SoW_fission_2.pdf

US Geological Survey

Alaska Science Center

Statement of Work

1.0 INTRODUCTION

The USGS needs to acquire apatite fission-track (AFT) and U-Pb analyses from geological samples collected from the Kuskokwim mountains (Km) region, Alaska during the 2026 field season.

2.0 BACKGROUND

The USGS conducts regional geologic and metallogenic framework studies that yield insight to the distribution of critical mineral resources in the United States. AFT thermochronometry is a technique of isotopic dating that can elucidate the timing, rates, and magnitude of rock exhumation. Combined AFT and apatite U-Pb analysis of bedrock samples from several mineral prospects in the Km will be used to test hypotheses about the depth of formation and post-formation exhumation of the deposit-critical information in refining conceptual models of ore deposit formation. Samples are also distributed along the Iditarod-Nixon Fork, Kaltag, and Denali fault systems. Phases of exhumation are anticipated to correspond to punctuated deformation along these fault systems as a function of the regional tectonic framework, with particular importance to understanding how and when ore deposits are potentially laterally translated. These constraints are important to both understanding the regional tectonic evolution of the area, as well as testing hypotheses regarding the spatiotemporal relation between multi-stage fault activity and mineralization in the region. The second planned use of this data is to create regional exhumation maps to inform next-generation mineral prospectivity mapping efforts.

3.0 SCOPE

AFT+U-Pb thermochronometry requires access to and expertise in the use of all supplies and instrumentation for preparing samples and counting and characterizing spontaneous fission-tracks (e.g., sample mounting, track etching by immersion in HNO3, and precision microscope capable of at least 2000x magnification). Critical to scientific objectives are measurements of track length that follow modern best practices (e.g., 252Cf irradiation), which additionally requires access to a vacuum chamber and fission fragments from a 252Cf source. Measurement of grain-by-grain Uranium and Calcium isotopes for final AFT age calculation and Uranium and Lead isotopes for complimentary U-Pb age calculation requires access to and expertise in a Nd: YAG (or similar) laser and sample cell linked to an inductively coupled plasma mass spectrometer (LA-ICP-MS)

USGS personnel will provide two types of samples: 14 samples of sedimentary and igneous bedrock and two samples of unconsolidated sediment. Submitted sample volumes will not exceed 5 kg of the freshest and least weathered rock available at each site of interest.

4.0 APPLICABLE DOCUMENTS

Publications documenting state-of-the-art analytical methods for collection and interpretation of AFT+U-Pb data:

Donelick, R. A., & Miller, D. S. (1991). Enhanced TINT fission track densities in low spontaneous track density apatites using 252Cf-derived fission fragment tracks: A model and experimental observations. International Journal of Radiation Applications and Instrumentation. Part D. Nuclear Tracks and Radiation Measurements, 18(3), 301-307.

Donelick, R. A., O’Sullivan, P. B., & Ketcham, R. A. (2005). Apatite fission-track analysis. Reviews in Mineralogy and Geochemistry, 58(1), 49-94.

Gleadow, A. J., Belton, D. X., Kohn, B. P., & Brown, R. W. (2002). Fission track dating of phosphate minerals and the thermochronology of apatite. Reviews in mineralogy and geochemistry, 48(1), 579-630.

Hasebe, N., Barbarand, J., Jarvis, K., Carter, A., & Hurford, A. J. (2004). Apatite fission-track chronometry using laser ablation ICP-MS. Chemical Geology, 207(3-4), 135-145.

Ketcham, R. A., Carter, A., Donelick, R. A., Barbarand, J., & Hurford, A. J. (2007). Improved modeling of fission-track annealing in apatite. American Mineralogist, 92(5-6), 799-

810.

Ketcham, R. A., Carter, A., Donelick, R. A., Barbarand, J., & Hurford, A. J. (2007). Improved measurement of fission-track annealing in apatite using c-axis projection. American

Mineralogist, 92(5-6), 789-798.

5.0 TECHNICAL REQUIREMENTS

The contractor shall have experience in all aspects of AFT+U-Pb thermochronometry using conventional track counting techniques, established calibrated against community standards, 252Cf irradiation, and LA-ICP-MS methods.

Of total submitted samples, data should conform to all aspects of standard AFT analysis and include age determinations for 40 grains and >100 track length measurements (where feasible, with the understanding that each sample will have natural variability in apatite yield and quality not able to be determined until mineral separations are complete). Two samples are from unconsolidated sediment collected from modern rivers and streams. For these samples, track lengths are not required but at least 80-100 grains should be measured for AFT age determinations.

6.0 DELIVERABLES

The contractor shall provide deliverables in a report in Excel spreadsheet format that includes:

Raw spontaneous fission-track counts and count area; associated kinetic parameters (e.g., Dpar and/or rmr0); raw and analytical standard-corrected measurements of 40Ca, 147Sm, 204Pb, 206Pb, 207Pb, 232Th, and 238U by LA-ICP-MS; fission-track lengths, angles to grain c-axis, and associated kinetic parameters; analyst calibration factors (e.g., ζ and uncertainty); and final calculated AFT and U-Pb dates.

7.0 PERIOD OF PERFORMANCE

Award to December 31, 2027