Australia is blessed with a wealth of mineral resources, and in terms of copper, the most famous deposit class present throughout the Australian continent would have to be Iron-Oxide-Copper-Gold (IOCG) deposits. I’ve been lucky to work on a variety of these deposits in one capacity or another. Although the IOCG classification term is a bit of a “bin”, it generally refers to deposits that have iron-oxide (magnetite and/or hematite), copper (chalcopyrite, bornite, chalcocite, & covellite) and gold (in whichever form is chooses). There are three main varieties of IOCG deposits found throughout Australia, which have a variety of characteristics and similarities, which we will outline in this article. The three IOCG types within Australia include: (1) Tennant Creek type; (2) Olympic Dam type and (3) Cloncurry type.
Tennant Creek Type
The Tennant Creek IOCG deposit type is found throughout the Tennant Creek gold fields, located in within the Northern Territory, Australia. Deposits included within this classification include the Juno (Cu-Au-Bi), Argo (Cu-Au), Peko (Cu-Au-Bi), Ivanhoe (Cu-Au), Warrego (Cu-Au-Cu-Au-Bi) & Nobles Nob (Au) deposits (Huston et al., 2020). In addition to copper and gold, these deposits often contain abundant bismuth. The deposits mentioned above are predominantly hosted within the 1860 Ma Warramunga Province, which consists of the three temporally equivalent units termed the Warramunga Formation, Junalki Formation and the Woodenjerrie Beds (Donnellan, 2013). The Warramnugi formation hosts over 700 ironstone bodies, which are comprised of hematite and/or magnetite and range from >15 mt to only a few tonnes (Donnellan, 2013). The formation of the ironstone bodies are interpreted to have formed during D1 deformation of the 1860–1845 Ma Tenant Event, are associated with abundant chlorite-sericite alteration and pre-date Cu-Au mineralisation (Donnellan, 2013; Rattenbury, 1992). Although less than 20% of these ironstones contain Cu-Au-Bi mineralisation, the ironstone bodies put the “IO” into “IOCG” in the case of the Tennant Creek style IOCG deposits. Cu-Au-Bi mineralisation occurs within overprinting veins containing variations of chalcopyrite, pyrrhotite, pyrite, bismuthate, Bi-tellurides, galena and sphalerite. Ore within these deposits are not so much as a breccia, as they are massive Fe-oxide overprinted by veining Cu-Au mineralisation. Muscovite Ar-Ar ages associated with Cu-Au ± Bi mineralisation from a range of deposits across the Tennant Creek Goldfield produce ages ranging from 1847–1851 Ma (Compston, 1995; Compston and McDougall, 1994; Fraser et al., 2008). Based on these ages, Cu-Au mineralisation is in interpreted to be contemporaneous with the later stages (D2) of the Tennant Event and the intrusion of the voluminous widespread Tennant Creek supersuite (Fraser et al., 2008).To throw a spanner into the works, there is potential for another significant Cu-Au event throughout the Tennant Creek goldfield at ca. 1660 Ma. A recent paper produced U-Th-Pb monazite ages of 1659 ± 13 Ma and 1659 ± 15 Ma for the Orlando East and Navigator 6 deposits produce respectively (Skirrow et al., 2019). The ca. 1660 Ma event is interpreted to be synchronous with brittle-ductile to brittle deformation, thus allowing the migration of Cu-Au bearing fluids into their current structural setting (Skirrow et al., 2019). However, the broader tectonic significance of the ca. 1660 Ma event is relatively unknown and further research is required.
Olympic Dam Type
The Olympic Dam IOCG deposit, situated in South Australia's Gawler Craton, is one of the most renowned and economically significant deposits in Australia. IOCG deposits within this classification include the Olympic Dam (Cu-Au-U), Prominent Hill (Cu-Au-Ag), Carrapateena (Cu-Au), Emmie Bluff (Cu-Au-U) & Oak Dam (Cu-Au-U) deposits. Unlike the deposits at Tennant Creek, these deposits are present throughout various litho-tectonic units of the Gawler Craton. One contrasting feature of deposits throughout the Gawler Craton, when compared to the Tennant Creek or Cloncurry-types, is that they do not crop out at the surface and are found beneath varying degrees of overburden. For example, the top of the Olympic Dam deposit is located ~300 m below overlying sediments, Carrapateena is located ~470 m below the overlying sediments and Prominent Hill is located under ~100 m of overlying sediments. This alone can make exploring for this deposit type enormously difficult. In the larger deposits, the dominant Fe-mineral found is hematite, with only trace abundances of magnetite present (Belperio et al., 2007; Ehrig et al., 2012; Porter, 2010). Cu-Au mineralisation occurs in the form of the various sulphide minerals chalcopyrite, bornite, covellite and chalcocite. In the aforementioned deposits, ore consists of hematite-dominated breccias containing the Cu-bearing minerals outlined above. The copper minerals are predominantly distributed heterogeneously throughout the breccias but can vary depending on the sample. These deposits are often enriched in REE’s (specifically LREE’s) and U, of which the mineralogy depends upon the specific deposit. Previous geochronology studies have shown that IOCG formation across the Gawler Craton is largely contemporaneous with the intrusion of the voluminous