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Pogo - Liese Zone Vein System |
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Alaska, USA |
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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 Pogo gold deposit, which comprises the Liese Zone intrusive related, shear hosted gold-quartz vein system within the regional Tintina Gold Belt is located 145 km SE of Fairbanks in eastern Alaska, USA (#Location: 64° 27' 28"N, 144° 54' 42"W).
The Tintina Gold Belt lies within the 1000 x 250 km Tintina-Tanana Terrane which extends from northern British Columbia in Canada, through the Yukon Territory of Canada to the south-west coast of Alaska in the USA, bounded to the north and south respectively by the regional Tintina and Denali Faults. The Tintina Gold Belt occupies much of this terrane. The terrane is composed of greenschist facies to high grade (amphibolite facies) gniesses derived from late Neoproterozoic to mid Palaeozoic (Siluro-Devonian) submarine arc and continental margin rocks deposited on continental basement rocks. Composite sub-terranes are made up of structurally stacked thrust slices. Granitoid emplacement into volcano-sedimentary suites in the Devonian to lower Carboniferous was followed by metamorphism in the early Cretaceous. Widespread granitoid intrusion which followed in the mid Cretaceous (107 to 89 Ma) was subsequently succeeded by extensive Tertiary intrusive activity. Important gold mineralisation in the belt appears to be related to both of these intrusive phases.
The country rocks in the Pogo deposit area include biotite feldspar gneiss, augen gneiss, mafic schist with metamorphic assemblages of quartz, feldspar, biotite, chlorite, muscovite, sillimanite, andalusite and garnet, intruded by small granitoid stocks and dykes, as well as the 50 km long Goodpaster Batholith. Ages of intrusions, which include granite, granodiorite, quartz-diorite and diorite, vary from 107 to 93 Ma. One age date from the metamorphic sequence was 128 Ma.
The economically significant gold mineralisation at Pogo comprises numerous massive auriferous quartz ±sulphide veins hosted in a sequence of amphibolite-grade, paragneiss and orthogneiss with lesser calc-silicate and serpentinite of Proterozoic to mid-Palaeozoic age. The first deformation, D1, was accompanied by Devonian to Early Carboniferous arc plutonism at 351 ±3 Ma. D2, from the Late Permian to Cretaceous accompanied WNW vergent convergence, with peak metamorphism at ~115 Ma. D3 produced Cretaceous brittle-ductile deformation, with plutonism introducing intrusive suites at 110, 104 and 95 Ma, accompanied by gold mineralisation. These Mid- to Late-Cretaceous age granitic plutons and dykes include, from oldest to youngest, two-mica granite, biotite granite, granodiorite and altered pegmatite dykes, followed by hornblende-biotite tonalite, biotite granodiorite and late mineral diorite. These granitoids intrude the gneisses, and are, in turn, cut by the mineralised veins. D4 brittle faulting was from the Paleocene to the present (Rogers, 2020 presentation).
The age of the gold mineralisation is post metamorphic at ~105 to 95 Ma in the late Lower Cretaceous and is controlled by a low angle regional shear and high angle faults. These structures are interpreted to be the hydrothermal fluid conduits. The region has undergone regional metamorphism that peaked at 110 Ma, resulting in moderate folding and foliation of the metamorphic host rocks. Cooling during retrograde metamorphism transformed the ductile deformation of the metamorphic fabric to semi-ductile to brittle, producing a low-angle shear across the region (Northern Star web page, 2021).
Gold in the quartz veins is characterised as 'Pogo Type' mineralisation and occurs as inclusions in arsenopyrite, inclusions in quartz, composite intergrowths with Bi-Te±S minerals in quartz and invisible Au (<1µm) inclusions to solid-solution atomically bonded in loellingite, coupled with 'low arsenic' arsenopyrite (Northern Star web page, 2021).
