Morphology and composition of zircons in rare metal granites from Brazilian tin provinces

Zircons of rare metal specialized granites from five large Brazilian tin provinces were compared morphologically and chemically through backscattered electron images and X-ray dispersive spectroscopy (SEM-EDS) in terms of Hf, Zr/Hf and Hf + Y + Th + U contents. Zircon crystals from the mineralized, evolved hydrothermalized rocks are fine-grained, anhedral, corroded and show little or no obvious zoning. They exhibit high Hf and Hf + Y + Th + U contents and low Zr/Hf values (generally <15) compared to those from less evolved unmineralized granites. Electron microprobe and LA-ICP-MS analyses of zircons from tin-mineralized and barren granites identified similar geochemical trends to those obtained by semiquantitative SEM-EDS. The Geochemical signature of zircon can be used as a prospective guide for specialized granites and zircon SEM-EDS analyses are a reliable tool in preliminary prospecting for this type of mineralization.