Geologic Environment
Arizona Strip
The "Arizona Strip" District is a region of northwestern Arizona near the Grand Canyon, in northern Mohave and Coconino counties. EFRC considers the "Strip" to be bounded on the north by the Arizona/Utah border; on the west by Grand Wash Cliffs; on the east by the Colorado River and Marble Canyon; and on the south by an east-west line roughly at the mid-point between the city of Flagstaff and the Grand Canyon.
Arizona Strip Claim Groups
The area is roughly 95 miles from north to south and 135 miles east to west, about 13,000 sq. miles in total. However, of this total area, over two-thirds (approximately 6.0 million acres) is withdrawn from mineral entry because of National Parks, Monuments, etc.
The Arizona Strip District is a relatively new uranium district that produced approximately 24 million pounds of U3O8 prior to the decline of uranium prices in the early 1990’s. Existing resources of about 30 million pounds of U3O8 contained in several breccia pipes have been reported. The pipes contain compact, high-grade uranium mineralization and individual pipes have been known to contain more than 7 million pounds of U3O8 at an average grade of about 0.70% U3O8.
The initial attraction to the Arizona Strip was the many circular structures which appeared on the satellite photos of the region. These structures indicated "pipe" features, which geologists recognized had the potential of hosting mineralization. Locating these structures was easy in some cases, as the circular delineations showed up readily in the geologic mapping, some of them as large as 2 ½ miles across. Others were far more challenging to locate, with only a 60 ft. diameter, and to further complicate matters, the pipes would occasionally occur in clusters.
The geologic setting of the Arizona Strip District is characterized by relatively flat-lying Triassic and older sediments overlying tilted Proterozoic sediments and Archean crystalline rocks. The figure below illustrates the geologic section exposed in the walls of the Grand Canyon. The rocks have been intruded locally by Cenozoic andesite, rhyolite, and basalt. The Kaibab Limestone is the most prominent surface formation as it forms the rim of the Grand Canyon and is exposed over the majority of the surface area in the flat terrain on both sides of the canyon.
Breccia pipes are collapse structures which consist of roughly cylindrical, nearly vertical columns of broken ("brecciated") rock. The geology specific to breccia pipes in northwest Arizona includes the Mississippian Redwall Limestone and younger rocks overlying the Redwall. The Redwall is key to breccia pipe geology as dissolution of the limestone formed voids that caused the eventual collapse of the rock units above. The collapse or "stoping" of beds higher in the column reached the Kaibab and Moenkopi formations at the surface resulting in a cylindrically-shaped nearly vertical column of broken (brecciated) rock up to 900m (3,000 feet) in vertical dimension and 40-200m (130-660 feet) in diameter in the subsurface. The stoping is believed to have begun soon after deposition of the Redwall Limestone, approximately 230 million years ago, and continued intermittently to the Triassic Period. Uranium-lead age dating of ore from the breccia pipes suggests that the main period of uranium mineralization occurred 200 to 220 million years ago.
Typical Breccia Pipe Section
The downward movement or collapse of rock material within the pipe structure can be demonstrated and confirmed by the presence of individual rock fragments containing fossils which are found in uncollapsed strata located several hundred feet vertically above the elevation of these fossils in the breccia pipe.
The history of breccia pipe mining near the Grand Canyon goes back to the turn of the 20th Century. The Orphan Mine was the first breccia pipe in the Arizona Strip District to be mined for uranium. This mine located on the escarpment below Maricopa Point (less than a mile from the current-day Grand Canyon Village) on the south rim of the Grand Canyon was staked for copper in 1893. The claim was patented in 1906 before the surrounding area became a national park protected by Federal law.
Uranium mining didn’t begin until the mid 1950’s. The last company to mine the Orphan was Cotter Corp. in 1969. In 1987, title to the mine reverted to the U.S. Government under terms of a 1962 public law. During the life of the Orphan Mine reported uranium production was approximately 4.4 million pounds of U3O8, at an average grade of 0.42%. Also production of about 6.68 million pounds of copper and 107,000 ounces of silver were reported.
Example of an Arizona Strip Breccia Pipe Mine
The North Kanab Mine operated by Energy Fuels Nuclear until 1991
Currently owned by Denison Mines and on Standby Status
During the 1980’s Energy Fuels Nuclear Inc. mined seven breccia pipe deposits and produced approximately 20 million pounds U3O8. No copper or silver was recovered from EFNI production. No other breccia pipes have been mined for uranium mineralization.
The high grade of the uranium ore from these breccia pipes; the significant experience of our people in locating, developing, and producing them; and the well defined and limited impact of breccia pipe uranium mines; all combine to make these extremely attractive targets for Energy Fuels.