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Origin of Minerals and Critical Metals in an Argillized Tuff from the Huayingshan Coalfield, Southwestern China Lei Zhao 1,2 , Qin Zhu 2 , Shaohui Jia 2 , Jianhua Zou 2,3 , Victor P. Nechaev 4 and Shifeng Dai 1,2, * 1 2 3 4

*

State Key Laboratory of Coal Resources and Safe Mining, China University of Mining and Technology, China; [email protected] College of Geoscience and Survey Engineering, China University of Mining and Technology (Beijing), Beijing 100083, China; [email protected] (Q.Z.); [email protected] (S.J.); [email protected] (J.Z.) Chongqing Key Laboratory of Exogenic Mineralization and Mine Environment, Chongqing Institute of Geology and Mineral Resources, Chongqing 400042, China Far East Geological Institute, 159 Pr 100-let Vladivostoku, Vladivostok 690022, Russia; [email protected] Correspondence: [email protected]

Academic Editor: Karen Hudson-Edwards Received: 3 April 2017; Accepted: 1 June 2017; Published: 5 June 2017

Abstract: This paper reports the minerals in an argillized tuff from the Lvshuidong mine, Huayingshan Coalfield, southwestern China. The clay assemblages of the samples are mainly kaolinite, varying proportions of illite and trace chlorite. Other minerals include various proportions of pyrite, small proportions of carbonates (calcite and ankerite), anatase, rutile, hematite, florencite and rare analcite. The clay mineralogy of the tuff profile changes vertically due to the change in the depositional environment. Although illite is minor in the middle and lower parts of the profile, it is relatively abundant in a few topmost samples where the proportion of illite is comparable to that of kaolinite. This is probably because the original volcanic ash was mainly deposited in a continental environment, but marine water may have percolated to the uppermost layers of the ash bed during early diagenesis, leading to the formation of concretions of concentric rings of kaolinite and illite. The samples of this study are derived from alkali mafic volcanic ash with relatively high concentrations of critical metals, including Nb, Ta, Zr, Hf, rare earth elements and Y. Keywords: minerals; critical metals; volcanic ash; tuff; immobile elements; rare earth elements; SW China

1. Introduction The Emeishan flood volcanism not only results in dominant basaltic lavas and subordinate pyroclastic rocks, but also flows and tuff of trachytic and rhyolitic composition in the west sub-province [1–3]. To the east of the Kangdian Upland, within the central Emeishan large igneous province, a tuff occurs as a possibly correlative band in a wide area covering western Yunnan, southern Sichuan, western Guizhou and Chognqing, where many major coalfields occur in this area (e.g., Songzao, Huayingshan and Zhongliangshan coalfields) (Figure 1). The thickness of the tuff varies from 0.2–21 m, commonly 3–5 m [4], representing a pyroclastic fall associated with the Emeishan flood volcanism. The tuff band can have various mineralogical and geochemical compositions due to different degrees of alteration. For example, Kramer et al. [5] showed that tuff bands from the Sydney Basin represent several cycles of eruptive activity and deposition resulting in a combination of pyroclastic fall, flow and surge with different chemical fingerprints.

Minerals 2017, 7, 92; doi:10.3390/min7060092

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On the other hand, altered alkaline volcanic ashes have been reported to be enriched in critical metals such asother Nb, Ta, Zr, Hf, Ga and rare volcanic earth elements and Y (REY, or REEtoifbe Y enriched is not included) [6,7]. On the hand, altered alkaline ashes have been reported in critical metals suchsequences as Nb, Ta,that Zr,have Hf, Ga and rare earthbyelements Y (REY, REEhave if Yalso is not Coal-bearing been influenced air-borneand volcanic ashorfalls been included) [6,7].some Coal-bearing sequences have beenand influenced air-borne ash particularly falls have reported to host critical metals [8].that Many coals the hostby strata in SWvolcanic China are alsocases, beenwhich reported to host critical metals [8]. Manyofcoals the host strata in SW China are such have beensome known as potential sources theseand critical metals [9–11]. particularly such cases, which have been known as potential sources of these critical metals [9–11].

Figure Provinces in in SW SW China China (A) and the Figure 1. 1.Provinces the location location and and tectonic tectonicmap mapofofthe theHuayingshan Huayingshan Coalfield (B), modified from Dai et al. [9]. Coalfield (B), modified from Dai et al. [9].

