Genesis of Pyrite Concretions: Constraints from Mineral and Geochemical Features of Longtan Formation in Anhui Province, Eastern China

Article Genesis of Pyrite Concretions: Constraints from Mineral and Geochemical Features of Longtan Formation in Anhui Province, Eastern China

Qing He 1, Yanfei An 1,* , Fangji Sun 2 and Chunkit Lai 3,4 1 School of Resources and Environmental Engineering, Anhui University, Hefei 230601, China 2 Third Geological Exploration Institute, Henan Bureau of Geo-exploration and Mineral Development, Zhengzhou 450001, China 3 Faculty of Science, University of Brunei Darussalam, Gadong BE1410, Brunei Darussalam 4 Centre for Excellence in Ore Deposits (CODES), University of Tasmania, Hobart 7001, Australia * Correspondence: [email protected]; Tel.: +86-0551-63861441

Received: 8 June 2019; Accepted: 26 July 2019; Published: 30 July 2019

Abstract: The occurrence of pyrite concretions in the Permian Longtan Formation sheds light on the paragenesis, formation conditions and regional paleoenvironment. We analyzed the mineral and geochemical characteristics of pyrite concretions using scanning electron microscopy-energy dispersive spectrometer (SEM-EDS), X-ray diffraction (XRD) and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) from the Longtan Formation shales in Anhui, Eastern China. These pyrite concretions consist of two types, each with a distinct nucleus and outer layer: The former is mainly made up of quartz, bivalve fragments and minor gypsum, ankerite, siderite and pyrite, the latter consists of pyrite (FeS2) in the voids of quartz. Based on the correlation matrix and geochemical/mineralogical affinity, trace elements in the pyrite concretions fall into three groups, that is, I (Sr, Ba, Rb and K) in calcic minerals from bivalve-bearing nucleus, II (Nb, Ta, Zr and Hf) in certain heavy minerals and III (V, Cr, Co and Ni) in pyrites. Mineral assemblage and paragenetic analysis show that the formation of pyrite concretions can be divided into three stages: (1) deposition of bivalve-bearing nucleus, (2) lithification of diatoms and (3) diagenesis of pyrite. Mineral and geochemical indicators suggest that the formation environment of pyrite concretions has undergone a major shift from lagoon with intense evaporation, to strong reducing marsh.

Keywords: pyrite concretions; Longtan Formation; Permian; LA-ICP-MS mineral geochemistry; sedimentary pyrite; Anhui (Eastern China)

1. Introduction Mineral concretions in sedimentary rocks, including ferromanganese (Fe-Mn), Co-rich, siderite and phosphorite ones, contain potential important (non)-metallic resources [1–6] and record many important diagenetic and paleoenvironmental information [5–8]. Although increasing studies tend to divide concretions into three main types, that is, syngenetic, diagenetic and epigenetic [9–11], all of them record the composition and conditions of the surrounding sediment layers [5,6]. Pyrite concretions record the sedimentary formation processes [7,8], the subsequent metasomatic growth and/or alteration [12,13], microbial sulphate reducing pyrite formation versus later-formed (metasomatic or hydrothermal related) pyrite formation [12,13] and thus determine the mineral and geochemical characteristics of pyrite concretions are highly important [8]. A large number of pyrite concretions were recently found from the Longtan Formation (Chaohu, Eastern China), whose genesis and sedimentary environment are still unclear [14,15]. In this paper, we tackle this issue from a mineralogical and geochemical perspective of pyrite concretions. Using scanning electron microscopy-energy dispersive spectrometer (SEM-EDS)

Minerals 2019, 9, 467; doi:10.3390/min9080467 www.mdpi.com/journal/minerals MineralsMinerals2019 2019, 9,, 4679, x FOR PEER REVIEW 2 of2 of 14 14

microscopy-energy dispersive spectrometer (SEM-EDS) and laser ablation inductively coupled and laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) analyses [16–18], plasma mass spectrometry (LA-ICP-MS) analyses [16–18], we identified the composition of nucleus we identiﬁed the composition of nucleus and rims of pyrite concretions and proposed that pyrite and rims of pyrite concretions and proposed that pyrite concretions were formed in multiple stages concretions were formed in multiple stages of diagenesis and metasomatism. This paper oﬀers new of diagenesis and metasomatism. This paper offers new insight into the sedimentary conditions of insight into the sedimentary conditions of the Longtan Formation. the Longtan Formation. 2. Geological Setting 2. Geological Setting The Upper Permian Longtan Formation is exposed in the Pingdingshan area (Chaohu city, Anhui The Upper Permian Longtan Formation is exposed in the Pingdingshan area (Chaohu city, Province) in Eastern China (Figure1a,b). Geologically, the study area lies on the northwestern margin Anhui Province) in Eastern China (Figure 1a,b). Geologically, the study area lies on the of the Yangtze Plate [19]. Local stratigraphy consists mainly of Silurian to Permian and Jurassic northwestern margin of the Yangtze Plate [19]. Local stratigraphy consists mainly of Silurian to sequences (Figure1c) The Permian sequences at Chaohu comprise (from bottom up) the Qixia, Gufeng, Permian and Jurassic sequences (Figure 1c) The Permian sequences at Chaohu comprise (from Longtan and Dalong Formations. The Permian sequences are overlain by the Lower Triassic Yinkeng bottom up) the Qixia, Gufeng, Longtan and Dalong Formations. The Permian sequences are Formation and overlie the Upper Carboniferous Chuanshan Formation [14,15]. The Longtan Formation overlain by the Lower Triassic Yinkeng Formation and overlie the Upper Carboniferous Chuanshan is dominated by terrigenous clastic rocks, approximately 60 m thick and includes three members: Formation [14,15]. The Longtan Formation is dominated by terrigenous clastic rocks, approximately The lower member consists of grayish-black siltstone, mudstone and shale with abundant fossils, 60 m thick and includes three members: The lower member consists of grayish-black siltstone, such as brachiopods and gastropods. The middle member consists of (dark)-gray thick-bedded mudstone and shale with abundant fossils, such as brachiopods and gastropods. The middle quartz sandstone with thin mudstone interbeds. The upper member consists of dark-grey mudstone member consists of (dark)-gray thick-bedded quartz sandstone with thin mudstone interbeds. The with interbedded thin coal seams and thick bioclastic/micritic limestone. The pyrite concretions are upper member consists of dark-grey mudstone with interbedded thin coal seams and thick exposed in the shale of the lower member (Figure2), which was interpreted to have deposited in a bioclastic/micritic limestone. The pyrite concretions are exposed in the shale of the lower member swampy environment. (Figure 2), which was interpreted to have deposited in a swampy environment.

