Regional Influence of Tectonics on Uranium Occurrences in Colorado Plateau Area: ABSTRACT

Abstract

Most uranium deposits of the Colorado Plateau are restricted to lenticular sandstone and conglomerates, and are characterized by relatively obvious local sedimentary controls. An argument can be made, however, that the areal and regional controls of uranium deposition are probably more closely related to the tectonic framework of the Plateau than to sedimentation. In addition, significant examples can be cited where even local controls are tectonic, such as the Woodrow Mine near Laguna, New Mexico, where ore is restricted to a breccia pipe, the Rajah Mine south of Gateway, Colorado, where ore occurs in and adjacent to a fault breccia, several fault-controlled deposits in Cane Creek Canyon, southeast Utah, and a deposit in Big Indian Wash, southeast Utah, that appears End_Page 793------------------------------ to be localized along a reflected deep-seated fault. Local control of uranium in the Todilto limestone north of Grants, New Mexico, is exercised by fractures and associated small folds and wrinkles in the limestone rather than by sedimentary features. Many additional examples are added when it is argued that the curved fractures bordering ore rolls pre-dated mineralization and thus restricted the distribution of ore. Although flexures and faults in many places control or influence primary ore distribution, joints do not. This is probably because joints are not through-going structures, but are restricted to brittle beds within the sedimentary pile, being absent from most of the numerous intercalated mudstones. Perhaps the best example of areal tectonic control on the Plateau is shown in the Big Indian Wash-Lisbon Valley uranium district. Here, all ore bodies are elongated approximately parallel with the strike of the beds along the Lisbon Valley anticline, and all except one occur in the elevation interval 6,200-6,700 feet above sea-level. The single exception, which is about 5,840 feet above sea-level, lies along the Lisbon Valley fault in the southern part of the district. A general anticlinal control appears also to exist in the Uravan Mineral Belt. On the regional scale, the peripheral distribution of most Plateau ore deposits about laccolithic mountains, plus a suggested zonal relationship between copper and uranium around the La Sal laccoliths, suggests that the intrusives may have both a structural and a petrogenic relationship to mineralization. Still broader structures appear to limit the occurrence and distribution of uranium in the Colorado Plateau. Significant deposits are not known to occur throughout the Plateau; rather they are restricted to the northwest-trending San Juan segment (as named by Vincent C. Kelley), which contains all the laccoliths. This structural segment is bounded by the Uncompahgre lineament on the northeast and by the Zuni lineament on the southwest. End_of_Article - Last_Page 794------------