Trace Element Concentrations and Mineralogy of Quartz Vein Deposits from Southeastern Hubei Province, China

Abstract

Hydrothermal quartz samples collected from the Fujiashan and Yipanqiu quartz deposits in southeastern Hubei Province, China have been investigated by analytical combination of optical microscopy, scanning electron microscopy, and inductively coupled plasma optical emission spectroscopy, in conjunction with conventional beneficiation processing to evaluate their potential as sources of high purity quartz (HPQ) from a commercial perspective. Microscopy efforts reveal that major mineral impurities associated with quartz are K-feldspar, muscovite, iron oxides, rutile with accessory kaolinite. Bulk trace element concentrations of the processed quartz products demonstrate that the Fujiashan-II quartz vein with cumulative impurities of less than 50 μg g⁻¹ with <30 μg g⁻¹ Al and <10 μg g⁻¹ Ti fits with the lattice-bound criteria for HPQ, meeting the requirement by a HPQ deposit. However, the Yipanqiu quartz deposits are not promising for HPQ production due to high fluid inclusion contents, intimate intergrowth texture with highly variable crystal size, and probably high lattice-bound element contents. The early Neoproterozoic Fujiashan quartz deposits have likely been experienced long-term retrograde metamorphism-related recrystallisation which might contribute to high-purity quartz formation. Due to a much younger crystallization age compared to the Fujiashan deposits, quartz grains in the middle Cretaceous Yipanqiu quartz vein retain high trace elements, leading to exclusion of being a HPQ deposit.

1. Introduction

Quartz is one of the most ubiquitous rock-forming minerals in the earth crust and can form various rocks in different geological environments [1,2], including hydrothermal (e.g., quartz vein, pegmatite), sedimentary (e.g., sandstone, chert), and metamorphic (e.g., quartzite) settings. High purity quartz (HPQ) is an essential material for high-tech applications, especially semiconductor, solar panel, and optic fibers to name a few [3,4]. Owing to rapid growth of these high-tech industries, demand for HPQ is and will be continually increasing dramatically [5]. However, natural quartz deposits suitable for HPQ production are rare [1,4,6]. Specifically, the quartz crucibles and semiconductor base materials consumed in photovoltaic and integrated circuit industries have been depending on HPQ produced from the Spruce Pine mining area for a long time [3,7,8], posing serious risks for world HPQ supply chain.

During crystallization and later metamorphism, quartz can incorporate several kinds of impurities, such as lattice-bound trace elements (ions Al³⁺, Fe³⁺, B³⁺, Ti⁴⁺, Ge⁴⁺, P⁵⁺ act as substitution ions for Si⁴⁺, and Li⁺, K⁺, Na⁺, H⁺, Fe²⁺ or even OH⁻ as charge compensators), micro-inclusions of minerals and trapped fluids [9,10,11,12,13]. Trace elements contents in quartz exert divergent influence on its quality. For instance, silica glass used for semiconductor base materials and crucibles requires low aluminum content, whereas for solar silicon used in photovoltaic industry, boron and phosphorus contents should be in the sub-ppm range [4,12]. Quality criteria of HPQ thus should be defined by trace element concentrations of quartz [6,14,15]. Although variable criteria have been proposed since various specific requirements of quartz quality differ from high-tech applications [3,6,15,16,17], in general, HPQ is designated as quartz that contains less than 50 μg g⁻¹ of impurities [15]. This HPQ definition was refined by Müller et al. [6] based on numerous reliable LA-ICP-MS analysis of elemental contents of lattice-bound impurities in quartz deposits from Norway. In their effort, the elemental concentration limits of HPQ were constrained at: Al < 30 μg g⁻¹, Ti < 10 μg g⁻¹, Na < 8 μg g⁻¹, K < 8 μg g⁻¹, Li < 5 μg g⁻¹, Ca < 5 μg g⁻¹, Fe < 3 μg g⁻¹, P < 2 μg g⁻¹ and B < 1 μg g⁻¹, whereby the sum of all elements should not exceed 50 μg g⁻¹. The refined HPQ definition was adopted by scientific communities [2,18,19].

There are several analytical techniques appropriate for detection of the trace element concentrations of quartz, and each technique has its own methodological limitation [4,20,21,22,23]. In situ techniques such as laser-ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), electron microprobe (EPMA), and secondary ion mass spectrometry (SIMS) applied to quartz have advantages in determining the lattice-bound element contents [11,19,20,23,24,25,26]. Other methods used to characterize chemical composition of quartz include bulk chemical analysis by solution ICP-OES/MS, scanning electron microscope-cathodoluminescence (SEM-CL), and electron spin resonance [2,23,27,28,29]. Bulk chemical analysis by solution ICP-OES/MS is generally used to detect trace elements concentration for quartz [4,30,31,32,33]. However, because fluid and mineral micro-inclusions are commonly incorporated within quartz crystal and/or occur along grain boundaries, bulk elemental data for raw quartz deposits cannot reflect genuine trace element concentrations in quartz [4,6]. Therefore, when applying bulk solution ICP-OES/MS technique, from a commercial point of view, beneficiation processes should be carried out and trace elements data of the processed quartz are more applicable for assessing the HPQ potential.

