The Portia - North Portia Cu-Au(-Mo) prospect, South Australia: timing of mineralization, albitisation and origin of ore fluid

Cu-Au(-Mo) mineralisation at the Portia-North Portia prospect is located under cover on the eastern flank of the Benagerie Ridge Magnetic Complex, within the Curnamona Province and approximately 125 km WNW of Broken Hill. The mineralisation is located within rocks which have a SHRIMP U-Pb zircon age of 1703 + 6 Ma, which is similar to Willyama Supergroup ages obtained from the Broken Hill and Olary Domains. The meta-sedimentary unit that hosts the mineralisation is approximately 200m thick and is overlain by carbonaceous phyllite, and underlain by a unit which is dominated by albite-magnetite-hematite. The host units contain numerous carbonate-rich domains intercalated with meta-evaporitic sediments. The sequence had undergone low-grade metamorphism and fabric development and was subsequently intensely albitised. Hydrothermal monazites formed during this albitisation event give SHRIMP II in situ U-Pb ages of ~1630 Ma. The albitised meta-sediments proved to be an excellent host to the later Cu-Au(-Mo) mineralisation. Abundant monazite associated with the mineralisation yield SHRIMP II ages of ~1605 Ma. It is possible that the numerous, highly fractionated and altered diorite bodies known to be present on the Benagerie Ridge, may have produced some of the metals. The "Hiltaba age" (-1585-1590 Ma) granites of the area cannot be considered as a source of the metals.