Figure 1. Map showing the location of concretions (a,b) and concretion-bearing sequences (c) in the Chaohu area (Anhui, Eastern China). Figure 1. Map showing the location of concretions (a,b) and concretion-bearing sequences (c) in the Chaohu area (Anhui, Eastern China). Minerals 2019, 9, 467 3 of 14 Minerals 2019, 9, x FOR PEER REVIEW 3 of 14

Figure 2. 2. StratigraphicStratigraphic column column of theof the Upper Upper Permian Permian Longtan Longtan Formation Formation at the Chaohu at the Chaohuarea, showing area, theshowing locations the locations of pyrite concretions.of pyrite concretions.

3. Samples and Methods Pyrite concretions concretions ( n(n= =20) 20 were) were sampled sampled from from outcrops outcrops of the of Longtan the Longtan Formation Formation in the Chaohu in the areaChaohu and area were and processed were processed at the Geological at the Geological Laboratory Laboratory of Anhui of Anhui University. University. All concretions All concretions were analyzedwere analyzed using scanning using scanning electron electron microscopy microscopy (SEM) and (SEM X-ray) and diﬀ Xraction-ray diffraction (XRD) and ( theXRD best-zoned) and the concretionbest-zoned (based concretion on SEM (base andd XRDon SEM results) and was XRD processed results) withwas LA-ICP-MSprocessed with spot analysesLA-ICP-MS (n =spot18). Analysisanalyses sites(n = were18). Analysis chosen on sites the were equatorial chosen plane on ofthe concretions equatorial parallelplane of to concretions the bedding. parallel Thin sections to the (aboutbedding. 60 Thinµm thick) sections were (about observed 60 µm with thick) scanning were observed electron microscopy with scanning (SEM, electron FE-ΣIGMA microscopy VP300, (SEM, Carl ZeissFE-ΣIGMA AG., Heidenheim, VP300, Carl Zeiss Germany) AG., forHeidenheim, the morphology Germany) and for distribution the morphology of pyrite and concretions distribution at the of Guangdongpyrite concretions Provincial at Key the Laboratory Guangdong of Provincial Mineral Resources Key Laboratory and Geological of Mineral Processes, Resources Sun Yat-sen and University.Geological Processes, The samples Sun were Yat- gold-coatedsen University. with The a Quorum samples Q150Twere gold ES sputtering-coated with coater. a Quorum The working Q150T distanceES sputtering of the coater. SEM-EDS The was working 10–16 mmdistance using of 10.0 the kV SEM beam-EDS voltage was 10 and–16 6 mm nm aperture using 10.0 with kV a beam 5 nm spotvoltage size. and The 6 images nm aperture were captured with a via 5 nm a retractable spot size. solid-state The images backscattered were captured electron via (BSE) a retractable detector. Mineralsolid-state compositions backscattered of the electron powdered (BSE) pyrite detector. samples Mineral (200 compositions mesh) were measured of the powdered with aRigaku pyrite SmartLabsamples (200 X-ray mesh powder) were di measuredﬀractometer with (XRD, a Rigaku SmartLab SmartLab 9kW, Rigaku X-ray powder Corporation, diffractometer Tokyo, Japan) (XRD at, theSmartLab Modern 9kW Experimental, Rigaku Corporation Technology, Toky Center,o, Japan Anhui) at University. the Modern The Experimental X-ray generator Technology comprises Center, a 9 kWAnhui rotating University. anode, The which X-ray was generator operated comprises at 40 kV, 100 a 9 mA kW and rotating equipped anode, with which a single-crystal was operated graphite at 40 monochromator.kV, 100 mA and Powerequipped samples with were a single scanned-crystal in the graphite 2θ range monochromator. of 5–90◦ with step Power size ofsamples 0.01◦. were scannedElemental in the 2 concentrationsθ range of 5–90 were° with measured step size withof 0.0 LA-ICP-MS1°. (laser ablation inductively coupled plasmaElemental mass spectrometry) concentrations at thewere Laboratory measured of with Sample LA- SolutionICP-MS (laser in Wuhan, ablation China. inductively The analysis coupled was performedplasma mass with spectrometry) a Resonetic RESOLution at the Laboratory M-50 ArF-Excimer of Sample laserSolution ablation in Wuhan system, (CoherentChina. The Inc., analysis Santa was performed with a Resonetic RESOLution M-50 ArF-Excimer laser ablation system (Coherent Inc., Santa Clara, CA, USA) coupled to an Agilent 7700 ICP-MS (Agilent Technologies, Santa Clara, Minerals 2019, 9, 467 4 of 14 Minerals 2019, 9, x FOR PEER REVIEW 4 of 14

Clara,CA, USA). CA, USA) Operating coupled conditions to an Agilent include 7700 ICP-MS193 nm (Agilent wavelength Technologies, and 80 mJ Santa laser Clara, energy, CA, USA).10 Hz Operatingfrequency, conditions 44 µm spot include size and 193 nmused wavelength He as the andcarrier 80 mJgas. laser Element energy, contents 10 Hz frequency, were calculated 44 µm spotwith sizethe and software used He ICPMS as the DataCal carrier gas. (Version Element 10.2 contents, China were University calculated of with Geosciences the software, Wuhan ICPMS, China DataCal), an (Versionin-house 10.2, program China developed University ofby Geosciences, the China University Wuhan, China), of Geosciences. an in-house The program detection developed limits of by trace the Chinaelements University were 2 of–8 Geosciences.ppb and the Theanalytical detection precision limits of was trace better elements than were5% , 2–8while ppb the and detection the analytical limits precisionof major waselements better were than greater 5%, while than the 0.1 detection wt. %. limits of major elements were greater than 0.1 wt. %.