Over the past decades, in order to alleviate growing tension regarding to global HPQ supply, many efforts have been made to look for potential HPQ resources around the world, especially in Europe, where several promising candidates were proposed, including kyanite quartzites, quartz veins, and granitic pegmatites in Norway [6,11,18], as well as hydrothermal veins from the southern Ural Region [19]. Owing to rapid growth of solar panel industry in China, the demand for HPQ is increasing nationwide. Although many domestic quartz deposits in China have been found and mined over decades, only few of them have been investigated and assessed with respect to HPQ potential on the basis of multiple analytical techniques [33]. In the present study, we applied a combination of analytical techniques including optical microscopy, scanning electron microscopy, conventional beneficiation process, and trace element analysis by inductively coupled plasma optical emission spectroscopy (ICP-OES) on the Fujiashan and Yipanqiu quartz deposits of economic interest in southeastern Hubei Province, central China. The aim of this study is to obtain detailed information about the type of impurities and trace elements of these quartz materials in order to evaluate their potential for HPQ deposits, and to discuss genesis of the investigated quartz veins. Here are presented the first results of a comprehensive mineralogical and geochemical study that would shed valuable insights on future tailored processing concepts, specially designed equipment, and commercial investment for these quartz deposits.

2. Materials and Methods

2.1. Geology of Quartz Deposits

Tectonically, the Fujiashan and Yipanqiu quartz deposits lie at southern margin of the Yangtze block (Figure 1), adjacent to the Jiangnan orogen which separates the Cathaysia block to the south and recorded the convergence history of Yangtze and Cathaysia blocks [34,35]. The quartz bodies are genetically associated with regional acid and intermediate intrusive granitic plutons, namely the Jiugongshan intrusion and the Shadian granite which emplaced asynchronously in early Neoproterozoic epimetamorphosed sedimentary sequences (Figure 1). The precise dates of the quartz deposits are uncertain. However, they are likely to be similar to their host granites and/or nearby plutons.

2.1.1. Fujiashan Quartz Deposits

The Fujiashan quartz deposits are located 25 km south of Tongshan county, Hubei Province (Figure 1), and are exposed at an altitude of 700–850 m a.s.l. These deposits comprise five quartz bodies (Figure 2a), numbered from I to V by local geology survey [37]. Two quartz bodies, I and II were quarried in the past to produce quartz lumps for glassware utilization. Mining activities were suspended currently due to regional environment and resources protection policies.

The I quartz body is the largest and exhibits dyke-like and tabular shape (Figure 3a). It is situated in the Jiugongshan intrusion along the NE-trending Huanghejian Fault which is the local dominate fault (the F1 fault in Figure 2a). The quartz vein extends about 4.4 km in a NE direction with an average width of 8 m, dipping 68–82° to NW (Figure 2a). Local geology survey has inferred a resource size of 4.8 million metric tons of quartz [38]. The vein is hosted by fine- to medium-grained, gneissic biotite monzogranite (Figure 3b) that has been dated at 830 ± 8 Ma by LA-ICP-MS zircon U-Pb technique [39].

The II quartz vein is emplaced in the fracture associated with the Luanjian Fault (the F2 fault in Figure 2a) that extends basically in a direction of EW. Its overlying country rocks are epimetamorphosed slate from the Lengjiaxi Group dated at 860–878 Ma [40], whilst the underlain are granitic rocks composed predominately of gneissic biotite monzogranite. Hydrothermal fluids mobilized and flanked over a minor upheaval that was induced by local magma ascent, crystallized a cap-like morphology for the quartz vein viewed from the top (Figure 2a). As a result of such configuration, the quartz body is exposed in a crescent pattern, plunging 8–25°. The hydrothermal vein is 260 m long in EW direction and 50 m wide NS with an average thickness of 13.8 m. The volume of this vein thus corresponds to 180,000 m³ (260 × 50 × 13.8 m³) and the estimated resource size is approximately 470,000 metric tons of quartz (assuming a density of 2.62 g cm⁻¹ for quartz).

2.1.2. Yipanqiu Quartz Deposits

The Yipanqiu quartz deposits lie 30 km SE of Tongshan county, just 20 km NE of the Fujiashan quartz deposits (Figure 1). The deposits outcrop at an altitude range of 200–800 m a.s.l, and contain I and II quartz bodies. The I quartz body is the largest and most dominant vein, and its resource is approximately 9.0 million tonnages quartz that constitute more than 99% of the deposits. The I quartz vein therefore is the focal point in present study due to its tremendous resource. At the present time, two active open-pit mines at Zimudong and Renyou localities are excavating quartz lumps on swelling bodies along the vein for glassware application.

