GEOLOGY, GEOCHRONOLOGY, AND GEOCHEMISTRY OF THE STIBNITE-YELLOW PINE GOLD-ANTIMONY-TUNGSTEN MINING AREA, IDAHO
Wintzer, Niki Everette
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The Stibnite-Yellow Pine mining area in central Idaho hosts gold, antimony, and tungsten in metasedimentary strata and Idaho batholith rocks. Garnet Lu-Hf geochronology and geothermobarometry yields an Early Cretaceous metamorphic age for the metasedimentary strata (112.8 ± 6.9 Ma) and mid-crustal pressures and temperatures of 7.5 kilobars and 775 °C, which are similar to the Salmon River suture zone 70 km to the west. In conjunction with stibnite fluid-inclusion determined pressures, ~24 km of crustal exhumation from 99 to 46 Ma is documented. New U-Pb zircon geochronology reveals magmatic events at 98, 88, 84, and 82 Ma that are mostly synchronous with the early metaluminous suite of Atlanta lobe in the Idaho batholith. U-Pb geochronology also reveals garnet-clinopyroxene skarn formation at 82 Ma and the youngest post-ore dike at 41 Ma within the mining area. Whole-rock major and trace element and radiogenic isotope Sr, Nd, Pb, and zircon Hf values of Cretaceous plutonic rocks from the mining area show they are transitional between the early metaluminous and Atlanta peraluminous suites. The same analyses but of mining area Eocene dikes compared to regional geochemical values of Challis volcanic rocks suggest that both were derived from a similar source. Similarities between whole-rock Pb isotopic signatures of plutonic mining area rocks and Idaho black shale belt (IBSB) galena reveal the IBSB is a potential lead and antimony source, which could be concealed at depth. Application of in situ laser ablation-inductively coupled plasma-mass spectrometry dating to scheelite in thin section from two ore deposits yielded U-Pb ages from 61.3 ± 4.1 to 55.1 ± 3.8 Ma and at 45.8 ± 2.6 Ma. Stibnite is intergrown with the 45.8 Ma scheelite so age constrains some stibnite mineralization. Isotope dilution thermal ion mass spectrometry dating of scheelite using the U-Pb decay system yielded an age of 57.52 ± 0.22 Ma for Yellow Pine pit scheelite. Scheelite and stibnite are Paleocene and Eocene but were long thought to be Cretaceous, so this geochronology resolves a long-standing debate about the formation age of tungsten and antimony within the largest United States antimony resource.