fluids involving in REE–Nb–Fe mineralization at Bayan Obo might be H2O–CO2–NaCl–(F–REE) system at least. The higher homogenization temperatures and salinities of some fluid inclusions indicate the initial existence of a dense brine, probably of magmatic/carbonatitic origin as described by Smith and Henderson (2000). The presence of REE-carbonates as solid phase in fluid inclusions shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo (Fan et al., 2004). Coexistences of brine inclusion and CO2-rich inclusion with similar homogenization temperatures suggests that these fluid inclusions resulted either from the trapping of boiling fluids or represent two unmixing/ immiscible fluids of original H2O–CO2–NaCl fluids with higher REE contents. Carbonatite-related fluids have been shown to include alkali-chloride and alkali-carbonate brines (Samson et al., 1995) and aqueous carbonic fluids (Ting et al., 1994), all of which may be comparable to the fluids at Bayan Obo. The hypersaline brine inclusions contain different numbers and types of solid phases at Bayan Obo. It is unclear whether the early hypersaline brines are pristine fluids, e.g. derived from carbonatite melts or result from interaction with wall rock. In other words, whether the external fluid composition was essentially constant during evolution is still an open question. The understanding of the origin and causes of compositional variation of the primary fluids with speculative REE-rich are underconstrained at present. Further work using LA-ICP-MS microanalyses on fluid inclusions will be done to decipher immiscible fluids related to this giant deposit. Acknowledgements Special thanks are due to the management and staff of the Bayan Obo mine for their hospitality during the fieldwork. Prof. Andrew Rankin and Dr. Martin Smith are acknowledged for their reviews and valuable
Bodnar, R.J., 1993. Revised equation and stable for determining the freezing point depression of H2O–NaCl solutions. Geochim. Cosmochim. Acta 57, 683–684. Brown, P.E., Hagemann, S.G., 1995. MacFlincor and its application to fluids in Archaean lode–gold deposits. Geochim. Cosmochim. Acta 59, 3943–3952. Chao, E.C.T., Back, J.M., Minkin, J.A., Tatsumoto, M., Wang, J., Conrad, J.E., MaKee, E.H., Hou, Z., Meng, Q., Huang, S., 1997. The sedimentary carbonate-hosted giant Bayan Obo REE–Fe–Nb ore deposit of Inner Mongolia, China: a cornerstone example for giant polymetallic ore deposits of hydrothermal original. U.S. Geol. Surv. Bull. 2143, 1–65. Fan, H.R., Xie, Y.H., Wang, K.Y., Tao, K.J., Wilde, S.A., 2004. REE daughter minerals trapped in fluid inclusions in the giant Bayan Obo REE–Nb–Fe deposit, Inner Mongolia, China. Int. Geol. Rev. 46, 638–645. Hao, Z.G., Wang, X.B., Li, Z., Xiao, G.W., Zhang, T.R., 2002. Bayan Obo carbonatite REE–Nb–Fe deposit: a rare example of Neoproterozoic lithogeny and metallogeny of a damaged volcanic edifice. Acta Geol. Sin. 76, 525–540 (in Chinese with English abstract). Le Bas, M.J., Spiro, B., Yang, X., 1997. Oxygen, carbon and strontium isotope study of the carbonatitic dolomite host of the Bayan Obo Fe–Nb–REE deposit, Inner Mongolia, N. China. Mineral. Mag. 61, 531–541. Meng, Q., 1982. The genesis of the host rock dolomite of Bayan Obo iron ore deposits and the analysis of its sedimentary environment. Geol. Rev. 28, 481–489 (in Chinese with English abstract). Samson, I.M., Liu, W., Williams-Jones, A.E., 1995. The nature of orthomagmatic hydrothermal fluids in the Oka carbonatite, Quebec, Canada: evidence from fluid inclusions. Geochim. Cosmochim. Acta 59, 1963–1977. Smith, M.P., Henderson, P., 2000. Preliminary fluid inclusion constraints on fluid evolution in the Bayan Obo Fe–REE–Nb deposit, Inner Mongolia, China. Econ. Geol. 95, 1371–1388. Ting, W., Burke, E.A.J., Rankin, A.H., Woolley, A.R., 1994. Characterisation and petrogenetic significance of CO2, H2O and CH4 fluid inclusions in apatite from the Sukulu carbonatite, Uganda. Eur. J. Mineral. 6, 787–803. Wang, Z., Li, S., 1973. The genetic features of the REE–Fe ore deposit related to sedimentation–metamorphism–hydrothermal metasomatism. Geochimica 1, 5–11 (in Chinese). Wang, J., Tatsumoto, M., Li, X., Premo, W.R., Chao, E.C.T., 1994. A precise 232Th–208Pb chronology of fine grained monazite: age of the Bayan Obo REE–Fe–Nb ore deposit, China. Geochim. Cosmochim. Acta 58, 3155–3169. Yang, X.M., Yang, X.Y., Zheng, Y.F., Le Bas, M.J., 2003. A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China. Mineral. Petrol. 78, 93–111. Yuan, Z., Bai, G., Wu, C., Zhang, Z., Ye, X., 1992. Geological features and genesis of the Bayan Obo REE ore deposit, Inner Mongolia, China. Appl. Geochem. 7, 429–442.
