Pogo, Liese Creek Prospects, Goodpaster Mining District, Southeast Fairbanks Census Area, Alaska, USAi

Regional Level Types
Pogo	Group of Mines (Active)
Liese Creek Prospects	- not defined -
Goodpaster Mining District	Mining District
Southeast Fairbanks Census Area	Census Area
Alaska	State
USA	Country

The Pogo project is a joint venture of Teck Corporation, Sumitomo Metal Mining, and Sumitomo Corporation. The Teck Corporation is the principal operator of the project and can earn a 40% interest in the property.

Location: The center of the Liese zone of the Pogo Mine that was the focus of mining as of early 2012 is just east of the Goodpaster River about 1.7 miles south of the mouth of Indian Creek. It is near the center of section 26, T. 5 S., R. 14 E, of the Fairbanks Meridian. Lamier and Puchlik (2011) have an image looking down on the mine and the zones of mineralization.

Geology: The rocks in the area are high grade gneiss intruded by Cretaceous granitic bodies. The area is cut by prominent northwest-trending high-angle faults but there are other high-angle faults with various orientations. To the north, the region is intruded by the Cretaceous Goodpaster batholith. At the Pogo deposit the host rock is predominantly amphibolite-grade biotite-quartz-feldspar gneiss. In the Liese Creek drainage, a series of granodiorite dikes intrude the gneiss. These dikes are interpreted to be apophyses of the batholith. The youngest geologic unit in the area is a northwest-trending, steeply dipping, diorite dike situated in Liese Creek. This dike partly cuts off mineralization on the northeast edge of the deposit (Teck Resources Inc., 1998).

Geochronology studies have focused on the granodiorite and diorite dikes in the Liese Creek drainage. The diorite dike has an 40Ar/39Ar biotite age of 94 Ma, and a U-Pb zircon age of 94 Ma. The granodiorite dike has a 40Ar/39Ar biotite age of 91.7 Ma, a 40Ar/39Ar white mica age of 91.2 Ma, and a U-Pb monazite age of 107 Ma (M. Smith, oral communication, 1999). Lamier and Puchlik (2011) suggest similarities of the Pogo deposit to other intrusion-related gold deposits in interior Alaska, e.g., the Fort Knox Mine (ARDF FB112). The mineral assemblages suggest deposition from rapidly-changing pulses of magmatic fluids rather than from a metamorphic source.
As of early 2012, mining is centered on the Liese zone that consists of at least three flat-lying, subparallel, stacked tabular bodies of massive quartz, he L1, L2, and L3. The Pogo deposit is marked by a flexure point with an almost sinusoidal wave shape. The west portion of the deposit strikes northeast and dips northwest about 30 degrees. The east portion of the deposit strikes east and dips north about 30 degrees. The upper Liese zone (L1) is the largest and shallowest; it is at least 4,000 by 2,000 feet in area and varies from 0 to 65 feet thick. The lower Liese zone (L2) lies 300 to 500 feet below the L1. It is generally thinner, but higher grade than the L1. The L3 zone is approximately 800 feet below L1 (Smith, 1999). Although there are no mappable thrust faults, the Liese zones intersect the foliation of the host rock at an angle of about 5 degrees and apparently mark low angle structures.

The mineralized layers in the Liese zone are predominantly quartz with about 3 percent sulfides. The ore minerals in the quartz include arsenopyrite, bismuth, bismuthinite, chalcopyrite, gold, loellingite, maldonite, pyrite, pyrrhotite, sphalerite, and tetradymite (Smith, 1999). The gold occurs as 1 to 25 micron grains in arsenopyrite along fractures, and as inclusions in bismuth, tetradymite, and other gold-lead-bismuth-tellurium minerals. Geochemical data suggest a strong correlation between gold and bismuth, and a weaker correlation between gold and other lithophile elements (Smith, 1999).
The Liese zones are all display vein and replacement textures. The veining textures are characterized by two styles of quartz veins and alteration assemblages. The early veins are characterized by white quartz with arsenopyrite, chalcopyrite, pyrite, pyrrhotite and loellingite, and secondary biotite in selvages up to 1 meter wide (Smith, 1998). The later veins are characterized by gray quartz in stockwork veins and replacement selvages. They contain arsenopyrite and pyrite, with secondary disseminated sericite and dolomite that often overprints earlier secondary biotite. Some silica flooding is observed in the gneiss and intrusive rocks (M. Smith, oral communication, 1999).

Work in the late 2000s and into 2011 identified three other areas of mineralization peripheral to the Liese zone (Lamier and Puchlik (2011). A North zone consists of three, steeply dipping quartz veins that strike northwest. they intersect the Liese zone and radiate out from it; they may be the feeder to it. The East Deep zone about 1,000 feet northeast of the Liese zone was the main focus of drilling in 2011. A 95 Ma post-mineral dike separates it from the Liese zone which it resembles in orientation and mineral assemblages. The 4021 target is about 2.5 miles southeast of the Liese Zone. It consists of two, stacked, shallow-dipping quartz veins in granitic gneiss. It possibly is a distal extension of the Liese Zone.

