Sedimentary Characteristics of the Member 1 of the Middle Devonian Suotoushan Formation and Its Ore-Bearing Potential in Northeastern Yunnan

Abstract

Super-large quartz sandstone deposit has been discovered in Northeastern Yunnan and the major ore-bearing bed is occurred in the Member 1 of the Middle Devonian Suotoushan Formation. Systematic study on the sedimentary characteristics of the Suotoushan Formation in Yiliang region, northeastern Yunnan Province has been conducted, Profile surveying, drilling and trenching show that, microscopic identification, the thickness of the Suotoushan Formation is 62.8 m ∼ 222.7 m and principally composed of gray-white thick-bedded medium-grained quartz sandstone with well developed parallel bedding, swash bedding, oscillation cross-bedding and wave ripple, showing remarkable wave action features. Thin section observation shows that quartz sandstone in the Member 1 of the Suotoushan Formation is grain supported, siliceous cemented, concave-convex and line contact and the pressure solution and secondary overgrowth of quartz sand grains are obvious. Grain size analysis shows that the grain size is concentrated between 1.5Φ and 3.5Φ with saltation population up to 99% which is composed by tow sub populations and lacking of traction and suspension populations, showing a typical foreshore grain size distribution. Chemical analysis shows SiO2 chemical purity of the quartz sandstone is 99.05 ∼ 99.39 with weighted average 98.99 (N = 11) which has exceeded the industry standard for first-grade raw material of plate glass. Based our studies above, it can be concluded that the quartz sandstone in Member 1 of Suotoushan Formation is high energy coastal foreshore sediments with high texture and mineral maturity, and has been further purified by diagenesis, and both the grain-size characteristic and chemical purity have reached the standard of high-purity quartz sandstone, which can be expected raw material for high-purity quartz. The results in this article can provide accurate prospecting target to enlarge the resources potential of high-purity quartz deposits in northeastern Yunnan.

1. Introduction

High purity quartz is a high-tech strategic mineral resource which is scarce in the world and in China，and the quartz sandstone of littoral facies is a potential high-purity deposit [1]. It is the raw material of high-grade quartz products and the material basis of high-end products in silicon industry. It can be widely used in photovoltaic electronic information electric light source and optical communication industry. It plays an important role in strategic emerging industries such as new materials and new energy [2]. High purity quartz sandstone is a characteristic mineral in Wumeng mountain area, mainly distributed in northeast Yunnan province, which is the pillar mineral for zhaotong Government to build “Yunnan Silicon Valley” [3]. The current domestic production of high purity quartz sandstone strata in fluvial facies (Xiaodian in Suqian city, Sidaoliangzi in Inner Mongolia, Hewan in Lanzhou city), lacustrine facies (Dengjiatun in Shuangliao city, Yamenying and Zhangwutai in Tongliao city, Songfeng in Jiangxi), less littoral facies is found and always mainly loose quartz sand (Dongfang city, Beihai city, Dongshan city), it’s low purity and impurity. The large-scale production of high-purity quartz sandstone produced in northeast Yunnan is restricted by the lag of mineral resources assessment and sedimentary facies research. Therefore, it’s the strategic demand to carry out the potential evaluation of high-purity quartz ore in northeast Yunnan and expand its resource stock to support “Yunnan Silicon Valley”. The Middle Devonian Suotoushan Formation distribute extensive in Yiliang area, northeast Yunnan, and it’s an important potential area of high purity quartz sandstone deposit [4]. Previous studies based on small scale and limited outcrop sequence stratigraphy considered that the sedimentary facies of middle Devonian in northeast Yunnan is barrier coast facies [5], and more detailed sedimentary facies analysis has not been carried out systematically. Therefore, the author carried out a systematic study on the sedimentary characteristics of the Member 1 of Suotoushan Formation, collected 7 samples for grain size analysis and 22 samples for chemical analysis (Figure 1), and discussed the sedimentary facies characteristics of the first member of Suotoushan Formation and the genetic mechanism of the high purity quartz sandstone in northeast Yunnan.