4.4. ResultsResults

4.1.4.1. TextureTexture of of Pyrite Pyrite Concretions Concretions MostMost pyrite pyrite concretionsconcretions areare well-preservedwell-preserved inin thethe blackblack shaleshale ofof thethe LowerLower LongtanLongtan Formation.Formation. TheThe shales shales are are laminated laminated (2–5 (2–5 mm mm thick) thick) and and comprise comprise predominantly predominantly clay clay minerals. minerals. Lenticular Lenticular coal coal seamseam (0.2 (0.2 ×0.5 0.5 mm~ ~ 0.50.5 × 2.52.5 m)m) andand abundantabundant sulfursulfur occuroccur inin thetheshales shales(Figure (Figure3 a).3a). Concretions Concretions cut cut × × throughthrough the the bedding bedding plane plane of of surrounding surrounding rocksrocks and and nono deformationdeformation waswas foundfound alongalong theirtheir contact.contact. EquatorialEquatorial planesplanes ofof concretionsconcretions areare parallel parallel toto the the bedding bedding (Figure (Figure3 b).3b). The The concretions concretions are are mostly mostly spheroidalspheroidal andand minorminor ellipsoidalellipsoidal oror irregularly-shape.irregularly-shape. TheyThey areare 4–15 4–15 cm cm in in diameter diameter withwith laterallateral spacingspacing of of about about 5–10 5–10 cm. cm. TheseThese concretionsconcretions preservepreserve thethe originaloriginal depositionaldepositional stratiﬁcationstratification ofof their their hostinghosting shales. shales. The The weathered weathered surfaces surfaces are are rough rough and and are are grayish grayish yellow yellow due due to oxidation to oxidation (Figure (Figure3c). The3c). concretions The concretions contain contain circumferential circumferential and radial and radial structures structures in their in cross their section cross section and their and fresh their surfacesfresh surfaces can be can divided be divided into the into dark-gray the dark euhedral-gray euhedral pyrite outer pyrite layer outer and layer the blackand the nucleus black organicnucleus matterorganic (Figure matter3d). (Figure Both 3d). the nucleusBoth the and nucleus outer and layer outer are relatively layer are denserelatively and dense contain and radial contain fractures radial betweenfractures the between former the two. former The central two. The part central of fractures part of is fractures not ﬁlled is (void) not filled and (void) there areand abundant there are mineralsabundant along minerals the void along margin. the void The margin nucleus. The occupies nucleus one-third occupies of one the-third concretions. of the concretions.

FigureFigure 3. 3.Photographs Photographs of of pyrite pyrite concretions concretions from from the the Longtan Longtan Formation Formation at Pingdingshan. at Pingdingshan. (a) Coal (a) andCoal abundantand abundant sulfur (smokesulfur (smoke emitted emitted from spontaneous from spontaneous combustion) combustion occur in) the occur shales; in (b the) Outcrops shales; of(b) pyriteOutcrops concretions of pyrite in theconcretions shales; (c in) Individual the shales pyrite; (c) Individual concretions; pyrite (d) Cross-section concretions; of(d) a Cross pyrite-section concretion, of a showingpyrite concretion, the nucleus showing and the outer the nucleus layer. White and thedots outer denote layer the. LA-ICP-MS White dots analysis denote spots.the LA-ICP-MS analysis spots. Minerals 2019, 9, 467 5 of 14

Minerals 2019, 9, x FOR PEER REVIEW 5 of 14 4.2. SEM Imaging 4.2. SEM Imaging SEM-backscattered electron (BSE) imaging of different parts of the concretion nucleus and outer layerS wereEM- conducted.backscattered The electron nucleus (BSE) mainly imaging consists of of bivalvedifferent fossils parts and of surrounded the concretion layers nucleus (Figure 4 anda). Whileouter layer the former were conducted. shows features The suchnucleus as symmetricmainly consists shells, of concentricbivalve fossils patterns and andsurrounded curved beakslayers (566(Figure to 877 4a).µ mWhile in shell the width; former Figure show4sb), features the latter such represents as symmetric a diatom shells, layer concentric with abundant patterns porous and (Figurecurved4 c).beaks Aggregated, (566 to 877 acicular µm in and shell platy width; crystals Figure are visible 4b), the in thelatter cavities represents of the bivalvesa diatom (Figure layer 4withd). Theabundant main constituent porous (Figure of the 4c). outer Aggregated, layer is fine-medium acicular and pyrite platy crystals, crystals which are visible are in thein the forms cavities of cube of a{100},the bivalves octahedron (Figure o{111} 4d). and The pentagonal main constituent dodecahedron of the e{210} outer (Figure layer is4 e). fine Pyrite-medium crystals pyrite are angular crystals, polyhedralwhich are and in the those forms closer of to cube the outer a{100}, layer octahedron margin are o{111} coarser and and pentagonal better crystallized. dodecahedron Fine-grained e{210} pyrite(Figure crystals 4e). Pyrite are visible crystals near are the angular nucleus. polyhedral Striations and on adjacent those closer planes toof the the outer porous layer pyrite margin crystals are arecoarser perpendicular and better to crystallized. each other andFine the-grai fracturesned pyrite between crystals them are arevisible distinct near under the nucleus. high magnification Striations on (adjacent1500) (Figure planes4 f). of the porous pyrite crystals are perpendicular to each other and the fractures between× them are distinct under high magnification (×1500) (Figure 4f).

FigureFigure 4. 4.Microstructure Microstructure of pyrite of pyrite concretions concretions under under scanning scanning electron electronmicroscope microscope (SEM). (a ) (AbundantSEM). (a) bivalveAbundant fossils bivalve in the fossilsnucleus; in ( b the) A bivalvenucleus; with (b) curvedA bivalve beak; with (c) Porous curved layer beak; in the (c) nucleus;Porous (layerd) Acicular in the andnucleus; platy ( crystalsd) Acicular in the and bivalve platy cavities; crystals ( e in) Di theﬀerent bivalve forms cavities; of individual (e) Differ pyriteent crystalsforms of in individual out layer; (pyritef) Porous crystals crystalline in out pyrite layer; crystals. (f) Porous Note: crystalline Spot 1–4 pyrite are the crystals. scanning Note: electron Spot 1 microscopy-energy-4 are the scanning dispersiveelectron microscopy spectrometer-energy (SEM-EDS) dispersive spots spectrometer and (b) and ((SEMc) are-EDS) the zoom-in spots and of the (b) pink-line and (c) boxes are the in (zooma) and-in (b of), respectively.the pink-line boxes in (a) and (b), respectively.

