Geochemistry and Mineralogy of Tuff in Zhongliangshan Mine, Chongqing, Southwestern China

Coal-bearing strata that host rare metal deposits are currently a hot issue in the field of coal geology. The purpose of this paper is to illustrate the mineralogy, geochemistry, and potential economic significance of rare metals in the late Permian tuff in Zhongliangshan mine, Chongqing, southwestern China. The methods applied in this study are X-ray fluorescence spectrometry (XRF), inductively coupled mass spectrometry (ICP-MS), X-ray diffraction analysis (XRD) plus Siroquant, and scanning electron microscopy in conjunction with an energy-dispersive X-ray spectrometry (SEM-EDX). The results indicate that some trace elements including U are enriched in the tuff from Zhongliangshan mine. The minerals in the tuff mainly include kaolinite, illite, pyrite, anatase, calcite, gypsum, quartz, and traces of minerals such as zircon, florencite, jarosite, and barite. The tuff is of mafic volcanic origin with features of alkali basalt. Some minerals including florencite, gypsum, barite and a portion of anatase and zircon have been derived from hydrothermal solutions. It is suggested that Zhongliangshan tuff is a potential polymetallic ore and the recovery of these valuable elements needs to be further investigated.


Introduction
With the depletion of traditional rare metal deposits, coal deposits as promising alternative sources for rare metals have attracted much attention in recent years [1][2][3][4][5][6][7][8][9].At present, germanium is the most successful rare metal element that has been extracted from coal ash [4,8,9].The three well-known coal-bearing strata hosted Ge deposits include Lincang (Yunnan Province) and Wulantuga (Inner Mongolia) of China, and Spetzugli of Russia, are the main sources for the industrial Ge at present and for the foreseeable future [1,4,8,9].The super-large coal-bearing strata hosted gallium deposit in the Jungar Coalfield (Inner Mongolia), China, is another typical example discovered in 2006 [10], which was considered as the third and the most outstanding discovery after the coal-bearing strata hosted uranium and germanium deposits [2,10].Moreover, aluminum is also enriched in Jungar coalfield [1,10].In 2010, another new type of coal-bearing strata hosted Nb (Ta)-Zr (Hf)-REE-Ga polymetallic deposit of volcanic origin was discovered in the late Permian coal-bearing strata of eastern Yunnan, southwestern China [11].Similar polymetallic deposits have since been discovered in some coalfields from southern China [1,3].Similar to most typical areas enriched in rare metals in coal-bearing strata, the tectonic controls on the localization of the metalliferous coal deposits and the mechanisms of rare-metal mineralization in south China and south Primorye of Russia have been studied comparatively in detail [3].The possible recovery of rare earth elements from coal and its combustion products such as fly ash is an exciting new research area [2,[12][13][14][15][16], because coal and its combustion derivation (fly ash) may have elevated concentrations of these rare metals.
The purpose of this paper is to discuss the mineralogical and geochemical compositions of tuff layer in late Permian coal-bearing strata of Zhongliangshan mine, Chongqing, southwestern China.It also contributes to the discussion on the origin and potential prospects of rare metals mineralization of the tuff.

Geological Setting
The Zhongliangshan mine is located in the urban area of Chongqing, southwestern China (Figure 1).The coal-bearing sequence is the late Permian Longtan Formation (P 3 l), which is composed of the light gray, gray, dark gray mudstone, sandy mudstone, siltstone, sandstone and coal seams.This formation is enriched in brachiopods, fern, cephalopods, bivalves, trilobite and other fossils.The Longtan Formation was deposited in a continental-marine transitional environment and has a thickness varying from 26.5 to 105.02 m, with an average of 71.08 m.It contains 10 coal seams, which are identified as K1 to K10 from top to bottom.The Changxin Formation conformably overlies the Longtan Formation and is mainly composed of thick layers of brown-gray, dark gray limestone intercalated with thin layers of mudstone and flint nodules.Some fossils including brachiopods, spindle dragonflies, sponges, corals, and trilobites are enriched in the Changxin Formation.The Maokou Formation disconformably underlies the Longtan Formation, which consists of thick layers of light gray to dark gray bioclastic limestone.
The tuff layer, with a thickness mostly of 2-5 m, light-gray or light-gray-white in color, and a conchoidal fracture and a soapy feel, is located at the lowermost Longtan Formation.The K10 coal seam conformably overlies the tuff layer, which has a disconformable contact with the underlying Maokou Formation (middle Permian) (Figure 2).The tuff is enriched in pyrite and shows massive bedding structure.The tuff was derived from the basalt eruption and deposited directly on the weathered surface of the Maokou Formation limestone, and then was subjected to weathering, leaching, and eluviation [17,18].It is usually described as bauxite or bauxitic mudstone during core sample identification or field lithological description [17,18].

