U-PB GEOCHRONOLOGICAL CONSTRAINTS ON THE GENESIS OF THE OLYMPIC DAM CU-U-AU-AG DEPOSIT, SOUTH AUSTRALIA

The Olympic Dam Cu-U-Au-Ag deposit is hosted by the Olympic Dam Brecci a Complex, a high-level, hematite-rich hydrothermal breccia system whi ch occurs entirely within, and clearly postdates, the Roxby Downs Gran ite. The U-Pb zircon age of the Roxby Downs Granite is well constraine d at 1588 +/- 4 Ma. Field evidence indicates that brecciation, hydroth ermal alteration, and Cu-U-Au-Ag-REE mineralization in the Olympic Dam Breccia Complex a ere penecontemporaneous, but this is inconsistent w ith differ;ng ages from previous U-Pb and Rb-Sr isotopic data on pitch blende and sericite. New SHRIMP U-Pb zircon data for three Olympic Dam Breccia Complex igneous rock units confirm that Cu-U-Au-Ag-REE minera lization was contemporaneous with brecciation and alteration and direc tly followed cooling of the Roxby Downs Granite by as little as severa l (<8) million years. Two of the dated Olympic Dam Breccia Complex roc k units are interpreted as autobrecciated felsic dikes. One crosscuts laminated hematite-quartz sandstones, and the other crosscuts Cu-U-min eralized hematite-rich breccias. Zircons from these dikes yielded ages of 1592 +/- 8 and 1584 +/- 20 Ma, respectively. The third unit dated is a disrupted ash-fall tuff horizon from within a diatreme which cros scuts hematite-quartz breccias near the center of the deposit. The nor thern extremities of this diatreme also crosscut hematitic breccias co ntaining a substantial Cu-U ore zone. Zircons from this tuff horizon h ave an age of 1597 +/- 8 Ma. The tuff and associated subaerial or suba queous volcaniclastics were unconsolidated or only weakly lithified at the time of their incorporation into the diatreme, so tile age of the tuff also reflects the age of diatreme activity. These new isotopic d ata indicate that the Olympic Dam Breccia Complex has an age of approx imately 1590 Ma and therefore imply that brecciation closely followed emplacement and cooling of the Roxby Downs Granite. Because these brec cias contain sulfide fragments and polyminerallic ore fragments, exhib it penecontemporaneous sulfide-oxide textural relationships, and show a close association between Cu +/- U +/- Ag +/- REE mineralization and Fe-rich breccias, we argue that the metals of economic interest were introduced during breccia formation at similar to 1590 Ma. Crosscuttin g relationships between ore zones and two of the dated rock units supp ort this conclusion. Given this age, the 1588 +/- 4 Ma Roxby Downs Gra nite and the similar to 1590 Ma Olympic Dam Breccia Complex can both b e correlated with the Gawler Range-Hiltaba volcano-plutonic event, a m ajor thermal event affecting much of the Gawler craton. The relatively short time interval between crystallization of the Roxby Downs Granit e and its subsequent brecciation, alteration, and mineralization in a near-surface environment implies that the granite was emplaced at a hi gh crustal level. This is consistent with established field evidence f or other granitoids of the Hiltaba Suite.