Potassium-rich brine deposit in Lop Nor basin, Xinjiang, China

Lop Nor potash deposit is the largest sulfate-type liquid potassium salt deposit in China, consisting of three areas: Xinqing platform, Luobei depression, and Tenglong platform. In this study, the geological background, basin structure, and fracture system of the deposit, along with the brine storage size of the three ore bodies are introduced in detail, using underground brine elevation models of the mining areas. We collected 91 samples of brine from long-term observation boreholes in the Lop Nor ore district, and analyzed their ion contents. The KCl content of the brine varies from 1% to 1.45% and that of B2O3 varies from 1900 to 4500 mg/L, which are higher than the cut-off for Chinese industrial mining grades. The spatial distributions of KCl and B2O3 contents in different ore beds were plotted, and the distributions of different grades of K+ and B3+ were revealed.


Research Area Overview
The Lop Nor basin is a secondary fault depression tectonic basin in the eastern Tarim Basin 16 , Central Asia's largest inland basin. Its development was controlled by different periods of tectonism. At the end of the Neogene, Himalayan movement caused the western Tarim Basin to uplift and the eastern part to sink, tilting the terrain from southwest to northeast as the elevation decreased from 1400 m to 800 m. Lop Nor is located at the lowest elevation in the basin, also known as the Lop Nor depression. This is why Lop Nor has become a catchment area, providing abundant source material for brine deposit formation. The Lop Nor area experienced complex tectonic movements from the Cretaceous to the Tertiary 17 , and is now surrounded by mountains on three sides (Fig. 1), which create a closed hydrological environment for brine deposit formation. The region has an arid continental climate and is one of the driest regions in the world, with annual rainfall of less than 20 mm and annual evaporation of approximately 3500 mm 10 . The annual average daily air temperature is 11.6 °C, with the highest temperatures reaching 50 °C and daily temperature differences of up to 25 °C. These high temperatures and long periods of daylight are beneficial to the formation of evaporite minerals 18 and are also favorable conditions for Lop Nor to become a salt deposition center. In this area, the prevailing winds are mainly from the east and northeast 19 . Average annual wind speed is 5.5 m/s, with maximum wind speed of up to 45 m/s, so wind erosion is strong and evidence can be seen everywhere of Quaternary erosional landforms called Yadan. The combination of structure, climate, and source supply in the Lop Nor area make the basin the largest production area for brine-type potash in China.

Research Techniques
Sampling method. The Lop Nor potassium-rich brine deposit consists of three mining areas-the Tenglong platform, Luobei depression, and Xinqing platform. In July and August, 2016, we collected brine samples from 22 drill holes in Tenglong platform, 43 in the Luobei depression, and 26 in the Xinqing platform. In situ measurements of brine burial depth, density and pH were made; GPS was used for geolocation and elevation measurements. Two sample bottles (white plastic) were used to collect 500 ml of brine per hole. To prevent evaporation of the brine in the extreme arid climate or leaks during transportation, bottles were sealed quickly with tape after sampling. Locations of the three mining areas are shown in Fig. 2, and the sampling points are shown in Fig. 3.
Sampling point location maps, elevation contour maps of the mining areas, and geochemical spatial distribution maps of brine in different ore beds were undertaken with Surfer 11 software 20 .

