Fluorite REE characteristics of the Diyanqinamu Mo deposit，Inner Mongolia，China

Long Jian·Hairui Sun·Jian'guo Gao

Fluorite REE characteristics of the Diyanqinamu Mo deposit，Inner Mongolia，China

Long Jian1·Hairui Sun2，3，4，5·Jian'guo Gao1

The Diyanqinamu Mo deposit，a newly discovered porphyry deposit in the northern-central part of the Great Xing'an Range，Inner Mongolia，China，is characterized by widely distributed fluorite.It is important to note that almost all the fluorite that is paragenetic with molybdenite is purple.The Tb/Ca—Tb/La ratios of these purple fluorite samples show that they have a hydrothermal origin. The unidirectional solidification texture at the apex of the aplitic granite and the low F contents in the andesite suggest that most of F in fluorite was derived from granitic melts.These observations suggest that the fluorite was related to the magmatic-hydrothermal fluids.All the fluorite separates have consistent total REE contents with LREE-depleted，HREE-enriched，negative Eu anomaly，unapparent Ce anomaly and positive Y anomaly.These characteristics are significantly different than those of country granite，andesite and tuff whole-rock.The positive Y anomaly of the fluorite separates implies that the hydrothermal fluids migrated a long distance，as suggested by the fact that the fluorite-molybdenite veins were mostly hosted in andesite and tuff，far from the Mo ore-forming granites.The features of LREE-depleted and HREE-enriched fluorite are due to the REE-complex in the F-enriched fluids during migration.The stronger negative Eu anomaly of fluorite than those of country rocks suggests that the Eu anomaly of the original hydrothermal fluid was enhanced by the high temperature（generally above 200 or 250°C）.The widespread magnetite in the studied deposit indicates that the magmatic-hydrothermal fluid was oxidized at early stage.On the other hand，the pyrite was also paragenetic，with the molybdenite and unapparent Ce anomaly implying that the hydrothermal fluid probably experienced oxygen fugacity decreasing during migration，which is important for Mo mineralization.

Fluorite·Rare earth elements·Geological implications·Diyanqinamu Mo deposit·Inner Mongolia

1 Introduction

The Diyanqinamu Mo deposit is located in the northerncentral part of the Great Xing'an Range，Inner Mongolia，China（Fig.1a）（Nie and Hou 2010；Shao et al.2011；Yan et al.2012；Sun et al.2014）.Ore bodies in this deposit are not hosted by porphyry，which puts the ore genesis of the Diyanqinamu Mo deposit into debate.Based on the geological features，geochronology，mineral geochemistry and Sr—Nd—Pb isotopic studies，Sun et al.（2015）and Leng et al.（2015）suggested that the Diyanqinamu deposit is a porphyry-related Mo deposit.Even so，the evolution process of hydrothermal fluids and the formation mechanism of the Diyanqinamu Mo deposit is still unclear.

Fig.1 Simplified regional geological map（modified after Shen et al.2012；Sun et al.2014）.1 Quaternary，2 Cretaceous，3 Jurassic，4 Permian，5 carboniferous，6 devonian，7 silurian，8 Ordovician，9 Pleistocene basalts，10 mMesozoic granites，11 Paleozoic granites，12 suture zone，13 proved or inferred fault，14 reverse fault，15 normal fault，16 transitional fault，F1 Erlian-Hegenshan，F2 Derbugan，F3 Xilamulun river，F4 North margin of North China Craton，F5 Dong-Ujimqin banner-YiheShabaer，F6 Baiyinhubuer-Mandubaolage，F7 Barunshabaer-northern Chaobuleng，F8 Chaobuleng-Wulagai，F9 Bayanmaodu ductile shear zone

Previous studies have shown that fluorite strongly enriches rare earth elements（REE）（Sallet et al.2005）and is widely distributed with ore minerals in some deposits（Hill et al.2000；Richardson and Holland 1979）.Fluorite REE is an important tool for tracing the source of ore-forming fluids and its mineralization processes（Bau 1991；Bau and Dulski 1995；Gagnon et al.2003；Richardson and Holland 1979；Smith et al.2000；Schwinn and Markl 2005）.This paper focuses on the fluorite REE features in order to determinethesourceandevolutionofore-forming hydrothermal fluids，and to understand the Mo mineralization mechanism in the Diyanqinamu Mo deposit.

