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Osborne / Trough Tank

Queensland, Qld, Australia

Main commodities: Au Cu
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The Osborne copper-gold deposit is located some 195 km south east of Mt Isa in North-west Queensland, Australia and 720 km west-southwest of the coastal city of Townsville.
(#Location: 22° 05' 43"S, 140° 34' 46"E).

The deposit occurs within a complex sequence of metamorphic, igneous and metasomatic rocks belonging to the Mesoproterozoic Eastern Fold Belt (Cloncurry Terrane) of the Mt Isa Inlier. The predominantly meta-sedimentary host sequence belongs to the Soldiers Cap Group, Mount Norna Quartzite, which overlies the quartz-mica psammites and minor pelites of the 2000 m thich Llewellyn Creek Formation, and is overlain in turn by the amphibolites, carbonaceous slates and scapolitic sandstones of the Toole Creek Volcanics. The Soldiers Cap rocks are intruded by the 1550-1500 Ma Williams-Naraku Batholith.

For details of the regional setting and geology of the northeastern Cloncurry district, see the Cloncurry IOCG Province record.

The host Mt Norna Quartzite unit comprises feldspathic psammites ± thin layers of pelite, stromatitic migmatites and local pre-metamorphic banded ironstone units and schists. Sheet intrusions of amphibolite and post metamorphic pegmatites are also present.

The psammites are characterised by sodic plagioclase comprising >95% albite and/or oligoclase + quartz. The ironstones are predominantly composed of varying proportions of magnetite, quartz and apatite. The deposit area is divided into two domains by the Awesome Fault. The bulk of the high grade Cu-Au mineralisation is focused along the contacts of the upper banded ironstone with feldspathic psammite in the Western Domain. Both ironstones in this domain contain disseminated Cu-Au mineralisation along their 1.3 km strike lengths and the gross distribution of ore mirrors the shape of the ironstone.

The Eastern Domain is largely devoid of ironstones but embraces a prolate high grade lens of ore in pegmatite, feldspathic psammite and meta-mafic intrusives. The majority of the Cu is present as chalcopyrite hosted by zones of coarse silica flooding. Within the ironstones, mineralisation is present with secondary hematite-pyrite-magnetite, whereas well mineralised silica flooded zones outside of the ironstones contain only magnetite-pyrite. In the Eastern Domain ore is characterised by pyrrhotite-pyrite-magnetite.

The ore is most closely related to quartz alteration, which can be traced down plunge into quartz and quartz-feldspar veins, and pegmatite dykes, while both the ore and the alteration fade out into barren, unaltered migmatite and gneiss. The zones of massive, coarse grained silicification / silica flooding with abundant wall rock relicts, hosts the bulk of the Cu-Au mineralisation. However, textural evidence indicates the main phase of Cu-Au deposition post dated the majority of the silica flooding and its temporally associated pre-Cu-Au pyrite±magnetite±siderite±talc and minor chlorine-bearing silicates.

Peak amphibolite facies metamorphism is dated at 1595 to 1568 Ma. Re-Os and U-Pb dating of the Cu-Au indicates it is broadly coeval with the peak metamorphism, although earlier dating suggested it was emplaced at around 1538 Ma. Fluid inclusion studies indicate the orebody formed at around 300°C from immiscible hyper-saline brines and carbon dioxide rich components.

In 1993, prior to mining, the total measured and indicated mineral resource was 11.2 Mt @ 3.5% Cu, 1.5 g/t Au (Adshead et al., 1998) within a larger global resource of 36 Mt @ 2% Cu, 1 g/t Au .

According to Ivanhoe Australia, 2010, the original resource announced by Placer Pacific, prior to mining, totalled 27 Mt @ 1.4% Cu, 0.8 g/t Au.

The total production + reserves to be mined as known in 2005 was 19 Mt @ 3.1% Cu, 1.1 g/t Au.

At December 31, 2007, reserves and resources quoted by Barrick, 2008, were:
    Proved + probable reserves - 4.26 Mt @ 0.63 g/t Au, 2.065% Cu,
    Proved + probable reserves - 3.67 Mt @ 0.86 g/t Au, 1.78% Cu,
    In 2007, production amounted to 1.3 t of recovered Au and 39 000 t Cu.

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.


Osborne

  References & Additional Information
   Selected References:
Adshead N D, Voulgaris P, Muscio V N  1998 - Osborne copper-gold deposit: in Berkman D A, Mackenzie D H (Eds),  Geology of Australian and Papua New Guinean Mineral Deposits The AusIMM, Melbourne    pp 793-799
Betts, P.G., Giles, D., Mark, G., Lister, G.S., Goleby, B.R. and Ailleres, L.,  2006 - Synthesis of the Proterozoic evolution of the Mt Isa Inlier: in    Australian J. of Earth Sciences   v.53, pp. 187-211.
Davidson G J, Large R R, Kary G L, Osborne R  1989 - The deformed iron-formation-hosted Starra and Trough Tank Au-Cu mineralization: A new association from the Proterozoic Eastern Succession of Mount Isa, Australia: in    Economic Geology Monograph 6    pp 135-150
Derrick G M  1996 - The geophysical approach to metallogeny of the Mt Isa Inlier - what sort of orebody do you want: in    Proc The AusIMM Annual Conference, Perth, 24-28 March, 1996 The AusIMM, Melbourne    pp 349-366
Fisher L A and Kendrick M A,  2008 - Metamorphic fluid origins in the Osborne Fe oxide–Cu–Au deposit, Australia: evidence from noble gases and halogens: in    Mineralium Deposita   v.43 pp. 483-497
Foster, D.R.W. and Austin, J.R.,  2008 - The 1800-1610 Ma stratigraphic and magmatic history of the Eastern Succession, Mount Isa Inlier, and correlations with adjacent Paleoproterozoic terranes: in    Precambrian Research   v.163, pp. 7-30.
Olivera, N.H.S., Butera, K.M., Rubenach, M.J.,Marshall, L.J., Cleverley, J.S., Mark, G., Tullemans, F. and Esser, D.,  2008 - The protracted hydrothermal evolution of the Mount Isa Eastern Succession: A review and tectonic implications: in    Precambrian Research   v.163 pp. 108-130
Williams, P. J., Kendrick, M.A. and Xavier, R.P.,  2010 - Sources of Ore Fluid Components in IOCG Deposits: in Porter T M, (Ed), 2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3, pp. 107-116.
Williams, P.J. and Pollard, P.J.,  2003 - Australian Proterozoic Iron Oxide-Cu-Au Deposits: An Overview with New Metallogenic and Exploration Data from the Cloncurry District, Northwest Queensland: in    Exploration & Mining Geology, CIM   v.10, No. 3, pp. 191-213.


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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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