Paper No. 9
Presentation Time: 10:15 AM
ZIRCON U/PB GEOCHRONOLOGY OF HOST ROCK AND MINERALIZATION AT THE COLES HILL URANIUM DEPOSIT, VIRGINIA
TAPPA, Michael, Department of Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, BODNAR, Robert J., Geosciences, Virginia Polytechnic Institute and State University, 4044 Derring Hall, Blacksburg, VA 24061, AYUSO, Robert A., 954 National Center, U.S. Geological Survey, Reston, VA 20192, AYLOR Jr, Joseph G., Virginia Uranium, Inc, 231 Woodlawn Heights, Chatham, VA 24531, WOODEN, Joseph L., USGS-Stanford Ion Microprobe Laboratory, Stanford University, Stanford, CA 94305 and VAZQUEZ, Jorge A., U.S. Geological Survey, 345 Middlefield Road MS 910, Menlo Park, CA 94025, mtappa@vt.edu
The Coles Hill uranium deposit, located in Pittsylvania Co., Virginia, contains a mineral resource of 119 million pounds U
3O
8 (cutoff grade = 0.025% U
3O
8) and is the largest undeveloped uranium deposit in the United States. Uncertainties in the age of the host rocks and the timing of mineralization have limited the development of a single unified genetic model for the deposit. The deposit is hosted within a highly brecciated and mylonitic section of the meta-igneous heterogeneous Leatherwood granite and is located within the regional Smith River Allochthon. The deposit lies immediately to the west of the Chatham Fault that separates the Mesozoic Danville basin to the east from the Paleozoic crystalline host rocks to the west. The tectonic and geographic setting of Coles Hill offers multiple potential sources for the uranium, including black shales or other sedimentary rocks in the adjacent Danville basin, the igneous and metamorphic host rocks, or an unidentified igneous or metamorphic source at depth. Determining a genetic model for the Coles Hill deposit is important for future uranium exploration in the region, because Triassic rift-related sedimentary basins bordered by crystalline rocks similar to those at Coles Hill occur throughout the central Appalachian region.
In an effort to constrain the age of the host rocks and to better understand potential sources for the uranium at Coles Hill, a study is underway to determine zircon U/Pb SHRIMP ages for the host rock and the mineralization. Two samples of the Leatherwood granite gneiss, collected from the immediate vicinity of the deposit, yield ages of 455.2 ± 2.3 Ma and 441.4 ± 2.4 Ma. A third, more vein-rich orthogneiss sample did not yield a coherent age group, but individual zircons yield ages that range from 458 to 416 Ma. No apparent age distinction exists among different zircon populations for all analyzed samples of the Leatherwood granite. Zircon has been isolated from two mineralized samples of drill core (41-90A) at Coles Hill and petrographically examined. SHRIMP analyses currently underway on these samples will help constrain the relative timing of mineralization and allow us to more realistically evaluate whether the Leatherwood granite is the uranium source at Coles Hill.
