Abstract: Abstract: Geological, Geochemical, and Petrophysical Features of a Roll Within a Tabular Uranium Body--Rajah 49 Mine, Mesa County, Colorado

Robert A. Brooks, Jeffrey C. Wynn

The Rajah 49 mine is located in the Gateway district, Colorado, near the northern end of the Uravan mineral belt. Uranium-vanadium ore occurs in the upper sandstone layer of the Salt Wash Member of the Jurassic Morrison Formation.

Three recognizable sandstone units coalesced to form the upper sandstone layer; the ore occurs along the edges of paleochannels within the middle unit. The deposit is a fairly typical tabular-type orebody. A crescentic roll feature within the deposit consists of a geochemical facies change in which the ore lies on the convex side and unmineralized sandstone lies on the concave side of a crescentic boundary zone having a radius of curvature of about 45 cm. The ore consists of impregnations of a variety of uranium and vanadium phases in a medium-grained sandstone.

Highest concentrations of vanadium occur on the concave side of the maximum uranium concentrations, and lead and selenium as clausthalite occur on the concave side of the vanadium ore. Copper and molybdenum concentrations are greater in the uranium ore zones. The distribution of elements is thus similar to that reported for other tabular uranium deposits, as well as to distributions reported for the roll-front deposits in Wyoming and Texas.

A series of petrophysical measurements was made on nearly 20 samples collected throughout the roll feature. These measurements included complex resistivity (magnitude and phase) over the 0.01 to 1,000-Hz range and magnetic susceptibility. The samples were all soaked in distilled water after collection and preparation, and this soaking fluid later was examined for both pH and leached-ion content.

Nonradiometric geophysical techniques did not respond well to uranium-vanadium mineralization. The usefulness of these techniques in exploration may be enhanced by combining them with geochemical and other approaches. In general, all of the samples showed anomalously high phase shifts compared to fresh sandstones, ranging as high as -53 milliradians at 0.1 Hz and -1,500 milliradians at 1,000 Hz. The spectral shapes did not vary significantly across the ore roll, but in nearly all cases they demonstrated a strong relaxation or unusual phase shift at 0.1 Hz and another at 100 Hz; both effects probably were due to minor clay content. Uncommonly high D.C. resistivities were recorded immediately adjacent to both sides of the ore zone. These high values may indicate either lower clay conten or blocked pore spaces. The magnetic susceptibility of all samples was uniformly low, typical of most sandstones, and ranged from 3 × 10^-7 emu/cc in the ore roll to as much as 6 × 10^-7 emu/cc on both sides. On the concave side of the roll feature, the pH of the soaking fluid ranged from 6 to 7, whereas on the convex side it remained uniform at about 5.

AAPG Search and Discovery Article #90969©1977 AAPG-SEPM Rocky Mountain Sections Meeting, Denver, Colorado