Unlike the Fujiashan quartz deposits, the I vein of Yipanqiu quartz deposits is embedded in the Shadian fault zone (the F1 fault in Figure 2b) which cuts early Neoproterozoic basement rocks southwest of the Shadian intrusion (Figure 2b). The Shadian fault exposed locally is a basal thrust which is 11 km long, 30–40 m wide, and dips to 145–170°. It was reactivated in late Yanshanian [41]. The Shadian granites are characterized by medium- to coarse-grained, porphyritic biotite monzogranite, similar to the Jiugongshan granites. By comparison to the Jiugongshan intrusion, however, LA-ICP-MS zircon U-Pb dating method has yielded a significantly younger magma crystallization age of 125 ± 1 Ma for the Shadian granites [42]. The vein is hosted by epimetamorphosed slate series of the Lengjiaxi Group (Figure 3c) and its formation could be tentatively linked to fluid mobilization during magma intrusion when the Shadian granite crystallized.

The I vein trends N–E, and is 10 km long, up to 10–18 m above ground surface (Figure 3d), and down to an average of 96.5 m at depth. It is approximately 20 m wide and dips 75–85° to SE. In Renyou and Zimudong sites (Figure 2b), silica-rich fluids probably encountered cavities along the Shadian fault zone and crystallized as swelling bodies, promoting the present mining activities.

2.1.3. Sampling

Because of the present ongoing and historic quartz lumps mining production, the investigated quartz bodies are well exposed. The sampling criterion applied on the quartz veins is established for the purpose to assess their potential as HPQ material. Consequently, quartz samples were collected from the core zone of the studied individual veins, instead of border and/or wall zones to ensure avoidance of possible impurities contamination which are likely to occur therein. Samples taken from the veins were also examined carefully in the field and compared to quartz lumps in quarries to improve their representativeness. In total four quartz samples were obtained.

2.2. Analytical Methods

2.2.1. Optical Microscopy

Double-polished thin sections were prepared for microscopic scrutiny from all the collected samples. A Leica DM4500 P polarization microscope (Leica Microsystems, Wetzlar, Germany) was used to characterize the petrological features of quartz samples, including granular size, mineral and fluid micro-inclusion assemblage, and microstructure.

2.2.2. Scanning Electron Microscope

After optical microscopical investigations, the thin sections were coated with carbon and placed into sample chamber of a scanning electron microscope (SEM). The SEM device used in this study is a FEI Versa 3D Dual Beam module (FEI Company, Hillsboro, OR, USA) equipped with an Oxford INCA energy dispersive spectrometer located at the Institute of Geology, Chinese Academy of Geological Sciences, Beijing. Acceleration voltage and current of the system were conditioned at 20 kV and 0.11 nA, respectively. Backscattered electron (BSE) images of quartz and micro-inclusions were captured during system operation to highlight impurities.

2.2.3. Raw Quartz Processing

Processing procedure has to be applied on natural quartz deposits to better understand the concentrations of trace-element impurities of quartz and further to constrain their potential as HPQ resources, because natural raw quartz commonly contains various types of impurities, such as foreign minerals, melt, and fluid micro-inclusions.

The collected raw quartz lumps (1–2 kg) were separated into decimeter-scale pieces with a hammer. All quartz fragments were then washed and brushed in ultrapure water (18.2 MΩ·cm), and air-dried to clean up quartz surface contamination, especially clay minerals and iron oxide coatings. Nearly 1 kg pre-cleaned quartz fragments were selected for further beneficiation.

A thermal treatment protocol was adopted for the quartz fragments to expose possible impurities enriched in micro fissures and along dislocations, as well as to remove fluid inclusions to some extent. The quartz fragments were placed into a muffle furnace and calcined at 1000 °C for 2 h and cooled with ultrapure water in a stainless steel sink. The calcined quartz pieces (~100 g) were then carefully crushed and milled in an agate mortar. Standard nylon sieves were used to divide quartz sands into different fractions, and grain sizes of 450–200 μm (40–80 mesh) and 200–125 μm (80–120 mesh) were collected separately for additional chemical treatment. Quartz powders of each fraction (~5 g) were put in pre-cleaned 50 mL Teflon breakers and acid-leached at 90 °C for 4 h in a shaking water bath (Julabo SW23, JULABO Technology, Beijing, China) by using a HCl-HF acid leaching system with 9 mL ultrapure HCl and 1 mL ultrapure HF. After acid leaching, quartz powders were washed ten times with ultrapure water and air-dried at room temperature.

