Workman's Bench

Prospect, Active

Alternative names

Wannemaker and Wortman

Commodities and mineralogy

Main commodities Au; Sb
Other commodities As; Hg; Pb
Ore minerals arsenopyrite; gold; stibnite
Gangue minerals ankerite; calcite; dolomite; quartz

Geographic location

Quadrangle map, 1:250,000-scale WI
Quadrangle map, 1:63,360-scale B-1
Latitude 67.4689
Longitude -150.2241
Nearby scientific data Find additional scientific data near this location
Location and accuracy This center of the Workman's Bench prospect is about 0.2 mile east of the junction of Smith Creek and Nolan Creek at the mine symbol on the Wiseman B-1 topographic map. The prospect is about 0.3 mile north of the center of section 33, T. 31 N., R. 12 W. The location is accurate.

Geologic setting

Geologic description

Before 1999, the Workman's Bench prospect was described as a 3-to-4-inch-wide quartz-stibnite vein exposed in a trench on the south side of Smith Creek. The stibnite fills open spaces between quartz crystals with euhedral terminations deposited along the margins of the vein (Ebbley and Wright, 1948). Mulligan (1974) noted that the vein occupies a vertical fissure that cuts flat-lying, blue-gray phyllite. Selected samples from this prospect contain as much as 12.2 parts per million gold, are anomalous in arsenic (Eden, 2000) and contain up to 58.3 percent antimony (Mulligan, 1974; Eden, 2000). Eden describes auriferous quartz veins containing stibnite and arsenopyrite elsewhere in the general area. Calcite, ankerite and minor dolomite accompany the quartz at the margins of these veins, which strike N44E and dip nearly vertically. The gold is mostly near the margins of the veins and rarely in the stibnite (Eden, 2000).
In 1999, the U.S. Bureau of Land Management began work in the area that included soil sampling and an airborne geophysical survey (Kurtak and others, 2002). Their data encouraged Silverado Gold Mines, Ltd., who was placer mining along Nolan Creek, to more aggressively search for the lode source of the placer gold (Bundtzen, 2008, 2009). They concentrated on Workman's Bench and on Pringle Bench (WI114), about 1500 feet to the northeast where auriferous stibnite also known occurs. From 1994 to 2008 in these two areas, Silverado drilled 15 reverse-circulation holes that totaled 3,230 feet and 58 shallow core holes that totaled 16,152 feet, dug numerous trenches, and drove 570 feet of underground workings on the Workman's Bench prospect that intersected the four stibnite or quartz-stibnite veins in two places along a northeast-trending structure.
The host rocks are Devonian phyllite, schist, and metasiltstone (Dillon and Reifenstuhl, 1990). The gold-stibnite mineralization occurs as distinct veins, four are recognized, parallel to and along a structural zone 120 to 350 feet wide that strikes northeast and dips 50 to 85 degrees southeast (Bundtzen, 2008 and 2009). Both stibnite (only) and quartz-stibnite veins are present; minor arsenopyrite occurs in them. At the Workman's Bench prospect, there are veins of massive stibnite up 15 inches thick that contain up to 2.86 ounces of gold per ton. Four, parallel veins across a distance of about 100 feet and a strike length of about 1,100 feet were cut in the 41 drill holes and in the underground workings of Workman's Bench; there are also numerous gold-stibnite(-quartz) veinlets between the major veins. Coincident with the work on Workman's Bench, Silverado was also working on similar mineralization at the Pringle Bench (WI114). The mineralization between Workman's Bench and Pringle Bench could not be visually followed other than in the drill core and cuttings but the attitudes and characteristics of the mineralization at the two sites demonstrate that the mineralization is continuous over a distance of at least 1,600 feet between the two prospects. Soil geochemistry and geophysical data suggests that the mineralization continues even further northeast.
Bundtzen, (2009) estimated that Workman's Bench has: 1) a 'probable reserve' of 42,412 tons with an average grade of 28 percent antimony and 0.408 ounce of gold per ton (at a cut-off grade of 4.0 percent antimony); and 2) an 'inferred resource' of 21,389 tons with an average grade of 12.11 percent antimony and 0.272 ounce of gold per ton (at a cut-off grade of 4.0 percent antimony.
Geologic map unit (-150.226848943337, 67.4684734779273)
Mineral deposit model Sb-Au vein (Nokleberg and others, 1987).
Age of mineralization The only constraint is the Devonian age of the host rocks.
Alteration of deposit No alteration is mentioned.

Production and reserves

Workings or exploration Before 1999, the Workman's Bench prospect was apparently only sampled by government and industry. In 1999, the U.S. Bureau of Land Management began work in the area that included soil sampling and an airborne geophysical survey (Kurtak and others, 2002). Their data encouraged Silverado Gold Mines, Ltd., who was placer mining along Nolan Creek, to more aggressively search for the lode source of the placer gold (Bundtzen, 2008, 2009). They concentrated on Workman's Bench and on Pringle Bench (WI114), about 1500 feet to the northeast where auriferous stibnite also known occurs. From 1994 to 2008 in these two areas, Silverado drilled 15 reverse-circulation holes that totaled 3,230 feet and 58 shallow core holes that totaled 16,152 feet, dug numerous trenches, and drove 570 feet of underground workings on the Workman's Bench prospect that intersected the four stibnite or quartz-stibnite veins in two places along a northeast-trending structure.
Indication of production None
Reserve estimates Bundtzen, (2009) estimated that Workman's Bench has: 1) a 'probable reserve' of 42,412 tons with an average grade of 28 percent antimony and 0.408 ounce of gold per ton (at a cut-off grade of 4.0 percent antimony); and 2) an 'inferred resource' of 21,389 tons with an average grade of 12.11 percent antimony and 0.272 ounce of gold per ton (at a cut-off grade of 4.0 percent antimony.
Production notes Probably none but see the nearby Pringle Bench (WI114).

References

MRDS Number A011941

References

Kurtak, J.M., Klieforth, R.F., Clark, J.M., and Maclean, E.A., 2002, Mineral investigations in the Koyukuk mining district, northern Alaska: Final Report: U.S. Bureau of Land Management Technical Report 50, v. 1 and 2, 845 p.
Reporters J.M. Britton (Anchorage); D.J. Grybeck (Contractor, U.S. Geological Survey)
Last report date 4/2/2010