Previous studies [9,12] have reported the mineralogy and geochemistry of the coal and host Previous studies [9,12] have reported the mineralogy and geochemistry of the coal and host rocks rocks (roof and floor strata of coal-seams) from the Huayingshan Coalfield, Sichuan Province, SW (roof and floor strata of coal-seams) from the Huayingshan Coalfield, Sichuan Province, SW China. China. The coals in this coalfield have been reported to be significantly enriched in critical metals The coals in Zr, thisNb, coalfield reported to be significantly enriched in critical including including Hf andhave REY,been which are attributed to alkali intra-seam tonsteins andmetals hydrothermal Zr,solutions Nb, Hf and REY, which are attributed to alkali intra-seam tonsteins and hydrothermal solutions [9]. It has been reported that not only the tonstein-bearing coals, but also the associated non-[9]. It has been reported that seams not only tonstein-bearing coals, butare also the associated non-coal strata coal strata distal to coal andthe that underlying the coal seam enriched in those critical metals distal coal seams and thatand underlying themechanisms coal seam are in those critical metalstuff [9]; are however, [9]; to however, the sources enrichment of enriched critical metals in the underlying not theclear. sources and enrichment mechanisms of critical metals in the underlying tuff are not clear. The present geochemicalcharacteristics characteristicsofofthe thealtered altered mafic The presentpaper paperreports reportsmineralogical mineralogical and geochemical mafic volcanic ash from the thegeological geologicalfactors factorsresponsible responsible volcanic ash from theHuayingshan HuayingshanCoalfield Coalfield and and discusses discusses the forfor thethe enrichment thecritical criticalmetals metalsand andmineral mineral compositions. compositions. ItItalso mafic chemical enrichment ofof the alsoaims aimstotoverify verifythe the mafic chemical composition thetuff tuffand andtotoevaluate evaluatethe the possible possible economic metals in in thethe composition ofof the economicsignificance significanceofofthe thecritical critical metals samples.The Theargillized argillizedtuff tuffbed bed at at the the lowermost lowermost Longtan in in SWSW samples. LongtanFormation Formationisiswidely widelydistributed distributed China, so that this study is expected to provide additional information on the depositional China, so that this study is expected to provide additional information on the depositional environment of the large area. of environment the large area. 2. Geological Setting 2. Geological Setting The Huayingshan Coalfield is located the southeastern Sichuan Province, SW(Figure China 1). The Huayingshan Coalfield is located in theinsoutheastern Sichuan Province, SW China (Figure 1). The coal-bearing sequences of the coalfield are Lopingian Longtan Formation (Figure 2), is The coal-bearing sequences of the coalfield are Lopingian Longtan Formation (Figure 2), which which is mainly composed of flint-bearing limestone, mudstone, siltstone, sandstone and 2–3 coal mainly composed of flint-bearing limestone, mudstone, siltstone, sandstone and 2–3 coal seams. seams. The lowermost coal seam in the Huayingshan Coalfield, which is indexed as K1, is also well The lowermost coal seam in the Huayingshan Coalfield, which is indexed as K1, is also well developed developed in the Lopingian strata of other coalfields in SW China. For example, the K1 coal seam of in the Lopingian strata of other coalfields in SW China. For example, the K1 coal seam of the the Huayingshan Coalfield is correlated to the No. 25 coal seam in the Guxu Coalfield and the No. 12 Huayingshan Coalfield is correlated to the No. 25 coal seam in the Guxu Coalfield and the No. 12 coal coal seam in the Songzao Coalfield (Figure 1). seam in the Songzao Coalfield (Figure 1). A tuff layer with a thickness of mostly 2–5 m, light-grey or light-grey-white in color, lies at the A tuff layer a thickness of mostly in 2–5 light-grey orCoalfield. light-grey-white color, lies at the lowermost levelwith of the Longtan Formation them,Huayingshan This tuff in is locally named lowermost levelinofthe thefield Longtan Formation in the Huayingshan This tuff underlain is locally named as “bauxite” lithologic descriptions. The tuff bed Coalfield. is disconformably by the as

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“bauxite” in the field lithologic descriptions. The tuff bed is disconformably underlain by the Maokou Formation, is a limestone of Guadalupian age. The Longtan is conformably Maokouwhich Formation, which is aunit limestone unit of Guadalupian age. The Formation Longtan Formation is overlain by limestones the Changxing Formation inFormation the studied area. The Changxing Formation is conformably overlainofby limestones of the Changxing in the studied area. The Changxing in turn overlainis by the Triassic (thesediments Feixianguan Formation).Formation). Formation in turn overlain sediments by the Triassic (the Feixianguan

Figure 2. Sedimentarysequences sequencesat atthe the Huayingshan Huayingshan Coalfield, China, modified from Dai Dai et al.et[9]. Figure 2. Sedimentary Coalfield,SW SW China, modified from al. [9].