2.2.4. Bulk Solution Chemical Analysis by ICP-OES

The trace element concentrations of bulk raw quartz powders and processed quartz powders were analyzed by solution ICP-OES. The coarser (40–80 mesh) and finer (80–120 mesh) quartz aliquots were chemically digested within a HNO₃-HF acid system using a microwave digestion instrument (MARS 6, CEM Corporation, Matthews, NC, USA), separately.

About 300 mg per sample was digested by 4 mL ultrapure HNO₃ and 1 mL ultrapure HF in a closed digestion vessel at 180 °C for 30 min. Remaining HF in produced solution was subsequently removed by heating the vessels on an electric heating plate at 150 °C to shrink the solution into droplets. During the heating procedure, 3 mL ultrapure water was added twice for further evaporation of the residual HF. The ultimate droplets were diluted to 15 mL or 50 mL depending on element concentrations for the ICP-OES measurements. The analysis was performed using a SPECTRO ARCOS ICP-OES analyzer (SPECTRO Analytical Instruments GmbH, Kleve, Germany). Nine elements which are common in quartz and pivotal for assessment of quartz quality were selected for elemental analysis, including Al, B, Ca, Fe, K, Li, Na, P, Ti. Measurement sequences revealed that procedural detection limits for Al, B, Ca, Fe, Li, and Ti range from 1 to 2 μg·L⁻¹, whereas for Na and K the procedural limits of detection range from 7 to 40 μg·L⁻¹. P has procedural detection limits ranging from 6 to 16 μg·L⁻¹. The analytical results are reproducible with a relative standard deviation below 10%.

3. Results

3.1. Petrography

3.1.1. Fujiashan Quartz Veins

The Fujiashan quartz veins are commonly fine-grained. Granular sizes of quartz crystals range from <1 μm to 1000 μm, with an average of 200–300 μm. Under polarized light, quartz grains exhibited a mortar texture (Figure 4a), showing deformation with dynamic recrystallization. The crystals are relatively clear, and fluid and melt inclusions are uncommon. Along the fine-grained intragranular quartz mortar borders, however micro-inclusion impurities do preferentially concentrate and outline grain appearances. Some quartz crystals show undulatory extinction and form a stripe-like pattern (Figure 4b). Primary fluid inclusions and pseudosecondary trails with liquid-vapor phases could be observed within quartz grains. In some cases, secondary fluid inclusion trails are visible and cut through crystal boundaries, showing later fluid alteration (Figure 4c).

The detected mineral inclusions in Fujiashan-II vein quartz are relatively rare, though including muscovite (10–600 μm in size), rutile (5–25 μm), iron oxides (10–200 μm), and minor K-feldspar (50–500 μm) in the order of abundance. Needle-like rutile inclusions generally demonstrate aggregated cluster patterns and are situated along intragranular rims (Figure 5a,b). Inclusions of muscovite are spotted occasionally along grain boundaries or embedded within crystals. By comparison, solid inclusions observed in Fujiashan-I vein quartz are K-feldspar (50–4000 μm), muscovite (10–700 μm), iron oxides (20–200 μm), and trace kaolinite (50–150 μm) which occurs in paragenesis with adjacent K-feldspar crystal (Figure 5c). Inclusions of K-feldspar are very common, and ubiquitously distributed along quartz grain boundaries and/or within quartz crystals. On certain occasions, large K-feldspar crystals enclave rounded quartz sub-grains, leading to variegated texture on the SEM images (Figure 5c).

3.1.2. Yipanqiu Quartz Veins

Grain sizes in the Yipanqiu quartz are highly variable and range from <1 μm up to 1 mm with an average of about 100 μm (Figure 4d), indicating a finer granular distribution than that of the Fujiashan vein quartz. Quartz grains generally form a close intergrowth assemblage with each other. Few larger crystals have undulatory extinction and stripe-like pattern, showing associated recrystallisation to some degree. Sub-micro fluid inclusions are extremely common. Some of these fluid inclusions occur in parallel to growth zones of quartz crystal, denoting a primary origin. In few instances, the fluid inclusions contain vapor bubbles and occasionally precipitated solid crystals which are tentatively interpreted as salt minerals (Figure 4e).

Microscopical observation on the Yipanqiu quartz found that mineral inclusions contain K-feldspar (10–2000 μm), muscovite (<10–300 μm), iron oxides (10–200 μm), and rutile (10–30 μm). K-feldspar minerals are very common and are the dominate micro-solid inclusions (Figure 5d). Muscovite has variable sizes and lies along grain boundaries and in some cases, co-occurs with K-feldspar (Figure 5e). Rutile inclusions are tiny needles in shape and embedded within quartz crystals as exsolution products (Figure 4f). On rare occasions, small apatite crystals (5–10 μm) in paragenesis with K-feldspar could also be detected.

