Partitioning and Leaching Behavior of Actinides and Rare Earth Elements in a Zirconolite-Bearing Hydrothermal Vein Syste
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Partitioning and Leaching Behavior of Actinides and Rare Earth Elements in a ZirconoliteBearing Hydrothermal Vein System Timothy E. Payne1, Reto Giere2, Kaye P. Hart1, Gregory R. Lumpkin1, and Peter J. McGlinn1 1 Australian Nuclear Science and Technology Organisation, PMB 1, Menai, 2234, Australia 2 Mineralogisch-Geochemisches Institut, Albert-Ludwigs-Universität, Freiburg, D-79104, Germany
ABSTRACT Chemical extraction techniques and scanning electron microscopy were used to study the distribution and behavior of actinides and rare earth elements (REE) in hydrothermal veins at Adamello (Italy). The six samples discussed in this paper were from the phlogopite zone, which is one of the major vein zones. The samples were similar in their bulk chemical composition, mineralogy, and leaching behavior of major elements (determined by extraction with 9M HCl). However, there were major differences in the extractability of REE and actinides. The most significant influence on the leaching characteristics appears to be the amounts of U, Th and REE incorporated in resistant host phases (zirconolite and titanite) rather than readily leached phases (such as apatite). Uranium and Th are very highly enriched in zirconolite grains. Actinides were more readily leached from samples with a higher content of U and Th, relative to the amount of zirconium. The results show that REE and actinides present in chemically resistant host minerals can be retained under aggressive leaching conditions. INTRODUCTION Synroc is a multiphase ceramic designed to incorporate the elements present in radioactive waste. Zirconolite (CaZrTi2O7) acts as a host phase for REE, U, Zr, Np and Pu in Synroc [1]. Natural zirconolites can provide information regarding the stability of individual mineral grains in geological environments, and their ability to retain actinides and REE. We are studying hydrothermal veins (Adamello, northern Italy), which are enriched in numerous elements including U, Th and REE; and contain Ti-rich minerals and zirconolite. Among the techniques being employed are chemical extraction studies, which are designed to complement mineralogical and other investigations. SAMPLES AND PROCEDURES In the study region, dolomitic marbles contain hydrothermal veins consisting of four distinct mineral zones; the titanian clinohumite, phlogopite, pargasite and forsterite zones [2,3]. Chemical extraction studies of three vein zones and the country rock (from the BR19 location) have been previously reported [4,5]. In this paper we focus on two separate locations in the phlogopite zone. The major minerals in this zone are phlogopite, calcite, and pyrrhotite. Minor minerals enriched in actinides and REE include zirconolite, titanite, and fluorine-rich apatite [2]. The zirconolite grains contain up to 22 wt.% U and up to 15 wt.% Th.
Samples were obtained from separate sections of the phlogopite vein zone, denoted BR19 and BR149. Samples of about 5 g were drilled out and crushed to fine powders. These samples were denoted S1, S6 and S12 (from
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