Yongbin Zhang·Qian Zhang·Yi Cai·Dapeng Wang·Kaiwen Li

Received：17 November 2014/Revised：12 March 2015/Accepted：9 June 2015/Published online：23 June 2015

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

The occurrence state of vanadium in the black shale-hosted vanadium deposits in Shangling of Guangxi Province，China

Yongbin Zhang1，2·Qian Zhang1·Yi Cai1，2·Dapeng Wang1·Kaiwen Li1

Received：17 November 2014/Revised：12 March 2015/Accepted：9 June 2015/Published online：23 June 2015

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

The Shangling vanadium deposit，which occurs intheLowerDevonianTangdingformationblackrockseries strata，has V2O5reserves of more than 1.5 million tons and prospectivereservesofmorethan2milliontons.Preliminary studies on the occurrence state of vanadium（V）in this deposit have been conducted by artificial heavy minerals concentrates，leaching experiments，scanning and transmission electron microscopy and X-ray powder diffraction. These experiments have revealed no independent vanadium mineral occurrences in the Shangling vanadium deposit and the percentages of water-soluble vanadium，hydrochloric acid soluble vanadium and HF soluble vanadium were 1.93%，21.42%and 76.47%，respectively.Based on our data and earlier research，we estimate that the valences state of V absorbed onto the surface of organic matter or clastic particles are+5 and+4，accounting for 10.00%and 13.35%of the total amount of V，respectively and the valences state of V that exist in the octahedral crystal lattice of authigenic illite include+3 and+4，accounting for 71.64%and 4.83%of the total amount of V，respectively. By calculating the correlation between the total organic carbon and V，we infer that after deposition and before entering the crystal lattice of illite，V occurs in the form of humate complex or is adsorbed by organic matter.About 4.24%of the Al is in the octahedral crystal lattice of illite，which was replaced bythevanadiumunderthe metallogenic environments of Shanglin.

Black rock series·Vanadium deposit·

1 Introduction

In China，black rock series type vanadium deposits are mainly distributed among eight provinces（i.e.，Zhejiang，Jiangxi，Anhui，Hubei，Hunan，Guangxi，Guizhou，and Shaanxi）with ages ranging from the Upper Sinian to the Permian.The largest number of deposits are from the Lower Cambrian；these studies are also the most detailed（Liu et al.2008；Yang et al.2009；Ye and Kong 2006；Bao et al.2002）.On the other hand，there are relatively few deposits occurring at other ages and these have received little attention（Liu 2011；Cao et al.2011）.At present，this type of deposits'utilization rate is relatively low and many deposits have not been effectively exploited，largely due to the potential for serious environmental pollution and high extraction cost.An important problem is that the occurrence state of V is unclear，therefore seriously inhibiting our understanding of the metallogenic origins of deposits of this type and the process design for V extraction and utilization.The Shangling vanadium deposit of Guangxi is a typical example of such deposits.

The Shangling vanadium deposit，located in Dafeng Town，Shangling County，Guangxi Province was discovered by the Guangxi Fourth Geologic Exploration Team in 1979，generally investigated from 1980 to 1986，and investigated in detailed depth from 2001 to 2003.Itsproven V2O5reserve is＞1.50 million tons and its prospective resource reserve is＞2.00 million tons.The deposit is the only large-scale vanadium deposit occurring in the Devonian black rock series in China.Because of environmental problems，cost and extraction rate issues，this deposit is currently in an‘‘off-production''state.Previous researchers pointed out that the utilization of V in this type depends on three aspects：the vanadium mineral composition，the occurrence state of V and the valence state of V（Yong et al.2007；Wang and Wang 2012；Xiao 2007）.Therefore，the occurrence state of V in the Shangling vanadium deposit needs to be identified in order to provide a theoretical basis for the exploitation and utilization of this deposit.Moreover，previous research studies of V in these types of deposits were restricted to only identifying the occurrence state.The emplacement mechanism of V was not discussed.In this paper the emplacement mechanism of V in the Shangling vanadium deposit will be discussed using the obtained data.

2 Geologic features of the deposit

2.1 Geologic background of the deposit

The ore district is located in the intersection between the western part of the South China Caledonian fold belt and the western wing of the Guangxi mountain-shaped structural frontal arc.The paleo-geographic study of this district by Meng et al.（2009）showed that，after the Caledonian orogeny and before the Devonian deposition，seawater slowly invaded the area from the northeastward Qinzhou trough.In the Early Devonian Lianhuashan period，seawater entered this district from the southwest direction，the hydrodynamic force was dominated by tides and the setting was onshore intertidal.From the Nagaoling to the Yujiang period，the sea area enlarged and the setting in the Late Yujiang period was a subtidal，semiclosed reducing one.In the Early Devonian Tangding period，the neritic subtidal zone facies formed in the Yujiang period changed itself into platform front slope facies and interplatform trough facies，and the Shangling vanadium deposit occurred in the interplatform trough facies.

