Structural control of uranium ore at the Monument No. 2 mine, Apache County, Arizona

Abstract

The Monument No. 2 uranium-vanadium deposit is in Monument Valley, Apache County, northeastern Arizona. The uranium and vanadium minerals are in sandstone and conglomerate of late Triassic age, the Shinarump member of the Chinle formation that fills an ancient stream channel cut through the Moenkopi formation of Triassic age and the Hoskinnini tongue of the Cutler formation into the DeChelly sandstone member of the Cutler formation of Permian age.Tyuyamunite, carnotite, becquerelite, corvusite, hewettite, metahewettite, rauvite, and uraninite are the principal ore minerals. They impregnate sandstone, fill fractures, and replace quartz, clay, and fossil plant fragments.The structural features lead to the interpretation that, during the Laramide orogeny in Late Cretaceous or Early Tertiary time, movement along the bedding planes brecciated the channel-filling sediments. Resistance of the thicker channel sediments to the bedding plane slippage set up stresses that formed a zone of en echelon strike-slip vertical faults along the channel. The ore-bearing solutions may have risen along the vertical faults from a deep source, and spread out to deposit ore in the highly permeable brecciated sandstone and conglomerate. Further upward movement of the solutions would have been retarded by the lenticular, relatively impermeable clayey siltstone that overlies the ore deposit.If this hypothesis is valid, zones of en echelon strike-slip faults cutting post-Moenkopi sedimentary rocks in Monument Valley may be an indication of buried channels and of ore deposits within the channels. Additional tectonic studies in the area are recommended to test the structural interpretation.