Science for Decision Making on Uranium Mining in Arizona

The uranium ore deposits in the breccia pipes of the Grand Canyon region are unusual, perhaps unique, and their genesis (processes that created them) remains mostly speculative. This study provides an opportunity to piece together a scientific puzzle, a model that explains their step-by-step creation. Constructing this model is not only an academic exercise but can also assist in the management of federal lands by improving understanding of where and why deposits can occur in the region. This allows land management agencies to plan for areas that have the potential for future exploration, discovery, and mining.

Objectives

Study several aspects of the breccia pipes of northern Arizona and their ore deposits to build a logical model that explains their formation, including the consideration of:

• Processes that created the host structures and set the stage for the deposition of the uranium and metallic minerals. Modern studies on the formation of collapse features provide insights that can help explain the creation of the vertical, pipe-like, solution-collapse structures ("breccia pipes") that host these uranium deposits. The regional geologic processes of the past that combined to create the geology of northern Arizona will be linked to the formation of the breccia pipes and their mineral deposits.
• Distinct assemblages of minerals in these deposits provide important clues into the source of the uranium and metals. Previous studies suggest that more than one phase of mineral deposition may have occurred, and these different phases may have occurred under different geologic conditions. Detailed microscopic studies utilizing scanning electron microscope analyses will determine the mineral phases present and examine their spatial, shape, size, and intergrowth relations in fine detail.
• Age(s) of the uranium deposition determined by isotopic analyses of the uranium minerals. Samples from different mines will be analyzed.¹
• Analyses of sulfur isotopes and fluid inclusions in ore minerals will provide insights into the character of the fluids that formed the assemblages of metal-rich minerals that occur with the uranium minerals.
• Ore bodies in these breccia pipe deposits consistently occur in the same rock units (formations). This observation provides another clue to the factors that influenced their formation.
• Geographic distribution of the known uranium-bearing breccia pipes will be analyzed, providing a view of the geologic factors that influenced their formation on a regional scale.

Photo credit: Karen Wenrich, USGS

Findings

• The study developed an updated “deposit model” that builds upon past USGS studies of the breccia pipe deposits of northern Arizona. In addition to describing the deposits, the paper produced by the study proposes a sequence of processes and geologic ingredients that led to their formation—the creation of the unusual host structures and their equally unusual mineral deposits.²
• The study considered a variety of observations ranging from regional to microscopic in scope. For example, the significant influences of stratigraphy and specific lithology at regional scales are explained. Characterization of the mineralization at detailed scales included microscopic analyses of mineralogy, mineral deposit chemistry, sulfur isotope systematics, and new uranium age determinations.^1-5
• High magnification showed that the uranium mineral, uraninite (uranium oxide), is complexly intergrown with at least 20 identified sulfur-metal-rich minerals, which variously contain copper, arsenic, cobalt, nickel, lead, zinc, and silver. The model shows that while the sulfur-rich minerals and the uranium oxide are intergrown, they represent different phases of mineralizing events due to different geologic processes at different episodes.
• The new proposed model could explain why some of the breccia pipes in the region contain uranium with metal deposits, while many are not mineralized. Perhaps most importantly, the model presents a new understanding useful to outline the region prospective for uranium-bearing breccia pipes in northwestern Arizona, an insight that substantially reduces and focuses the area for exploration.

Status

Completed

Specific Tasks

Task 1: Distribution and stratigraphic positions of breccia pipes
Task 2: Estimate the number of mineralized breccia pipes
Task 4: Compare breccia pipe to sandstone deposits

Sources

Principal Investigators

Bradley S. Van Gosen
Geologist
Central Mineral Resources Team
303.236.1566
bvangose@usgs.gov

Partners

Energy Fuels Incorporated

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