Pogo - Liese Zone Vein System

Alaska, USA

Main commodities: Au
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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 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 (metamorphic assemblages of quartz, feldspar, biotite, chlorite, muscovite, sillimanite, andalusite and garnet), 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 on the project occurs in two gently dipping, sub-parallel, massive quartz veins, each of up to 30 m in thickness (generally 4 to 12 m), L1 and L2 of the Liese Zone, while a third, L3 has been 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.

Exploration activity in the area commenced in 1991, with the Liese Zone ore deposits discovered in 1995. The first gold from the deposit was produced in February, 2006.

The indicated+inferred resource in 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).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary 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.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
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.   v101 pp 311-327
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   v38 pp 863-875
Roberts P S, Smith M T, Thompson J F H, Bressler J R  2001 - The Pogo gold deposit - a discovery case history: in Yates K (Ed.), 2001 NewGenGold 2001, Conference Proceedings AMF, Adelaide    pp 161-170

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