K, Sr isotopes, and trace element to constrain potash origin in the Simao Basin, southwestern China, and insight into K isotope geochemical behavior in evaporite

Abstract

With the advancement of emerging potassium isotope testing techniques, the characteristics of δ⁴¹K in evaporite have garnered increasing attention. Its application in constraining the genesis of potash deposits and tracing the process of potash mineralization holds significant prospects. A comprehensive analysis of the geochemical characteristics of K, Sr isotopes, and trace elements provides an opportunity to address the ongoing debate regarding the mineralization mechanism of potash in the Simao Basin. In this study, we collected 64 potash samples from the Simao and Khorat basins. The analytical results revealed that the major ions in the samples consist of Na⁺, K⁺, Mg²⁺, Ca²⁺, Cl⁻, and SO₄^2-, while the trace elements Br, Sr, Rb, B, Li, V, Cr, Mn, Ba, As, and Zn are relatively enriched. The δ⁴¹K values range from −0.12 ‰ to 0.20 ‰ with an average of 0.01 ‰. The ⁸⁷Sr/⁸⁶Sr ratios range from 0.707553 to 0.708565 with an average of 0.708020, which is lower than that of the river water in the drainage basin. Additionally, the Br content ranges from 316.3 × 10⁻⁶ to 1709.1 × 10⁻⁶, with an average of 633.8 × 10⁻⁶. These data indicate that the Early Cretaceous Albian-Middle Jurassic Bajocian seawater serves as an important source of potassium for potash deposition in the Simao Basin. The characteristics of the ratios of the Br/Cl (mmol/mol), K/Cl (mmol/mol), Rb/Sr (mol/mol), along with the contents of the Br and Rb, indicate that the most of potash in the Simao Basin is primary origin. Moreover, we have also provided new insight into the geochemical behaviors of K isotopes during the evolution of the evaporites. The precipitation of potash may be accompanied by K isotope fractionation, as evidenced by the lighter K isotope composition of carnallite compared to that of sylvinite. K isotope fractionation occurs between the secondary sylvite and its parent source, and newly precipitated solids are characterized by relatively light K isotope. Episodic freshwater inflow into the evaporative basin stimulates microbial activity, leading to fluctuations in K isotopes in the precipitated potash during the same evaporation stage. Therefore, this work not only provided direct evidence from the K isotopes for determining the origin of the potash in the Simao Basin but also further perceived the geochemical behaviors of K isotopes during the evolution of evaporite.