3.2. Chemical Composition

Trace element compositions for all the collected raw quartz samples and their processed products determined by solution ICP-OES analysis are summarized in

Table 1. Trace element concentrations in μg g⁻¹ of raw and processed quartz samples from the Fujiashan and Yipanqiu quartz veins in southeastern Hubei Province (China) determined by bulk solution ICP-OES analysis.

Sample Number	Quartz Powder	Grain Size (Mesh)	Al	B	Ca	Fe	K	Li	Na	P	Ti	Total
Fujiashan-I vein
FJS-I	raw	40~80	650.60	1.96	144.79	23.31	636.93	5.06	39.46	<0.8	10.21	1512.33
	processed	40~80	196.67	<0.10	8.57	2.92	153.02	5.17	4.55	<0.3	3.78	374.66
	raw	80~120	764.97	1.85	39.73	67.67	814.84	4.99	47.44	<0.8	7.98	1749.47
	processed	80~120	185.42	<0.10	7.27	2.91	144.58	4.85	5.15	<0.3	3.57	353.74
Fujiashan-II vein
FJS-II	raw	40~80	69.13	1.83	17.99	13.78	6.92	3.99	28.12	<0.8	9.46	151.22
	processed	40~80	29.82	0.88	2.68	1.22	<2.10	3.70	6.86	<0.8	5.40	50.54
	raw	80~120	70.60	1.95	30.37	58.89	7.91	3.95	27.42	<0.8	9.07	210.16
	processed	80~120	28.18	0.37	4.66	0.68	<2.10	3.72	6.60	<0.8	4.45	48.66
Yipanqiu quartz at Renyou
YPQ-R	raw	40~80	2448.93	0.96	32.93	14.68	2884.48	15.00	51.95	1.65	4.25	5454.83
	processed	40~80	137.39	<0.10	8.33	5.73	35.63	9.92	2.05	<0.30	4.41	203.47
	raw	80~120	2910.47	1.04	42.20	13.65	3564.08	15.00	56.94	<0.80	4.67	6608.04
	processed	80~120	141.95	<0.10	7.55	4.27	30.91	10.79	1.64	<0.30	4.97	202.09
Yipanqiu quartz at Zimudong
YPQ-Z	raw	40~80	2425.13	0.77	24.15	28.05	1547.74	39.54	38.15	1.79	4.62	4109.95
	processed	40~80	568.46	<0.10	8.40	1.32	66.26	44.65	15.11	<0.80	2.01	706.20
	raw	80~120	2296.15	1.00	34.36	41.86	1916.98	45.48	46.42	1.33	6.84	4390.42
	processed	80~120	550.23	<0.10	7.69	1.68	62.98	44.45	14.78	<0.80	2.17	683.97

. The total concentrations of elemental impurities in raw and processed quartz from different veins are highly variable.

3.2.1. Raw Quartz

An interesting phenomenon is that for each of the investigated raw quartz samples, the finer fraction (80–120 mesh) has generally a higher sum of the analyzed elements compared with the coarser fraction (40–80 mesh). Major contributors to the data discrepancy are elements of Al, K, and Fe (Table 1). For instance, total concentration of 80–120 mesh fraction of raw quartz from Renyou site in Yipanqiu deposit is 6608 μg g⁻¹, while the 40–80 mesh fraction has a concentration of 5455 μg g⁻¹ which is evidently lower than the coarser grains. A favorable explanation could be due to the existence of large-sized mineral inclusions, such as K-feldspar, muscovite, as well as iron-oxides (Figure 4 and Figure 5) which feature perfect cleavage characteristics, the smaller cleaved flakes of these minerals produced from milling therefore tend to stay in finer fraction, i.e., the 80–120 mesh. The analytical discrepancy thus may be attributed to the artificial mechanic handling of bulk quartz samples. For better understanding of the impurities within raw quartz crystals, trace element concentrations of the 40–80 mesh fraction are considered in the following discussion.

Though data of raw quartz in Fujiashan-II vein illustrate the lowest concentrations of elements compared with other three samples, total sum of impurities from its 40–80 mesh fraction still reach up to 151 μg g⁻¹, which is significantly higher than the HPQ criteria defined by Harben [15] and Müller et al. [6] despite the fact that these criteria are more suitable to assess lattice-bound impurities in quartz. Al and K concentrations of the 40–80 mesh fractions in Fujiashan-I and Yipanqiu-I vein quartz are very high, falling within ranges of 651–2449 μg g⁻¹ and 637–2885 μg g⁻¹ respectively, indicative of the presence of mineral inclusions of feldspar and muscovite (Figure 4 and Figure 5). Those of Fe and Ti are 15–28 μg g⁻¹ and 4.3–10.2 μg g⁻¹ separately, indicating the presence of iron-oxides and rutile inclusions, but alternatively to lattice-bound Fe and Ti. Na and Ca contents of all the raw samples are relatively high and separately spans from 28 to 52 μg g⁻¹ and from 18 to 145 μg g⁻¹, which could be referred to fluid inclusions and/or undetected solid inclusions, such as calcite, albite. P contents for all the samples from Fujiashan quartz are below detection limit with exception of the Yipanqiu-I vein where elevated P content was detected, probably corresponding with apatite inclusions.