Figure 1.

Devonian tectonic setting, paleogeographic framework, brief geological map of Northeastern Yunnan and sampling position (after reference [5]).

The study area is located in the Kang-Dian fault depression belt at the western edge of the South China block, with kang-dian uplift in the west and Upper Yangtze uplift in the north, forming a southward opening bay, known as Zhaotong-Weining Depression (called Zhaotong Bay in this paper) [6]. In the late Middle Devonian, the study area is a littoral shallow sea clastic sedimentary area, mainly composed of quartz sand with high maturity in the littoral zone, distributed in a ring band on the east side of the Kang-Dian uplift and the southwest side of the Upper Yangtze uplift. Due to the influence of the Middle Devonian Haikou movement, small uplift is seen in the interior of the bay (Figure 1) [6]. Created by Xian Si-yuan in 1974 its mountain group name in Suotoushan of Qingmen in zhaotong city, the Middle Devonian strata group of Qingmen which is the shelf sea sedimentary contact the Late-Middle Devonian on Hongyapo group of tidal flat shelf sea sedimentary contact, main rock is fine-grained quartz sandstone, pure texture, well sorting and roundness. The lower and upper as a pure quartz sandstone, the middle part is sandstone with siltstone and mudstone [7]. The Suotoushan Formation in the area can be roughly divided into three members. The Member 1 and Member 3 of Suotoushan Formation are mainly quartz sandstone. The Member 2 of Suotoushan Formation contains more siltstone and mudstone. The quartz sandstone in the first member of Yutoushan Formation has the purest texture and is the host layer of high purity quartz sandstone in the study area.

2. Type section

In order to reveal the sedimentary characteristics and lithofacies paleogeographic evolution of quartz sandstone in the Member 1 of Suotoushan Formation in the study area, the author measured several sections，The type sections are listed as follows.

