Shang et al. Journal of Palaeogeography (2021) 10:4 https://doi.org/10.1186/s42501-021-00083-3 Journal of Palaeogeography ORIGINAL ARTICLE Open Access Characteristics and origin of a new type of polyhalite potassium ore in the Lower Triassic Jialingjiang Formation, Puguang area, northeastern Sichuan Basin, SW China Wen-Jun Shang1,2, Mian-Ping Zheng1,2, Yong-Sheng Zhang1,2,3, Jia-Ai Zhong1,2*, En-Yuan Xing1,2, Yuan Peng1,2, Bao-Ling Gui1,2 and Kong Li4 Abstract A new type of polyhalite potassium ore (NTPPO) was found in the Lower Triassic Jialingjiang Formation, NE Sichuan Basin, SW China. It is water soluble, therefore can be exploited using the water-solution method, and is of great potential of economic value and research significance. Based on cores, thin sections, energy spectrum and SEM analyses, its microfeatures, macrofeatures and origin are discussed, and a genetic model is established to provide a scientific basis for future evaluation, prediction and exploration of potassium ore in the Sichuan Basin. It is proposed that the NTPPO was caused by storm activities: (1) the storm broke the original sedimentary polyhalite–gypsum beds, whose fragments were transported into the salt basin with high content of K+ and Mg2+; (2) in the basin, the polyhalite continued to be formed from gypsum by metasomatism with K- and Mg-rich brine; (3) during diagenesis, under high temperature and high pressure, K–Mg-rich brine from halite continued to replace anhydrite (or gypsum) to form polyhalite. Keywords: Puguang area, Lower Triassic, New type of polyhalite potassium ore, Microfeatures and macrofeatures, Storm event deposition 1 Introduction potential reserve as much as 10 billion tons of K2O (Jin Polyhalite is used as a high-quality soil-friendly chlorine- 1989). However, the polyhalite found in earlier periods free fertilizer, which is rich in potassium and magne- mostly occur as layers or lenses in gypsum or anhydrite sium, and therefore is good for growing crops (Chen strata (Huang 1996), which is of relatively high mining 1999; Zheng et al. 2018). Polyhalite has been found in cost due to poor water solubility (An et al. 2004); more- the Sichuan Basin, Lop Nur, Qaidam Basin, Bohai Bay over, its economic value decreases to zero with increas- Basin and Jianghan Basin in China (Wang 1982; Zhao ing burial depth. So far, only the polyhalite from shallow et al. 1987; Lin and Yin 1998; Liu et al. 2008, 2015; Niu strata in the Nongle area, Quxian county, Sichuan Basin, et al. 2015). Among them, the polyhalite from the SW China, has been exploited (Huang 1996; Lin and Yin Triassic in the Sichuan Basin exhibits the widest distri- 1998), while those polyhalites in other areas, moderately bution and the highest degree of enrichment, with a to deeply buried, mostly remain unminable. Recently, our team found a large amount of clastic polyhalite, * Correspondence: [email protected] which is distributed in the thick halite of the Lower 1Institute of Mineral Resources, Chinese Academy of Geosciences, Beijing 100037, China Triassic Jialingjiang Formation in the Puguang area, 2Key Laboratory of Salt Lake Resources and Environment, Ministry of Natural Xuanhan county, northeastern Sichuan Province, SW Resources, Beijing 100037, China China. High solubility of the halite makes this polyhalite Full list of author information is available at the end of the article © The Author(s). 2021 Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article's Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article's Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. Shang et al. Journal of Palaeogeography (2021) 10:4 Page 2 of 13 easy to be exploited via the water-solution method, late Eocene–Oligocene polyhalite in the Dawenkou which greatly reduces its mining cost; therefore, this de- Depression, Bohai Bay Basin, was precipitated from a posit has great potential economic value. This polyhalite mixture of a Ca2+-rich fluid surrounding the playa and a is considered as a new type of polyhalite potassium ore K+-andMg2+-rich brine in the playa. Based on polyha- (hereinafter called the ‘NTPPO’, Zheng et al. 2018). This lite characteristics in the Jianghan Basin, Wang (1982) paper mainly focuses on microfeatures and macrofea- summarized the genetic mechanism as follows: when a tures of the NTPPO and analyzes its genesis in order to brine becomes sufficiently concentrated to deposit provide a scientific basis for future prospecting, predic- potassium and magnesium salts, the calcium ions trans- tion and evaluation of polyhalite in the Sichuan Basin. ported by relatively low-salinity water combining with The results of evaporation experiments (Han et al. the potassium and