Sediment-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 μm) 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.