Structural Geology of the Nadaleen Trend, Northeastern Yukon Territory, Canada: Implications for Recent Au Discoveries

Justin C. Palmer, Colorado School of Mines
Yvette D. Kuiper, Colorado School of Mines

Recent Au discoveries in the eastern Nadaleen Trend of northeastern Yukon Territory are hosted in strongly deformed, unmetamorphosed, Neoproterozoic carbonate and siliciclastic rocks and display characteristics similar to Carlin-type deposits in Nevada. The complicated structural geology primarily results from mid-Cretaceous, thin-skinned fold-thrust deformation imposed upon a mechanically heterogeneous package of sedimentary rocks. Four generations of deformation resulted in tilting of stratigraphy to the SSW (D1), several moderately- to steeply-SSW-, SW- and E-plunging hundred-meter scale chevron folds and the E-trending dextral Osiris Fault (D2), the N-dipping reverse Nadaleen Fault zone (D3), and the N-trending dextral Conrad Fault (D4). Similar combinations of structures have not been recognized regionally, perhaps because the Nadaleen Trend is located in a zone of regional stratigraphic and structural transitions. Neoproterozoic- and Paleozoic-aged platform rocks occur west and north of the Nadaleen Trend; similarly aged basinal rocks occur to the east and south. Regionally extensive east-trending thrust faults, including the Dawson Thrust and Kathleen Lakes Fault, end within 10 km of the Nadaleen Trend. The combination of S-vergent thrust faults at and north of the Nadaleen Trend, and N-vergent thrust faults elsewhere in the region have the geometry of an E-trending triangle zone across the Nadaleen Trend. We suggest that these geologic transitions reflect the influence of a previously unrecognized, south-dipping basement normal fault (the Rackla Fault) that occurs west of the Nadaleen Trend and terminates below it. This fault is interpreted to have enough offset west of the Nadaleen Trend to obstruct north-directed fold-thrust movement, resulting in a lack of significant shortening to the north. The triangle zone is interpreted as a result of north-directed motion interacting with and overriding the minor obstruction at the eastern end of the Rackla Fault. Overriding the obstruction may have led to tectonic wedging and backthrusting. East of the Nadaleen Trend, fold-thrust movement was unobstructed.

Antiformal SSW-plunging D2structures host the Conrad, Osiris, and Isis East Au zones within the eastern Nadaleen Trend. The folds are oriented oblique to the general easterly trend of folds and thrust faults in the region. Their oblique orientation perhaps made them act as locally favorable channels for upwelling ore fluids, while the E-trending triangle zone across the Nadaleen Trend may have acted as a more regional structural funnel.