Fluid unmixing/immiscibility as an ore-forming process in the giant REE–Nb–Fe deposit, Inner Mongolian, China: Evidence from fluid inclusions Hong-Rui Fan ⁎, Fang-Fang Hu, Kui-Feng Yang, Kai-Yi Wang Key Laboratory of Mineral Resources, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China Received 29 August 2005; accepted 4 November 2005 Available online 9 March 2006
Abstract The giant Bayan Obo REE–Nb–Fe deposit consists of replacement bodies hosted in dolomite marble made up of magnetite, REE fluorocarbonates, fluorite, aegirine, amphibole, calcite and barite. Two or three phase CO2-rich, three phase hypersaline liquid–vapor–solid, and two phase liquid-rich inclusions have been recognized in mineralized fluorite and quartz samples. Microthermometry measurements indicate that the carbonic phase in CO2-rich inclusions is nearly pure CO2. Fluids involving in REE–Nb–Fe mineralization at Bayan Obo might be mainly of H2O–CO2–NaCl–(F–REE) system. Coexistences of brine inclusions and CO2-rich inclusions with similar homogenization temperatures give evidence that immiscibility happened during REE mineralization. An unmixing of an original H2O–CO2–NaCl fluid probably derived from carbonatitic magma. The presence of REE-carbonates as an abundant solid in fluid inclusions shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo. © 2006 Elsevier B.V. All rights reserved. Keywords: Fluid inclusion; Fluid unmixing/immiscibility; REE mineralization; Bayan Obo
1. Introduction The Bayan Obo REE–Nb–Fe deposit is located approximately 50 miles south of the China and Mongolia border, at 109°57′13″E and 41°46′22″N. The deposit hosts the world's largest known rare earth element (REE) resource, as well as being a major Nb and Fe producer in China. REE reserves in the deposit, which represents 70% of the world's REE resources, are 57.4 Mt with an ⁎ Corresponding author. Tel.: +86 10 62008088; fax: +86 10 62010846. E-mail address: [email protected] (H.-R. Fan). 0375-6742/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.gexplo.2005.11.039
average grade of 5.17 to 6.19 wt.% REE2O3, and Nb reserves are estimated at 2.2 Mt with an average grade of 0.126 to 0.141 wt.% Nb2O5 (Chao et al., 1997; Hao et al., 2002). Smith and Henderson (2000) have characterised fluid inclusions at Bayan Obo, and REE daughter minerals (might be cebaite and bastnaesite) are identified in fluid inclusions (Fan et al., 2004). The present study focuses on the nature of the fluids involved in REE–Nb–Fe mineralization in the deposit, and on the possibility that fluid unmixing/immiscibility of an original H2O–CO2– NaCl–REE fluid may have contributed to ore genesis for this giant REE–Nb–Fe deposit.
