DOI: 10.5066/p95cpdad
title: U-Pb age determinations of uraninite by electron microprobe analyses of ore samples from two solution-collapse breccia pipe uranium deposits, Grand Canyon region, northwest Arizona, USA

description: This data release compiles the electron microprobe spot analyses of U, Th, and Pb concentrations in uraninite (U oxide) particles, and corresponding calculated age determinations, measured in samples of ore from two uranium-copper breccia pipe ore bodies, the Canyon (Pinyon Plain) and Hack II deposits. The U-rich samples that were analyzed typify the deposits hosted by solution-collapse breccia pipes in the Grand Canyon region of northwestern Arizona. Applying procedures outlined by Bowles (1990), the U, Pb, and Th measurements from each spot analysis were used to calculate a model age for the formation of each uraninite particle. The U, Pb, and Th analyses and calculated age determinations are provided as additional information on the timing and origin of the uranium deposition within the unusual breccia pipe deposits of northwestern Arizona. One of the analyzed samples (CMCH-053-21A) was selected from drill core of a U-Cu ore body of the Canyon deposit, hosted in a solution-collapse breccia pipe. This deposit lies about 750 to 2,000 ft (230 to 610 m) below the surface about 6.1 miles (10 km) south-southeast of Tusayan, Arizona, at latitude 35.88333 North, longitude -112.09583 West (datum WGS 1984). Energy Fuels Inc., owner and operator of the property, conducted extensive drilling into the Canyon deposit, delineating the extent and uranium and copper content of the ore bodies (Mathisen and others, 2017). Mining facilities, including a shaft, have been developed by Energy Fuels at the deposit. The company renamed the Canyon mine as the ?Pinyon Plain mine? in 2021. As of October 2021, they await favorable economic conditions to resume mining operations and recover the ore. An earlier-published data release (Van Gosen and others, 2020a) provides the geochemical analyses of 63 elements for 35 drill core samples of the Canyon deposit that were collected by the USGS. X-ray diffraction (XRD) analyses were performed on 28 of these samples to examine their mineralogy; the raw XRD data are provided in Van Gosen and others (2020a). In addition to the XRD analyses, ore mineralogy was also determined by examinations of thin sections of 21 of the ore samples using a scanning electron microscope equipped with an energy dispersive spectrometer (SEM-EDS). The mineralogical analyses are published in Van Gosen and others (2020c). The bulk geochemistry and mineralogy of Canyon deposit sample CHCH-053-21A, analyzed in this study, is provided in Van Gosen and others (2020a, 2020b). The geochemical and mineralogical analysis of ore samples collected from the Hack II deposit, also hosted by a solution-collapse breccia pipe, are published in another data release (Van Gosen and others, 2020b). That data release includes the bulk geochemistry and mineralogy of samples 84-HJW-12 and 84-HJW-3A, which were examined by this study. The Hack II deposit is one of four breccia pipes mined in Hack Canyon near its intersection with Robinson Canyon, approximately 30 miles (48 km) southwest of Fredonia and 9 miles (14.5 km) north-northwest of Kanab Creek, at latitude 36.58219 north, longitude -112.81059 west (datum of WGS84). Mining began at Hack II in 1981 and ended in May 1987. The USGS collected the samples from the Hack II mine in 1984 from underground exposures during active mining. The Canyon and Hack II deposits are representative of numerous other uranium deposits hosted by solution-collapse breccia pipes in the Grand Canyon region of northwest Arizona. These U-Cu deposits occur within matrix-supported, vertical columns of breccia (a "breccia pipe") that formed by solution and collapse of sedimentary strata (Wenrich, 1985; Alpine, 2010). The breccia pipes average about 300 ft (90 m) in diameter and can extend vertically for as much as 3,000 ft (900 m), from their base in the Mississippian Redwall Limestone to as stratigraphically high as the Triassic Chinle Formation. The regions north, south, and east of the Grand Canyon host hundreds of solution-collapse breccia pipes (Van Gosen and others, 2016). Six decades of exploration across the region has found that most of these breccia