2.2 Strata in the ore district

The outcropping strata are mainly from the Cambrian，the Devonian，the Carboniferous and the Quaternary with no strata from the Silurian or Jurassic（Fig.1）.Previous researchersredividedtheoriginalMiddleDevonian Yujiang formation（roughly equivalent to the Lower Emsian Stage on the international stratigraphic table）and MiddleDevonianDongganglingformation（roughly equivalent to the Givetian Stage on the international stratigraphic table）within the range of the ore district into the Lower Devonian Yujiang and Tangding formations，the Middle Devonian Nabiao and Luofu formations and the Upper Devonian Liujiang formation from old to new. There is conformable contact between the strata of various formations.Moreover，the Tangding formation is the main occurrence horizon of the vanadium deposit.

These formations have the following characteristics：

（1） The Yujiang formation（D1y）has a thickness of＞105.31 m.Its top is mainly a set of biodetritusbearing argillaceous limestone，with local siliceousbanded limestone and a thin layer of limestone sandwiched with mudstone.Its lower portion is calcic mudstone，mudstone and siliceous mudstone，etc.Fossils of brachiopods and pelecypoda occur throughout.

（2） The Tangding formation（D1t）has a thickness of 22.09—129.39 m.It is primarily a set of black rock series sandwiched with siliceous rock.The lithology is black mudstone，black silty mudstone，black pelitic siltstone，black siltstone and carbon-bearing siliceous rock，with star-like and lumpy pyrite.The V is mainly enriched in the first four rock series in the middle of this layer，and the carbon-bearing siliceous rock contains a very low V content.

（3） The Nabiao formation（D2n）has a thickness of 31.46—128.62 m.It is black mudstone，siliceous mudstone and calcic mudstone，with star-like and lumpy pyrite；its bottom portion has a lithology similar to that of the top of the Tangding formation，which is continuous deposition.

（4） The Luofu formation（D2l）has a thickness of 28.91—57.97 m.It is gray，gray yellow and yellow brown siliceous rock sandwiched with mudstone and siliceous shale.There are fewer amounts of carbon in this formation.

（5） The Liujiang formation（D3l）has a thickness of＞168.07 m and exhibits incomplete outcropping.Its bottom portion is siliceous rock sandwiched with siliceous shale，with one to three layers of manganiferous siliceous rock.

2.3 Structure of the ore district

The ore district is located on the northeast wing of the Damingshan anticline；except for the sub-first-order fold Bijiashan anticline，the other structures are undeveloped. The strata in the ore district have a strike between 300°and 330°in an eastward monocline，with dip angles generally being 20°—35°.The nose-shaped Bijiashan anticline occurs in the central part of the ore district；it has a short shaftshape and its planar shape looks like a‘‘nose''（Fig.1）. Within the range of the ore district，no regional fault structure is found，and small faults and interlayer crushed zones are developed only near the Bijiashan anticline.This is the section with the best V mineralization（Fig.1；ore body number 3）.

Fig.1 Position and ore-body distribution of the Shangling vanadium ore district

Furthermore，no geologic phenomena of contemporaneous magmatic or hydrothermal activity are found in the ore district and adjacent areas.

2.4 Features of the ore bodies

This deposit has seven controlled industrial ore bodies（Fig.1），whose overall length is 27.0 km；all of them have conformable contact with the strata，with consistent occurrence and stable horizon.The largest ore body，i.e.，ore body number 3，has a length of 14.0 km and a thickness of 1.0—40.0 m（averaging 15.0 m）and a V2O5content of 0.71%—1.81%（averaging 1.06%）.Ore body number 7 has a length of 3.5 km and a thickness of 6.5—25.0 m（averaging 14.0 m）；its maximum depth extent is 780.0 m，and its V2O5content averages 0.99%.In addition to the above 7 industrial ore bodies，there are more than 20 small ore bodies in the five layers，with grades ranging from 0.50%to 0.70%，located in the top or bottom of the industrial ore bodies.Their lengths are generally＜1.0 km and their thickness are several meters.The ore body in the northernmost end of the ore district continues to extend by＞3.0 km，and the reserve has not yet been calculated. There are still V-bearing black rock series outcropping intermittently within＞20.0 km with the ore body extending southward to Xinqiao，Binyang County，and a sampling analysis for V2O5content gives a value of＞0.70%. Therefore，it is estimated that the V resource quantity in this deposit may be far more than 200.0 million tons and the deposit is likely to reach the ultra-large scale.

