Jones reported in November 1978 that a classification of uranium deposits in sedimentary rocks, presented earlier (Jones, 1978), contained nine major classes. In the present report, the first eight of these classes (placers, quartz-pebble conglomerates, marine black shales, phosphorites, water, lignite and related rocks, evaporative precipitates, and limestones) are described in greater detail and summarized by a set of recognition criteria. Recognition criteria for the ninth class, sandstone deposits, are presented elsewhere (Austin and D'Andrea, this volume).
Headings for the recognition criteria include geologic setting, host-rock characteristics, uranium mineralogy, and geometry of the ore. The recognition criteria are followed by a, list of selected references.
Mickle and Mathews reported that a classification of uranium deposits in sedimentary rocks, presented earlier (by Jones, 1978), contained nine major classes. In the present report, the first eight of these classes (placers, quartz-pebble conglomerates, marine black shales, phosphorites, water, lignite and related rocks, evaporative precipitates, and limestones) are described in greater detail and summarized by a set of recognition criteria.
Recognition criteria for the ninth class, sandstone deposits, are presented elsewhere (Austin and D'Andrea, this volume).
Headings for the recognition criteria include geologic setting, host-rock characteristics, uranium mineralogy, and geometry of the ore. The recognition criteria are followed by a, list of selected references.
The above is followed by Austin and D 'Andrea, Jr., who reported that three overall factors are necessary for formation of uranium deposits in sandstone: a source of uranium, host rocks capable of transmitting uranium-bearing solutions, and a precipitant. Possible sources of uranium in sandstone-type deposits include ground- waters emanating from granitic highlands, arkosic sediments, tuffaceous material within or overlying the host rocks, connate fluids, and overlying black shales. The first three sources are considered the most likely.
Host rocks are generally immature sandstones deposited in alluvial-fan, intermontane-basin or marginal-marine environments, but uranium deposits do occur in well-winnowed barrier-bar or eolian sands. Host rocks for uranium deposits generally show coefficients of permeability on the order of 1-100 gal/day/ft2.
Precipitants are normally agents capable of reducing uranium from the uranyl to the uranous state. The association of uranium with organic matter is unequivocal; H2S, a powerful reductant, may have been present at the time of formation of some deposits but may go unnoticed today. Vanadium can serve to preserve the tabular characteristics of some deposits in the near-surface environment, but is considered an unlikely primary precipitant for uranium.
Uranium deposits in sandstone are divided into two overall types:
1) pene-concordant deposits, which occur in locally reducing environments in otherwise oxidized sandstones; and
2) roll-type deposits, which occur at the margin of an area where an oxidized groundwater has permeated an otherwise reduced sandstone.
Uranium deposits are further broken down into four subclasses:
- Subclass 241: Roll-type deposits that occur in intermontane basins and (or) close to the source of the host sediments (example: Gas Hills, Wyoming).
- Subclass 242: Roll-type deposits that occur in coastal-plain environments and (or) at some distance from the source of the host sediments (example: South Texas).
- Subclass 243: Peneconcordant deposits that occur in discrete channels scoured into underlying strata (example: Monument Valley-White Canyon Districts, Utah-Arizona), and
- Subclass 244: Peneconcordant deposits that do not occur in discrete scours or channels (example: Uravan Mineral Belt, Colorado-Utah).
Uranium deposits in sandstone account for most of the production and reserves in the U.S. Productive sandstones are fluvial or marginal marine, rarely eolian. The ages of sandstone host rocks range from Paleozoic to Tertiary. However, most sizeable deposits in the U.S. are in the Chinle Formation, of Triassic age; the Morrison Formation, of Jurassic age; or various Tertiary formations which range in age from Paleocene to Miocene.
Three factors are necessary for formation of a uranium orebody in sandstone:
1. A source of uranium,
2. host rocks capable of transmitting uranium-bearing solution, and
3. a precipitant (s).
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