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Exploration Techniques for Locating Uranium-Mineralized Breccia Pipes in Northern Arizona

Karen J. Wenrich

Thousands of solution-collapse breccia pipes may crop out in the canyons and on the plateaus of northern Arizona, and more than 80 contain uranium or copper mineralized rock; however, their small size and limited rock outcrop make them difficult to locate. Recognition of the pipes on the plateaus is particularly important because mining access to the plateaus is far easier than to the canyons.

Several reconnaissance geochemical methods have been tested to help locate suspected and mineralized breccia pipes. (1) A hydrogeochemical survey conducted on the 1,500 mi2 (4,000 km2) Hualapai Indian Reservation appears to yield anomalous values downstream from regions, such as Mohawk Canyon, where clusters of mineralized pipes occur. Pigeon Spring, east of the Pigeon mine, also had anomalous uranium (44 ppb). (2) A stream-sediment survey was not made on the Hualapai Reservation because the dilution factor that results from the large volume of country rock, compared to that of mineralized rock, swamps out any low-level geochemical signature contributed to streams by rock or soil overlying breccia pipes.

Several types of detailed geochemical and geophysical surveys, made over individual collapse features located through examination of aerial photographs and later field mapping, have generally been successful at delineating collapse features from the surrounding host rock: (1) rock geochemistry commonly shows Ag, As, Ba, Cu, Pb, Se, and/or Zn enrichments of from 3 to 100 times background levels over mineralized breccia pipes; (2) soil surveys appear to have the greatest exploration potential of the geochemical methods (e.g., samples collected from the centers of several collapse features show consistently twice the background value for a number of elements); (3) B. cereus bacterial surveys over a known mineralized pipe show significantly more anomalous samples within the ring fracture han outside the breccia pipe; (4) helium soil-gas surveys were over seven collapse features, but only two of the seven showed significantly higher values in the center of the pipe; and (5) geophysical surveys indicate that scalar audio-magnetotelluric (AMT) and E-field telluric profile data show diagnostic conductivity differences over mineralized pipes as compared to the surrounding terrane. These geophysical and geochemical surveys can help evaluate breccia pipe targets for drilling.

AAPG Search and Discovery Article #91043©1986 AAPG Annual Convention, Atlanta, Georgia, June 15-18, 1986.