Geologic descriptionThe Yakima prospect was discovered in the early 1880s and was developed by a 175-foot shaft, a 500-foot crosscut and a 500-foot drift at the bottom of the shaft, a 60-foot adit, and another shallow shaft (Redman and others, 1989). The prospect was explored by Occidental Petroleum and Noranda in the 1970s and 1980s, and was drilled by Hecla Minerals in 1990 and 1991 (Bundtzen and others, 1991; Swainbank and others, 1991). The host rocks at this prospect are quartz-sericite phyllite and interbedded volcanic metaconglomerate. The phyllite contains 1 to 3 percent disseminated pyrite, with traces of galena and sphalerite, and concordant layers and seams of galena and sphalerite up to 0.25 inch thick (Redman and others, 1989). The phyllite is also cut by quartz veins that contain albite, pyrite, galena, and sphalerite. The deposit is approximately 3000 feet long and 1000 feet wide. U.S. Bureau of Mines samples contained up to 11.3 ppm gold, 11.0 ppm silver, 830 ppm copper, 1,120 ppm lead, and 3160 ppm zinc (Redman and others, 1989). Newberry and others (1997), have interpreted the Yakima prospect as a Cretaceous volcanogenic massive sulfide deposit. The deposit is geologically similar to the Alaska Treasure Mine (JU228), Red Diamond (JU232), Mammoth (JU229), and Homestake prospects (JU231) (Redman and others, 1989). The rocks in the general area are Upper Jurassic or Cretaceous marine argillite and graywacke, interbedded with andesite or basalt (Brew and Ford, 1985). The bedded rocks are regionally metamorphosed to prehnite-pumpellyite or greenschist grade, and cut by diorite or gabbro dikes and sills. |
