Abstract
The petrography and geochemistry of zircon offers an exciting opportunity to better understand the genesis of, as well as identify pathfinders for, large magmatic–hydrothermal ore systems. Electron probe microanalysis, laser ablation inductively coupled plasma mass spectrometry, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) imaging, and energy-dispersive X-ray spectrometry STEM mapping/spot analysis were combined to characterize Proterozoic granitic zircon in the eastern Gawler Craton, South Australia. Granites from the ~1.85 Ga Donington Suite and ~1.6 Ga Hiltaba Suite were selected from locations that are either mineralized or not, with the same style of iron-oxide copper gold (IOCG) mineralization. Although Donington Suite granites are host to mineralization in several prospects, only Hiltaba Suite granites are considered “fertile” in that their emplacement at ~1.6 Ga is associated with generation of one of the best metal-endowed IOCG provinces on Earth. Crystal oscillatory zoning with respect to non-formula elements, notably Fe and Cl, are textural and chemical features preserved in zircon, with no evidence for U or Pb accumulation relating to amorphization effects. Bands with Fe and Ca show mottling with respect to chloro–hydroxy–zircon nanoprecipitates. Lattice defects occur along fractures crosscutting such nanoprecipitates indicating fluid infiltration post-mottling. Lattice stretching and screw dislocations leading to expansion of the zircon structure are the only nanoscale structures attributable to self-induced irradiation damage. These features increase in abundance in zircons from granites hosting IOCG mineralization, including from the world-class Olympic Dam Cu–U–Au–Ag deposit. The nano- to micron-scale features documented reflect interaction between magmatic zircon and corrosive Fe–Cl-bearing fluids in an initial metasomatic event that follows magmatic crystallization and immediately precedes deposition of IOCG mineralization. Quantification of α-decay damage that could relate zircon alteration to the first percolation point in zircon gives ~100 Ma, a time interval that cannot be reconciled with the 2–4 Ma period between magmatic crystallization and onset of hydrothermal fluid flow. Crystal oscillatory zoning and nanoprecipitate mottling in zircon intensify with proximity to mineralization and represent a potential pathfinder to locate fertile granites associated with Cu–Au mineralization.
Highlights
Ore deposits falling within the broad iron-oxide-Cu–Au (IOCG) clan are a fascinating, and in some cases, poorly understood, family of magmatic–hydrothermal ore systems (References [1,2] and references therein)
Screening of zircon populations in each sample shows a range of grain sizes from 30 to ~300 μm, displaying both primary and secondary overprinting textures (Figure 2A–D)
Oscillatory zoning patterns can be produced via metasomatism in the absence of open fractures
Summary
Ore deposits falling within the broad iron-oxide-Cu–Au (IOCG) clan are a fascinating, and in some cases, poorly understood, family of magmatic–hydrothermal ore systems (References [1,2] and references therein) Debate on their genesis includes the relative timing and evolution of fluids during transition from the magmatic to hydrothermal stage. Contemporary consensus views the incorporation of other, “non-formula” components in zircon, including Ca, Fe, Al, and H2 O, as being unrelated to primary magmatic processes (Reference [9], and references therein). These are most often measured within altered zircon (e.g., [10]) and are attributed to metamictization-related effects.
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
