Rackley, Shockey, and Dahill reported in 1968 that a geologic concept that guides exploration for uranium in sandstone-type deposits can succeed only if that concept deals effectively with the major generic factors concerning ore concentrations. These factors concern:
1) the source and method of transportation of the uranium,
2) the relationship of the deposits to structurally positive areas,
3) the physical-chemical nature of the host rock,
4) the character of the concentration process and its capabilities to produce the minerals in the quantities observed,
5) the amount and distribution of carbonaceous matter, and
6) the shape of the orebodies.
Exploration guides for sandstone-type uranium deposits were developed empirically by the U.S. Atomic Energy Commission from experience in the early exploration of the late 1940's and early 1950's. The development of uranium deposits during the past twelve to fifteen years has added substantially to the knowledge of uranium geology. There are complex relationships in the deposition of a uranium orebody, which are commonly unrecognized and invariably poorly understood. There has been much written on the association between carbonaceous material or organic matter and sandstone-type uranium deposits. Pyrite is also present in amounts which make one think that it too plays a role in the deposition of uranium and again, the part played is not clearly understood.
The sulfur isotope studies have indicated that some of the pyrite is biogenic and that there is a relationship of unknown significance between the biogenic sulfides and uranium deposits. There have been a number of papers on solution fronts or rolls, some of which attribute the fronts to contrasts in oxidation-reduction potential (by a number of geoscientists), while Melin (1964) attributed the front to neutralization of an acidic solution. Some geologists think that tuffaceous sediments are the uranium source. Others, including the authors, think that the granitic rocks and sediments derived from them are the uranium source. Still others think that the uranium is from deep seated igneous sources. While it cannot be denied that all of these possibilities exist, the authors prefer the granitic source because it eliminates the need for external sources and avoids some problems of transportation.
The key to the source is probably found in the process that produces the deposit, therefore, the events which take place in the leading edges of geochemical cell, or in the solution front, shall be examined. It is easy to say that a solution front is the combined product of chemical, physical and biochemical reactions. To say what these reactions are is an entirely different matter. The need to combine the available knowledge into a working hypothesis is obvious if the renascent uranium exploration effort is to fulfill the anticipated demand for nuclear fuel. The concept elucidated here is a genetic hypothesis used to guide exploration for sandstone-type uranium deposits. The hypothesis may be applicable broadly in the search for vanadium, copper, silver and other metaliferous sandstone-type deposits. This concept has been used successfully to explore for uranium within known districts and in the discovery of new districts, the magnitudes of which are as yet largely undefined. In a more detailed sense, the concept is applicable to mine planning and ore control.
An orebody is a unique geological occurrence in which one or more elements have been concentrated a few times to a few hundred or more times their average abundance. The average abundance of uranium is 4 ppm for all rock. Ore bodies of 0.20% or 2,000 ppm uranium contain 500 times as much uranium as the average rock of the earth's crust. It is interesting to note that 4 ppm is equivalent to more than 40,000 tons of uranium metal per cubic mile or 96,000,000 pounds of U3O8. Uranium is plentiful but it is concentrated into orebodies only under uncommon conditions. The authors believe that this concept provides a skeleton on which to hang the details of uranium concentration processes yet to be developed. The authors hope to generate thought and discussion leading to more understanding of these processes.
The authors concluded that hat the concept discussed in this paper is effective in exploration, in mine planning and in ore control but how well it works depends directly on how much the geologist works the geology. Some areas have readily recognizable alteration, while other areas have very subtle differences between the altered and the protore portions of the cells. The principal criteria used are color, pyrite morphology, kaolinization of feldspar, cementation, low gamma ray background of altered zones commonly accompanied by gamma-ray anomalies at the upper and lower margins of the altered zone, and tripolitic alteration of chert. Analyses to determine the presence and distribution of trace elements may also be useful.
Pyrite morphology is one of the more consistent guides in determining alteration or protore. In the details of ore control and mine planning, the wedge shape of the overall cell is important, principally when the upper and lower limbs begin to approach commercial ore. From this point of the edge of the cell, whether it's the leading edge or the lateral edge, the entire thickness of the altered zone can be expected to have ore grade deposits on its surface. An abrupt change in the thickness of the altered zone indicates a possible ore-bearing interval, which has been missed between drill holes.
A good way to assist in the definition of potential ore areas is to isopach the interval between the upper and lower limbs of the cell. A more definitive technique is to contour the upper and lower surfaces of the alteration individually. Ore trends which may be commercially significant can be missed by the drill holes themselves but their presence can be predicted on the basis of the contours and further tested by drilling to determine their presence.
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