Accepted Manuscript Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation Shang Liu, Hong-Rui Fan, Kui-Feng Yang, Fang-Fang Hu, Kai-Yi Wang, FuKun Chen, Yue-Heng Yang, Zhan-Feng Yang, Qi-Wei Wang PII: DOI: Reference:
S0301-9268(17)30699-X https://doi.org/10.1016/j.precamres.2018.04.021 PRECAM 5071
To appear in:
Precambrian Research
Received Date: Revised Date: Accepted Date:
15 December 2017 20 April 2018 23 April 2018
Please cite this article as: S. Liu, H-R. Fan, K-F. Yang, F-F. Hu, K-Y. Wang, F-K. Chen, Y-H. Yang, Z-F. Yang, Q-W. Wang, Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation, Precambrian Research (2018), doi: https://doi.org/10.1016/j.precamres.2018.04.021
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Mesoproterozoic and Paleozoic hydrothermal metasomatism in the giant Bayan Obo REE-Nb-Fe deposit: constrains from trace elements and Sr-Nd isotope of fluorite and preliminary thermodynamic calculation Shang Liua,b, Hong-Rui Fana,b,c*, Kui-Feng Yanga,b,c, Fang-Fang Hua,b,c, Kai-Yi Wanga, Fu-Kun Chend, Yue-Heng Yanga,b,c, Zhan-Feng Yange, Qi-Wei Wange
a
Key Laboratory of Mineral Resources, Institute of Geology and
Geophysics, Chinese Academy of Sciences, Beijing 100029, China b
College of Earth and Planetary Sciences, University of Chinese
Academy of Sciences, Beijing 100049, China c
Institutions of Earth Science, Chinese Academy of Sciences, Beijing
100029, China d
Key Laboratory of Crust-Mantle Materials and Environments, School of
Earth and Space Sciences, University of Science and Technology of China, Hefei 230026, China e
Baotou Research Institute of Rare Earth, Baotou 014030, China
*Corresponding author: E–mail:
[email protected], Address: as above; Phone: (+86) 10 82998218; Fax: (+86) 10 62010846
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Abstract Fluorite of various occurrences could portray the hydrothermal metasomatism occurred in the giant Bayan Obo REE-Nb-Fe ore deposit. Two
Mesoproterozoic and
Paleozoic hydrothermal
events
with
distinguished fluid origin and geochemical properties have been studied. In Mesoproterozoic, discrete fluorite from the dolomite-dominating REE-Nb ores and banded fluorite aggregates from the banded REE-Nb-Fe ores are extremely enriched in LREE and depleted in HFSE compared with the primitive mantle. In Paleozoic, fluorite from veined-breccia REE-Nb ores varies significantly in trace element components, ranging from LREE-enriched HFSE-depleted fluorite to fluorite with “flat” REE pattern and extremely depletion of Zr. The veined fluorites have variable and higher radiogenic Sr isotopic components (initial 87Sr/86Sr ratios: 0.703464-0.708208) than the discrete and
banded
fluorite
(0.703183-0.705047),
indicating
crustal
contamination in the Paleozoic hydrothermal fluids from Precambrian basement rocks and Bayan Obo Group. Fluorites of different occurrences and associated bastnäsite and monazite have similar Nd isotopic components and Nd evolution history, which is mostly consistent with whole rock of the ore-hosting dolomite. The Sm-Nd system of some fluorite and bastnäsite were reset during the Paleozoic alteration triggered by the closure of Paleo-Asian Ocean. In such geological setting,
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according to the δ34S of the dominating sulfur-bearing mineral, Paleozoic veined sulfides (~0±3‰), the prime sulfur source was oceanic crust/MORB. Relatively high δ34S of barite (~13-14‰) was caused by fractionation of sulfur isotopes at an equilibrium temperature of 400-430 °C, rather than contribution from ancient seawater. The trace element, Sr-Nd isotope of fluorite and sulfur isotope characters of sulfides indicate carbonatitic origin of the Mesoproterozoic hydrothermal fluids, while the Paleozoic fluids, derived from subducted Paleo-Asian oceanic crust, carried sulfur components from MORB, scavenged high radiogenic Sr components from Precambrian basement rocks or Bayan Obo group, may leached a deep-seated carbonatite stock and finally altered the Mesoproterozoic mineralized ore-hosting dolomite. Both the Mesoproterozoic and Paleozoic hydrothermal fluids had initial temperature of ~400–500 °C, and gangue minerals (fluorite, sulfide and sulfate) precipitated at the temperature of ~320–440 °C. Preliminary thermodynamic calculations of the REE-F-C-Ca system constrained that the pressure of both fluids would not exceed 2 kbar. The drop of temperature and increase of pH are two critical factors to REE mineralization. Paragenesis of Fe-bearing minerals indicates an increase of fO2 in the Mesoproterozoic hydrothermal fluids and a drop of fS2 in the Paleozoic fluids in the Fe-O-S system. Furthermore, in the Ca-F-C-O system, the Mesoproterozoic hydrothermal fluids evolved with increasing