As can be shown in the number 32 prospecting line profile map（Fig.1），the V ore bodies are laminar，quasilaminar，and lentoid，do not have evident boundary lines with the surrounding rock and extend downward to gradually pinch out.On the ground surface of Dafeng，ore body number 3 is divided into two layers.The upper layer is 5.0—9.0 m thick，with very low carbon content，and is gray white，gray yellow and gray black mudstone，with an average V2O5content of 1.42%.The lower layer is the main ore body，with an average thickness of 17.8 m，very high carbon content and an average V2O5grade of 1.06%. Other ore bodies do not exhibit such stratification.We inferthat the differences in the carbon content may be the result of late oxidation.

2.5 Lithology，texture，and structure of samples

The samples were mainly black mudstone，black pelitic siltstone，black silty mudstone，black siltstone and black siliceous rock.The ore textures were mainly pelitic，cryptocrystal—pelitic and microscopic scale—pelitic.The ore structures were mostly plicated and vesicular，with some oriented structure and biodetritus pelitic texture.The mineral particle size in the samples was very small，so the minerals（other than pyrite）could be distinguished by the naked eye.

3 Sample collection，preparation，and analysis methods

3.1 Sample collection

The samples in this study were collected from the Tangding formation stratum in two areas，i.e.，Dafeng，and Shibiao（as shown in Fig.1），and all of them were fresh samples collected with an uncovered surface weathered layer.Because the lithology changes little in the lower layer of Dafeng，the Dafeng sample was collected at equidistant intervals；however，the Shibiao sample was collected at lithological intervals.The collected samples were dried in an oven at 70°C for 48 h to remove moisture and then were stored in dry bags for later analysis.

3.2 Scanning electron microscopic analysis

Scanning electron microscopic analysis was performed usingaJSM-6460high-resolutionscanningelectron microscope（JEOL，USA），equipped with an Oxford Link ISIS energy dispersive spectrometer，at an operating voltage of 25 kV.The sample was prepared by using the section method：one takes a small piece of the flat section of the dry sample，which has not been treated in any way so as to avoid artificial influence on the texture and structure，uses a vacuum carbon sprayer to coat a layer of carbon film onto the sample observation surface（with a film thickness of～500 A°）and then places the prepared sample into the instrument for observation.The carbon（C）content in the energy spectrum composition analysis results with the scanning electron microscope will therefore be affected by the background value of C.Backscattered electron（BSE）images are sensitive to atomic number and relatively pure facies can be identified with BSE images（Meunier 1994），so all scanning electron microscope images given in this paper are BSE images.This part of the work was

completed at the State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences.

3.3 Transmission electron microscopic analysis

Transmission electron microscopy was performed using a JEM-2000FX-IIhigh-resolutiontransmissionelectron microscope（JEOL，USA），equipped with an EM-ASID20 type scanning imaging system and an Oxford Link ISIS energy dispersive spectrometer，at an operating voltage of 160.0 kV.The sample was prepared as follows：one crushes a dry sample to below 100 mesh using an agate grinding bowl，adds absolute ethyl alcohol，evenly disperses and suspends the crushed sample in the absolute ethyl alcohol using an ultrasonic oscillator and then wets a micro-sieve copper wire mesh specially designed for transmission electron microscopy with some of this ethyl alcohol solution；after the sample is air dried，it is placed into the instrument for observation.The copper（Cu）in the energy spectrum composition analysis results from the transmission electron microscope can therefore be regarded as a background value.This part of the work was completed at the State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences.

3.4 Leaching experiment

The leaching experiments and relevant analysis were all completed at the State Key Laboratory of Environmental Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences.The sample named Z14 was used for these experiments，because it was fresh and its carbon content was moderate，and 2.5 Kgs of the bulk sample were ground to 100—200 mesh for a follow-up study.The concentrations of V2O5，Al2O3and K2O were determined by atomic absorption spectrometry（AAS）.

3.4.1 Water leaching

Three samples with equal masses of 20 g were separated from the bulk sample and placed into three 250 mL beakers，each with 200 mL deionized water，and electromagnetically stirred for 3 h at 60°C，70°C，and 80°C，respectively.The solution in each beaker was transferred into a centrifuge tube and centrifuged at high velocity for 30 min，and the supernatant was the put into the 500 mL volumetric flask.The lower residue was washed with 100 mL of deionized water and centrifuged for 20 min. This step was repeated once again.Then，the washing solutions were merged into a 500 mL volumetric flask，and then the volume was fixed to 500 mL with deionized water.

Table 1 The content of TOC，V and Al2O3in whole-rock samples

3.4.2 HCl acid leaching

Six samples with equal masses of 20 g were separated from the bulk sample and placed into six 250 mL beakers with 200 mL of hydrochloric acid（HCl）at 10%，20%，30%，40%，50%and 60%，respectively，and the remaining steps were the same as those for water leaching.

