Shumagin

Prospect, Active

Alternative names

Choumagin
Unga

Commodities and mineralogy

Main commodities Ag; Au
Other commodities As; Cu; Hg; Pb; Te; Zn
Ore minerals arsenopyrite; galena; gold; pyrite; sphalerite
Gangue minerals calcite; quartz

Geographic location

Quadrangle map, 1:250,000-scale PM
Quadrangle map, 1:63,360-scale A-2
Latitude 55.2253
Longitude -160.5737
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Shumagin prospect is a southwest-trending mineralized zone approximately 1.4 miles southwest of the seaplane anchorage at Squaw Harbor on Baralof Bay. It is about 0.4 mile southeast of the center of section 19, T. 57 S., R. 74 W. of the Seward Meridian. The location is near the northeast end of drilling that has been completed along the strike of mineralization and is accurate to within 500 feet.

Geologic setting

Geologic description

The host rock of the Shumagin deposit is Eocene to Oligocene (Popof) volcanic rocks (Wilson and others, 1995). The deposit is in the northeast section of the Aquila-Shumagin fault system, which strikes southwest across Unga Island.
The deposit consists of auriferous, sulfide-bearing quartz-calcite veins in a fault that trends N 60 E and dips 80 to 85 SE. The veins and fault can be traced approximately 4,000 feet along strike over a vertical distance of 765 feet. The mineralization is interpreted as a fault-controlled epithermal, volcanic-hosted vein deposit (White and Queen, 1989). The hanging wall is tuff and the footwall is andesite. Movement on the fault is probably high-angle reverse.
The deposit consists of four veins, two vein systems, and two fault breccias. Within the fault, the mapped units from oldest to youngest are pyrite-rich cataclastic, clast-supported fault breccia, the Union vein, and matrix-supported quartz breccia (White and Queen, 1989). The total thickness of these units averages about 40 feet. Veins that are peripheral to or cut the main vein-fault system include the Lucky Friday and Greenbaum veins, the Vuggy Watercourse vein, and several carbonate veins. Gold is most abundant in the Union vein, which generally is considered to be the principal gold deposit.
The Union vein is a composite vein consisting of a 10-foot-wide zone of closely-spaced quartz veins in the hanging wall tuff. Individual veins are sinuous, vuggy, and generally less than 30 inches wide. Native gold occurs, with finely crystalline arsenopyrite, pyrite, galena, and sphalerite. Precious-metal grades range from a trace in wide zones to more than 10 ounces of gold and 150 ounces of silver per ton in narrow zones. There is a strong correlation between gold, silver, and tellurium, indicating that some of the precious metals probably occur as tellurides (White and Queen, 1989). Singer (1999) suggests that this is a Sado-type gold deposit.
In 2014 after a detailed review of the existing drill core, Redstar Gold Corp. (Redstar) observed evidence of a phreatomagmatic (hydrothermal-magmatic) breccia system interpreted to be intimately associated with a nearby dacitic, hypabyssal dome mapped along Bloomer Ridge to the southeast. The phreatomagmatic breccia phases are interpreted to be pre- to syn-mineralization. A protracted multi-phase, anastomosing and bifurcating, precious metal-bearing quartz ± carbonate (rhodochrosite) + adularia hydrothermal system is part of, and is superimposed upon the breccia phases. The hydrothermal system either intensely replaced and/or re-brecciated the phreatomagmatic breccias evident by open space veins, vein breccias and stockwork zones exhibiting crustiform, cockade and banded textures. At depths of between 30 meters to 105 meters below sea level, the hydrothermal system (as well as the breccia bodies) widens and bifurcates exhibiting an increase in stockwork veining within the hanging wall to the breccia system. Above this level, the hydrothermal system narrows and has been telescoped up section along the footwall and along specific breccia phases (Redstar Gold Corp., 2014b).
Sulfide minerals when present occur as very fine-grained pyrite, sphalerite, galena, visible gold and blue-grey metallic sulfides locally within crustiform layers or in stockwork and crackle breccia systems. Pervasive phyllic alteration (quartz-sericite-pyrite), zones of variable silicification, quartz-adularia-carbonate stockworks and patchy disseminated pyrite occurs throughout the breccia and outbound into the hanging wall overprinting epiclastic sediments and dacite (Redstar Gold Corp., 2014b).
After drilling several holes in 2015, Redstar discovered an older, Ginguro-style (crustiform-colloform pyrite +/- marcasite) epithermal breccia vein system in the footwall below the younger Shumagin-style quartz veining. This new vein system is anomalous in gold and silver (1 to 4 grams per tonne gold) where Ginguro-sulfides are encountered (both veins and clasts) and exhibits contrasting epithermal geochemical signatures to Shumagin-style breccia veins with distinctly higher levels of arsenic, antimony, and mercury (Redstar Gold Corp., 2015a).
Geologic map unit (-160.575735728269, 55.2244709367856)
Mineral deposit model Sado-type epithermal gold vein (Cox and Singer, 1986; model 25d).
Mineral deposit model number 25d
Age of mineralization Eocene or younger, based on the age of the host rock (Wilson and others, 1995).
Alteration of deposit Strong argillic alteration is present as much as 148 feet from the Shumagin deposit. Quartz-sericite-pyrite alteration extends outward from Union vein and the matrix-supported quartz breccia (White and Queen, 1989). Zones of variable silicification, quartz-adularia-carbonate stockworks and patchy disseminated pyrite occurs throughout the breccia and outbound into the hanging wall overprinting epiclastic sediments and dacite (Redstar Gold Corp., 2014b).

