Geological Survey Bulletin 1229 Geology of the Circle Cliffs Area, Garfield and Kane Counties, Utah

The presence of uranium-mineral concentrations in the Circle Cliffs area was the primary reason for remapping the area, and mineral prospecting by commercial interests and by the U.S. Atomic Energy Commission continued during the present study. Other metals that may attract future prospecting in this area are manganese, selenium, and titanium.

GENERAL CHARACTER AND DISTRIBUTION

Uranium minerals occur in the Moenkopi, Chinle, and Morrison Formations and are associated with some of the faults in the Circle Cliffs area. The only minable deposits are in a mineralized zone a few feet thick at the base of the Shinarump Member of the Chinle Formation and in a zone near the base of the Salt Wash Sandstone Member of the Morrison Formation.

Uranium deposits at the base of the Shinarump are associated almost entirely with channels (fig. 14), which generally range from a few tens of feet to 8,000 feet in width and from 10 to 100 feet in depth. The deposits are not restricted to any preferred position relative to the channel cross section, but all are restricted to a zone a few feet thick at the Shinarump-Moenkopi contact. The Moenkopi is always slightly radioactive in a zone a few inches to 1 foot thick adjacent to the Shinarump channel contact, but minable concentrations of uranium occur only in irregular elongate ridges, probably former streambanks, that extend as much as a few feet above the channel. In places the rock in the ridges is fractured, perhaps as a result of slumpage, and some of the wider fractures are filled with sandstone that probably sifted in when the Shinarump was being deposited in the channel.

FIGURE 14—Uranium prospects and distribution and channel pattern of the Shinarump Member of the Chinle Formation. Compiled by L. D. Carswell and E. S. Davidson, 1956-57. Inset (prospect 20) is 5 miles north of its actual position. Uranium prospects named on plate 1 and described on pages 71-91.

Most of the uranium ore mined in the Circle Cliffs area before 1958 was derived from these mineralized ridges in the Moenkopi. The mineralized ridges from high-grade ore pods that are a few square feet in cross section and as much as several hundred feet long. Where the ridge flattens out or merges into the general slope of the channel flank, the uranium is dissipated along a wide zone at the Shinarump-Moenkopi contact and is not minable. This type of deposit is shoestring-shaped in plan view, and because of the limit in cross-sectional area, it presents a very difficult drilling target and does not constitute a large ore body. The mined bodies are exposed at the surface and small enough to be mined profitably by two or three men. The Rainy Day, Stud Horse, Yellow Jacket, and Sneaky-Silver Falls prospects are typical of this type.

In channels, the most uniformly mineralized deposits are those that occur mainly in the Shinarump, particularly where the few feet of basal sandstone contains abundant 1/4-inch to 1-foot fragments of mudstone and at least moderate amounts of charcoaly wood. Such deposits contain widely disseminated uranium minerals and may be large enough to constitute practicable drilling targets. The grade of ore allowing, deposits of this type are the only ones that can be expected to exceed 10,000 tons in size. The Centipede and Horsehead prospects are typical of this type.

Several deposits in Shinarump channels consist of uranium minerals localized in scattered fragments and large logs of charcoaly wood. The uranium mineral concentrations are very spotty in such deposits because only a small proportion of the total amount of charcoaly wood is mineralized. The deposits generally are not minable. The Lone B and Cool prospects are typical of this type.

Deposits near the base of the Morrison Formation are confined to the lowermost sandstone unit of the Salt Wash Member. Uranium in the Dream prospect, the most thoroughly explored deposit, is evenly disseminated in a 3- to 4-foot-thick sandstone bed that contains abundant small pieces and flakes of charcoaly wood. Why uranium is concentrated in nearly minable deposits in some places near the base of the Salt Wash is not entirely clear; sandstone of the Salt Wash is not mineralized at other places where geologic conditions appear identical.