pipes are not mineralized or substantially mineralized, and only a small percentage of the breccia pipes contain ore-grade uranium deposits. The mineralized breccia pipes contain concentrations of uranium, arsenic, copper, silver, lead, zinc, cobalt, and nickel minerals (Wenrich, 1985), which is reflected in the data sets of Van Gosen and others (2020a, 2020b, 2020c). The Canyon mine sample (CMCH-053-21A) yielded generally consistent age results whether spots were divided by textural type or lumped together. Only those analyses from uraninite as inclusions in chalcopyrite (Cu sulfide) had unusual scatter, possibly due to Pb loss to the sulfide host. The full CMCH-053-21A data set provide a remarkably consistent estimated date at 118.0 � 4.5 million years ago (Ma) with a mean square of weighted deviates (MSWD) of 0.35 based on 44 of 47 measurements (the low MSWD suggests that the individual errors may have been overestimated). The number after the +/- is one standard deviation of the age in Ma. The Hack II mine sample (84-HJW-12) yielded a wide range of calculated dates. Fine-grained uraninite contained virtually no Pb and gave first-pass model ages all less than 3 Ma. These were not included in further analyses. The remaining 32 analyses, all from uraninite with the droplet-like texture, gave consistently older ages ranging 72 to 233 Ma. Individual droplets or clusters of grains typically gave more coherent age ranges. There was no compelling reason to reject any of these data; however, the aggregate weighted date of 151 � 16 Ma (MSWD = 12) is probably geologically meaningless. Examination of histograms of the whole data set and an older subset show evidence of several maxima. Grouping of these into younger, intermediate, and older groups is suggestive of coherent age ranges of 112 � 6, 175 � 9, and 213 � 10 Ma, respectively. A second sample from this deposit, 84-HJW-3A, yielded no usable geochronological data on microprobe analysis due to Pb concentrations below detection limits. The microprobe analyses of this sample are included in this data release as the chemical results may be useful. References cited above: Alpine, A.E., ed., 2010, Hydrological, geological, and biological site characterization of breccia pipe uranium deposits in northern Arizona: U.S. Geological Survey Scientific Investigations Report 2010-5025, 353 p., 1 pl., scale 1:375,000, at http://pubs.usgs.gov/sir/2010/5025/. Bowles, J.F.W., 1990, Age dating of individual grains of uraninite in rocks from electron microprobe analyses: Chemical Geology, v. 83, nos. 1-2, p. 47-53, https://doi.org/10.1016/0009-2541(90)90139-X. Mathisen, M.B., Wilson, V., and Woods, J.L., 2017, Technical report on the Canyon mine, Coconino County, Arizona, U.S.A.: NI 43-101 Report, prepared by Roscoe Postle Associates Inc. for Energy Fuels Resources (USA) Inc., October 6, 2017, 139 p., accessed October 18, 2021, at https://www.energyfuels.com/pinyon-plain-mine. Van Gosen, B.S., Johnson, M.R., and Goldman, M.A., 2016, Three GIS datasets defining areas permissive for the occurrence of uranium-bearing, solution-collapse breccia pipes in northern Arizona and southeast Utah: U.S. Geological Survey data release, https://doi.org/10.5066/F76D5R3Z. Van Gosen, B.S., Benzel, W.M., and Campbell, K.M., 2020a, Geochemical and X-ray diffraction analyses of drill core samples from the Canyon uranium-copper deposit, a solution-collapse breccia pipe, Grand Canyon area, Coconino County, Arizona: U.S. Geological Survey data release, https://doi.org/10.5066/P9UUILQI. Van Gosen, B.S., Benzel, W.M., Kane, T.J., and Lowers, H.A., 2020b, Geochemical and mineralogical analyses of uranium ores from the Hack II and Pigeon deposits, solution-collapse breccia pipes, Grand Canyon region, Mohave and Coconino Counties, Arizona, USA: U.S. Geological Survey data release, https://doi.org/10.5066/P9VM6GKF. Van Gosen, B.S., Benzel, W.M., Lowers, H.A., and Campbell, K.M., 2020c, Mineralogical analyses of drill core samples from the Canyon uranium-copper deposit, a solution-collapse breccia pipe, Grand Canyon area, Coconino County, Arizona, USA: U.S. Geological Survey data release, https://doi.org/10.5066/P9F745JX. Wenrich, K.J., 1985, Mineralization of breccia pipes in northern Arizona: Economic Geology, v. 80, no. 6, p. 1722-1735, https://doi.org/10.2113/gsecongeo.80.6.1722.