(1) The measured stratigraphic profile of Suotoushan Formation of Middle Devonian (D2st) in Caijiawuji, Yiliang County (P14)
Overlying strata: Hongyapo Formation of Middle Devonian (D2hy)
30. Dark gray shale, the upper part is gray white thin bedded quartz sandstone, containing mud and gravel, the grain size becomes coarser upward	0.9m
—————————————conformable contact————————— —————
Member 3 of Suotoushan Formation(D2st^3)
29. Dark gray shale, the upper part is gray white thin bedded quartz sandstone, containing mud gravel, upward grain size coarsening	9.8m
28. The grayish white thin bedded fine-grained siltstone sandstone intercalated with gray mud shale and wave-forming ripple developed	3.7m
27. The lower part is grayish white medium grained quartz sandstone with 8cm grayish yellow thin bedded argillaceous siltstone. the upper part is gray-white thin bedded fine grained quartz sandstone	8.8m
26. The gray-white thin bedded fine-grained quartz sandstone interbedded with dark gray mud shale in a ratio of approximately 3:1–5:1	30.7m
25. Gray-white middle-grained fine-grained quartz sandstone, with bluish silty mudstone at the bottom	8.8m
24. Gray-white thin bedded fine grained quartz sandstone	5.9m
23. Gray-white fine-grained quartz sandstone mixed with gray-green and purple mudstone, mud shale	12.6m
22. Gray thick bedded fine-grained quartz sandstone with cross bedding	14.4m
21. Gray thin-medium grained quartz sandstone, with argillaceous masses, oblique bedding and scour surface, gray thin-medium lithic quartz sandstone, silty mudstone intercalated mudstone can be seen locally	11m
20. Greyish-white thick bed – massive fine grained quartz sandstone	4.5m
—————————————conformable contact————————— —————
Member 2 of Suotoushan Formation(D2st^2)
19. Interbedded gray mudstone and gray-yellow medium siltstone with unequal thickness	20.1m
18. Gray – white meso-grained quartz sandstone developed in parallel bedding	36.5m
17. The lower part is gray and black-dark, yellow-gray, green mud shale, the upper part is dark gray mudstone	39.2m
16. Greyish-white mesomorphic powder-fine quartz sandstone	2.5m
15. Thin bedded mudstone and silty mudstone interspersed with gray-white medium – grained quartz sandstone	57.3m
14. The lower part is dark-gray thin-medium quartz siltstone and dark-gray argillaceous siltstone, and the middle and upper part is gray-yellow medium quartz siltstone. Pyrite clumps are seen in the quartz siltstone at the top	20.7m
13. The lower part is grayish yellow medium lithic siltstone, and the middle and upper part is grayish white medium fine siltstone sandstone	14.7m
12. Dark gray mudstone	14.9m
—————————————conformable contact————————— —————
Member 1 of Suotoushan Formation(D2st^1)
11. Gray-white thick bedded fine grained quartz sandstone	6.8m
10. The lower is yellow-gray medium siltstone and gray-white thick bedded fine grained quartz sandstone, the middle is covered by mantle rock, the upper is gray thin bedded argillaceous siltstone and gray-white thin bedded fine-grained quartz sandstone	45.6m
9. The gray-white thick bedded medium-grained quartz sandstone.the lower part contains 1–3cm purplish thin mudstone and 30cm argillaceous siltstone	31.1m
8. Gray-white thick bedded fine grained quartz sandstone	43.5m
7. Gray-white thick bedded fine-grained quartz sandstone with gray-white thin bedded fine-grained quartz sandstone in the middle, grayish yellow silty mudstone and mid-bedded fine-grained quartz sandstone can be seen at the top	4.4m
6. Gray-white medium-thick bedded fine-grained silty quartz sandstone	7m
5. Gray-white thick bedded fine grained quartz sandstone	28.8m
4. Gra-white thick bedded fine-grained quartz sandstone Light flesh-red, grayish white thick bedded medium-grained quartz sandstone, occasionally parallel bedded. The upper quartz sandstone is obliquely bedded. Gray argillaceous siltstone about 60cm in the lower part, gray-green mudstone 30–40cm in the middle, and purple red quartz sandstone 50cm thick in the top	48.2m
3. Gray-white thick bedded medium-fine grained quartz sandstone, with 15cm iron bearing fine grained quartz sandstone at the bottom. The upper medium-grained quartz sandstone reaches the industrial standard of silica ore	6.1m
—————————————conformable contact——————————————
Lower strata: Middle Devonian Qingmen Formation (D2qm)
2. Gray-green thin bedded siltstone intermixed with dark gray shale, 3–6cm purplish red argillaceous siltstone at the top, and 1.2m dark gray argillaceous mass at the cross section	1.2m
(2) Measured stratigraphic profile of Yutoushan Formation of Middle Devonian (D2st) in Songlinpo, Yiliang County(P6)
Overlying strata: Hongyapo Formation of Middle Devonian (D2hy)
