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Wirrda Well
South Australia, SA, Australia
Main commodities: Cu Ag Au

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The Wirrda Well iron oxide copper-gold mineralised system, which is located within the Olympic IOCG Province on the eastern rim of the preserved Gawler craton in northern South Australia, is ~25 km SSE of Olympic Dam, ~17 km ESE of Acropolis and 530 km NNW of Adelaide (#Location: 30° 38' 55"S, 136° 57' 02"E).

Wirrda Well, Acropolis, 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.

Hayward and Skirrow (2010) note that the Olympic Dam, Wirrda Well and Acropolis mineralised systems are found within an ENE-trending fault-bounded ~40 x 100 km block that partially cuts second order NNW-trending faults, and is characterised by a broad magnetic anomalism that is interpreted to represent widespread magnetite alteration. This block is bounded by ENE- to NE-trending faults, 10 to 15 km to the NW of Olympic Dam and a similar distance to the SE of Wirrda Well, and by major regional NNW-trending faults systems, one of which is the Elizabeth Creek Fault Zone just to the SW of Acropolis. Within this block, there is a cluster of ENE- to NE-elongated Hiltaba Suite plutons, including the Roxby Downs Granite (which hosts Olympic Dam) and the Burgoyne Batholith (which is host to the eastern extremities of both Acropolis and Wirrda Well). The lopolithic Burgoyne Batholith has a base at ~5 km depth, above interpreted Hutchison Group metasedimentary rocks (Lyons and Goleby, 2005).

The Gawler Craton and Olympic IOCG Province record and images included therein illustrate these features and provides an overview of the regional geological, metallogenic and geophysical setting and framework of the province.

The primary host to mineralisation at Wirrda Well is a sheared, to locally mylonitised, megacrystic K feldspar-bearing, ~1850 Ma Donington Suite granite and similarly deformed and aged mafic dykes (Ehrig, 2013). The Donington Suite granite occurs as a NE trending corridor, bounded on either side by Hiltaba Suite granites of the Burgoyne Batholith, and by 1591±10 Ma (U-Pb zircon) Gawler Range Volcanics (see the image in the Gawler Craton and Olympic IOCG Province record). The Burgoyne Batholith to the NE, on either side of the Donington Suite corridor, is separated from the Gawler Range Volcanics to the SW by a set of NE-dipping, NW-trending reverse faults. The Gawler Range Volcanics unconformably overlie the Donington Suite intrusions (Hayward and Skirrow, 2010).

The Wirrda Well mineralised system is reflected by a near-circular ~4 km diameter magnetic anomaly with an ~1800 nT peak, and a near-coincident, but more diffuse gravity response of ~6 mGal above regional background (Vella and Cawood, 2006). Within this large concentric gravity and magnetic anomaly, the deposit is composed of a vertically-plunging breccia pipe with multiple apophyses that are separated into the main North and South mineralised zones, both characterised by weak brecciation and intense hydrothermal iron-oxide alteration. These two main zones are centred on steeply NNW-dipping, ENE-trending faults located near intersection zones with second-order NW-trending faults. The principal hematite altered breccias are ~3.5 x 0.8 and 2 x 0.5 km in size respectively. The eastern third of the latter is hosted by the Burgoyne Batholith, with the remainder within Donington Suite granitoids and partially within Gawler Range Volcanics to the south. A large 2 x 2 km mass of magnetite alteration (the probable main source of the magnetic response) occurs in an uplifted block along a northwest-trending fault immediately to the north of the main hematite mineralised zones (Hayward and Skirrow, 2010).

A significant proportion of the brecciation and mineralisation is concentrated along the contact zones of the intrusive rocks. A post-mineralisation fault block of Gawler Range Volcanics rocks also occurs within the North Zone. Pre- syn-and post-mineralisation mafic dykes are found throughout the deposit, including post-mineralisation picrites (Huang et al., 2016). These mineralised and altered Palaeo- and Mesoproterozoic rocks are overlain by a ~330 m thick cover sequence of Mesoproterozoic (post 1424 Ma) Pandurra Formation, and Neoproterozoic and younger sedimentary rocks.

Fe-Cu (U-Au) mineralisation occurs as magnetite/hematite vein networks and alteration zones in sericitised, brecciated and magnetite/hematite altered Gawler Range Volcanics. Within the Hiltaba Suite Burgoyne Batholith and the Donington Suite granites, massive granite breccia is developed with pervasive and vein magnetite/hematite, and various combinations and proportions of siderite, quartz, K feldspar, sericite, chlorite, barite, fluorite, apatite, phlogopite, sulphides (pyrite-chalcopyrite) and uraninite, with bornite more spatially confined to local zones, accompanied by extensive chlorite-sericite alteration.

