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The AAPG/Datapages Combined Publications Database

AAPG Bulletin


Volume: 65 (1981)

Issue: 4. (April)

First Page: 768

Last Page: 768

Title: Geology of Aurora Uranium Prospect, Malheur County, Oregon: ABSTRACT

Author(s): Michael W. Roper, Andy B. Wallace

Article Type: Meeting abstract


The Placer Amex Aurora uranium prospect is located 2,000 ft (610 m) southwest of the old Bretz mercury mine and adjacent to the Cordex Syndicate Bretz uranium prospect, within the northern rim of the McDermitt caldera complex. Drilling has defined 17 million tons (Mg) of mineralization at a grade of 0.05% U3O8. The shallow mineralized zone is 500 by 1,500 ft (152 by 457 m) in area and up to several hundred feet thick. The long axis of the deposit is subparallel with the northwest-trending caldera rim fracture passing through the Bretz open pits.

Uranium mineralization occurs dominantly as very fine-grained uraninite and coffinite localized in highly altered vesicular to scoriaceous flow tops and breccia layers within a complex, intermediate lava sequence. Volcanic rocks are locally covered by several hundred feet of tuffaceous lacustrine sediments. Rhyolitic rocks beneath the Aurora lavas possess an asymmetric, anticlinal upper surface, the axis of which coincides with the trend of mineralization.

Associated alteration minerals include montmorillonite, leucoxene, opal, clinoptilolite, and framboidal pyrite. Mineralization was apparently introduced by an epithermal mechanism which added altering fluids and uranium-bearing solutions along a complex, steeply dipping fracture system coincident with the axis of the mineralized zone. A supergene mechanism may then have spread the altering and mineralizing solutions laterally along the more permeable layers within the lava sequence. Mineralization may have occurred during the time of deposition of 50 to 70 ft (15 to 21 m) of uranium-enriched tuffaceous sediments which directly overlie the Aurora lavas.

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