A regression, related to the so-called Dongwu Movement, which is a regional uplift event in SW A regression, to theage, so-called Movement, uplift event in China at the laterelated Guadalupian exposedDongwu the Yangtze Block and which resultedisinaaregional depositional hiatus. SW China at the late Guadalupian exposed the Yangtze Block to and resulted in a depositional Subsequently, the elevated Maokouage, Formation limestone was subjected intensive erosion. A mafic volcanic eruption the then occurred, and volcanic ashlimestone fell on to eroded to surface of the hiatus. Subsequently, elevated Maokou Formation wasthe subjected intensive erosion. Maokou Formation [13]. A mafic volcanic eruption then occurred, and volcanic ash fell on to the eroded surface of the Maokou Extensive Formation [13]. transgression, occurring probably during or after the main Lopingian eruption phase of the Emeishan basalts [3], resulted in the thick carbonate and siliciclastic deposits of the Xuanwei Extensive transgression, occurring probably during or after the main Lopingian eruption phase of and Longtan Formations in eastern Yunnan and western Guizhou, respectively. These strata the Emeishan basalts [3], resulted in the thick carbonate and siliciclastic deposits of the Xuanwei and unconformably overlie the Emeishan Basalts, except in local areas of eastern Chongqing and western Longtan Formations eastern Yunnan and western Guizhou, These strata unconformably Guizhou, whereinthe Longtan Formation directly rests respectively. on the erosional surface of the overlie the Emeishan Basalts, except in local areas of eastern Chongqing and western Guizhou, where Maokou Formation. the Longtan directly restssequences on the erosional surface ofdeposited the Maokou Formation. The Formation Lopingian coal-bearing in SW China were within a range of facies The Lopingian coal-bearing in SW China were deposited a range of facies spanning from fluvial, throughsequences continental-marine transitional, to marinewithin carbonate platform environments [13]. According Dai et al. [4], before transitional, the depositiontoof marine the K1 coal seam in platform the spanning from fluvial, throughtocontinental-marine carbonate Huayingshan Coalfield, the paleogeographic environment of the area was of mainly a tidal flat. Afterin the environments [13]. According to Dai et al. [4], before the deposition the K1 coal seam the accumulation of the of K1 coal seam,environment transgressionof occurred in was the eastern [13], and Huayingshan Coalfield, thepeat paleogeographic the area mainlycoalfield a tidal flat. After the the studied area was again mainly occupied by a tidal flat environment [4].

accumulation of the peat of K1 coal seam, transgression occurred in the eastern coalfield [13], and the studied area was again mainlyTechniques occupied by a tidal flat environment [4]. 3. Samples and Analytical Twenty argillized tuff bench samples were collected from a working surface of the Lvshuidong 3. Samples and Analytical Techniques mine, Huayingshan Coalfield. The tuff bed is generally massive bedding, light-grey or

Twenty argillized tuff bench samples were collected from a working surface of the Lvshuidong mine, Huayingshan Coalfield. The tuff bed is generally massive bedding, light-grey or light-grey-white