Production and reserves

Workings or exploration
Atwood (1909) first described the prospect, whose workings at that time consisted of two tunnels 150 feet apart vertically. The lower one was 365 feet in length with a 75-foot crosscut. The upper tunnel was 79 feet in length with a 169-foot crosscut.
From 1983 to 1987, Alaska Apollo Gold Mines diamond drilled 23 holes totaling 9,269 feet, drilled 44 percussion holes, and cut 13 trenches through the vein and fault. At that time the deposit was estimated to contain 300,000 tons of ore grading 0.5 ounce of gold per ton. In 1989 Ballatar Exploration core-drilled approximately 6,000 feet.
In 1986 Battle Mountain Exploration Company mapped and sampled the deposit and relogged the core. The data indicated that an ore shoot approximately 400 feet long, 5 to 30 feet thick, plunging 45 degrees to the northeast in the plane of the fault (Ellis and Apel, 1990). A deep hole drilled by Alaska Apollo Gold Mines 600 feet down the dip of the vein intersected 27 feet of rock that averaged 0.46 ounce of gold per ton. The deposit was estimated by Battle Mountain to contain 352,000 tons of ore with an average grade of 0.299 ounce of gold per ton. In 1990 Battle Mountain Exploration Company drilled a 1,022-foot core hole collared on Aleut Corporation land that was inclined to hit the deposit at depth. At a depth of 900 feet, the hole cut 18 feet of rock that averaged 0.47 ounce of gold per ton.
In 1987 during road building, a 500-foot-long, faulted eastern extension of the Shumagin vein was uncovered, mostly on Aleut Corporation land. Battle Mountain Exploration Company mapped and sampled this section of vein. It was found to be a large, weakly mineralized, south-dipping veined structure, locally as much as 50 feet thick. Samples contained up to 680 parts per billion (ppb) gold, 12.1 parts per million (ppm) silver, 234 ppm arsenic and 2,000 ppb mercury (Ellis and Apel, 1990).
In 2011, Redstar Gold Corp. (Redstar) (2012a; 2012b) began work in the area in a joint venture with Full Metal Minerals Inc. on land they were purchasing and on land leased from the Aleut Native Corporation. They drilled 10 holes and did extensive surface sampling. Some notable intercepts in the holes were: 1) 21 meters that averaged 4.02 grams of gold per tonne, this included a 1 meter interval with 43.90 grams of gold per tonne; and 2) 30 meters that averaged 14.98 grams of gold per tonne, this included 0.55 meter with 738 grams of gold per tonne.
In 2014 Redstar completed a soil sample grid of approximately 750 m by 200 m at Shumagin to test the western extension of known surface and sub-surface mineralization along inferred structures, as documented by historic geological maps and trench data. Results from the survey showed a gold and silver geochemical soil signature that averaged 20 to 50 parts per billion (ppb) gold with up to 615 ppb gold and averaged over 0.5 grams per tonne (g/t) silver with up to 6.8 g/t silver over areas of historical trenching. This anomaly has been extended to the southwest by approximately 400 meters, where anomalous values occur along the southwestern flank of a low lying ridge that abuts against an active, north draining flood plain (Redstar Gold Corp., 2014a). Rock chip sampling collected from quartz vein breccias and stockwork found in surface outcrops produced results of up to 9.93 g/t gold and 74.4 g/t silver (Redstar Gold Corp., 2014b).