The main iron-oxide and Cu-(Fe)-sulphides are vertically zoned, with magnetite dominating at depth, grading up into hematite rich alteration. Similarly, sulphides pass upwards from bornite-chalcocite, through chalcopyrite-bornite to pyrite-chalcopyrite. Magnetite-pyrite-apatite-chlorite assemblages are dominant at depth and grade upwards into zones of bornite-chalcocite with increasingly abundant of hematite. Bornite-chalcocite and chalcopyrite-dominant zones are most strongly developed in the North Zone, where they also extend to greater depths than in the South Zone (Ehrig, 2013). In contrast to Olympic Dam, however, they are weakly mineralised and of more limited spatial extent, both laterally and vertically.

Brecciation and hematite development are more widespread than at Acropolis, more closely resembling that at Olympic Dam. However, unlike at Olympic Dam, many of the breccias contain clasts derived from the host deformed (Donington Suite) granite and Hutchison Group metasediments.

The deposit contains conspicuous hydrothermal apatite which characterise the host assemblage and define a geochemical signature. Chondrite-normalised rare earth element (REE) fractionation patterns for this apatite reflect the evolution of hydrothermal fluids during ore formation. These patterns comprise an early generation of abundant apatite associated with the magnetite-dominant mineralisation that have a characteristic light-REE enriched fractionation trend. Subsequent hydrothermal overprinting of this early high-temperature assemblage results in the loss of REE and Y, as well as Cl, along fractures within the apatite, accompanied by the formation of a new generations of apatite that is also depleted in these elements. The transition from early reduced, to later oxidised assemblages is accompanied by an evolution of the REE and Y signature of apatite in which LREE-enriched zones with negative Eu-anomalies are replaced by convex middle-REE (MREE)-enriched patterns, positive Eu-anomalies and the development of a conspicuous negative Y-anomaly. This trend is interpreted to suggest that hydrothermal conditions led to a decrease in salinity, pH and temperature in association with hematite-sericite alteration to produce MREE-enriched apatite (Krneta et al., 2017).

The best reported intersection from the mineralised hematite breccias is 248 m @ 0.86% Cu, 4.6 g/t Ag from 419 m (WMC Limited unpublished memoranda, 1985).

The most recent source geological information used to prepare this decription was dated: 2017.     Record last updated: 22/11/2017
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.

  References & Additional Information
   Selected References:
Courtney-Davies, L., Ciobanu, C.L., Verdugo-Ihla, M.R., Dmitrijeva, M., Cook, N.J., Ehrig, K. and Wade, B.P.,  2019 - Hematite geochemistry and geochronology resolve genetic and temporal links among iron-oxide copper gold systems, Olympic Dam district, South Australia: in    Precambrian Research   v.335, doi.org/10.1016/j.precamres.2019.105480.
Dmitrijeva, M., Ciobanu, C.L., Ehrig, K., Cook, N.J., Verdugo-Ihl, M.R., Metcalfe, A.V., Kamenetsky, V.S., McPhie, J. and Carew, M.,  2022 - Geochemical Data Analysis of Iron Oxide Copper-Gold Mineralization, Wirrda Well Prospect, South Australia: in    Econ. Geol.   v.117, pp. 853-874.
Hayward N and Skirrow R,  2010 - Geodynamic Setting and Controls on Iron Oxide Cu-Au (±U) Ore in the Gawler Craton, South Australia: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3 pp. 119-146
Heinson, G., Didana, Y., Soeffky, P., Thiel, S. and Wise, T.,  2018 - The crustal geophysical signature of a world-class magmatic mineral system: in    Scientific Reports   2018:8, DOI:10.1038/s41598-018-29016-2.
Katona, L.,  2020 - Geophysical signatures of IOCG prospects in the Olympic Cu-Au Province: in    Mesa Journal   v.92, pp. 26-37.
Krneta, S., Cook, N.J., Ciobanub, C.L., Ehrig, K. and Kontonikas-Charos, A.,  2017 - The Wirrda Well and Acropolis prospects, Gawler Craton, South Australia: Insights into evolving fluid conditions through apatite chemistry: in    J. of Geochemical Exploration   v.181, pp. 276-291.

Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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