Strontium and boron isotopic constraint on the marine origin of the Khammuane potash deposits in southeastern Laos

Since 2006,we have conducted a geological exploration of the Khammuane area in southeastern Laos.Several thick sequences of potash deposits containing sylvite and carnallite were discovered from drilling Paleocene strata of the Thangon Formation(E1tg).To constrain the origin of the brine that formed thick sequences of salt mineral deposits,we measured the strontium and boron isotope composition of some salt minerals in the potash deposit.Boron isotope composition of halite-and potash-associated minerals in the Khammuane potash deposit varied from 19.91%to 31.01‰,which differs significantly from continental evaporates(-6.83‰--5.79‰)but is very close to that of salts precipitated from evapo-concentrated seawater(10‰-36‰).The 87Sr/86Sr ratios(0.707542 to 0.709461)of salts were also close to the values of Paleocene seawater(0.70772 to 0.707830).The content of Br-(0.01%-0.25%)and Br×103/Cl ratios(0.10-0.52)of halite were>10 times higher than that of common continental salts.These characteristics indicate clearly that the Laos potash deposits were deposited from seawater.The small variation in boron and strontium isotopes in all salt sedimentary sequences(gypsum-halite-potash-halite)also suggests that the recharge source should be stable and the effects of other continental water mixing are negligible.Large-scale potash deposits are commonly precipitated from seawater.Hence,the results of recent regional drilling work and geochemical research suggest extensive potash resources could occur in the Thangon Formation of southeastern Laos.