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in color, with a conchoidal fracture and a soapy feel. The total sampled thickness of the profile is 2 m, 10 cm for each bench sample. These samples are named 1 to 20 from top to bottom. Due to the underground working condition, the top and bottom parts of the tuff bed were not accessible and thus not sampled. Each sample was also ground to fine powder (about 200 mesh) and split into representative portions for mineralogical and geochemical analyses. The fine powders of the samples were subjected to X-ray diffraction (XRD) analysis, using a Rigaku D/max-2500/PC powder diffractometer with Cu-Kα radiation and a scintillation detector. Mineralogical compositions were quantified from the X-ray diffractograms of tuff samples using the Rietveld-based Siroquant™ software developed by Taylor [14]. More details related to the use of the Siroquant technique for coal and non-coal materials from coal-bearing sequences are given by Ward et al. [15], and many studies have indicated the reliability of this technique (e.g., [15–17]). Percentages of major element oxides (SiO2 , TiO2 , Al2 O3 , Fe2 O3 , MgO, CaO, MnO, Na2 O, K2 O and P2 O5 ) in the ashes (815 ◦ C) of the powder samples were determined by X-ray fluorescence spectrometry (XRF; ARL ADVANT'XP+). Concentrations of most trace elements in the samples were determined by inductively-coupled plasma-mass spectrometry (ICP-MS). Prior to ICP-MS analysis, the samples were subjected to microwave dissolution in a mixed acid. For each sample (50 mg), the digestion reagent was a mixture of 2 mL 65% HNO3 and 5 mL 40% HF. Description of the ICP-MS techniques for determination of trace elements in rock samples have been discussed by Dai et al. [18]. Fluorine in the samples was determined using a pyrohydrolysis/fluoride ion-selective electrode technique. Mercury in the samples was analyzed using a Milestone DMA-80 Hg analyzer (Milestone, Milan, Italy); the detection limit of Hg is 0.005 ng, and the linearity of the calibration is in the range 0–1000 ng. Thin-sections and/or polished sections were also made from chips of selected samples. Thin-sections were subjected to petrographic analysis using a Leica DM4500P polarizing microscope equipped with Leica LAS digital imaging software. The minerals in the polished sections were studied using a field emission-scanning electron microscope (FE-SEM, FEI Quanta™ 650 FEG, FEI, Hillsboro, OR, USA), equipped with an EDAX energy-dispersive X-ray spectrometer (EDAX, Mahwah, NJ, USA). Prior to the SEM examination, specimens were coated with carbon to avoid charging effects and then mounted on an aluminum stub, with carbon tape being used to ensure good electrical contact between the specimen and the stub. 4. Results and Discussion 4.1. Comparison between Mineralogical and Chemical Data Quantitative mineralogical results from XRD analysis and Siroquant interpretation for the tuff samples from the Lvshuidong mine are given in Table 1. Major element geochemical data of high temperature ashes (HTAs), based on XRF analysis, as well as trace element geochemical data are given in Table 2. The relationship between the chemistry of HTAs and mineralogy was studied to check the reliability of the quantitative XRD data, following the calculation procedure described by Ward et al. [19]. The chemical composition was modified for each sample by deducting the CO2 and H2 O+ to derive an equivalent to an ash analysis. The actual chemical composition determined by XRF was also normalized to an SO3 -free basis. The correlations of major oxides are shown in Figure 3. The plots for Al2 O3 , SiO2 and Fe2 O3 of the tuff samples show a high degree of correlation between the respective proportions from the XRD + Siroquant analysis and chemical data. This indicates that Siroquant gives consistent results for those major minerals (e.g., kaolinite, illite and pyrite) that contribute to the major element oxides in the analyzed rocks.


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Table 1. Mineralogy of the samples from the Lvshuidong mine by XRD and Siroquant (wt %; the thickness of each bench is 10 cm). Sample

Quartz

Kaolinite

Illite

Chlorite

Pyrite

Calcite

Ankerite

Hematite Analcite

Anatase

Rutile

Florencite

Jarosite

Bassanite

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

0.3 0.1 0.1 0.3 0.2

42.5 41.7 34.4 57.1 63.5 79 70.2 79.6 80.9 78.7 81.6 81.2 81 76 65.5 71.2 63.5 67.6 58.4 47.2

25.7 42 42.4 34.7 27.7 7.6 6 6.3 5.4 3.7 4.3 5.4 4.3 4.7 4 3.5 4.3 6.8

3.8 -

25.6 10.7 14.2 1.4 2.9 5.1 15.2 5.3 4.7 7.1 3.4 3.8 4.8 11.1 23.7 20.8 27.7 25.3 31.6 39.6

0.3 1 0.1 0.1 0.5 0.4 0.6 0.5 0.4 0.4 0.6 1.3 0.7 0.5 0.5 0.3

0.3 1.2 0.6 -

1.3 1.3 1.1 1.2 1.3 0.8 1 0.9 -

3.7 4.1 4.4 5.5 5 5.7 5.2 6.1 6.2 7.3 8.1 6.9 6.6 5.5 3.6 4.2 3.7 4.2 3.5 3.6

0.6 0.2 0.5 0.5 0.4 0.6 0.5 0.4 0.6 0.2 0.6 0.5 0.3 -

0.7 0.5 0.4 0.6 0.8 0.9 0.8 0.7 0.7 0.8 0.8 0.7 0.7 0.6 0.5 0.6 0.4 0.6 0.5 0.3

1 0.1 0.5 0.5 1.1 1.1 0.6 1.1 0.9 0.7

0.8 1.1

0.6 0.5 -

Table 2. Concentrations of major element oxides (%) and trace elements (µg/g, Hg in ng/g) in the studied samples. Sample

