Geologic description
The country rocks in the general area of the Cone Mountain prospect are a Miocene (?) alkali feldspar granite stock and associated quartz-porphyritic rhyolite dikes (Koch, 1996, 1997). The prospect is on the western margin of the stock, which intrudes Eocene granodiorite and quartz monzonite plutons and Mesozoic or Paleozoic pelitic schist and paragneiss. A group of 145 claims was staked in 1976, presumably for uranium-bearing and related minerals (Elliott and Koch, 1981, loc. 4; Koch and others, 1981; U. S. Geological Survey, 1979). Geochemical surveys [(Koch and Elliott, 1981 (OFR 81-728C-K)] show high values of silver, beryllium, molybdenum, niobium, lead, tin, yttrium, rare earth elements, and zinc in rock, stream-sediment and heavy-mineral concentrate samples from the immediate area of the stock. Differentiation enrichment of these elements in such a highly-evolved silicic magma may account for most of the values reported, but a few high lead and zinc values suggest local sulfide deposition. In 1976, Pacific Coastal Minerals staked 176 claims in the area; they held them several years and did some drilling. Still and others (2002) collected samples of granite at two localities between Cone Mountain and Black Crag; they contained up to 140 parts per million (ppm) cerium, 75 ppm lanthanum, 48 ppm neodymium, and 66.9 ppm thorium. They also collected 8 samples of a variety of felsic volcanic and intrusive rocks. The highest values were in a sample of molybdenite-bearing rhyolite breccia that contained 1,740 ppm cerium, 954 ppm lanthanum, 440 ppm neodymium, 60 ppm samarium, 72.6 ppm thorium, and 2,098 ppm molybdenum. |