3.4.3 HF acid leaching

Six samples with equal masses of 20 g were separated from bulk sample and placed into six 250 mL beakers with 200 mL of HF at 0.2%，1%，3%，5%，15%and 20%，respectively，and the remaining steps were the same as those for water leaching.

3.5 Analysis of V，Al3O2，K2O，and total organic carbon of whole rock

The element V in the whole rock was determined by using inductively coupled plasma mass spectroscopy.The Al2O3in the whole rock was determined by using the chemical acid—alkali quantification method；the K2O in the whole rock was determined by using AAS；the Al2O3and K2O contents in the illite resulted from the energy spectral composition analysis.This part of the test was completed at the State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences.

The total organic carbon（TOC）analysis was completed by using the carbon and sulfur analyzer at the Geochemical Test Department，Lanzhou Center for Oil and Gas Resources，Institute of Geology and Geophysics，Chinese Academy of Sciences.

3.6 X-ray powder diffraction

The X-ray powder diffraction（XRD）analysis was completed using a D/Max-2200 X-ray diffractometer（Rigaku，Japan）at the State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences.

4 Results and discussion

4.1 Occurrence state of V

The samples had small particles，reaching the micron level，and contained large amounts of organic matter（as shown in Table 1，with a TOC content of 8.16—19.10 wt%）.This made it relatively difficult for conventional microscopic identification，so we employed heavy minerals concentrate identification，XRD，leaching experiments，scanning electron and transmission electron microscopy and energy spectral composition analysis to determine the occurrence state of V in this deposit.

4.1.1 XRD analysis results

The sample was broken into equal parts with different particle sizes using 60，80，100 and 200 mesh sieves and then elutriated using the artificial elutriating method. Heavy concentrate separation and heavy concentrate identification were conducted for the elutriated sample parts，and suspect minerals were sorted for microscopic identification and XRD analysis.Our analysis indicated that the sample did not contain any independent V mineral. The XRD analysis results of a whole-rock sample（Table 2）revealed that the minerals with the highestcontentswerequartz（12.65%—80.89%）andillite（17.25%—53.75%），followedbymontmorillonite（～2.06%），kaolinite（～0.62%）andironmineral（1.30%—21.54%，mainly pyrite）.There was some calcite，dolomite，plagioclase and potash feldspar，but no independent V mineral was found.

Table 3 Water leaching experimental results of sample number Z14 at different temperatures

Table 4 Acid leaching experimental results of sample number Z14 at different HCl concentrations and at 25°C

4.1.2 Leaching experiment

The water leaching results at different temperatures（Table 3）indicate that，with increasing temperature，the leaching rate of V2O5changed very little（＜2%）.Although we need more data for water leaching，we suggest that the proportion of water-soluble vanadium（Vwater）in the sample is 1.93%（Table 3）.We infer that the V leached by water is slightly absorbed onto the surface of the organic matter or detrital mineral.

The acid leaching（HCl）results at different acid concentrations and a constant temperature（25°C）（Table 4）show that the leaching rate of V2O5is relatively low（≤23.35%），and it changes little with acidity（18.36%—23.35%）.We infer that the V leached by HCl is strongly absorbed onto the surface of organic matter or detrital mineral（e.g.，Wehrli and Stumm 1989；Emerson and Huested 1991），because HCl cannot dissolve the main V-bearing minerals in the sample，so V cannot be released in large quantities.In addition，when the concentration of acid increased from 50.0%to 60.0%，the change of leaching rate is small.It can be inferred that the proportion of HCl soluble vanadium is 21.42%（VHCl=Vabsorbed-Vwater）and the sum of the proportion of water soluble vanadium and HCl soluble vanadium is about 23.35%，whichrepresentstheV intheformofabsorption（Vabsorbed=VHCl+Vwater）.

The acid leaching（HF）results at different acid concentrations and at a constant temperature（25°C）（Table 5）show that the leaching rate of V2O5increases gradually with the increase in acid concentration，and that，when the acid concentration increases to 20.0%，the leaching rate reaches 99.82%（VHF）.We infer that the V leached by HF is in the crystal lattice of the silicate minerals，because HF can dissolve the clayish minerals in the sample to different extents and destroy the crystal lattice of mineral（Zhang et al.2001），so the V can be released in large quantities.As shown in Table 5，when the concentration of acid increased from 15.0%to 20.0%，the leaching rate of V2O5change was very small relative to the other data in Table 5，so we can infer that the proportion of vanadium in thecrystallatticeisabout76.47%

In conclusion，the proportion of water soluble vanadium，HCl soluble vanadium and vanadium in the crystal lattice are 1.93%（Vwater），21.42%（VHCl）and 76.47%（Vcrystal），respectively（Tables 3，4，5）.

