THE COMPOSITION OF GAHNITE IN GRANITIC PEGMATITES

Gahnite (ZnAl₂O₄), the zinc end-member of the spinel group of minerals, is a common accessory phase in a large variety of rock types, including Fe-Al-rich metasedimentary and metavolcanic rocks spatially associated with metamorphosed massive sulfide deposits, aluminous metasedimentary rocks, metabauxites, marble, skarns, granite, and granitic pegmatites. The major element composition of zincian spinel in metamorphosed massive sulfide deposits has been investigated extensively and is actively used as an indicator of mineralization. However, available data for this mineral in granitic pegmatites, including rare-metal (Li, Be, Ta) pegmatites, worldwide is very limited. Previous studies suggest that the composition of gahnite in granitic pegmatites can provide information about the degree of pegmatite evolution and/or fractionation of the original melt. In this study we have investigated the major element composition of gahnite in rare element class (LCT) pegmatites from the Borborema Pegmatite Province (BPP; Capoeira 2, Alto Mirador, Boqueirao, and Carrascao pegmatites), northeaster Brazil, Maryland (Ben Murphy pegmatite), Maine (Lord Hill pegmatite), and North Carolina (Burleson, mica-type). The compositions of gahnite in most pegmatites, expressed in terms of mol % hercynite (Hc), gahnite (Gah), and spinel (Spl) end members, fall in the field of granitic pegmatites and have a range of Hc_0.6-9.2Gah_89.8-98 Spl_0-4.7. In contrast, gahnite from Maine has the highest Fe and lowest Zn contents yet reported, and its composition is described as Hc_13-27Gah_72-87Spl_0-0.96. This indicates that gahnite from Maine has the lowest degree of pegmatite fractionation among the studied pegmatites. A plot of mol (Fe+Mg)/Al vs. (Zn+Mn)/Al also evidences the lowest degree of fractionation of this pegmatite, whereas the pegmatites from the BPP show the highest degree of fractionation. Gahnite from North Carolina and Maine are similar in composition to that from the Carrascao and Alto Mirador pegmatites, respectively. Additional studies, including trace elements, are needed to understand the anomalously high Hc component in the pegmatite from Maine and the relationships among the chemical composition of gahnite, the type of pegmatite, and the presence of rare metal mineralization.