Western Australia, WA, Australia

Main commodities: W Zn Pb Cu
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The O'Callaghans tungsten deposit is located ~400 km ESE of Port Hedland and ~10 km SE of the Telfer gold mine in Western Australia (#Location: 21° 48' 38"S; 122° 14' 50"E).

Discovery was the result of targeted drilling from 1985 by Newmont Australia, which had followed up geophysical data indicating both concealed granite and discrete magnetic anomalies to the south of Telfer. The first skarn mineralisation was intersected in 1991. By 2009, a maiden Mineral Resource was released by Newcrest Mining Ltd. O'Callaghans is one of the largest tungsten resources in Australia and is situated within the intracontinental Yeneena Basin of the Paterson Province. The basin was filled by the Neoproterozoic Yeneena Supergroup, made up of the Throssell Range and overlying Lamil groups, intruded by extensive granites that are 200 to 250 m.y. younger.

For detail of the regional to district scale tectonic and geological setting is described in more detail in the separate Telfer record.

Mineralisation at the O'Callaghans deposit is developed at the flat lying contact between the Puntapunta Formation and the underlying O'Callaghans Granite, at a depth of ~350 m below surface. The Puntapunta Formation occurs towards the top of the Lamil Group, and comprises up to 1500 m of laminated to thinly bedded, dark grey dolomitic sandstone, dolomitic siltstone, chert, shale and limestone. It overlies the sandstones and lesser siltstones and mudstones of the Malu Formation, host to the Telfer gold deposit. The O'Callaghans Granite, which is not exposed at surface, is one of a widespread suite of highly fractionated, I-type granite bodies distributed over an area in excess of ten thousand square km within the Yeneena Basin. These granitic plutons have been divided into an earlier 645 to 630 Ma magnetite-stable, and a later ~605 Ma ilmenite-stable, more reduced suite. The latter includes the O'Callaghans Granite.

Mineralisation is directly related to a complex zonation of high temperature metasomatic calc-silicate alteration overprinting folded Puntapunta Formation sedimentary carbonate rocks. The O'Callaghans Granite is a medium-grained biotite monzogranite with accessory titanite, zircon and hornblende, and has high SiO2, Na2O, K2O, and low MgO contents, as commonly observed in granites associated with skarn-type tungsten mineralisation. It has been affected by two hydrothermal alteration events. The first is an early white mica-chlorite alteration of plagioclase and biotite respectively, while the second produced veins up to 1 m thick of quartz±fluorite±pyrite±chalcopyrite±molybdenite±pyrrhotite±scheelite, fringed by 1 to 5 cm alkali feldspar-muscovite alteration selvages. The mineralised and altered granite is overlain by a narrow zone of weakly foliated granite and an outer, poorly mineralised, rind of quartz flooding on the intrusive contact, representing a relatively sharp contact.

The overlying 5 to 70 m thick mineralised exoskarn, which is characterised by >10% sulphides and an assemblage of amphibole, pyroxene, garnet, biotite, chlorite and sulphides, covers a NW elongated area of ~1300 x 900 m. It passes upward into an outer, irregular, zone of calc-silicate altered carbonate rocks of the Puntapunta Formation, with <10% sulphides. The exoskarn was generated in three stages:
• High temperature, prograde, calc-silicate alteration, corresponding to the initial granite intrusion. It is defined by an assemblage of clinopyroxene, garnet, amphibole, calcite, quartz and titanite, with accessory pyrite and pyrrhotite;
• The main mineralising potassic altered assemblage, dominated by biotite and alkali feldspar, muscovite, fluorite, sulphides (pyrite-pyrrhotite-chalcopyrite-sphalerite-scheelite±molybdenite), magnetite and wolframite. This phase coincided with ascent of the main hydrothermal fluids into the apex of the underlying granite intrusion, and the concomitant mineralised veining stage in the granite; and
• A later, overprinting and lower temperature retrograde alteration, generating an assemblage of chlorite, sericite and hematite, accompanied by pyrite and chalcopyrite, possibly related to interaction with meteoric fluids.

The principal tungsten minerals are wolframite and scheelite, with associated sphalerite, galena and chalcopyrite, accompanied by an assemblage that reflects elevated fluorine, bismuth, molybdenum and tin. No economically significant gold is present. Two zones of elevated zinc and lead are found at the transition between the tungsten species wolframite and scheelite. The deposit is characterised by sharp footwall and hanging wall contacts, but also sharply defined internal lenses of high grade sulphides such as sphalerite and galena in particular.

At 31 December, 2016, the deposit contained estimated
    Indicated + Inferred Mineral Resources of 78 Mt @ 0.33% WO3, 0.50% Zn, 0.25% Pb, 0.29% Cu,
      inclusive of a
    Probable Ore Reserve of 44 Mt @ 0.36% WO3, 0.65% Zn, 0.32% Pb, 0.29% Cu (Newcrest Mining, Annual Report, 2017).

The most recent source geological information used to prepare this summary was dated: 2012.    
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
 References to this deposit in the PGC Literature Collection:
Schindler, C., Hagemann, S.G., Batt, G. and Maxlow, J.,  2012 - The geological setting and petrography of the OCallaghans granite and W skarn mineralization in the Telfer area, Paterson Orogen, Western Australia: in    SEG 2012 Conference, Lima Peru,   Abstracts volume, 2p.

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