InIn addition,addition, fourfour energyenergy dispersivedispersive spectrometricspectrometric (EDS)(EDS) pointspoints ofof diatomdiatom layerlayer inin thethe nucleusnucleus (Spot(Spot 1), 1), acicular acicular and and platy platy crystals crystals in the in bivalvethe bivalve cavities cavities (Spot 2)(Spot and 2) individual and individual pyrite crystals pyrite (Spot crystals 3) and(Spot porous 3) and pyrite porous (Spot pyrite 4) in the (Spot outer 4) layer in the were outer analyzed layer in were this analyzedstudy. The in EDS this spectrum study. Theof Spot EDS 1 arespectrum composed of Spot of oxygen 1 are composed (61.05 atom of oxygen %) and (61.05 silicon atom (36.02 %) atom and %)silicon (Figure (36.025a), atom indicating %) (Figure that the5a), mineralindicating of porous that the diatom mineral layer of around porous the diatom bivalve layer is most around probably the bivalve quartz is (SiO most2). Results probably of Spot quartz 2 is(S composediO2). Results of of oxygen Spot 2 (40.91 is composed atom %), of sulfur oxygen (27.51 (40.91 atom atom %) and%), sulfur calcium (27.51 (30.63 atom atom %) %) and show calcium that the(30.63 acicular atom and%) show platy that crystals the acicular in the bivalve and platy are gypsumcrystals in (CaSO the bivalve4) (Figure are5 b).gypsum Meanwhile, (CaSO4 the) (Figure EDS results5b). Meanwhile, of Spot 3 conﬁrm the EDS that results individual of Spot pyrite 3 confirm in the outer that layerindividual comprises pyrite predominantly in the outer pyrite layer (FeScomprises2) (Figure predominantly5c), whilst those pyrite of spot(FeS 42) of(Figure porous 5c), pyrite whilst (contains those of sulfur, spot 4 iron of porous and minor pyrite silicon (contains and oxygen)sulfur, iron suggest and theminor presence silicon ofand diatom oxygen) remnants suggest/pseudomorphs the presence of (Figure diatom5d). remnants/pseudomorphs (Figure 5d). Minerals 2019, 9, x FOR PEER REVIEW 6 of 14

Minerals 2019, 9, 467 6 of 14 Minerals 2019, 9, x FOR PEER REVIEW 6 of 14

Figure 5. SEM-EDS spectra of different minerals in pyrite concretions. (a) Diatom layer in the FigureFigure 5. SEM-EDS 5. SEM-EDS spectra spectra of di ofﬀerent different minerals minerals in pyrite in pyrite concretions. concretions. (a) Diatom(a) Diatom layer layer in the in the nucleus nucleus (Spot 1); (b) acicular and platy crystals in the bivalve (Spot 2); (c) pyrite crystals in the outer (Spotnucleus 1); (b) ( acicularSpot 1); (b and) acicular platy and crystals platy incrystals the bivalve in the bivalve (Spot ( 2);Spot (c 2);) pyrite (c) pyrite crystals crystals in in the the outer outer layer layer (Spot 3); (d) porous pyrite in the outer layer (Spot 4). (Spotlayer 3); (d (Spot) porous 3); (d pyrite) porous in pyrite the outer in the layer outer (Spot layer 4).(Spot 4).

4.3.4.3. XRDXRD4.3. XRD XRDXRDXRD patternspatterns patterns ofof theofthe the pyritepyrite pyrite concretionsconcretions concretions areare shownshownshown in inin Figure FigureFigure 6.6 .6.The The The results results results show show show that that thethat themain the main main mineralmineralmineral ofof thethe of concretionstheconcretions concretions isis quartzisquartz quartz andand and pyrite. pyrite.pyrite. TheThe nucleus nucleusnucleus comprises comprisescomprises mainly mainlymainly quartz, quartz,quartz, pyrite pyritepyrite with withwith minorminorminor gypsum, gypsum, gypsum, ankerite ankerite ankerite and and and siderite siderite siderite (Figure (Figure(Figure6a), 6a), whilst6a), whiwhi thelstlst outerthe the outer outer layer layer compriseslayer comprises comprises predominately predominately predominately pyrite pyrite with trace quartz (Figure 6b). The proportion and degree of crystallization of pyrite in the withpyrite trace with quartz trace (Figure quartz6 b).(Figure The proportion6b). The proportion and degree and of crystallizationdegree of crystallization of pyrite in of the pyrite outer layerin the outer layer is markedly higher than that in the nucleus. isouter markedly layer is higher markedly than thathigher in thethan nucleus. that in the nucleus.

Figure 6. X-ray diffraction (XRD) patterns for the pyrite concretions from Pingdingshan: (a) Nucleus; (b) Outer layer. Abbreviations: Qtz, Quartz; Py, Pyrite; Gy, gypsum; Ank, Ankerite; Sd, Siderite. Figure 6. X-ray diﬀraction (XRD) patterns for the pyrite concretions from Pingdingshan: (a) Nucleus; Figure 6. X-ray diffraction (XRD) patterns for the pyrite concretions from Pingdingshan: (a) Nucleus; (b) Outer layer. Abbreviations: Qtz, Quartz; Py, Pyrite; Gy, gypsum; Ank, Ankerite; Sd, Siderite. (b) Outer layer. Abbreviations: Qtz, Quartz; Py, Pyrite; Gy, gypsum; Ank, Ankerite; Sd, Siderite. Minerals 2019, 9, 467 7 of 14 Minerals 2019, 9, x FOR PEER REVIEW 7 of 14

