Pb isotope geochemistry of stratabound Zn-Pb(-Ag-Au-Ba-F) deposits and occurrences in the Nome Complex, Seward Peninsula, Alaska: Metal sources and regional comparisons

Abstract

A detailed study of the Pb isotope geochemistry of Zn-Pb(-Ag-Au-Ba-F) stratabound sulfide deposits within metasedimentary rocks of the Neoproterozoic to Mississippian(?) Nome Complex provides key information for understanding deposit genesis and crustal evolution. A total of 106 new analyses of galena (and other sulfi des) and metasedimentary rocks hosting the deposits shows that (1) Pb isotope signatures of the deposits are heterogeneous when considered as a group; (2) the stratabound Nelson deposit, and deformed veins at Quarry and Galena, are isotopically similar; (3) stratabound and locally stratiform lenses such as Wheeler North and Aurora Creek had different isotopic evolutions; and (4) the occurrence at Bluff and the postmetamorphic, undeformed Pb-Zn-Ag veins and replacements at Hannum, Independence, Foster, and Omilak show the highest values of 206 Pb/ 204 Pb in the region. Pb isotope data for the stratabound Zn-Pb deposits and occurrences do not lie along similar secondary or anomalous lead evolution lines, and there is no shared, two-stage lead line that would provide intersections with a primary or single-stage lead isotope growth curve. Lead isotopic characteristics of the Nelson stratabound deposit and the deformed veins at Quarry and Galena indicate that they largely shared metal and fluid sources. Quarry and Galena also display sufficient Pb isotopic contrast compared to Aurora Creek and Wheeler North to eliminate such veins as subsurface “feeders” for these stratabound deposits, if the deformed veins and deposits formed as closed isotopic systems (without a contribution from externally derived lead). The Pb isotope composition of galena from Aurora Creek formed by a multistage process. It is thus possible that the Aurora Creek deposit originally contained Pb isotope compositions that resembled those from Quarry and Galena. That early-formed Pb was probably remobilized and mixed with radiogenic lead contributed by Mesozoic hydrothermal fluids similar to those associated with the gold-quartz veins in the region. Values of 207 Pb/ 204 Pb and 206 Pb/ 204 Pb from each of the deposits and occurrences plot within the Pb isotope fields of the host metasedimentary rocks and Mesoproterozoic basement rocks of Seward Peninsula; Pb isotope compositions in the deposits thus reflect a local source control. The processes that generated the premetamorphic Zn-Pb(-Ag-Au-Ba-F) sulfide deposits in the Nome Complex differed from those that generated Zn-Pb-Ag deposits in the western Brooks Range, such as the giant Red Dog ore body. Taken as a group, the stratabound lenses and deformed veins in the Nome Complex did not form in a single, widespread, homogeneous hydrothermal system. The Brooks Range deposits, which consist of a range of host rock types and styles of mineralization distributed over a large area, have a high degree of regional Pb isotope homogeneity. The Wheeler North deposit is isotopically similar to Red Dog and related deposits and may have formed in a related hydrothermal system. A preliminary comparison of the Pb isotope compositions of sedimentary-exhalative (SEDEX)–type deposits within the Arctic Alaska–Chukotka terrane and deposits in crustal blocks of Laurussia shows: (1) noteworthy Pb isotopic overlap exists between some of Zn-Pb-Ag deposits in Ireland and the deposits in Arctic Alaska ; but (2) no exact isotopic match exists between any of the deposits in Arctic Alaska and any deposit in crustal blocks involved in the Paleozoic evolution of Laurussia.