magnesium in brine can result in 1982) show that polyhalite can be directly formed when formation of polyhalite. Niu et al. (2015) suggested that the six-element system involving K+,Na+,Mg2+,Ca2+/ formation of polyhalite in the Bieletan area of the − 2− Cl ,SO4 and H2O reaches a certain equilibrium, or Qarhan Salt Lake, Qinghai, occurred through meta- can be metasomatically formed by contact between a somatic replacement of early-formed salts by magnesium concentrated brine with high K+ and Mg2+ concentra- sulfates in lacustrine facies. Liu et al. (2015) proposed tions and calcium sulfate salts, such as gypsum or an- two genetic models for the polyhalite in Kunty Salt Lake, hydrite (Freyer and Voigt 2003; Wollmann et al. 2008, northwestern Qaidam Basin: the reaction between 2009; Wollmann 2010). The first type is called the pri- highly-concentrated brine and anhydrite; and the reac- mary sedimentary polyhalite, and the second is called tion between highly concentrated brine and K+-rich clay. the metasomatic polyhalite. The latter is further divided into: (1) parasyngenetic metasomatic polyhalite, that is, 2 Geological setting in the late stage of evaporation, polyhalite forms through 2.1 Tectonic background reaction between concentrated brine with high K+ and The NTPPO was discovered in the Puguang area of Mg2+ concentrations and previously precipitated calcium Xuanhan county, northeastern Sichuan (Fig. 1a) in mar- sulfate salts (such as gypsum and sodium sulfate) (Yuan ine evaporative strata of the Jialingjiang Formation, top- 1974); and (2) postdiagenetic metasomatic polyhalite, most Lower Triassic (Fig. 2). This area is located in a formed by metasomatism of potassium- and highly elevated and steep tectonic belt, which has been magnesium-rich brine transported through structural superimposed and reconstructed by several phases of tec- cracks into contact with calcium sulfate salts, such as tonic activity, resulting in very complex structures (Tang gypsum or anhydrite (Yuan 1974; Liao et al. 1984). et al. 2008). Reverse faults in NW and NE directions are Based on laboratory studies, Chinese researchers have best developed (Fig. 1b). The gypsum salt layer in the proposed the following views on the genesis of polyha- Lower Triassic Jialingjiang Formation and Middle Triassic lites deposited in anhydrite in the Sichuan Basin: (1) Pri- Leikoupo Formation experienced plastic deformation due mary deposition, in which polyhalite is formed by to tectonic activities, forming a slippage zone; and its evaporation of a fluid mixture composed of Ca2+- and upper and lower strata show significantly different struc- 2− SO4 -rich drainage water from the gypsum succession tural characteristics (Fig. 1c) (Zhou et al. 2013). and K+- and Mg2+-rich residual concentrated brine (Huang 1996). (2) Penecontemporaneous metasomatism, 2.2 Salt-forming background in which, during the process of deposition, the concen- The largest extinction event in the Phanerozoic Eon oc- trated residual K+- and Mg2+-rich brine reacts with curred at the Permian–Triassic boundary (Raup 1979; gypsum to form polyhalite (Huang 1996). (3) Metasoma- Erwin 1994; Mundil et al. 2004; Alroy et al. 2008; Ogg tism, in which during diagenesis K–Mg-rich brine from and Gradstein 2008; Yin and Song 2013), when more leaching of a salt layer (Li and Han 1987; Pan 1988)or than 90% of marine species disappeared, damaging mar- from an external hydrothermal environment reacts with ine and terrestrial ecosystems (e. g., Retallack 1995; gypsum or anhydrite to form polyhalite. Additionally, in Payne et al. 2006; Algeo et al. 2011). This was followed the Lop Nur Salt Lake, Xinjiang Uygur Autonomous Re- by a dramatic change in the global palaeoenvironment gion, NW China, Liu et al. (2008) suggested that there and palaeoclimate (Stanley 1988). The Early Triassic are two kinds of polyhalite precipitation mechanisms: (1) temperature increased (Sun et al. 2012), and large-scale hydrothermal events, which might result in polyhalite monsoon systems developed (Parrish 1993). At this time, precipitation through replacement of glauberite, or even the Sichuan Basin was within the influence range of low- direct precipitation from brine; (2) during the playa latitude monsoons, which is between latitudes of 12.5° N stage, K+- and Mg2+-rich intercrystal brine mixes with and 15° N (Jiang et al. 1987; Zhu et al. 1988), subjected Ca2+-rich surface water, and precipitates polyhalite to hot and arid climates (Qian et al. 2010). During the inter-halite crystals. Zhao et al. (1987) inferred that the Jialingjiang stage, the Sichuan Basin changed from an Shang et al. Journal of Palaeogeography (2021) 10:4 Page 3 of 13 Fig.