Table 5 Acid leaching experimental results of sample number Z14 at different HF concentrations and at 25°C

Fig.2 Detrital mineral quartz under an electron microscope and energy spectral composition analysis results.a Detrital quartz with very high psephicity under a scanning electron microscope（BES，×2000），where the block dots on the quartz surface are energy spectral composition analysis dots.b Detrital quartz with very high psephicity under a transmission electron microscope（×20，000），where the white dots on the quartz surface are energy spectral composition analysis dots.c Energy spectral composition analysis results of quartz in panel a（voltage：25 kV）. d Energy spectral composition analysis results of quartz in panel b（voltage：169 kV）

4.1.3 Scanning electron microscopic，transmission electron microscopic，and energy spectral analysis results

The quartz in the sample observed under the scanning electron microscope（Fig.2a）and the transmission electron microscope（Fig.2b）has very small particles，reaching the micronlevel（0.4—5.0 μm），whichhaveveryhigh psephicity.Therefore，we can infer that the quartz is detrital mineral.The energy spectral composition analysis results（Fig.2c，d）show that there are only energy peaks of Si（Kα：1.74 keV）and O（Kα：0.52 keV）.No energy peak of V（Kα：4.95 keV）is found，so it is inferred that the quartz does not contain V.

Fig.3 Organic matter with different forms under a transmission electron microscope and energy spectral composition analysis results. a Cellular organic matter and quartz（×10，000），where the white dots on the organic matter surface are energy spectral composition analysis dots. b Energy spectral composition analysis results of organic matter in panel a（voltage：160 kV）.c Spherical organic matter and quartz（×60，000），where the white dots on the organic matter surface are energy spectral composition analysis dots.d Energy spectral composition analysis results of organic matter in panel c（voltage：160 kV）.e Flocculent organic matter and quartz（×30，000），where the white dots on the organic matter surface are energy spectral composition analysis dots.f Energy spectral composition analysis results of organic matter in panel e（voltage：160 kV）

The organic matter in the sample observed under a transmissionelectronmicroscopeexhibitedcellular（Fig.3a），spherical（Fig.3c）and flocculent（Fig.3e）forms. The energy spectral analysis results indicate that none of the organic matter in these three forms contains V（Fig.3b，d，f）；however，this can only indicate that there is no V complexed with the organic matter and cannot indicate that there is no V adsorbed by the organic matter，as the transmissionelectronmicroscopicanalysissample treatment process may cause the V adsorbed by organic matter to separate from the organic matter and enter the solution.

Through observation under a scanning electron microscopic and a transmission electron microscope，it is found that the there are two kinds of illite in the sample.One is flaky or platy illite，or illite with extremely fine crystal growing on the surface of other minerals（Fig.4a，c，e）；such illite with high euhedral degree，good crystal form andevident edges and corners may be authigenic illite.The other type is xenomorphic illite，exhibiting breaking，abrasion，corrosion and oriented arrangement（Fig.5a，c）；it may be detrital illite（e.g.Liu 1987），and the content of such illite in the sample is very small.It can be seen from the energy spectral analysis results that the detrital illite contain little V（Fig.5b，d），whereas the energy spectral graph of the authigenic illite has an obvious energy peak of V（Fig.4b，d，f）.This is one of the distinguishing marks between detrital illite and authigenic illite in this deposit and also means that V occurs in the authigenic illite.

Fig.4 V-bearing authigenic illite under electron microscope and energy spectral composition analysis results.a Authigenic illite，pyrite，and rutile with fine crystal structure under a scanning electron microscope（BES，×700），where the white dots on the illite surface are energy spectral composition analysis dots.b Energy spectral composition analysis results of authigenic illite in panel a（voltage：25 kV）.c Lamellar authigenic illite under a scanning electron microscope（BES，×1600），where the white dots on the illite surface are energy spectral composition analysis dots.d Energy spectral composition analysis results of authigenic illite in panel c（voltage：25 kV）.e Flaky authigenic illite under a transmission electron microscope（×40，000），where the white dots on the illite surface are energy spectral composition analysis dots. f Energy spectral composition analysis results of authigenic illite in panel e（voltage：160 kV）

Throughenergyspectralanalysis，itisfoundthatthereisV inilmeniteand goethite，with a content ofgenerally～0.5%. These minerals account for a very small content ratio in the sample，sotheyarenegligiblewithrespecttothetotalVinthe whole deposit.There is no V information detected in the pyrite，montmorillonite，kaolinite and potash feldspar，etc.

4.1.4 Relationships of V with Al2O3and K2O

In the HF acid leaching experiment，the leaching curves of V2O5，Al2O3and K2O are highly coupled（Fig.6），indicating that，during leaching，V2O5，Al2O3and K2O are separated from the crystal lattice of the mineral simultaneously and proportionally，and the main Al-and K-enriched mineral in the sample is illite（Table 2）.Therefore it can be inferred that the V primarily occurs in illite.