Workings: The Goodpaster region was first explored for placer gold in 1915. Thomas (1970) reports a stampede of prospectors that quickly ended when little gold was found. In 1991, a regional stream sediment sampling program identified gold, arsenic, and tungsten anomalies in Liese Creek and Pogo Creek. From 1991 to 1994, exploration consisted of soil sampling, minor prospecting, and geophysics. The Pogo deposit underlies a 1-square-mile area that contain more than 100 parts per billion (ppb) gold in soils. Three core drill holes were completed on Liese Creek in 1994; 13 additional core holes were drilled in 1995. These holes identified the Liese L1 zone. Work in 1996 consisted of 22 drill holes that further defined the zone. From 1991 to 1996, the exploration program drilled a total of 36,703 feet of core with 4,142 core samples and 3,520 geochemical samples. An additional 41 drill holes in 1997 enlarged the deposit and found the lower Liese L2 zone. Drilling in 1998 focused on better defining the L1 and L2 and the Liese L3 zone that was first identified in 1991. As of 1999, a total of 91,263 feet of drilling had been completed and 3,404 core samples and 1,500 geochemical samples were collected (M. Smith, oral communication, 1999). Extensive exploration continued through 2011 when three new ore bodies outside the Liese zone were announced (Lamier and Puchlik, 2011).

All the permits for the mine were received by the middle of 2004 (Szumigala and Hughes, 2005). By January 2006, the deposit had been fully developed with a large permanent camp, ore processing facilities, water treatment plant, power transmission lines, provisions for tailings disposal, and underground workings (Szumigala and Hughes, 2007). The first ore arrived at the mill on January 12, 2006 and the first gold pour was on February 12, 2006. Until 2009, the Pogo project was a joint venture of the Teck Resources Corporation, Sumitomo Metal Mining, and Sumitomo Corporation, with Teck as the operator. In July, 2009, Teck sold their interest in the mine and it is now fully owned by Sumitomo (Szumigala and others, 2011).

In 2012, surface exploration included four surface drills. Underground drilling included four drills with over 100,000 feet drilled as of late 2012. The areas of focus were definition drilling of Liese Zone, and East Deep, the latter of which also had underground exploration drilling performed. Geotechical and hydrological drilling programs were also undertaken. Twenty more prospects were identified by gold geochemical anomalies, geophysical data, and quartz vein outcrops (Puchlik and others, 2012).
To ensure continued mining into the future, Sumitomo Metal Mining Pogo LLC invested $10 million in exploration at Pogo mine in 2016, following $15 million in 2015. At least five high-grade gold zones (Liese, East Deep, North, Fun, and South Pogo) have been discovered within 1 mile of the mill; these zones are currently contributing ore to the mill, or are expected to in the near future (Athey and Werdon, 2017)..

Age: Mineralization at the Pogo deposit is thought to be plutonic-related. Geochronology studies have focused on the granodiorite and diorite dikes in the Liese Creek drainage. A diorite dike has an 40Ar/39Ar biotite age of 94 Ma, and a U-Pb zircon age of 94 Ma. A granodiorite dike has an 40Ar/39Ar biotite age of 91.7 Ma, an 40Ar/39Ar white mica age of 91.2 Ma, and a U-Pb monazite age of 107 Ma. (M. Smith, oral communication, 1999).

Alteration: The Liese Zones all are associated with vein and replacement type textures. The veining textures are characterized by two styles of quartz veins and alteration assemblages. The early veins are typified by white quartz containing arsenopyrite, chalcopyrite, pyrite, pyrrhotite and loellingite with secondary biotite in selvages up to 1 meter in width. The later veins are typified by gray quartz as stockwork veins and replacement selvages containing arsenopyrite and pyrite, with secondary disseminated sericite and dolomite. It is common to find the sericite-dolomite alteration overprinted upon the earlier secondary biotite. Some silica flooding is observed in the gneiss and intrusive (M. Smith, oral communication, 1999).

Reserve Estimates: As of December 31, 2011, the Pogo Mine has a total reserve and resource of 13.594 million tons with an average grade of 0.366 ounces of gold per ton (or 4.973 million ounces of gold) (Sumitomo Metal Mining Co., Ltd., 2012). This total included 1.283 million ounces of gold in the East Deep target that was identified in 2011.

Production: In 2010, the Pogo Mine mined 900,585 tons of ore from which 383,434 ounces of gold was produced with a recovery rate of 89.6 percent (Szumigala and others, 2011). The total production of the Pogo Mine from 2006 when mining began to 2010 was 1,483,645 ounces of gold.
In 2011, the Pogo Mine produced 385,000 ounces of gold (Puchlik and others, 2012).

Pogo mine celebrated its 10th anniversary of production in 2016, producing 269,342 ounces of gold from 1,515,117 tons of ore and waste materials mined; 941,856 tons were milled, and 12,812,069 cubic feet of paste fill was placed back in the underground workings (Athey and Werdon, 2017).

Commodities (Major) - Au; (Minor) - Ag, As, Bi, Cu, Mo, Pb, Te, Zn
Development Status: Yes; large
Deposit Model: Low-sulfide Au-quartz veins? (Cox and Singer, 1986; model 36a).

Other Databases

Link to USGS - Alaska:	BD033

Other Regions, Features and Areas containing this locality

References