24. Purplish-red and gray-green silty mudstone interbedded with grey thin bedded quartz silt-fine sandstone. The ratio of the two lithologies is about 1:1	9.77m
—————————————conformable contact——————————————
Member 3 of Suotoushan Formation(D2st^3)
23. Gray-white thin-medium grained quartz sandstone	1.26m
22. Gray, thin-medium quartz sandstone. The quartz sand layers are interspersed with gray-green and 1–8cm purplish red thin-layer siltstone. From bottom to top, quartz sandstone becomes thinner and siltstone thickens. In the middle and upper part, the ratio of quartz sandstone to gray-green and purple siltstone approaches 1:1	2.84m
21. Light gray thick bed to massive quartz sandstone	5.77m
20. The bottom is gray-white thin bedded fine-grained quartz sandstone(gravels at the bottom), the middle is gray-yellow thin bedded siltstone, the top is gray-green thin bedded mudstone	1.29m
—————————————conformable contact——————————————
Member 2 of Suotoushan Formation(D2st^2)
19. Gray white thin to medium grained fine-grained quartz sandstone interbedded with mud shale and very thin argillaceous siltstone	1.03m
18. Thick layers of gray-white massive fine grained quartz sandstone. 40cm thick gray-green silty mudstone in the middle and upper part, and it is separated from the next layer by 1–3cm thick gray-green silty mudstone at the top	24.63m
17. Light grey mudstone interbedded with grey white quartz siltstone	0.58m
16. Greyish medium grained quartz sandstone	2.02m
15. Thick gray-white massive fine and medium-grained quartz sandstone	20.73m
14. It is a set of siltstone, quartz sandstone, argillaceous siltstone and silty mudstone assemblage, which can be divided into five parts. The bottom is purplish red siltstone, the middle and lower part is gray white thin bedded quartz sandstone, the middle part is sandwiched with 50cm thick gray white thin bedded argillaceous siltstone, the middle and upper part is purple red thin bedded argillaceous siltstone, the top is gray green thin bedded argillaceous mudstone	5.37m
—————————————conformable contact————————— —————
Member 1 of Suotoushan Formation(D2st^1)
13. Thick layer of gray-white massive fine grained quartz sandstone	12.4m
12. Light grey thin – mesomorphic fine-grained quartz sandstone bounded by grey-green extremely thin mudstone at the bottom	5.72m
11. Grayish-white medium-thick fine-grained quartz sandstone	0.71m
10. Gray, thin – medium quartz sandstone	3.57m
9. This layer is a combination of mud shale and siltstone, true thickness of this layer is about 70cm. It can be divided into three parts. The bottom (25cm) is gray green thin layered mud shale; The middle part(25cm) is gray-yellow medium-lithic quartz sandstone. The upper part (25cm) is purplish red thin bedded siltstone	0.67m
8. Greyish medium – thick fine grained quartz sandstone	1.8m
7. Gray-yellow thin – intermediate lithic quartz sandstone	3.14m
6. Thick greyish – massive fine grained quartz sandstone	0.9m
5. Grayish white thin – medium grained quartz sandstone interbedded with very thin grayish green mudstone	10.53m
4. Gray thin bedded fine grained quartz sandstone	4.66m
—————————————conformable contact————————— —————
Lower strata: Qingmen Formation of Middle Devonian (D2qm)
3. Gray-green silty mudstone	5.5m
(3) Measured stratigraphic profile of Suotoushan Formation (D2st) in Haibatou, Daguan County(TP1)
Overlying strata: Hongyapo Formation of Middle Devonian (D2hy)
13. Gray-green mudstone and silty mudstone interspersed with a small amount of gray-white thin bedded fine-grained quartz sandstone	28.40m
—————————————conformable contact————————— —————
Member 3 of Suotoushan Formation(D2st^3)
12. Gray – white thick bedded fine grained quartz sandstone	14.0m
—————————————conformable contact————————— —————
Member 2 of Suotoushan Formation(D2st^2)
11. Gray mudstone with little iron	1.30m
10. Gray thin bedded fine grained quartz sandstone	0.80m
9. Gray mudstone with little iron	0.90m
—————————————conformable contact————————— —————
Member 1 of Suotoushan Formation(D2st^1)
8. Gray medium-thick bedded fine-grained quartz sandstone, little pyrite nodules included	8.10m
7. Gray thick and blocky fine-grained quartz sandstone. fossil plant fragment included, large oblique bedding developed	16.50m
6. Gray medium-thick fine grained quartz sandstone	5.30m
5. Thin-bedded fine-grained quartz sandstone interbedded with iron argillaceous quartz siltstone of unequal thickness	8.40m
4. Thick gray blocky fine-grained quartz sandstone, horizontal bedding developed	19.60m
3. Gray thin bedded fine-grained quartz sandstone	2.10m
2. Gray medium-thick fine-grained quartz sandstone	2.80m
—————————————conformable contact————————— —————
Lower strata: Qingmen Formation of Middle Devonian (D2qm)
1. Gray silty mudstone	14.60m