Results and Discussion
Mining area structures. The Lop Nor basin is situated in the eastern part of the Tarim Basin, at the junction of the Beishan terrane and eastern Tianshan fold belts (Fig. 1) where tectonic movements were strong and folds, fractures, joints, and fissures are well developed. In this area, the main fractures are the XingDi dextral fracture, Konqi River dextral fracture, Cheerchen sinistral fracture, and Altun sinistral fracture 21 . The study area is located in the northern Lop Nor basin (Fig. 1). Boundaries of each mining area and the brine storage layers are controlled by five faults 22 . The F1 tensional fault marks the border between the Luobei depression and Xinqing platform, with a strike of 30° and dip of approximately 120°, with linear extension. It rises in the west and forms a platform, while it sinks in the east and forms a depression. The F2 normal fault is located between the Tenglong platform in the east and the Luobei depression in the west, with a strike of 30° and dip of approximately 300°. The F5 normal fault is located in the eastern Tenglong platform, with a strike of 30° and dip of approximately 300°; it declines in the west to become a platform and uplifts in the east to become low mountains and hills. It also forms the line of demarcation between the Tenglong platform and the Beishan terrane. The F3 confined aquifer passes through the south-central Tenglong platform to divide the platform into a southern part and a northern part; it has a strike of 70° and a dip of 135°; the northern part of the fault sinks and the southern part uplifts. It controls the development   Characteristics of the potassium-rich brine reservoir. The reservoir of the Lop Nor potassium-rich brine deposit is mainly glauberite with a gravel layer, but the existing and proven brine beds are mostly in glauberite layers. Only a few gravel strata contain potassium-rich brines, and their distribution and reserves are not clearly defined. Therefore, this paper only discusses glauberite reservoirs. Currently, 200 boreholes have been drilled in the Lop Nor mining area, and the Late-Middle Pleistocene to Late Pleistocene strata are distributed as a massive, continuous, thick layer of glauberite with well-developed intercrystal porosity (Fig. 4C), forming a good storage medium for potassium-rich brine. However, due to the influence of buried faults, there are differences in the brine storage layers among the three mining areas. There are seven brine beds (Fig. 4B) exposed by drill holes in the Luobei depression, including a phreatic aquifer, W 1L , and six artesian aquifers, W 2L , W 3L , W 4L , W 5L , W 6L , and W 7L ; however, due to the limitations of mining depth, at present, only W 1L , W 2L , W 3L , and W 4L seams have been mined. There are two artesian brine aquifers, W 2X and W 3X , exposed by drill holes in the Xinqing mining area 24 . There are three beds in the Tenglong mining area, including a phreatic aquifer, W 1T , and two artesian aquifers, W 2T and W 3T (Fig. 4A).
Luobei depression mining area. The Luobei depression is the main storage site of the Lop Nor potassium-rich brine, with a north-south length of about 60 km, east-west width of 32.5 km, and a total area of 1411 km 2 25 . There are stable salt-bearing clay aquiclude layers separating the seven brine layers in the mining area. The potassium-rich brines occur in glauberite rocks; lithologies of each brine storage layer with gypsum and glauberite contents can be seen in Fig. 4(B). W 1L is a phreatic aquifer with layered distribution across the whole Luobei depression, with average thickness of 17.54 m, water table depths of 1.7 to 2.3 m, porosities of 6.98% to 38.45%, and specific yields of 4.57% to 25.89%. Water yield is the highest in the central and northeast, with unit brine overflows of more than 5000 cubic meters per day per meter of water table depth (m 3 /d · m), while in the rest of the aquifer the unit brine overflows range from 1000 to 5000 m 3 /d · m 25 . The W 2L artesian aquifer is confined, nearly horizontal with a stratified distribution, and has an average thickness of 10.18 m, unit brine overflows of 10 to 100 m 3 /d · m, water table depths of 20 to 40 m, porosities of 4.34% to 37.8%, and specific yields of 1.08% to 21.04%. The W 3L artesian aquifer is confined, with stratified distribution and an average thickness of 8.50 m, unit brine overflows of 10 to 100 m 3 /d · m, water table depths of 40 to 70 m, porosities of 2.85% to 19.97%, and specific yields of 1.10% to 13.37%. The W 3L aquifer is also confined with stratified distribution, with an average thickness of 7.28 m, unit brine overflows of 10 to 100 m 3 /d · m, water table depths of 70 to 100 m, porosities of 5.22% to 24.72%, and specific yields of 1.03% to 9.91% 16 . The lithologies of the four brine storage layers are dominated by glauberite, and occasional lacustrine sedimentary clastic rocks, such as gypsum.
Xinqing platform mining area. The Xinqing platform consists of two confined potassium-bearing brine aquifers (Fig. 4A). Confined brines have layered or stratified distributions. The average thicknesses of the aquifers are 4.38 to 7.52 m. Due to control by the F1 fault, there is no phreatic aquifer in the Xinqing platform, but this does not affect the continuity of the brine storage layer between the mining areas. The W 2X aquifer is confined, stratified, and distributed in the eastern part of this ore district with a north-south length of 77.78 km, east-west width of 16.82 km, and total area of 1100 km 2 . Unit brine overflows are 2.25 to 541.51 m 3 /d · m, water table depths are 10 to 20 m, porosities are 3.89% to 40.69%, and specific yields are 2.01% to 21.15%. The W 3X aquifer is also confined and stratified, with a north-south length of 76.10 km, east-west width of 18.81 km, and total area of 1444 km 2 . Unit brine overflows are 1.67 to 293.99 m 3 /d · m, water table depths are 11.3 to 38 m, porosities are 4.16% to 26.43%, and specific yields are 2.11% to 14.19% 23 .
Tenglong platform mining area. The Tenglong platform consists of a phreatic aquifer and two confined aquifers. W 1T is a phreatic, stratified aquifer and is the main ore body, bounded by the F3 fault. It is distributed the northern part of the mining area, with a north-south length of about 33 km, east-west width of about 20 km, and total area of 610 km 2  Geochemical characteristics of potassium-rich brines. A total of 91 brine samples were collected from boreholes in the three mining areas, and analyzed for their chemical compositions, as listed in Table 2.
As described in Table 2, the brines of the three mining areas are all high salinity, ranging from 247 to 385 g/L. Luobei depression has the highest salinity, averaging 367 g/L, followed by Xinqing with an average of 332 g/L, and then Tenglong with an average of 325 g/L Cl − , and Na + are the main components followed by Mg + , K + , and SO 4 − , while other elements are found in very small amounts and their abundances in the brine are not more than 1%. According to the classification system of Valyashko 26 (1965), the Lop Nor brine deposit hydrochemistry is of the magnesium sulfate subtype.
The main targets for mining are potassium and boron. The K + contents in the brines of the three ore districts range from 5.5 to 9.8 g/L, which are all higher than the industrial mining grade of KCl (1%) 27 . The average content of K + is 7.2 g/L in the Xinqing mining area, 8.0 g/L in the Luobei mining area, and 7.7 g/L in the Tenglong mining area. The average coefficient of variation is 12.3%, indicating that the grade of potassium is stable. The content of B 3+ in the three mining areas ranges from 277.3 to 755.6 mg/L, which are all higher than the comprehensive utilization grade of 150 mg/L, and in most brines the B 3+ content is higher than the industrial mining grade of 300 mg/L 27 . Comprehensive development and use of brine can be carried out. The contents of bromine and lithium are generally low in the whole mining area, ranging from 1.9 to 48.8 mg/L for Br − and from 3.4 to 25.3 mg/L for Li + , which are far lower than the comprehensive utilization grade of 150 mg/L for bromine and 13.1 mg/L for lithium. These latter two elements are not yet of mining value.
Spatial distribution trends of KCl contents. We plotted isoline maps of KCl contents from measured results, using Surfer software (Fig. 5). The figure shows the spatial distributions of KCl contents for three layers (W 1 , W 2 , and W 3 ) in the Lop Nor potassium brine deposit.
According to Fig. 5, the grades of KCl in the three ore beds are above the industrial mining grade (KCl content 1%) and the salinity and content of K + of the brines increased with burial depth. The layers with KCl grades ≥1.2% make up more than 50% of the whole mining area, and as depth increases, the area increases. For the upper W 1 layer, KCl contents of more than 1.2% in the brine are mainly found in the south-central part of the mining area, with the highest value 1.56%, and an average value of 1.3% (Fig. 5a). The contents of KCl in the middle, W 2 ore layer, exhibit two high areas and one low area. The areas with more than 1.2% of KCl are mainly found in the north-central and southern regions of the mining area, with KCl contents of 1.2% to 1.45% and 1.2% to 1.55%, respectively. The average value is 1.21%. Between the two high value areas is a low value area, with KCl contents of 1% to 1.2% (Fig. 5b). In the W 3 ore layer, the KCl grade is generally high, with KCl contents of more than 1.2% across most of the mining area, ranging from 1.2% to 1.5%, with an average of 1.31% (Fig. 5c).
The KCl grades in the three ore beds are more than the industrial grade. Since the Lop Nor potassium brine deposit was opened in 2004, it has been continually mined for 13 years. In comparison with the KCl grade identified before mining, the grade of KCl has decreased by 0.2% to 0.3%.
Spatial distribution trends of B 2 O 3 contents. The isoline maps (Fig. 6) of B 2 O 3 contents in different ore beds indicate contents are higher than that of the industrial mining grade of 1000 mg/L, and that most B 2 O 3 contents are more than 2400 mg/L. Content of B 2 O 3 in the W 1 reservoir (Fig. 6a) Table 2. Chemical compositions of potassium-rich brine in Lop Nor.  an average of 3200 mg/L. The highest value area (more than 3900 mg/L) is concentrated in the middle of the mining area. The content of B 2 O 3 in the W 2 reservoir (Fig. 6b) varies from 1900 to 4100 mg/L, with an average of 2900 mg/L. There are lower values in the northeastern and southwestern parts of the mining area and higher values in the northern and southeastern parts. The content of B 2 O 3 in the W 3 (Fig. 6c) reservoir ranges from 1900 to 4300 mg/L, with an average of 3000 mg/L. Higher values are distributed from the southeastern part of the mining area toward the northwest. Although the contents of B 2 O 3 in the three layers are higher than that of the industrial mining grade, at present, only the potassium in the brine is mined.

