Genesis of Polyhalite and Its Significance of Jialingjiang Formation in Dianjiang Salt Basin, Eastern Sichuan Basin
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摘要:
四川盆地三叠纪是主要的成钾期，目前发现的主要含钾矿物为杂卤石，有关早三叠世嘉陵江组四段石盐岩中杂卤石成因一直存在争议. 采用薄片鉴定、扫描电镜、稀土元素和锶同位素等手段，分析了四川盆地东部垫江盐盆长寿地区嘉陵江组四段石盐岩中杂卤石矿物形态特征，初步探讨了该杂卤石成因及对寻找海相钾盐的指示意义. 扫描电镜下与石盐岩共生的杂卤石主要为针状和长柱状，自形程度较高；含杂卤石盐岩相对富集轻稀土，亏损重稀土，无明显的Eu和Ce异常，表现出与海水稀土元素配分模式一致的特征，表明其具有海水来源. 长平3井石盐岩样品87Sr/86Sr值为0.708 151~0.708 286，变化较小，与三叠纪海水值基本一致，说明杂卤石是海水高度浓缩的产物. 垫江盐盆嘉陵江组四段石盐岩中杂卤石主要为原生杂卤石，表明该地区石盐岩沉积时古卤水已经接近或达到钾盐析出阶段，原生杂卤石接近了盐湖的浓缩中心，因此垫江盐盆是有利的找钾区.
Abstract:The Triassic witnessed the significant potash formation in Sichuan Basin. So far, the main potash mineral reported is mostly polyhalite, while the origin of polyhalite in the fourth member of Jialingjiang Formation in eastern Sichuan Basin still remains controversial. In this paper, it employs the thin section identification, scanning electron microscope (SEM) and geochemical measurements (including rare earth element and strontium isotope), analyzing the polyhalite samples from the fourth member of Jialingjiang Formation, eastern Sichuan Basin in detail. The microscope and SEM photographs show that the polyhalite which is in intergrowth with halite has well euhedral and few structural defects. The characteristics of rare earth element are relatively enriched in LREE and depleted in HREE, with weak negative Eu and Ce abnormities, which are similar to those of the seawater. In addition, strontium isotopic values of the halite samples (Changping 3 Well) are between 0.708 151 and 0.708 286, which are basically consistent with the seawater values in the Triassic. All these evidences indicate that the source of halite and polyhalite was the products of highly concentrated seawater. In this paper, it holds that the polyhalites in the fourth member of Jialingjiang Formation in eastern Sichuan Basin were mainly primary polyhalites, and the seawater was highly concentrated and had approached or reached the critical point of potassium and magnesium salts precipitation. It infers that Changping 3 Well is close to the concentration center of Dianjiang Salt Basin, and the nearby area is the suitable place for the formation and preservation of potash deposit.
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图 1 四川盆地东部含盐盆地分布及采样钻孔位置示意图
Fig. 1. Distribution of salt basins in the eastern Sichuan Basin and location of sampling site
图 2 长平3井嘉陵江组四段岩性及采样位置
Fig. 2. Lithological column and sampling position of Jialingjiang Formation in Changping 3 Well, eastern Sichuan Basin
图 3 石盐岩样品照片
a. 桔红色；b. 浅红色；c. 无色透明；d. 浅灰色；e. 深灰色
Fig. 3. Halite photos of Jialingjiang Formation
图 8 四川盆地东部嘉陵江组石盐岩样品锶同位素组成与全球其他地区对比
Fig. 8. Comparison of Sr isotopic values among halite samples in eastern Sichuan Basin and other samples in the world