LOI

SiO2

TiO2

Al2 O3 Fe2 O3 MgO CaO

MnO Na2 O K2 O

P2 O5

Total

Li

Be

F

Sc

V

Cr

Co

Ni

Cu

Zn

1 2 3 4 5 6 7 8 9 10 11 12 13 14

16.7 13.8 13.7 11.8 12.6 13.9 18.4 14.6 14.6 16.0 14.0 14.1 14.5 17.9

32.4 37.1 36.7 42.2 41.1 39.9 31.8 38.6 39.1 38.3 39.2 39.3 38.9 34.7

2.9 3.8 3.8 5.6 5.5 5.6 4.2 5.8 6.5 6.5 8.0 6.7 6.6 4.9

27.0 29.9 30.3 33.5 33.3 33.1 28.3 32.1 32.6 31.8 32.4 32.4 32.4 29.9

0.02 0.05 0.003 0.005 0.003 0.004 0.05 0.05 0.02 0.02 0.02 0.04 0.02 0.003

0.14 0.13 0.13 0.21 0.21 0.21 0.15 0.2 0.2 0.19 0.23 0.21 0.2 0.16

82.5 84.5 85.4 87.2 86.4 85.1 80.5 84.3 84.3 82.5 84.6 84.4 83.9 81.2

994 1348 2231 1826 1628 1420 968 1073 1131 1000 1029 951 944 927

4.9 4.9 4.5 5.7 5.8 5.8 4.6 5.6 5.9 5.3 5.4 5.1 5.1 4.3

796 718 637 625 580 566 401 511 522 451 412 390 380 338

23 25 22 21 19 19 11 17 16 14 19 19 17 14

298 391 348 562 355 595 451 583 514 582 655 452 594 500

267 302 252 746 629 520 468 568 453 473 512 471 377 710

77 68 37 32 38 51 60 65 100 96 62 45 48 24

154 113 67 67 72 103 110 106 225 249 147 123 139 91

62 197 221 346 520 49 163 130 276 265 314 252 338 243

22 30 33 35 16 40 93 73 100 118 73 73 70 142

16.2 8.8 9.9 1.9 3.5 4.6 14.6 5.8 4.4 4.3 3.6 3.9 4.4 10.6

0.35 0.37 0.37 0.26 0.21 0.16 0.18 0.21 0.19 0.17 0.15 0.23 0.18 0.14

0.25 0.79 0.11 0.15 0.1 0.19 0.35 0.37 0.3 0.34 0.39 0.68 0.35 0.11

0.19 0.2 0.2 0.16 0.14 0.12 0.11 0.12 0.12 0.11 0.09 0.1 0.11 0.1

2.99 3.55 3.94 3.14 2.42 1.27 0.71 0.97 0.83 0.69 0.61 0.77 0.7 0.69


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Table 2. Cont. Sample

LOI

SiO2

TiO2

Al2 O3 Fe2 O3 MgO CaO

MnO Na2 O K2 O

P2 O5

Total

Li

Be

F

Sc

V

Cr

Co

Ni

Cu

Zn

15 16 17 18 19 20

20.4 19.3 20.4 20.0 22.2 23.5

29.6 30.8 29.3 29.8 26.3 23.7

3.2 3.4 3.1 3.2 2.6 2.4

26.8 27.9 26.7 27.1 24.5 22.2

17.9 16.3 18.6 17.8 22.4 25.5

0.13 0.14 0.14 0.15 0.18 0.19

0.46 0.41 0.29 0.32 0.25 0.54

0.01 0.01 0.01 0.01 0.005 0.01

0.08 0.1 0.1 0.1 0.11 0.1

0.43 0.48 0.43 0.47 0.49 0.66

0.12 0.12 0.1 0.12 0.08 0.08

78.6 79.7 78.8 79.1 77.0 75.3

365 946 824 637 443 370

3.2 3.6 3.4 3.3 2.9 2.2

414 414 390 429 351 318

20 26 24 25 21 19

314 328 307 281 252 251

453 510 514 415 436 432

25 32 28 29 18 11

109 169 179 148 125 78

208 205 185 196 173 147

123 120 71 70 356 233

Sample

Ga

Ge

Rb

Sr

Y

Zr

Nb

Mo

Cd

In

Sn

Sb

Cs

Ba

Hf

Ta

Hg

Tl

Pb

Bi

Th

U

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

43 51 53 46 41 42 37 47 49 44 51 50 50 43 31 35 32 33 28 27

2.5 3.7 3.9 3.8 2.7 2.3 3.4 3.1 3.2 3.5 3.7 2.4 4.0 4.0 2.0 1.9 1.8 1.7 1.7 1.2

38.7 31.5 46.9 35.5 23.2 14.7 10.0 10.9 9.8 7.5 5.5 4.6 5.2 8.1 6.8 7.8 7.3 7.7 7.9 8.4

730 242 534 569 186 543 647 464 388 429 221 325 247 357 482 518 430 498 320 334

72 50 50 42 38 36 21 31 30 26 31 25 22 20 37 41 35 30 24 25

614 738 724 958 873 948 784 983 1041 1162 1241 1034 1037 888 628 657 600 609 555 469

83 96 94 125 122 126 105 133 140 158 170 150 112 130 82 86 78 80 68 93

0.67 0.59 0.38 0.52 0.02 1.22 0.81 0.85 0.73 0.49 0.88 0.02 0.53 0.71 0.43 1.51 0.51 0.55 0.56 0.91

