Depositional Environment during Late Salt-Forming Period in Potash Deposits of Vientiane Basin, Laos: Evidence from Borate Minerals
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摘要:
老挝万象盆地蕴含着丰富的钾资源，是研究古蒸发岩和古海水形成演化的热点地区之一. 老挝万象盆地含盐建造包含3个蒸发岩-碎屑岩沉积韵律，而钾镁盐矿赋存在下盐段顶部. 通过对老挝通芒钾盐矿区钾镁盐矿层的硼酸盐矿物的提取，发现在成钾晚期硼酸盐矿物有方硼石单晶和鲕粒方硼石两种形式. 钻孔中方硼石单晶含量较少，而鲕粒方硼石分布较多且基本赋存于钾镁盐矿层顶部，鲕粒方硼石的规律性分布表明成钾晚期为动荡的浅水环境. 无论是方硼石单晶还是鲕粒方硼石都在高能环境下形成，其中鲕粒方硼石对于反演盐湖沉积环境有明显的指示意义.
Abstract:There exist abundant potassium resources in Vientiane Basin of Laos. These huge evaporites or potash deposits are important materials for studying formation and evolution of ancient evaporites and seawater. The salt-bearing construction of the Vientiane Basin in Laos contains three evaporite-clastic sedimentary rhythms, while the potassium-magnesium salt deposits are present at the top of the lower salt section. Through the extraction of borate minerals from the potassium-magnesium salt deposits in the Thongmang potash mine in Laos, it is found that there are two forms of borate minerals in the late stage of potassium formation, namely boracite single crystal and oolitic boracite. According to the analysis, the content of boracite single crystal in the borehole is less; while the oolitic boracite is more distributed and basically present on the top of the potassium-magnesium salt layer, and the regular distribution of oolitic boracite shows that the sedimentary environment is a turbulent shallow water environment. Both the boracite single crystal and oolitic boracite are formed in a high-energy environment, and the oolitic boracite is of great significance to invert the salt-forming environment.
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图 1 区域地质构造简图
1.二叠系；2.中‒下侏罗统；3.下白垩统；4.塔贡组；5.班塔博组；6.背斜及其编号；7.向斜及其编号；8.整合界线；9.角度不整合界线；10.平行不整合界线；11.平移断层；12.隐伏断层；13.性质不明断层；14.河流或水库；15.白垩纪二长花岗斑岩；16.研究区；17.行政村
Fig. 1. The regional geological structural diagram of study area
表 1 样品的矿物组成（%）
Table 1. Mineral compositions(%)of the sample
样品编号 方硼石 硬石膏 石英 方解石 白云石 水氯硼钙石 菱镁矿 碳酸镁钙 T1‒01 82 10 3 5 T1‒02 25 67 1 T1‒03 20 49 21 9 T2‒07 11 82 3 1 2 T2‒08 11 86 2 1 1 T2‒09 14 70 9 2 2 2 FJ‒13 49 21 9 20 FJ‒14 82 3 1 2 11 FJ‒15 86 2 1 1 11 FJ‒16 70 9 2 2 2 14 
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表 2 方硼石单晶颗粒SEM⁃ED元素分析结果（%）
Table 2. Elemental analysis of boracite single crystal by SEM-ED(%)
颗粒编号 方硼石颗粒形态 主要元素 岩性 O Mg Cl Ca Si Al a1 晶体 69.91 23.13 6.96 钾盐 a2 晶体 71.09 22.17 6.58 0.16 b1 晶体 63.25 22.99 11.88 0.36 1.26 0.27 b2 晶体 64.72 22.13 10.13 0.49 2.53
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表 3 鲕粒方硼石颗粒SEM⁃EDS元素分析结果（%）
Table 3. Element analysis of oolitic boracite crystal by SEM-ED (%)
颗粒编号 方硼石颗粒形态 主要元素 岩性 O Mg Cl Ca Si Al Na Fe Br S Mo Pb c1 鲕粒 67.86 22.78 9.15 0.21 岩盐 c2 鲕粒 71.93 21.39 5.94 0.40 0.33 c3 鲕粒 66.97 22.56 9.22 0.48 0.29 0.47 d1 鲕粒（边缘) 63.38 21.32 11.92 0.61 0.54 0.25 0.25 0.87 0.40 0.48 d2 鲕粒（内部） 64.73 22.64 11.35 0.36 0.20 0.21 0.52 e1 鲕粒（内部） 64.73 22.64 11.35 0.36 0.20 0.21 0.52 e2 鲕粒（边缘) 63.38 21.32 11.92 0.61 0.54 0.25 0.25 0.87 0.40 0.48 f1 鲕粒（内部） 66.66 23.08 9.51 0.34 0.41 f2 鲕粒（中部） 66.47 21.85 9.44 0.40 0.86 0.59 0.39 f3 鲕粒（边缘） 71.08 20.94 6.50 0.18 0.28 0.39 0.62 f4 鲕粒（边缘） 72.75 19.94 5.42 0.23 0.64 0.60 0.41
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