Geologic description
The rocks along the middle portion of the Seventymile River are Paleozoic gneiss, amphibolite, schist, and greenschist and Upper Cretaceous to Pliocene terrigenous sedimentary rocks (Foster, 1976). The main trace and subsidiary strands of the Tintina Fault run along and parallel the Seventymile River.
The falls of the Seventymile River are located about 20 miles upstream of the Yukon River; they are about 9 feet high (Prindle, 1905). Above the falls well-developed gravel benches are found on both sides of the river; low benches are best developed on the north side of the river. By 1936, small-scale mining had been carried out intermittently for many years on gravel on the lower benches (Mertie, 1938). One hundred holes were drilled on low benches on the north side of the river, from the falls upstream to within a mile of Barney Creek. The depth to bedrock averaged 19 feet to bedrock, with a maximum depth of 27 feet. Two assays of placer gold from the Seventymile River averaged of 828 parts of gold and 163 parts of silver per thousand. A sample of placer gold from the Seventymile River at the mouth of Broken Neck Creek contains 80.59 percent gold, 18.21 percent silver, and 0.20 percent platinum (Mertie, 1942). No platinum grains were observed; the platinum is presumably alloyed with gold and silver. Placer gold has been produced on many of the tributaries entering the Seventymile River, including Rock Creek (EA030), Fox Creek (EA021), Crooked Creek (EA026), Broken Neck Creek (EA024), Canyon Creek (EA027), Sonickson Creek (EA022), Barney Creek (EA019), Nugget Creek (EA014), Alder Creek (EA011), and Flume Creek (EA008). In 1902, a hydraulic plant was used to mine on the Seventymile River about 15 miles upstream from the Yukon River (Brooks, 1903). Bar mining and sniping are reported on the Seventymile River in most years from 1902 to 1940. A small hydraulic outfit operated at the falls in 1903 (Brooks, 1904; Prindle, 1905), and in 1910 placer mining occurred at the falls and Curtis Bar (Ellsworth and Parker, 1911). Mining at the falls ceased in 1912, but more mining took place at Curtis Bar than in any recent year (Ellsworth and Davenport , 1913). In 1936, a drilling crew of Gold Placers, Inc. prospected benches along the Seventymile River. They drilled 100 holes above the falls, but results of the drilling were not as encouraging as had been expected (Mertie, 1938). |
