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

www.nature.com/scientificreports OPEN Potassium-rich brine deposit in Lop Nor basin, Xinjiang, China Ming-guang Sun & Li-chun Ma Received: 19 January 2018 Accepted: 1 May 2018 Published: xx xx xxxx 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. Global potash resource reserves are rich, estimated at 250 billion tons (K2O)1, but most are subsurface solid minerals of marine origin. Although potassium resources are abundant, their distribution is uneven. Potash deposits are mainly found in Europe, North America, Central Asia, and Southeast Asia. China’s potash resources account for only 1.8% of the world’s potassium reserves2. China is also a large agricultural country, where fertilizer application has been ranked first in the world. Potash fertilizer consumption in China has reached 10 million tonnes per year. In 2014, China’s potash production was 4,300,000 t3, leaving a large gap between output and consumption, which although increases the value of domestic potash resources also resulted in the importation of large quantities of potassium resources. Solid potassium deposits are only found in the Mengyejing potassium deposit in Yunnan, China, with proven KCl reserves of 16.76 Mt4. Total proven reserves of potash in China are 0.99 Gt (KCl)5, of which brine type potash resources account for more than 98% of the total. Brine deposits are mainly distributed in the Qaidam Basin of Qinghai and the Lop Nor Basin in Xinjiang. The Lop Nor Basin is a Quaternary dry saline lake, located in a low-lying area at the eastern edge of the Tarim Basin in Xinjiang, with coordinate ranges of 39° to 41°N and 88° to 92°E (Fig. 1A). Since the 1995 discovery of the Lop Nor potassium-rich brine deposit, the amount of potassium chloride resources has been proven up to 145 Mt6. The Lop Nor deposit is the second largest brine potash deposit after that of the Qaidam Basin in Qinghai, China. Previous studies have been done on glauberite (CaSO4·Na2SO4) reservoir characteristics of brine potash deposits at Lop Nor7, chemistry of the potassium-rich brine8–10, and brine mining technology11–15, but studies of spatial distribution and geochemical characteristics of the Lop Nor deposit are less common. This paper introduces metallogenic geological background and distribution characteristics of ore bodies in the Lop Nor potassium-rich deposit. The spatial distribution of ore bed geochemistry and spatial enrichment trends of K and B are introduced through brine data from 91 drill holes, providing a basis for development and use of potash deposits in the Lop Nor basin. Research Area Overview The Lop Nor basin is a secondary fault depression tectonic basin in the eastern Tarim Basin16, 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 Tertiary17, 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 mm10. 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 MLR Key Laboratory of Metallogeny and Mineral Assessment, Institute of Mineral Resources, CAGS, Beijing, 100037, China. Correspondence and requests for materials should be addressed to L.-c.M. (email: ) SCientifiC REPOrTS | (2018) 8:7676 | DOI:10.1038/s41598-018-25993-6 1 www.nature.com/scientificreports/ Figure 1. Structural map of the Tarim Basin in Xinjiang, China21. 1. Xing Di Dextral fracture, 2. Konqi River Dextral fracture, 3. Cheerchen Sinistral fracture, 4. Altun Sinistral fracture, 5. Lop Nor sub-basin, 6. Depression, 7. Uplift, 8. Tarim Basin, 9. Research area, 10. Fracture, 11. Strike-slip directions. of daylight are beneficial to the formation of evaporite minerals18 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 northeast19. 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. Analytical and mapping methods. Main elements Na+, K+, Mg2+, Ca2+, Cl−, SO42−, CO32−, and HCO3− and trace elements Li+, B3+, Br−, I−, Rb+, Cs+, and Sr2+ were tested and analyzed in the brine samples. Na+, K+, Mg2+, Ca2+, Sr2+, Rb+, Li+, Cs+ were determined by atomic absorption spectrophotometry (error less than 2%). Br−, I−, CO32−, HCO3−, and Cl− were (...truncated)