Larsen Ridge

Prospect, Probably inactive

Alternative names

Lucky 13

Commodities and mineralogy

Main commodities Cu; Mo; W
Other commodities Ag; Pb; Zn
Ore minerals chalcopyrite; galena; magnetite; molybdenite; powellite; pyrite; pyrrhotite; scheelite; sphalerite
Gangue minerals amphibole; calc-silicates; epidote; garnet; pyroxene; quartz

Geographic location

Quadrangle map, 1:250,000-scale EA
Quadrangle map, 1:63,360-scale A-6
Latitude 64.2396
Longitude -143.8916
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Larsen Ridge prospect is located on a northeast-trending ridge about 1.5 miles west of Mount Harper, in the headwaters of the Healy River. Larsen Ridge is not labeled on the U.S. Geological Survey topographic map of the Eagle A-6 quadrangle (1956). VABM Good is on a 6,077-foot-high peak at the southwest end of Larsen Ridge. Several skarns have been identified at the Larsen Ridge prospect; the largest is the Lucky 13 skarn, which is the location of the coordinates, in the NW1/4 section 12, T. 8 S., R. 19 E., of the Fairbanks Meridian; the location is accurate. The Mount Harper area is locality 6 of Cobb (1972 [MF-393]). The Larsen Ridge prospect is located within Doyon, Ltd. selected or conveyed land.

Geologic setting

Geologic description

The Larsen Ridge prospect is in Paleozoic amphibolite-facies biotite gneiss, amphibolite, and quartzite (Foster, 1976). These rocks occur as a large roof pendant over a Cretaceous igneous body informally named the Mount Harper batholith. On Larsen Ridge, tungsten skarns are spatially associated with tungsten- and molybdenum-bearing intrusions (Dashevsky and others, 1986). The predominant intrusive rock in the Larsen Ridge area is quartz monzonite (U.S. Bureau of Mines, 1995). Biotite from the Mount Harper granite gives a 40Ar/39Ar plateau age of 105.8 +/- 0.4 Ma (Newberry and others, 1998). Biotite from a granitic body on Larsen Ridge gives a 40Ar/39Ar plateau age of 94.2 +/- 0.3 Ma; the location of the Lucky 13 skarn within 100 m of this granitic body suggests that it formed at approximately the same time (Newberry and others, 1996).
Tungsten-bearing garnet-, pyroxene-, and epidote-bearing skarns are found in irregular and discontinuous surface exposures over a 460 m by 1,220 m area on Larsen Ridge (Huskey, 1981). Several sulfide-bearing skarns have been identified on Larsen Ridge, including Lucky 13, Middle Tactite, Rubble Ridge, and Amphibolite Band (Dashevsky and others, 1986). The largest skarn is Lucky 13; it is 90 meters long by 3 meters wide, as much as 7.6 meters thick, and composed of mixed biotite-amphibolite and calc-silicate skarn (Huskey, 1981). The Larsen Ridge skarns contain scheelite with accessory pyrite, pyrrhotite, magnetite, and chalcopyrite and minor molybdenite, powellite, galena, and sphalerite. WGM Inc. estimates that there are approximately 20,000 tons of material that averages 1.5 percent WO3 exposed at the surface in the Larsen Ridge area; local high-grade areas contain as much as 4.2 percent WO3 and 2.2 percent copper. There are also small discontinuous exposures of sulfide-bearing amphibolite in the Larsen Ridge area that average less than 0.25 percent WO3. Two holes were drilled at the prospect, but they did not reach target depth (WGM Inc., 1998 [DLR 98-10]). There are several other molybdenum-tungsten prospects in the Mount Harper area, including VABM Good (EA092), Quartz-Porphyry (EA094), Airplane Ridge (EA093), and Section 21 (EA095).
Molybdenite in a quartz vein on the southern slope of Mount Harper was first reported by Chapin (1919). Anomalous tungsten, copper, and molybdenum were found during a stream-sediment reconnaissance in the Mount Harper area by WGM Inc. in 1975; this work was followed by geologic mapping and rock sampling in 1976 (Dashevsky and others, 1986). The Larsen Ridge prospect was discovered in 1977. In 1979, Union Carbide Corp. sampled rocks at the Larsen Ridge prospect and flew an airborne magnetic survey of the area. Two core holes drilled by Union Carbide Corp. in 1981 were aborted early due to adverse ground conditions and mechanical failures.
Geologic map unit (-143.893919890171, 64.2392423841283)
Mineral deposit model W skarn (Cox and Singer, 1986; model 14a).
Mineral deposit model number 14a
Age of mineralization Dating by 40Ar/39Ar of biotite from a granitic body on Larsen Ridge gives a plateau age of 94.2 +/- 0.3 Ma; the location of the Lucky 13 skarn within 100 meters of this granitic body suggests that it formed at approximately the same time (Newberry and others, 1996).

