The early Mesoproterozoic Olympic Cu-Au province extends over 500 km along the eastern margin of the Gawler Craton. Although the boundaries are not yet well defined, and much of the province is concealed beneath Neoproterozic to Cainozoic cover, the metallogenic belt is inferred to transgress several tectonic domains of mainly Palaeoproterozoic meta-sedimentary and meta-igneous basement. Iron oxide - rich hydrothermal systems of the Peake and Denison Inlier, Mabel Creek Ridge, and south-central Gawler Craton may prove to be extensions of the Cu-Au province.
We have identified three major regions of early Mesoproterozoic hydrothermal and magmatic activity: in the Mount Woods Inlier in the north, in basement to the Stuart Shelf (which hosts the Olympic Dam Cu-U-Au deposit), and in the Moonta-Wallaroo-Roopena region in the south of the metallogenic province. Each of the regions contains high- to low-temperature Fe-oxide bearing alteration, Cu-Au±U mineralisation, and felsic to mafic magmatism of the ~1590 Ma Hiltaba Suite intrusions, with or without Gawler Range Volcanics. The three regions are inferred to represent the imprint of separate crustal-scale thermal anomalies.
Re-logging of drill core, and petrological studies reveal important similarities and systematic variations in hydrothermal mineral assemblages along the length of the metallogenic belt. The key assemblages are:
a. calcsilicate - alkali feldspar ± magnetite ± pyrite ± pyrrhotite ± chalcopyrite (CAM);
b. magnetite - biotite ± pyrite ± chalcopyrite (MB), and
c. hematite - sericite - chlorite - carbonate ± pyrite ± Cu-Fe sulfides ± U, REE minerals (HSCC)
Higher grade and more extensive Cu-Au±U mineralisation is generally associated with the relatively oxidised and lower temperature HSCC assemblage (250-300°C, or less, based on fluid inclusion data). In most cases, this assemblage overprints the CAM and MB assemblages, which represent the products of high- to moderate-temperature (~500° - 350°C) hydrothermal fluids of intermediate, or in places, reduced oxidation state (i.e. magnetite-pyrite or magnetite-pyrrhotite stability). Based on drill holes examined to date, albite and biotite are the dominant alkali alteration products in the Moonta-Wallaroo district, whereas K-feldspar or sericite are the main alkali silicates in basement to the Stuart Shelf. All three assemblages (CAM, MB and HSCC) including albite and K-feldspar are well represented in the Mount Woods Inlier (e.g., CAM & MB at the Manxman and Joes Dam prospects; HSCC at the Prominent Hill prospect). Sodium- and chlorine-bearing varieties of scapolite, or pseudomorphs of scapolite, have been identified in all three regions of hydrothermal activity. The sources of halides and metals in the hydrothermal fluids are currently under investigation using fluid inclusion microanalytical techniques.
The crustal levels of the hydrothermal systems as they are currently exposed at the base of cover rocks are inferred to vary dramatically, even within the three regions of hydrothermal activity. Brittle-ductile shear-hosted CAM and MB assemblages, such as those in the Moonta-Wallaroo district, developed at deeper crustal levels than breccia-hosted HSCC assemblages (e.g., Olympic Dam). We suggest that uplift or unroofing of some of the hydrothermal systems occurred during and/or after their development, resulting in ‘telescoping’ of deeper and shallower alteration patterns. CAM, MB, HSCC alteration and associated Cu-Au±U mineralisation represent a possible spectrum of settings from deeper, higher-temperature environments to near-surface, low-temperature settings where there was greater involvement of surficial fluids in the hydrothermal systems.
