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The Mount Elliott IOCG System, Eastern Fold Belt, Mount Isa Inlier, Northwest Queensland
 
by
Mathew Brown, Ivanhoe Australia, Cloncurry, Qld, Australia,   T. M. (Mike) Porter, Porter GeoConsultancy Pty Ltd., Adelaide, South Australia.

in - Porter, T.M. (ed.), 2010 - Hydrothermal Iron Oxide Copper-Gold & Related Deposits: A Global Perspective, v. 3, Advances in the Understanding of IOCG Deposits; PGC Publishing, Adelaide.   pp. 219-232.

ABSTRACT

   The Mount Elliott iron oxide copper-gold (IOCG) system is located within the Eastern Fold Belt (EFB) of the Palaeo- to Mesoproterozoic Mount Isa inlier. It lies within a major high-strain corridor, the Mount Dore Fault (MDF) zone, one of a network of anastomosing structural zones that are developed within the EFB over intervals of tens to hundreds of kilometres, and which appear to influence the distribution of regional calcic-sodic alteration and IOCG mineralisation. The Mount Elliott IOCG system is hosted within a succession of altered siliciclastic and carbonatic sediments and volcanics within the upper of three rift basin cover sequences that overlie a Palaeoproterozoic to Neoarchaean basement, and occur within an enclave between batholiths of 1550 to 1500 Ma A-type granitoids and gabbroids. The steeply east-dipping, reverse MDF, also defines the boundary between the broadly equivalent eastern siliciclastic dominated Soldiers Cap Group and the carbonate-bearing Young Australia Group of the 1680 to 1610 Ma Cover Sequence 3, and as such may represent a rejuvinated synsedimentary rift basin margin structure.
    Mineralisation is hosted by both calc-silicates of the Staveley Formation of the Young Australia Group, and intensely skarn altered shales and meta-mafic rocks of the structurally overlying, but stratigraphically older Kuridala Formation of the Soldiers Cap Group. The bulk of the deposit is hosted within breccias, including a polymictic pipe-like mass cutting calc-silicates and amphibolites (after "diorite") within the Staveley Formation, and a megabreccia with clasts of from 0.1 to 20 m across within phyllites and meta-mafic rocks of the Kuridala Formation. Mineralisation also occurs as a replacement of adjacent banded calc-silicates, replacement of infill to breccias and as late carbonate-sulphide veins. Alteration comprises early, pre-breccia pervasive hematite-stained albite-silica, followed by multiple pulses of fracturing, brecciation and alteration, each composed of initial diopside-scapolite, followed by the deposition of actinolite and mineralisation, resulting in an assemblage of chalcopyrite, actinolite, scapolite ±andradite ±tourmaline ±allanite ±apatite ±magnetite ±pyrite ±pyrrhotite and very abundant calcite and anhydrite, as well as minor biotite, chlorite and K feldspar. Skarn alteration closely associated with the main copper-gold-bearing sulphides has been dated at 1510±3 Ma, close to the age of the nearby batholithic granitoids, while stable isotope data are consistent with dominantly magmatic fluids during mineralisation, possibly influenced by a metamorphic fluid component.
    Mineralisation has been known and sporadically exploited at Mount Elliott since 1899, while an offshoot of the historic deposit, the Corbould zone, was discovered in 1995. Exploration in the vicinity during the 1980s and 1990s had also encountered the SWAN and SWELL prospects, neither of which have been brought to production. A reevaluation of the existing resource and an intense deep drilling campaign focused on the SWAN resource since 2003 has shown that it coalesces at depth with the other three zones of mineralisation, all of which represent higher grade cores within a large envelope of copper-gold mineralisation, as defined by a 0.25% Cu
 equiv. cut-off. Published total resources are 570 Mt @ 0.44% Cu, 0.26 g/t Au at 0.3% Cu equiv. cut-off, extending over a strike length of more than 1.5 km, width of ~500 m and to a depth of 1200 m, and includes 62 Mt @ 1.01% Cu, 0.4 g/t Au at a cut-off of 1.0% Cu equiv.

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