Samples and Analytical Procedures
A total of 21 bench samples were taken from the tuff layer in the Zhongliangshan mine, following the Chinese Standard GB/T 482-2008 [19].Each tuff bench sample was cut over an area 10-cm wide, 10-cm deep and 10-cm thick.All collected samples were immediately stored in plastic bags to minimize contamination and oxidation.Large chips were selected at random from each sample for preparation of polished sections and also kept for later reference if required.The remainder of each sample was crushed and ground to pass through the 200-mesh sieve for analysis.
The loss of ignition (LOI) of each sample was determined according to ASTM standard D3174 [20].All samples were analyzed by X-ray diffraction (XRD) using a D8 advance powder diffractmeter with Ni-filtered Cu-Kα radiation and a scintillation detector.The XRD pattern was recorded over a 2θ interval of 2.6 ˝-70 ˝, with a step size of 0.02 ˝.X-ray diffractograms of the tuff samples were subjected to quantitative mineralogical analysis using Siroquant™ of China University of Mining and Technology (Beijing), a commercial interpretation software developed by Taylor [21] based on the principles for diffractogram profiling set out by Rietveld [22].Further details indicating the use of this technique for coal-related materials are given by Ward et al. [23,24] and Ruan and Ward [25].A Scanning Electron Microscope in conjunction with an energy-dispersive X-ray spectrometer (SEM-EDX, JEOL JSM-6610LV+OXFORD X-max, Tokyo, Japan), with an accelerating voltage of 20 kV, was used to study morphology and microstructure of minerals, and also to determine the distribution of some elements in tuff samples under a high vacuum mode in Chongqing Institute of Geology and Mineral Resources.Percentages of major element oxides including SiO2, TiO2, Al2O3, Fe2O3, MnO, MgO, CaO, Na2O, K2O, and P2O5 in the tuff samples were determined by X-ray fluorescence spectrometry (XRF) The procedures of ICP-MS were: weigh 0.25 g sample in a 50 mL Teflon beaker; add 20 mL HNO 3 -HClO 4 -HF (volume ratio of 4:1:5) and 2 mL H 2 SO 4 ; place on a temperature controlled heating plate and heat to 230 ˝C until like wet salt; then heat to 280 ˝C and evaporate to dryness; turn off the heating plate to cool the sample for 3 min; add 8 mL concentrated aqua regia; incubate for 10 min; transfer the solution to a 25 mL plastic flask; mix and volume; take 5 mL of solution to a 25 mL volumetric flask and dilute to the mark; and study using high resolution inductively coupled plasma mass spectrometry.

Minerals
The proportion of each crystalline phase of the tuff identified by X-ray diffractometry plus Siroquant is given in Table 1.The minerals in Zhongliangshan tuff mainly include kaolinite, illite, pyrite, anatase, calcite, gypsum, and quartz.Some trace minerals such as zircon, florencite, jarosite, and barite, are observed under SEM-EDX.Kaolinite is the dominant mineral of the tuff in Zhongliangshan (Figure 3).The average content of kaolinite is up to 65.3%, and all the samples are richer than 50% except for samples S140SE7-13, S140SE7-14, and S140SE7-19.Kaolinite occurs mainly as matrix material (Figure 4A), and to a lesser extent, as vermicular (Figure 4B) and individual massive (Figure 4C).Illite occurs at the lower part of the profile (Figure 3).

Pyrite, Jarosite and Barite
Pyrite distributes widely in the tuff samples and its content varies from 0.4% to 38.2% (12.6% on average).Its content is gradually decreasing from top to bottom (Figure 3), suggesting that the upper portion has been more subjected to seawater.Pyrite mainly occurs as discrete particles (Figure 4A,D), lumps (Figure 4D), and in some cases, as cubic crystal and pentagonal dodecahedron (Figure 4E).Jarosite occurs as fracture-fillings (Figure 4F), indicating a weathering product of pyrite.Barite is located on the edge of jarosite (Figure 4F), which may be formed by the reaction of jarosite with the hydrothermal solution containing Ba.

