Location: The Monarch prospect is at the head of an unnamed northwest-flowing tributary of the Sinuk River. It is mainly in the W1/2 section 33, T. 8 S., R. 35 W., Kateel River Meridian. It is 1.5 miles southwest of the Mogul prospect (NM020) and accurately located. It is locality 3 of Cobb (1972 [MF 463],1978 [OFR 78-93]).
Geology: The Monarch prospect occurs in marble overlying mica schist; it is the largest of the iron deposits of the Sinuk River region. The main Monarch prospect (called the East Gap) contains abundant limonite and some hematite in an area about 2,000 feet long and about 700 feet across. High-grade iron ore, about 50 percent or more iron, is contained within a lower grade zone about 3,000 feet long and as much as 1,500 feet across. The West Gap portion of the prospect is 1,500 feet by 300 feet and has a north-northwest trend. Exposures are poor, but some near vertical limonite veins are as much as 30 feet across and extend downward at least 12 feet. Shallit (1942; Mulligan and Hess, 1965, table 3) estimated that there are 50,000 long tons of rock containing 30 to 45 percent iron and about 500,000 tons of rock containing 15 to 25 percent iron here. Most of the iron ore has only 1 percent or less of manganese, but Mulligan and Hess (1965, p. 14) cite one analysis indicating about 15 percent iron and 11 percent manganese. The West Gap body seems to have more manganese. A representative sample of the East Gap body contained 78.30 percent ferric oxide (about 55 percent iron) and 1.37 percent manganese oxide (Eakin, 1915, p. 363). Soil samples collected here by Herreid (1970, table II, figure 4) locally are highly anomalous in lead and zinc. One sample at the basal marble contact below the West Gap zone contained 1,000 ppm zinc. Samples over the East Gap zone contain as much as 340 ppm lead and 1,200 ppm zinc. Sample 111 from West Gap contained 280 ppm lead. The prospect appears to lie along north-trending high-angle faults (Herried, 1970). 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 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 possibly transitional to some of the lead-zinc-barite (as at the Quarry prospect, NM135) deposits of the area. 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. 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).
Workings: Open cuts, a shallow shaft, and a short adit were driven before 1914. There are at least 12 patented claims over this prospect (Mulligan and Hess, 1965).
Age: Late Cretaceous or Early Tertiary; post mid-Cretaceous metamorphism.
Alteration: Dolomitization and oxidation.
Reserves: Shallit (1942; Mulligan and Hess, 1965, table 3) estimated that this prospect contains 50,000 long tons of 30 to 45 percent iron and about 500,000 tons of 15 to 25 percent iron. Most of the iron ore has only 1 percent or less of manganese, but Mulligan and Hess (1965, p. 14) cite one analysis indicating about 15 percent iron and 11 percent manganese. The West Gap body seems to have more manganese. A representative sample of the East Gap body contained 78.30 percent ferric oxide (about 55 percent iron) and 1.37 percent manganese oxide (Eakin, 1915, p. 363).

Commodities (Major) - Fe; (Minor) - Au, Mn, Pb, Zn
Development Status: None
Deposit Model: Carbonate-hosted, iron oxide deposit.

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4 valid minerals.