四川盆地三叠系杂卤石的时空分布、显微特征及成因研究

Study on the temporal and spatial distribution,microscopic characteristics and genesis of Triassic polyhalite in Sichuan Basin

四川盆地是我国重要的成盐聚钾盆地,三叠系嘉陵江组、雷口坡组发育大量的杂卤石,具分布广泛、层数多、累计厚度大的特点。前人对杂卤石的微观结构及成因机制研究较少,影响了四川盆地杂卤石钾盐资源的勘查及深部化学开采。本次研究通过野外调查,结合薄片、电子探针、定量光谱分析及蒸发试验等方法,开展杂卤石的微观组构特征及成因机制研究。研究发现四川盆地杂卤石的分布与盐岩分布并不一致,杂卤石主要分布在广安—武胜、邛崃—浦江、长寿—垫江地区,四川盆地以发育"硬石膏—岩盐—杂卤石—岩盐—硬石膏"、"硬石膏—杂卤石—硬石膏"两种成钾序列为主,微观形态以(1)呈块状、片状、集合体状、不规则粒状与石盐、硬石膏共生;(2)呈放射状、纤维状、花瓣状与硬石膏共生;(3)呈星点状、碎片状、港湾状与硬石膏共生三种为主。研究认为杂卤石主要有原生沉积及后生交代两种成因,"硬石膏—岩盐—杂卤石—岩盐—硬石膏"型的杂卤石应为从饱和含钾卤水中直接析出,"硬石膏—杂卤石—硬石膏"型杂卤石为交代成因,杂卤石交代硬石膏,具明显的交代残余结构,钾石膏可能作为交代过程的中间产物。盐类矿物在沉积成岩—埋藏变形变质—差异隆升改造等漫长的过程中会出现溶解、交代、重结晶等改造和重塑,形成现今四川盆地含盐剖面的组合特征。

Objectives:Sichuan Basin is an important salt forming and potassium accumulating basin in China.There are a lot of polyhalite in the Triassic Jialingjiang Formation and Leikoupo Formation,which is characterized by wide distribution,multiple layers and large accumulated thickness.Previous studies on the microstructure and genetic mechanism of polyhalite have little influence on the exploration and deep chemical exploitation of polyhalite in Sichuan Basin.Methods:In this paper,through field investigation,thin section,electron microprobe,quantitative spectral analysis and evaporation test,combined with lithofacies paleogeography and sedimentary geochemistry,the micro fabric characteristics and genetic mechanism of polyhalite are studied.Results:The results show that the distribution of polyhalite in Sichuan Basin is not consistent with that of salt rock.Polyhalite is mainly distributed in Guang’an wusheng,Qionglai Pujiang and Changshou Dianjiang areas.In Sichuan Basin,there are two kinds of K-forming sequences:anhydrite rock salt polyhalite rock salt anhydrite and anhydrite polyhalite anhydrite.The microscopic morphology is mainly as follows:(1)it is massive,flaky,aggregate and irregular granular,coexisting with halite and anhydrite;(2)it is radial,fibrous and petal shaped;and(3)it is star like,fragmentary and bay shaped,and coexists with anhydrite.Conclusion:Based on the results from the analysis and arrangement for the collected.The results show that polyhalite has two main Genesis:primary deposition and epigenetic metasomatism.The"Anhydrite—Halite—Polyhalite—Halite—Anhydrite"type polyhalite should be directly separated from the saturated potassium bearing brine.The"Anhydrite—Polyhalite—Anhydrite"type polyhalite is metasomatic origin,which has obvious metasomatic residual structure.Potassium gypsum may be used as the intermediate product of metasomatism.In the long process of sedimentary diagenesis,burial deformation,metamorphism and differential uplift,salt minerals will be transformed and remolded,such as dissolution,metasomatism and recrystallization,forming the combination characteristics of salt bearing sections in Sichuan Basin.