Zirconolite, aeschynite-(Ce), titanite and apatite have been found as minor or accessory minerals in a Ti-rich (TiO2 = 2.1-4.5 wt.%) hydrothermal vein occurring in dolomite marbles at the contact with a tonalite intrusion of the Tertiary Adamello batholith (northern Italy). The vein consists of four distinct mineral zones, comprising from margin to center: (1) forsterite + calcite, (2) pargasite + calcite + titanite + sulfides, (3) phlogopite + calcite + titanite + sulfides, and (4) titanian clinohumite + spinel + calcite + sulfides. Zirconolite occurs in two vein zones only: in the phlogopite zone it is invariably anhedral, often corroded, and exhibits complex chemical zonation patterns. In the titanian clinohumite zone zirconolite is idiomorphic and characterized by a pronounced discontinuous chemical zoning, but shows no evidence of corrosion. The considerable compositional variation observed for zirconolite (in wt.%: SIGMA(REE2O3) = 0.74-16.8, UO2 = 0.59-24.0, ThO2 = 0.67-17.1) is due to the zoning, and may be attributed to four major substitutions described by the exchange vectors: 1. (Th, U) (Mg, Fe2+) Ca-1 Ti-1 2. REE Al Ca-1 Ti-1 3. REE Fe2+ (Nb, Ta) Ca-1 Ti-2 4. Hf Zr-1. Exchange vector (2) is effective at total REE2O3 contents up to approximately 5 wt.%, whereas vector (3) is operating at higher concentrations. Both titanite and aeschynite-(Ce) exhibit, like zirconolite, complex chemical zonation patterns which document that the trace element content of the metasomatic fluid was variable during the vein-forming process. As indicated by thermodynamic analysis of the phase assemblages, the vein zones containing the REE-bearing minerals formed at 500-600-degrees-C (P(total) almost-equal-to 2 kbar) from a reducing fluid rich in H2S, HCl-degrees, HF-degrees and phosphorus, but relatively poor in CO2 (X(CO2) almost-equal-to 0.2). Geochemical and isotopic data are consistent with the interpretation of the fluid as being derived from the nearby tonalite intrusion. The abundance of idiomorphic fluor-apatite as well as textural relations between apatite, the other REE-bearing minerals and the fluorine-bearing hydrous silicates suggest F- and PO43-to be the most likely ligands for complexing REE, Ti, Zr and other high-field-strength elements in the vein-forming fluid. The corrosive features observed for zirconolite demonstrate that hydrothermal fluids are able to dissolve zirconolite, which is one of the main components of SYNROC-C, the most promising disposal option for high-level nuclear waste. Therefore, immobilization of radioactive waste in zirconolite can be guaranteed only if an effective sealing material prevents any hydrothermal fluid from access to the final disposal site.