Geology of the Acropolis prospect, South Australia, constrained by high-precision CA-TIMS ages

Abstract

Acropolis is an Fe-oxide–copper–gold prospect ∼20 km from Olympic Dam, South Australia, and marked by near-coincident gravity and magnetic anomalies. Prospective Fe-oxide–apatite ± sulfide veins occur in Mesoproterozoic and Paleoproterozoic volcanic and granitoid host units beneath unmineralised sedimentary formations. We have produced a geological map and history of the prospect using data from 16 diamond drill holes, including LA-ICPMS and high-precision CA-TIMS ages. The oldest unit is megacrystic granite of the Donington Suite (ca 1850 Ma). A non-conformity spanning ca 250 My separates the Donington Suite and felsic lavas and ignimbrites of the Gawler Range Volcanics (GRV; 1594.03 ± 0.68 Ma). The GRV were intruded by granite of the Hiltaba Suite (1594.88 ± 0.50 Ma) and felsic dykes (1593.88 ± 0.56 Ma; same age as the Roxby Downs Granite at Olympic Dam). The felsic dykes are weakly altered and lack Fe-oxide–apatite–sulfide veins, suggesting that they post-date the main hydrothermal event. If correct, this relationship implies that the main hydrothermal event at Acropolis was ca 1594 Ma and pre-dated the main hydrothermal event at Olympic Dam. The GRV at Acropolis are the same age as the GRV at Olympic Dam and ca 3–7 My older than the GRV exposed in the Gawler Ranges. The gravity and magnetic anomalies coincide with sections through the GRV, Hiltaba Suite and Donington Suite that contain abundant, wide, Fe-oxide veins. The GRV, Hiltaba Suite and Donington Suite are unconformably overlain by the Mesoproterozoic Pandurra Formation or Neoproterozoic Stuart Shelf sedimentary formations. The Pandurra Formation shows marked lateral variations in thickness related to paleotopography on the underlying units and post-Pandurra Formation pre-Neoproterozoic faults. The Stuart Shelf sedimentary formations have uniform thicknesses.

KEY POINTS

1. Fe-oxide–apatite ± sulfide veins are hosted by the Gawler Range Volcanics (1594.03 ± 0.68 Ma), the Hiltaba Suite granite (1594.88 ± 0.50 Ma) and Donington Suite granite (ca 1850 Ma).

2. The age of felsic dykes (1593.88 ± 0.56 Ma) interpreted to be post-mineralisation implies that the main hydrothermal event at Acropolis was ca 1594 Ma.

3. The Gawler Range Volcanics at Acropolis are the same age as the Gawler Range Volcanics at Olympic Dam and ca 3 to 7 My older than the Gawler Range Volcanics exposed in the Gawler Ranges.

Keywords:

• Acropolis prospect
• Olympic Cu–Au–U province
• CA-TIMS geochronology
• geophysical anomaly
• Gawler Range Volcanics
• Hiltaba Suite
• Mesoproterozoic
• Olympic Dam

Acknowledgements

The drill core logs presented here depend partly on logs completed by WMC geologists R. K. Uppill, R. Evans and H. Paterson (1979–1984). Assistance from BHP Olympic Dam drill core processing staff is gratefully acknowledged. We thank Ken Cross and Elizabeth Jagodzinski for reviews of the manuscript.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This project was funded by an Australian Research Council Linkage Grant to VK and JM and by BHP Billiton Olympic Dam.