In summary, because of common occurrence of K-feldspar, muscovite, and other solid inclusions within the raw quartz samples, bulk chemical compositions of these raw quartz are superimposed by mineral inclusions and the concentrations of impurities within quartz crystals could not be authentically documented.

3.2.2. Processed Quartz

Data of trace element concentrations of processed quartz for all the quartz samples reveal noticeable purification efficiencies (Table 1). Sum of the analyzed elements for all samples is well below 1000 μg g⁻¹. Analytical discrepancies related to grain sizes are no longer evident, suggesting that milling-produced flakes of mineral inclusions inside the sample powders were likely removed to a large extent during processing. Total impurities contents of the 80–120 mesh fraction are slightly lower than those of the 40–80 mesh fraction, and the main contributors are Ca, K, and Na which may be referred to fluid inclusions. The causes are probably related to calcination and subsequent acid-leaching that could remove more impurities contained within fluid inclusions along micro-tracks in finer quartz grains. The analyzed element contents of the processed 80–120 mesh quartz sands therefore could reflect chemical compositions of the quartz crystals more representatively compared to the coarser fraction, and therefore are discussed in the following section.

Processed quartz sample from the Fujiashan-II vein demonstrates the lowest total content of trace elements compared to the other deposits discussed in this study, indicating the highest quality of quartz. The sum content of impurities of the processed 80–120 mesh fraction in the Fujiashan-II vein is 48.7 μg g⁻¹ (Table 1), slightly below the upper concentration limit of HPQ [15]. The processed quartz has moderately low Al (28.2 μg g⁻¹), moderately high Ti (4.5 μg g⁻¹), and very low Fe (0.7 μg g⁻¹) concentrations. The Li content is 3.7 μg g⁻¹ and B is 0.4 μg g⁻¹. The alkali and alkali earth elements K, Na, and Ca are all below the upper content limits of HPQ designated by Müller et al. [6]. P concentration is less than 0.8 μg g⁻¹, and also meet the HPQ requirement.

The processed quartz from Fujiashan-I vein is characterized by moderately high Li (4.9 μg g⁻¹) and Fe (2.9 μg g⁻¹) and moderately low Ti (3.6 μg g⁻¹) contents. Although the Li, Na, Ti, and Fe concentrations in quartz are below the upper limits of HPQ, similar to those of the Fujiashan-II vein quartz; however, Al and K contents are very high and well above the HPQ limits, probably resulting from residual K-feldspar inclusions that were not removed during beneficiation. In comparison, the processed quartz sands of the Yipanqiu-I vein show variable concentrations of impurities for the two presented samples from the Renyou and Zimudong localities. These processed quartz sands have very high Li (11–44 μg g⁻¹), Al (142–550 μg g⁻¹), K (31–63 μg g⁻¹), and moderately high Ti (2–5 μg g⁻¹) contents. The sums of the elements for these processed quartz sands are 202 and 684 μg g⁻¹, which considerably exceed the 50 μg g⁻¹ impurities upper limit of HPQ.

4. Discussion

4.1. Economic Assessment with HPQ Aspect

The main objective of this study is to assess and identify the potential of vein quartz deposits as HPQ resources in southeastern Hubei. According to the criteria defined by Harben [15] and Müller et al. [6], total impurities of high-purity quartz should not exceed 50 μg g⁻¹ with specific upper limits for different elements.

It should be noted that because chemical analysis technique is limited to bulk solution ICP-OES in this study, trace element contents of raw quartz crystals and their processed products determined in this investigation cannot represent the contents of lattice-bound trace elements within quartz grains [4,6]. Even though processing flowsheet used in this effort did reduce the level of impurities of raw quartz considerably as illustrated in Table 1, mineral and fluid inclusions incorporated within or in paragenesis with quartz crystal cannot be removed completely during processing. Advanced quartz processing techniques such as flotation as well as hot chlorination which could further remove fluid inclusions and foreign minerals in quartz are needed to better constrain the concentrations of structurally incorporated trace elements. An alternative solution to this problem might rely on in situ spot analytical techniques such as LA-ICP-MS, EPMA, and SIMS in the future, though specific instrumental parameters are required to ensure that the ablated mass volume should be sufficient for sample representation [4,20,24].