4.4. LA LA-ICP-MS-ICP-MS Mineral Geochemistry The LA-ICP-MSLA-ICP-MS geochemical results of the pyrite concretionsconcretions are shown in Table1 1.. AllAll mainmain elements ((>0.1>0.1 wt. %) areare shownshown asas oxides oxides except except iron iron and and sulfur sulfur in in the the form form of pyriteof pyrite (FeS (FeS2). Based2). Based on theon the 2:1 2:1 ratio ratio between between water water (H2 (HO)2O) and and quartz quartz (SiO (SiO2), it2), is it considered is considered that that the the water water (H 2(HO)2O) should should be crystallinebe crystalline water water in quartzin quartz (SiO (SiO) and2) and record record as SiOas SiO2H2·2HO.2O. Trace Trace elements elements are are expressed expressed in ppm.in ppm. 2 2· 2 Except for Spots 8 and 9 in the fissurefissure between the nucleus and outer layer (Figure(Figure3 3d),d), the the FeS FeS22 and SiO SiO2·2H2H2OO contents account for 91.79–99.41 91.79–99.41 wt. wt. %. %. The nucleus contains contains mainlymainly SiO SiO2·2H2H 2O 2· 2 2· 2 (66.37–91.14(66.37–91.14 wt. %), FeS2 (1.89–25.81(1.89–25.81 wt. %) and AlAl22O3 (2.07(2.07–3.66–3.66 wt. %) and minor KK22O (0.43–1.07(0.43–1.07 wt. %), %), CaO CaO (0.46 (0.46–1.78–1.78 wt. wt. %) %) and and P2O P25O (0.385 (0.38–1.03–1.03 wt. wt.%), %),while while the outer the outer layer layer contains contains mainly mainly FeS2 FeS(57.38(57.38–98.12–98.12 wt. %) wt. and %) with and a with very a verywide widerange range SiO2·2H SiO2O 2H(1.29O– (1.29–39.1539.15 wt. %) wt. and %) low and Al low2O3 Al(0.23O– 2 2· 2 2 3 (0.23–1.891.89 wt. %). wt. Major %). Major element element contents contents in the in thenucleus nucleus are are significantly significantly higher higher than than those those of of the the outer layer, whereaswhereas thethe FeSFeS22 content of the nucleus is lower (Table1 1).). In addition, trace elements in the nucleus and outer layer mainly fall into three groups based on the correlation correlation matrix matrix or or geochemical geochemical (Figure (Figure 7)7 )and and mineralogical mineralogical affinity a ffinity [20]. [20 The]. The 1st 1stgroup group (Sr, (Sr,Ba, Ba,K and K and Rb) Rb) is correlated is correlated with with CaO CaO and and P2 PO25O in5 in the the nucleus nucleus and and is is mostly mostly associated associated wi withth the bivalve nucleus [[21].21]. The 2nd group (Nb, Ta, ZrZr andand Hf)Hf) isis relatedrelated toto diatomitediatomite (SiO(SiO2·2H2H 2O) across 2· 2 the whole concretion.concretion. We speculatespeculate thatthat thethe 2nd2nd group group elements elements may may have occurred in diatomite [[22].22]. Meanwhile, the 3rd3rd group group (Co, (Co, Ni, Ni, V V and and Cr) Cr) is is ass associatedociated with pyrite (FeS 2)) centralized centralized in in the entire concretion and their contents gradually increaseincrease fromfrom insideinside outout [[23].23].

Figure 7.7. ClusteringClustering analysisanalysis diagramsdiagrams of of trace trace elements elements in thein the pyrite pyrite concretions concretions from from the Chaohu the Chaohu area. area. Minerals 2019, 9, 467 8 of 14

Table 1. Laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) geochemical results (ppm) of the pyrite concretions from Pingdingshan.

Number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 Position Nucleus Outer Layer

FeS2 1.89 5.85 5.52 8.72 10.25 11.41 25.81 25.71 12.18 57.38 65.33 79.64 82.61 87.72 92.58 91.87 97.01 98.12 SiO 2H O 91.14 86.51 86.69 83.07 82.45 81.91 66.37 65.17 61.43 39.15 30.04 17.37 13.64 8.01 4.50 5.59 2.28 1.29 2· 2 Al2O3 3.37 3.66 2.90 2.07 3.59 3.17 3.54 2.17 4.20 0.79 1.45 1.89 1.25 1.36 0.87 1.00 0.38 0.23 K2O 0.54 0.47 0.43 1.07 0.56 0.58 0.49 0.39 0.68 0.14 0.26 0.02 0.23 0.63 0.18 0.18 0.01 0.01 CaO 0.59 0.67 1.37 1.78 0.46 0.62 0.79 2.87 11.66 0.66 1.14 0.05 0.74 0.48 0.72 0.24 0.04 0.05 P2O5 0.49 0.43 0.97 1.03 0.38 0.47 0.41 0.99 2.02 0.20 0.42 0.07 0.33 0.54 0.39 0.40 0.04 0.01 K2O 0.54 0.47 0.43 1.07 0.56 0.58 0.68 0.49 0.39 0.14 0.26 0.02 0.23 0.63 0.18 0.18 0.01 0.01 SrO 0.20 0.12 0.50 0.05 0.08 0.09 0.14 1.30 3.20 0.04 0.06 0.00 0.07 0.02 0.09 0.04 0.00 0.00 BaO 0.05 0.03 0.12 0.01 0.03 0.03 0.04 0.24 0.59 0.01 0.01 0.00 0.02 0.01 0.02 0.01 0.00 0.00 total 98.82 98.22 98.92 98.88 98.36 98.84 98.27 99.31 96.35 98.52 98.97 99.05 99.12 99.38 99.54 99.52 99.79 99.73 K 4514 3935 3551 8917 4624 4772 5635 4066 3219 1149 2138 134 1949 5190 1506 1527 114 111 Sr 1665 1004 4208 383 717 782 1159 11032 27051 378 520 3.62 593 176 768 319 8.11 5.81 Ba 486 258 1107 106 232 248 338 2116 5275 94.6 133 5.51 158 53.9 214 77.1 4.11 3.66 Rb 25.86 23.53 19.86 53.7 26.5 27.6 24.0 18.4 32.06 6.46 13.1 0.47 13.1 40.6 8.92 10.6 0.70 0.74 V 142 107 287 68.7 101 91.4 112 227 507 42.4 82.3 42.7 73.0 57.1 88.8 120 15.6 6.07 Co 0.69 1.53 1.67 4.77 2.31 2.04 3.27 4.65 20.3 5.44 6.40 0.29 5.76 5.11 7.56 3.83 0.21 0.67 Ni 4.27 8.70 11.6 21.2 13.2 13.5 19.5 33.2 70.2 29.8 34.4 4.70 29.1 19.3 28.2 16.6 3.34 3.21 As 3.50 7.69 9.98 3.49 8.67 9.51 9.81 14.2 16.4 12.4 10.2 6.69 10.6 28.9 16.5 25.5 6.05 9.20 Nb 2.26 3.43 1.89 1.67 2.23 2.41 3.06 3.98 2.70 0.82 2.20 0.01 3.19 1.35 2.20 1.52 0.18 0.27 Ta 0.20 0.26 0.14 0.13 0.16 0.21 0.20 0.29 0.18 0.06 0.22 0.00 0.35 0.13 0.24 0.13 0.02 0.02 Zr 84.6 75.5 89.2 71.1 84.2 86.4 76.2 62.2 100 28.2 45.0 5.23 43.1 73.7 53.4 22.1 2.71 1.44 Hf 1.07 1.07 1.07 0.87 1.09 1.27 1.09 0.85 1.41 0.50 0.84 0.11 0.79 1.85 1.16 0.47 0.10 0.04 Minerals 2019, 9, x FOR PEER REVIEW 9 of 14