Gawler Range Volcanics and Hiltiba Suite Granites from 1594-1586 Ma (Courtney-Davies et al., 2019; Jagodzinski, 2005; Reid et al., 2013). Unlike at Tennant Creek, hematite and Cu-mineralisation are interpreted to have formed during the same mineralisation event (Courtney-Davies et al., 2019). There are some important notes to this model though. Often, within a single deposit, more than one mineralising event is recorded. At Olympic Dam, a significant amount of uranium, as well as some Cu-Au mineralisation, is interpreted to have been deposited from 700-500 Ma, which is up to a billion years after the primary IOCG mineralisation event (Ehrig et al., 2021). At the Vulcan Cu-Au prospect, also located within the northern Gawler Craton, three events at ca. 1590 Ma, ca. 1100 Ma and ca. 500 Ma have been recorded via apatite U-Pb geochronology (Wade, 2021). Although most mineralisation throughout the Gawler Craton is attributed to a main mineralogical event at ca. 1590 Ma, overprinting (enriching?) events appear to be a common phenomenon. While we’re here, it should be noted that some new research has shown the potential for an additional Cu event with IOCG-like characteristics at 1440 Ma (unpublished data; To Be Published). Furthermore, hydrothermal alteration throughout the Peake and Denison domain is relatively later, coinciding with the age of IOCG mineralisation and associated alteration throughout the Mount Isa domain (Bockmann et al., 2023). And with that segway…
Cloncurry Type Deposits
This deposit type is concentrated along the western side of the Mount Isa Inlier, located in northwest Queensland. The most notable example of this deposit type is the Ernest Henry IOCG deposit, located 38 km northeast of Cloncurry. In the Ernest Henry IOCG deposit, chalcopyrite comprises Cu-mineralisation while gold is located as “free gold”, as well as inclusions within pyrite (Mark et al., 2006). The “IO” part of the deposit is comprised of magnetite. In the Cloncurry region, much of the current geometry, grade of metamorphism and presence of metasomatism can be attributed to having occurred during some point of the (1610-1500 Ma) Mount Isa Orogeny (Bell and Hickey, 1998; Davis et al., 2001). Although the deformation scheme varies depending upon the place and/or author, three broad deformation events can be attributed to having occurred throughout the region. However, for the purpose of this writing D2 and D3 are considered important, while D1 is considered there, but not significant here. D2, which occurred at ca.1580 Ma, consisted of large-scale E-W deformation throughout the Mount Isa Inlier and produced a wide array of alteration, of which at least some is found within the Ernest Henry IOCG deposit (Cave et al., 2018; Giles and Nutman, 2002). This metamorphic event also resulted in the formation of the Osborne Cu-Au skarn deposit, which is dated at ca. 1594 Ma (Gauthier et al., 2001). D3 deformation occurred broadly from 1530-1500 Ma but could also be defined within the period of 1550-1480 Ma (Davis et al., 2001; Page and Bell, 1986). D3 is considered the most important event and is temporally coeval with widespread Na-Ca alteration throughout the Eastern Fold Belt (Marshall and Oliver, 2008; Oliver, 1995; Oliver et al., 2004), and the formation of a vast array of deposits including Ernest Henry (Cu-Au), E1 (Cu-Au), Monakoff (Cu-Au), Swan (Cu-Au) and Mount Elliot (Cu-Au) deposits (Duncan et al., 2011; Garrett, 1992; Mark et al., 2006). Interestingly, this is also consistent with the age of the Merlin Re-Os deposit (Babo et al., 2017). The age of these deposits is temporally associated with widespread magmatism of the 1530-1500 Ma Williams-Naraku Batholiths (Davis et al., 2001; Wyborn, 1998). Ore-stage mineralisation is typically composed of chalcopyrite and magnetite with bornite and hematite less common. However, the mineralogy of each deposit can vary significantly. In some instances, these deposits are classified as skarns in their own right (Wang and Williams, 2001), blurring the boundaries of deposit classification. It is noteworthy that Cloncurry also hosts another type of deposit termed an “ISCG”, which lack the iron-oxide part of the equation but is compensated by the presence of the sulphides, most notably pyrrhotite (Skirrow, 2022). These deposits formed during the same period of IOCG mineralisation. Deposits within this class include the Mount Colin, Greenmount & Jericho Cu-Au deposits (Haynes and Porter, 2000; Skirrow, 2022). Although controversial, the Mount Isa Cu deposit may also be considered an ISCG deposit.
Conclusion
In this article we briefly introduce the three main types of IOCG deposits present within Australia. All three deposits formed during different time periods, ca. 1860 Ma for Tennant-Creek type, ca. 1590 Ma for Olympic Dam type and ca. 1520 Ma for Cloncurry type. However, each deposit type correlates with large-scale magmatism and coeval deformation. Each type of IOCG deposit has a unique mineralogy, paragenesis and presents its own challenge for exploration. Lastly, each deposit class has its own unique hidden gem, weather it be the ca. 1660 Ma Cu-Au in Tennant Creek, 1440 Ma mineralisation in the Gawler Craton, or an ISCG deposit in the Mount Isa Inlier.
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