rocks of the Triassic-Jurassic Culpeper/Barboursville Basin lack the requisite permeability to have acted as hosts for uranium deposits. Most of the sandstones and conglomerates contain a large amount of silt- and clay-sized material, which results in extremely low permeability.
There appears little likelihood that economic uranium deposits associated with these sandstones will be discovered in the foreseeable future. The Pennsylvanian and Mississippian sandstones have been extensively drilled and mined for coal without the discovery of significant uranium mineralization, and the Triassic basin in Virginia does not appear to contain suitable lithologies. Consequently, the use of ISL/ISR technology to mine sandstone-hosted uranium deposits in Virginia is unlikely in the foreseeable future.
Comparable uranium deposits. Roll-front-type deposits in Wyoming (Finch, 1996) represent equivalents of deposits that may occur in Pennsylvanian, Mississippian, or Triassic sandstones. Carboniferous sandstones in the Arlit area of Niger, belonging to the tectonolithologic category of uranium deposits, may also have some similarities to the continental sandstones in Virginia. They contain more than 150,000 tU at grades of 0.2 to 0.5 percent. The climatic conditions in this area are extremely arid, with high average temperature and extremely low rainfall.
Hydrothermal Metasomatic Deposits Associated with Alkali Metasomatism (Type 6A)
The Coles Hill deposit, located in the Pittsylvania County, occurs within a fault-bounded wedge of the sheared and highly potassic calcalkaline Leatherwood Granite (Figures 3.13, 3.14), along the Chatham Fault Zone at the northwest margin of the Triassic age Danville Basin (Jerden, 2001). The Leatherwood Granite, a component of the Martinsville Igneous Complex, was emplaced during the Late Ordovician (~442 Ma) in the Chopawamsic Volcanic Belt (Figure 3.5). Amphibolite layers are common within the granite. The deposit is partly covered by Danville Basin sedimentary rocks (Figure 3.13). The mineralized orebodies are characterized by intense sodium metasomatic alteration associated with quartz dissolution. The ore deposit is mainly contained within two approximately 350-m-long and 250-m-wide cylindrical bodies, within which the orebodies form lenticular layers below the Chatham Fault Zone (Figure 3.14).
The enclosing rocks are dominantly granitoids, with ~30 percent quartz by volume. The mineralized rocks and their alteration envelope are poor in quartz because the hydrothermal processes associated with the genesis of the deposit lead to nearly complete quartz leaching and albitization of these rocks.
Uraninite and coffinite are the main ore minerals—these are easy to leach, but they are hosted by a hard rock (Figure 3.15) that is difficult to crush. The Coles Hill ore contains high concentrations of phosphorus, with most ore grade samples ranging from 1 to 9 weight percent P2O5, but the concentrations of other trace elements are similar to those of the enclosing granitic gneisses (Jerden,