U-PB SCHEELITE LA-ICP-MS DATES FROM THE HISTORIC YELLOW PINE PIT, YELLOW PINE AU-SB-W MINING AREA, CENTRAL IDAHO, USA

Two polished thin sections from the Yellow Pine pit ore body in the Stibnite District, Idaho yielded 44.33 ± 0.41 Ma and 44.7 ± 1.3 Ma U-Pb scheelite dates. We applied in situ laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) to co-existing ore minerals scheelite (calcium tungstate; CaWO₄) and stibnite (antimony sulfide; Sb₂S₃) from the historic Yellow Pine pit of central Idaho. Concentrations of U were variable (<1 to >10 ppm) but initial Pb concentrations were low (<0.01 ppm) in scheelite, such that individual spot U-Pb crystallization dates could be obtained via the ²⁰⁷Pb concordance correction using stibnite to anchor the initial Pb isotope composition. Groups of spot analyses within petrographically defined scheelite growth phases were combined to determine U-Pb dates via Tera-Wasserburg semi-total Pb regression analysis. Although we have not found an adequately homogeneous primary U-Pb scheelite standard material, the effects of mineral matrix and ablation geometry on the U/Pb fractionation factor were investigated and constrained by ablating a variety of mineral and glass standards at variable spot diameters.

These are the second set of U-Pb scheelite dates from the Yellow Pine mining area/Stibnite District and are statistically indistinguishable from the circa 45 Ma U-Pb scheelite dates from the Hangar Flats ore body, which is roughly 2 miles south of the Yellow Pine pit along the ore-controlling Meadow Creek Fault. Age of scheelite and stibnite mineralization has been a point of geologic disagreement since the early 1930's, with some interpreting all Au-Sb-W mineralization as Cretaceous, others suggesting all was Eocene, still others thought Au was Cretaceous and Sb-W was Eocene. Mineralization during the Eocene can be related temporally to abundant intrusive and extrusive rocks of the Challis magmatic province. Further work will attempt to correlate ore mineralization with specific events in the regional volcanic history.