Precise geochronological constraints on the origin, setting and incorporation of ca. 1.59 Ga surficial facies into the Olympic Dam Breccia Complex, South Australia

The Olympic Dam Breccia Complex (host to the Olympic Dam Cu-U-Au-Ag deposit) is derived largely from granite (Roxby Downs Granite) but includes clasts and domains derived from surficial facies (felsic volcanic rocks and bedded clastic facies). New highprecision CA-TIMS geochronology is used to constrain the ages of the granite, the volcanic rocks and bedded clastic facies. The felsic volcanic rocks (1594.73 Ma ± 0.30 Ma) are slightly older than the granite (1593.87 Ma ± 0.21 Ma). These ages, together with the absence of any older country rock in the breccia complex, suggest the felsic volcanic rocks that were originally present at Olympic Dam were intruded by the granite. The deposition of the bedded clastic facies (tuffaceous mudstone) has been constrained to 1590.97 ± 0.58 Ma, indicating the presence of a basin around 3 myr after emplacement of the granite and felsic volcanic rocks. Extrusive correlatives of maficultramafic dykes intruding the breccia complex are proposed as a local source of detrital Cr-spinel in the bedded clastic facies. The granitoid-dominated provenance of some of the bedded clastic facies implies that ca. 1590 Ma granitoids were exposed when the bedded clastic facies were accumulating; the absence of ≥1740 Ma detrital zircons indicates the provenance did not extend beyond granitoids of the Burgoyne Batholith in the vicinity of Olympic Dam. An initial stratigraphic sequence is proposed for the BCF involving a transition in provenance from volcanic-dominated to granitoid-dominated. Faults are proposed to have been responsible for the segmentation and incorporation of the bedded clastic facies into the Olympic Dam Breccia Complex. The formation of the Olympic Dam Breccia Complex and Olympic Dam hydrothermal system post-dated the emplacement of the Roxby Downs Granite (at 1593.87 ± 0.21 Ma). The bedded clastic facies (1590.97 ± 0.58 Ma) have been altered and mineralised.