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The AAPG/Datapages Combined Publications Database
AAPG Bulletin
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Empty or partly empty shells that conform to detrital rather than original crystal shapes of sanidine grains are present in host sandstones of the Morrison Formation. This somewhat paradoxical situation is explained by removal of sodium from a surface layer of the detrital grain during weathering, with concomitant conversion of this layer to microcline, which resists dissolution under conditions prevailing after sedimentation. During compaction, dissolution of this outer layer occurs at pressure points; once this layer is penetrated, dissolution of the interior proceeds along crystallographic directions and removes all or part of the unaltered sanidine.
Untwinned microcline also is present as minute crystals within shells and as outgrowths on both detrital potash feldspar grains and (rarely) on the shells. Uraniferous organic material occurs both under and over some outgrowths on detrital microcline, suggesting contemporaneity of outgrowths and organic material. Elsewhere, chlorite, reportedly contemporaneous with coffinite, coats both exteriors and interiors of feldspar shells and thus succeeds feldspar dissolution; calcite supersedes rather than replaces feldspars. Quartz outgrowths are commonly earlier than uraniferous organic material but later than jordisite. Locally, chlorite and/or hematite form total or partial pseudomorphs after pyrite. Rarely, marcasite is partially pseudomorphous after, and forms outgrowths on, pyrite.
These and similar observations by others reveal fragments of a paragenetic sequence complicated by the presence of both primary and redistributed ore. Further investigations may complete a sequence useful in determining conditions of mineralization, and thus in the discovery of similar ore deposits.
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