Abstract

This paper describes the occurrence of Th-rich hellandite-(Ce) and britholite-(Ce) within a volcanic ejectum from a pyroclastic formation of the Vico volcanic complex, north of Rome, Italy. The host rock is a saturated alkali-syenite which consists mainly of potassium feldspar and plagioclase, together with fluor-biotite, plagioclase, titanite, clinopyroxene and magnetite. Hellandite-(Ce) and britholite-(Ce) are associated with this mineral assemblage which contains also several accessory phases including zircon, fluorapatite, 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 unusual REE, Th, U minerals are secondary to the formation of the host rock and the compositions of the associated minerals suggest that they originated from fluids rich in F, P, and possibly S and C. The presence of fractures filled by clays containing dispersed REE-minerals, probably fluorocarbonates, suggests a very late-stage hydrothermal remobilization, transportation and redeposition of the REE. An important implication of such widespread secondary mineralization is that great care should be exerted when using bulk-rock trace-element data from these volcanic samples for geochemical modelling.

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