Through analysis of the correlation between Al2O3and V contents in whole rock（Fig.7a），it can be seen that V and Al2O3have a positive correlation.This also indicates that V occurs in illite，because the main Al-enriched mineral in the sample is illite.The discreteness of data in Fig.7a may imply that a portion of the V in the samples occurs in other forms（adsorbed by clay mineral or organic matter）in addition to occurring in illite.To verify thecorrectness of the correlation of samples in Fig.7a，we analyzed the V2O5and Al2O3contents in illite（Table 6）using the energy spectral analysis method.As shown in Fig.7b，the Al2O3and V2O5in illite also have similar positive correlations to the data of the whole rock.

Fig.5 Detrital illite under electron microscope and energy spectral composition analysis results.a Detrital illite with abrasive corrosion phenomenon under a transmission electron microscope（×20，000），where the white dots on the illite surface are energy spectral composition analysis dots. b Energy spectral composition analysis results of detrital illite in panel a（voltage：160 kV）. c Detrital illite with oriented arrangement phenomenon under a scanning electron microscope（BES，×1000），where the white dots on the illite surface are energy spectral composition analysis dots.d Energy spectral composition analysis results of detrital illite in panel c（voltage：25 kV）

Fig.6 Leaching rate curves of V2O5，Al2O3，and K2O in sample number Z14 in acid leaching at different HF acid concentrations

4.1.5 The valences of V in different occurrence state

The valences of V in nature are mainly+3，+4 and+5. According to previous studies（Shannon 1976），since the effective ionic radii of V3+（0.64 A°）is close to that of Fe3+（0.67 A°）and Al3+（0.57 A°），V3+almost does not form an independent mineral but mainly occurs in magnetite（Evans and Landergran 1975）and Al-bearing clay minerals（Peacor et al.2000）in the form of isomorphism or by combining with organic matter to form vanadyl porphyrin（Lewan and Maynard 1982）.V4+occurs among clay minerals in the form of adsorption（Emerson and Huested 1991），combines with organic matter to form a compound（Cheshire et al.1977；Perrin 1979）or occurs in the crystal lattice of the clay mineral（Maylotte et al.1981）.V5+generally occurs in oxidized water in the form of dissoluble vanadate or adsorbs onto the surface of the solid particles or organic matter（Emerson and Huested 1991）.In ligand field theory，V5+，V4+，and V3+have increasing ligand field stabilization energy on an octahedral crystal lattice（Ahrens 1952），so V3+can even more easily enter the octahedral crystal lattice position of the mineral under clay mineral recrystallization（Breit and Wanty 1991；Tosca et al.2010）during diagenesis.

Given the above，we infer that the valence of Vwaterand VHCl，which adsorbed by the solid particles or organic matter is mainly+5 and secondly is+4（Emerson and Huested 1991），the valences of Vcrystalinclude+3 and+4（Peacor et al.2000；Maylotte et al.1981）.Moreover，a previous study on V extraction from the Shangling vanadium ore using the black shale wet method（Xiao 2007）showed that the V in the sample has three valences，with their contents being 71.82%（V3+），18.18%（V4+），and 10.00%（V5+），respectively.Based on our data（Vwater：1.93%，VHCl：21.42%，Vcrystal：76.47%）and the results of previous studies（Xiao 2007），we estimate that theproportion of V that is V4+absorbed by particles or organic matter is 13.35%and the proportion of V4+in crystal lattice is 4.83%The proportion of V that is V3+in the crystal lattice of illite is 71.64%and the proportion of V5+absorbed by particles or organic matter is 10.00%of the total V.

Fig.7 Sample composition correlation curve diagrams.a Correlation curve diagram between Al2O3and V contents in whole-rock samples. b Correlation curve diagram between Al2O3and V2O5contents in authigenic illite.c Correlation curve diagram between TOC and V contents in whole-rock samples.d Correlation curve diagram between V and Vcrystal+Al×0.7778