0.91 1 0.96 1.23 1.17 1.24 1.4 1.55 1.72 1.95 1.88 1.65 1.58 1.83 1.37 1.42 1.14 1.21 2.25 3.95

0.25 0.55 0.75 0.57 0.69 0.25 0.36 0.39 0.5 0.51 0.62 0.55 0.49 0.66 0.46 0.4 0.38 0.35 0.62 0.39

7.9 9.5 8.8 10.3 2.1 9.3 9.0 10.8 13.5 13.3 13.4 2.4 11.2 11.7 8.7 9.0 7.8 8.4 6.8 5.6

1.77 1.78 0.6 0.93 0.36 2.58 3.58 3.51 3.74 2.97 2.16 0.15 1.45 1.32 1 2.87 2.63 2.42 1.92 2.4

4.2 3.7 3.5 4.0 3.8 3.4 3.0 3.4 3.3 2.9 2.9 2.7 2.8 2.9 2.5 2.9 2.9 2.9 2.5 1.8

99 59 78 86 51 90 100 92 95 104 58 70 57 73 82 90 75 96 63 57

17 19 19 25 22 25 21 25 27 29 33 27 27 24 17 18 16 16 14 12

5.4 5.9 5.8 7.7 7.7 7.9 6.6 8.1 8.9 9.6 10.7 8.9 5.7 9.0 5.1 5.5 4.8 5.1 4.4 5.8

190 202 65 85 84 255 249 167 158 162 118 108 106 157 254 153 196 204 194 127

0.37 0.17 0.1 0.04 0.04 0.13 0.45 0.11 0.08 0.07 0.06 0.04 0.05 1.37 7.53 2.35 2.72 5.24 2.56 0.82

30 23 9 13 11 13 19 16 36 28 26 23 18 29 28 80 94 67 274 66

1.57 1.11 1.63 0.97 1.26 1.94 1.05 1.09 1.48 1.53 1.54 0.86 1.1 1.23 2.11 1.51 1.58 2.88 0.68 0.61

19 16 14 16 16 11 10 16 16 15 19 16 10 13 20 21 21 19 16 14

5.6 2.7 2.3 3.0 3.0 5.0 3.2 4.8 3.1 2.9 3.6 3.1 3.0 2.9 3.0 3.6 3.8 3.3 4.0 3.3

LOI = Loss on ignition.


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Figure 3. Comparison between proportions of major element oxides in tuff samples from the

Figure 3. Comparison between proportions of major element oxides in tuff samples from the Lvshuidong mine, inferred from Siroquant and determined by XRF. The diagonal line represents Lvshuidong mine, inferred from Siroquant and determined by XRF. The diagonal line represents equality in each plot. Relevant trend lines and correlation coefficients (r), obtained from linear equality in each plot. Relevant trend lines and correlation coefficients (r), obtained from linear regression regression analysis, are also shown in each case. analysis, are also shown in each case.

A relatively strong correlation is also observed for the plots of TiO2, K2O and P2O5, with all of the beingstrong relatively close to the 1:1 diagonal 3). This indicates also A points relatively correlation is also observedline for(Figure the plots of TiO andSiroquant P2 O5 , with all of 2 , K2 O that consistent results for those (anatase and florencite). The plot for CaO shows aalso thegives points being relatively close tominor the 1:1minerals diagonal line (Figure 3). This indicates that Siroquant relatively broad scatter, most of the points being under the equality line. gives consistent results forwith those minor minerals (anatase and florencite). TheThis plotindicates for CaOthat shows carbonate minerals are relatively underestimated by the quantitative techniques. Ca ions may also a relatively broad scatter, with most of the points being under the equality line. This indicates that have been absorbed clay minerals. Titanium may occur as atechniques. separate Ti-bearing carbonate minerals areon relatively underestimated by partially the quantitative Ca ions phase may also (nano particles) precipitated in conjunction with kaolinite [20]. Such a phase is difficult to identify by have been absorbed on clay minerals. Titanium may partially occur as a separate Ti-bearing phase XRD, which leads to an underestimation of TiO2 by XRD relative to the chemical analysis.

