Elemental distribution in apatite, titanite and zircon during hydrothermal alteration: Durability of immobilization mineral phases for actinides

In situ isotopic analyses of U, Pb and rare earth elements (REE) in apatite and titanite were conducted for tuff samples collected from Bidoudouma, the Republic of Gabon, in order to assess the stability of accessory minerals during hydrothermal alteration and to deduce the alteration conditions. The U–Pb results reveal that titanite of detritus origin (2.8–2.7 Ga) released Pb by igneous activity at ca. 560 Ma, in association with the dolerite dyke intrusion. The hydrothermal fluid associated with the igneous activity interacted with titanite and zircon; this resulted in elemental redistribution. The Ti content of the corroded titanite is 9–24% lower than that of unaltered titanite, whereas the Al, Fe and Ca contents are higher than those of unaltered titanite (Ca: >10% of the Ca contents in unaltered titanite; Al and Fe: >twice the Al and Fe contents in unaltered titanite); this indicates that Ca was incorporated into the corroded titanite together with Ti substitution for Al and Fe. Ca enrichments in the corroded titanite and altered zircon led to the incorporation of Pb and Eu2+ during the alteration, and this resulted in positive Eu anomalies of the corroded titanite and altered zircon. The U–Pb data and REE distributions suggest that Pb was incorporated into both the minerals; however, U was released from the corroded titanite and incorporated into the altered zircon during the hydrothermal alteration. On the other hand, apatite grains were not affected by the hydrothermal alteration at ca. 560 Ma. Analyzing the mobilization of Ti and Zr and the solubility of the apatite, it can be inferred that the redistribution of REE, U and Pb in titanite and zircon occurred under alkaline conditions. The U–Pb and REE data suggest that apatite is a good immobilization phase for REE, Pb and U under alkaline conditions (pH > 8).