From the above three representative measured sections, the boundary between top and bottom of Suotoushan Formation is clearly marked and the internal tripartite is extremely obvious, which can be compared regionally (Figure 2). The Member 1 of Suotoushan Formation is mainly composed of gray-white medium-thick bedded quartz sandstone with relatively few muddy interbeds, which is a concentrated production layer of high purity quartz sandstone. Many muddy interlayers in Member 2 of Suotoushan Formation, and the quartz sandstone is dark in color and mainly thin bedded, which is not the enrichment layer of high purity quartz sandstone. Although Member 3 of Suotoushan Formation contains more quartz sandstone, there are more mud interlayers, which is not the ideal formation for high purity quartz sandstone production. Therefore, only the sedimentary structure, tectonic characteristics and sedimentary environment of Member 1 of Suotoushan Formation are discussed detailedly in this paper.

Figure 2.

Histogram correlation diagram for the Middle Devonian Suotoushan Formation in Northeastern Yunnan.

3. Sediment structure

Member 1 of Suotoushan Formation in the study area is mainly gray-white medium thick bedded quartz sandstone. Microscopic identification results show that the minerals are mainly medium-fine quartz, with a content of 99% the roundness is subedge, grain support, Moderate sorted, concave-line contact, and the cementation mode is press-inlay – pore type. Field profile shows the Member 1 of Suotoushan Formation is dominated by gray-white medium-thick bedded quartz sandstone with sheet sand body. Parallel bedding (Figure 3a) and wash bedding (Figure 3b) are extremely well developed. Straight ridge wave-forming marks (Figure 3c), small flowing wave marks and exfoliated lineation can be seen on the plane. Skolithos in the vertical plane can also be seen in the Luohanba section (Figure 3e). Sedimentological studies show [8, 9] that parallel bedding, wash bedding and wave-forming ripple marks are typical structures of the foreshore wash zone, while small flow ripple marks are products of wave backflow deposition. Therefore, it can be considered that the sedimentary environment of Member 1 of Suotoushan formation is the foreshore zone. The existence of Skolithos [10] also indicates that the sedimentary environment of Member 1 of Suotoushan Formation is dominated by the foreshore zone.

Figure 3.

Typical sedimentary texture and structure of the Member 1of the Suotoushan Formation.

4. Grain size analysis

The grain size distribution characteristics of sedimentary rocks can reflect the hydrodynamic conditions of sedimentation, and the results of grain size analysis are an important means to distinguish the sedimentary environment [11]. Sediment can be transported in three ways in normal traction flow: suspension, saltation, bottom passage or traction. Different sedimentary environments have different transport modes, among which the eolian dune sand and beach sand have narrow grain size interval, saltation is specially developed in general, while the passage is not developed in general and there is a lack of suspension. The biggest characteristic of beach sand is that there is a cut-off point inside the saltation whole, which represents the slight change of wave round-trip energy [11]. The quartz sandstone in the Member 1 of Suotoushan Formation is mostly medium and fine grained quartz sandstone (Figure 3f), with excellent sorting, grain support, voidaceous cementation, and void filled by secondary enlarged quartz (Figure 3g). Its mineral maturity and structural maturity are extremely high. In order to quantitatively reveal the grain size distribution characteristics of quartz sandstone in the Member 1 of Suotoushan Formation, seven samples were randomly selected for grain size analysis in this paper (Table 1, Figure 4a–c).