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aCa2+ and decreasing aF-, while the Paleozoic fluids present characteristic of mixing origin again with significantly variable aCa2+ and aF-. Key words: Hydrothermal; Fluorite; Geochemistry, Thermodynamic; Bayan Obo REE-Nb-Fe deposit
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1. Introduction Fluorite is one of the most common hydrothermal minerals in global REE-(Nb) deposits, especially in the carbonatite-related REE deposits (Temple et al., 1965; Wall et al., 1995; Andradeab et al., 1999; Groves et al., 2001; Wang et al., 2001). Considering its close association with REE-bearing minerals, fluorite has been conventionally regarded as resultant or by-product of REE mineralization processes (Williams-Jones et al., 2000; Gültekin et al., 2003). The role of fluorine that has played in the transport and deposition of REE in the hydrothermal fluids has been discussed and affirmed (Richardson and Holland, 1979; Migdisov et al., 2007, 2009, 2014; Williams-Jones et al., 2012). Therefore, fluorite has its inherent advantage in understanding the REE mineralization process and the properties of the ore-forming fluids. The Bayan Obo REE-Nb-Fe deposit is the largest REE deposit in the world currently, with REE reserves over 57.4 million metric tons (Mt) (average grade 6% of REE oxides), Nb reserves of 2.16 Mt (average grade 0.13% of Nb2O3) and Fe reserves of at least 1500 Mt (average grade 35% of iron oxides) (Drew et al., 1990; Hao et al., 2002; Fan et al., 2016). The Bayan Obo REE-Nb-Fe deposit is hosted in a dolomite unit, whose origin has been controversial. Abundant research on geochemistry, geochronology and paleontology has been carried out, in order to
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distinguish the origin of the ore-hosting dolomite among hypotheses including 1) carbonatite intrusion (Liu et al., 1985; Yuan et al., 1992; Le Bas et al., 1997; Hao et al., 2002; Wang et al., 2002; Yang et al., 2004; Xu et al., 2008; Yang et al., 2011; Sun et al., 2012, 2013), 2) volcanic exhalative-sedimentary carbonatite (Bai et al., 1996; Xiao et al., 2003, 2012) or 3) a sedimentary carbonate formation (Meng et al., 1982, 1992; Liang et al., 1993; Wei et al., 1994; Qiao et al., 1997; Yang et al., 2009; Zhang et al., 2012; Lai et al., 2012, 2016). In despite of these divergence, it has long been accepted that the hydrothermal processes have made significant and indispensable contribution to the REE mineralization at Bayan Obo (Chao et al., 1991; Campbell and Henderson, 1997; Smith et al., 1999; Xu et al., 2012). Extensive alteration to the ore-hosting dolomite was presumed to be multi-stages (Chao et al., 1991; Fan et al., 2016). There are mainly two alteration events occurred in Bayan Obo that were related to the REE enrichment process, a Mesoproterozoic alteration event and a Paleozoic one (Campbell et al., 2014; Fan et al., 2016), supported by geochronological of hydrothermal minerals and their associated REE-bearing minerals (Ren et al., 1994; Liu et al., 2004; Hu et al., 2009; Zhu et al., 2015). The Mesoproterozoic hydrothermal event is characterized by the formation of discrete hydrothermal minerals in dolomite-dominating ores and banded ores. Discrete hydrothermal fluorite, monazite and apatite in
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the dolomite-dominating ores presented a Sm-Nd isochron age of 1250±210 Ma (Zhang et al., 2001). Other Mesoproterozoic dating results of the dolomite-dominating ores include a deduced La-Ba isochrone age of monazite (1350±149 Ma) (Nakai et al., 1989), Sm-Nd isochron age of discrete monazite and bastnäsite (1313±41 Ma) (Ren et al., 1994), Th-Pb isochron age of discrete monazite (1231±200 Ma) (Liu et al., 2005). Meanwhile, hydrothermal monazite and parisite grains from the banded REE-Fe ores present Sr-Nd isochron age of 1700±480 Ma (Cao et al., 1994). Indirect dating of the Mesoproterozoic hydrothermal event concluded from fenitization around Wu carbonatite dyke, which occurred instantly after the intrusion of carbonatite at 1354±59 Ma (Sm-Nd dating of whole rock) (Yang et al., 2011). Our former study has proved that the aegirine-rich and Na-amphibole-rich massive REE-Fe ores were product of fenitization similar to that around carbonatite dykes (Liu et al., 2018). A Mesoproterozoic hydrothermal event was also recorded by a Na-amphibole vein in wallrock of the ore-hosting dolomite which presented
40
Ar-39Ar age of 1.26 Ga (Conrad and Mckee, 1992). In
addition, a heat event was recorded by zircon in the ore-hosting dolomite with Th-Pb age of 1301±12 Ma (Zhang et al., 2017). There were two heat events recorded by another discrete zircon from one banded REE ore sample, which has a Th-Pb age of 1325±60 Ma at the core and 455.6±28.3 Ma at the rim (Campbell et al., 2014).
