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Acropolis

South Australia, SA, Australia

Main commodities: Fe Cu Au U
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The Acropolis magnetite deposit, which is located within the Olympic IOCG Province on the eastern rim of the preserved Gawler craton in northern South Australia, is ~25 km SW of Olympic Dam, ~17 km WNW of Wirrda Well, and 530 km NNW of Adelaide (#Location: 30° 37' 20"S, 136° 44' 51"E).

Acropolis, Wirrda Well, Olympic Dam, Carrapateena, Prominent Hill and Hillside, and all of the significant known IOCG mineralised systems of the Gawler craton, are hosted within Palaeo- to Mesoproterozoic rocks, and are distributed along the eastern edge of the currently preserved craton to define the Olympic IOCG Province.

See the Carrapateena record for a summary of the geological setting of the Olympic IOCG Province.

The Acropolis deposit is reflected by a complex, ~15 km long, overall NW-SE oriented, multi-source magnetic anomaly of up to ~6000 nT (Paterson, 1986) and a near coincident gravity response of ~22 mGal (Vella and Cawood, 2006) above regional background. The ~10 km long, gravity anomaly has an east-west trend, with a SE elongated ridge on its eastern margin. The main peak of the gravity anomaly is offset ~2 km to the SW of the peak magnetic response, which also has a SE trending ridge on its southeastern margin. These anomalies had been tested (to 1986) by more than 11 drill holes unevenly distributed over 50 km2 below a flat lying pile comprising the Mesoproterozoic (post 1424 Ma) Pandurra Formation, Neoproterozoic and younger sedimentary cover, that varies from 450 to >600 m in thickness (Paterson, 1986).

Acropolis was one of the high priority targets selected by Western Mining Corporation (WMC) in 1975, as part of the program that resulted in the discovery of Olympic Dam.

The mineralisation at Acropolis is hosted within pervasively altered felsic (dacitic) units of the Gawler Range Volcanics (GRV), comprising ash flow tuffs and felsic to intermediate volcanic rocks, bounded to the east by granitoid rocks. The GRV rocks have a porphyritic texture, although sericitisation has obliterated the mineralogy of the original phenocrysts. Strong chloritic alteration is locally developed, while in the vicinity of the major iron oxide masses, the volcanics are invaded by pervasive hematite alteration (Paterson, 1986).

The GRV volcanic rocks overlie laminated siltstones, most likely of the ~1770 and 1740 Ma Wallaroo Group (Cross 1993).

The granitoid rocks vary, and include diorite, potassic (biotite) granite to granodiorite to tonalite, gneissic granite and a late alkali quartz-syenite. The latter is sericitised and is interpreted to be a late stage intrusion into altered GRV, probably belonging to the Hiltaba Suite, while the other granitoids, which are overlain by the GRV, are generally medium grained to porphyritic, foliated (syn-Kimban orogeny) and are equated with the ~1850 Ma granitoids of the Donington Suite (Paterson, 1986; Cross 1993; Skirrow et al., 2007).

Mineralisation at Acropolis occurs as large bodies of massive iron oxides, primarily magnetite with hematite formed by subsequent oxidation. The two main bodies are 7 km apart. The magnetite is usually accompanied by apatite, chlorite and minor quartz, with intersections of up to 200 m of close to 60% Fe. The massive oxide sheets are surrounded by a zone of smaller veins of the same mineralogy within a halo of pervasive hematite alteration. A thick interval of coarse granite breccia with a hematite-magnetite matrix, pyrite and minor chalcopyrite was intersected in one drill hole towards the eastern margin of the deposit, with adjacent unbrecciated granite intruded by magnetite veins to the southeast (Paterson, 1986).

Sulphide mineralisation is associated with the iron oxides, occurring as pyrite with local develoments of chalcopyrite, and associated barite, fluorite and phlogopite. Sulphides are more common in the zones of strong iron oxide vein networks within the altered GRVs surrounding the massive magnetite±hematite masses, although the massive, fine grained hematite-magnetite-apatite rock may contain veins, clots and disseminations of suphides. The sparse drilling has suggested a possible zonation from pyrite to chalcopyrite and locally to bornite. Uranium, and to a lesser extent rare earth mineralisation is mostly found within the main iron rich lenses. The bulk of the known Cu (-U-Au) mineralisation is in the northwestern zone, where magnetite, hematite, K feldspar, quartz and apatite occur in vein networks and as massive replacements in sericitised GRV dacite.

The mineralistion at Acropolis is unlike that at Olympic Dam, although there are similarities in the styles of alteration, with the former having been deposited at higher maximum temperatures and under more reduced conditions (Paterson, 1986; Cross 1993; Skirrow et al., 2007). Oreskes and Einaudi (1992) suggest that the predominance of magnetite over hematite, and the presence of hydrothermal K feldspar in addition to sericite, suggests that the Acropolis veins could represent a deeper level analogue to the Olympic Dam deposit and/or may have preserved evidence of a higher temperature ore-forming event. The same authors measured oxygen isotope equilibrium values for two samples of magnetite-K feldspar±quartz±apatite veins from Acropolis suggesting temperatures of formation of 550 and 440°C compared to a range of 500 to 200°C from a larger population of magnetite and hematite samples from Olympic Dam. They concluded however, that the magnetite in both formed under similar conditions, with hematite resulting from interaction with a separate lower temperature fluid.

Low grade intersections of up to 66 m @ 0.7% Cu and 35 ppm U
3O8 (including lesser intervals of >400 ppm U3O8) have been encountered at Acropolis (Paterson, 1986).

Direen and Lyons (2007) quote a geological resource of ~500 Mt @ ~60% Fe at Acropolis.

This summary is sourced from Paterson (1986), Cross (1993) and Direen and Lyons (2007).

The most recent source geological information used to prepare this summary was dated: 2007.    
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


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