Pyrite, Jarosite and Barite
Pyrite distributes widely in the tuff samples and its content varies from 0.4% to 38.2% (12.6% on average).Its content is gradually decreasing from top to bottom (Figure 3), suggesting that the upper portion has been more subjected to seawater.Pyrite mainly occurs as discrete particles (Figure 4A,D), lumps (Figure 4D), and in some cases, as cubic crystal and pentagonal dodecahedron (Figure 4E).Jarosite occurs as fracture-fillings (Figure 4F), indicating a weathering product of pyrite.Barite is located on the edge of jarosite (Figure 4F), which may be formed by the reaction of jarosite with the hydrothermal solution containing Ba.

Anatase
Anatase is present evenly in the tuff samples and varies from 0.8% to 14.3% with an average of 6.2%.The content of Nb in the anatase is up to 0.18% determined by SEM-EDX.Anatase occurs mainly as irregular fine particles (Figure 4B,C) or as colloidal (Figures 4A and 5A) in the kaolinite matrix.Anatase is present evenly in the tuff samples and varies from 0.8% to 14.3% with an average of 6.2%.The content of Nb in the anatase is up to 0.18% determined by SEM-EDX.Anatase occurs mainly as irregular fine particles (Figure 4B,C) or as colloidal (Figures 4A and 5A) in the kaolinite matrix.

Calcite and Gypsum
Calcite distributes at the lower portion of the profile (Table 1, Figure 3), similar to that of illite.Gypsum occurs as radiating forms in the tuff and is present on the edge of fractures (Figure 5B), indicating an epigenetic origin.

Calcite and Gypsum
Calcite distributes at the lower portion of the profile (Table 1, Figure 3), similar to that of illite.Gypsum occurs as radiating forms in the tuff and is present on the edge of fractures (Figure 5B), indicating an epigenetic origin.

Zircon and Florencite
Although zircon and florencite are at concentration below the detection limit of the XRD and Siroquant analysis, they have been observed under SEM-EDX in the tuff samples of the present study.Zircon occurs as subhedral (Figure 5C) and long axis (Figure 5D) in the kaolinite matrix.Florencite occurs as ellipsoidal form in kaolinite; however, minerals containing medium (M-REE) and heavy-rare earth elements (H-REE) have not been observed (M-REE include Eu, Gd, Tb, Dy, and Y; and H-REE include Ho, Er, Tm, Yb, and Lu [7]).

Major Elements
The loss of ignition of the tuff samples varies from 13.94% to 23.56%, with an average of 17.7%.The major element oxides are mainly represented by SiO 2 (35.3% on average) and Al 2 O 3 (29.23%),followed by Fe 2 O 3 (10.95%)and TiO 2 (3.82%) (Table 2).The ratio of SiO 2 /Al 2 O 3 is from 1.16 to 1.26 and averages 1.21, higher than the theoretical value of kaolinite (1.18).The ratio of TiO 2 /Al 2 O 3 is from 0.09 to 0.15, with an average of 0.13.

Trace Elements
Compared with the average concentration of the Upper Continental Crust (UCC) [26], some trace elements are enriched in the tuff samples from Zhongliangshan mine (Table 2).The concentration coefficients (CC, the ratio of the trace-element concentrations in investigated samples vs. UCC) of trace elements higher than 10 include Li, Cr, Cu, Cd, Sb and Re; whereas the elements with CC between 5 and 10 include V, Ni, Zr, Hf, and U. Elements Be, Sc, Co, Zn, Ga, Nb, REE, Ta, and Th, have a CC between 2 and 5. Elements Rb, Sr, Ba, and Tl are depleted, with a CC < 0.5.Other trace elements have concentrations close to the UCC, with CC between 0.5 and 2.

Scandium
The average content of Sc in tuff samples is 30.1 µg/g, which is close to these of the tuffs from Songzao (29.8 µg/g), Nanchuan (26.3 µg/g) and the mafic rocks (29 µg/g, 1060 samples) [27].Scandium is immobile during weathering and alteration and thus can be used as a reliable indicator for the source of tonsteins in coal-bearing strata system [28,29].