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The Paleozoic hydrothermal event is characterized by ores consisting of veins, blocks or mega-crystals of hydrothermal fluorite, aegirine, pyrite, barite, Na-amphiboles and REE/Nb-bearing minerals (e.g. bastnäsite, huanghoite, pyrochlore). The Th-Pb dating of huanghoite and pyrochlore monominerals from aegirine-rich veins gave an isochron age of 438 Ma (Chao et al., 1991). Meanwhile, the Sm-Nd isochron age of fluorite, aegirine, calcite and bastnäsite monominerals from veined-breccia type REE-Nb ores is approximate 440 Ma (Hu et al., 2009). The Re-Os dating of pyrite and molybdenite in the veined-breccia type REE-Nb ores is ~436 Ma (Liu et al., 1996; Liu et al., 2004). All above ages constrain a Paleozoic hydrothermal event distinct from the one caused by Mesoproterozoic ore-forming fluids. A subsequent Permian skarnization event presents in the contact zone between the Mesoproterozoic ore-hosting dolomite and Permian granitic plutons (Zhang et al., 2003; Fan et al., 2009). These magmatic activities destroyed the REE-Nb-Fe ore bodies where it had intruded. Therefore, the skarnization has been regarded as irrelevant to the REE mineralization and beyond concerns of this study. In most former studies, the Mesoproterozoic hydrothermal event was regarded as the major REE mineralization stage (Yang et al., 2011; Zhu et al., 2015; Wang et al., 2018). Besides, some other research paid attention to the contribution of Paleozoic hydrothermal fluids (Chao et al., 1992;
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Ling et al., 2013; Yang et al., 2017). Fluid inclusion studies support a carbonatitic origin of the ore-forming fluids (Smith and Henderson, 2000), and REE-fluorocarbonate-bearing fluid inclusions in fluorites from banded and veined type ores indicates the fluoritization occurred simultaneously with the REE mineralization (Fan et al., 2004). On the contrary, subduction released fluids were proposed as major ore-forming fluids according to oxygen isotope disequilibrium (Ling et al., 2013). In current studies, direct comparison between these two hydrothermal events is still insufficient, especially on their origin and geochemical characteristics. Fluorite is one of the ubiquitous hydrothermal minerals both in the Mesoproterozoic and the Silurian hydrothermal events (IGCAS, 1988). Nearly all hydrothermal silicates with hydroxyl in the ores, e.g. Na-amphiboles and fluorophlogopite, suffered varying degree of fluorine exchange (Xu et al., 2005). Other fluorine-rich minerals in the ores include apatite and fluorocarbonates, the primary REE-bearing minerals (Smith et al., 2007). All these hydrothermal minerals reveal that the ore-forming fluids are intrinsically REE-rich and F-enriched as well. In this study, fluorites of different occurrences formed in two alteration events were analyzed for their trace elements and Sr-Nd isotopic components. Bastnäsite and monazite associated with these fluorites were analyzed for Nd isotopic components. Besides, new sulfur isotope data of
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sulfide and barite associated with the Paleozoic fluorite were provided for further discussion on the systematic similarities and differences in origin and geochemical properties of the two hydrothermal fluids separated temporally by ~0.9 Ga. Preliminary thermodynamic calculations were conducted to explore the physico-chemical conditions and precipitation process of fluorocarbonates and fluorite in these two alteration events.
