Ore at the Mi Vida mine, San Juan County, Utah, consists of uraninite, coffinite, montroseite, a new vanadium oxide mineral, 2 corvusite, and minor secondary minerals. The ore occurs within the matrix of the lower Chinle calcareous sandstones and conglomerates, which are colored dark gray to black by ore minerals and carbonaceous material. The primary ore minerals fill interstices in the sandstone and replace calcite and fossil wood. The replacement process was partly selective: much uraninite replaced carbonaceous debris, calcite, and detrital grains, and the vanadium minerals principally replaced calcite of the sandstone matrix. A few sulfides, pyrite, galena, and greenockite, appeared at the same time or slightly later than the primary ore. Replacement textures are common in the ore, but age relations are not clear in all cases.