Britholite-hellandite intergrowths and associated REE-minerals from the alkali-syenitic ejecta of the Vico volcanic complex (Latium, Italy): petrological implications bearing on REE mobility in volcanic systems

This paper describes the occurrence of Th-rich hellandite-(Ce) and britholi te-(Ce) within a volcanic ejectum from a pyroclastic formation of the Vice volcanic complex, north of Rome, Italy. The host rock is a saturated alkali -syenite which consists mainly of potassium feldspar and plagioclase, toget her with fluor-biotite, plagioclase, titanite, clinopyroxene and magnetite. Hellandite-(Ce) and britholite-(Ce) are associated with this mineral assem blage which contains also several accessory phases including zircon, fluora patite, allanite, and silico-phosphates of Th, U, REE close in composition to cheralite-brabantite. Hellandite-(Ce) and britholite-(Ce) are typically intergrown with hellandite in all case observed rimming britholite. These u nusual REE, Th, U minerals are secondary to the formation of the host rock and the compositions of the associated minerals suggest that they originate d from fluids rich in F, P and possibly S and C. The presence of fractures filled by clays containing dispersed REE-minerals, probably fluorocarbonate s, suggests a very late-stage hydrothermal remobilization, transportation a nd redeposition of the REE. An important implication of such widespread sec ondary mineralization is that great care should be exerted when using bulk- rock trace-element data from these volcanic samples for geochemical modelli ng.