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TitleThe importance of siliceous radiolarian-bearing mudstones in the formation of sediment-hosted Zn-Pb ± Ba mineralization in the Selwyn Basin, Yukon, Canada
AuthorMagnall, J M; Gleeson, S A; Paradis, S
SourceEconomic geology and the bulletin of the Society of Economic Geologists vol. 110, 2015 p. 2139-2146,
Alt SeriesEarth Sciences Sector, Contribution Series 20140090
PublisherSociety of Economic Geologists
Mediapaper; on-line; digital
File formatpdf
AreaMacMillan Pass
Lat/Long WENS-130.0000 -128.0000 63.0000 62.0000
Subjectseconomic geology; stratigraphy; mineral occurrences; mineral deposits; zinc; lead; barium; sulphides; sulphide deposits; mineralization; sedimentary rocks; Selwyn Basin; Tintina fault; Kechika Trough; Tom deposit; Jason deposit; SEDEX; Paleozoic
Illustrationslocation maps; photomicrographs; ternary diagrams; plots
ProgramTargeted Geoscience Initiative (TGI-4), Sedimentary Exhalative Ore Systems
Released2015 11 09
AbstractSediment-hosted massive sulfide (SHMS) mineralization occurred in three main episodes during the evolution of the Selwyn Basin, Yukon, and Northwest Territories, Canada. In the Macmillan Pass district (Tom and Jason deposits), the Upper Devonian strata of the Earn Group host the youngest, least deformed examples of SHMS mineralization. The Earn Group comprises gray to black, homogeneous mudstones, which have been deformed in the Cordilleran fold-and-thrust belt, are weathered and poorly exposed; this makes stratigraphic correlations and the recognition of prospective units very challenging. The host rock (siliceous mudstone) and mineralized intervals at the Tom and Jason Zn-Pb ± Ba deposits have been sampled from drill core. Major element chemistry and total organic carbon (TOC) data are presented for the host-rock samples from two drill holes (TYK-5, n = 18; 76-17, n = 19). All mudstones contain between 2 and 5 wt % TOC and 67 to 83 wt % SiO2. Subtle alteration, marked by potassium loss (K2O/Al2O3 <0.2), is present in both footwall and hanging-wall samples from TYK-5, a drill-hole intersecting mineralization at Tom. Notably, samples from a regional drill hole (76-17) are unaltered, suggesting these samples represent the background mudstone composition. The SiO2 content of the host rock has no obvious relationship with the hydrothermal activity at Macmillan Pass, and most likely represents an increased flux of biogenic silica. Radiolarians (30-500 ?m) have been identified within the host rock, either replaced by sulfide or hydrothermal carbonate, or recrystallized to silica spheroids. They are identifiable with a hand lens and form distinctive 10-cm-thick radiolarian- rich beds (>50% radiolarians by volume) within the mineralized horizons and in barren, time equivalent strata, and therefore are a useful stratigraphic marker unit in otherwise homogeneous, featureless fine-grained mudstones. The presence of highly siliceous, carbonaceous mudstone units (with preserved radiolarians) suggests that in the Late Devonian there is a link between biological productivity in the water column, the resultant sedimentation, and the SHMS deposits. These biosiliceous lithologies may be particularly favorable host rocks to SHMS mineralization for the following reasons: (1) high porosities during early diagenesis preserve volume and enhance permeabilities in partially lithified sediments, which are important parameters for subseafloor replacement mineralization; (2) these units are rich in organic materials due to their deposition in productive settings, and have a high capacity for the generation of sulfide via sulfate reduction. Therefore, we suggest there is a link between times of high biological activity and productivity in oceans, sediments derived from those processes, and SHMS deposits.
Summary(Plain Language Summary, not published)
The Targeted Geoscience Initiative (TGI-4) is a collaborative federal geoscience program that provides industry with the next generation of geoscience knowledge and innovative techniques to better detect buried mineral deposits, thereby reducing some of the risks of exploration. In the Macmillan Pass district, sediment-hosted base-metal mineralization (Tom and Jason deposits) occurred in the Late Devonian strata of the Earn Group of the Selwyn Basin, Yukon, Canada. X-ray imaging reveals barium enrichments that are uniquely confined to their skeletal structure. This is suggestive of a primary, biogenic origin for barium, and is the first direct evidence for productivity-derived barium. The association of mineralized horizons and radiolarians is a significant feature of the Selwyn Basin. The recognition of radiolarians within the siliceous mudstones can be used as a marker horizon for mineralization at Macmillan Pass and has the potential to form a vector towards base metal enrichments in otherwise apparently homogenous, featureless mudstones.