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Osborne, Trough Tank, Kulthor
Queensland, Qld, Australia
Main commodities: Au Cu


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The Osborne (originally known as Trough Tank) and nearby Kulthor copper-gold deposits are 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).

In 1985, CSR Limited was granted exploration titles centred on a prominent magnetic anomaly, and entered into a joint venture with Billiton Limited to search for copper-gold hosted within ironstone. By the end of 1987, a total of 74 reverse circulation and 23 diamond drill holes had been drilled through cover into the Osborne anomaly without encountering any significant economic intersections. In 1988, Placer Exploration Limited acquired CSR Limited and drilled the discovery hole of 32 m @ 5.8% Cu and 3.2 g/t Au in TTHQ0029 in late 1989. Placer then undertook intensive drilling and in early 1991 established a resource of 27 Mt @ 1.4% Cu and 0.8 g/t Au. The Osborne Mine was developed and Placer Pacific Ltd commenced mining in August 1995, initially as a small open pit followed by extensive underground operations. In early 2006, Barrick Gold Corp acquired Placer Dome and became the sole owner of the Osborne operation. Ivanhoe Australia Limited acquired the Osborne Copper Gold Project from Barrick Gold in September 2010. Subsequently, Ivanhoe was renamed Inova Resources in 2012, and Inova was acquired by Shanxi Donghui Coal Coking and Chemicals Group Co. Ltd to become Chinova Resources in 2013. Known reserves were exhausted in 2015 and mining operations were suspended, although potential still remains at the Kulthor deposit. The Osborne processing plant continued operations from the other sources listed above, and from the 'Osborne Underground Remnant Mining Project' but when the latter was completed, went into a care and maintenance in February 2021.

The Osborne and Kulthor deposits occur 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.

Osborne

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 immediate host to mineralisation is a package of metasomatic rocks (Rubenach et al., 2001) that includes: i). albitites that are largely derived from psammitic gneiss; ii). sillimanite-rich rocks from the pelites; iii). magnetite-rich ironstones and biotite schists derived from amphibolite; iv). cummingtonite and anthophyllite rocks; and v). calc-silicate rocks. The calc-silicate assemblages have a variable distribution relative to the Cu-Au mineralisation. The diagnostic minerals of the alteration assemblage include epidote, andradite and titanite, with sporadic clinopyroxene, calcic amphibole, magnetite, hematite, calcite, quartz, pyrite and pyrrhotite.

The psammitic facies 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 Western Domain comprises two dominant mineralised bodies made up of four lodes that are moderate to steeply dipping sheets, with dimensions of 1100 x 350 m wide x 20 m thick in No. 1; and 700 x 250 x 20 m thick in No. 2. The Western Zone 3E orebody is a smaller flat-lying lensoid ~300 x <100 m x 20m thick (Gow, Valenta and Fox, 2020).

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.

The main Osborne mineralisation includes a copper sulphide assemblage, with chalcopyrite being by far the dominant copper sulphide, while bornite only occurs in accessory to minor amounts. Other dominant sulphides include pyrite and pyrrhotite, with associated magnetite or hematite. Minor to accessory sulphides and oxides include molybdenite, scheelite and various cobalt sulphides. A zonation is documented within the deposit from the Western to the Eastern domains in sulphide and oxide mineralogy and the copper/gold ratio (Adshead 1995). Pyrite is the ubiquitous Fe-sulphide in the western domain, whilst pyrrhotite is restricted to the high-grade mineralisation of the Eastern High Grade Zone in the eastern domain. ron oxides, The Western domain hosts hematite-magnetite-pyrite alteration of the banded ironstones, whilst the Eastern domain typically hosts a magnetite-pyrite association. Hematite-magnetite-pyrite altered ironstone of the Western domain has a relatively low Cu/Au ratio compared to the pyrrhotite-rich silica-flooding-associated Cu-Au mineralisation of the Eastern domain.

The majority of the ore grade copper mineralisation occurs as very coarse grained chalcopyrite within massive sulphide aggregates replacing silica flooded rocks. The majority of gold occurs as 1 to 10 µm inclusions in chalcopyrite and as solid solutions in pyrite and/or chalcopyrite. Lesser gold occurs along chalcopyrite and pyrite grain boundaries (Tullemans et al., 2001).

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.

Kulthor

The Kulthor copper gold deposit is located 2.3 km WSW of the Osborne copper mine. It comprises a steeply dipping tabular zone with several high grade shoots contained within a broader 0.1 to 0.5% Cu Equiv. halo. The mineralised zone has a strike length of at least 900 m and persists to a depth of ~700 m.

