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.
TAN HongBingMA HaiZhouLI BinKaiZHANG XiYingXIAO YingKai
溢晶石作为一种极易溶的盐类矿物,不能从正常海水蒸发的最终产物中得到(Herrmann et al.,1973;Hardie,1990),在一些存在富钙镁卤水的现代环境中,即使在干旱条件下,溢晶石也不会沉淀(ElTabakhetal.,1999)。但是,在白垩纪的蒸发岩中却发现了大量该矿物,它们主要分布在3个地区(Vysotskiy,1988):南美洲巴西的Sergipe-Alagoas盆地,西非的Congo-Gabon盆地。
The Khorat Plateau on the Indochina Terrane is known to have formed during the closure of the Tethys Ocean, although the origin of its potash mineral deposits is a topic of current debate. Data from a borehole on Savannakhet Basin is used in this study to re-define the evaporation processes of the study area. Geochemical analyses of halite from various borehole-derived evaporite strata have elucidated the fluid sources from which these ores formed. Measured δ11B indicated that ore deposits formed primarily due to evaporation of seawater, although non-marine fluids affected the later stages of the evaporation process. Fluctuations in B and Br concentrations in carnallite- and sylvite-rich strata indicate the influence of fresh water. Boron concentration in carnallite unit indicated the influence of hydrothermai fluids. From the relative timings of these various fluid influxes, the evolution of these evaporates can be divided into four stages： （1） an initial marine evaporation at the beginning of the deposit＇s formation, where seawater （and minor fresh water） trapped on the uplifted Khorat Plateau produced sediments and salts with Br contents lower than those of normal marine-derived evaporites; （2） a transgression stage, where seawater recharged the basin; （3） a hydrothermal infiltration stage, which was coeval with the late Yanshan movement; and （4） a stage of fresh water supply, as recorded by fluctuations in B and Br contents, inferring intermittent fresh water influx into the basin. Thus, although evaporites on the Savannakhet Basin primarily formed via marine evaporation, they were also influenced to a significant degree by the addition of non-marine fresh water and hydrothermal fluids.
