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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 5. (May)

First Page: 963

Last Page: 963

Title: Relation of Calcite Cementation and Uranium Mineralization in South Texas: ABSTRACT

Author(s): Steven M. Murray, Harald S. Poelchau, Stephen D. Meyers

Article Type: Meeting abstract


Host sandstones for uranium ore in the Miocene Oakville Formation commonly show strong calcite cementation. These hard, limy ledges are found in the area of the mineralization front and over the barren interior. They commonly occur at the top and the base of the host sandstone body, while the sand in between is only slightly calcareous and highly permeable. Outside the mineralization front, in chemically reduced zones, cementation is less pronounced or absent.

Precipitation of calcite cements occurred both before and after uranium mineralization, and has replaced some of the framework mineral grains. Uranium is usually found in the uncemented part of the sand; in only a few places is it in direct contact with calcite.

Carbon isotope values of the cements are interpreted to indicate that the carbon is the result of contribution from organic matter. Calcite in the highly cemented zone (> 30% calcite) has ^dgrC13 values of -11 to -20 ppt whereas values in the slightly cemented sands of the proto-ore and altered interior range from 0 to -5 ppt. The light carbon in these cements associated with the uranium ores probably entered the sediments as light hydrocarbons associated with the H2S which precipitated pyrite. Pyrite has been documented in other south Texas deposits as resulting from H2S generation in the Edwards Limestone at depth with subsequent migration.

It appears that the occurrence of calcite cement is genetically related to the emplacement of uranium ore. The distribution of limy ledges, which can be mapped from electric logs, elucidates the geologic history of a uranium deposit and serves as an aid in exploration.

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