ZIRCONIUM AND HAFNIUM IN MINERALS OF THE COLUMBITE AND WODGINITE GROUPS FROM GRANITIC PEGMATITES

Niobium-tantalum-oxide minerals from five localities with moderately to highly fractionated granitic pegmatites were examined for their Zr and Hf contents. Wodginite was found to contain up to 9.59 ZrO2 and 1.15 wt.% HfO2 and columbite-group minerals, up to 1.26 and 0. 12 wt.%, respectively. The Zr/Hf value is decidedly high in columbite-group minerals (13 to 6) relative to wodginite (4 to 1). Coexisting wodginite-group and columbite-group minerals behave similarly with respect to the overall behavior of Zr + Hf and with respect to the partitioning of Zr versus Hf. In individual pegmatite bodies, no systematic changes in Zr + Hf or Zr/Hf are discernible among the various zones. However, a positive correlation of Zr + Hf with Sri, Mn and Ta is locally observed, and a negative correlation with Ti. No clear trends are shown by Zr/Hf. The Zr/Hf value in the (Nb,Ta)-oxide minerals generally corresponds to its range in associated zircon-hafnon. Considerable depolymerization of highly fractionated, (F, B, P)- and H2O-rich pegmatite-forming melts, in part alkaline in boundary layers (particularly significant in final stages of consolidation), is responsible for shifting oxygen coordination of Zr and Hf from [8]- to [6]-fold. Consequently, the abundance of zircon decreases, and the (Nb,Ta)-oxide minerals become geochemically significant carriers of Zr and Hf. This effect apparently operates not only in granitic pegmatites but also in other granite-related environments.