2 Geology

2.1 Regional geology

The Diyanqinamu Mo deposit，which is about 120 km northeast of the town of Uliastai，Inner Mongolia，China，is located in the north central part of the Great Xing'an Range（Fig.1a）.The basement rocks are discontinuously exposed in this region，and mainly consist of the Paleozoic strata，which are uncomfortably overlain by the Mesozoic strata，whereas the Precambrian rocks are absent in the basements（Xue et al.2009）.The Paleozoic strata include the Ordovician，Silurian，Devonian，Carboniferous and Permian，and mainly consist of volcaniclastic rocks（Nie et al. 2007）.The Triassic strata not exposed in this region.The Jurassic strata are uncomfortably overlain by the Permian，which consists mostly of volcaniclastic rocks，intercalated mafic volcanic-sedimentary rocks.The Cretaceous rocks consist mainly of sedimentary rocks such as sandstone，mudstone and conglomerate with interlayers of lignite.The magmatic rocks in this region were generally aged from the Paleozoic to the Mesozoic（Fig.1b），particularly the Late Paleozoic and Early Mesozoic.These magmatic rocks are closely related to the poly-metallic deposits in the studied area（Hong et al.2003；Nie et al.2004）.The NE-and NNE-striking faults are developed and structurally controlled by the distribution of ore deposits（Fig.1b）.

2.2 Ore deposit geology

In the Diyanqinamu district，the hosting rocks consist mainly of the Late Jurassic Chagannuoer Formation volcanic and volcanoclastic rocks，which are uncomfortably overlain by the Middle Ordovician Hanwula Formation weakly metamorphic rocks.The Chagannuoer Formation rocks include andesite，rhyolite，dacite and tuff，and are exposed in the center of the Diyanqinamu deposit（Fig.2）. Most of Mo ore bodies are hosted by these rocks.Orehosting andesite has a zircon U—Pb age of 165±3 Ma（2σ，n=11，MSWD=1.8；Leng et al.2015）.Detailed lithological descriptions are available in Leng et al.（2015）.

Fig.2 Simplified geological map of the Diyanqinamu Mo deposit

The Diyanqinamu district was subjected to intensive tectonic and magmatic activity.Structures（include transcurrent，normal and thrust faults）in the mining area are predominantly NW-and NE-trending（Fig.2）.The intrusive bodies located in the southeast，including the porphyritic granites and the aplitic granites，were revealed by drill holes（ZK9701 and ZKp2104 and ZK4522）（Fig.2）. These granites intrude into the ore-hosting volcanic and volcaniclastic rocks when they are barren in Mo mineralization（Fig.3b，3c）.Detailed information about the granitic intrusions could be obtained from Sun et al.（2014）.Particularly，unidirectional solidification texture（UST）was observed at the apex of the aplitic granites and is an important record on the fluid exsolving from the granitic melt（Sun et al.2015）.

In the Diyanqinamu Mo deposit，Mo is the only economic metal，with a Mo metal reserve of 0.79 Mt（average grade：0.099%）.The ore bodies display a flattened circular shape in a plane with its long axis extending northeastward，and are present as branch recombination and thinning out at depth（Fig.3a，c）.Ore bodies are mainly located above the 400 m level，whereas some individual ore bodies may reach-250 m，as revealed by drill holes（Fig.3c）.The molybdenite Re—Os ages indicate that the timing of the Mo mineralization is at～156 Ma（Leng et al.2015），which is similar to the zircon U—Pb ages of the porphyry granite and aplitic granite（Sun et al.2014）.The Mo mineralization mainly occurs at veins with minor disseminated grains in the host rocks and filling in the fractures.The ore minerals are mainly pyrite and molybdenite，with minor chalcopyrite，galena，sphalerite，bismuthinite，arsenopyrite，scheelite and magnetite.The gangue minerals are quartz，feldspar，sericite，fluorite and calcite，etc.

