Formation of marine sylvite on the Khorat Plateau, Southeast Asia: Evidence from B isotopes, trace elements, and petrography

Abstract

The potash minerals on the Khorat Plateau consist of carnallite and sylvite. The origin of sylvite is still a matter of debate due to a lack of diagnostic evidence. In this study, we analyzed the petrography, trace element compositions, and B isotope compositions of various salt facies, including carnallites, sylvites, carnallite-associated halites, and sylvite-associated halites. The results show that the δ11B values of all of the salt samples range from 16.66 ‰ to 32.78 ‰, which are consistent with those of marine precipitates. Nevertheless, the sylvite-associated halites have higher B contents and elevated δ11B values, which differ greatly from those of the carnallite-associated halites but are similar to those of the potash facies. The variations in the B isotope compositions from the anhydrites to the carnallite-associated halites to the carnallites are consistent with the trend of marine salts from seawater evaporation. The Br contents of the sylvites and sylvite-associated halites are lower than those of their primary marine counterparts, suggesting a secondary origin. In contrast, the carnallites and carnallite-associated halites have Br contents within the range of the primary marine salts, denoting a primary origin. The trace element compositions, B isotopes, and petrography of the different salt facies suggest that the sylvites are secondary minerals and formed via the dissolution of primary carnallites. The carnallites were dissolved or leached by unsaturated waters, forming K and Mg enriched brine, from which the secondary sylvites and halites subsequently precipitated. This process usually occurred in the flanks of the salt domes where fractures were widely developed. We speculate that sylvites in those global giant Cretaceous potash deposits were most likely caused by extensive dissolution and recrystallization processes, and the initial and predominant potash mineral was carnallite.