Minerals5. Discussion2019, 9, 467 9 of 14

5.1. Formation Stage of Pyrite Concretions 5. Discussion Pyrite concretions in sedimentary rocks, including deposited, lithificated and diagenetic type, 5.1.are Formation formed Stage by the of aggregationPyrite Concretions of authigenic minerals [9,12]. These minerals usually consist of several kinds and formed in different stages [24,25]. For this reason, paragenetic association of mineralsPyrite concretionsin pyrite concretions in sedimentary can be used rocks, to including reveal the deposited, formation lithificatedof concretion and [7, diagenetic26]. According type, to are formedthe SEM by- theEDS aggregation and XRD results, of authigenic the mineral minerals compositions [9,12]. of These the nucleus minerals and usually outer layer consist are of clearly several kindsdifferent and formed from each in di other:fferent the stages former [24 is,25 mainly]. For this made reason, up of paragenetic quartz, with association minor gypsum of minerals, pyrite, in pyriteankerite concretions and siderite can be, whereas used to the reveal latter the consists formation mainly of concretionof pyrite and [7 ,quartz.26]. According to the SEM-EDS and XRDGypsum results,, ankerite the mineral and compositions siderite crystals of inthe the nucleus nucleus and are outer mainly layer preserved are clearly in di thefferent bivalve from eachfossils other: and the the former quartz is mainly occurs mainlymade up as of porous quartz, diatomite with minor filling gypsum, with somepyrite, pyrite ankerite grains and in siderite, the whereasbivalves. the Based latter on consists the distributions mainly of pyrite of minerals and quartz. and bivalves in the nucleus, it is inferred that the gypsumGypsum, was ankerite likely be and formed siderite before crystals the bivalve in the nucleusfossils [10] are and mainly minerals preserved around in the the bivalve bivalve (e.g., fossils andpyrite, the quartz quartz) occurs may have mainly formed as porous after diatomitethe burial fillingof the bivalves with some [9]. pyrite In thegrains outer layer, in the pyrites bivalves. occurs Based onmainly the distributions as fine-grained of minerals pyrite andfilled bivalves in diatomite in the pore nucleus, or cubic it is pyrite inferred with that minor the gypsum diatom wasresidue likely on be formedthe crystal before surface. the bivalve This indicates fossils [10 that] and the minerals pyrite was around formed the after bivalve the porous (e.g., pyrite,diatomite quartz) [1]. may have formedMeanwhile, after the burial the relationship of the bivalves between [9]. In pyrite the outer (FeS2) layer, and diatomite pyrites occurs (SiO2·2H mainly2O) has as fine-grained also been pyriteclarified filled from in diatomite all the concretions. pore or cubic FeS2 pyriteand SiO with2·2H minor2O show diatom negative residue correlations on the (Figure crystal 8a surface.) and the This indicatespercentage that of the SiO pyrite2·2H2 wasO decline formed and after FeS the2 ratio porous accordingly diatomite increase [1]. from the nucleus to the outer layer (Figure 8b). The components of FeS2 and SiO2·2H2O in the concretions indicate that the Meanwhile, the relationship between pyrite (FeS2) and diatomite (SiO2 2H2O) has also been diatomite is mainly enriched in the nucleus, while the pyrite is primarily concentrated· in the outer clarified from all the concretions. FeS2 and SiO2 2H2O show negative correlations (Figure8a) and layer. Distribution characteristics of the diatomite· may be related to the growth habits of diatoms, the percentage of SiO 2H O decline and FeS ratio accordingly increase from the nucleus to the which gradually become2· 2looser from the nucleus2 to outer layer [27]. Pyrite enrichment in the outer outer layer (Figure8b). The components of FeS and SiO 2H O in the concretions indicate that the layer is probably the result of pyrite filling in the2 voids of2 ·diatom,2 which becomes gradually denser diatomite is mainly enriched in the nucleus, while the pyrite is primarily concentrated in the outer from the nucleus to the outer layer [1]. layer. DistributionAccording to characteristics the paragenetic of relations the diatomite of minerals may be and related other to components, the growth the habits formation of diatoms, of whichpyrite gradually concretions become can be looser divided from into the three nucleus main to stages: outer (1) layer deposition [27]. Pyrite of bivalve enrichment-bearing in nucleus the outer layerand is gypsum; probably (2) the lithification result of pyrite of diatomite filling in around the voids the of bivalve diatom,-bearing which nucleus; becomes (3) gradually diagenesis denser of frompyrite the in nucleus the voids to theof diatomite. outer layer [1].

FigureFigure 8. 8.Diagrams Diagrams of of pyrite pyrite (FeS (FeS2)) and and diatomite diatomite (SiO (SiO2·2H2H2O)O) contents contents in in the the pyrite pyrite concretions concretions 2 2· 2 fromfrom the the Pingdingshan Pingdingshan area. area. ( a()a) Pyrite Pyrite (FeS (FeS2)) versus versus diatomite diatomite (SiO2·2H2H2O)O) contents contents in in the the pyrite pyrite 2 2· 2 concretions;concretions; (b ()b The) The contents contents of of pyritepyrite (FeS(FeS2)) and and diatomite diatomite (SiO (SiO2·2H2H2O)O) in inthe the pyrite pyrite concretions. concretions. 2 2· 2 5.2.According Formation toEnvironment the paragenetic of Pyrite relations Concretions of minerals and other components, the formation of pyrite concretions can be divided into three main stages: (1) deposition of bivalve-bearing nucleus and gypsum; (2) lithiﬁcation of diatomite around the bivalve-bearing nucleus; (3) diagenesis of pyrite in the voids of diatomite. Minerals 2019, 9, x FOR PEER REVIEW 10 of 14