H.-R. Fan et al. / Journal of Geochemical Exploration 89 (2006) 104–107
2. Geological setting The Bayan Obo deposit is hosted within the sediments of the Middle Proterozoic Bayan Obo Group, which consist predominantly of Hb1 to Hb7 and Hb9 units of slate, sandstone and quartzite, and Hb8 unit of dolomite (Fig. 1). The origin of the Hb8 dolomite hosting the majority of the REE–Nb–Fe ores is still disputed; it has been proposed to be either sedimentary (Meng, 1982; Chao et al., 1997), or the result of carbonatite magmatism (Yuan et al., 1992; Le Bas et al., 1997; Hao et al., 2002; Yang et al., 2003). The paragenesis of the deposit is extremely complex with at least 11 stages from deposition, through metamorphism and mineralization, to the intrusion of Hercynian granitoids to the south of the deposit (Chao et al., 1997). The Th–Pb dates of REE mineralization from monazite range from 555 to ∼400 Ma (Wang et al., 1994). At the simplest level, the main-stage banded mineralization (∼430 to 420 Ma) shows a generalized paragenetic sequence of strongly banded REE and Fe ores showing alteration to aegirine, fluorite and minor alkali amphibole. These rocks are cut by aegirine-rich veins containing apatite, bastnaesite, fluorite and quartz, and later aegirine-rich veins and pods associated with barite, bastnaesite and Ba–REE–fluorocarbonates. The final stage in REE mineralization is associated with the formation of Nb-bearing minerals such as columbite, ilmenorutile, aeschynite and fergusonite. Several possible formation modes have been proposed for the deposit, including syngenetic sedimentary
105
deposition (Meng, 1982), metasomatism associated with granitic magmatism (Wang and Li, 1973), and deposition from exhalative, possibly carbonatite related, hydrothermal fluids (Yuan et al., 1992). Most recently the works of Chao et al. (1997) have demonstrated an epigenetic origin for the deposit via multistage hydrothermal metasomatism. On this basis models involving metasomatism by fluids derived from either subduction (Wang et al., 1994), or carbonatite, or alkaline, magmatism (Yang et al., 2003) have been proposed. The interpretation of the carbonatite source for the metasomatic fluids is supported by the presence of carbonatite dykes cutting the sediments to the north and southeast of the deposit and the apparent carbonatitic affinities of the host dolomite (Le Bas et al., 1997). 3. Samples and analytical techniques 42 fluorite and quartz samples examined in this study were taken from orebodies of the East Pit and the Main Pit at Bayan Obo (Fig. 1). Doubly polished sections were prepared for fluid inclusion optical examination and microthermometric analysis. Heating and cooling experiments were carried out using a Linkam THMS 600 programmable heating–freezing stage at the Institute of Geology and Geophysics, Chinese Academy of Sciences (IGGCAS). The stage was calibrated with pure CO2 (− 56.6 °C) and pure H2O (0 and 374.1 °C) synthetic fluid inclusion standards supplied by FLUID INC. Carbonic phase melting (Tm,CO2) and clathrate melting
Fig. 1. Geological sketch map of the Bayan Obo area (modified after Fan et al., 2004).
106
H.-R. Fan et al. / Journal of Geochemical Exploration 89 (2006) 104–107
(Tm,clath) were determined with extreme care by temperature cycling, and heating rate near Tm,CO2 and Tm,clath was 0.1 to 0.2 °C/min. The reproducibility of measurements was ± 0.2 below + 30 °C and ± 2 above 100 °C, where the chips were centered in the specimen holder. Salinity is expressed as equivalent wt.% NaCl and was calculated from measurements of the ice melting temperature (Tm,ice) using the equations of Bodnar (1993) for aqueous inclusions. Salinities of carbonic and hypersaline inclusions were calculated from the clathrate melting and halite dissolution temperatures, respectively, using the MacFlincor program package (Brown and Hagemann, 1995). In order to confirm the suggested fluid inclusion volatile species, representative samples were analyzed using a Renishaw 2000 Laser Raman microspectrometer equipped with a CCD detector and an Ar ion laser with a wavelength of 514.5 nm at IGGCAS. 