American

Prospect, Inactive

Commodities and mineralogy

Main commodities Fe
Other commodities Au; Mn; Zn
Ore minerals goethite; limonite; pyrite
Gangue minerals dolomite; quartz

Geographic location

Quadrangle map, 1:250,000-scale NM
Quadrangle map, 1:63,360-scale D-2
Latitude 64.777
Longitude -165.87
Nearby scientific data Find additional scientific data near this location
Location and accuracy The American prospect is on a north-sloping face of a hill about 1 mile south of American Creek at an elevation of 600 feet. The prospect is about 2 miles west of a south-flowing part of Sinuk River and 2.3 miles northeast of a low divide near hill 660 on the Nome-Teller road. The location shown is taken from Herreid (1970) and is accurate within about 500 feet. This is locality 1 of Cobb (1972 [MF 463], 1978 [OFR 78-93]).

Geologic setting

Geologic description

Extensive amounts of limonite occur near a contact between marble and schist, probably along a high-angle fault that strikes west and is nearly coincident with the lithologic contact (Herreid, 197, figure 8). Surface rubble locally contains more than 30 percent goethite. The goethite-bearing rocks are associated with crackled dolomite and bleached, very light colored calcitic marble. The largest goethite-rich mass is about 600 feet long and as much as 150 feet across. The general iron-rich zone is about 1,800 feet long. The American lode is probably the second largest iron occurrence in the area, second only to the Monarch prospect (NM017). According to Shallit (1942), there are about 40,000 long tons of rock here that contain between 20 and 40 percent iron. Some of the deposit is anomalous in zinc. Herreid (1970, Appendix II) found as much as 800 ppm in zinc in soil samples collected north of the iron accumulation. One sample of silicified schist from this locality contained 0.06 ppm gold (Herreid, 1970). The host rock to the iron accumulation is primarily marble and some intercalated metasedimentary schist.
This prospect and other iron deposits of the Sinuk River area are at or near the base of massive marble whose protolith age is probably lower Paleozoic (Sainsbury, Hummel, and Hudson, 1972; Bundtzen and others, 1994). The deposits are locally contolled by high angle faults or folds but they are in general crudely stratabound within the basal massive marble or underlying calc-schist (Mulligan and Hess, 1965; Herreid, 1970). This stratigraphic interval also hosts base metal sulfide-fluorite-barite deposits at the Galena (NM130) and Quarry prospects (NM135).
The origin and age of the iron deposits of the Sinuk River area are uncertain. The deposits may be, in part, gossan developed on oxidized sulfide deposits (Eakin, 1915 [B 622-I, p. 361-365]; Mertie; 1918 [B 662-I, p. 425-449]; Cathcart, 1922; Mulligan and Hess, 1965; Herreid, 1970). Several of the iron deposits, including American (NM014) and Monarch (NM017), are locally highly anomalous in zinc and lead. Arguing against a simple gossan origin is the paucity of diagnostic textures and structures in boxworks that would suggest derivation from specific sulfide minerals. Alternatively, these deposits could be hypogene iron oxide and carbonate deposits that are transitional to some of the lead-zinc-barite deposits (as at the Quarry prospect, NM135) at the same stratigraphic horizon.
The age of the iron deposits of the Sinuk River area is most likely post mid-Cretaceous because faults that crosscut mid-Cretaceous metamorphic rocks are an important control on them. A Late Cretaceous age for the iron deposits was suggested by Brobst and others (1971) as this is the age of fluorine-rich tin granites of northwestern Seward Peninsula (Hudson and Arth, 1983). (Fluorite is abundant at the Quarry prospect.) The youngest possible age appears to be early Tertiary, when deep weathering, sandstone-type uranium mineralization, and possibly karst formation occurred to the east in the Solomon quadrangle (Hudson, 1999).
Geologic map unit (-165.87263735303, 64.7762321613849)
Mineral deposit model Iron deposit localized by marble-schist contact and intersecting fault.
Age of mineralization Late Cretaceous or Early Tertiary; post mid-Cretaceous metamorphism.
Alteration of deposit Dolomitization, silicification, and oxidation.

Production and reserves

Workings or exploration A few surface prospecting pits to 6 or 8 feet deep are present here.
Indication of production None
Reserve estimates Shallit (1942) estimated that 40,000 long tons of rock are present that contain 20 to 40 percent iron with minor manganese (also see Mulligan and Hess, 1965, p. 18).

References

MRDS Number A012777

References

Hudson, T.L., and Arth, J. G., 1983, Tin granites of Seward Peninsula, Alaska: Geological Society of America Bulletin, v. 94, p. 768-790.
Reporters C.C. Hawley and Travis L. Hudson
Last report date 10/22/1999