Petrographic and chemical study of prefault uranium ores from the Section 30, Section 30-west, and Section 23 mines in the Ambrosia Lake mining district, New Mexico has revealed that prefault ores commonly contain several authigenic phases including a new V-Ti mineral, which formed from destruction and remobilization of primary constituents in Ti-magnetites. High-grade ore samples also contain diagenetic clausthalite. Microprobe and SEM/EDS study indicate high U concentrations along the contacts of organic matter and surrounding detrital grains. The cores of the organic matter which fill pore spaces are commonly very low in U, as well as Si, Al, V, and Fe. Petrographic relationships as well as the chemistry and U distribution of the titanomagnetite grains and organic matter imply that the U was introduced to the sediments after the organic matter was emplaced and before the sediments were compacted.