Rare earth element mobility during granite alteration: Evidence from southwest England

Geochemical analyses of granitic rocks from southwest England reveal that the rare earth elements (REE) were potentially mobile during hydrothermal and supergene alteration. In particular, trivalent REE were removed from the system during K-silicate alteration, Eu was lost during sericitic alteration; all REE were lost during tourmalinization, and light REE were lost during chloritization and argillic alteration. The fluids themselves had low concentrations of REE; in only one case (chloritization) were heavy REE introduced during alteration. Analysis of separated minerals indicated that the behaviour of the REE could be partly explained in terms of their different affinities for the primary and secondary assemblages. Thermodynamic calculations indicated that REE mobility is enhanced by the presence of fluorine in the alteration fluids partly because REE form more stable complexes with F than with Cl and partly because elements such as Ti, Zr and P that form REE-bearing minor phases are themselves potentially mobile.