Abstract

Most Plateau deposits occur in sedimentary strata which range from Paleozoic to Tertiary, with major concentrations in Triassic and Jurassic formations. A number are primary and retain the characteristics of original deposition, but in many localities secondary action has obliterated the original features.

Four hypotheses of uranium emplacement on the Colorado Plateau justify consideration: (1) the hydrothermal theory, (2) the circulatory ground-water theory, (3) the ash-leach theory, and (4) the syngenetic theory. Observations indicate that (1) is most plausible in the light of recent evidence.

Additional wall-rock alteration data confirm the permeation of Plateau strata by hydrothermal solutions associated with uranium emplacement. Chrome-mica clay distributed along vertical fractures in sandstone, certain mica polymorphs associated with ore introduction, recrystallization of the clay-mineral constituents of sedimentary strata, and clay haloes associated with ore provide constantly accumulating criteria. Recently discovered alteration sequences in which dolomitization, argillization, and silicification accompany emplacement provide further evidence.

Mineral deposits of the Plateau yield 11 principal associated metallic elements: Se, Mo, As, Cu, K, V, Co, Cr, Pb, Zn, and S. In the interpretation of the temperatures of emplacement, the minerals which contain Mo, As, Cu, Co, and Cr particularly provide hydrothermal data. In these, distribution, association, occurrence in both hypogene and supergene forms, and general mineral history favor original precipitation from solution in the temperature range 100° to 350°C.

Organic materials heavily impregnated with uranium indicate uranium emplacement at temperatures essential to produce cracking of hydrocarbons. Hydrocarbon fragments from sediments entrapped in sulfide-uranium-bearing vein material yield clusters of spherical gas pockets, an apparent product of natural cracking. This would imply temperatures above 275°C. in places.

Uranium mineralization and alteration are associated with breccia pipes and collapse features which vertically pierce horizontal Plateau strata. Similar mineralization and alteration may be found near by and occupying channels of wide horizontal extent. Uranium-bearing diatremes, associated volcanic vents, and intrusives occur near by. The igneous features in places furnish regional patterns.

Acceptance of the data on absolute age, wall-rock alteration, mineral association, organic-ore emplacement, and vertical ore-bearing conduits with the time and temperature factors implied casts serious doubt on both the syngenetic and ash-leach theories. Extensive modification of the circulatory ground-water theory is also required to provide an original magmatic source for the uranium ions. The weight of evidence supports uranium introduction into Plateau strata by hydrothermal activity. Nevertheless, subsequent distribution by ground-water and superimposed supergene action must be recognized.

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