基于流体包裹体分析的四川盆地早三叠世古海水化学特征及其对钾盐沉积的约束研究

Potassium salt is a scarce and strategic resource in China. The ancient marine basin is the key direction for China to find potassium in the future. The Sichuan Basin is one of the most important marine-forming salt-forming basins in China. A large number of polyhalites and potassium-rich brines have been found in the Triassic strata, but no soluble potassium deposits have been found so far. Therefore, whether the Sichuan Basin can form a soluble salt deposit is a long-standing debate in the Chinese sedimentology community, especially the potash industry. The precipitation of potash minerals is restricted by many factors. Among them, the chemical conditions such as the pH and composition of the ancient brine are important constraints, which have an important impact on the potassium deposition of the Triassic paleo-sea waters in Sichuan. The evaporites in Hechuan area are studied. Through the study of the primary fluid inclusions of sulfate and rock salt, the pH value and chemical composition of the paleo-seawater in the early Triassic salt formation period are inverted to determine the pH and chemical composition. The mechanism of restricting the type of potassium salt minerals in seawater. The implementation of the project not only has important scientific significance for the study of potassium-forming mechanism in the Sichuan Basin, but also has practical application value for finding soluble potassium salts in the region. It also provides relevant information for the study of the ancient Tethys Ocean.

钾盐是中国紧缺、战略性资源，古代海相盆地是中国今后找钾的重点方向。四川盆地是中国最重要的海相成盐成钾盆地之一，其中三叠系地层已经发现了大量的杂卤石和富钾卤水，但是至今仍然没有发现易溶性钾盐矿床。因此，四川盆地能否形成易溶性钾盐矿床，是中国沉积学界尤其钾盐界长期争论的问题。钾盐矿物的析出受多种因素制约，其中，古卤水的酸碱度和成分等化学条件是重要的制约因素，对四川三叠纪古海水的钾盐沉积有重要影响，本项目拟以长寿地区和合川地区的蒸发岩为研究对象，通过蒸发岩的原生流体包裹体的研究，以反演早三叠世成盐期古海水的pH值和化学组分，进而确定酸碱度和化学组分对海水析出钾盐矿物类型制约机理。该项目的实施不仅对四川盆地成钾机理研究具有重要的科学意义，而且对在该地区寻找易溶性钾盐具有实际应用价值，同时还可为古特提斯洋的研究提供相关资料。

本项目在总结四川盆地蒸发岩地层的沉积过程与成盐时间、规模的基础上，通过对川东地区早中三叠世地层中盐类矿物演化序列的研究，结合碳酸盐岩的碳氧同位素和盐类矿物流体包裹体分析，查明碳酸盐的碳氧同位素特征、石盐岩的化学成分、矿物成分和石盐岩群体包裹体的同位素特征，研究其古环境指示意义。四川盆地嘉陵江组含盐系地层中石盐的流体包裹体的氢氧同位素组成明显偏离全球大气降水量线，说明在嘉陵江组四亚段，沉积环境为强烈的蒸发环境，成盐物质的主要来源为海水补给型。四川盆地在早三叠世时期气温都普遍处在高温的气候环境中。根据碳氧同位素计算，飞仙关组的最低温度为31.09℃，最高温度为52.4℃，平均温度为42.30℃；嘉陵江组的最高温度为72.48℃，最低温度为25.47℃，平均温度为43.89℃；巴东组的最高气温为61.399℃。最低温度为31.7403℃，平均温度为43.61℃。而与巴东组同一时期的雷口坡组的温度，温度最高为67.19℃，最低为22.966℃，平均温度为46.84℃。