Vanadium, Cr, Co and Ni
The average contents of V, Cr, Co and Ni in the investigated samples are 576, 360, 39.8, and 114 µg/g, respectively, close to the tuff from Songzao (V, Cr, Co, and Ni being 576, 549, 37.9, and 164 µg/g, respectively) [1,3] and the normal detrital sediments (888 samples) in the south of Sichuan Province surrounding Chongqing (V, Cr, Co, and Ni being 442, 206, 31, and 61 µg/g, respectively) [30].The contents of V and Cr have the same variations through the seam section, gradually increasing from top to bottom (Figure 6).However, the contents of Co and Ni are higher in the middle relative to the upper and lower portions (Figure 6).The terrigenous source of the inorganic matter in the late Permian coals and normal sediments in southwestern China is the Emeishan Basalt of the Kangdian Upland, which is enriched in V, Cr, Co, and Ni [31,32].The values of tuff samples in the Zhongliangshan mine are close to those in normal sediments, indicating the normal sediments in southwestern China and tuff in Zhongliangshan have the same magmatic sources (the Emeishan basalt magma enriched in V, Cr, Co, and Ni).Dai et al. [1,3,18] suggested that some dark minerals such as basic plagioclase and pyroxene in the basalt rocks could be easily decomposed under weathering conditions and then transported into coal-bearing basin as complex anions.UCC, the Upper Continental Crust; CC, concentration coefficient of trace elements in the tuff, normalized by average trace element concentrations in UCC [26]; nd, no data.

Niobium, Ta, Zr and Hf
The average contents of Nb, Ta, Zr, and Hf of tuff in Zhongliangshan mine are 123, 8.67, 1361, and 35.2 µg/g, respectively, being close to those of the tuff from Songzao (Nb, Ta, Zr and Hf being 118, 9.46, 1377, and 41.5 µg/g, respectively).The Nb and Zr display a similar trend, both gradually decreasing from top to bottom (Figure 6).
The common Nb-, Zr-, REE-, and Ga-bearing minerals have rarely been observed in the tuff, and thus it is suggested that these rare metals probably occur as absorbed ions [11,29].However, Nb may occur as isomorph in the Ti-bearing minerals (Figure 4B,C) and Zr occurs as zircon (Figure 5C,D) in studied samples.

Gallium
The concentration of Ga in Zhongliangshan tuff varies from 19 to 68.7 µg/g and averages 38.2 µg/g, higher than the minimum industrial grade in bauxite (20 µg/g) and coal (30 µg/g) [34], but lower than the concentration of the late Permian "Nb(Ta)-Zr(Hf)-Ga-REE" polymetallic deposit in eastern Yunnan, southwestern China [11].From top to bottom, the concentration of Ga gradually decreases, consistent with those of the Nb and Zr.Because the geochemical nature of Ga is similar to Al [1,8], it may occur as isomorph in Al-bearing minerals (e.g., kaolinite).