The Kulthor deposit has been interpreted to lie on the western limb of a fold nose within the Mt Norna Quartzite, with the Osborne deposit on the eastern limb, and a core of Llewellyn Creek Formation. The Kulthor mineralised zone occurs between the Western and Central Ironstones (Osborne lies on the eastern of the three main ironstones), within a fault-bound block of amphibolite and altered psammite that is 4 km long and a ~400 m wide, surrounded by a broader zone of schist-rich and granofels-rich migmatite (Morrison and Orr, 2002). The mineralised zone coincides with intervals of thick, steeply-dipping, quartz-dolomite ±feldspar veins which are, in turn, cut by most of the economic mineralisation, and occurs within a late brittle fracture system (Gunter, 2018). The veins comprise a series of finger like projections localised along the Western and Eastern shears that sandwich the block, and are also found centrally within the mineralised zone. The domains of denser veining are up to 80 m wide, although individual veins are rarely >10 m in true width. They overprint the amphibolite and psammite but lie in shears that are sub-parallel to the older foliation in both of these units. The mineralisation occurs a series of upright, lenticular sulphide-rich shear and replacement lodes that overprint and are aligned parallel to these veins. At the deposit-scale the block-bounding structures at Kulthor are all pre- and syn-mineralisation brittle shears, characterised by gouge, breccia and peripheral stylolitic shears, locally altered and enclosing shear veins.

The amphibolites have variable thickness, and are difficult to correlate from hole to hole, although their contacts are parallel to other units. They are commonly sheared, granoblastically recrystallised and altered, but locally have fine-grained margins at contacts, and interpreted to have originally been intrusive (Morrison and Orr, 2002). The mineralised zone is predominantly bounded to the west by migmatite, and by amphibolite-psammite that overlies granofels-migmatite in the east. The latter encloses the northern end of the mineralised zone. As at Osborne, the mineralised zone is narrow, altered and partly recrystallised, surrounded by more strongly recrystallised and partially melted rocks, i.e. granofels, migmatite and gneiss (Morrison and Orr, 2002).

In contrast to the iron oxide-rich Osborne, Kulthor is predominantly a sulphide body. The mineralisation occurs as a series of upright, lenticular sulphide-rich shear and replacement lodes comprising a network of mineralised sulphidic shears, stylolites, and bladed replacement and comb veins overprinting the dolomite-quartz-feldspar veins. The sulphide-rich zones are zoned from chalcopyrite-dominant at shallow levels to pyrite-dominant centrally, and pyrrhotite-dominant at depth, although chalcopyrite occurs throughout. Outside the main mineralised zone, sparse sulphides are localised in shears, occurring almost exclusively in centimetre-scale dolomite veins with local quartz, pyrite, chalcopyrite, chlorite, and calcite over-print.

Alteration is best recognised in the least recrystallised rocks (i.e. the psammite and amphibolite); partially developed in the partly recrystallised rocks (i.e. migmatites); and generally visually absent from the most re-crystallised rocks (i.e. gneiss). There is also a strong spatial association of alteration with veining and discordant pegmatite dykes. The overall spatial paragenetic relationships are consistent with mineralisation, alteration and veins being linked via the pegmatites to the partial melting process.

Reserves, Resources and Production

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 in the Barrick Annual Report, 2007, were:
    Measured + Indicated Mineral Resources - 3.268 Mt @ 0.84 g/t Au, 2.192% Cu,
    Inferred Mineral Resources - 4.318 Mt @ 0.594 g/t Au, 1.44% Cu,
    Proved + Probable Ore Reserves - 3.793 Mt @ 0.60 g/t Au, 2.065% Cu.

Prior to mining the Kulthor Resource was, in 2012 (Gow, Valenta and Fox, 2020):
    Measured + Indicated Mineral Resources - 7.5 Mt @ 1.0 g/t Au, 1.6% Cu,
    Inferred Mineral Resources - 5.4 Mt @ 0.9 g/t Au, 1.3% Cu,
Following 2 years of mining and further drilling, a new resource was estimated in 2017 at a cut-off grade of 0.75% Cu
Equiv.:
    Measured + Indicated Mineral Resources - 33.29 Mt @ 0.58 g/t Au, 0.86% Cu,
    Inferred Mineral Resources - 7.9 Mt @ 0.54 g/t Au, 0.79% Cu.

Between 1995 and 2021, the Osborne processing plant treated 35.6 Mt @ 2.19% Cu, 0.89 g/t Au, with an average of 92% Cu and 76% Au recoveries (Chinova website, viewed April 2023). This included production from the:
• Osborne Open Pit and Underground - 29.5 Mt @ 2.31% Cu, 0.91 g/t Au;
• Kulthor underground - 2.78 Mt @ 1.66% Cu, 1.05g/t Au;
    of which 24.2 Mt @ 2.49% Cu, 0.93 g/t Au was from the combined Osborne/Kulthor underground orebodies between 1996 and October 2015.
• Trekelano Open Pit - 2.36 Mt @ 1.55% Cu, 0.42 g/t Au;
• Starra 276 - 0.86 Mt @ 1.65% Cu, 0.93 g/t Au,
The Trekalano and Starra 276 ore was trucked to Osborne, as was additional ore from the Starra 254 and 222 underground and open pit mines after 2015.

The most recent source geological information used to prepare this decription was dated: 2020.     Record last updated: 17/4/2023
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.
Gow, P., Valenta, R. and Fox, N.,  2020 - NW Mineral Province 3D Deposit Atlas - Osborne and Kulthor: in    The University of Queensland. Data Collection,    37p. https://doi.org/10.14264/uql.2020.1
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, 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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