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TitlePaleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 1 - lithogeochemistry of proximal and distal shales
AuthorLeighton, C; Leybourne, M I; Layton-Matthews, D; Peter, J MORCID logo; Gadd, G MORCID logo; Voinot, A
SourceCanadian Mineralogist vol. 59, no. 5, 2021 p. 1207-1232,
Alt SeriesNatural Resources Canada, Contribution Series 20200586
PublisherMineralogical Association of Canada
Mediapaper; on-line; digital
RelatedThis publication is related to Paleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 2 - pyrite paragenesis and mineral chemistry
File formatpdf; html
NTS105I; 105J; 105K; 105N; 105O; 105P
AreaMacMillan Pass
Lat/Long WENS-134.0000 -128.0000 64.0000 62.0000
Subjectseconomic geology; geochemistry; tectonics; Science and Technology; Nature and Environment; mineral exploration; mineral potential; mineral deposits; base metals; zinc; lead; silver; barium; uranium; molybdenum; ore mineral genesis; mineralization; lithogeochemistry; bedrock geology; lithology; sedimentary rocks; clasts; shales; tectonic history; hydrothermal systems; fluid flow; fluid dynamics; host rocks; geochemical anomalies; thermal analyses; mineral enrichment; organic carbon; isotopic studies; isotopes; sulphur; strontium; barite; remobilization; precipitation; MacMillan Pass District; Earn Group; Tom Deposit; Jason Deposit; Phanerozoic; Paleozoic; Devonian
Illustrationslocation maps; geoscientific sketch maps; photographs; tables; profiles; plots; ternary diagrams
ProgramTargeted Geoscience Initiative (TGI-5) Volcanic and sedimentary systems - sedimentary exhalative ore systems
Released2021 10 07
AbstractThe MacMillan Pass District in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Bulk geochemical paleoredox proxies (Eu/Eu*, Ce/Ce*, Mo, Re/Mo, and Ni/Co) indicate anoxic-dysoxic water column and sulfidic porewater conditions persisted during the Late Devonian deposition of the Lower Earn Group host rocks. Positive Eu/Eu* anomalies (up to 3.31) in sulfide mineralization at the Tom deposit are consistent with relatively high temperature (probably >250 °C), reducing, acidic hydrothermal fluids that infiltrated laterally through unconsolidated sediments proximal to the hydrothermal upflow zone and/or exhaled at the seafloor as moderate- to high-density brines. Molybdenum and U enrichment factors (relative to upper continental crust) and Mo/organic C values are consistent with a moderately restricted basin; Mo/C values fall between those of the Black Sea (highly restricted) and the Framvaren Inlet (moderately restricted). A Ba-rich shale was identified in rocks that are distal and time-equivalent units to the Pb-Zn mineralization; based on the bulk chemical compositions and on previous S and Sr isotope studies, we interpret the baryte in this unit to be largely hydrothermal in origin and perhaps remobilized and reprecipitated during hydrothermal base-metal mineralization.
Summary(Plain Language Summary, not published)
This contribution focuses on one of Canada's premiere lead-zinc mineralized districts in Yukon. The impetus for this work was to decipher the mechanisms of deposit formation and development of methods and methodologies that can be used to explore for additional deposits in the district, and elsewhere. Whole-rock geochemical analyses of the immediate host rocks (above and below mineralization) both within the deposit and further away were used to determine the conditions of the mineralizing fluid (hot, reducing, acidic), the water column (nil to little free oxygen), and the immediate subsurface muds (sulfidic). Certain geochemical parameters indicate that the deposits formed in a moderately restricted basin. Barium in the host rocks is interpreted to have originated from the mineralizing fluid. Similar analyses of other areas can be used to make inferences about the mechanisms of emplacement of mineralization, and the setting under which these occurred.

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