2. Geological setting The Bayan Obo REE-Nb-Fe deposit is located on the north margin of North China Craton (NCC), adjacent to the south margin of Central Asian Orogenic Belt (Fig. 1a, 1b). Like Xiong’er and Yanliao rift basin on the north margin of NCC, the Bayan Obo-Langshan rift basin is regarded as part of the rift system formed during the breakup of the Columbia Supercontinent (Zhao et al., 2004; Yang et al., 2012; Liu et al., 2015; Zhong et al., 2015). During the formation of the Bayan Obo rift, basement rocks of NNC, including Archaean gneiss and Paleoproterozoic TTG
are
uncomfortably
covered
by
a
set
of
Mesoproterozoic-Neoproterozoic terrigenous-volcanic sedimentary rocks, termed as Bayan Obo Group (Wang et al., 2001; Liu et al., 2011; Fan et al., 2010, 2014; Liu et al., 2017) (Fig. 1c). The Bayan Obo Group, mainly containing two sedimentary cycles of clastic sedimentary, shale-mudstone and dolomite-limestone, with small quantity of volcanics (600℃ but REE minerals precipitate at 227-453℃ (Xie et al., 2009). The formation temperature of the veined fluorite could be estimated using the equilibrium temperature of sulfur isotopes of associated sulfide and sulfate in the veined REE-Nb ores, following equations (1) and (2):
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(1) (2) Fractionation factors used in these equations were proposed by Ohmoto and Rye (1979) and Ohmoto and Lasaga (1982). Associated pyrite and barite in sample 16BY-36 and 16BY-78 have equilibrium temperature of sulfur isotopes around 435 and 418 °C, respectively. Associated barite and galena in sample 16BY78 gave equilibrium temperature of ~429 °C. Fan et al. (2004) observed REE fluorocarbonate-bearing fluid inclusions in the vein ores, which have Th of 420–480 °C and fluorocarbonate precipitated at 400–320 °C during cooling. Qin et al. (2007) heated the CO2±H2O±carbonate inclusions in the veined fluorite, which have Th of 324-350 °C. In the study of IGGCAS (1988), the barite from veined ores contain two types of fluid inclusions: H2O-rich inclusions (Th: 253–361 °C) and CO2-rich inclusions (Th: 339–421 °C). As for the dolomite-calcite thermometry calculations, Wang et al. (2010) proposed that carbonates in the ore-hosting dolomite altered by the Paleozoic fluids present equilibrium temperature of 432–507 °C. Therefore, we refer the initial Paleozoic fluid has temperature of ~400–500 °C, and the veined fluorite, sulfide and barite precipitated at ~320–440 °C. In spite of divergent origins, the Mesoproterozoic and Paleozoic hydrothermal fluids have similar initial temperature, and fluorite in the dolomite-dominating ores, banded ores and veined ores precipitated at similar temperature
P a g e | 36
during the hydrothermal alteration to the ore-hosting dolomite. The Fe-O-S system is sensitive to the even slight change of fO2 and fS2 in the hydrothermal fluids (Fig. 9). In fluorite-rich massive Fe ores, the hematite was observed developing along the cleavage of magnetite crystals under the reflection light (Fig. 3f). Sample massive Fe ores present no sign of alteration from the meteoric water. Hydrothermal fluids derived from Permian granite intrusions are not likely to be responsible for the formation of hematite, because intense skarnization was only observed in the east contact zone between the ore-hosting dolomite and granite stocks, rather than in the East and Main open pits. Caledonian fluids forming the veined ores are not likely to be responsible for the formation of hematite in our sample, because pyrite, galena and pyrrhotite veins or stockwork were not observed in our sample. Recent in situ study on Fe-oxide of Bayan Obo concluded that the associated hematite and magnetite in the massive ores had magmatic-hydrothermal origin, with similar REE patterns and trace element characteristics (Huang et al., 2015). Therefore, we believe the associated magnetite and hematite in the fluorite-rich massive Fe ores indicate an increase of fO2 during the evolution of the Mesoproterozoic fluids. While in the Paleozoic formed veined ores, fluorite vein or blocks are always associated with pyrite, pyrrhotite, galena and barite. In several samples, pyrrhotite cements euhedral pyrite crystals or grows surrounding the euhedral pyrite crystals
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(Fig. 3h). This phenomenon indicates a drop of fS2 from the precipitation of pyrite to pyrrhotite. The barite associated with pyrite in the veined ores constrains the Paleozoic hydrothermal fluids to higher fO2 region in the sulfide domain. 6.3 Preliminary thermodynamic constrains on the precipitation of fluorocarbonates and fluorite Since the fluorite and REE-bearing minerals are cogenetic in rare earth elements, with the understanding of temperature of Mesoproterozoic and Paleozoic hydrothermal alteration, we could discuss the deposition process and conditions by preliminary thermodynamic calculations. Williams-Jones et al. (2012) and Migdisov et al. (2014) proposed that the rare earth elements could deposit in acid fluids with increasing pH. At 1000 bar, with the increasing of pH, Nd begins to deposit in the form of NaF3 (s) at the pH of 2.0, 2.7, 3.5 when the temperature is 200 °C, 300 °C and
400
°C,
respectively.