Figure 6 illustrates the measured concentrations of Al, Ti, and Li of all the investigated samples, including raw and processed quartz powders. Processed quartz from Fujiashan-II vein plots into the high-purity quartz field. In addition, concentrations of all the other trace elements listed in Table 1 for this vein are below the upper concentration limits of HPQ proposed by Müller et al. [6]. Since portion of micro-impurities such as K-feldspar and fluid inclusions still remain in the processed quartz, superimposing the concentrations of lattice-bound trace elements, it is confident to speculate an even lower level of impurities if advanced processing techniques such as flotation and hot chlorination would be applied in the future to further remove non-lattice-bound impurities.

Moreover, the Fujiashan-II quartz vein has a resource estimation of approximately 470,000 metric tons, falling into the designated field of economic interest deposits as illustrated in Figure 7. The resource estimation is similar to that of the Kvalvik quartz vein, a possible HPQ deposit in west Norway [6]. In addition, foreign minerals assemblage of this vein is comparable to those in the hydrothermal quartz veins from southern Ural Region and Norway which were deemed as potential HPQ resources [6,19]. In accordance with the HPQ definition [6,15], and certain deposit size requirement, the medium-sized Fujiashan-II quartz vein can be ranked as potentially economic HPQ deposit.

The processed quartz in the Fujiashan-I vein has a sum of elemental concentrations above 350 μg g⁻¹, with high contents of Al and K (Table 1), which are well above the upper concentration limit of HPQ [6,15]. Therefore, the Fujiashan-I quartz vein cannot be deemed as HPQ deposit in a strict sense. However, the high concentrations of Al and K in the processed quartz are probably caused by ubiquitous occurrence of K-feldspar (Figure 5). Large portion of the K-feldspar in the raw materials could not be removed during processing in this study, though hot acid leaching with 1 mL HF designed did have an ability to attack and decompose K-feldspar to a minor extent [43], leading to decline in impurities concentrations for the processed quartz (Table 1). Further processing for removal of K-feldspar would significantly reduce the impurities level. Except Al and K, the contents of other analyzed elements in the Fujiashan-I quartz such as Ca, Li, Na, and Ti meet the HPQ requirement and are comparable to those of the Fujiashan-II quartz (Table 1). Specifically, concentrations of B and P are below the limits of detection (0.1 and 0.3 μg g⁻¹, respectively), indicating potential application for solar panel industry. Given the immediate vicinity of the Fujiashan-II quartz vein (Figure 2a) and the similarity in their geogenesis, it is reasonable to postulate that the Fujiashan-I quartz may still contain structurally incorporated impurities less than 50 μg g⁻¹. Another promising aspect of economic interest of the Fujiashan-I quartz deposit derives from its inferred resource size which is near 4.8 million metric tons according to local mapping results, and thus, the deposit can be considered as potentially economic (Figure 7).

Trace element concentrations of processed quartz from the Yipanqiu deposit demonstrate unfavorable outlook. The processed products of 80–120 mesh quartz powders from Renyou and Zimudong localities have total trace element concentrations of 202 μg·g⁻¹ and 684 μg·g⁻¹, well above the upper concentration level of 50 μg·g⁻¹. Although the high Al and K contents may still be caused by residual K-feldspar inclusions remained in the processed quartz, because of the high Li contents (Table 1), the Yipanqiu quartz cannot be graded as a HPQ resource. Li contents are mostly constant between raw quartz and processed quartz for all the investigated samples (Figure 6), suggesting that the analyzed data of Li likely reflect the contents of lattice-bound Li [21,44]. Moderately high Na and Ca concentrations possibly indicate the presence of remaining fluid inclusions [20,21,45,46]. Additional challenges will be the separation of the quartz crystals due to the intimate intergrowth texture and various grain sizes (Figure 4). The deposit size is approximately 9.0 million metric tons and plots into the potentially economic field, but way beyond the HPQ field in the diagram (Figure 7). Thus, the Yipanqiu quartz deposit is not considered as a source for HPQ material.

4.2. Genetic Implications for the Fujiashan and Yipanqiu Quartz Deposits

The genesis of high-purity quartz with low trace-element concentrations was proposed by many authors [11,47,48]. These studies have demonstrated that during multiple regional metamorphism and related deformation, retrograde recrystallisation of quartz occurring at lower temperature could reduce grain-boundary area due to lattice recovery and stain-reduced grain boundary migration [48]. This mechanism could heal lattice defects and expel trace elements to the grain boundaries and/or concentrate them into the micro-inclusions, and thus purify the quartz crystals [6].

