蒸发岩序列中氯化物盐的氯同位素分馏效应及应用——兼论塔里木盆地、柴达木盆地古代岩盐的沉积阶段

Fractionation of chlorine isotope in salt mineral sequences and application:Research on sedimentary stage of ancient salt rock deposit in Tarim Basin and western Qaidam Basin.

卤水蒸发析盐到晚期阶段,才有可能析出溶解度极大的钾盐类矿物,因此在一个成盐盆地要找到钾盐矿床,除判断物源特征外,还必须找到最晚沉积阶段的区域和层位。本文通过对实验室配制溶液和盐湖卤水蒸发析出的不同氯化物矿物、钾盐矿床沉积序列不同阶段盐类矿物对比分析,讨论了氯同位素的分馏规律。结果表明,不同蒸发浓缩阶段的氯化物盐,氯同位素发生显著分馏。一般钾盐沉积层位的石盐δ^(37)Cl值小于0,钾石盐及其以后沉积的氯化物δ^(37)Cl值小于-0.5‰,特别是钾石盐以后析出的氯化物更为偏负,光卤石一般小于-1.00‰,而钾盐沉积阶段之前早期沉积石盐的δ^(37)Cl值显著偏正。因此,δ^(37)Cl值是判断岩盐沉积阶段的有效指标。基于此,对塔里木库车盆地第三纪、莎车盆地白垩-第三纪部分代表性盐矿点以及柴达木盆地西部不同构造单元、不同层位第三纪岩盐沉积做了氯同位素分析。初步结果表明塔里木莎车盆地晚白垩世岩盐的沉积阶段明显晚于库车盆地第三纪沉积,特别是莎车盆地喀什次级凹陷δ^(37)Cl值最低,大部分样品小于-0.5‰,推测该区成盐古卤水已浓缩到晚期或接近晚期钾盐沉积阶段,应是今后一重点开展钾盐找矿的区域。柴达木盆地油墩子、南翼山地区中新统、上新统蒸发岩沉积序列δ^(37)Cl值普遍偏正,推测当时卤水浓缩演化可能只达到石盐沉积阶段,浓缩到后期阶段的富K^+、Mg^(2+)盐溶液随西部隆起迁移到柴达木盆地南部地区,因此认为第三纪地层虽然石盐沉积层位多,厚度大,但找到大规模钾盐矿床的可能性不大。

The brine must have evaporated to reach the late stage of sylvinite precipitation and must have continued to concentrate the salts for a long time. Then, it is possible to form potash deposit. Hence, this paper, examines the isotopic composition of different sequences of halite, sylvite and carnallite minerals for man made chemical solutions and salt lake brine evapo-concentrated precipitations, ancient potash deposits in an attempt to examine whether chlorine isotopic ratios can be used to study potash deposits and then be utilized to prospect for new potash deposits in the Tarim Basin and western Qaidam Basin. The results show that delta(37) Cl values of the precipitates or salt deposits decreased systematically during the main phase of halite crystallization. Generally, the sequence of delta(37) Cl values ill salt rock is early precipitated halite (> 0 parts per thousand) > late precipitated halite (< 0 parts per thousand) > sylvite (<-0.5 parts per thousand) > carnallite (< - 1.0 parts per thousand). Hence, delta(37) Cl in salts can be used as an indicator of evaporation stage. Based on this, we conclude that the concentration of ancient brines formed ancient salt rocks is much more concentrated in Tarim-Shache subbasin than that in Tarim-Kuche subbasin. Particularly, the delta(37) Cl values of salt rock in Kashi depression of western Shache subbasin is greatly negative (< - 1.00 parts per thousand), which may predicate that the ancient brine has been concentrated to the terminal stage of salt deposit or even later stage of sylvinite deposit. Then it is possible to find sylvite hearing deposits around the western region of the Shache subbasin. But to the western Qaidam Basin, all the delta(37) Cl values of Tertiary salt rock deposit samples from Youdunzi, Nanyishan, Shizigou and Tulingou depressions show obviously positive, which indicate that they are early halite deposits and the primary brine do not evapo-concentrate to reach the late stage of potash hearing minerals precipitation. Therefore, there maybe have no much more hope to find potash deposit in Tertiary salt rock deposit of western Qaidam Basin. The late stage of highly evapo-concentrated brines possibly moved to the south area.