Rare Earth Elements (REE)
In this study, REE is used to specifically represent the elemental suite La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, Er, Tm, Yb, and Lu [35].The abundances and geochemical parameters of REE in the tuff samples are listed in Tables 3 and 4 respectively.The concentration of REE varies from 234 to 1189 µg/g, with an average of 548 µg/g.The concentration of REE gradually decreases from top to bottom, similar to that of Nb and Zr.Yttrium is closely associated with lanthanides in nature, because its ionic radius is very similar and its ionic charge is equal to that of Ho [7].For this reason, yttrium is generally placed between Dy and Ho in normalized REE patterns [36].Based on Seredin-Dai's classification [7], a three-fold geochemical classification of REE was used in the present study, including light (L-REE: La, Ce, Pr, Nd, and Sm), medium (M-REE: Eu, Gd, Tb, Dy, and Y), and heavy (H-REE: Ho, Er, Tm, Yb, and Lu) REE [7].Accordingly, three enrichment types are identified, L-type (light REE; La N /Lu N > 1), M-type (medium REE; La N /Sm N < 1, Gd N /Lu N > 1), and H-type (heavy REE: La N /Lu N < 1), in comparison with the upper continental crust [7].This classification has been widely adopted and used in recent years [2].
Table 4 and Figure 7 illustrate that the tuff in the Zhongliangshan mine is mainly enriched in heavy REE.Only samples S140SE7-1, S140SE7-3, S140SE7-4, S140SE7-6, and S140SE7-8 are enriched in light REE; and samples S140SE7-2, S140SE7-10, and 140SE7-14 are enriched in medium REE.From top to bottom, the light REE enrichment only occurs in the upper portion of the profile, while the lower portion is enriched in heavy REE and the medium REE enrichment occasionally occurs in the middle portion.
The Ce-anomaly (expressed as δCe) values vary from 0.70 to 1.77, with an average of 1.41, indicating a well-pronounced Ce positive anomaly.The REE distribution patterns of the tuff display positive Ce anomalies, owing to the in-situ precipitation of Ce 4+ in the process of weathering, leaching, and eluviation [35].The Eu-anomaly (δEu) values varying from 0.76 to 1.51, with an average of 1.06, show a slight Eu positive anomaly, indicating the tuff and the Emeishan basalt have the same origin [35].From top to bottom, δCe and δEu markedly increase.The distribution of REE of the tuff in the Zhongliangshan mine appears as a sawtooth shape, the portions of La-Sm and Gd-Lu occurring gentle and small slope, which indicates that the fractionation of REE is low.(Gd/Lu) N , ratio of (Gd) N and (Lu) N ; δCe = Ce N /(La N ˆPr N ) 1/2 ; δEu = Eu N /(Sm N ˆGd N ) 1/2 ; N, REE are normalized by Upper Continental Crust (UCC) [26].Two reasons may be responsible for the H-REE enrichment of the tuff samples in the Zhongliangshan mine.First, L-REE can be easily leached by groundwater than H-REE; Second, L-REE can be easily adsorbed on the organic matter than the H-REE [37], which may be adsorbed by the coal seam overlying the tuff.The REE enrichment mode in the Zhongliangshan tuff is similar to that of Songzao Coalfield.Some studies have shown that L-REE are more easily to be leached by groundwater and are more apt to be adsorbed by organic matter [38][39][40][41][42].

Origin of Tuff
In the late Permian Age, the Dongwu movement, one of the most important tectonic events in southern China, caused the upper Yangtze basin uplifting and the subsequent sea regression, which led to an extensive erosion in the area.The upper part of Maokou limestone of Sichuan Basin had been subjected to a serious erosion, resulting in the formation of a vast weathering residual plain, where peat subsequently accumulated.Meanwhile, the Emeishan basalt volcano began erupting and reached a climax in the early stage of late Permian, leading to a tuff layer overlying the Maokou limestone [17].
Al2O3 and TiO2 are both stable components in the rock and would be little altered during alteration, so the ratio of TiO2/Al2O3 (KAT) would be kept constant and can frequently be used to study the origin of volcanic ash [29,42].It is suggested that KAT values for silicic volcanic ash are <0.02, and those for mafic and alkali volcanic ashes are >0.08 and between 0.02 and 0.08, respectively [43,44].The KAT ratios of the tuff in the Zhongliangshan mine are >0.08 (Figure 8), Two reasons may be responsible for the H-REE enrichment of the tuff samples in the Zhongliangshan mine.First, L-REE can be easily leached by groundwater than H-REE; Second, L-REE can be easily adsorbed on the organic matter than the H-REE [37], which may be adsorbed by the coal seam overlying the tuff.The REE enrichment mode in the Zhongliangshan tuff is similar to that of Songzao Coalfield.Some studies have shown that L-REE are more easily to be leached by groundwater and are more apt to be adsorbed by organic matter [38][39][40][41][42].

Origin of Tuff
In the late Permian Age, the Dongwu movement, one of the most important tectonic events in southern China, caused the upper Yangtze basin uplifting and the subsequent sea regression, which led to an extensive erosion in the area.The upper part of Maokou limestone of Sichuan Basin had been subjected to a serious erosion, resulting in the formation of a vast weathering residual plain, where peat subsequently accumulated.Meanwhile, the Emeishan basalt volcano began erupting and reached a climax in the early stage of late Permian, leading to a tuff layer overlying the Maokou limestone [17].
Al 2 O 3 and TiO 2 are both stable components in the rock and would be little altered during alteration, so the ratio of TiO 2 /Al 2 O 3 (KAT) would be kept constant and can frequently be used to study the origin of volcanic ash [29,42].It is suggested that KAT values for silicic volcanic ash are <0.02, and those for mafic and alkali volcanic ashes are >0.08 and between 0.02 and 0.08, respectively [43,44].The KAT ratios of the tuff in the Zhongliangshan mine are >0.08 (Figure 8), suggesting a mafic volcanic origin.In addition, the tuff samples fall in the area basalt to alkali basalt from the La/Yb-REE diagram (Figure 9), indicating a feature of alkali basalt.
suggesting a mafic volcanic origin.In addition, the tuff samples fall in the area basalt to alkali basalt from the La/Yb-REE diagram (Figure 9), indicating a feature of alkali basalt.

