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TitlePaleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 2 - pyrite paragenesis and mineral chemistry
AuthorLeighton, C; Layton-Matthews, D; Peter, J MORCID logo; Gadd, M GORCID logo; Voinot, A; Leybourne, M I
SourceCanadian Mineralogist vol. 59, no. 5, 2021 p. 1233-1259,
Alt SeriesNatural Resources Canada, Contribution Series 20210230
PublisherMineralogical Association of Canada
Mediapaper; digital; on-line
RelatedThis publication is related to Paleoredox conditions, hydrothermal history, and target vectoring in the MacMillan Pass base-metal district, Yukon, Canada: 1 - lithogeochemistry of proximal and distal shales
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; mineralogy; stratigraphy; Science and Technology; Nature and Environment; mineral exploration; mineral potential; mineral deposits; base metals; zinc; lead; silver; barium; sulphide deposits; ore mineral genesis; mineralization; ore controls; tectonic history; hydrothermal systems; metamorphism; paragenesis; pyrite; core samples; morphology, crystal; diagenesis; crystallization; recrystallization; trace element geochemistry; trace element distribution; whole rock geochemistry; mass spectrometer analysis; stratigraphic analyses; stratigraphic correlations; models; fluid dynamics; barite; mineral occurrences; MacMillan Pass District; Tom Deposit; Jason Deposit; Phanerozoic; Paleozoic; Devonian
Illustrationslocation maps; geoscientific sketch maps; photomicrographs; plots; tables
ProgramTargeted Geoscience Initiative (TGI-5) Volcanogenic massive sulphide ore systems - base metal sources and processes
Released2021 10 07
AbstractThe MacMillan Pass district in Yukon, Canada, hosts the Tom and Jason clastic sediment-hosted Zn-Pb-Ag-(Ba) deposits. Pyrite-bearing drill core samples were collected from seven drill holes that intersected sulfide mineralization and time-stratigraphically equivalent rocks at varied spatial distances extending up to 3 km away from the deposits to assess the relative timing of pyrite mineralization and the chemistry of pyrite paragenesis. There are four pyrite morphologies: framboids and polyframboids (Py1), subhedral to euhedral inclusion-free crystals (Py2a), silicate inclusion-bearing nodules with serrated edges (Py2b), and euhedral idiomorphic overgrowths on preexisting pyrite morphologies (Py3). These morphological varieties correspond in time from syngenetic to earliest diagenetic growth (Py1), early to late diagenetic growth (Py2a, Py2b), and metamorphic crystallization and/or recrystallization of previous textural varieties (Py3). A representative subset of pyrite grains was analyzed for trace element contents and distributions by laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS). Analyses by LA-ICP-MS reveal that each textural variety of pyrite has a distinct trace element composition that also varies depending on stratigraphic unit. A suite of clastic sediment-hosted sulfide mineralization-related elements was incorporated into Py2 within sulfide mineralized units at greater abundances than that in unmineralized units (e.g., Zn, As, Pb, Tl, Bi). Lead abundances and Pb/Se and As/Mo values in pyrite are the most robust vectoring tools documented. The timing for clastic sediment-hosted Zn-Pb mineralization was syn and/or post late diagenesis (Py2b).
A Ba-enriched horizon was identified in rocks and this is interpreted to be the distal time-stratigraphic equivalent unit to Zn-Pb mineralization. The Ba-enriched horizon contains Py2 with anomalous metal (Tl, Co, Mn, Cd, Zn, Sb) contents and abundant macroscopic baryte, and it is interpreted to represent the distal expression of sulfide mineralization-forming hydrothermal activity. Four genetic models for mineralization are reviewed; however, the only model that is consistent with our whole rock and pyrite geochemistry involves venting of buoyant hydrothermal fluid, mixing with ambient seawater, and remaining or sinking into unconsolidated sediments, with lateral migration up to 2-3 km from the vent source.
Summary(Plain Language Summary, not published)
Pyrite from Pb-Zn deposits and the enclosing host rocks from MacMillan Pass, Yukon was subjected to analyses by laser probe mass spectrometry. The goal was to determine the impact of hydrothermal fluids on putatively diagenetic and hydrothermal pyrite, and to characterize their geochemical differences. These geochemical differences were then applied to developing novel vectors to mineralization in a proximal to distal profile. The profile is both vertical (above and below mineralization) and lateral (close to the metalliferous fluid conduit and away from it).

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