LEAD ISOTOPE DATA FOR THE MURRAY AU-AG SUBDISTRICT OF THE COEUR D'ALENE PB-ZN-AG MINING DISTRICT, WITH COMPARISON TO THE STIBNITE MINING DISTRICT AND LEMHI PASS REGION, IDAHO

Thirteen new lead isotope analyses for galena-bearing veins from Murray, Idaho, are combined with values from the Coeur d’Alene District (CDA) (Leach, et al., 1998). Murray samples include Au-bearing veins, some with Te, from the East Fork of Eagle Creek and Bear Creek drainages. Lead isotope graphs show 3 clusters. Group 1 is the main CDA Ag-Pb-Zn mine samples; the plots define steep slopes with ²⁰⁶Pb/²⁰⁴Pb from 16 to 16.6, ²⁰⁷Pb/²⁰⁴Pb from 15.3 to 15.59, and ²⁰⁸Pb/²⁰⁴Pb from 35.62 to 36.53. Group 2 (Middle Group) includes multiple gold-bearing prospects near Murray with ²⁰⁶Pb/²⁰⁴Pb values of 17.76 to 18.3, ²⁰⁷Pb/²⁰⁴Pb values of 15.52 to 15.61, and ²⁰⁸Pb/²⁰⁴Pb from 38.0 to 38.55 Group 2 overlaps or is below a Stacey-Kramers curve. Group 3 (Silver Strand) plots at higher ²⁰⁶Pb/²⁰⁴Pb values of 19.0 to 19.53, ²⁰⁷Pb/²⁰⁴Pb from 15.66 to 15.69, and ²⁰⁸Pb/²⁰⁴Pb from 39.24 to 40.4. Our Group 3 (youngest) matches Group 3 of Zartman and Stacey (1971) and plots right of a Stacey-Kramer curve. Group 1 represents Mesoproterozoic leads of the main CDA. Groups 2 and 3 include outlier deposits listed in Leach et al. (1998), who also suggested the silver-rich systems were Cretaceous to early Tertiary in age. Group 2 leads could be derived from mixing of Group 1 and Group 3 leads, but the discrete clusters and model ages suggest a possibility of two Phanerozoic ore-forming systems, one Cretaceous and/or Paleozoic, and one Tertiary. The Murray data was compared to isotopic data for the Stibnite Au-Sb-W Mining District in central Idaho and the Lemhi Pass REE-Th-Fe District in Lemhi County. Groups 1 and 2 data plot significantly left of mineralized samples from either Stibnite or Lemhi Pass. Group 3 is close to values of a post-mineral dike (45.9 Ma) from Stibnite, but far left (i.e. lower 206/204) of mineralized samples. Group 2 overlaps partly with Idaho intrusive leads compiled by Gashnig, but Group 2 is also close to a few samples of oxides, sulfides and feldspars from the Lemhi region, including a fresh sample of Paleozoic-age Leadore Granite. Two Cambro-Ordovician alkaline intrusives plot slightly to the right. A more general observation is that both of the districts south of the Belt Basin rift contain diverse and radiogenic lead values, perhaps reflecting a distinct mantle and crustal architecture from that of the Coeur d’Alene and Murray districts.