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. |