Table 6 Al2O3and V2O5（wt%）contents in illite of sample

4.1.6 The occurrence states of V before entering the illite The TOC and V contents in whole rock have an evident correlation（r=0.89；Fig.7c），but the organic matter in the samples do not contain V.Based on this contradiction，we infer that before V enters the crystal lattice of illite，the high enrichment of V in the sediments has a close relationship with the organic matter（e.g.，Peacor et al.2000）. The metal—organic complexes of V mainly include two types：the humate complex and the tetrapyrrole（porphyrin）complex.The latter has high thermal stability，is resistant to strong acid and does not easily undergo ion exchange reactions（Barnes and Dorough 1950；Rosscup and Bowman 1968；Stott 2011）；therefore it can be retained under diagenesis and even under metamorphism.Due to the fact that the organic matter in the samples does not contain V（Fig.3b，d，f），we can infer that，before V enters the crystal lattice of illite，the V-enriched metal—organic complex is the humate complex.In addition，V could be absorbed by the organic matter（Emerson and Huested 1991）.With the sediments being separated from the bottom-layer water and entering the diagenesis stage，under the condition of the existence of H2S，the thermal evolution of organic matter drives the humate complex to decarboxylation（Pauli 1975），releasing V4+，which is reduced into V3+under the effects of H2S and the organic matter（Breit and Wanty 1991）.It then enters the more stable crystal lattice of the clay mineral.In other words，the organic matter plays a role in the enrichment of V.

4.2 Emplacement rate of V

Fromthediscussioninthesectionabove，weknowthattheV in the Shangling deposit replaces the Al3+in the illite to enter the octahedral crystal lattice，forming V-bearing illite（Peacor et al.2000；Wilson and Pittman 1977）. It is known that the chemical formula of illite isK0.75（Al1.75R）［Si3.5Al0.5O10］（OH）2，soweestimatedthatthe Al in the octahedral positions of normal illite accounted for 77.78%of total Al in the normal illite and the Al in the tetrahedron sites of the normal illite comprised 22.22%of the total Al in the normal illite；we regarded the value of Al+VcrystalasthetotalAlinthenormalillite.Basedonthis，thecontentofAlintheoctahedralpositionscanbecalculated according to the following formula：

As showed in Fig.7d，the V is highly positively correlated with the content of Al in the octahedral positions（r=0.93）and the slope of the line is 0.0424，which is identified as the replacement rate of V in the illite，and meansthatunderthemetallogenic environmentsof Shanglin，about 4.24%of the Al in the octahedral crystal lattice of illite was replaced by the vanadium.

5 Conclusions

（1） There are two occurrence states of V in the Shangling vanadium deposit，Guangxi：a.absorption on the surface of organic matter or clastic particles（23.35%）；b.existence in the octahedral crystal lattice of authigenic illite（76.47%）.

（2） The valences state of V absorbed on the surface of organic matter or clastic particles are+5 and+4，accounting for 10.00%and 13.35%of the total amount of V，respectively.The valences state of V existed in the octahedral crystal lattice of authigenic illite include+3 and+4，accounting for 71.64 and 4.83%of the total amount of V，respectively.In addition，the detrital illite in the samples contain little vanadium.

（3） Before entering the crystal lattice of illite，the V primarily occurs in the form of the humate complex or absorber by organic matter，so the organic matter play a role in the enrichment of V.Under the metallogenic environmentsofShanglin，about 4.24%of the Al in the octahedral crystal lattice of illite was replaced by the vanadium.

AcknowledgmentsWe are grateful to the State Key Laboratory of Ore Deposit Geochemistry，Chinese Academy of Sciences，for supporting this project.This study was also funded by the National Natural Science Foundation of China（Grant No.41372105）and the 12th Five-Year Plan Project of the State Key Laboratory of Ore Deposit Geochemistry，Chinese Academy of Sciences（SKLODGZY125-04）.We also thank the skilled staff at the Institute of Geochemistry，Chinese Academy of Sciences，and Gong Guohong，Liu Shirong，and Tang Yang for their assistance in XRF，TEM，and SEM. In addition，comments from Ye Lin greatly improved the manuscript.

Ahrens LH（1952）The use of ionization potentials.1.ionic radii of the elements［J］.Geochim Cosmochim Acta 2：155—169

Bao Z，Wan R，Bao J（2002）Vanadium deposits of biack shaie in upper Yangtze Platform［J］.Yunnan Geol 2：175—182（in Chinese with English abstract）

Barnes JW，Dorough GD（1950）Exchange and replacement reactions of alpha，beta，gamma，sigma-tetraphenyl-metalloporphins［J］. J Am Chem Soc 72：4045—4050

Breit GN，Wanty RB（1991）Vanadium accumulation in carbonaceous rocks—a review of geochemical controls during deposition and diagenesis［J］.Chem Geol 91：83—97

Cao W，F(xiàn)u Q，Zhong J，Yang T（2011）Ore potential of black shale series in Gufeng formation of Permian in Xuanen County，Hubei Province［J］.Resourc Environ Eng 25：304—309（in Chinese with English abstract）

Cheshire MV，Berrow ML，Goodman BA，Mundie CM（1977）Metal distribution and nature of some Cu，Mn and V complexes in Humic and Fulvic-acid fractions of soil organic-matter［J］. Geochim Cosmochim Acta 41：1131—1138

Emerson SR，Huested SS（1991）Ocean anoxia and the concentrations of molybdenum and vanadium in seawater［J］.Mar Chem 34：177—196

