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Pine Creek Goldfield - Enterprise |
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Northern Territory, NT, Australia |
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Au
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Pine Creek Goldfield comprises a narrow, ~1 km wide by 6 km long, northwest-trending belt centred about 0.5 km west of Pine Creek township in the Northern Territory, Australia. It includes some 15 hard rock and numerous alluvial workings, which together, constitute the most productive goldfield in the Pine Creek Orogen. It hosts a number of small to medium sized gold deposits, one of the more significant of which is Enterprise. Other deposits include Coxs, Czarina, South Czarina, South Enterprise, North Gandys Hill, International and Kohinoor (#Location: Enterprise - 13° 49' 33"S, 131° 49' 43"E).
From the discovery of gold in the early 1870s to 1915, 124 960 t of ore were treated, yielding an average of 32 g/t Au from the batteries and 7.8 g/t Au from the cyanide works. Reported total production to 1915 is 2.300 tonnes of Au (Hossfeld 1936).
The goldfield falls within the 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 to 1870 Ma Palaeoproterozoic supracrustal meta-sediments and volcanics that are >4 km, possibly 10 to 14 km thick, overlying Neoarchaean ~2670 to 2500 Ma 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 the 1835-1800 Ma Cullen Granite that has imposed a broad contact metamorphic aureole.
The Pine Creek goldfield is developed within a 35 x 5 km, south-east trending lobe of Burrell Creek Formation (Finniss River Group) occupying an embayment within the Cullen Granite, which in turn lies within the 20 to 25 km wide Noonamah-Katherine Lineament.
The Pine Creek Goldfield lies adjacent to the western margin of the Pine Creek Shear Zone in sheared and contact-metamorphosed shale, siltstone and greywacke of the Mount Bonnie and Burrell Creek formations. The Mount Bonnie Formation, exposed in the northwestern part of the field, consists of pelitic sedimentary rocks interbedded with greywacke and tuffaceous units. A thin carbonaceous shale interval, containing chert nodules, has been usewd as a marker horizon in defining a moderately tight, southeast-plunging F3 anticline (Enterprise anticline) at the Enterprise mine. Two other similar anticlines (Czarina and Kohinoor anticlines) are present to the east. The axes of all three structures trend northwest and plunge southeast (Ahmad and Hollis, 2013).
Mineralisation is predominantly structurally controlled, with economic lodes occurring as saddle reefs and less commonly as discordant quartz veins, or in faults and shear zones. Minor amounts of gold are also disseminated in the wallrock adjacent to quartz veins. Hossfeld (1936a) listed 16 saddle reefs (eight in the Enterprise anticline, three in the Czarina anticline, five in the Kohinoor anticline), which were mined extensively. The western limbs of these reefs are usually more persistent laterally and at depth. Old workings have usually followed the contact zone between the reef and wallrock, leaving the central part of the quartz veins (Ahmad and Hollis, 2013).
Mineralisation occupies the transitional zone between the Mount Bonnie and Burrell Creek formations. The wallrock to the gold-quartz lodes comprises greywacke, shale, mudstone and chert. Most of these rocks commonly contain up to 1% disseminated sulphides, mainly pyrite, but also arsenopyrite and pyrrhotite.
The Enterprise deposit occurs at the boundary between the 200 to 300 m thick Mount Bonnie Formation, a transition facies between the low energy cherts, banded iron formation and fine grained pyritic and carbonaceous sediments of the Koolpin Formation (East Alligator Group) and the overlying high energy greywacke, siltstone and mudstone of the Burrell Creek Formation. In the deposit area the sequence is composed of: i). a 50 m thick lower mine greywacke, ii). an 18 m thick nodular chert, iii). a 65 m thick spotted silt, and iv). the >50 m thick upper greywacke, the first unit of the Burrell Creek Formation.
The dominant structure at Enterprise is the moderately tight, upright Enterprise anticline, cut by 60 to 70° W dipping faulting parallel to the fold axes. The Enterprise anticline strikes at 315° and plunges 5 to 10° SE (Cannard and Pease 1990). The largest of the faults is the Eastern Fault Zone, which can be traced for 600 m and strikes northwest. This fault is displaced by younger north-trending faults, and in the southern part, it is displaced by a near-vertical 060° trending fault. These younger faults do not carry auriferous quartz veins, but may host some late-stage galena-quartz veins.
Mineralisation consists of quartz-sulphide veining (15 to 20% of the rock, and 2 to 80%, averaging 10% sulphides in veins) with pervasive vein margin alteration and the introduction of associated sulphides (pyrite, pyrrhotite and arsenopyrite with lesser sphalerite, galena and chalcopyrite) in the enclosing sediments. The veins range from millimetres to metres in thickness and from a metre to tens of metres in strike and down dip length. Most of the mineralisation is wthin 50 m of the axis of the Enterprise anticline. Typically mined ore grade widths are of 20 to 80 m. The vein density is greatest within 20 m either side of the fold axis and resembles a stockwork. In general mineralisation is stratabound, following or bounded by particular units, although such controls are not persistant along strike. Oxidation and clay mineral alteration is around 20 to 40 m deep.
The gold is free milling or is contained in arsenopyrite. Sulphide minerals include pyrite, arsenopyrite, pyrrhotite, marcasite, chalcopyrite, galena, sphalerite, bismuthinite, tetrahedrite and covellite. In addition, rare native copper and bismuth are present.
Total production from Enterprise to 1987 was 4.043 t Au, including 1.9 t Au from 1.0 Mt @ 2.3 g/t Au extracted from the current operation from 1985 to 1987.
Reserves in 1987 were 9.2 Mt @ 2.7 g/t Au.
Resources and production (where mined) for deposits of the goldfield in 2009, were (Gillman et al., 2009):
Coxs - 0.53 Mt @ 1.4 g/t Au,
Czarina - 1.844 Mt @ 1.69 g/t Au,
South Czarina - 0.17 Mt @ 1.5 g/t Au,
Enterprise - 1.241 Mt @ 2.6 g/t Au, plus production of 19.4 Mt @ 2.7 g/t Au, (McKibben et al., 2008)
South Enterprise - 1.24 Mt @ 2.6 g/t Au,
North Gandys Hill - 0.163 Mt @ 2.8 g/t Au,
International - 1.141 Mt @ 1.7 g/t Au,
Kohinoor - 0.26 Mt @ 2.6 g/t Au, plus production of 0.001 Mt @ 30.7 g/t Au.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1990.
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.
Enterprise
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Selected References:
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Cannard C J, Pease C F D 1990 - Enterprise 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 773-778
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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
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Rasmussen B, Sheppard S and Fletcher I R, 2006 - Testing ore deposit models using in situ U-Pb geochronology of hydrothermal monazite: Paleoproterozoic gold mineralization in northern Australia: in Geology v34 pp 77-80
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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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