Unnamed (east of Bonanza Bar)

Occurrence, Inactive

Commodities and mineralogy

Main commodities Ag; Au; Cu; Pb
Other commodities Bi; W
Ore minerals galena; pyrite

Geographic location

Quadrangle map, 1:250,000-scale EA
Quadrangle map, 1:63,360-scale B-1
Latitude 64.259
Longitude -141.1684
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is exposed at river level along the north side of the Fortymile River 0.4 mile downstream from Bonanza Bar and about 1.0 miles upstream from the mouth of Nugget Creek. The occurrence is in section 34, T. 7 S., R. 33 E., of the Fairbanks Meridian. The location is accurate.

Geologic setting

Geologic description

The rocks in the vicinity of this occurrence are Paleozoic amphibolite-facies quartzite of the Fortymile River assemblage that has been intruded by granitic and ultramafic rocks of Jurassic age (Foster, 1976; Dusel-Bacon and others, 2002).
Sulfides occur as layers within biotite + quartz +/- staurolite +/- garnet schist and biotite quartzite. The sulfides are present in two modes: (A) a finely laminated, rusty-weathering, approximately 1-meter-thick oxidized layer, including a 1- to 2-centimeter-thick layer of massive pyrite or limonite after pyrite; and (B) a massive, foliation-parallel, quartz layer (quartz sweat?), in which layers of galena are apparently concordant with metamorphic layering. Petrographic analysis shows that the quartz-rich layer contains minor white mica and rare patches of sericitized and (or) kaolinitized feldspar; quartz grains are strained and have sutured contacts; tiny, thin, scattered white mica crystals occur between some quartz grains. A sample from mode A contains 260 ppm copper, 130 ppm lead, 2.8 ppm silver, and 20 ppb gold. A sample from mode B contains 6,350 ppm lead, 137 ppm copper, 13 ppm silver, and 295 ppb gold. Slightly elevated bismuth (as much as 10 ppm) and tungsten (as much as 680 ppm) in one quartz-rich rock suggests mineralization may be pluton related (Cynthia Dusel-Bacon, USGS, written communication, 2002).
Geologic map unit (-141.170677072599, 64.2586968249363)
Mineral deposit model Slightly elevated bismuth (as much as 10 ppm) and tungsten (as much as 680 ppm) in one quartz-rich rock suggests mineralization may be pluton related (Cynthia Dusel-Bacon, USGS, written commun., 2002).
Alteration of deposit Sericitized/kaolinitized feldspar?

Production and reserves

Indication of production None

References

References

Dusel-Bacon, Cynthia, Lanphere, M.A., Sharp, W.D., Layer, P.W., and Hansen, V.L., 2002, Mesozoic thermal history and timing of structural events for the Yukon-Tanana Upland, east-central Alaska: 40Ar/39Ar data from metamorphic and plutonic rocks: Canadian Journal of Earth Sciences., v. 39, no. 6, p. 1013-1051.
Reporters M.B. Werdon
Last report date 8/27/2002