(nano particles) precipitated in conjunction with kaolinite [20]. Such a phase is difficult to identify by XRD, leads to Samples an underestimation of TiO2 by XRD relative to the chemical analysis. 4.2.which Minerals in the The mineral assemblage in the samples is dominated by kaolinite, varying proportions of illite 4.2. Minerals in the Samples and pyrite and small proportions of carbonates (calcite and ankerite), anatase, rutile, florencite,

The mineral assemblage in the samples is dominated by kaolinite, varying proportions of illite and pyrite and small proportions of carbonates (calcite and ankerite), anatase, rutile, florencite, jarosite and

Minerals 2017, 7, 92 Minerals 2017, 7, 92 Minerals 2017, 7, 92

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jarosite and bassanite. Figure 4 shows the XRD patterns of selected samples. Quartz either below bassanite. Figure 4 shows the XRD patterns of selected samples. Quartz is either belowisthe detection jarosite and bassanite. Figure 4 shows the XRD patterns of selected samples. Quartz is either below the detection limit orinpresent in a trace proportion. Figure 5 vertical illustrates the vertical trends in limit or present only a traceonly proportion. Figure 5 illustrates the trends in abundance of the detection limit or present only in a trace proportion. Figure 5 illustrates the vertical trends in abundance of different minerals in the tuff band. Vitroclastic texture is not evident in thin-sections, different minerals in the tuff band. Vitroclastic texture is not evident in thin-sections, due to complete abundance of different minerals in the tuff band. Vitroclastic texture is not evident in thin-sections, due to completeKaolinite devitrification. Kaolinite and illite the samples are products and of devitrification devitrification. and illite in the samples areinproducts of devitrification make up mostand of due to complete devitrification. Kaolinite and illite in the samples are products of devitrification and make upgroundmass. most of theNo rock groundmass. No other volcanogenic minerals (e.g., quartz, or the rock other volcanogenic minerals (e.g., quartz, biotite or sanidine) were biotite observed make upwere mostobserved of the rock groundmass. No other volcanogenic minerals (e.g., quartz, biotite or sanidine) under the optical microscope or the SEM. under the optical microscope or the SEM. sanidine) were observed under the optical microscope or the SEM.

Figure 4. XRD patterns showing minerals identified in sample 3 (upper) and 18 (lower). Cl = chlorite; Figure 4. XRD patterns showing minerals identified ininsample 3 (upper) and 1818(lower). ClCl==chlorite; patterns minerals identified sample (upper) chlorite; I Figure = illite; 4. K XRD = kaolinite; F =showing florencite; A = anatase; C = calcite; R =3rutile; P =and pyrite;(lower). J = jarosite. I I==illite; K = kaolinite; F = florencite; A = anatase; C = calcite; R = rutile; P = pyrite; J = jarosite. illite; K = kaolinite; F = florencite; A = anatase; C = calcite; R = rutile; P = pyrite; J = jarosite.

Figure 5. Column sections showing vertical variations inminerals major minerals in thetuff studied Figure 5. Column sections showing vertical variations in major (wt %) in(wt the%) studied samples. Figure 5. Column sections showing vertical variations in major minerals (wt %) in the studied tuff samples. tuff samples.

4.2.1. Clay Minerals 4.2.1.Clay ClayMinerals Minerals 4.2.1. Kaolinite mainly occurs as groundmass (Figures 6 and 7) and, to a lesser extent, as vermicular Kaolinite mainly occurs groundmass (Figures and and, lesser extent, vermicular Kaolinite mainly occurs asasgroundmass (Figures 66and 7)7)and, totoaalesser asasvermicular aggregates, which have probably formed after the biotite group (Figure 6A) extent, and pumice clasts or aggregates, which have probably formed after the biotite group (Figure 6A) and pumice clastsoror aggregates, which have probably formed after the biotite group (Figure 6A) and pumice clasts micro-lapilli (chalazoidite) (Figure 6B–D). XRD patterns show a relatively poorly-ordered structure micro-lapilli (chalazoidite) (Figure 6B–D). XRD patterns show a relatively poorly-ordered structure micro-lapilli (chalazoidite) (Figure 6B–D). XRD patterns show a relatively poorly-ordered structure of of the kaolinite in the samples, which reflects the fine poorly-crystallized kaolinite in the groundmass. of the kaolinite in the samples, which reflects the fine poorly-crystallized kaolinite in the groundmass. the kaolinite in the samples, which reflects the fine Kaolinite veins (Figure 7E) also occur, indicating anpoorly-crystallized epigenetic origin. kaolinite in the groundmass. Kaolinite veins (Figure 7E) also occur, indicating an epigeneticorigin. origin. in the samples from a few Kaolinite veins (Figure 7E) also occur, indicating an epigenetic Illite occurs almost in all of the samples, but is relatively abundant Illite occurs almost in all of the samples, but is relatively abundant thesamples samples from few Illite occurs almost in all of the samples, but is relatively abundant ininproduct the from aafew topmost samples of the profile. In those samples, kaolinite formed as a basic of the alteration samples the profile. those samples,kaolinite kaolinite formed basicproduct product the alteration topmost samples ofofthe profile. InInthose samples, formed asasaa7A,B). basic ofofthe alteration oftopmost micro-lapilli, while illite commonly accentuated their rims (Figure To a lesser extent, illite of micro-lapilli, while illite commonly accentuated their rims (Figure 7A,B). To a lesser extent, illite


