Roosevelt Hills

Prospects, Inactive

Commodities and mineralogy

Main commodities Ag; Sn
Other commodities As; Bi; Cu; Mn; Mo; Pb; U; W; Zn
Ore minerals pyrite
Gangue minerals coronodite; fluorite; quartz; sericite; tourmaline

Geographic location

Quadrangle map, 1:250,000-scale KH
Quadrangle map, 1:63,360-scale A-4
Latitude 64.022
Longitude -151.502
Nearby scientific data Find additional scientific data near this location
Location and accuracy The Roosevelt Hills prospects cover about 10 square miles and are situated on the southeast flank of a rounded hill, locally termed hill 1910. The approximate center of the prospect area is in SW1/4SE1/4 section 29, T. 10 S., R. 20 W., of the Fairbanks Meridian. The Kantishna River flows within 11 miles, and there is an airstrip at Lake Minchumina approximately 30 miles southwest of the prospects. The Roosevelt Hills are situated on land selected by or conveyed to Doyon, Limited and in the Denali National Preserve.

Geologic setting

Geologic description

The Roosevelt Hills are composed of an elliptical, east-trending, muscovite-biotite granite stock and hornfelsed, pelitic schists (Burleigh, 1989 [DLR 90-24]). The age of the schist is unknown; the granite is Tertiary in age (Bond, 1985). The granite underlies several rounded hills and an adjacent eroded basin, covering approximately 10 square miles (Burleigh, 1989 [DLR 90-24]). Bond (1985) reported that the stock comprises early to late intrusive phases of granite, semi-porphyritic granite, equigranular granite, aplite, and quartz-tourmaline veins. The Roosevelt Hills stock is interpreted to be a deeply weathered, calc-alkaline, peraluminous granite (Burleigh, 1989 [DLR 90-24]). Regionally, the area contains a system of northeast-trending strike-slip and related conjugate faults associated with Cretaceous to Tertiary plutonic rocks and coeval or younger volcanic rocks (Clautice and others, 1993).
The prospects consist of silicified-sericitized shear zones in the granite. One such zone, about 70 feet long and 30 feet wide, contains a black coating in vugs and on joint surfaces. According to x-ray fluorescence analysis, this coating is coronodite: (Mn, Pb)Mn3O7. (Burleigh, 1989 [DLR 90-24]) inferred that the coating could be from downward-percolating supergene fluids. Assays from the coronodite-bearing shears contained as much as 5,000 ppm Sn, 39.7 ppm Ag, 20,013 ppm Mn, and 2,688 ppm Pb (Burleigh, 1989 [DLR 90-24]). Bond (1985) reports that assays from the same zone contain 61 percent Pb, 1,600 ppm Cu, 6,500 ppm Zn, 300 ppm Mo, 300 ppm As, 785 ppm Sn, and 60 ppm W. Adjacent areas with no coronodite coating contain as much as 3,100 ppm Pb, 500 ppm Zn, 16 ppm Mo, 7 ppm Ag, 1,400 ppm As, 6,800 ppm Sn, and 105 ppm W (Bond, 1985). In the northern half of the granite, widely spaced sets of coarse drusy quartz veins, 0.1 to 3 feet wide, trend 15 to 150 degrees and dip 70 degrees. According to x-ray fluorescence, these veins contain as much as 110 ppm Sn, with minor pyrite. Late-stage veins contain purple fluorite (Burleigh, 1989 [DLR 90-24]). Bond (1985) reported a rock chip that contained 12,600 ppm Sn. Pan concentrates from streams draining the granite contain as much as 4,200 ppm Sn, 9 ppm Pb, 66 ppm Zn, 9.8 ppm As, and 6 ppm U (Burleigh, 1989 [DLR 90-24]). Burleigh (1989 [DLR 90-24]) reported that the upper portions of the granite, which commonly contain large amounts of Sn, have been eroded. There are numerous alluvial deposits in the area with potential for Sn placers.
Regional stream sediment sampling with airborne radiometrics in 1975-76 identified geochemical anomalies (Bond, 1985). In 1977, exploration efforts consisted of a scintillometer survey, mapping, and a stream sediment, rock chip, and soil sampling grid. Additional stream-sediment, rock chip, and water sampling was completed in 1980 (Bond, 1985). In 1981, an airborne radiometric program was conducted. Further rock chip sampling occurred in 1984. In 1985, two days of rubble and outcrop sampling identified hill 1910 as a Sn-rich zone in the granite (Bond, 1985). Burleigh (1989 [DLR 90-24]) conducted mapping activities and collected 18 rock chip and 10 pan concentrate samples. There are no indications of production.
Geologic map unit (-151.504352459534, 64.0214766768304)
Mineral deposit model Shear-hosted veining, lower portion of Sn greisen deposits? (Cox and Singer, 1986; model 15c?).
Mineral deposit model number 15c?
Age of mineralization Veins cut Tertiary intrusion.
Alteration of deposit Numerous shear zones in granite are silicified and sericitized and contain a black coating in vugs and on joint surfaces. According to x-ray fluorescence analysis, this coating is coronodite: (Mn, Pb)Mn3O7. Burleigh (1989 [DLR 90-24]) inferred that the coating could be from downward-percolating supergene fluids.

Production and reserves

Workings or exploration Regional stream sediment sampling with airborne radiometrics in 1975-76 identified geochemical anomalies (Bond, 1985). In 1977, exploration efforts consisted of a scintillometer survey, mapping, and a stream sediment, rock chip, and soil sampling grid. Additional stream-sediment, rock chip, and water sampling was completed in 1980 (Bond, 1985). In 1981, an airborne radiometric program was conducted. Further rock chip sampling occurred in 1984. In 1985, two days of rubble and outcrop sampling identified hill 1910 as a Sn-rich zone in the granite (Bond, 1985). Burleigh (1989 [DLR 90-24]) conducted mapping activities and collected 18 rock chip and 10 pan concentrate samples.
Indication of production None
Production notes There are no indications of production.

Additional comments

The Roosevelt Hills are situated on land selected by or conveyed to Doyon, Limited. For more information, contact Doyon, Limited. The site is also in Denali National Preserve.

References

MRDS Number 10307707

References

Bond, James, 1985, Geologic field examinations in Doyon, Limited, regional selection block 9: Doyon Limited Report 85-19, 179 p. (Report held by Doyon, Limited, Fairbanks, Alaska).
Reporters Cameron S. Rombach (ADGGS)
Last report date 11/8/1999