Conclusion
The Lop Nor brine deposit is a Quaternary continental saline lake, where a large amount of potassium-rich brine is contained in porous glauberite rock. This is very rare in Quaternary potassium basins, and the genesis of the deposit is still unclear. The deposit formed three mining areas under the control of tectonic action. These are, in order from west to east, the Xingqing platform, Luobei depression, and Tenglong platform. The Luobei depression is the main ore body, with the largest area and umber of storage aquifers (seven in the first 200 meters). However, only four layers, W 1L to W 4L , are currently exploited. Potash reserves have not been ascertained for strata below 200 meters. Lop Nor potassium-rich brine deposit contains high salinity brines, with salinity ranges of 247 to 385 g/L. KCl grade ranges from 1.2% to 1.45% with an average of 1.27%. B 2 O 3 grade varies from 1900 to 4500 mg/L, which is higher than the industrial mining grade. The deposit is now mined by the SDIC Xinjiang Luobupo Hoevellite Co. Ltd and the main product is potassium sulfate, with an annual production capacity of 1.3 million tons. From the plotted KCl and B 2 O 3 contour maps, an obvious desalination phenomenon is seen in the northern part of the W1 ore layer, possibly related to the low-salinity water submersible recharge in front of the Kulutage in the northern part of the basin. Therefore, it will be necessary to further exploit isotope tracer methods to construct models of the brine flow fields and take measures to ensure the quality and continued production of the deposit.