Sample	Y170588001	Y170588002	Y170588003	Y170588004	Y170588005	Y170588006	Y170588007
separation coefficient(S0)	0.78	0.90	0.89	0.80	0.83	0.81	0.92
Standard deviation (σ)	0.41	0.35	0.38	0.34	0.32	0.64	0.81
Skewness (SK)	0.37	0.23	0.03	0.23	0.14	−0.01	4.76
Kurtosis (K)	2.92	3.22	3.52	2.90	3.11	3.07	29.92
Average (Mz)	2.31	2.27	2.13	2.25	2.40	1.99	3.01
Coarse sand(0 < Φ < 1)	0.00per	0.00per	0.00per	0.00per	0.00per	6.06per	0.00per
Medium sand(1 < Φ < 2)	23.24per	21.61per	34.76per	25.39per	9.77per	43.27per	0.00per
Fine sand(2 < Φ < 3)	71.63per	76.63per	63.61per	73.38per	86.83per	45.05per	52.15per
Very fine sand(3 < Φ < 4)	5.12	1.76per	1.63per	1.23per	3.39per	5.59	45.03per
Coarse silt(4 < Φ < 5)	0.01per	0.00per	0.00per	0.00per	0.00per	0.00per	0.82per
Fine powder sand(5 < Φ < 8)	0.00per	0.00per	0.00per	0.00per	0.00per	0.00per	0.00per
Clay(≥8Φ)	0.00per	0.00per	0.00per	0.00per	0.00per	0.00per	0.00per

Table 1.

Grain size parameters of the quartz sandstone from the Member 1 of the Suotoushan Formation.

Figure 4.

Grain size analysis diagram of the quartz sandstone from the Member 1 of the Suotoushan Formation (diagram (d) is after reference [11]) (NPA-Normal probability accumulation).

The results showed that the samples were dominated by fine sand, followed by medium sand, containing a certain proportion of very fine sand, most of which did not contain coarse silt, only one sample detected coarse sand (Y170588006, 6.06per), completely missing gravel particles, silt and clay particles. The separation coefficient (S₀) is between 0.78 and 0.92. Comparison sorting coefficient grading standard (Fine: 1–1.23; Good: 1.23–1.41; Medium: 1.41 to 1.74; Poor: 1.74–2.0; Bad: > 2.0) [11], and the sorting degree of the analyzed samples was better than “very good” grade. Standard deviation (σ) can also describe the sorting degree, with most samples ranging from good (0.35–0.50) to excellent (< 0.35). The skewness (SK) values of the samples were 2 near symmetrical (−0.10 ∼ +0.10), 3 positive skewness (+0.10 ∼ +0.30) and 2 polar positive skewness (+3 ∼ +1.0), and the grain size peak value was inclined to the fine end. It is generally believed that beach sand is mostly negatively skewed and dune sand is mostly positively skewed [11]. The characteristics of sample skewness of the Member 1 of Suotoushan Formation may be related to the dissolution and fining of quartz particles during diagenesis. Kurtosis mainly describes the convexity of grain size curve, so as to reflect the concentration degree of grain size, and is also an indicator of the degree of sediment sorting. The classification levels are: very wide (< 0.67 =, wide (0.67 ∼ 0.90), narrow (1.11 ∼ 1.50), very narrow (1.50 ∼ 3.00), and very narrow (> 3.0) [11]. The kurtosis of the quartz sand in the Member 1 of Suotoushan Formation is very narrow and the grain size distribution is unusually concentrated, indicating that the quartz sand has undergone very sufficient sorting and transformation, thus laying the foundation of high purity quartz sandstone. Appears above a mountain group quartz sandstone grain size analysis results show that the majority of samples to a single modal nearly normal distribution, the lack of time overall, overall and suspended saltation overall content above 99per, and can distinguish two general, show wave flow and reflow sedimentary characteristics, and the ideal of foreshore with particle size distribution (Figure 4d). It should be noted that the internal intercept points of the saltation of quartz sandstone in the Member 1 of Suotoushan Formation are not clear, indicating that the sorting ability of quartz sandstone is close to eolian dune sand and superior to ordinary beach sand. This is also an obvious feature of quartz sand in the Member 1 of Suotoushan Formation, which is different from other quartz sandstone along the shore.