Low trace-element concentrations of the Fujiashan-II high-purity quartz might be caused by metamorphic process. Regional metamorphic history of the Fujiashan area however has never been scrutinized before, leading to difficulty and obscurity for the genetic correlation between metamorphic events and high-purity quartz formation. Nevertheless, because the intrusion timing of the Jiugongshan granite has been constrained at early Neoproterozoic [39], the associated quartz veins in this area thus have the possibilities to undergo long-term deformation and recrystallization. In fact, the common mortar texture, stripe-like pattern, and concentration of micro-inclusions along the grain boundary observed in the Fujiashan quartz reveal that the deposit has experienced regional or dynamic metamorphic deformation (Figure 4), indicating retrograde overprints. Ti exsolution illustrated by occurrence of intragranular rutile needles may provide further support for grain recrystallisation and thus may result in low trace concentrations in the quartz.

The Yipanqiu quartz deposit contains higher trace-element contents compared to data of the Fujiashan quartz, excluding its potential as a HPQ source. Because the Shadian intrusion formed at 125 ± 1 Ma [42], the genetically associated Yipanqiu vein quartz thus has a much younger crystallization age and a probably less metamorphic degree. Quartz samples from this vein are characterized by highly variable grain sizes with intimate intergrowth texture and random distribution of micro-inclusions (Figure 4), likely suggesting weak regional metamorphism in the Shadian area. Due to a short-term metamorphic history, the recrystallization process occurred may not be able to expel sufficient trace elements to the grain boundaries to purify the quartz crystals. Thus, with the exception of solid inclusions, large portions of incipient trace elements are still structure-incorporated and cannot be readily removed by conventional processing.

In summary, the results of the present study through microscopy observation and trace-element analysis of the Fujiashan and Yipanqiu quartz deposits show significant discrepancy in the formation age of the two deposits probably resulting in distinct metamorphism levels. The higher-level recrystallisation of the Fujiashan vein perhaps promotes formation of high-purity quartz deposit, whereas relatively lower-level metamorphic deformation of the Yipanqiu vein leads to high detrimental element concentrations and thus low-quality quartz deposit.

5. Conclusions

Trace-element concentrations of the Fujiashan and Yipanqiu quartz deposits in the southeastern Hubei Province, China were determined in the present study for the first time. Raw quartz samples were subjected to beneficiation process by thermal treatment and calcination, in combination of hot acid leaching. The raw quartz samples and their equivalent processed products were analyzed by using a combination of analytical techniques of optical and SEM microscopy, as well as bulk solution ICP-MS. These efforts characterize the types and trace-element concentrations of impurities in quartz and necessitate the assessment of high-purity quartz potential for the investigated quartz deposits. The conclusions are drawn as follows.

Microscopical investigation carried out on the Fujiashan and Yipanqiu quartz deposits reveal that major micro-inclusions in quartz contain solids of K-feldspar, muscovite, iron-oxides, rutile and trace apatite, as well as fluid inclusions. The Fujiashan-II vein are characterized by low content of micro-inclusions. In contrast, mineral inclusions especially K-feldspar are very common in the Yipanqiu quartz deposits.

Bulk solution ICP-OES analysis demonstrates that the processed quartz grains from the Fujiashan-II vein contain cumulative trace-element concentrations of <50 μg g⁻¹ with <30 μg g⁻¹ Al and <10 μg g⁻¹ Ti, which define the vein as a high-purity quartz resource in accordance with the HPQ definition [6,15]. The concentrations of detrimental elements other than Al and K of the processed quartz grains from the Fujiashan-I vein also satisfy the HPQ definition; high Al and K contents above the limits in the processed quartz are probably caused by residual K-feldspar inclusions which could be removed by further beneficiation process, e.g., flotation and hot chlorination. Considering the immediate vicinity of the Fujiashan-II high-purity quartz vein and the similarity in their geogenesis background, it is still promising to speculate that the Fujiashan-I vein could be deemed as a potential HPQ deposit. However, chemical analysis results of the processed samples in Renyou and Zimudong localities for the Yipanqiu quartz deposit show an unbright image with respect to high-purity quartz due to high impurities contents, especially the likely lattice-bound Li contents. From an economic point of view, the intergrowth texture with highly variable grain sizes in conjunction with ubiquitous fluid inclusions incorporated within quartz grains from the Yipanqiu vein deposit are also challenges for quartz separation.

Genetic examinations on the Fujiashan and Yipanqiu quartz veins reveal that the discrepancy in trace-element contents for the two deposits might rely on distinct metamorphism processes. The Fujiashan veins are genetically linked to nearby early Neoproterozoic Jiugongshan granites and have experienced long-term metamorphism-related deformation. The recrystallisation and reorganization processes of quartz lattice thus probably caused defects healing and expelling of trace elements, leading to reduction of impurities concentrations. In contrast to the Fujiashan quartz veins, the Yipanqiu deposit crystallized at middle Cretaceous has a much younger formation age and the subsequent metamorphic deformation occurred may be less significant. Therefore, the quartz grains in the Yipanqiu quartz vein perhaps retain high trace elements, excluding its potential for being high-purity quartz deposit.