陕北盐盆奥陶系马家沟组五段蒸发岩沉积环境与作用——来自石盐流体包裹体的证据

Sedimentary environment and sedimentation of evaporite in the fifth Member of Majiagou Formation of Salt Basin in Northern Shaanxi Ordovician strata:Evidence from fluid inclusions

陕北盐盆地因发育全球罕见的奥陶系马家沟组五段含盐地层受到广泛关注。文章运用X射线粉晶衍射矿物组分半定量分析、流体包裹体的均一温度、拉曼光谱、扫描电镜能谱和包裹体氢同位素分析等多种手段,对陕北米脂县境内两口钻井奥陶系马家沟组石盐岩进行了分析和测试。流体包裹体均一温度测试结果表明,石盐岩中原生单一液相包裹体、次生正方形单一液相包裹体的均一温度平均值分别为27.9℃、30.6℃,峰值均为25-30℃,次生与原生的流体包裹体的形成环境基本一致。在重结晶程度高的层段,石盐岩流体包裹体的δD值和石盐岩K＋含量的变化趋势相反,结合该时期内含盐层段宏观地质特征以及钾石盐颗粒常呈圆粒状赋存在石盐晶间裂缝等微观证据,笔者认为研究区石盐岩沉积时因外来水体频繁侵入导致原生石盐岩发生溶解和重结晶,降低了形成大规模钾盐矿床的概率。

Ordovician was a period of extreme scarcity of evaporites, during which the potash or even halite deposits were very rarely formed. In this study, however, a great thickness of salt-bearing strata was developed in the Ordovician Majiagou Formation in northern Shaanxi salt basin, and even potash deposit was found in the fifth member of Majiagou Formation, which attracted much attention from researchers. In this paper, salt samples of the fifth member of Majiagou Formation from two cores in Mizhi County of Shanbei （northern Shaanxi） were analyzed and tested through homogenization temperature of fluid inclusion, combined with X-ray diffraction （XRD）, Raman spectrum （Rs） , energy spectrum analysis of scanning electron microscope （SEM） and hydro- gen isotope of fluid inclusion. The result shows that the mean inclusion homogenization temperature （ThAVG） of primary single-fluid phase and secondary square single-fluid phase inclusion of the rock salt was 27.9 ℃ and30.6℃ respectively. The histogram of homogenization temperature shows that the peaks of both types of fluid inclusions ranges from 25℃ to 30℃, suggesting that the formation environment of secondary fluid inclusion was very close to that of primary fluid inclusion. The analytical results confirm that the D and K＋ show opposite variation trend in the highly re-crystallized strata, as revealed by the hydrogen isotope method of fluid inclusions from rock salt. In combination with macro geological characteristics of the salt-bearing strata in this period and the microscopic geological evidence from the occurrence state of sylvinite that granular crystals often exist in the marginal fissure between the halite crystals, it is concluded that the frequent and fast invasions of foreign water bodies into saline lake mainly led to the dissolution and re-crystallization of the primary halite of the fifth member of Majiagou Formation, and also decreased the probability of forming large-sized potash deposit.