川东北普光地区下三叠统蒸发岩硫同位素地球化学特征及其指示意义

Sulfur isotope geochemistry of Lower Triassic evaporites and its indicative significance in Puguang area,northeast Sichuan basin

海相硫酸盐矿物的硫同位素研究不仅可以解析当时的古海水特征,还可为物质来源分析、地层对比及其沉积环境条件分析提供重要线索。川东北地区中-下三叠统海相蒸发岩地层中除大量膏-盐以外,还发育与石盐共伴生的硫酸盐型钾盐矿物杂卤石(被命名为“新型杂卤石钾盐矿”),因此,对该区域硫同位素的研究不仅可以为蒸发岩的沉积环境及其演化进行解密,还可为钾盐矿物杂卤石的物源分析提供证据。但当前对于研究区内硫同位素研究多存在于大时间尺度上的演化规律以及全球性的地层对比,对于杂卤石发育的嘉四-五段地层内δ^(34)S的高精度演化规律及其指示意义研究相对匮乏。本文利用川宣地1井中的硬石膏、杂卤石及二者的混合样品开展δ^(34)S的高精度分析。研究显示,硬石膏与杂卤石样品不存在明显的硫同位素分馏。同时,δ^(34)S结果证实,钾盐矿物杂卤石及其伴生硬石膏的物源主要是海水,但受到细菌还原作用和蒸发岩矿物沉淀的影响。此外,δ^(34)S演化规律与嘉四-五段地层的层序演化规律较一致,这表明δ^(34)S测试对蒸发沉积过程的沉积演化分析及区内地层的高精度对比具有重要意义,在杂卤石型钾盐矿勘查工作中可用于精细对比和矿层定位的重要手段。

Studies on sulfur isotopes of marine sulfate minerals can indicate the characteristics of ancient seawater at that time,and provide important evidence for material source analysis,stratigraphic correlation and sedimentary environment conditions.Except for a large amount of anhydrite-salt deposits,sulfate-type potassium mineral polyhalite associated with salt,which has been called“a new type of polyhalite potassium ore”,is also developed in the Middle and Lower Triassic marine evaporite strata in northeast Sichuan.Therefore,the study of sulfur isotope in this area can reveal the sedimentary environment and its evolution of the evaporite;at the same time it can also provide evidence for the provenance analysis of potassium salt mineral polyhalite.However,researches on sulfur isotopes in the study area mostly existed in the evolution law of large time scale and global stratigraphic correlation.Studies on the evolution ofδ^(34)S in the fourth and fifth member of Jialingjiang formation where polyhalite is developed and its indicative significance are rare.In this paper,a high-precision analysis ofδ^(34)S has been carried out using anhydrite,polyhalite and mixed samples of the two above from Well CXD1.It shows that there is no obvious sulfur isotope fractionation between anhydrite and polyhalite.Meanwhile,δ^(34)S results show that the main source of potassium salt mineral polyhalite and its associated anhydrite is seawater,but it is also affected by bacterial sulfate reduction and evaporite mineral precipitation.In addition,theδ^(34)S evolution pattern is obviously consistent with the sequence evolution of the 4th and 5th embers of Jiangling Formation,which indicates theδ^(34)S analysis is of great significance for the analysis of evaporative sulfates sedimentary evolution and the high-precision correlation of the strata in the region.It can be used as an important means of high-resolution correlation and ore layer location in the further exploration of polyhalite-type potassium salt deposits.