Hydrothermal Solution
Some researchers have shown that there have been activities of low-temperature hydrothermal solutions in the late Permian Age in southwestern China, which resulted in enrichment of trace elements and minerals in some coal [1,[45][46][47][48][49][50][51].Similarly, some minerals of tuff in Zhongliangshan are formed owing to the influence of hydrothermal solution.
In addition to the derivation from volcanic ash, anatase and zircon might have been derived from hydrothermal alteration in the Zhongliangshan tuffs.Anatase of various particle sizes is distributed in the kaolinite matrix (Figure 4B,C).Figures 4A and 5A illustrate that part of anatase could be formed by hydrothermal alteration.Zircon from Figure 5D displays long axis and could be formed by the effect of hydrothermal alteration.Zircon in Figure 5D exclusively contains Zr, Si and O determined by the SEM-EDX.Finkelman [52] has demonstrated that Hf, Th, U, Y and HREE occur in the volcanogenic zircon, but were not identified in authigenic ziron, in accordance with the results of the Zhongliangshan tuff samples.suggesting a mafic volcanic origin.In addition, the tuff samples fall in the area basalt to alkali basalt from the La/Yb-REE diagram (Figure 9), indicating a feature of alkali basalt.

Hydrothermal Solution
Some researchers have shown that there have been activities of low-temperature hydrothermal solutions in the late Permian Age in southwestern China, which resulted in enrichment of trace elements and minerals in some coal [1,[45][46][47][48][49][50][51].Similarly, some minerals of tuff in Zhongliangshan are formed owing to the influence of hydrothermal solution.
In addition to the derivation from volcanic ash, anatase and zircon might have been derived from hydrothermal alteration in the Zhongliangshan tuffs.Anatase of various particle sizes is distributed in the kaolinite matrix (Figure 4B,C).Figures 4A and 5A illustrate that part of anatase could be formed by hydrothermal alteration.Zircon from Figure 5D displays long axis and could be formed by the effect of hydrothermal alteration.Zircon in Figure 5D exclusively contains Zr, Si and O determined by the SEM-EDX.Finkelman [52] has demonstrated that Hf, Th, U, Y and HREE occur in the volcanogenic zircon, but were not identified in authigenic ziron, in accordance with the results of the Zhongliangshan tuff samples.

Hydrothermal Solution
Some researchers have shown that there have been activities of low-temperature hydrothermal solutions in the late Permian Age in southwestern China, which resulted in enrichment of trace elements and minerals in some coal [1,[45][46][47][48][49][50][51].Similarly, some minerals of tuff in Zhongliangshan are formed owing to the influence of hydrothermal solution.
In addition to the derivation from volcanic ash, anatase and zircon might have been derived from hydrothermal alteration in the Zhongliangshan tuffs.Anatase of various particle sizes is distributed in the kaolinite matrix (Figure 4B,C).Figures 4A and 5A illustrate that part of anatase could be formed by hydrothermal alteration.Zircon from Figure 5D displays long axis and could be formed by the effect of hydrothermal alteration.Zircon in Figure 5D exclusively contains Zr, Si and O determined by the SEM-EDX.Finkelman [52] has demonstrated that Hf, Th, U, Y and HREE occur in the volcanogenic zircon, but were not identified in authigenic ziron, in accordance with the results of the Zhongliangshan tuff samples.
Florencite, the main carrier of REE in the Zhongliangshan tuff samples, occurs as ellipsoidal in the kaolinite matrix (Figure 5E,F), indicating a syngenetic or early diagenetic hydrothermal origin.Dai et al. [14] have also demonstrated that florencite is one of the important carriers of REE in the late Permian coals in southwestern China [3].
Gypsum (Figure 5B) and barite (Figure 4F) occur as crack-fillings, the former occurring as radiating and the latter on the edge of jarosite, indicating an epigenetic hydrothermal origin.

