Summary:
The Bullfrog Project consists of two separate contiguous deposits, also known as Copper Bench and Indian Bench.
Sandstone-type uranium deposits typically occur in fine- to coarse-grained sediments deposited in a continental fluvial environment. The uranium is either derived from a weathered rock containing anomalously high concentrations of uranium, leached from the sandstone itself, or leached from an adjacent stratigraphic unit. It is then transported in oxygenated water until it is precipitated from solution under reducing conditions at an oxidation-reduction front. The reducing conditions may be caused by such reducing agents in the sandstone as carbonaceous material, sulfides, hydrocarbons, hydrogen sulfide, or brines.
There are three major types of sandstone hosted uranium deposits: tabular vanadium-uranium Salt Wash type of the Colorado Plateau, uraniferous humate deposits of the Grants, New Mexico area, and the roll-type deposits of Wyoming. The differences between the Salt Wash deposits and other sandstone hosted uranium deposits are significant. Some of the distinctive differences are as follows:
• The deposits are dominantly vanadium, with accessory uranium.
• One of the mineralized phases is a vanadium-bearing clay mineral.
• The deposits are commonly associated with detrital plant trash, but not redistributed humic material.
• The deposits occur entirely within reduced sandstone, without adjacent tongues of oxidized sandstone.
The vanadium content of the Henry Mountains Basin deposits is relatively low compared to many Uravan deposits. Furthermore, the Henry Mountains Basin deposits occur in broad alluvial sand accumulations, rather than in major sandstone channels as is typical of the Uravan deposits of Colorado. The Henry Mountains Basin deposits do have the characteristic geochemistry of the Uravan deposits and are classified as Salt Wash type deposits.
Extensive research by Northrop and Goldhaber (1990) shows that the Henry Mountains Basin deposits were formed at the interface of an underlying brine with overlying oxygenated flowing waters carrying uranium and vanadium in solution. Reduction and deposition of the mineralization were enhanced where the interface occurred within sandstones containing carbonaceous debris. The multiple mineralized horizons developed at favourable intervals as the brine surface migrated upwards. Geochemical studies indicate the uranium and vanadium were leached either from the Salt Wash sandstone or the overlying Brushy Basin Member.
Uranium Mineralization
The uranium-vanadium mineralization in the Henry Mountains Basin area is similar to the mineralization observed elsewhere in the Colorado Plateau.
It occurs as intragranular disseminations within the lower fluvial sand facies (Lower Rim) of the Salt Wash Member containing detrital organic debris. Mineralization primarily consists of coffinite, with minor uraninite, which usually occurs in close association with vanadium mineralization. Mineralization occurs as intergranular disseminations, as well as coatings and/or cement on and between sand grains and organic debris.
The Lower Rim of the Salt Wash Member has been subdivided into an upper, middle, and a lower unit designated as the Upper-Lower, Middle-Lower, and Lower-Lower. Each of these subunits, in turn, have been subdivided into upper, middle, and lower horizons. The Bullfrog deposit primarily occurs in upper and lower portions of the Middle-Lower unit (i.e., 60 ft to 100 ft above the base) with minor mineralization found in the upper portion of the Lower-Lower unit. Minor mineralization is also seen in the underlying Tidwell formation.
The Bullfrog deposit extends approximately 3.5 mi along a northwesterly trend to the northeast of the Tony M-Southwest deposit. Mineralization of the Bullfrog deposits ranges in thicknesses of three feet to six feet but occasionally is shown on radiometric logs exceeding 12 ft in some portions of the Project area.
Vanadium Mineralization
Vanadium occurs as montroseite (hydrous vanadium oxide) and vanadium chlorite in primary mineralized zones located below the water table (i.e., the northern portion of the Tony M deposit). Montroseite is the only vanadium oxide mineral identified in this interval. An unusual vanadium-bearing chlorite or an interlayered vanadium-bearing chlorite-smectite is the only authigenic clay mineral(s) recognized. The grain size and sorting characteristics of detrital quartz grains vary within the host rocks; cross-bed laminae with coarser grains and better sorting are invariably more highly mineralized.
Above the water table vanadium chlorite is absent, while montroseite and a suite of secondary uranium-vanadium minerals are present. These include tyuyamunite, metatyuyamunite, rauvite, and carnotite all of which have been identified in samples from the southern part of the Tony M deposit. Carnotite is a secondary hydrous potassium-vanadium-uranium mineral, while the other three are similar minerals with calcium replacing potassium. The later minerals occur above the water table in the zone that has been subjected to near surface secondary oxidation.