RE-OS DATING OF ARSENOPYRITE FROM THE GALENA MINE — EVIDENCE FOR MESOPROTEROZOIC AGE OF THE AG-PB-ZN MINERALIZATION IN THE COEUR D'ALENE DISTRICT, IDAHO

Since 1879 mines in the Coeur d’Alene district of Northern Idaho, USA, have yielded more than 7 Mt of Pb, 3 Mt of Zn, and ~1.3 Moz of Ag, making the district the second largest silver province in the world. The Ag-rich sulfide ore (tetrahedrite & galena) is mainly hosted by steeply-dipping siderite-quartz veins intersecting folded quartzite and siltstone from the Ravalli Group of the Mesoproterozoic Belt Supergroup. The age of mineralization in the district has been controversial since the first geological studies. Both Precambrian (~1.0 Ga) and/or Late Cretaceous-early Tertiary ages have been proposed based on radiogenic isotope methods (i.e., K/Ar, ⁴⁰Ar/³⁹Ar) used to determine ages of non-sulfide vein minerals. Here we report the first Re-Os geochronological data on arsenopyrite (aspy) extracted from a quartz-siderite vein from the 4000 ft. level in the Galena Mine, Idaho. Arsenopyrite formed during the late stages of massive siderite precipitation — the main gangue mineral of the Ag-rich veins. Siderite formation was closely followed by that of Ag-rich tetrahedrite and/or galena. Minor aspy also precipitated during tetrahedrite deposition. Hence, we interpret the Re-Os aspy age to correspond to that of the main stage silver ore. Arsenopyrite shows typical Re contents for crustal hydrothermal sulfides of 5-10 ppb, Re/Os ratios are high to very high (1200-5000), and common Os contents are very low (>97% radiogenic ¹⁸⁷Os). Samples processed with a conventional ¹⁹⁰Os+¹⁸⁵Re spike yield an isochron age of ca. 1200 Ma with a very high initial ¹⁸⁷Os/¹⁸⁸Os ratio ≈ 6, indicating a crustal source of Os in the hydrothermal fluids. One sample was processed with a mixed double spike and yields a corroborating Re-Os Model Age of ~1220 Ma. The new Re-Os data show that arsenopyrite from silver-rich veins at the Galena Mine is much older than previously thought, and cannot be related either to a 1.0 Ga event, or emplacement of the Idaho batholith in the Late Cretaceous-early Tertiary. The data also suggest that significant tectonic deformation of the Belt metasediments in and around the district, which predates the emplacement of the Ag-Pb-Zn veins, was associated with an orogenic event older than 1200 Ma, likely the 1.3 Ga East Kootenay orogeny, and not with the accretion of the Wallowa-Seven Devils terrane in the Early Cretaceous.