A tuff interlayer in deep potash-bearing salt rocks and its implication for potash mineralization in the Simao Basin, southwestern China

Abstract

The lithology and genesis of a dark grey clastic interlayer first encountered within the deepest potassium-rich salt body in the Simao Basin, southwestern China, were analysed. Analyses of the petrography, mineralogy, and element geochemistry of the layer revealed that (1) the layer contains quartz crystals with gulf corrosion edges and explosion cracks and angular volcanic ash-sized glasses; (2) the main mineral components of the crystal fragments are chlorite, illite, biotite, quartz, anhydrite, gypsum, magnesite, pyrite, molybdenite, clinopyroxene, and zircon; (3) the rare earth element patterns, Zr/TiO2 and Nb/Y diagrams as well as boron content all indicate a volcanic origin for the layer. Based on these observations, the layer is suggested to be an altered tuff associated with various volcanic fragments dominated by chlorite and formed after alteration of a parent tuff in an alkaline, salty, and low-temperature water body. Discovery of the layer indicates that the potash-bearing salt rocks could have taken in volcanic materials during these volcanic activities and provides the possibility of reliable zircon U‒Pb dating to determine the absolute age of the host rock, which is fundamental in studying the genetic mechanism of this deeply buried salt body.

Genesis of Pyrite in Clastic Rocks of Deep Salt-Related Strata in the Simao Basin and Its Implication for Potash Mineralization: A Case Study of the Well MK-3

“…The δ 34 S V-CDT values of anhydrite with similar depth (2524.4 to 2526.1 m) range from 15.1‰ to 15.5‰. The δ 34 S V-CDT value of anhydrite deposited in the early stage (at a depth of 2627.8 m) is 15.5‰ [3]. It is concluded that pyrite is of sedimentary origin and H 2 S generated by anhydrite formed in the early biodegradation stage is an important sulfur source.…”

“…The Simao Basin can be divided into four uplifts and four depressions, a total of eight second-order tectonic units (see Figure 1), and the sedimentary strata are mainly Mesozoic-Cenozoic [13,14]. Salt rocks discovered in the basin are mainly found in the Mengyejing Formation of the Lower Cretaceous, with a maximum burial depth above 1251 m. From west to east, salt rocks can be divided into the Jinggu salt-bearing belt, Zhengdong-Mengla salt-bearing belt, and Jiangcheng salt-bearing belt [2,15]. The surrounding rock of salt rock is mainly mud-clast conglomerates, and there is no carbonate rock and sulfate rock of corresponding scale in normal evaporative deposition sequence, and mud-clast conglomerates often appear in salt rock [16,17].…”

“…There is an anhydrite layer at 2394.4 to 2396.6 m depth above the salt layer in the well MK-3, with δ 34 S V-CDT values ranging from 13.9‰ to 15.1‰ (see Figure 10) [3]. The anhydrite provides a sulfur source with heavy sulfur isotope content for pyrite through thermochemical sulfate reduction (TSR) under hydrothermal action [39].…”

“…Recently, important progress has been made in the investigation of deep potash outside the mining area. Potash-rich rock salt with a cumulative thickness of 149 m was discovered at a depth of 2390 to 2650 m, and the maximum content of KCl can reach 13.4% [2,3]. Miao et al [3] proved that the provenance of the rock salt was seawater by analyzing the strontium (Sr), sulfur (S), and potassium (K) isotopic geochemical characteristics of the rock salt and its related anhydrite rocks.…”

“…Potash-rich rock salt with a cumulative thickness of 149 m was discovered at a depth of 2390 to 2650 m, and the maximum content of KCl can reach 13.4% [2,3]. Miao et al [3] proved that the provenance of the rock salt was seawater by analyzing the strontium (Sr), sulfur (S), and potassium (K) isotopic geochemical characteristics of the rock salt and its related anhydrite rocks. Du et al [4] also reached the same conclusion by analyzing the geochemical characteristics of trace elements and chlorine (Cl) isotopes in rock salt.…”