The main wall rock alteration types in the Diyanqinamu Mo deposit are propylitic，phyllic and argillic alterations，and the propylitic alteration belt is developed around the major Mo ore bodies（Leng et al.2015）.In addition，fluorite has been found to be another important alteration，and shows a wide variety of colors including purple，green and white.However，it is necessary to note that the fluorite coexisting with the molybdenite are almost purple and always occurred as veins（Figs.4，5），which is extremely obvious in the no.33 exploration line（Fig.4）.In the fluorite-molybdeniteveins，fluoriteisparageneticwith molybdenite，K-feldspar，quartz，pyrite andscheelite（Figs.5，6）.Detailed information about the mineralization and alteration can be obtained in Sun et al.（2015）and Leng et al.（2015）.

Fig.3 Three-dimensional morphology of the ore body（A）and geological profile of the exploratory lines of 45 and 97（B and C）（after Leng et al.2015）.

3 Sampling and analytical methods

Eight purple fluorite samples were selected from drilling holes ZK3314 and ZK2505（Table 1）.All fluorite samples were crushed to 40—80 meshes and hand picked for separation under a binocular microscope.The separated fluorite grains were rinsed with distilled water.All of them were pulverized into granules smaller than 200 meshes in agate mortars to minimize potential contamination and were then analyzed for REE contents.The REE were determined using a PE Elan 6000 ICP-MS at the State Key Laboratory of Ore Deposit Geochemistry，Chinese Academy of Sciences.The powdered samples（50 mg）were dissolved in high-pressure Teflon bombs，using a HF+HNO3mixturefor 48 h at≈190°C（Qi and Gre´goire 2000）.Rh was used as an internal standard to monitor the signal drift during counting.The international standards GBPG-1，OU-6 and the Chinese National standards GSR-1 and GSR-3 were used for analytical quality control.The analytical precision was generally better than 5%for the REE.

Fig.4 Geological profile of no.33 exploratory line

4 Results

The REE and Y contents of the fluorite samples and statistical REE data of the country tuff，andesite and granites whole-rock in the Diyanqinamu deposit are listed in Table 1 and shown in Fig.7.

The major country rocks in the Diyanqinamu Mo deposit are andesite，tuff and granites.These country whole-rock samples showed different REE（including Y）chondritenormalized patterns from those of the fluorite.The granites whole-rock samples were relatively enriched in LREE，depletedinHREEwithnegativeEuanomalies.TheAndesite whole-rock samples displayed REE chondrite-normalized patterns similar to those of the granites，but showed contrasting Eu and Y anomalies.The Tuff whole-rock samples had a wide range ofPREE（including Y），with LREE-enriched，HREE-depleted and negative Eu anomalies.

Fig.5 Photos of fluoritemolybdenite vein in the Diyanqinamu deposit.Mo molybdenite，Fl fluorite，Sh scheelite，Sp sphlerite，Py pyrite.A—F thick and thin fluorite-molybdenite vein in tuff，G molybdenites are paragenetic with pyrite in fluorite vein，H molybdenite scattered in fluorite vein，I molybdenite are paragenetic with scheelite in fluorite vein

Fig.6 Photomicrographs of ore minerals.Mag magnetite，Ccp chalcopyrite，Py pyrite.A—B at early stage，molybdenites are paragenetic with magnetite，C—D at the late stage，molybdenites are paragenetic with pyrite

5 Discussion

Recent studies demonstrate that fluorite usually shows wide fractionations between LREE and HREE，with variable Eu，Ce，and Y anomalies，which is used for tracing the source and evolution of ore-forming fluids，and discussing the cause of REE fractionation and redox conditions in the hydrothermal fluids（Bau and Moller 1992；Bau and Dulski 1995；Bau et al.2003；Castorina et al.2008；Dill et al. 2011；Ehya 2012；Kempe et al.2002；Mo¨ller et al.1976；Sallet et al.2005；Schwinn and Markl 2005）.

