PB-SR-ND ISOTOPE CONSTRAINTS ON METAL SOURCES OF ZN-PB-AG DEPOSITS, BROOKS RANGE, ALASKA

^{Pb-Sr-Nd isotope data were collected on the Mississippian shale-hosted Zn-Pb-Ag Red Dog deposits, other shale-hosted deposits near Red Dog, and Zn-Pb-Ag sulfide and barite deposits from the western and central Brooks Range (BR). Data also were obtained on Lower Mississippian to Upper Devonian clastic marine-deltaic rocks of the Endicott Group (EG) underlying the deposits in western BR, rocks correlative with the EG (and older) from the North Slope (NS) northeast of Red Dog, and Late Proterozoic probable basement rocks from southwestern BR. Galena from deposits in the western BR has a distinctive range (206Pb/204Pb = 18.364-18.428, 207Pb/204Pb = 15.553-15.621, and 208Pb/204Pb = 38.083-38.323) that overlaps age-corrected data (Red Dog deposits at 338 Ma) for clastic rocks in western BR (206Pb/204Pb = 17.325-18.515, sup>207}Pb/²⁰⁴Pb = 15.525-15.564, ²⁰⁸Pb/²⁰⁴Pb = 37.924-38.022), and NS (²⁰⁶Pb/²⁰⁴Pb = 17.243-18.783, ²⁰⁷Pb/²⁰⁴Pb = 15.476-15.591, ²⁰⁸Pb/²⁰⁴Pb = 38.048-38.853). The Pb data link the deposits to the EG and correlative rocks from the NS, suggesting an extensive hydrothermal system, and indicating the EG as a likely metal source and fluid conduit. In the western BR, barite Sr isotopes (⁸⁷Sr/⁸⁶Sr = 0.708615-0.709899) overlap values in the Kuna Fm. (host of deposits, ⁸⁷Sr/⁸⁶Sr₃₃₈ = 0.708032-0.711582) and EG (⁸⁷Sr/⁸⁶Sr₃₃₈ = 0.707424-0.709575). In the central BR, Sr in barite (Howard Pass quad, ⁸⁷Sr/⁸⁶Sr = 0.707487-0.707867) is less radiogenic than in barite from the western BR and rocks from the NS (⁸⁷Sr/⁸⁶Sr₃₃₈ = 0.708989-0.712754). The Sr isotope compositions do not reflect a single fluid, or a unique mix of fluids in the western and central BR, as established by the similar range of Pb isotopes linking all the deposits. The Late Proterozoic rocks have values of ²⁰⁶Pb/²⁰⁴Pb = 17.787-19.282, ²⁰⁷Pb/²⁰⁴Pb = 15.465-15.660, and ²⁰⁸Pb/²⁰⁴Pb = 37.419-39.475, ⁸⁷Sr/⁸⁶Sr = 0.706010-0.709560, and ¹⁴³Nd/¹⁴⁴Nd = 0.511879-0.513014 (ε_Nd338 = 7.07 to -11.47) that overlap the EG. Nd isotope compositions of the Kuna Fm. and EG in the western BR, and from the NS overlap (ε_Nd338 = -1.57 to -9.57, -4.23 to -9.42, -4.99 to -10.73, respectively) suggesting the sediments were derived from an evolved, older crustal source. The source terrane of the clastic sediments may have included similar Proterozoic materials.