Abstract

A drill log database accumulated from over 30 years of uranium exploration drilling has been utilized for subsurface mapping of individual fluvial channel facies in the upper Fort Union Formation in the vicinity of the Highland uranium mine, Converse County, Wyoming. The distribution and high density of drilling in the four township study area facilitates accurate resistivity log correlation and permits the geometry, dimensions and paleodrainage directions of preserved sand channels to be readily defined. A phase of infiltration of these paleochannel aquifers by recharging meteoric water occurred after uplift of the basin margins to the south and southwest. The progress of this event can be mapped from the distribution of zones of hematitic alteration which have been preserved in the sandy facies. The alteration can be traced in the subsurface using a distinctive gamma-ray signature on the uranium exploration logs. The patterns of alteration demonstrate that the infiltrating fluids migrated in a general northeasterly direction, in contrast to the northwesterly paleoflow direction of the host channel facies. In the Highland area, multiple stacked fluvial channels have been infiltrated by fluids flowing first through a single regional sand horizon, which acted as a feeder, then downwards through relatively small zones of interconnection to penetrate the underlying channels. Trace quantities of dissolved uranium in these fluids accumulated at selected locations along the boundaries of these alteration zones forming economic deposits of this element.