Mitchell and three co-authors reported in 1981 that uranium deposits occur in Cretaceous limestone at Sierra Gome z, Chrhuahua. Very few other localities are known to contain uranium in limestone, except in Arizona. One of these, the Tbdilto Formation in the Grants District in New Mexico, consists of quadravalent uranium, primarily pitchblende, in organic-rich limestone.
Another occurrence is the calcrete deposits of Yeelirrie, West Australia, which contain hexavalent uranium minerals syngenetically precipitated with the calcrete material. A third consists of hexavalent uranium deposits in solution channels and paleokarsts in the Madison Limestone (Mississippian) in Montana and Wyoming (Childers and Bailey, 1979). Only those deposits in the Madison Limestone appear to be analogous to those at Sierra Gomez.
McEldowney and others (L977) describe mineralized zones of three types in the Madison Limestone: (1) layered uranium miner alization interbedded
with silt and clay as karst fill; (2) angular fragments of country rock in clay and calcite matrix with uranium disseminated as crusts and coatings; and (3) vuggy and disseminated uranium associated with sulfides in brecciated rock.
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