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The AAPG/Datapages Combined Publications Database

AAPG Bulletin

Abstract


Volume: 64 (1980)

Issue: 5. (May)

First Page: 795

Last Page: 795

Title: Tabular Uranium Ore in Poison Canyon Area, Morrison Formation, San Juan Basin, and Application of Lacustrine-Humate Model: ABSTRACT

Author(s): C. E. Turner-Peterson

Article Type: Meeting abstract

Abstract:

Surface ore trends in the Poison Canyon sandstone (of economic usage) coincide with a distinct facies in the underlying "K" shale. Uranium mineralization of the sandstone occurs only where the unit is underlain by an offshore-lacustrine, gray, pyritic mudstone facies of the "K" shale. Where the "K" grades laterally into a nearshore-lacustrine red mudstone facies, the overlying Poison Canyon sandstone is unmineralized.

These relations suggest that the lacustrine-humate model may account for the origin of tabular ore in the Grants Mineral Belt. The basic premise of the model is that humate in tabular sandstone ore deposits originated as soluble humic substances in the pore fluids of nearby offshore-lacustrine gray mudstones.

Gray mudstones deposited in reducing, alkaline conditions are considered potentially favorable humate "source rocks" because reducing conditions favor preservation of humic matter in the pore water of lake-bottom sediments and because alkaline conditions favor solubilization of these humic substances so they can be expelled with the pore fluids during compaction. In the "K" shale in the Poison Canyon area early diagenetic reducing conditions are indicated by the presence of pyrite and the dark gray color. Alkaline conditions resulted from alteration of volcanic ash incorporated in the muds; the pore water pH rose sufficiently high to strip the muds of the humic matter and move it, during compaction, into the overlying Poison Canyon sandstone where it was fixed as a humate deposit. Sub equently, uranium delivered by groundwater was fixed by the humate to form the tabular ore deposit.

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