A COMPARISON BETWEEN THE CHEMICAL COMPOSITION OF GAHNITE IN GRANITIC PEGMATITES FROM THE PAMPEAN PEGMATITE PROVINCE AND A METAMORPHOSED MASSIVE SULFIDE DEPOSIT: A POSSIBLE NEW EXPLORATION GUIDE

Granitic pegmatites are commonly hosts to economic deposits of rare metals, including Li, Ta, and rare earth elements, among others. Rare metals are becoming increasingly economically important with insufficient global supplies. The major element composition of some minerals, in particular gahnite (ZnAl₂O₄), has been used as an exploration guide for metamorphosed massive sulfide deposits (MMSDs). Gahnite is also an accessory mineral in pegmatites, including those with rare metal mineralization, but detailed studies on the major element chemical composition of gahnite in pegmatites are very scarce and there is only one trace element study of gahnite in pegmatites. In this study we investigated the major and trace element chemical composition of gahnite in the LCT (lithium, cesium, tantalum) rare element class, beryl-columbite-phosphate subtype pegmatites from the Comechingones (Blanca Dora, Juan Román, Magdalena, La Ona, and Sin Nombre pegmatites) and Conlara (Nancy pegmatite) pegmatite districts of the Pampean Pegmatite Province, Argentina, and compared them with gahnite from the small mid Proterozoic Nine Mile Pb-Zn MMSD, Curnamona Province, Australia. The compositions of gahnite from the pegmatites, expressed in terms of mol % hercynite (Hc), gahnite (Ghn), and spinel (Spl) end members, fall within the pegmatite field and are defined by Ghn_78.49-90.35Hc_9.07-20.52Spl_0.25-3.37. Gahnite in the La Ona pegmatite has the highest Zn content, which is typical of more fractionated and evolved magmas, and this suggests the highest degree of evolution for this pegmatite among the Argentina pegmatites. Gahnite compositions from the Nine Mile deposit (Ghn_56.17-77.22Hc_16.20-36.72Spl_3.83-10.83) fall within the MMSD field characterized by higher Mg and Fe and lower Zn contents compared to gahnite from pegmatites. Laser ablation ICP-MS analysis show that the trace elements present in gahnite hosted in the pegmatites and the Nine Mile deposit include Li, Cr, Co, Cu, Ga, and Cd. Gahnite in the Nine Mile deposit also contains V, Cr, Ni, Ti, and Pb, which are not present in gahnite in the pegmatites. These compositional differences between gahnite in pegmatites and the Nine Mile MSD suggest that major and trace element contents in gahnite could be used to discriminate between the two rock types and used as exploration guides.