Evans HT，Landergran S（1975）Vanadium.In：Wedepohl KH（ed）Handbook of Geochemistry，vol 23.SpringerVerlag，Berlin

Lewan MD，Maynard JB（1982）Factors controlling enrichment of vanadium and nickel in the bitumen of organic sedimentary rocks［J］.Geochim Cosmochim Acta 46：2547—2560

Liu G（1987）Study Of The relationship between characteristics of clay minerals and depositional environments［J］.Acta Sedimentol Sin 1：48—57（in Chinese with English abstract）

Liu L（2011）Geological characteristics of Vanadium deposit in Yangou area with discussion on its genesis［J］.Glob Geol 30：186—191（in Chinese with English abstract）

Liu G，Yin H，F(xiàn)u Y（2008）Vanadium deposits in the lower Cambrian black rock series in Hunan Province［J］.Geolo Resourc 3：194—201（in Chinese with English abstract）

Maylotte DH，Wong J，Stpeters RL，Lytle FW，Greegor RB（1981）X-ray absorption spectroscopic investigation of trace Vanadium sites in coal［J］.Science 214：554—556

Meng Y，Gan Q，Zhen Y（2009）The paleogeography of metallogenic period and mineralization process of the Vanadium Deposit of Shanglin［J］.Land Resourc South China，27—29（in Chinese with English abstract）.

Meunier JD（1994）The composition and origin of vanadium-rich clay-minerals in Colorado Plateau Jurassic sandstones［J］.Clays Clay Miner 42：391—401

Pan Y，Yu J，Wu H（2007）Process evaluation of vanadium extraction from stone coal［J］.Express Inf 23：10—13（in Chinese with English abstract）

Pauli FW （1975）Heavy-metal humates and their behavior against hydrogen-sulfide［J］.Soil Sci 119：98—105

Peacor DR，Coveney RMJ，Gengmei Zhao（2000）Authigenic illite and organic matter：the principal hosts of Vanadium in the Mecca quarry shale at Velpen，Indiana［J］.Clays Clay Miner 48：311

Perrin DD（1979）Stability constants of metal ion complexes，Part B. Organic ligands［M］.IUPAC Chemical Data Series 22.Pergamon Press，New York，p 1264.

Rosscup RJ，Bowman DH（1968）Thermic stability of Vanadium and Nickel Petroporphyrins［J］.Erdol und Kohle Erdgas Petrochem 21：28

Shannon RD （1976）Revised effective ionic-radii and systematic studies of interatomic distances in Halides and Chalcogenides［J］.Acta Crystallogr Sect A 32：751—767

Stott JRW（2011）International Bulletin of Missionary Research［J］. Nature 35：205

Tosca NJ，Johnston DT，Mushegian A，Rothman DH，Summons RE，Knoll AH（2010）Clay mineralogy，organic carbon burial，and redox evolution in Proterozoic oceans［J］.Geochim Cosmochim Acta 74：1579—1592

Wang M，Wang X（2012）State-of-art Development of Vanadium Extraction from Stone Coal［J］.Iron Steel Vanadium Titan 1：8—14（in Chinese with English abstract）

Wehrli B，Stumm W（1989）Vanadyl in natural-waters—adsorption and hydrolysis promote oxygenation［J］.Geochim Cosmochim Acta 53：69—77

Wilson MD，Pittman ED（1977）Authigenic clays in sandstones—recognition and influence on reservoir properties and paleoenvironmental analysis［J］.J Sediment Petrol 47：3—31

Xiao D（2007）Mineralogy of Stone Coal from Shanglin of Guangxi and Vanadium Extraction with Hydrometallurgical Process［J］. Nonferrous Met 3：85—90（in Chinese with English abstract）

Yang J，Yi F，Qian Z（2009）Study of Paleotemperature of the lower Cambrian black shale series and their implications in Northern Guizhou Province［J］.Acta Mineral Sin 29：87—94（in Chinese with English abstract）

Ye S，Kong F（2006）The geology and formation of vanadium deposit in black sequence of Xiuwu region［J］.Resourc Environ Eng 5：501—504

Zhang Y，F(xiàn)an B，Peng D（2001）Research of precipitation polyammonium vandate from extraction solution of acid-leaching bone coal［J］.Chin J Rare Met 2：157—160（in Chinese with English abstract）

10.1007/s11631-015-0060-8

? Yongbin Zhang

275947159@qq.com

Qian Zhang

zhangqian@vip.gyig.ac.cn

1State Key Laboratory of Ore Deposit Geochemistry，Institute of Geochemistry，Chinese Academy of Sciences，Guiyang 550081，Guizhou，China

2University of Chinese Academy of Sciences，Beijing 100049，China

Leaching experiments·Occurrence state·Emplacement rate of V