Characteristics and Significance of Rare Earth Elements in Lithic Cuttings of Gypsum Salt Section of Geothermal Well in Gaotan Town, Linshui, Sichuan
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摘要: 膏盐稀土元素研究不仅可以查明成矿物质来源, 而且对于评价钾盐的成矿潜力也具有重要意义。四川盆地华蓥山东翼四川邻水县高滩镇倒碑村地热井嘉陵江组—雷口坡组钻遇多层膏盐夹层, 对其岩屑开展了岩石学、稀土元素和相关微量元素测试分析。研究表明: (1)该区嘉陵江组—雷口坡组膏盐层成分主要为石膏, 含少量钾长石、石英、黏土矿物等陆源碎屑, ΣREE 较低, 为7.18×10–6～77.63×10–6, 均值20.03×10–6, 明显低于泥岩及砂岩稀土总量, 与江西信江盆地晚白垩世膏盐以及安徽北部等地碳酸盐岩REE 总量相近。ΣREE与Al 的主要载体(黏土矿物)呈正相关关系, HREE 与LREE 亦呈明显正相关关系, 稀土元素来自附近泸州古隆起陆源碎屑态黏土矿物以及黏土矿物对水体中稀土的吸附作用, 这两种作用可能是该区膏盐类稀土元素的主要来源。稀土元素含量或可作为膏盐和盐岩盆地与古陆距离的评价指标。膏盐岩屑矿物组成及稀土元素特征反映该区靠近泸州古隆起, 来自古陆的地表淡水的淡化作用, 是该区未能形成规模性钾盐矿床的主要原因; (2)该区膏盐层段岩屑稀土元素为相对平坦的页岩标准化曲线形态, LREE/HREE 介于3.02～3.72 之间, 平均值3.37。(La/Yb)N 介于1.15～1.63 之间, 平均值1.29, 曲线略向右倾斜, 轻稀土弱富集。δEu 介于1.14～6.89 之间, 平均值2.65。δCe 介于0.81～0.87, 平均值0.85, 具明显的Eu 正异常和较弱的Ce 亏损, 表现为缺氧、还原、强酸的环境, 地层中或广泛存在热化学硫酸岩还原作用(TSR); (3)La/Ce 值、Zn-Ni-Co 三角图、logTh-logU 相关图显示膏盐地层为高温环境, 这促进了TSR 作用的发育。高温除与正常地温梯度相关外, 还可能与峨眉山玄武岩喷发后续效应以及雷口坡组底部火山喷发(绿豆岩)有关。这些研究有助于膏盐类稀土元素聚集机理的认识, 对于膏盐沉积、成岩过程中稀土元素的地球化学行为的认识也有积极意义。Abstract: The study of rare earth elements in paste salts can help identify the source of ore-forming materials, and have important significance for evaluating the ore-forming potential of potassium salts. A multi-layer paste salt interlayer was drilled in the Jialingjiang Formation–Leikoupo Formation of a geothermal well in Daobei Village, Gaotan Town, Linshui County, Sichuan, and petrological, rare-earth elements, and related trace elements were tested and analyzed. The study shows: (1) The gypsum salt beds in this area are mainly composed of gypsum and contain a small amount of terrigenous debris, such as potassium feldspar, quartz, and clay minerals. ΣREE is low, ranging from 7.18×10–6–77.63×10–6 with an average of 20.03×10–6, which is significantly lower than the total REE of mudstone and sandstone, and similar to the total REE of gypsum salt in Xinjiang Basin of Jiangxi Province and carbonate rocks in northern Anhui Province. ΣREE is positively correlated with the main carrier of Al (clay minerals), and HREE and LREE are also significantly positively correlated. Ree elements come from terrigenous clastic clay minerals in the nearby Luzhou palaeouplift and the adsorption of clay minerals on REE in water, which may be the main sources of REE elements in paste salts. The content of rare earth elements may be used as an index to evaluate the distance between paste salt and salt basin and ancient land. The mineral composition and rare earth element characteristics of the paste rock debris indicate that this area is close to the Luzhou palaeouplift, and the desalting effect of surface fresh water from the ancient land is the main reason for the failure to form large-scale potassium salt deposits in this area; (2) The rare earth elements of the paste salt formation in this area are relatively flat standardized curve of shale. LREE/HREE ranges from 3.02 to 3.72, with a mean value of 3.72. (La/Yb)N was between 1.15–1.63 with a mean value of 1.29. The curve inclined slightly to the right, and light rare earth were weakly enriched. δEu ranged from 1.14 to 6.89 with a mean of 2.65.δCe ranges from 0.81 to 0.87 with a mean of 0.85. It has an apparent positive Eu anomaly and weak Ce deficit, which is characterized by anoxic, reductive and strong acid environment, and hydrothermal sulfite reduction (TSR)exists in the formation or widely; (3) The La/Ce values, Zn-Ni-Co triangle diagram and LogTh-LogU correlation diagram show that the paste salt formation is a high-temperature environment, which promotes the development of TSR effect. In addition to the normal geothermal gradient, the high temperature may be more importantly related to the follow-up effect of the Emei basalt eruption and the volcanic eruption (mung bean rock) at the bottom of the Lekoupo Formation. These findings help understand the accumulation mechanism of earth elements in paste salt and positively promote the understanding of the geochemical behavior of earth elements in the process of paste salt deposition and diagenesis.
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Key words:
- Linshui County /
- Gaotan Town /
- geothermal well /
- gypsum salt /
- rare earth element
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