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

Abstract

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 (E 1 tg).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 87 Sr/ 86 Sr 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×10 3 /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. potash deposit, strontium isotope, boron isotope, marine origin, Laos Citation:Tan H B, Ma H Z, Li B K, et al. Strontium and boron isotopic constraint on the marine origin of the Khammuane potash deposits in southeastern Laos.

Origin of the late Cretaceous potash-bearing evaporites in the Vientiane Basin of Laos: δ11B evidence from borates

“…Sulfur isotopes from anhydrites (El Tabakh et al, 1999) falling in the range of marine evaporites implied their seawater origin. Evidence from boron isotope composition in fluid inclusions of halite showed that it differed significantly from that of continental evaporites and is very close to that of salts precipitated from evapo-concentrated seawater (Tan et al, 2010). Based on the lack of evidence suggesting transgression, some authors insisted that the Maha Sarakham Formation is not of marine origin (Utha-Aroon, 1993;Meesook, 2000;Racey and Goodall, 2009).…”

“…Through studies on the borates, we attempted to use the boron isotopic composition of borates to reveal the origin of potash-bearing evaporites on the Khorat Plateau. Because of the significantly different range of marine and nonmarine borates (averaging +25 ± 4‰ and À7 ± 10‰, respectively), the boron isotopic composition of borates has widely been used to distinguish marine and nonmarine sedimentary environments or the origin of fluids from which they precipitate in many evaporite deposits (Swihart et al, 1986;Oi et al, 1989;Vengosh et al, 1992;Grice et al, 2005;Tan et al, 2010). In this paper, we also attempted to characterize the changes of boron isotopic composition and discuss its influencing factors.…”

The origins of the Mengye potash deposit in the Lanping–Simao Basin, Yunnan Province, Western China

“…Br/Cl molar ratios have also frequently been used by many studies to determine whether the origin of water is marine or non-marine (Swihart and Moore, 1986;Utha-Aroon, 1993;Vengosh et al, 1995;El Tabakh et al, 1999;Taberner et al, 2000;Rahimpour-Bonab and Kalantarzadeh, 2005;Kendrick et al, 2006;Tan et al, 2010). The typical Br/Cl × 10 −3 molar ratio is~0.11 mmol/mol in the halite stage during seawater evaporation, 0.31 mmol/mol in the sylvite stage, 0.45 mmol/mol in the carnallite stage and 0.68 mmol/mol in the final stage ( Fig.…”

“…Warren, 1989Warren, , 2006Warren, , 2010Utha-Aroon, 1993;Qian et al, 1994;El Tabakh et al, 1998Rahimpour-Bonab and Kalantarzadeh, 2005;Cendón et al, 2008;Talbot et al, 2009aTalbot et al, , 2009bTan et al, 2010;Tong et al, 2013;Zhang et al, 2013). Potash deposits typically originate from brines of marine or/and land origin(s), along with varying inputs from deeplycirculating meteoric, basinal and hydrothermal fluids (e.g.…”

“…Qu et al, 1998;Li and Hu, 2001;Li et al, 2003), due to the occurrence of similar salt minerals in both these areas within the same tectonic belt. Surprisingly, in~50 years of exploration, no large potash deposit has been discovered in the area, and no consensus has therefore been reached regarding the fluid origin of the LSB, or indeed that of the potash deposits in the SNB and KB (Utha-Aroon, 1993;El Tabakh et al, 1998Tan et al, 2010).…”