Geologic description
Still and others (2002) found possible evidence of claim staking here and were told of drilling for molybdenum in the 50s but there is no information about exploration here prior to their work. They briefly examined the area and collected several mineralized samples as part of a regional mineral assessment for the Bureau of Land Management. The site might be part of the prospect several miles to the southwest (BC004) that was explored in the 1970s.
The country rocks in the general area of Black Crag are a Miocene (?) alkali-feldspar granite stock and associated quartz-porphyritic rhyolite dikes (Koch, 1996). The prospect is near the margin of the stock, which intrudes Eocene granodiorite and quartz monzonite plutons and Mesozoic or Paleozoic pelitic schist and paragneiss. On the east side of the valley, Still and others (2002) collected several samples of float that consisted of: 1) quartz porphyry with molybdenite along fractures; 2) fine-grained silicified felsite cut by a 0.1-foot-thick seam of molybdenite; and 3) silicified granite with pyrite and chalcopyrite. Samples of the molybdenite-bearing rocks contained 735 and 1,348 parts per million (ppm) molybdenum and the copper-bearing sample contained 1,508 ppm copper. A sample of silicified breccia cemented by pyrrhotite contained 93 parts per billion gold, 43.5 ppm silver, and 558 ppm zinc. At the head of the valley under a hanging glacier, volcanic rock with molybdenite along fractures is abundant. Samples contained 23 to 2,529 ppm molybdenum, 119 to 713 ppm zinc, and up to 1,740 ppm cerium, 954 ppm lanthanum, 440 ppm neodymium, 60 ppm samarium, 72.6 ppb thorium, and 16 ppm uranium. |
