Harshman and Adams reported in 1980 that the study of roll-type deposits during the past 20 years, since the first description of a deposit in the United States, has developed general concepts of ore formation which are accepted widely and are compatible with available data. If this were not the case the concepts would not have endured and could not have been so successfully applied to exploration using the relations of altered-unaltered sandstone.
The comparative simplicity of the model, and the ease with which it has been applied to exploration have, oddly enough, probably inhibited detailed studies of ore districts that would have provided data now needed for refinement of ore controls for exploration and resource assessment programs. The most thorough study of a roll-type district was that of the Shirley Basin (Harshman, 1972) which is drawn on heavily in this report.
The general concept of roll-type formation provides a strong basis for the development of geological observations and guides, or recognition criteria, for resource studies and exploration. Indeed, industry has been developing and using them for 20 years. As the objective of this study was to identify the most useful recognition criteria and develop a method for their systematic use in resource studies and exploration, the study is best summarized by reference to the important geological observations about roll-type deposits.
REFLECTIONS AND CONTINUING STUDIES
The application of geologic information to resource studies and exploration for roll-type uranium deposits continues to evolve. Resource studies are only
now being developed, both in theory and in practice, and much room exists for improvement. By contrast, considerable exploration experience has been accumulated, and some discoveries will continue to result from pushing the "state-of-the-art" to deeper targets, geological analogous provinces, and lower grade ores. We expect, however, that critical review of "explored" productive and neighboring areas, and geologic environments with slightly but significantly different geologic characteristics, will also yield exploration rewards. It is toward both of these possibilities that we have attempted to identify geologic guides to aid in selecting targets and choosing priorities.
In spite of the abundance of data for roll-type uranium deposits, there are numerous inadequacies and ambiguities in the data, and therefore, in the conclusions one might draw from them. In this report, we have attempted to document the data for roll-type deposits, so that the uncertainties in the
interpretations we have drawn are readily obvious. Some of the more serious inadequacies in the database for these deposits are tabulated below, as a guide for future studies:
1) Too few roll-type deposits have been thoroughly and systematically studied. In our opinion, this should once again become a high priority of the
U. S. Geological Survey. Studies should not be limited to the deposits, or to their environments of formation, but should document and integrate the two. "Environment of formation" is used in the broadest sense to incorporate regional structural settings, uranium source rocks, and all other factors
responsible for important aspects of ore formation.
2) The economic characteristics of roll-type deposits, such as ore grade distribution, size and shape of ore pods, and frequency and distribution along
roll fronts, are geologically controlled. Geologic criteria should be developed to better anticipate these characteristics, for use in resource studies,
exploration, and mining. For example, the character of alteration zones in the Great Divide Basin seems to relate to the grade and width of the ore,
which probably reflects the geology and hydrology of the system. In the Powder River Basin, wider ore tends to be lower in grade, but there is no
indication that this is related to the character of the alteration zone. The documentation of such relations would undoubtedly improve all aspects of
resource exploration and mining activities.
3) Alteration associated with roll fronts is now recognized to take such a variety of forms that the model for ore formation is oversimplified. A geologist must be familiar with the range of examples, some of which are difficult to recognize, without much guide as to where the various types are likely to
occur. The habits and geologic controls of various alteration assemblages should be documented as an aid to anticipating ore occurrences in different
geologic environments. Why, for example, were iron-rich clays produced in greenish-yellow to yellow alteration assemblages in the Shirley Basin, whereas, reddish, hematitic alteration resulted in presumably roll front process in the Powder River Basin?
4) The abundance of certain elements associated with roll-type deposits varies between districts, particularly vanadium (higher in the Black Hills District and the Kaycee Area) and molybdenum (higher in the Gas Hills District and the Crooks Gap Area). Study of such variations will provide information on the source of elements and mechanisms of roll front formation.
5) Perhaps the most important single control on the habit and character of roll-type deposits is the abundance and distribution of reductant within the
host sandstone. The distribution of pyrite and organic material should be studied in the unaltered sandstones of all major districts, so as to ascertain
what observations will help anticipate the size, shape, and grade characteristics of deposits in new areas or in new host rocks.
6) The investigation of fluvial depositional environments, and its application to uranium deposits, has expanded significantly in recent years, much to
the credit of those who have popularized this work. It is now generally accepted as a tool in exploration. More emphasis and training are required, however, to transfer the theory of depositional environments to the interpretation of sedimentary systems in practical field problems.
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