Rubicon

Prospect, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Pb; Zn
Ore minerals arsenopyrite; galena; pyrrhotite; sphalerite
Gangue minerals quartz

Geographic location

Quadrangle map, 1:250,000-scale JU
Quadrangle map, 1:63,360-scale B-1
Latitude 58.3014
Longitude -134.3185
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Rubicon prospect is at an elevation of approximately 2,500 feet, 1/4 mile south of upper Gold Creek. It is 1.2 miles north of Roberts Peak and 1 mile west of Clark Peak, in the SE1/4NE1/4 section 20, T. 41 S., R. 68 E. of the Copper River Meridian. The location is accurate.

Geologic setting

Geologic description

The Rubicon prospect was discovered in 1906 and has been developed by 5 adits and a 400-foot trench. The deposit consists of an older set of foliation-concordant boudinaged, quartz veins in black phyllite and felsic schist (Redman and others, 1989). These veins are up to 10 feet thick and 200 feet long and contain disseminated pyrrhotite. A series of en echelon faults intersect these veins and contain siderite, quartz , and brecciated wallrock. The late-stage, fault-controlled, quartz-siderite veins contain arsenopyrite, galena, pyrrhotite, and sphalerite. U.S. Bureau of Mines samples contained up to 5.3 ppm gold, 797.1 ppm silver, 2.47 percent lead, and 3.65 percent zinc. The U.S. Bureau of Mines estimates a resource of 40,500 tons with an average of 2.37 ounces of silver and 0.03 ounce of gold per ton (Redman and others, 1989).
This prospect is in the Juneau Gold Belt, which consists of more than 200 gold-quartz-vein deposits that have produced nearly 7 million ounces of gold. These gold-bearing mesothermal quartz vein systems form a zone 160 km long by 5 to 8 km wide along the western margin of the Coast Mountains. The vein systems are in or near shear zones adjacent to west-verging, mid-Cretaceous thrust faults. The veins are hosted by diverse, variably metamorphosed, sedimentary, volcanic, and intrusive rocks. From the Coast Mountains batholith westward, the host rocks include mixed metasedimentary and metavolcanic sequences of Carboniferous and older, Permian and Triassic, and Jurassic-Cretaceous age. The sequences are juxtaposed along mid-Cretaceous thrust faults (Miller and others, 1994). The sequences are intruded by mid-Cretaceous to middle Eocene plutons, mainly diorite, tonalite, granodiorite, quartz monzonite, and granite. Sheetlike tonalite plutons emplaced just east of the Juneau Gold Belt and undeformed granite and granodiorite bodies that are emplaced farther to the east are between 55 and 48 Ma (Gehrels and others, 1991). The structural grain of the belt is defined by northwest-striking, moderately to steeply northeast-dipping, penetrative foliation that developed between Cretaceous and Eocene time (Miller and others, 1994). The majority of the veins in the Juneau Gold Belt strike northwest. Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).
Geologic map unit (-134.320266613901, 58.3010734542512)
Mineral deposit model Low-sulfide Au-quartz vein (Cox and Singer, 1986; model 36a)
Mineral deposit model number 36a
Age of mineralization Isotopic dates indicate that the auriferous veins in the Juneau Gold Belt formed between 56 and 55 Ma (Miller and others, 1994; Goldfarb and others, 1997).

Production and reserves

Workings or exploration The deposit at the Rubicon prospect was discovered in 1906 and was developed by 5 adits and a 400-foot trench.
Indication of production None
Reserve estimates The U.S. Bureau of Mines estimates a resource of 40,500 tons of ore with an average of 2.37 ounces of silver and 0.03 ounce of gold per ton (Redman and others, 1989).

References

References

Gehrels, G.E., McClelland, W.C., Samson, S.D., and Patchett, P.J., 1991, U-Pb geochronology of detrital zircons from a continental margin assemblage in the northern Coast Mountains, southeastern Alaska: Canadian Journal of Earth Sciences, v. 28, no. 8, p.1285-1300.
Goldfarb, R.J., Miller, L.D., Leach, D.L., and Snee, L.W, 1997, Gold deposits in metamorphic rocks in Alaska, in Goldfarb, R.J., and Miller, L.D., eds., Mineral Deposits of Alaska: Economic Geology Monograph 9, p. 151-190.
Miller, L.D., Goldfarb, R.J., Gehrels, G,E., and Snee, L.W., 1994, Genetic links among fluid cycling, vein formation, regional deformation, and plutonism in the Juneau gold belt, southeastern Alaska: Geology, v. 22, p. 203-206
Reporters J.C. Barnett and L.D. Miller (Juneau, Alaska )
Last report date 12/15/2001