Brookins reported in July 1979 that the sedimentary uranium deposits in the Grants mineral belt (GMB) occur in a strip nearly one hundred miles long and twenty miles wide that extends from near Gallup on the west to Languna on the east. Of the sedimentary rocks that are exposed in the area, mineable uranium deposits are found only in those of Jurassic and Cretaceous ages. These rocks are, in ascending order:
- the Entrada Sandstone,
- Todilto Limestone,
- Summerville Formation,
- Bluff Sandstone of the San Rafael Group, and
- the Morrison Formation, all of Late Jurassic age, and
- the Dakota Sandstone of Late Cretaceous age.
Deposits in the Morrison Formation, in particular, have been mined extensively and have been the subject of much research since the early 1950's. However, the origin of these deposits is still controversial and many uncertainties exist, even though there are pronounced similarities for many of the deposits.
Most uranium occurrences are closely associated with accumulations of organic matter, and the host sediments either contain granitic or volcanic detritus or are associated with tuffaceous material (Finch, 1967). The most striking feature of the GMB is the similar geochemistry and mineralogy in most of the deposits; iron (as pyrite and, more rarely marcasite), uranium (coffinite; lesser uraninite, U-C complex (?), vanadium (oxides and silicates), molybdenum (jordisite), selenium (native and ferroselite), and organic carbon (humates and fossil wood) appear in essentially all the uranium deposits (Kendall, 1972) although in differing proportions. In addition, ubiquitous clay mineral assemblages of montmorillonite, mixed-layer illite-montmorillonite, kaolinite and
chlorite exist in most deposits (Keller, 1959; Nash, 1968; Granger, 1962; Kendall, 1972; Brookins 1976a; 1977~).
Several hypotheses (or models) have been developed to explain the origin of the ore deposits (ash leach theory, granite-leach theory syngenetic, epigenetic, etc. ) and agreements are lacking i n many aspects of mineralization. Furthermore, although some agreement has been reached on the particular role of organic matter to uranium mineralization, its role on the dissolution and reprecipitation of the host silicates, its nature and origin have largely been ignored.
Recent investigations (Huang and Keller, 1970, 1971, and 1972a; Ong and others, 1970) show that naturally occurring organic acids, commonly called humic acids, possess significant cation exchange properties (sorption) and are effective agents in the colloidal transport of metallic elements. Since the organic acids are known to dissolve (or reprecipitate rock-forming silicate minerals (especially clay minerals which also have large cation exchange properties), detailed mineralogical and geochemical studies for both barren and mineralized zones should set further limitations on the origin of the sandstone-type uranium deposits.
The author (and his team) continued combined sedimentary petrologic, clay mineralogic, and geochemical and geochronical studies with the samples collected from selected localities in the Grants mineral belt.
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