5. Chemical purity

Chemical purity is one of the evaluation factors of high purity quartz sand [1], and also a standard to measure whether it meets the plate glass industry index [12]. In view of this, the chemical composition of quartz sandstone in the first and Member 3 of Suotoushan Formation in the study area was analyzed (Table 2). The analysis results show that, except for a small amount of Fe₂O₃, the SiO₂ index of the Member 1 of Suotoushan Formation quartz sandstone exceeds the standard of the first grade industrial index for flat glass, while the Fe₂O₃ and SiO₂ quartz sandstone of the Member 3 of Suotoushan Formation basically cannot reach the standard of the first grade. Therefore, the Member 1 of Suotoushan Formation is a favorable horizon for high purity quartz sandstone deposit.

Member 3 of Suotoushan Fomation	Sample	SiO2	Al2O3	Fe2O3	Member 1 of Suotoushan Fomation	Sample	SiO2	Al2O3	Fe2O3
	TC1-H1	90.21	4.20	2.35		TC0-H1	99.07	0.22	0.25
	TC1-H2	94.64	1.88	2.29		TC0-H2	99.05	0.21	0.23
	TC1-H3	97.08	0.41	2.25		TC0-H3	98.83	0.16	0.51
	TC1-H4	98.28	0.45	1.07		TC0-H4	99.14	0.26	0.26
	TC1-H5	98.18	0.33	1.32		TC0-H5	99.35	0.21	0.10
	TC1-H6	97.31	0.37	2.11		TC0-H6	99.26	0.19	0.14
	TC1-H7	97.84	0.40	1.48		TC0-H7	99.39	0.14	0.14
	TC1-H8	97.02	0.63	2.05		TC0-H8	99.20	0.14	0.32
	TC1-H9	98.50	0.56	0.60		TC0-H9	98.97	0.22	0.42
	TC1-H10	90.20	4.65	2.07		TC0-H10	98.28	0.43	0.33
	TC1-H11	98.06	0.29	1.51		TC0-H11	98.34	0.48	0.32
Industry standard of Flat glass	Superior	≥98.5	≤1.0	≤0.05	Standard according to the quality requirements of silicon materials for flat glass (DZ/T027-2002)
	Level 1	≥98.0	≤1.0	≤0.10

Table 2.

Chemical analysis data of the Member 1 and Member 3 quartz sandstone of the Soutoushan Formation (%).

6. Discussion

6.1 Correlation with grain size characteristics of high-purity quartz sand

The grain size and its distribution are two of the four basic characteristics of high purity quartz sand (chemical purity, grain size, grain size distribution and particle shape) [1] and an important factor which directly affects the equilibrium melting of high purity quartz sand and a material control index to reduce the defects of quartz glass [13]. Therefore, Shi Bin et al. established a set of grain size parameter evaluation system for high purity quartz sand [14]. The grain size parameters of typical high-purity quartz sand are as follows: average grain size (Mz) = 3.125Φ; Standard deviation (σ) = 0.274; Partial degrees (SK) = 0.000. The grain size parameters of samples (ZOZ) are as follows: average grain size (Mz) = 2.64Φ; Standard deviation (σ) = 0.33; Partial degrees (SK) = 0.36. The comparison results show that the grain size, grain size distribution and grain size curve of quartz sand in the Member 1 of Suotoushan Formation reach or approach the index of high quality and high purity quartz sandstone. Considering the common grain size characteristics of foreshore deposits, the quartz sandstone of the Member 1 of Suotoushan Formation should be the most ideal high purity quartz sandstone.

