Palaeoproterozoic evolution of the Challenger Au deposit, South Australia, from monazite geochronology

Monazite in granulite facies metatexite migmatites (Christie Gneiss) hosting the Challenger Au deposit, South Australia, records a series of growth and resorption stages over a c. 60 Myr period between 2470 and 2410 Ma. A combination of electron microprobe X-ray mapping and in situ ion-microprobe dating was used to delineate and date five compositional domains. The oldest prograde metamorphic components are preserved in granoblastic gneisses surrounding the deposit, and as small high-Y cores in large monazite grains in Au-bearing migmatites. In metatexite leucosomes, these cores were partially resorbed prior to the growth of large high-Th monazite domains that crystallized during partial melting and stromatic migmatite development at c. 2443 Ma. Subsequent heating to biotite dehydration conditions (c. 850 °C at 7 kbar) caused further partial melting roughly 10-15 Myr later, giving rise to c. 2428 Ma domains surrounding partly resorbed 2443 Ma grains that were entrained in the higher-temperature melts. This period of partial melting coincided with isoclinal folding culminating in dextral transpression and represents the most likely window for remobilization of Au-bearing polymetallic sulphide melts into low-strain domains. Localized reaction of residual melt with the granulite facies assemblage during cooling gave rise to narrow high-Y rims dated at 2414 ± 7 Ma. Although monazite from unmineralized granoblastic gneisses and migmatitic ore zones display the same range of U-Pb dates, monazite in migmatites displays a higher overall Ca + Th + U content, indicating that compositional heterogeneities between ore zones and host rocks developed prior to 2470 Ma, perhaps a consequence of the hydrothermal alteration inferred to have accompanied gold mineralization.