Different parts of the pyrite concretions in the Longtan Formation were likely formed in different sedimentary environments and recorded the corresponding environmental information [28,29]. The presence of gypsum in the bivalve fossils indicates that the nucleus has likely come from an arid and oxidizing environment [30–32]. The main composition of the concretions around the nucleus is diatomite (SiO2·2H2O) with no detrital particles and carbonate rocks. This supports that these diatoms were formed in shallow water translucent zone [9] but suggests that the seawater in thatMinerals environment2019, 9, 467 contains few clastic materials [33,34]. These barren seas was so nutritionally10 of 14 deficient for diatoms that they have to grow around the nucleus to get nutrients [35,36]. Differently, diagenetic5.2. Formation pyrite Environment filled in ofthe Pyrite voids Concretions of diatomite represents restore environment. It is speculated that the environment of pyrite diagenesis may be marsh facies in Longtan Formation [9]. SalinityDifferent of parts water of and the pyritesedimentary concretions facies in can the be Longtan determined Formation by the were 1st group likely formedelements in and diff erenttheir ratiossedimentary (e.g., Sr/Ba environments ratio and andthe Rb/K recorded ratio) the in corresponding sedimentary rocks. environmental The Sr/Ba information and Rb/K ratios [28,29 of]. river and freshThe presence-water lake of gypsumare generally in the lower bivalve than fossils those indicates of marine that sediments the nucleus [37 has,38] likely. Sr/Ba come ratios from of the an bivalvearid and nucleus oxidizing are environment of 1.59 to [ 5.2130–32 (all]. The >1), main which composition may suggest of the salty concretions-water and around marine the nucleus deposits is diatomite (SiO 2H O) with no detrital particles and carbonate rocks. This supports that these diatoms [38,39]. The Rb/K2· 2 ratio of bivalve nucleus (0.0056–0.006) is above 0.004 but below 0.006, which wereindicate formed sea-land in shallow transitional water translucentfacies [37]. zoneTherefore, [9] but it suggests is inferred that that the seawaterthe nucleus in thatof concretion environment of thiscontains study few should clastic materials be formed [33 in,34 the]. These evaporative barren seas microfacies was so nutritionally of marine deficient environments for diatoms near that the seathey-land have transit to growional around facies, the possibly nucleus lagoonal, to get nutrients environment. [35,36 ]. Differently, diagenetic pyrite filled in the voidsThe 2nd of diatomite group elements represents (Nb, restore Ta, Zr environment. and Hf) occur It iscommonly speculated in that certain the environmentheavy minerals of pyrite (e.g., columbites,diagenesis may zircon) be marsh and faciescan infer in Longtan the element Formation source [9]. and formation environment [40]. The concentrationsSalinity of of water Nb and and sedimentary Ta, Zr and Hffacies are can positively be determined correlated by the with 1st eachgroup other elements in this and study their (Figureratios (e.g., 9), showing Sr/Ba ratio that and the the nucleus Rb/K and ratio) outer in sedimentary layer should rocks. not have The similar Sr/Ba and provenance. Rb/K ratios A marked of river differenceand fresh-water of ratio lake and are contents generally of lowerZr and than Hf, thoseNb and of marineTa between sediments the nucleus [37,38 ].and Sr/ Bathe ratiosouter oflayer the inferbivalve that nucleus the diatom are of in 1.59 the to nucleus 5.21 (all and>1), pyrite which in may the suggestouter layer salty-water were probably and marine formed deposits in different [38,39]. Theenvironments Rb/K ratio of[41 bivalve]. The narrowernucleus (0.0056–0.006) deviation in is the above nucleus 0.004 but than below the outer 0.006, layer which suggests indicate sea-land that the formationtransitional environment facies [37]. Therefore,of bivalve- ithosting is inferred diatom that nucleu the nucleuss is calmer of concretion than the outer of this pyrite study layer should [42 be]. formedElements in the evaporativeof the 3rd group microfacies (V, Cr, ofCo marine and Ni) environments could replace near Fe2+ the of pyrites sea-land and transitional the Co/Ni facies, ratio possiblycan indicate lagoonal, the genetic environment. types of pyrites [43]. The Co/Ni ratios in these pyrite concretions are of 0.06 to 0.27The (<1), 2nd representing group elements a sedimentary (Nb, Ta, Zr origin and [43 Hf)]. Theoccur strong commonly linear in Co certainversus heavyNi relationship minerals (e.g.,suggests columbites, that these zircon)pyrite grains and canwere infer deposited the element in a similar source time. and Compared formation to Ni, environment V is more easily [40]. Theaccumulated concentrations in anoxic of Nbcondition and Ta, or Zr in and organic Hf are-rich positively sediments. correlated Therefore, with the each high other V/(V in + thisNi) studyratios (Figurerepresent9), a showing strong hypoxic that the nucleusenvironment. and outer V/(V layer + Ni) should ratios not of havethe pyrite similar concretions provenance. are A of marked 0.59 to 0.97,difference showing of ratio predominantly and contents hypoxic of Zr and and Hf, sulfidic Nb and environments Ta between the based nucleus on the and classification the outer layer of Zhanginfer that et the al. (2011)diatom [44 in]. the Com nucleusbined and with pyrite mineralogical in the outer evidence, layer were we probably suggest formedthat the in formation different environmentenvironments of [41 pyrite]. The in narrower the voids deviation of diatomite in the nucleus was likely than athe hypoxic outer layer and suggests sulfidic that marshy the environment.formation environment of bivalve-hosting diatom nucleus is calmer than the outer pyrite layer [42].

Figure 9. Binary diagrams of Ta, Nb, Hf, Zr contents in the nucleus and outer layer of pyrite concretions Figure 9. Binary diagrams of Ta, Nb, Hf, Zr contents in the nucleus and outer layer of pyrite concretions from the Chaohu area. (a) Ta versus Nb; (b) Hf and Zr. from the Chaohu area. (a) Ta versus Nb; (b) Hf and Zr. Elements of the 3rd group (V, Cr, Co and Ni) could replace Fe2+ of pyrites and the Co/Ni ratio can indicate the genetic types of pyrites [43]. The Co/Ni ratios in these pyrite concretions are of 0.06 to 0.27 (<1), representing a sedimentary origin [43]. The strong linear Co versus Ni relationship suggests that these pyrite grains were deposited in a similar time. Compared to Ni, V is more easily accumulated in anoxic condition or in organic-rich sediments. Therefore, the high V/(V + Ni) ratios represent a strong hypoxic environment. V/(V + Ni) ratios of the pyrite concretions are of 0.59 to 0.97, showing predominantly hypoxic and sulﬁdic environments based on the classiﬁcation of Zhang et al. (2011) [44]. Minerals 2019, 9, 467 11 of 14

