Release of uranium from highly radiogenic zircon through metamictization: The source of orogenic uranium ores

Abstract

Although circumstantial evidence from ore deposit mineralogy and geochemistry can imply potential sources for fluids and metals, rarely is direct evidence for metal leaching from source rocks seen in the vicinity of deposits. Here, we investigate the source of metals for a series of fault zone– and shear zone–hosted uranium occurrences in the Mount Isa inlier, Australia. As well as containing uranium, these deposits are enriched in Zr and rare earth elements (REEs), requiring that unusual fluids were responsible for addition of these typically immobile elements. During the Isan orogeny, highly saline metamorphic fluids infiltrated the sheared margin of a highly evolved granite intrusion, which contains elevated U, Th, F, Zr, and REEs. Synchrotron X-ray fluorescence spectroscopy and high-resolution electron microprobe mineral mapping show that this radioactive element–rich characteristic caused zircon crystals to become highly metamict, allowing the elements therein to become mobile. Thus, when orogenic fluids pervasively infiltrated along shear zones ?100 m.y. after granite intrusion, their unusually saline character allowed enhanced dissolution of regional carbonates and fluorite from the granite, providing the ligands needed for transport of uranium and the normally immobile elements from the metamict zircons.