STRUCTURAL CONTROLS ON CARLIN-TYPE MINERALIZATION IN THE CONRAD ZONE OF THE NADALEEN TREND, RACKLA GOLD BELT, YUKON TERRITORY, CANADA

Recently discovered gold (Au) ore in the Nadaleen Trend within the Rackla Gold Belt of Yukon Territory resembles high-grade Carlin-type Au ore found in northern Nevada that accounts for ~6% of global Au production. Spatial relations from detailed surface mapping and drill core analysis suggest that Au ore in the trend is primarily structurally controlled with local stratigraphic control. We determined that the Au-rich Conrad, Osiris, and Isis East Zones within the trend are controlled primarily by moderately to steeply SSW-plunging antiformal chevron folds that occur over a 3km² area. The Conrad Zone is the more complex of the three and the focus of our research.

The plunging antiform hosting the Conrad Zone was cut off to the north prior to mineralization by the N-dipping, Au-poor Nadaleen Fault that probably slowed the ascent of ore fluids beyond the fault because of its fine-grained, dense nature. Also, fine-grained pyritic siliciclastic rocks and matrix-supported quartz pebble conglomerate that overlie the primary silty carbonate host rock in the Conrad anticline probably also significantly inhibited ore fluid upwelling. The combination of low-permeability structural and stratigraphic features formed a setting where fluid upwelling was repressed, and ultimately led to higher grades through increased fluid-rock interaction.

We interpret the trends complex pre-mineralization structure to be the result of three unique shortening events beginning in the Late Triassic. Rocks of the trend lie in the lower plate of the SSW-dipping Dawson Thrust fault, which separates off shelf rocks of the Neoproterozoic Mackenzie Platform to the NNE and Selwyn Basin rocks to the SSW. Late Triassic to Early Jurassic ocean closure and terrane accretion resulted in the Dawson and other regional thrust faults and also accounts for the moderate SSW-dip of rocks in the Nadaleen Trend. Later dextral shear, possibly related to slip along the regional-scale, dextral strike-slip Tintina Fault, caused the steeply SSW-plunging chevron folds. Continued NW-directed shortening resulted in the formation of the E-trending, dextral strike-slip Osiris Fault that offset the antiforms hosting the Osiris and Isis East zones. Subsequent NNE-directed shortening activated the Nadaleen Fault, completing the structural setting before ore formation.