Application of a Selective Phase Extraction Procedure to Samples from the Adamello Contact Aureole (Italy)
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APPLICATION OF A SELECTIVE PHASE EXTRACTION PROCEDURE TO SAMPLES FROM THE ADAMELLO CONTACT AUREOLE (ITALY) T.E. PAYNE, G.R. LUMPKIN, P.J. McGLINN, and K.P. HART Australian Nuclear Science and Technology Organisation, PMB 1, Menai, 2234, Australia ABSTRACT Hydrothermal veins, rich in Ti, Zr, rare earth elements (REE's), and actinides, occur in the pure dolomitic marbles of the Adamello contact metamorphic aureole. A selective phase extraction using 9M HCl was applied to samples from within and near these veins to chemically separate acid-soluble phases from residual phases, and to study the associations of U, Th, and REE's with these phases. The samples were from the phlogopite, titanian clinohumite, and forsterite vein zones, and from the country rock. The effects of the extraction were studied by SEM/EDS and by chemical analysis. Isotopes of U and Th were analysed by alpha-spectrometry. The chemical data and SEM/EDS results indicated that dolomite, calcite, apatite, and much of the pyrrhotite were dissolved by the 9M HCI, whereas spinel, phlogopite, titanite, chalcopyrite, and zirconolite were among the acid-resistant phases. In all vein samples, the REE-patterns of acid-soluble phases were consistent with the dissolution of REE-rich apatite. In samples from the phlogopite zone, the majority of U, Th, and REE's were in residual phases, and the REE pattern of the residue was similar to that of REE-rich titanite. In the titanian clinohumite zone, a substantial proportion of these elements were in acid-soluble phases, and the REE pattern of the residual phases resembled that of zirconolite. Clinohumite was partially dissolved by the HCI treatment. The sample from the forsterite zone contained substantial amounts of REE's in acid-soluble phases, whereas Th and U were mostly in residual phases. The dolomite (country rock) samples contained small amounts of an acid-resistant, uranium-rich phase which, while only comprising about 0.2% of the mass, accounted for 40-60% of the uranium present. INTRODUCTION In order to predict the mobility of actinides and trace elements in the geologic environment, a detailed knowledge of their interactions with component mineral phases is required. One way of gaining information about elemental distributions in natural systems is to use selective phase extractions, in which chemical reagents are used to dissolve different mineral phases. The solutions obtained by chemical extractions can be analysed by sensitive analytical techniques such as inductively coupled plasma mass spectrometry (ICPMS) and alpha-spectrometry. Thus, extractions can provide quantitative information about the distribution of minor components between various mineral phases. Extraction techniques have been used in several natural analogue investigations, in which geologic systems of relevance to the behavior of nuclear waste are being examined [1,2]. In the present study, selective extractions are being applied to samples from the Adamello contact aureole in Northern Italy. Hydrothermal veins at Adamello are enriched i
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