According to the Tb/Ca—Tb/La diagram of fluorite（Mo¨ller et al.1976），all fluorite samples in this study are plotted into the hydrothermal field（Fig.8）.As important petrography evidence，unidirectional solidification structure（UST）in the aplitic granite implies that the granite has experienced magmatic fluid exsolving，which is similar to that of the typical porphyry deposits（Kirkham and Sinclair 1988；Shannon et al.1982；Yang et al.2008）.In general，andesite is characterized by having a low F content，implying that the andesite，as one of the important hosting rocks，is unable to provide sufficient F for the widespread fluorite in the Diyanqinamu Mo deposit. Based on the evidences of Sr，Nd，Pb and the apatite chemical composition（Sun et al.2015），previous studies thought that most of the F in fluorite was sourced from the granitic melt.Therefore，we propose that the purple fluorite-molybdenite veins were of magmatic hydrothermal origin and associated with the porphyry granites in the Diyanqinamu Mo deposit.

The chondrite-normalized REE（including Y）patterns show that the fluorite samples were LREE-depleted and HREE-enriched（Fig.7A）.Generally，the causes of LREE fractionations can be explained using the dissolution of phosphate minerals（Hannigan and Sholkovitz 2001），fluids mixes（Aubert et al.2001）and/or complications during fluid migration（Bau 1991；Sallet et al.2005）.However，the HREE-enriched strata in the Diyanqinamu Mo deposit werw absent（Fig.7b，c，d）.Therefore，it is difficult to attribute the HREE-enriched feature to the dissolution of phosphate minerals.Fluids mix is common in porphyry deposits（e.g.，Selby et al.2000），and meteoric water has extremely（LREE）UCC-depleted patterns（Aubert et al. 2001；Sallet et al.2005）.Therefore，fluids mix could not be a main factor in the formation of LREE-depleted and HREE-enriched for fluorite.In addition，the REE-complexation prevailing over the REE-adsorption for the mineralizing solutions is also available to explain the characteristic of the HREE-enriched in fluorite.In alkaline fluids with carbonate species and/or F as complexing ligands，the HREE are enriched in solution and the REE patterns show a（La/Lu）nratio＜1（Schwinn and Markl 2005）.As noted above，these fluorites were sampled from the fluorite-molybdenite veins，so it is reasonable to conclude that an F-enriched hydrothermal fluid could be able to bring about the distinct pattern of fluorite compared to that of the major country rocks in the Diyanqinamu Mo deposit.The REE（including Y）patterns between the fluorites and hosting rocks suggest that the Y and Ho fractionation is not a source-related phenomenon but dependson the fluid compositions（Bau and Dulski 1995）.In general，fluorite derived from and deposited near igneous rocks apparently displayed Y/Ho ratios close to those of igneous rocks（Bau and Dulski 1995）.The intensely positive Y anomaly in the fluorite（Fig.7）indicates that the fluoriteforming fluid has experienced a long migration distance. This opinion is consistent with the fact that the fluoritemolybdenite veins are of a magmatic hydrothermal origin， although they were mainly concentrated in the andesite and tuff and far away from the granites（Sun et al.2015）.It is possible that the hydrothermal fluid in the Diyanqinamu deposit probably experienced fluids mix，but the unique chondrite-normalized REE patterns of the studied fluorite mostly depended on the REE-complexation and a long migration distance.

