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Isotopic ratios and element concentrations of U-Pb and Sm-Nd for scheelite petrochronology

February 17, 2022

Instrument, standard, and sample data from application of in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICPMS) dating of scheelite in thin section, and sample data from isotope dilution thermal ionization mass spectrometry (ID-TIMS) scheelite dating are in this data release. Fractionation factor and ratio data in between NIST glass and natural scheelite as well as isotope dilution Sm-Nd scheelite data are also included. Scheelite is from two ore deposits within the Stibnite-Yellow Pine mining area, and analyses yielded bimodal U-Pb ages with major growth of scheelite in the range of 61.3 +- 4.1 to 55.1 +- 3.8 Ma and a second growth event involving both scheelite and stibnite at 45.8 +- 2.6 Ma. In situ analyses were calibrated and confirmed by ID-TIMS U-Pb lower intercept ages of two coarsely crystalline scheelite samples yielded ages of 56.62 +- 0.16 Ma and 57.52 +- 0.22 Ma. These results, in good agreement with 40Ar/39Ar ages of 57.08 +- 0.12 to 56.2 +- 1.2 Ma from adularia petrographically linked to scheelite, confirm primary scheelite mineralization occurred ca. 57.5 Ma. This geochronology resolves a long-standing debate about the formation age of scheelite and stibnite within the United States' largest antimony source. Calibration with ID-TIMS provided reliable ages, and crystals from the same sample were used as a matrix-matched primary standard material for LA-ICPMS. No matrix-match standard was previously characterized accentuating the importance of this newly characterized matrix-matched scheelite standard.