Summary:
Paleozoic Era sedimentary rocks of northern Arizona are host to thousands of breccia pipes. The pipes extend from the Mississippian Redwall Limestone up to the Triassic Chinle Formation, a total of approximately 4,000 ft of section. However, due to erosion and other factors, no single pipe has been observed cutting through the entire section. No pipe occurs above the Chinle Formation or below the Redwall Limestone. Breccia pipes mineralized with uranium are called Solution-Collapse Breccia Pipe Uranium deposits, which are defined as U.S. Geological Survey Model 32e.
Breccia pipes within the Arizona Strip District are vertical or near-vertical, circular to elliptical bodies of broken rock composed of slabs, rotated angular blocks, and fragments of surrounding and stratigraphically higher formations. The inclusion of breccia made of stratigraphically higher formations suggests that the pipes formed by solution collapse of underlying calcareous rocks, such as the Redwall Limestone. Surrounding the blocks and slabs making up the breccia is a matrix of fine material comprised of surrounding and overlying rock from various formations. For the most part, the matrix consists of siliceous or calcareous cement.
Breccia pipes are comprised of three interrelated features: a basinal or structurally shallow depression at the surface (designated by some as a collapse cone); a breccia pipe that underlies the structural depression; and annular fracture rings that occur outside but at the margin of the pipes. Annular fracture rings are commonly, but not always, mineralized. The structural depression may range in diameter up to 0.5 miles or more, whereas breccia pipe diameters can range up to approximately 600 ft, but normally range from 200 ft to 300 ft in diameter.
Mineralization in the breccia pipes takes place by water flowing along fractures and through porous materials that provide conduits for fluid flow and typically takes place in stages. Wenrich and Sutphin (1989) identified at least four separate mineralizing events within the Arizona Strip District pipes, with uranium and copper mineralization occurring during the last two.
To date, mineralized breccia pipes appear to occur in clusters or trends. Spacing between pipes ranges from hundreds of feet within a cluster to several miles within a trend. Pipe location may have been controlled by deep-seated faults, but karstification of the Redwall Limestone in the Mississippian and Permian Periods is considered to have initiated formation of the numerous and widespread pipes in the region.
Mineralization
Mineralization at the Project extends vertically approximately 1,700 ft, both inside and outside the pipe, but high-grade uranium and copper mineralization is found primarily in the collapsed portions of the Coconino, Hermit, and Esplanade horizons and at the margins of the pipe in fracture zones. Sulfide zones are found scattered throughout the pipe but are especially concentrated (within a sulfide cap) near the Toroweap-Coconino contact, where the cap averages 20 ft thick and consists of pyrite and bravoite, an iron-nickel sulfide. The ore assemblage consists of uranium-pyrite-hematite with massive copper sulfide mineralization common in and near the high-grade zone. The strongest mineralization appears to occur in the lower Hermit-upper Esplanade horizons in an annular fracture zone.
The metal of interest at the Project is uranium, though significant copper mineralization co-exists in the breccia pipe. As the breccia within the pipe consists entirely of sedimentary rocks, mineralization typically occurs in the matrix material (primarily sand) surrounding the larger breccia clasts.
Uranium Mineralization
Uranium mineralization at the Project is concentrated in three stratigraphic levels or zones (Upper/Cap, Main, and Juniper) within a collapse structure ranging from 80 ft to 230 ft wide with a vertical extension from a depth of 650 ft to over 2,100 ft, resulting in approximately 1,450 ft of mineralization. Mineralized intercepts range widely up to several tens of feet with grades in excess of 1.00% U3O8 . In previous reports and EFR news releases, the mineralization was subdivided into six distinct zones; those six have been combined into the three listed above for simplicity. The Upper/Cap Zone combines the previously reported Upper and Cap Zones. The Main Zone combines the previously reported Main and Main-Lower zones, and Juniper combines the previously reported Juniper I and Juniper II zones.
Age dating of mineralization (U-Pb) indicates a range of 101-260 million years, suggesting that the earliest uranium mineralization occurred in the Permian Period, before the pipes fully formed in the Triassic Period.
Consistent with other breccia pipe deposits, in the mineralized zone, the uranium mineralization occurs largely as blebs, streaks, small veins, and fine disseminations of uraninite/pitchblende (UO2 ). Mineralization is mainly confined to matrix material, but may extend into clasts and larger breccia fragments, particularly where these fragments are of Coconino sandstone. Uranium mineralization occurs primarily as uraninite and various uranium-phase minerals (unidentified minerals), with lesser amounts of brannerite and uranospinite.
Copper Mineralization
Currently, there is no reasonable prospect of economic copper extraction at the Project.
Significant copper mineralization occurs at the Project within the Main Zone and, to a lesser extent, in the Main-Lower zone, both with and without uranium mineralization.
Copper mineralization can be disseminated throughout the matrix material (commonly replacing calcite cement) with higher-grade mineralization typically occurring as vug fills, blebs, or streaks within the matrix and sometimes zoning the breccia clasts. The highest-grade copper mineralization completely replaces the matrix cement or replaces the matrix material altogether.
Copper mineralization occurs primarily as tennantite, chalcocite, and bornite with lesser amounts of covellite. Pyrite and sphalerite are also found throughout the pipe. Silver is commonly associated with the copper mineralization in the Main Zone. Assay values of silver greater than one ounce per short ton are common where copper grades are high. Arsenic is present where tennantite mineralization occurs. Additionally, lower quantities of silver, zinc, lead, molybdenum, copper, nickel, and vanadium are present and scattered throughout the pipe.