Preliminary Evaluation of Rare Metals
Coal and coal-bearing strata have recently become alternative sources for recovery of rare metals [2,3,7,8].The U.S. Department of Energy's National Energy Technology Laboratory has selected 10 projects to receive funding for research in support of the lab's program on recovery of rare earth elements from coal and coal byproducts since 2015 [2,53].
Based on the Chinese industry standards [33], the required (Nb,Ta) 2 O 5 concentrations for marginal and industrial grade Nb(Ta) ore deposits of weathered crust type are 80-100 and 160-200 µg/g, respectively; equivalent concentrations are 40-60 and 100-120 µg/g for Nb(Ta) ore deposits of river placer type.The concentration of (Nb,Ta) 2 O 5 varies from 47 to 324 µg/g, with an average of 186 µg/g, higher than the marginal and industrial grade for weathered crust and placer deposit types.Concentration of TiO 2 varies from 1.55% to 5.28% and averages 3.82%, higher than the industrial grade of Chinese industry standard [54].The average concentration of Ga (38.2 µg/g) is also up to the standards for industrial utilization in bauxite (20 µg/g) and coal mining (30 µg/g) [34].In addition, the concentrations of REE vary from 234 to 1189 µg/g and averages 548 µg/g, higher than the cut-off grade of Chinese weathering crust ion adsorption type rare earth elements deposits (500 µg/g) [55].
The Nb, Ti, Ga, and REE all exceed their respective industrial grade of China in the tuff of the Zhongliangshan mine.It is considered that the Zhongliangshan tuff is a potential polymetallic ore worth in-depth study.

Conclusions
Compared with the Upper Continental Crust, some trace elements including Li, Be, Sc, V, Cr, Co, Ni, Cu, Zn, Ga, Zr, Nb, Cd, Sb, REE, Hf, Ta, Re, Th, and U are enriched in tuff from Zhongliangshan mine, Chongqing, southwestern China.The minerals mainly include kaolinite, illite, pyrite, anatase, calcite, gypsum, quartz, and traces of minerals such as zircon, florencite, jarosite, and barite.The tuff is of mafic volcanic origin with features of alkali basalt.The H-REE enriched in the tuff due to L-REE being leached easier by groundwater and adsorbed in the organic matter of the coal seam overlying the tuff.Some minerals including florencite, gypsum, barite, and a portion of anatase and zircon are precipitated from hydrothermal solution.It is suggested that Zhongliangshan tuff is a potential polymetallic ore and the opportunity for recovery of these valuable elements needs to be studied in depth.

Figure 3 .
Figure 3. Vertical variations of minerals from the tuff in the Zhongliangshan mine.Figure 3. Vertical variations of minerals from the tuff in the Zhongliangshan mine.

Figure 3 .
Figure 3. Vertical variations of minerals from the tuff in the Zhongliangshan mine.Figure 3. Vertical variations of minerals from the tuff in the Zhongliangshan mine.

Figure 6 .
Figure 6.Vertical variations of selected trace elements of the tuff in Zhongliangshan mine, Chongqing.Figure 6. Vertical variations of selected trace elements of the tuff in Zhongliangshan mine, Chongqing.

Figure 6 .
Figure 6.Vertical variations of selected trace elements of the tuff in Zhongliangshan mine, Chongqing.Figure 6. Vertical variations of selected trace elements of the tuff in Zhongliangshan mine, Chongqing.

Figure 7 .
Figure 7. Distribution patterns of REE in the tuff samples from Zhongliangshan mine.REE are normalized by Upper Continental Crust [26].

Figure 7 .
Figure 7. Distribution patterns of REE in the tuff samples from Zhongliangshan mine.REE are normalized by Upper Continental Crust [26].

Figure 9 .
Figure 9. Relation between REE and La/Yb of tuff samples in the Zhongliangshan mine, Chongqing.

Figure 8 .
Figure 8. Plot for TiO 2 vs. Al 2 O 3 of tuff samples in the Zhongliangshan mine, Chongqing.

Figure 9 .
Figure 9. Relation between REE and La/Yb of tuff samples in the Zhongliangshan mine, Chongqing.

Figure 9 .
Figure 9. Relation between REE and La/Yb of tuff samples in the Zhongliangshan mine, Chongqing.

Table 2 .
Elemental concentrations in Tuff samples from the Zhongliangshan Mine (elements in µg/g, Oxides in %).

Table 3 .
Rare earth elements in the tuff samples collected from the Zhongliangshan Mine (µg/g).

Table 4 .
Rare earth elements geochemical parameters of Zhongliangshan tuff.