However,
considering
the lack
of
thermodynamic properties of fluorocarbonate then, the alternative deposition forms of REE, bastnäsite and parisite, were excluded from the calculations. Not until in recent study of Migdisov et al. (2016), it was proved that REE could deposit in the form of fluocarbonate in acid aqueous environment. In this section, our calculation will focus on the deposition process of fluorocarbonate, fluocerite and fluorite at different
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temperature (300 °C, 500 °C) and pressure (1kbar, 2kbar). Since two hydrothermal events have similar temperature and mineral associations in the REE-F-C-Ca system, this calculation provides an integrate discussion. And this calculation is just a preliminary research because there has been no quantitative record of major or trace components of primary fluid inclusions in fluorite, barite or other gangue minerals that were formed during two hydrothermal events, except salinity (NaCl wt%). The initial components of the modeled fluid were set as following. According to the microthermometry study of fluid inclusions by Smith et al. (2000), Fan et al., (2004) and Qin et al. (2007), the salinity of initial fluids is chosen as 10 wt% NaCl equivalent, which is representative for fluid inclusions in dolomite
(4.6
–
15.4
wt%
for
Qin
et
al.
(2007))
from
dolomite-dominating ores, apatite from banded ores (6–10 wt% for Smith et al. (2000)), and fluorite and barite from veined ores (5–15 wt% for Fan et al., (2004)). To set the concentration of HF and Ce3+, we referred to component of carbonatitic fluid (Bühn et al., 2002) and fluid inclusion in Captain Mountains REE ores (Banks et al., 1994), which were also cited in the calculation of Migdisov et al. (2016). Our calculation was conducted in an initial fluid with 500 ppm HF (aq) and 240 ppm Ce. The initial pH of the modeled fluid is 2. Migdisov et al. (2016) designed a deposition model in an acid REE-F-rich fluid that was neutralized by gradual addition of dolomite. This model
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was modified in this study to discuss the deposition process of Ce-fluorocarbonate (bastnäsite, parisite), Ce-fluoride (fluocerite) and fluorite under different temperature and pressure conditions (Fig. 10). Standard thermodynamic properties of bastnäsite-Ce and parisite-Ce by experiment are from Gysi and Williams-Jones (2015). Data of REE fluorides (CeF2-, CeF2-, CeF3) and chloride (CeCl2-, CeCl2-) are from Migdisov et al. (2009). Data of REE oxide and hydroxide (Ce(OH)2-, Ce(OH)3) are from Haas et al. (1995). Standard thermodynamic properties of other aqueous species (Ce3+, H+ CO32-, HCO3-, CO2 (aq)) and mineral (fluorite) were from the updated database of SUPCRT 92 (Johnson et al., 1992). Detailed standard thermodynamic properties have been summarized in supplementary table 1. Thermodynamic properties of all aqueous species and minerals mentioned above, as well as the solubility of fluocerite, bastnäsite, parisite and fluorite, were calculated to high temperature and pressure following the methods of Shock et al. (1989, 1992). Under specific temperature and pressure, with stepwise addition of dolomite, the concentration of all aqueous species at equilibrium in the modeled fluid were calculated by equations and the iteration method documented in Crerar (1975). At 300 °C and 1 kbar, with the addition of dolomite into the acid fluid, bastnäsite begins to deposit instantly the dolomite reacted with the fluid (Fig. 10a). Less than 30 mg of dolomite is needed in 1kg solution to
P a g e | 40
trigger the precipitation of bastnäsite, and fluorite has not deposit until addition of over 540 mg of dolomite. Once REEs are deposited in form of bastnäsite, there would be no parisite or fluocerite formed. Bastnäsite and fluorite begin to deposit at pH = ~2 and ~3, respectively. At 500 °C and 1 kbar, the REEs would still precipitate as bastnäsite, but the deposition could not initiate unless more than 430 mg dolomite reacting with the solution (Fig. 10b). Fluorite begins to deposit when 500 mg dolomite were neutralized in the solution. Bastnäsite and fluorite begin to deposit at pH = ~3 and ~3.3, respectively. At 300 °C and 2 kbar, REEs would precipitate in form of fluocerite, which means no bastnäsite or parisite would appear in the solution during the whole reaction (Fig. 10c). The fluocerite will deposit instantly when dolomite is added to the solution, while the precipitation of fluorite would consume over 760 mg dolomite. Both fluocerite and fluorite begin to deposit at pH
