Factors that localized uranium deposition in the Dakota Sandstone, Gallup and Ambrosia Lake mining districts, McKinley County, New Mexico

Abstract

Geologic studies were made at all of the uranium mines and prospects in the Dakota Sandstone of Early(?) and Late Cretaceous age in the Gallup mining district, McKinley County, N. Mex. Mines in rocks of Dakota age of the adjacent Ambrosia Lake mining district were visited for comparative purposes. Mines in the eastern part of the Gallup district and in the Ambrosia Lake district are on the Chaco slope of the southern San Juan Basin in strata that dip gently northward toward the central part of the basin. Mines in the western part of the Gallup district are along The Hogback (Nutria monocline) in strata that dip steeply westward into the Gallup sag. Geologic factors that controlled formation of the uranium deposits in the Dakota Sandstone are (1) the presence of a source of uranium, believed to be uranium deposits of the underlying Morrison Formation of Late Jurassic age; (2) the accessibility to the Dakota of uranium-bearing solutions from the Morrison; (3) the presence in the Dakota of permeable sandstone beds overlain by impermeable carbonaceous shale beds; and (4) the occurrence within the permeable Dakota Sandstone beds of carbonaceous reducing material as bedding-plane laminae or as pockets of carbonaceous trash. Most of the uranium deposits in the Dakota Sandstone are found in the lower part of the formation in marginal-marine distributary-channel sandstones. However, the Hogback No. 4 (Hyde) mine (Gallup district) is found in sandy paludal shale, and another deposit, the Silver Spur (Ambrosia Lake district), is found in what is interpreted to be a massive beach or barrier-island sandstone in the upper part of the Dakota. The most favorable sedimentary depositional environments for the accumulation of uranium are the areas lateral to main distributary channels, where levee, splay, and some distributary-channel sandstones intertongue with gray carbonaceous shales and siltstones of the well-drained swamp environment. Black carbonaceous shale beds formed in the poorly drained swamps in the interfluve areas are not favorable host rocks for uranium. The depositional energy levels of the various environments in which the sandstone and shale beds of the Dakota were deposited govern the relative favorability of the strata as uranium host rocks. In the report area, uranium commonly is in carbonaceous sandstone deposited in medium-energy channels. A prerequisite, however, is that the sandstone be overlain by impermeable shale beds. 1 2 URANIUM DEPOSITION IN THE DAKOTA SANDSTONE Medium-energy fluvial conditions result in the deposition of sandstone beds having detrital carbonaceous material distributed in laminae or in trash pockets on bedding planes. The carbonaceous laminae and trash pockets provide the necessary reductant to precipitate uranium from solution. High-energy fluvial conditions result in the deposition of sandstones having little or no carbonaceous material. Low-energy swampy conditions result in carbonaceous shale deposits, which are generally barren of uranium because of their relative impermeability ...