An improved method to oxidize volcanogenic massive sulphide mineralization and isolate barite for radiometric dating using 226Ra, 228Ra, 228Th and 210Pb

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Ditchburn, R.G. 2018 An improved method to oxidize volcanogenic massive sulphide mineralization and isolate barite for radiometric dating using 226Ra, 228Ra, 228Th and 210Pb. Lower Hutt, N.Z.: GNS Science. GNS Science report 2018/04. 17 p.; doi: 10.21420/G2K927

Abstract: Potassium bromate (KBrO3), a powerful oxidizing agent, is employed to decompose samples of volcanogenic massive sulfide (VMS) mineralization without significantly affecting the barite that contains 226Ra, 228Ra, 228Th and 210Pb, the isotopes needed for radiometric dating. The sulfides, typically sphalerite, chalcopyrite and pyrite, are oxidized to produce a solution of Cu, Fe and Zn sulfate that is decanted when the barite has settled. The oxidation starts in the cold and proceeds to near completion overnight, far easier than evaporation with concentrated HCl and HNO3 or fusion with a mixture of Na2SO4, K2SO4 and H2SO4. Galena, including the initial 210Pb decaying from the time of mineralization, is oxidized to form a PbSO4 precipitate. This is removed as a solution in EDTA leaving just the new 210Pb grown from 226Ra within the barite. An age can be derived from the barite 210Pb/226Ra value if the 210Pb is chemically separated for measurement. Silicate impurities are decomposed using a hot mixture of H2SO4, HNO3 and HF. The waste acid is decanted rather than evaporated so that pyrosulfate transposition to eliminate residual fluoride is not needed. Processing time has been halved and the procedure is less hazardous. Minimal corrosive chemicals are vented to the atmosphere so the fume cupboard does not require an elaborate scrubber. (auth)