4. Fluid inclusions In the Bayan Obo REE–Nb–Fe district, abundant carbonic (CO2-rich) and aqueous inclusions have been reported in ores (Smith and Henderson, 2000; Fan et al., 2004). So far, at least three types of fluid inclusions have been recognized on the basis of their appearance at room temperature: two or three phase CO2-rich, three phase hypersaline liquid–vapor–solid with one or more daughter minerals (L–V–nS), and two phase aqueous liquid-rich (L–V) inclusions. 4.1. CO2 CO2 inclusions occur as isolated cavities or planar arrays in healed microfractures in fluorite and quartz. The inclusions are generally less than 20 μm in diameter and occur as two or three phases consisting of an aqueous liquid, a carbonic liquid and/or vapor with relatively constant phase ratios. Melting of the carbonic phase occurs either at the CO2 triple point of − 56.6 °C, or over a small interval with depressed melting temperatures between − 57.0 and − 56.7 °C. These measurements indicate that the carbonic phase in these inclusions is nearly pure CO2. Laser Raman analyses with only sharp around 1386 and 1285 cm− 1 peaks have revealed the presence of CO2 and almost no traces of other compositions like CH4 and/or N2. Melting of the CO2 clathrate in the presence of CO2 liquid occurs between 4.1 and 8.5 °C. Calculated salinities of the inclusions are in the range of 3.0 to 10.3 wt.% NaCl eq. Partial homogenization of CO2 liquid + CO2 vapor to liquid CO2 occurs between 20.2 and 29.5 °C. Upon heating, more
than one-half of the studied inclusions decrepitated prior to final homogenization, at temperatures from 200 to 280 °C. Total homogenization temperatures to liquid, obtained mainly from inclusions with lower CO2 contents and smaller diameters, range from 250 to 320 °C. 4.2. Hypersaline brines (L–V–nS) Hypersaline brine (L–V–nS) inclusions are commonly observed in samples, and are concentrated mainly in fluorite, but are also found in quartz cores and in REE fluorocarbonates. They are isolated and primary. During heating experiments some L–V–S inclusions decrepitated or stretched prior to total homogenization. Only a few of them survived, and the hexagonal or irregular shaped daughter minerals were nearly always the last phase left in the inclusions. With continuous heating and then cooling, the hexagonal shaped daughter minerals displayed the following behavior: (1) complete dissolution at temperatures of 420∼480 °C, these are also the total homogenization temperatures of the inclusions; (2) re-crystallization to one or more new hexagonal shaped daughter minerals at about 400–320 °C. Cubic halite, if present, melted from 224 to 352 °C during heating and formed again during cooling. Salinities calculated for these inclusions based on halite dissolution temperature range from 33 to 42 wt.% NaCl. Fan et al. (2004) had identified REEcarbonates, halite, sylvite, barite, calcite and pyroxene (?) on the basis of crystal habit (microscopic and SEM) and EDX analysis in the hypersaline brine inclusions. By comparison with Raman spectra of reference REEcarbonate mineral crystals, the hexagonal or irregular shaped daughter minerals in the L–V–S inclusions might be cebaite and bastnaesite (Fan et al., 2004). The presence of REE-carbonates as a solid in the L–V–nS inclusions from ore-forming veins shows that the original ore-forming fluids are very rich in REE. 4.3. Liquid-rich fluids (L–V) L–V inclusions occur in fluorite, quartz, barite and REE fluorocarbonates, and range up to 20 μm in diameter. Primary L–V inclusions are most abundant in the latest coarse-grained fluorite veins, which cut obviously banded REE and Fe ores. These primary L– V inclusions are related to the latest fluids. Most other L–V inclusions in quartz and REE fluorocarbonates occur along secondary planes. The inclusions do not nucleate a recognizable clathrate phase, indicating minor amounts (b2.7 wt.%) of CO2. L–V inclusions
H.-R. Fan et al. / Journal of Geochemical Exploration 89 (2006) 104–107
showed final homogenization to liquid at temperatures between 153 and 325 °C and melting of ice in the range of − 3.4 to − 11.2 °C, corresponding to salinities from 5.6 to 15.2 wt.% NaCl eq.
