EOCENE U-PB SCHEELITE LA-ICP-MS DATES OF STIBNITE-SCHEELITE MINERALIZATION IN THE YELLOW PINE AU-SB-W MINING AREA, CENTRAL IDAHO, USA

Gold, antimony, and tungsten ore minerals are localized along the Meadow Creek fault zone that cuts Late Cretaceous granodiorite of the Idaho Batholith and Neoproterozoic to Ordovician strata of the Stibnite roof pendant in the Yellow Pine mining area, central Idaho. Paragenetic studies show the gold mineralization, as disseminated auriferous pyrite and arsenopyrite, is cut by stibnite-scheelite breccia and veins. Ar-Ar adularia geochronology constrained gold mineralization to circa 51 Ma.

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).