Hematitic oxidation in the Westwater Canyon Member of the Morrison Formation extends along the outcrop from the Pipeline fault northeast of Gallup, New Mexico, to the San Mateo fault north of Grants, New Mexico. The hematitic sandstone forms a broad lobe in the subsurface to a depth of 730 m (2,400 ft). The downdip edge arcs eastward from northeast Church Rock through Crownpoint, and southeastward to the west edge of the Ambrosia Lake district. The red sandstone is bordered on the downdip side by a band of limonitic oxidation which interfingers with reduced sandstones basinward. The limonitic oxidation forms a relatively narrow band along the north and west sides of the hematitic lobe, but expands progressively on the east and southeast. Weak limonitic oxidation, as indic ted by the absence of pyrite and a bleached to faint yellowish-gray color, appears to extend from the San Mateo fault eastward under Mount Taylor to the Rio Puerco.

The hematitic oxidation is epigenetic and is believed to be of late Miocene to early Pliocene age. The limonitic oxidation follows the present groundwater flow pattern and probably dates from late Pliocene to recent. The oxidation patterns are important in uranium exploration because the hematitic area is essentially barren, whereas the limonitic areas contain ore deposits which are in the process of being destroyed by oxidation.

End_of_Article - Last_Page 694------------