EOCENE U-PB SCHEELITE LA-ICP-MS DATES OF STIBNITE-SCHEELITE MINERALIZATION IN THE YELLOW PINE AU-SB-W MINING AREA, CENTRAL IDAHO, USA
In situ laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) was performed on scheelite from 4 polished thin sections. Spot measurements were taken in different mineral zone types as revealed by cathodoluminescent images of the naturally blue fluorescing scheelite. Three zone types are: (1) moderate brightness brecciated fragments, (2) brighter veins and disseminated grains replacing carbonate, and (3) oscillatory zoned subhedral grains in veins and vugs. Scheelite has relatively high and variable U concentrations (<1 to >10 ppm) and low initial Pb (<0.01 ppm), making it amenable to U-Pb geochronology. Coexisting stibnite was analyzed to confirm and measure initial Pb isotopic compositions.
Analyses yielded reproducible U-Pb dates. Zone type 1 yielded mostly circa 45 Ma dates. Zone types 2 and 3 yielded mostly early 40’s Ma dates with a relatively younger mid 30’s Ma date. Scheelite grains of type 2 and 3 are clearly intergrown with stibnite grains and in apparent isotopic equilibrium, so stibnite mineralization occurred in the late Eocene. Mid to late Eocene U-Pb scheelite dates prove that some of the stibnite-scheelite mineralization was coeval with Challis volcanism associated with the neighboring Thunder Mountain caldera complex eruptions (50-43 Ma).