Geologic description
The Cosna Dome area is characterized by low rounded hills having less than 1 percent outcrop (Burleigh, 1989 [OFR 11-89]). The country rocks are Precambrian to Cambrian quartzite, metasiltstone, slate, phyllite, and grit (Chapman and others, 1975). 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 Cosna Dome prospects are sparsely intruded by thin aplitic dikes (Burleigh, 1989 [OFR 11-89]).
The Pb-Ag prospect is characterized by numerous east-trending subparallel zones of limonite-stained vuggy quartz veins in open-space fractures. The quartz veins contain hydrothermal sericite and various combinations of galena, chalcopyrite, pyrite, sphalerite, and stibnite. Massive galena samples from two prospect pits contained 65 to 147 ounces Ag per ton. A cobble of stibnite from a trench contained 0.7 ppm Hg, 0.8 ppm Ag, and 68.95 percent Sb. Some of the best assays of galena yield 71 percent Pb, 91.3 ounces Ag per ton, and 0.06 percent Sn. A pan concentrate from gravels downstream of the prospect contained 7 ppm Ag, and 1,150 ppm Pb (Burleigh, 1989 [OFR 11-89]).
The Sn prospect is characterized at the surface by subtle linear benches and depressions trending approximately 145 degrees. These benches and depressions contain brecciated graywacke with irregular quartz stringers, goethite-limonite gossan, and some quartz-tourmaline veining. An iron-stained quartz-veined breccia sample contained 45 ppm Ag, 830 ppm Sn, 2,200 ppm Pb, and 181 ppm As. A sample consisting of gossan and quartz-chlorite veins crosscutting a massive quartz lens contained 54 ppm Ag, 5,800 ppm Pb, 897 ppm As, 0.320 ppm Au, and less than 200 ppm Sn. A zone of dense gossan material contained 20,000 ppm Sn, 110 ppm Ag, 2,130 ppm As, and 3,950 ppm Pb. Regional stream-sediment sampling has found detrital cassiterite. A pan concentrate from gravels downstream of the prospect contained 0.090 ppm Au, less than 5 ppm Ag, 2,100 ppm Sn, and 195 ppm Pb (Burleigh, 1989 [OFR 11-89]).
The regional northeast-trending fault and plutonic system that characterizes Cosna Dome and the Bitzshtini Mountains also characterizes Haystack Mountain (KH004) approximately 20 miles southwest and the Chitanatala Mountains to the north and northwest. Silberman and others (1978) determined a K-Ar biotite age of 64.2 plus or minus 1.9 Ma for the pluton on Haystack Mountain, and a K-Ar biotite age of 91.3 plus or minus 2.7 Ma and a K-Ar hornblende age of 92.5 plus or minus 2.8 Ma for the pluton in the Chitanatala Mountains. Results of airborne radiometric and magnetic surveys of the Bitzshtini Mountains are similar to the geophysical expression of the Haystack Mountain pluton and associated hornfels. Burleigh (1989 [OFR 11-89]) interprets the Bitzshtini Mountains, including Cosna Dome, as the surface expression of an aureole of hornfels around a shallow, unexposed pluton with high-level felsic dikes and Pb-Ag-Sn mineralization. It is reported that Tom Arnstrom first discovered lode cassiterite at the head of a gulch on the northwest flank of Cosna Dome. It is unclear what year the discovery occurred. Using information supplied by Arnstrom, Charles Holky prospected the area for tin (Burleigh, 1989 [OFR 11-89]). Holky reported good prospects of placer cassiterite in the headwaters of the Cosna River (Joesting, 1943). Holky and Colbert discovered lead and silver vein mineralization at the Pb-Ag prospect and hand-dug a 100-foot adit, three trenches, and several prospecting pits. The U.S. Bureau of Mines conducted a brief examination, consisting of rock, soil, and pan concentrate sampling, of the Pb-Ag and Sn prospects in 1964 and a follow up Cu-Pb-Zn soil survey in 1966. In 1980, the Department of Energy completed 1:250,000-scale airborne radiometric and magnetic reconnaissance surveys for the region. During and after 1985, Charles Woodruff conducted various trenching operations and constructed a 1,100-foot airstrip on Cosna Dome. There is no reported production for the Cosna Dome Pb-Ag and Sn prospects (Burleigh, 1989 [OFR 11-89]). |
