Uranium ore deposits in the Grants Mineral Belt, New Mexico, occur in fluvial sandstones in the Jurassic Morrison Formation.
Uranium mineralization is concentrated by a dark-gray to black substance that has been identified as humate derived from decaying vegetation. Black ore is truncated by overlying sandstone in at least two ore deposits, documenting an early age of mineralization. Ore deposits in the Grants Mineral Belt vary greatly in size and shape, generally occur in clusters, and often are difficult targets for drilling.
Current exploration is largely a process of drilling in stages to (1) delineate favorable from unfavorable ground on a wide-spacing, (2) seek mineralization in favorable ground, and (3) conduct closely spaced drilling in mineralized areas. Criteria for favorability differ among exploration groups but generally includes (1) the presence of a host sandstone, (2) anomalous mineralization, (3) color of host rock, (4) presence of carbonaceous matter, and (5) position of area with respect to mineralized trends.
A description of the sequence of drilling, from ore discovery to a mine on a one-square mile area at the Johnny M uranium deposit located in the east part of the Ambrosia Lake District, provides an example of the problem of predicting ahead of discovery where in a certain area, orebodies may occur. A study of the drill data at the Johnny M indicates the uranium ore is not related to specific features other than the presence of humate which is commonly associated with coalified plant fragments in mudstone-rich parts of the host sandstone.