表 1 石盐岩样品稀土元素含量（10-6）分析结果
Table 1. The contents of rare earth elements (10-6) of halite samples
样品号 CP3-1 CP3-2 CP3-3 CP3-4 CP3-5 CP3-6 CP3-7 CP3-8 CP3-9 La 10.96 31.63 11.54 8.46 7.21 1.04 42.93 0.27 1.03 Ce 28.20 67.24 22.38 15.33 15.69 3.37 108.23 0.89 1.99 Pr 4.00 7.88 2.71 2.26 2.67 1.41 13.19 1.25 1.38 Nd 17.62 27.83 9.82 6.00 8.23 3.90 46.38 2.23 1.41 Sm 3.95 4.77 2.65 1.27 1.37 1.68 7.19 1.72 0.19 Eu 0.62 0.89 0.40 0.24 0.36 0.44 1.19 0.31 0.05 Gd 2.06 2.87 2.07 1.44 1.64 2.22 3.64 1.81 0.34 Tb 0.24 0.40 0.35 0.39 0.46 0.51 0.64 0.47 0.25 Dy 0.62 0.83 0.86 0.60 0.74 1.08 0.99 0.74 0.18 Ho 0.06 0.11 0.08 0.05 0.07 0.11 0.10 0.07 0.01 Er 0.09 0.15 0.10 0.07 0.08 0.19 0.12 0.08 0.03 Tm 0.01 0.01 0.02 0.01 0.01 0.02 0.01 0.00 0.01 Yb 0.04 0.03 0.05 0.06 0.02 0.03 0.08 0.02 0.02 Lu 0.01 0.00 0.01 0.00 0.02 0.01 0.01 0.00 0.01 Y 1.75 3.64 3.06 2.03 2.34 4.36 3.24 1.91 0.39 ΣREE 68.48 144.64 53.04 36.19 38.59 16.01 224.72 9.86 6.89 LREE/HREE 20.92 31.87 13.98 12.77 11.67 2.84 39.11 2.09 7.29 δEu 0.66 0.73 0.52 0.54 0.74 0.70 0.71 0.53 0.64 δCe 1.03 1.03 0.97 0.85 0.86 0.67 1.10 0.37 0.40 样品号 CP3-10 CP3-11 CP3-12 CP3-13 CP3-14 CP3-15 CP3-16 CP3-17 CP3-18 La 0.29 2.81 5.96 0.55 0.18 0.09 0.17 0.20 0.21 Ce 1.00 4.97 12.35 1.63 0.70 0.88 0.62 1.48 1.22 Pr 1.07 1.62 2.74 0.76 1.03 0.96 1.10 1.25 1.07 Nd 1.26 2.38 9.72 0.91 0.99 0.44 0.67 2.26 1.10 Sm 0.61 0.69 4.62 0.63 0.75 0.18 0.39 0.60 0.38 Eu 0.16 0.15 1.12 0.15 0.25 0.07 0.11 0.17 0.10 Gd 0.80 0.78 5.46 0.41 1.38 0.31 0.50 0.65 0.37 Tb 0.26 0.32 0.91 0.11 0.26 0.12 0.15 0.13 0.10 Dy 0.33 0.57 2.15 0.16 0.58 0.17 0.29 0.29 0.18 Ho 0.03 0.06 0.22 0.03 0.08 0.02 0.03 0.04 0.03 Er 0.04 0.05 0.23 0.09 0.11 0.05 0.07 0.07 0.06 Tm 0.01 0.01 0.02 0.00 0.02 0.01 0.01 0.01 0.00 Yb 0.02 0.05 0.02 0.07 0.06 0.03 0.04 0.02 0.03 Lu 0.01 0.00 0.00 0.00 0.01 0.00 0.00 0.01 0.00 Y 1.09 2.35 6.20 0.89 2.67 0.60 1.05 1.24 0.76 ΣREE 5.88 14.45 45.52 5.52 6.40 3.33 4.15 7.18 4.84 LREE/HREE 2.94 6.84 4.05 5.23 1.56 3.74 2.80 4.94 5.27 δEu 0.70 0.63 0.68 0.90 0.74 0.93 0.79 0.82 0.80 δCe 0.44 0.56 0.74 0.61 0.39 0.70 0.34 0.72 0.63 
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表 2 石盐岩样品锶同位素分析结果
Table 2. Sr isotopic values of halite samples
样品号 CP3-1 CP3-2 CP3-3 CP3-4 CP3-5 CP3-6 CP3-7 CP3-8 CP3-9 87Sr/86Sr 0.708 234 0.708 166 0.708 151 0.708 194 0.708 185 0.708 204 0.708 207 0.708 257 0.708 226 2σ 0.000 008 0.000 009 0.000 009 0.000 009 0.000 008 0.000 007 0.000 007 0.000 033 0.000 008 样品号 CP3-10 CP3-11 CP3-12 CP3-13 CP3-14 CP3-15 CP3-16 CP3-17 CP3-18 87Sr/86Sr 0.708 208 0.708 190 0.708 220 0.708 200 0.708 218 0.708 286 0.708 173 0.708 176 0.708 171 2σ 0.000 009 0.000 009 0.000 011 0.000 010 0.000 010 0.000 010 0.000 010 0.000 008 0.000 008
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