Combined with mineralogical evidence, we suggest that the formation environment of pyrite in the voids of diatomite was likely a hypoxic and sulfidic marshy environment. Minerals 2019, 9, x FOR PEER REVIEW 11 of 14 5.3. Three-Stage Formation Model of Pyrite Concretions 5.3. Three-Stage Formation Model of Pyrite Concretions Depositional formation of the gypsum-bearing bivalve: the paleoenvironment of the lower Yangtze regionDepositional in the Chaohu formation area was of possibly the gypsum a sea-land-bearing transitional bivalve: the lagoonal paleoenvironment environment of [33 the]. In lower the lagoon,Yangtze there region are in large the Chaohu bivalves area with was gypsum possibly precipitated a sea-land on transitional them during lagoonal evaporation environment events [ [3345]. TheseIn the bivalves lagoon, were there swept are large into the bivalves sea by with waves gypsum and turned precipitated into the on bivalve them nucleus during via evaporation abrasion (Figureevents [1045a).]. These bivalves were swept into the sea by waves and turned into the bivalve nucleus via abrLithificationasion (Figure formation 10a). of diatomite concretion: Some diatoms are nourished by organic-rich bivalveLithification in the sea formation [22]. They of grew diatomite and aggregated concretion: around Some diatoms the bivalve are nourished nucleus, forming by organic diatom-rich nodulesbivalve in [46 the]. Presumably, sea [22]. They the grew bivalves and are aggregated the actual around physical the nucleation bivalve nucleus, points. Informing these diatom diatom nodules,nodules [the46]. earlier Presumably, diatoms the are bivalves generally are denser the actu in theal physical interior and nucleation the later-formed points. In diatoms these diatom in the nodules, the earlier diatoms are generally denser in the interior and the later-formed diatoms in the outer layer are looser than the interior (Figure 10b). outer layer are looser than the interior (Figure 10b). Diagenetic formation of pyrite: Sedimentary environment in the study area quickly turned Diagenetic formation of pyrite: Sedimentary environment in the study area quickly turned into into a marshy environment. The diatom nodules were buried rapidly by fine carbonaceous detrital a marshy environment. The diatom nodules were buried rapidly by fine carbonaceous detrital sediments [12]. During the diagenesis, this biodetritus may have released sulfur-bearing and strongly sediments [12]. During the diagenesis, this biodetritus may have released sulfur-bearing and reducing fluids [9,47]. They combined with ferrous ions to form pyrite and fill the voids of diatoms [48]. strongly reducing fluids [9,47]. They combined with ferrous ions to form pyrite and fill the voids of The pyrites formed in the outer layer prevented seawater from entering into the concretion interior, diatoms [48]. The pyrites formed in the outer layer prevented seawater from entering into the resulting in lesser amount of pyrite in the nucleus (Figure 10c). concretion interior, resulting in lesser amount of pyrite in the nucleus (Figure 10c).

FigureFigure 10. 10. SketchSketch map map of formation of formation model model of pyrite of pyrite concretions concretions from Chaohu. from Chaohu. (a) Depositional (a) Depositional formation formationof gypsum of-bearing gypsum-bearing bivalve, bivalve,(b) Lithification (b) Lithification formation formation of diatomite of diatomite concretion, concretion, (c (c) )D Diageneticiagenetic formationformation ofof pyritepyrite concretion.concretion. 6. Conclusions 6. Conclusions (1) Compositions between the nucleus and outer layer of pyrite concretions are clearly different (1) Compositions between the nucleus and outer layer of pyrite concretions are clearly from each other. The former is mainly made up of quartz, bivalve fragments and minor gypsum, different from each other. The former is mainly made up of quartz, bivalve fragments and minor ankerite, siderite and pyrite, whilst the latter consists of pyrite in the voids of quartz. gypsum, ankerite, siderite and pyrite, whilst the latter consists of pyrite in the voids of quartz. (2) Mineral paragenesis indicates that the formation of pyrite concretions has undergone three (2) Mineral paragenesis indicates that the formation of pyrite concretions has undergone three stages: (1) deposition of gypsum-bearing bivalve, (2) lithification of diatomite concretion, (3) diagenesis stages: 1) deposition of gypsum-bearing bivalve, 2) lithification of diatomite concretion, 3) of pyrite concretion. diagenesis of pyrite concretion. (3) We speculate that the formation environment of pyrite concretions has undergone a major shift (3) We speculate that the formation environment of pyrite concretions has undergone a major from sedimentary to diagenetic stage. The early sedimentary environment was likely a nutrient-poor shift from sedimentary to diagenetic stage. The early sedimentary environment was likely a sea-land transitional lagoon with intense evaporation, while the diagenetic environment was likely nutrient-poor sea-land transitional lagoon with intense evaporation, while the diagenetic strongly reductive marshy environment with abundant clastic/carbonaceous detritus supply. environment was likely strongly reductive marshy environment with abundant clastic/carbonaceous detritus supply.

Author Contributions: conceptualization, Y.A.; methodology, Q.H. and Y.A.; validation, Q.H. and F.S.; investigation, Y.A.; resources, F.S.; data curation, Y.A. and Q.H.; writing—original draft preparation, Q.H.; writing—review and editing, Y.A. and C.L.; supervision, C.L.; funding acquisition, Y.A. and Q.H.

Funding: This work was funded by the National Natural Science Foundation of China (41802006, 41602173), the Natural Science Foundation of Anhui Province (1708085QD86) and the University Natural Science Research Project of Anhui Province (KJ2018A0005). Minerals 2019, 9, 467 12 of 14

Author Contributions: Conceptualization, Y.A.; methodology, Q.H. and Y.A.; validation, Q.H. and F.S.; investigation, Y.A.; resources, F.S.; data curation, Y.A. and Q.H.; writing—original draft preparation, Q.H.; writing—review and editing, Y.A. and C.L.; supervision, C.L.; funding acquisition, Y.A. and Q.H. Funding: This work was funded by the National Natural Science Foundation of China (41802006, 41602173), the Natural Science Foundation of Anhui Province (1708085QD86) and the University Natural Science Research Project of Anhui Province (KJ2018A0005). Acknowledgments: We thank Yixiang Chen, Sen Yang, Qin Jiang, Qianwen Sun, Yu Wang, Qikuan Zhu and Yuangao Sun (Anhui University) for their sampling support. Conﬂicts of Interest: The authors declare no conﬂict of interest.

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