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

Abstract

Abstract. 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 stepwise 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(M g2 + F e2 + z n 2 + m n 2+ ca2+),,T•4+ + 0.12(OH ) where Al' and Fe' are held constant. The average composition of the Bergell hoegbomites is given by the formula Fe u 7 Mg0.69 Zn 0.04 Ti 0.17 A1 3.94 F4.+060 7.98( 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 (Ti', Zr4+ , U4+, Th', Nb', Y 3+ ) 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 P01 -as likely ligands for complexing of the above mentioned elements.

Hydrothermal mobility of Ti, Zr and REE: examples from the Bergell and Adamello contact aureoles (Italy)

“…Geological setting and detailed mineralogy of the skarn were given by Giere (1986). The mineralogy is briefly summarized here.…”

“…This three-stage crystallization seems to reflect the three-stage alteration of spinel. However, these two processes can only be linked to one another through one textural relationship observed in thin section; the crystallization of zirconolite zone El preceeds the third alteration stage, during which both the metasomatic phlogopite and the spinel have been chloritized (see Giere 1986). This means that at least one zirconolite zone formed contemporaneously with phlogopite.…”

“…Although few data are available, it has been demonstrated both empirically (in field studies, references cited) and experimentally (Kosterin, 1959;Balashov and Krigman, 1975;Flynn and Burnham, 1978;Bilal and Becker, 1979;Bilal and Koss, 1980a,b;Nguyen, 1985), that the anions F -, Cl -, 0H-, CO32-, SO42-and PO43-can be effective ligands for complexing REE and U. Mobility of Ti and Zr seems to be favoured by relatively high activities of CO2 (Hynes, 1980), B - (Alderton et al, 1980;Konev, 1978) or F -and PO43 (Kraynov et al, 1969;Giere, 1986) in metasomatic fluids; they also migrate in surface waters at ambient temperatures and pressures, mainly because of complex formation with humic and fulvic acids (e.g. Samchuk and Sushchik, 1986).…”

“…Rudashevskiy, 1969;Rekharskiy and Rekharskaya, 1969;Sinyakov and Nepeina, 1969;Pavelescu and Pavelescu, 1972;Zhuravleva et al, 1976;Staatz et al, 1977;Graf, 1977;Hellman and Henderson, 1977;Van Wambeke, 1977;Konev, 1978;Martin et al, 1978;Hellman et al, 1979;Ludden and Thompson, 1979;Alderton et al, 1980;Dostal et al, 1980;Hynes, 1980;Taylor et al, 1981;Fryer, 1980, 1983;Baker and De Groot, 1983;Nystrom, 1984;Robins, 1984;Giere, 1986;Cathelineau, 1987;Vocke et al, 1987). The occurrence of U-REE daughter crystals in fluid inclusions infers that the REE are present in high concentrations in certain hydrothermal fluids (Kwak and Abeysinghe, 1987).…”

ALLANITE-(Ce) FROM THE EOCENE CASTO GRANITE, IDAHO: RESPONSE TO HYDROTHERMAL ALTERATION

“…Furthermore, the trivalent REE and Th 4+ are reasonable geochemical analogues of the trivalent actinides (e.g., Am 3+ , Cm 3+ , Bk 3+ , Pu 3+ ) and the tetravalent actinides (e.g., Pu 4+ ), respectively (cf. Choppin 1983, 1986, 1989, Brookins 1984, Millero 1992, which are not present in significant quantities in natural geological environments.…”