Howards Pass is a major, 38 km-long district of shale-hosted zinc-lead deposits in western Northwest Territories and eastern Yukon. Fifteen separate deposits are hosted by carbonaceous lime mudstone and mudstone of the Ordovician-Silurian Duo Lake Formation. The existing stratigraphic framework for that unit was based on lithostratigraphic correlation of multiple poorly exposed, partial sections of outcrop and drill core; and the structural model of a simple syncline has been challenged by the suggestion that the district lies within a thrust duplex. Successful prediction of ore distribution will rely on a valid structural model and a robust framework for the internal stratigraphy of the host unit.
This study aims to test the usefulness of high-resolution carbon-isotope stratigraphy in detecting imbricate thrusts within the Duo Lake Formation. In 2016, an outcrop section and drill core from two separate deposits were logged and sampled.
There is a zone of high strain at the base of Duo Lake Formation in all three sections and numerous smaller faults throughout the formation. Detailed lithological study found insufficient evidence to recognize previously described members in the studied sections. Although compositional units could be identified, none of the units was present in all three sections, which could reflect facies changes, modification by secondary processes, or fault disruption of the stratigraphy.
Identifiable graptolites were found at several levels within the outcrop section, including faunas of Darriwilian (Middle Ordovician) age near the sheared basal contact with Rabbitkettle Formation, Rhuddanian (earliest Silurian) graptolites at the top of the section, and some Katian (Late Ordovician) collections in between. Unfortunately, it is not possible to reliably correlate these strata into the core sections.
A total of 392 samples from the three sections were analyzed for the carbon isotope composition of their organic matter. In all three sections there are intervals with δ13Corg values of ~-27.0 to -28.5 ‰ near the base. In the core sections these values gradually decline to ~-30 to -31 ‰ near the middle, then gradually return to higher values near the top. Superimposed on these overall patterns are shorter stratigraphic intervals of weak positive excursions. Higher in the sections, both cores show a positive shift in values coinciding with the highly mineralized unit, overlain by a stronger positive shift at or near the top. These similarities suggest that the two core sections span a similar stratigraphic interval, supported by similarities in the lithologies and patterns of variation in their total organic carbon content. None of the studied sections show any evidence of stratigraphically repeated patterns of δ13Corg values that would be expected if there were stacked sets of strata repeated by thrust faulting.
The positive shift in δ13Corg values in the mineralized interval shows strong similarities in both pattern and magnitude to shifts seen in the Aeronian (mid-Llandovery) strata in Arctic Canada and other parts of the world. Although not conclusive, this is consistent with the evidence from published biostratigraphic data that the strata of the main mineralized interval at Howards Pass is primarily Aeronian in age.