6.2 Relationship between sea level rise and high purity quartz sandstone

The Qingmen Formation in the lower strata of Suotoushan Formation is dominated by gray-green, yellow-green and yellow-gray mudstone, intermixed with sandy mudstone, siltstone, argillaceous limestone and limestone. The fossils of normal shallow sea creatures are abundant, mainly brachiopods and corals. Trilobites, ammonites and bambusitic stones are found in the shallow sea shelf environment [7]. The Member 1 of Suotoushan Formation shoreline quartz sandstone is obviously lower than Qingmen Formation shallow sea shelf deposition, which records an event of sea level decline and shallow sea shelf uplift and exposure. Extensive shelf exposure can lead to quartz sand sorting, transformation and redeposition, which may be one of the reasons for the high structural and compositional maturity of quartz sandstone in the Member 1 of Suotoushan Formation. The Member 1 of Suotoushan Formation in the study area is gray medium fine quartz sandstone, with almost no argillaceous interlayer, indicating that the sedimentation of sedimentary basement and sedimentary filling were in a long-term equilibrium state, and the sedimentary environment was always in the foreshore zone, which was conducive to the full sorting and enrichment of quartz sand. The Member 2 of Suotoushan Formation is mainly composed of thin bedded sandstone and mudstone, which is estimated to be littoral zone-inner shelf deposition, indicating that the sea level begins to rise and the weakening of hydrodynamic conditions is not conducive to quartz sand sorting and enrichment. Although the sea level of the Member 3 of Suotoushan Formation fell again during the sedimentary period, the sedimentary environment was not stable and the medium thickness bedded quartz sandstone was not developed, which was not conducive to the formation of high purity quartz sandstone deposits. Chemical analysis results (Table 2) also prove that the purity of quartz sandstone in the Member 1 of Suotoushan Formation is higher than that in the Member 3 of Suotoushan Formation.

6.3 Diagenetic purification process of quartz sandstone in the Member 1 of Suotoushan Formation

The microscopic observation shows that the contact relationship between quartz particles in the Member 1 of Suotoushan Formation has developed from simple grain support to concave and convex intercalation, and the intergranular pores are filled by secondary expansion edge of quartz. It is generally believed that pressure dissolution first occurs at the contact position of quartz particles. Simple pressure can locally improve the solubility of minerals or rocks, and pressure on a certain point on the surface of dry solid material can improve the solubility of the material at that point, which is the basic theoretical basis of pressure dissolution [15]. In the process of pressure dissolution, elements such as silicon, aluminum, sodium, potassium and calcium enter pore water with mineral dissolution, leading to material redistribution [15]. SiO₂ can be precipitated around quartz sand particles again, forming quartz enlarged edge, while other materials may be taken away with pore water, further purification of quartz sandstone. Pressure solution phenomenon not only exists between particles, but also generally exists between layers and cracks. Sutures and pressure solution pits are relatively developed (Figure 3h). It can be seen that the quartz sandstone of the Member 1 of Suotoushan Formation has undergone the diagenetic process of pressure dissolution and SiO₂ reprecipitation, which undoubtedly further purified the quartz sandstone of the Member 1 of Suotoushan Formation. It can be considered that the high purity quartz sandstone of the Member 1 of Suotoushan Formation not only has the deposition of high purity quartz sand but also has the diagenetic purification process. The specific diagenetic purification process and mechanism need to be further studied.

7. Conclusion

1. The Member 1 of Suotoushan Formation is a quartz sandstone deposit with high mineral maturity and structural maturity in the foreshore zone, which is a favorable horizon for the exploration of high-purity quartz sandstone deposit.

2. The high-purity quartz sandstone of the Member 1 of Suotoushan Formation is recycled quartz sand, which is the result of uplifting, exposure and redeposition of the underlying shallow sea shelf.

3. The diagenesis (pressure dissolution and siliceous cementation) of the Member 1 of Suotoushan Formation improves the purity of SiO₂ further.

4. The grain size characteristics of the Member 1 of Suotoushan Formation are up to or close to the index of high quality and high purity quartz sandstone, and the chemical purity exceeds the standard of the industrial index Grade I for flat glass, so it can be used as the raw material of high-purity quartz sand.