The Selective Dissolution of Carbonate Minerals for Fe Isotope Determination——A Case Study on the Ore-hosting Dolomite Marble in the Bayan Obo Ore Deposit
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摘要: 碳酸盐矿物铁同位素测试的选择性溶解方法包括稀盐酸溶解法和稀醋酸溶解法，已应用于一般地质样品。但是对于白云鄂博矿床这类矿物组成复杂的特殊样品。本文在常温下对白云石、磁铁矿、赤铁矿、黄铁矿、钠闪石和黑云母等白云鄂博矿床中常见的含铁矿物进行稀盐酸(0.5~1 mol/L 盐酸)溶解条件实验。结果显示，稀盐酸在短时间内(约3 h)能够有效地溶解碳酸盐矿物中的铁，而铁氧化物、硫化物、硅酸盐矿物的铁溶解率基本都< 1%。实验选取典型的白云鄂博矿床赋矿白云岩，对比了稀盐酸溶解法、稀醋酸溶解法与分离白云石单矿物进行全部溶解所获得的铁同位素组成：赋矿白云岩E-16的δ56Fe值分别为-0.69‰、-0.62‰、-0.69‰；赋矿白云岩E-26的δ56Fe值分别为-0.19‰、-0.18‰、-0.12‰，三种方法在误差(不确定度)范围内结果一致。此外，稀醋酸对碳酸盐的部分溶解过程中没有观察到明显的铁同位素分馏现象，表明稀盐酸溶解法和稀醋酸溶解法都能用于提取白云鄂博矿床样品碳酸盐矿物的铁同位素信息。本研究对于碳酸盐矿物的选择性提取技术在地质上的应用具有借鉴和指导意义。Abstract: With the development of Fe isotope technique, the carbonate minerals in normal carbonate whole rocks have been selectively extracted by lower concentration HCl and HAc solution, respectively, for Fe isotope determination. However, the applicability of the two methods for those samples with complex mineral compositions such as Bayan Obo ore samples requires further examination. The experiment carried out in this study can be separated into two steps. In the first one, lower concentration 0.5-1 mol/L HCl was used to dissolve different minerals, including dolomite, magnetite, hematite, pyrite, riebeckite, and biotite, at room temperature for about 3 hours. The results indicated that the lower concentration HCl can effectively dissolve the carbonate minerals, while the solubility of Fe released from Fe oxides, sulfides and silicate minerals were all less than 1%. Secondly, the lower concentrations of HCl and HAc were both used to dissolve the typical ore-hosting dolomite marble in Bayan Obo and then the Fe isotopes were measured. The results showed that the δ56Fe values of Fe-bearing carbonates extracted by HCl and HAc were in good agreement with those of separated dolomite mineral within uncertainties. They were -0.69‰, -0.62‰ and -0.69‰, respectively, for sample E-16, and -0.19‰, -0.18‰ and -0.12‰, respectively, for sample E-26. Additionally, Fe isotope fractionation was not observed during the partial dissolution of carbonate minerals using HAc, which indicated that both lower concentration HCl and HAc were reliable to partially dissolve Fe-bearing carbonate minerals from the Bayan Obo dolomite samples for the Fe isotopic determination. This study provided a good example for the application of a selective extraction technique for carbonate minerals from geological samples.
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Key words:
- Bayan Obo deposit /
- dolomite /
- carbonate minerals /
- diluted HCl dissolution /
- diluted HAc dissolution /
- Fe isotope
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表 1 不同相矿物在盐酸中的溶解
Table 1. The dissolution of various phases of minerals with HCl
样品
名称样品
性质称样量
m/mgc(HCl)/
(mol·L-1 )溶样
时间
t/h溶样
温度
θ/℃Fe溶解
率/%白云石 粉末 ~20 0.5 3 20 100.0 磁铁矿 粉末 15.2 0.5 3 20 0.13 赤铁矿 粉末 11.1 0.5 3 20 0.02 黄铁矿 粉末 11.6 0.5 3 20 0.01 白云石 粉末 ~20 1 3 20 100.0 磁铁矿 粉末 9.0 1 3 20 0.27 赤铁矿 粉末 12.4 1 3 20 0.05 黄铁矿 粉末 19.0 1 3 20 0.01 钠闪石 粉末 18.4 1 3 20 0.00 黑云母 粉末 12.3 1 3 20 0.32 钠闪石 粉末 25.0 6 3 20 0.18 
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表 2 白云鄂博矿床白云岩不同处理方式的Fe同位素组成对比
Table 2. Comparison for Fe isotope ratio measurements of Bayan Obo ore-hosting dolomite marble with different dissolution methods
样品
编号样品性质 溶样方式 δ56Fe
/‰δ57Fe
/‰w(Fe)
/%Fe的
溶解率
/%E-16 全岩粉末 全溶 -0.26 -0.41 6.5 100.0 全岩粉末 0.75 mol/L盐酸，常温3 h -0.69 -1.06 3.3 51.3 全岩粉末 10%醋酸，常温40 h -0.62 -0.91 2.4 36.7 白云石
单矿物6 mol/L盐酸，常温5 h -0.69 -1.04 5.8 100.0 E-26 全岩粉末 全溶 -0.01 0.05 6.2 100.0 全岩粉末 0.75 mol/L盐酸，常温3 h -0.19 -0.27 4.8 77.1 全岩粉末 10%醋酸，常温40 h -0.18 -0.24 4.0 64.2 白云石
单矿物6 mol/L盐酸，常温5 h -0.12 -0.18 5.4 100.0 M-63 全岩粉末 全溶 -0.24 -0.32 3.3 100.0 全岩粉末 10%醋酸，常温40 h -0.20 -0.33 2.2 64.8 白云石
单矿物6 mol/L盐酸，常温5 h -0.31 -0.44 5.1 100.0 M-64 全岩粉末 全溶 0.01 -0.01 5.6 100.0 白云石
单矿物6 mol/L盐酸，常温5 h -0.23 -0.30 5.4 100.0
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