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REE-bearing minerals in a Ti-rich vein from the Adamello contact aureole (Italy)

Gieré R., Williams C.T.
1 992
Contributions to Mineralogy and Petrology
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 discontinous chemical zoning, but shows no evidence of corrosion. The considerable compositional variation observed for zirconolite (in wt.%: ∑(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-1 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°C (Ptotal≈2 kbar) from a reducing fluid rich in H2S, HCl°, HF° and phosphorus, but relatively poor in CO2(XCO 2 ≈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 PO 4 3- to be the most likely ligands for complexing REE, Ti, Zr and other high-field-strength elements in the veinforming 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.
EES Authors: 
Reto Gieré

Department of Earth and Environmental Science / University of Pennsylvania, 251 Hayden Hall, 240 South 33rd Street, Philadelphia, PA 19104-6316