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Abstract: Assessment of Magnetic Measurements in Uranium Exploration

Bruce Smith, James Scott, Richard Reynolds

Measurement of the magnetic properties of rocks, combined with geochemical studies, indicates that magnetic surface and borehole measurements will provide useful supplemental information for solving many uranium exploration problems.

In the area studied, there are systematic variations in the distribution of magnetic minerals (principally magnetite and ilmenite) relative to uranium mineralization in sandstone. Variations in the quantity of strongly magnetic detrital minerals partly result from their replacement by either oxidation or reduction to nonmagnetic or weakly magnetic minerals. The composition of the replacement phases and degree of alteration are related directly to the geochemical setting and thus provide useful control in interpreting borehole susceptibility and surface magnetic-field intensity measurements in and over some uranium deposits. Borehole-susceptibility logs obtained from holes drilled into a few sedimentary uranium deposits of Tertiary age indicate the presence of anomalously low susceptibility rocks associated with the uranium. Laboratory measurements on core samples show very low susceptibility values (generally in the range from 0 to 50 × 10-6 cgs units); thus, borehole magnetic-logging measurements must be very precise if optimum interpretation capability is to be achieved.

Where systematic variations in susceptibility are associated with uranium deposits, surface magnetic-field-intensity measurements may provide a simple, fast, and inexpensive geophysical method to delineate uranium deposits if they are at a suitable depth and if other paleosedimentary factors have not led to a variation in concentration of magnetic minerals which obscures the magnetic signature of the uranium deposit. Surface measurements, like borehole measurements, require high precision and low background-noise levels. Field data show magnetic lows associated with uranium mineralization.

The present data indicate that borehole and surface magnetic measurements may have an important, but generally overlooked, application in uranium exploration.

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