U-Pb geochronological constraints on the genesis of the Olympic Dam Cu-U-Au-Ag deposit, South Australia

Abstract

The Olympic Dam Cu-U-Au-Ag deposit is hosted by the Olympic Dam Breccia Complex, a high-level, hematite-rich hydrothermal breccia system which occurs entirely within, and clearly postdates, the Roxby Downs Granite. The U-Pb zircon age of the Roxby Downs Granite is well constrained at 1588 + or - 4 Ma. Field evidence indicates that brecciation, hydrothermal alteration, and Cu-U-Au-Ag-REE mineralization in the Olympic Dam Breccia Complex were penecontemporaneous, but this is inconsistent with differing ages from previous U-Pb and Rb-Sr isotopic data on pitchblende and sericite. New SHRIMP U-Pb zircon data for three Olympic Dam Breccia Complex igneous rock units confirm that Cu-U-Au-Ag-REE mineralization was contemporaneous with brecciation and alteration and directly followed cooling of the Roxby Downs Granite by as little as several (<8) million years. Two of the dated Olympic Dam Breccia Complex rock units are interpreted as autobrecciated felsic dikes. One crosscuts laminated hematite-quartz sandstones, and the other crosscuts Cu-U-mineralized hematite- rich breccias. Zircons from these dikes yielded ages of 1592 + or - 8 and 1584 + or - 20 Ma, respectively. The third unit dated is a disrupted ash-fall tuff horizon from within a diatreme which crosscuts hematite-quartz breccias near the center of the deposit. The northern extremities of this diatreme also crosscut hematitic breccias containing a substantial Cu-U ore zone. Zircons from this tuff horizon have an age of 1597 + or - 8 Ma. The tuff and associated subaerial or subaqueous volcaniclastics were unconsolidated or only weakly lithified at the time of their incorporation into the diatreme, so the age of the tuff also reflects the age of diatreme activity. These new isotopic data indicate that the Olympic Dam Breccia Complex has an age of approximately 1590 Ma and therefore imply that brecciation closely followed emplacement and cooling of the Roxby Downs Granite. Because these breccias contain sulfide fragments and polyminerallic ore fragments, exhibit penecontemporaneous sulfide-oxide textural relationships, and show a close association between Cu + or - U + or - Ag + or - REE mineralization and Fe-rich breccias, we argue that the metals of economic interest were introduced...