Some uranium-mineral claims have been located in the faulted rocks west of Wagon Box Mesa, and a radioactivity survey of several square miles in the faulted area was made in 1956. Most of the following discussion is abstracted from the resulting unpublished reports by D. D. Schultz and G. E. Thomas, who were members of the U.S. Geological Survey party in the Circle Cliffs area at that time. The faults are as much as a mile long and have a maximum displacement of about 40 feet. Nearly all are vertical or high-angle normal faults. The units displaced by the faults are the lower part of the Moenkopi Formation, the Kaibab Limestone, and the upper part of the Cutler Formation. Bleached zones a few feet wide occur along the fault traces. Locally, the fault surfaces are mineralized by copper carbonate minerals, pyrite, marcasite, and a mixture of hematite and limonite that may be a weathering product of the iron sulfides. No uranium minerals were identified in the mineralized rock, but a uranium mineral is probably responsible for the radioactivity along the fault surfaces. Most fault surfaces are more radioactive than the country rock, and those in the Moenkopi are more radioactive than surfaces of the same fault in the underlying Kaibab Limestone or sandstone of the Cutler Formation. However, uranium is not abundant in the fault zones, and the grade of the deposit is nowhere high enough to encourage mining.

I think that the assemblage and distribution of the minerals along the fault zones indicate that the mineralization resulted from redeposition of chemicals leached by ground water from overlying rocks. Other workers believe that the faults were channelways for hydrothermal solutions that rose from below and that the minerals were deposited from such solutions. Some drilling was done by the U.S. Atomic Energy Commission near the faulted zones, but the data thus obtained did not indicate whether downward-leaching ground water, upward-rising hydrothermal solutions, or laterally traveling solutions were responsible for the occurrences.

Members of the Chinle Formation other than the Shinarump locally contain low-grade concentrations of uranium. A few discrete sandstone lenses in the Monitor Butte Member contain uranium, especially in Wolverine Canyon and on the west side of Moody Creek. The lenses generally consist of sandstone containing chips and pellets of carbonaceous mudstone. The lithology is favorable for uranium deposition, but the lenses are widely separated. The mineralization, consequently, is sporadic and low in grade. In places, large charcoaly and silicified logs in beds near the base of the Petrified Forest Member of the Chinle Formation are heavily impregnated with carnotite. This type of occurrence is not minable and constitutes little more than a mineralogical oddity. Most of the mineralized logs have been found on the west side of the Circle Cliffs area.

MINERALOGY OF DEPOSITS

The primary uranium mineral in radioactive deposits in the Shinarump Member of the Chinle Formation and in the Moenkopi Formation probably is uraninite; but in the weathered parts of the deposits and also in the Monitor Butte Member of the Chinle, uranium occurs in carbonate, phosphate, sulfate, silicate, arsenate, and hydrated oxide minerals which were described by Weeks and Thompson (1954, p. 21). The deposits also contain pyrite, marcasite, sphalerite, and galena. Spectrographic analyses show that nickel, silver, molybdenum, cobalt, yttrium, and ytterbium also are associates of uranium ore in the area (Davidson, 1959, p. 444-446).

Carnotite is the only uranium mineral noted in logs in the Petrified Forest Member of the Chinle and in the Salt Wash Sandstone Member of the Morrison.

RELATION OF ASPHALT TO URANIUM DEPOSITS

The lowermost beds of the Shinarump Member contain asphalt in places. Generally the asphalt occurs as brown specks disseminated in the rock, but locally it is so abundant that brownish-black liquid asphalt seeps out along the contact with the Moenkopi. Where asphalt occurs in the Shinarump, some has penetrated into the upper few feet of the Moenkopi, and the ridges of Moenkopi on the channel contacts are heavily impreguated. A few selected samples of asphaltic Moenkopi from the Rainy Day mine and the Rocky Mountain Co. prospects were analyzed for organic carbon content in an effort to correlate the carbon content with the uranium content (table 1). The gray Moenkopi contains 0.16-1.68 percent organic carbon, but there seems to be no direct correlation between organic material and uranium content. One sample of red unaltered and virtually nonradioactive Moenkopi collected in the mine area contained 0.17 percent organic carbon.

TABLE 1.—Uranium and organic carbon content in rocks of the Moenkopi Formation at the Rainy Day mine
[Analyst, Wayne Mountjoy]

CORRELATION OF TECTONIC STRUCTURE AND URANIUM DEPOSITS

Although considerable work was done in an effort to correlate several deposits in the Circle Cliffs area with some phase of the regional geologic structure, no correlation was established. Deposits having a production of several hundred tons or more occur in many different positions on the Circle Cliffs anticline, and none seem related either to faults or to joints.

