Cosmo Howley ( Cosmopolitan Howley )

Northern Territory, NT, Australia

Main commodities: Au
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The Cosmo Howley (previously Cosmopolitan Howley) gold deposit, which is located 160 km south of Darwin, lies within the Archaean to Palaeoproterozoic Pine Creek Inlier which covers an ENE elongated area of 400 x 150 km immediately to the south and east of Darwin in the Northern Territory, Australia.

The Pine Creek Inlier is composed of 2200-1870 Ma Palaeoproterozoic supracrustal meta-sediments and volcanics that are around 10-14 km thick, overlying Archaean crystalline basement exposed as three separate domal highs surrounded by the supracrustals.   The sequence commences with the Namoona Group conglomerates, meta-arkose, gneiss, schist and dolomite, which were metamorphosed to amphibolite facies from 2100 to 2000 Ma, then overlain by further sediments, the Mount Partridge Group of principally fluviatile sandstone, dolomitic siltstone and mudstone.   These were then unconformably followed by the South Alligator Group of carbonaceous mudstone, iron formation, greywacke and acid tuff which passed up into the greywackes and mudstone of the Finniss Group.   All of these sequences were intruded by Zamu dolerite.

Following a break the sequence was subjected to deformation and greenschist metamorphism from 1870 Ma to produce a series of tight, non-cylindrical folds with considerable associated brittle deformation and extensive axial plane slaty cleavage in pelite.   This was followed by intrusion of a number of pulses of granite from1820 to 1800 Ma

The Cosmo Howley deposit straddles the axis of the Howley Anticline, a regionally continuous 30 km long structure.   In the mine area the fold axis plunges at 50° towards 325° with a well developed consistent slaty cleavage.   Intense faulting, sometimes with associated brecciation, parallels the cleavage through the axial zone and east limb in zones from 1 to 10 m wide.

The principal units in the vicinity of the deposit are the Wildman Siltstone of the Mount Partridge Group and the Koolpin Formation and Gerowie Tuff of the East Alligator Group.   The main host to mineralisation at Cosmo Howley is the Koolpin Formation, although other mineralisation occurs along the axis and limbs of the Howley Anticline in lithologies of the Gerowie Tuff (Fleur de Lys and Chinese Howley) and Wildman Siltstone (Big Howley) over an interval of more than 5 km.

The Koolpin Formation is divided into three members:  1). a 250 m thick Lower Member of carbonaceous and micaceous mudstone and siltstone,  2). a Middle Member which is up to 100 m thick and mainly composed of interbedded iron formation (10 to 15% Fe as oxide and sulphide), carbonaceous mudstone and mudstone, and  3). a 50 to 150 m thick Upper Member of carbonaceous mudstone.   Significant gold ore is developed at a number of levels within the Middle Member, although substantial economic mineralisation is also found in ironstones and mudstone in both the Upper and Lower Members.   40% of the reserves are in the Lower Member.

Mineralisation takes the form of quartz veins that occur within 50 m of the non-carbonaceous footwall of carbonaceous slates.   Although veins are found within the carbonaceous slate, and further than 50 m from the contact, these are generally barren.   Except for the hydrofracture stockworks of the fold hinge, the mineralised veins are bedding concordant in hornfels and slates on the steeply dipping limbs of the anticline.   The veins formed near the peak of contact metamorphism following intrusion of the Cullen Batholith to the south.   Veins show crack-seal textures and are discontinuous, tapering at their extremities.   Veins of <5 cm thickness generally have a length to thickness ratio of >10, while those that are thicker than 5 cm have a ratio of >50.

Gold is closely associated with sulphides (pyrite and arsenopyrite with traces of sphalerite and chalcopyrite - with pyrrhotite below 300 m), weathered to goethite, hematite and other limonites in the oxide zone.   Fine pyrite follows cleavage and is disseminated in the iron formations (5 to 15%), mudstone (10%) and carbonaceous mudstones (5 to 30%), while thin 'beds' of fine arsenopytite are found in the iron formations.   Later coarse pyrite occurs in shears and with coarse arsenopyrite in quartz veins.   Most of the gold occurs in quartz veins, breccias and shears, with higher grades in BIF adjacent to discordant structures, and lesser in mudstones in similar structures.

Several other gold mines occur along the crest of the Howley Anticline. They include Chinese Howley, Chinese South, Big Howley, Howley North, Bridge Creek and Mount Paqualin. Together with Cosmo Howley these mines form a 24 km-long line of lodes known as the Howley Line. The mines are located close to the hinge zone of the Howley Anticline in the Koolpin Formation (Mount Paqualin), Gerowie Tuff (Chinese Howley) or Mount Bonnie Formation (Big Howley). In these mines, mineralisation is essentially not associated with ironstone beds.

The pre-mining reserve totalled:
    Oxide ore - 1.6 Mt @ 1.7 g/t Au
    Sulphide ore - 2.5 Mt @ 2.5 g/t Au
By 1989 the mining reserve had increased to: 6 Mt @ 2.54 g/t Au.
Historic production to 1989 had totalled 1.487 t Au. Total open pit production to 1994 was 7.7 Mt @ 2.3 g/t Au to a depth of 120 m.
The global resource, including underground ores was stated at 10 Mt @ 2.75 g/t Au in 1995.

Resources and production (where mined) for deposits of the goldfield in 2009, were (Gillman et al., 2009):
    Big Howley resource - 0.647 Mt @ 1.6 g/t Au,
    Chinese Howley resource - 5.32 Mt @ 1.4 g/t Au,
    Chinese South resource - 4.2 Mt @ 1.5 g/t Au,
    Cosmo Howley production - 6.98 Mt @ 2.1 g/t Au (McKibben et al., 2008).
    Cosmo Deep resource - 8.743 Mt @ 4.38 g/t Au.

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1995.    
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
   Selected References:
Alexander K R, Kavanagh, Rolfe G L  1990 - Cosmo Howley Gold deposit: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 751-753
Matthai S K, Henley R W, Bacigalupo-Rose S, Binns R A, Andrew A S, Carr G R, French D H, McAndrew J, Kananagh M E  1995 - Intrusion-related, high-temperature Gold Quartz veining in the Cosmopolitan Howley metasedimentary rock-hosted Gold deposit, Northern Territory, Australia: in    Econ. Geol.   v90 pp 1012-1045
Matthai S K, Henley R W, Heinrich C A  1995 - Gold precipitation by fluid mixing in bedding-parallel fractures near Carbonaceous slates at the Cosmopolitan Howley Gold deposit, northern Australia: in    Econ. Geol.   v 90 pp 2123-1242
Nicholson P M, Eupene G S  1990 - Gold deposits of the Pine Creek Inlier: in Hughes F E (Ed.), 1990 Geology of the Mineral Deposits of Australia & Papua New Guinea The AusIMM, Melbourne   Mono 14, v1 pp 739-742

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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