Fig.7 Chondrite-nomalized REE（include Y）patterns of fluorite and the country whole-rock（Rudnick and Gao 2003）

Fig.8 Tb/Ca—Tb/La relationships for different genetic fluorite（After Mo¨ller et al.1976）

Figure 7 also shows that the fluorite samples are consistent by displaying an almost weak Ce anomaly and a distinctly negative Eu anomaly showing that the fluorite is paragenetic with neither magnetite nor pyrite.Although the Eu anomaly is controlled by the chemical and redox conditions，in some cases（Castorina et al.2008），the Eu3+/Eu2+redox potential of the hydrothermal fluids strongly depends on the temperature（Bau et al.2003；Schwinn and Markl 2005；Zhou et al.2011）.In general，the precipitation temperature of molybdenite in a porphyry Mo deposit exceeds 200°C（Clark 1972）.At T＞200°C（Schwinn and Markl 2005）or250°C（Bau1991；Wood1990），Eu3+isreducedto Eu2+，which prevents Eu from entering into the fluorite lattice，triggering the decoupling of the Eu from its trivalent REE neighbors，and eventually resulting in a negative anomaly in the fluorite due to a crystal controlledfractionation during precipitation（Bau 1991；Bau and Moller 1992；Mo¨ller et al.1998）.However，the Eu anomaly in the fluorite can also reflect the REE pattern of its parent fluid（e.g.Schwinn and Markl 2005）.This is in view of their magmatic origin and the stronger negative Eu anomaly of fluorite compared to that of granites，so we suggest that the fluorite crystallized at a temperature above 200 or 250°C from a primitive fluid with a negative Eu anomaly.

Based on the classifications according to the Fe2O3/FeO ratios（Wood1990），theMomineralization-relatedgranitesin the Diyanqinamu deposit were magnetite-series I-type granites（Sun et al.2014），indicating that these granites were oxidized.In addition，magnetite is paragenetic with molybdenite（Fig.5a，b），indicatingthatthemagmatichydrothermal fluidsassociatedwiththegraniteswerealsohighlyoxidizedat the early mineralization stage（Leng et al.2015；Sun et al. 2015）.Therefore，the absence of positiveCe anomalies inthe fluorite（Fig.7a and Table 1）and the scattered pyrite paragentic with molybdenite in the fluorite-molybdenite veins（Fig.5g）implies that the fluids have changed from an oxidized to a reduced condition，which is an important mechanism for the Mo mineralization in the Diyanqinamu deposit.

6 Conclusion

Based on new geological and mineral geochemical evidences，the ore-forming hydrothermal fluids in the Diyanqinamu deposit were sourced from F-enriched magmatic water and experienced a long migration distance.The studiedfluoriteformedattemperaturesabove200or250°Cfrom the magmatic fluids with a negative Eu anomaly.The hydrothermal fluids in the Diyanqinamu deposit have experienced a decreasing oxygen fugacity，which is an important mechanism for the Mo mineralization.

AcknowledgmentsThis study was financially supported by the 12th Five-Year Plan Project of the State Key Laboratory of Ore Deposit Geochemistry，Chinese Academy of Sciences（SKLODGZY125-02）and the National Natural Science Foundation of China（41272111）.The authors would like to thank Jincang Mining Company for providing assistance in fieldwork.We are very grateful to Jing Hu and Guangping Bao（from the Institute of Geochemistry，Chinese Academic of Sciences）for their instructions in the analytical process.Comments and suggestions by anonymous reviewers also greatly improved the quality of this paper.

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10.1007/s11631-015-0073-3

9 June 2014/Revised：24 July 2015/Accepted：10 September 2015/Published online：23 September 2015

✉ Hairui Sun

HaiRuiSun@126.com

1Faculty of Land Resource Engineering，Kunming University of Science and Technology，Kunming 650093，China

2Development and Research Center of China Geological Survey，Beijing 100037，China

3National Exploration and Development Planning Technical Guidance Center in Ministry of Land and Resorces，Beijing 100120，China

4China University of Geosciences，Beijing 100083，China

5State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，ChineseAcademy of Sciences，Guiyang 550081，China

©Science Press，Institute of Geochemistry，CAS and Springer-Verlag Berlin Heidelberg 2015