CRITERIA AND ORIGIN OF URANIUM DEPOSITS

In the Circle Cliffs area as elsewhere on the Colorado Plateau, the occurrence of uranium can generally be correlated with certain lithologic and geologic features, but these features are not always associated with economically minable deposits. Most of the geologic conditions around any mine or prospect are duplicated in barren areas of the Circle Cliffs district, and an ore body should not be expected in all places having the geologic attributes of the more mineralized prospects. Therefore, favorable criteria, which are simply the factors that aid in locating uranium deposits sufficiently concentrated to encourage prospecting, are not specific evidence of an ore body. But if the criteria are present, the chances of finding an ore body are much greater than if they are not. Botanical prospecting methods have been investigated (Kleinhampl and Koteff, 1960), but they have not proved to be any more specific than the geologic criteria.

In the Circle Cliffs area the principal criteria of favorable environments for uranium deposits, primarily in the Shinarump Member of the Chinle Formation, are—

1. A channel eroded in the top of the Moenkopi and filled with sandstone of the Shinarump. Channels about 3,000 feet or less wide and about 40 feet or less deep probably are most favorable for minable uranium concentrations, mainly because these channels contain more mudstone than the larger channels.

2. Material that is capable of precipitating uranium from solution. Charcoaly material (Moore, 1954; McKelvey and others, 1955; Gruner, 1956, p. 514) or hydrogen sulfide and the sulfide ion (Gruner, 1952; 1956, p. 514; Miller and Kerr, 1954) released upon decay of organic matter have long been regarded as effective precipitants; more recently Gruner (1958) pointed out that crude oils and "dried" crude oils also may precipitate uranium. The intimate association of iron and copper sulfides with uranium indicates that the precipitating atmosphere was reducing in nature. As discussed above, no direct unit-for-unit correlation between organic carbon content and uranium content was found in the Circle Cliffs area.

3. A lenticular sandstone that is continuous with the rest of the sandstone and that contains abundant fragments and thin beds of mudstone and abundant organic material or some other reducing material. In the Circle Cliffs area this type of sandstone occurs only along the base of the Shinarump channels, and it is not very thick or continuous there. This rock would probably be a more effective host rock if it were capped or overlain by a few feet of mudstone; unfortunately, this type of occurrence is rare in the area.

4. Lenses and blanket deposits of mudstone interbedded with and capping a mudstone-fragment conglomerate are major criteria in uranium mines of other districts, but such lenses are very uncommon in the Circle Cliffs district.

5. Proximity to the regional pinchout of the Shinarump, as suggested by Johnson (1959, p. 83, 91, pl. 6), because the lithologies outlined in 3 and 4 above commonly occur within 10-20 miles of such a regional pinchout.

The nature of uranium-transporting solutions has long been a subject of controversy and will not be reviewed here. The relative merits of hydrothermal solutions, ground water, or a combination of these, and of petroleum and carbon dioxide-charged solutions were discussed in detail by McKelvey, Everhart, and Garrels (1955) and by Gruner (1956). The distribution and position of uranium deposits in the Circle Cliffs area indicate that the ore solutions migrated along the beds and did not rise from below in the immediate vicinity of the prospects. The temperatures or the possible natures of the solutions are not known.

FUTURE OF THE AREA

The most favorable ground for additional prospecting is considered to be in the western and northern parts of the area, mainly because there the Shinarump channels are moderate in size, and the proportion of mudstone interbeds and fragments in the channeling sandstone is greater than in other parts of the area. However, no mines of consequence have yet been developed in the Shinarump in these parts of the area, and the depth of burial of the unit would probably preclude its exploration in many places. No pattern of ore bodies of significant size exists to allow confident prediction of more ore bodies.

In the Salt Wash Sandstone Member of the Morrison, the regional change in lithology in the Circle Cliffs area from a blanket-type sandstone to the south to a more lenticular sandstone to the north may partly cause uranium ore bodies to be concentrated near the area of lithologic change. The Dream and Solitude prospects are in the area of change; however, no minable deposits have been found, and the Salt Wash is too deeply buried to encourage exploration.