Production and reserves

Workings or exploration Molybdenite in a quartz vein on the southern slope of Mount Harper was first reported by Chapin (1919). Anomalous tungsten, copper, and molybdenum were found during a stream-sediment reconnaissance in the Mt. Harper area by WGM Inc. in 1975; this work was followed by geologic mapping and rock sampling in 1976 (Dashevsky and others, 1986). The Larsen Ridge prospect was discovered in 1977. In 1979, Union Carbide Corp. sampled rocks at the Larsen Ridge prospect and flew an airborne magnetic survey of the area. Two core holes drilled by Union Carbide Corp. in 1981 were aborted early due to adverse ground conditions and mechanical failures.
Indication of production None
Reserve estimates WGM Inc. estimates that there are approximately 20,000 tons of material with an average grade of 1.5 percent WO3 exposed at the surface in the Larsen Ridge area; local high-grade areas contain as much as 4.2 percent WO3 and 2.2 percent copper.

Additional comments

The Larsen Ridge prospect is located within Doyon, Ltd. selected or conveyed land. For more information contact Doyon, Ltd., Fairbanks, Alaska.

References

References

Dashevsky, S.S., Nicol, D.L., and Bond, J., 1986, Mines, prospects, and geochemical anomalies on Doyon Limited regional overselection lands, Alaska, Blocks 1-8: Doyon, Ltd. Report 86-01a, 300 p. (Report held by Doyon, Ltd., Fairbanks, Alaska).
DiMarchi, J.J., Weglarz, T.B., Dickey, C.F., Laux, D.P., Smith, M.T., Toro, J., and Kurtz, J.P., 1991, Central Alaska Gold Co. 1990 annual report, Alaska field operations - Doyon option volume 1: Doyon, Ltd. Report 91-8a. (Report held by Doyon, Ltd., Fairbanks, Alaska).
DiMarchi, J.J., Weglarz, T.B., Dickey, C.F., Laux, D.P., Smith, M.T., Toro, J., and Kurtz, J.P., 1991, Central Alaska Gold Co. 1990 annual report, Alaska field operations - Doyon option volume 2, analytical results, geochemical results part A: Doyon, Ltd. Report 91-8b. (Report held by Doyon, Ltd., Fairbanks, Alaska).
DiMarchi, J.J., Weglarz, T.B., Dickey, C.F., Laux, D.P., Smith, M.T., Toro, J., and Kurtz, J.P., 1991, Central Alaska Gold Co. 1990 annual report, Alaska field operations - Doyon option volume 2, analytical results, geochemical results part B: Doyon, Ltd. Report 91-8c. (Report held by Doyon, Ltd., Fairbanks, Alaska).
DiMarchi, J.J., Weglarz, T.B., Dickey, C.F., Laux, D.P., Smith, M.T., Toro, J., and Kurtz, J.P., 1991, Central Alaska Gold Co. 1990 annual report, Alaska field operations - Doyon option volume 7 plates, Fortymile central, Fortymile south, mid-Yukon, Nikolai, and East Holy Cross blocks: Doyon, Ltd. Report 91-8d. (Report held by Doyon, Ltd., Fairbanks, Alaska).
Hughs, M.A., and Siems, P.L., 1981, 1980 Lucky 13 project area report: Doyon, Ltd. Report 81-03. (Report held by Doyon, Ltd., Fairbanks, Alaska).
Huskey, G.F., 1981, 1981 Lucky 13 project area report: Doyon, Ltd. Report 82-49, 7 p. (Report held by Doyon, Ltd., Fairbanks, Alaska).
Newberry, R.J., Allegro, G.L., Cutler, S.E., Hagen-Levelle, D.D., Adams, D.D., Nicholson, L.C., Weglarz, T.B., Bakke, A.A., Clautice, K.H., Coulter, G.A., Ford, M.J., Myers, G.L., and Szumigala, D.J., 1997, Skarn deposits of Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 355-395.
Union Carbide Corp., 1980, 1979 Lucky 13 project area report: Doyon, Ltd. Report 80-16 (Report held by Doyon, Ltd., Fairbanks, Alaska).
WGM Inc., 1977, Doyon Project, 1976 annual progress report, volume 1a, Blocks 1, 4, 5, 7, 8: Doyon, Ltd. Report 77-02a. (Report held by Doyon, Ltd., Fairbanks, Alaska).
WGM Inc., 1998, Veta property, 1997 annual report, volume 1: Doyon, Ltd. Report 98-10, 227 p. (Report held by Doyon, Ltd., Fairbanks, Alaska).
Reporters R.L. Flynn; M.B. Werdon
Last report date 5/1/2002