Zirconolite, allanite and hoegbomite in a marble skarn from the Bergell contact aureole: implications for mobility of Ti, Zr and REE

Zirconolite, allanite and hoegbomite are present as accessory phases in a metasomatically altered spinel-calcite-marble from the contact with the Bergell intrusives (Switzerland/Italy). Textural relationships indicate a step-wise alteration of spinel to 1) hoegbomite or corundum + magnetite, 2) margarite and 3) chlorite. Replacement of spinel by hoegbomite can be described by the substitution 1.94(Mg2+, Fe2+, Zn2+, Mn2+, Ca2+)⇋Ti4+ +0.12(OH−) where Al3+ and Fe3+ are held constant. The average composition of the Bergell hoegbomites is given by the formula Fe 0.97 2+ Mg0.69Zn0.04Ti0.17Al3.94Fe 0.06 3+ O7.98(OH)0.02 and seems to be imposed by the composition of pre-existing spinel. During the first two steps of spinel alteration, calcite was replaced by anorthite+phlogopite, and the rare earth element(REE)-bearing minerals zirconolite, allanite and sphene were formed. Allanites have characteristic chondrite-normalized REE patterns with enrichment in the light REE. The zirconolite patterns show a marked increase in concentration from La to Ce, followed by an almost constant section. Sphene lacks detectable La, and its REE patterns vary from grain to grain. Contemporaneous formation of phlogopite, REE-bearing minerals and hoegbomite during replacement of the spinel-calcite-marble indicates that the metamorphic fluid introduced potassium along with REE and other high valence cations (Ti4+, Zr4+, U4+, Th4A3804265, Nb5A3804265, Y3A3804265) possibly as polynuclear complexes. The abundance of fluorine-bearing phlogopite and fluor-apatite as well as their close association with REE-bearing minerals and hoegbomite suggests F− and PO 4 3− as likely ligands for complexing of the above mentioned elements.