PHASE ANALYSES OF URANIUM BEARING MINERALS FROM THE NOPLA I DEPOSIT, PEÑA BLANCA, MEXICO

The Nopal I uranium deposit is located in the Peña Blanca district, approximately 50 miles north of Chihuahua city, Mexico. The deposit was formed by hydrothermal processes within the fracture zone of welded silicic volcanic tuff. The ages of volcanic formations are between 35 to 44 m.y.. Most of the formations were subsequently silicified. The uranium ore body was up-lifted above the water table and is presently exposed at the surface. Detailed petrographic characterization, electron microprobe backscatter electron (BSE) imaging, and selected x-ray maps for the samples from Nopal I high grade ore document a complex uranium mineral paragenesis. There are apparently at least two stages of uranium mineral precipitation. Stage one uranium minerals precipitated from hydrothermal fluids that interacted with existing silicate minerals in the welded tuff. These uranium minerals were mainly precipitated along cleavages of feldspars. The major uranium enrichment process resulted in the alteration of stage one uranium minerals and the precipitation of stage two uranium minerals. Stage 2 uranium minerals occur along fractures in the tuff. Energy dispersive spectrometry and x-ray maps suggest that uranophane and weeksite are the dominant stage two uranium minerals. Variations in Ca and K contents in both minerals imply that the chemistry of stage one minerals influence the chemistry of second stage uranium minerals. Continued study is needed to fully characterize uranium behavior associated with the Nopal I deposit.