Unnamed (at head of Rulby Creek)

Occurrence, Probably inactive

Commodities and mineralogy

Main commodities Fe
Ore minerals limonite

Geographic location

Quadrangle map, 1:250,000-scale NM
Quadrangle map, 1:63,360-scale D-2
Latitude 64.7724
Longitude -165.7371
Nearby scientific data Find additional scientific data near this location
Location and accuracy This occurrence is about 0.8 mile south of the Tub Mountain prospect (NM018) near the head of Rulby Creek (NM016). The occurrence is at an elevation of 760 feet and is accurately located from Herreid (1970).

Geologic setting

Geologic description

This limonite occurrence is about 0.8 mile south of the Tub Mountain prospect (NM018). It is a few hundred feet across (Herreid, 1970) and one of several carbonate-hosted, iron-rich deposits in the area (NM014, NM014, NM017, NM018, and NM020). The limonite can be massive to granular, botryoidal, mammillary, or fibrous. The iron content ranges to as much as 59 percent in analyzed samples (Eakin, 1915 [B 622-I, p. 361-365]). Hematite, pyrolusite, and calcite are present locally.
This occurrence 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 controlled 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 lead-zinc-barite deposits such as the Quarry (NM135).
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) because this is the age of fluorine-rich tin granites of northwestern Seward Peninsula (Hudson and Arth, 1983). 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 (-165.739737540278, 64.7716363294854)
Mineral deposit model Carbonate-hosted, iron oxide deposit.
Age of mineralization Late Cretaceous or Early Tertiary; post mid-Cretaceous metamorphism.
Alteration of deposit Dolomitization and oxidation.

Production and reserves

Workings or exploration There may be small prospecting pits at this locality.
Indication of production None


MRDS Number 10307910


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