Redstar drilled eight holes along the Shumagin vein system in May 2015. The first four drill holes were infill holes across an area of historic drilling used in a non-43-101 compliant resource estimation. The last four drill holes target high-grade mineralization down plunge and along strike of the known Shumagin vein system. These drill holes revealed an older, Ginguro-sulfide textured epithermal breccia vein that was not destroyed by late-stage Shumagin-style vein breccias. Notable intercepts from this drilling were: 1) 1.9 meters that averaged 202 g/t gold and 82 g/t silver; 2) 6.35 meters that averaged 9.45 g/t gold and 103 g/t silver, including 3.0 meters with 16.95 g/t gold and 183 g/t silver; 3) 4 meters that averaged 11.62 g/t gold and 95.6 g/t silver, including 1.0 meter with 20.90 g/t gold and 232 g/t silver; and 4) 5 meters that averaged 9.35 g/t gold and 27.62 g/t silver, including 1.0 meter with 41.2 g/t gold and 130 g/t silver (Redstar Gold Corp., 2015a).
Redstar Gold Corp. conducted two phases of exploration in 2016 on their Unga project, which includes the Shumagin (PM064), Apollo (PM079; PM084), Centennial, Orange Mountain (PM067), Zachary Bay (PM054), Amethyst (PM092), and Aquila (PM086) epithermal gold-silver prospects. The Shumagin trend parallels the historically mined Apollo–Sitka vein system (PM079; PM084; PM076), which between 1886 and 1922 produced approximately 150,000 ounces of gold at a grade of approximately 0.292 ounce of gold per ton (Athey and Werdon, 2017).
In June 2016, Redstar Gold Corp.’s Phase I surface program included classifying alteration assemblages and conducting other geologic work to identify drill targets. From October through November, Phase II’s drilling program tested the down-dip and along-strike expansion potential of high-grade vein/breccia mineralization within the Shumigan Gold Zone (PM064). A total of 1,505 meters were drilled in 7 holes spaced over ~750 meters of strike length. All drill holes intersected the target structure, which includes multi-generational phreatomagmatic breccias, hydrothermal breccias, and late breccias and veins with colloform-crustiform- to cockade-textured quartz-adularia-carbonate (± rhodochrosite, ± green clay). Select intervals from drill holes 16SH019 and 16SH020 include 0.9 meter at 14.95 grams of gold per tonne and 1.15 meters at 11.3 grams of gold per tonne; release of full assay results is planned for late January 2017 (Athey and Werdon, 2017).
Indication of production None
Reserve estimates A resource of 300,000 tons grading 0.5 ounce of gold per ton was estimated from the exploration work done by Alaska Apollo Gold Mines in 1983 to 1987. In 1986, Battle Mountain Exploration Company estimated that the deposit contained 352,000 tons of material that averages 0.299 ounce of gold per ton. Queen (1988) estimated a resource of 270,000 tons that averages 0.49 ounce of gold per ton and 1.97 ounces of silver per ton.

Additional comments

The Shumagin prospect lies on state mining claims however the surrounding surface rights belong to the Unga Corporation and the subsurface rights belong to the Aleut Corporation (Redstar Gold Corp., 2015b).

References