quartzite of the group is white, has a specularite-rich zone at the base, and is considered to be eolian. The rocks have been metamorphosed to the amphibolite faciès only near granites.
The undeformed Apache Group, deposited after the Mazatzal orogeny, has been intruded by swarms of diabase dikes and sills. A basal conglomerate, partially fluvial, is locally rich in hematitic debris and an overlying shale is red and hematitic. Up section is the Dripping Spring Formation, which includes a lower siltstone and conglomerate and an upper siltstone member.
Small uranium mines and occurrences in the upper siltstone are considered to be genetically tied to pyritic siltstone. Uranium enrichment is due to remobilization caused by diabase intrusions [45, 46, 47]. Within the Mescal limestone, above the Dripping Spring Formation, are local radiometric anomalies in chert, limestone breccia, and conglomerate [45, 46].
All sediments have been determined by radiometric dating to be younger than 1.74 b.y., and there is abundant evidence of a strongly oxidizing atmosphere during deposition. In the conglomerates most highly enriched in uranium (33 ppm uranium) [44], uranium and iron in specularite correlate positively. There is a lack of correspondingly high thorium content. Most conglomerates that have the slightly elevated uranium geochemical values are only weakly radioactive (2 or 3 times background), and none are pyritiferous.
The Precambrian conglomerates in the central Arizona Arch are considered unfavorable for uraniferous quartz-pebble conglomerates due to the absence of Archean rocks and favorable source rocks for detrital uraninite and to the young ages of the Precambrian sediments.
CONCLUSIONS
Comparisons of the geologic characteristics of selected exposed Precambrian terranes in the conterminous United States with those of known uraniferous quartz-pebble conglomerates elsewhere indicate that the U.S.
terranes as presently known have low potential for paleoplacer uranium deposits. Field investigations conducted as part of the NURE program, which included extensive geochemical sampling in all areas and drilling in two, focused on selected potentially favorable Precambrian rocks. These investigations failed to identify a significantly favorable geologic environment for uranium deposits in uraniferous quartz-pebble conglomerates.
Quartz-pebble conglomerates that have some characteristics in common with those of the Witwatersrand or Elliot Lake districts have been identified, but they lack significant concentrations of uranium or other metals. Conglomerate deposition between 2.0 and 3.2 b.y. ago on an Archean craton plus source-rock, depositional-environment, and preservation factors must all be met for favorable concentrations of uranium to be probable. For the known U.S.
conglomerates that meet time and depositional-site criteria, other crucial requirements either are not met or are indeterminable (Table 1).
The presence of uraniferous quartz-pebble conglomerates in the United States that do not meet the time and depositional-site criteria established for the economic deposits cannot be precluded, but the authors consider the
probability of their existence to be low. The information generated tends to support the time-space restriction as observed in the known uraniferous conglomerate deposits. Additional dating may reveal that lower Proterozoic rocks are more widespread than presently known. For example, the first lower Proterozoic rock in Alaska has recently been reported—an augen gneiss dated as 2.3 b.y. [49]. However, available information suggests that Arehean craton limits will remain approximately as currently known.
The most promising known U.S. areas that meet the age-of-deposition criterion are southeastern Wyoming, the Nemo area of the Black Hills, and southwestern Montana. However, in these areas the thorium and uranium are in thorium-rich heavy-mineral placer concentrations that have characteristics typical of conglomerates of any age. No detrital uraninite concentrations were found or indicated to have been present in appropriate hydraulic-equiva-lency depositional regimes. In addition, no fluvial oligomictic conglomerates in geologic settings favorable .for mechanical concentration and preservation of uraninite were found or indicated. In general, the rocks are metamorphosed and/or deformed to such a degree that the preservation of any placer uranium oxides that might have existed is doubtful.
Within the known archean cratons, in the Lake Superior and Wyoming regions, the presence of favorable conglomerates cannot be ruled out because investigations largely did not include the subsurface, and very little detailed geophysical and drilling information is available. Additional investigations proposed for both regions [4, 5, 6] deserve serious consideration.
The quartz-pebble conglomerate target having the most uranium potential in the United States may be the possible extension of members of the Huronian Supergroup beneath Paleozoic rocks in the Upper Peninsula of Michigan, as proposed by Button and Adams [5]. On the other hand, Karlstrom and others [5, 7] presented sedimentation studies that led them to suspect most quartz-pebble conglomerates were deposited south of present outcrops in Minnesota and Wisconsin. However, drilling in buried Precambrian rocks outside the generalized boundaries of the Archean craton is not known to have cut favorable metasedimentary strata. Indications based on available information are that the Proterozoic rocks examined are all too young to host deposits and that any older quartz-pebble conglomerates, if present, will be below younger cover on the craton.
There are many unanswered questions, especially in regard to age, concerning many, in most places poorly exposed, metasedimentary Precambrian rocks in the United States. Although favorable geologic environments have not been outlined to date, placer quartz-pebble conglomerate uranium deposits may be discovered in the Precambrian of the United States as geologic information becomes available and subsurface exploration targets are identified and
evaluated.
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