Geochemistry of hydrothermal veins containing zirconolite and betafite at Adamello, Italy

Hydrothermal veins containing zirconolite and betafite occur in dolomite marbles at the contact with a tonalite intrusion of the Adamello batholith, Italy. The veins display four distinct mineral zones and are highly enriched in Th, U, Ti, Zr, Nb, and rare earth elements (REE) relative to their host rock. Wide ranges in concentration of these elements and distinct inter- element fractionation trends exist across the four vein zones. The behavior of Th closely reflects that of P, Ti, Nb, and heavy REE, but was distinct from that of U, Zr and light REE (La to Sm). The presence and composition of refractory minerals such as zirconolite, betafite, uraninite, thorianite, baddeleyite, rutile, allanite, and aeschynite provide evidence for the transport of Ti, Zr, Nb, REE, and actinides by a fluid, which was further characterized by a low pH and high contents of F, P, Cl and H2S. Thermodynamic analysis of these veins indicates that they were formed at 550-600°C, 200 MPa total pressure, and from a fluid derived from the tonalite. Mineralogical and textural observations suggest that crystallization of the refractory minerals was in part induced by precipitation of fluorapatite and sulfides. The geochemical data further indicate that precipitation of light REE was induced by the fluid/wall-rock interaction which led to a significant dilution of the fluid by CO2. The studied veins provide an example of high- temperature transport of actinides and REE in the Earth's crust and show that these elements can be precipitated from a fluid and subsequently immobilized by zirconolite and betafite, two important actinide hosts in ceramic nuclear waste forms.