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of micro-lapilli, while illite commonly accentuated their rims (Figure 7A,B). To a lesser extent, illite occurs as of the (Figure 6B,C). 6B,C). Smectite is also observed under the optical asa acore core of concretions the concretions (Figure Smectite is also observed under microscope the optical inside the illite-rimmed clasts of altered volcanic glass (Figureglass 7C). (Figure 7C). microscope inside the illite-rimmed clasts of altered volcanic Although optical microscope, chlorite is generally below the Although sometimes sometimesbeing beingobserved observedunder underthe the optical microscope, chlorite is generally below detection limit of the quantitative XRD techniques except one sample (3). Chlorite occurs as a core of the detection limit of the quantitative XRD techniques except one sample (3). Chlorite occurs as a core the concretions (Figure 7D)7D) thatthat formed probably after thethe original volcanic material. of the concretions (Figure formed probably after original volcanic material.

Figure 6. 6. SEM SEM backscattering backscattering images images of ofthe theclay clayminerals. minerals.(A) (A)Vermicular Vermicularaggregates aggregatesofofkaolinite, kaolinite,inin Figure a a kaolinite groundmass of sample 12. Small white anatase particles disseminated in the groundmass. kaolinite groundmass of sample 12. Small white anatase particles disseminated in the groundmass. (B–D) Associations Associationsof ofkaolinite kaoliniteand andillite illiteininsample sample5.5.The The spectra Spots 1 and 2 are spectra of of Spots 1 and 2 are thethe EDSEDS datadata for for kaolinite in (D). kaolinite andand illiteillite in (D).


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Figure 7. Optical photomicrographs photomicrographs of of minerals minerals in in the samples. samples. (A,B) (A,B) Crossed Crossed polarized polarized light light (XPL) (XPL) images images of kaolinite with multiple illite rims, probably after micro-lapilli (chalazoidite), (chalazoidite), in samples 5 and 4, respectively; (C) XPL image of illite-rimmed clay mineral, probably after fragments of pumice and glass shards in sample 4; (D) XPL image of kaolinite and chlorite probably after a bubbled Plane polarized polarized light light (PPL) (PPL) image image of of kaolinite kaolinite vein vein cutting cutting through through clay clay volcanic glass in sample 4; (E) Plane concretions in sample 4; (F) XPL image of Illite and possible hematite after volcanic glass in sample 5.

4.2.2. Pyrite Pyrite Pyrite occurs in significant proportions proportions in the upper and lower parts of the tuff section. Pyrite commonly occurs occursas as euhedral crystals (Figure 8A)ineither in the groundmass postdating euhedral crystals (Figure 8A) either the groundmass or postdatingorearly-formed early-formed concretions (Figure 8D). It also occurs as clay overgrowth on (Figure the clay concretions concretions (Figure 8D). It also occurs as overgrowth on the concretions 8C,D) and as a (Figure and as a nucleus of the surrounding minerals 8B). Some rounded pyrite nucleus 8C,D) of the surrounding clay minerals (Figure 8B).clay Some rounded(Figure pyrite might be replacement after might be replacement after some organics (Figure 8B). some organics (Figure 8B). 4.2.3. Carbonate Minerals Carbonate minerals, minerals, including including calcite calcite and and ankerite, ankerite, are are present present in in all all of of the the Lvshuidong Lvshuidong tuffs, tuffs, Carbonate Carbonate minerals minerals replacing replacing the previously-formed previously-formed concretions are although in minor proportions. Carbonate common (Figure 8B,E,F).


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Figure 8. Images of minerals in the sample 12. (A) PPL image of pyrite overprint clay concretions; Figure 8. Images of minerals in the sample 12. (A) PPL image of pyrite overprint clay concretions; (B) (B) SEM backscattering image of pyrite surrounded by kaolinite and carbonate minerals; (C,D) SEM SEM backscattering image of pyrite surrounded by kaolinite and carbonate minerals; (C,D) SEM backscattering images of kaolinite with pyrite rim(s); (E) PPL image of concretion consisting of analcite, backscattering images of kaolinite with pyrite rim(s); (E) PPL image of concretion consisting of kaolinite and ankerite; (F) XPL image of (E). analcite, kaolinite and ankerite; (F) XPL image of (E).

4.2.4. 4.2.4. Anatase Anatase and and Rutile Rutile Anatase relatively significant significant proportions, proportions, Anatase and and rutile rutile persistently persistently occur occur in in all all the the tuff tuff samples samples in in relatively with the former being more abundant (up to 8.1%) than the latter (