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

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.