107
comments. This work was supported by The National Natural Science Foundation of China (Project No. 40472057). References
5. Discussion and conclusions The main REE ores at Bayan Obo are related with fluorite and magnetite/hematite mineralization. Combined with fluid inclusion results above, fluids involving in REE–Nb–Fe mineralization at Bayan Obo might be H2O–CO2–NaCl–(F–REE) system at least. The higher homogenization temperatures and salinities of some fluid inclusions indicate the initial existence of a dense brine, probably of magmatic/carbonatitic origin as described by Smith and Henderson (2000). The presence of REE-carbonates as solid phase in fluid inclusions shows that the original ore-forming fluids are very rich in REE, and therefore, have the potential to produce economic REE ores at Bayan Obo (Fan et al., 2004). Coexistences of brine inclusion and CO2-rich inclusion with similar homogenization temperatures suggests that these fluid inclusions resulted either from the trapping of boiling fluids or represent two unmixing/ immiscible fluids of original H2O–CO2–NaCl fluids with higher REE contents. Carbonatite-related fluids have been shown to include alkali-chloride and alkali-carbonate brines (Samson et al., 1995) and aqueous carbonic fluids (Ting et al., 1994), all of which may be comparable to the fluids at Bayan Obo. The hypersaline brine inclusions contain different numbers and types of solid phases at Bayan Obo. It is unclear whether the early hypersaline brines are pristine fluids, e.g. derived from carbonatite melts or result from interaction with wall rock. In other words, whether the external fluid composition was essentially constant during evolution is still an open question. The understanding of the origin and causes of compositional variation of the primary fluids with speculative REE-rich are underconstrained at present. Further work using LA-ICP-MS microanalyses on fluid inclusions will be done to decipher immiscible fluids related to this giant deposit. Acknowledgements Special thanks are due to the management and staff of the Bayan Obo mine for their hospitality during the fieldwork. Prof. Andrew Rankin and Dr. Martin Smith are acknowledged for their reviews and valuable
Bodnar, R.J., 1993. Revised equation and stable for determining the freezing point depression of H2O–NaCl solutions. Geochim. Cosmochim. Acta 57, 683–684. Brown, P.E., Hagemann, S.G., 1995. MacFlincor and its application to fluids in Archaean lode–gold deposits. Geochim. Cosmochim. Acta 59, 3943–3952. Chao, E.C.T., Back, J.M., Minkin, J.A., Tatsumoto, M., Wang, J., Conrad, J.E., MaKee, E.H., Hou, Z., Meng, Q., Huang, S., 1997. The sedimentary carbonate-hosted giant Bayan Obo REE–Fe–Nb ore deposit of Inner Mongolia, China: a cornerstone example for giant polymetallic ore deposits of hydrothermal original. U.S. Geol. Surv. Bull. 2143, 1–65. Fan, H.R., Xie, Y.H., Wang, K.Y., Tao, K.J., Wilde, S.A., 2004. REE daughter minerals trapped in fluid inclusions in the giant Bayan Obo REE–Nb–Fe deposit, Inner Mongolia, China. Int. Geol. Rev. 46, 638–645. Hao, Z.G., Wang, X.B., Li, Z., Xiao, G.W., Zhang, T.R., 2002. Bayan Obo carbonatite REE–Nb–Fe deposit: a rare example of Neoproterozoic lithogeny and metallogeny of a damaged volcanic edifice. Acta Geol. Sin. 76, 525–540 (in Chinese with English abstract). Le Bas, M.J., Spiro, B., Yang, X., 1997. Oxygen, carbon and strontium isotope study of the carbonatitic dolomite host of the Bayan Obo Fe–Nb–REE deposit, Inner Mongolia, N. China. Mineral. Mag. 61, 531–541. Meng, Q., 1982. The genesis of the host rock dolomite of Bayan Obo iron ore deposits and the analysis of its sedimentary environment. Geol. Rev. 28, 481–489 (in Chinese with English abstract). Samson, I.M., Liu, W., Williams-Jones, A.E., 1995. The nature of orthomagmatic hydrothermal fluids in the Oka carbonatite, Quebec, Canada: evidence from fluid inclusions. Geochim. Cosmochim. Acta 59, 1963–1977. Smith, M.P., Henderson, P., 2000. Preliminary fluid inclusion constraints on fluid evolution in the Bayan Obo Fe–REE–Nb deposit, Inner Mongolia, China. Econ. Geol. 95, 1371–1388. Ting, W., Burke, E.A.J., Rankin, A.H., Woolley, A.R., 1994. Characterisation and petrogenetic significance of CO2, H2O and CH4 fluid inclusions in apatite from the Sukulu carbonatite, Uganda. Eur. J. Mineral. 6, 787–803. Wang, Z., Li, S., 1973. The genetic features of the REE–Fe ore deposit related to sedimentation–metamorphism–hydrothermal metasomatism. Geochimica 1, 5–11 (in Chinese). Wang, J., Tatsumoto, M., Li, X., Premo, W.R., Chao, E.C.T., 1994. A precise 232Th–208Pb chronology of fine grained monazite: age of the Bayan Obo REE–Fe–Nb ore deposit, China. Geochim. Cosmochim. Acta 58, 3155–3169. Yang, X.M., Yang, X.Y., Zheng, Y.F., Le Bas, M.J., 2003. A rare earth element-rich carbonatite dyke at Bayan Obo, Inner Mongolia, North China. Mineral. Petrol. 78, 93–111. Yuan, Z., Bai, G., Wu, C., Zhang, Z., Ye, X., 1992. Geological features and genesis of the Bayan Obo REE ore deposit, Inner Mongolia, China. Appl. Geochem. 7, 429–442.