The initial resource was calculated from the Liese Zone where there are two gently dipping, sub-parallel, massive quartz veins, each of up to 30 m in thickness (generally 4 to 12 m), known as L1 and L2, while a third, L3 was indicated at depth. These veins are within mylonitic shear zones associated with thrusting, and are locally accompanied by sheeted, en echelon or stockwork developments. The princpal hosts are biotite-quartz-feldspar paragneiss, although all other lithologies are cut. Where the veins cross intrusives they tend to split and become stockwork zones. Each vein is made up of 0.5 to 5 cm thick bands, parallel to the vein margins, composed of quartz with pyrite-arsenopyrite ±pyrrhotite ±chalcopyrite, cut by dolomite-sericite and Fe-Mg carbonate. Approximately 96% of the gold is free. The veins contain around 3% sulphides (arsenopyrite, pyrite, pyrrhotite, loellingite, chalcopyrite, bismithunite, sphalerite, galena, molybdenite, tetradymite, maldonite and a variety of Bi-Pb-Ag sulphosalts (Roberts et al., 2001).
In 1976 a USGS regional sampling survey reported As-Bi anomalism along Goodpaster River. In 1981, WGM Inc., funded by Phillips Petroleum, conducted a stream sediment and panned concentrate sampling program in the Goodpaster River area. This program identified Pogo Creek and to a lesser extent Liese Creek as anomalous in Au, As and W. Follow-up confirmed this anomaly and identified mineralised quartz float. No further work was conducted until 1991 when the Sumitomo Metal Mining Co. led Stoneboy joint venture conducted a soil sampling program on Pogo Ridge, with follow up drilling intersecting 7.6 m @ 28.4 g/t and 5.2 m @ 9.8 g/t Au in two holes. The Liese Zone ore deposits discovered in 1995. In 1997, Sumitomo Metal Mining Co., Ltd and Teck Resources Inc. signed a Joint Venture agreement whereby Teck could earn a 40% interest in the property and assumed operator status from 1998. The first gold from the deposit was produced in February, 2006. In mid-2009, Sumitomo Metal Mining Co., Ltd bought Teck’s 40% share after Teck divested its global gold assets. Sumitomo then became the operator of the Pogo Mine. In mid-2018, Northern Star purchased the Pogo Mine, becoming the sole owner and operator in September 2018. Total production to June 2020 was 130 t of gold.
The indicated+inferred resource in 2001 (Roberts et al., 2001) was:
8.964 Mt @ 18.86 g/t Au for 169 tonnes of gold.
Reserve and resource figures in 2003 were (source: TeckCominco web site):
Probable reserve - 7 Mt @ 16.2 g/t Au.
Indicated resource - 0.77 Mt @ 8.92 g/t Au.
Inferred resource - 1.23 Mt @ 16.9 g/t Au.
Remaining reserves in 2012 (Sumitomo Metal Mining Company, 2012) were:
12.332 Mt @ 12.6 g/t Au for 155 tonnes of gold
(following the discovery in 2011 of the "East Deep" or E1 deposit with 40 t of contained Au, 300 m NE of the Liese Zone).
Remaining Mineral Resources and Ore reserves at 31 March 2021 (Northern Star Resources, Resource and Reserve Satement, 2021) were:
Indicated + Inferred resource - 22.897 Mt @ 9.4 g/t Au for 215 t of contained gold.
Probable reserve - 5.852 Mt @ 8 g/t Au for 46.6 t of contained gold (included in resources).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2003.
Record last updated: 1/8/2013
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.
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Selected References:
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Baker, T., Ebert, S., Rombach, C. and Ryan, C.G., 2006 - Chemical Compositions of Fluid Inclusions in Intrusion-Related Gold Systems, Alaska and Yukon, Using PIXE Microanalysis: in Econ. Geol. v.101, pp. 311-327.
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Rhys, D., DiMarchi, J., Smith, M., et al., 2003 - Structural setting, style and timing of vein-hosted gold mineralization at the Pogo deposit, east central Alaska: in Mineralium Deposita v.38, pp. 863-875.
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Roberts, P.S., Smith, M.T., Thompson, J.F.H. and Bressler, J.R., 2001 - The Pogo gold deposit - a discovery case history: in Yates K (Ed.), 2001 NewGenGold 2001, Conference, Perth, Western Australia, 26-27 November 2001, AMF, Adelaide, Proceedings volume, pp. 161-170.
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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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