Structural geology and geochemistry of sedimentary rock-hosted gold in the eastern Nadaleen Trend, Yukon Territory, Canada

Recently discovered gold (Au) in the eastern Nadaleen Trend of northeastern Yukon Territory is hosted in unmetamorphosed Neoproterozoic carbonate and siliciclastic rocks that were subject to intense deformation prior to Au mineralization. Intense deformation in the region resulted from mid-Cretaceous NNE-vergent, thin-skinned fold-thrust deformation. Structures within a 6 km2 area at the eastern Nadaleen Trend are complex and are not recognized elsewhere in northeastern Yukon Territory. Field observations suggest these rocks are the product of NNE-SSW-directed shortening (D1), NW-SE-oriented dextral simple shear (D2), and N-S-directed shortening (D3) followed later by minor dextral slip (D4). The complicated geometry of the area may result from the presence of basement structures and/or competency contrasts between rock units within the sedimentary package.;The eastern Nadaleen Trend lies within an E-trending triangle zone bounded by S- and N-vergent reverse faults to the north and south, respectively. The triangle zone does not continue to the west or east. To the west, rocks consist of multiple NNE-verging structures in the south and only one map-scale fold in the north indicating that strain decreases northwards. Rocks in the east are consistently NNE-vergent. This change in structural style suggests that NNE-directed transport in the rocks to the west was obstructed at the latitude of the eastern Nadaleen Trend while transport in the east was unobstructed. Potential causes for the obstruction include rheological heterogeneity or a subsurface structure. The obstruction is nearly co-spatial with the steep southward transition from both Neoproterozoic and Paleozoic platform rocks to basin rocks. This abrupt platform-to-basin transition occurred in this location throughout much of geologic history, suggesting that the location of the transition may be pinned by a pre-existing basement feature. For this reason, we suggest that mid-Cretaceous deformation patterns were influenced by a subsurface basement fault that obstructed NNE-directed fold-thrust movement to the west of the eastern Nadaleen Trend.;Major Au deposits within the eastern Nadaleen Trend occur within moderately to steeply S- to SSW-plunging anticlines and not within folds in other orientations or synclines. The orientation of the folds that host Au is oblique to the regional trend and they occur within the E-trending triangle zone. Reverse faults bounding the triangle zone are interpreted to have acted as an aquitard below which ore fluid migrated updip. Within the eastern Nadaleen Trend, ore fluid migration was concentrated into hinge zones of SSW-plunging anticlines, which may have acted as local fluid conduits.;One of the Au zones, the Conrad zone, is situated in the northeast part of the eastern Nadaleen Trend and within a steeply SSW-plunging anticline that folds fine-grained siliciclastic rocks overlying silty limestone. This anticline is cutoff to the north by a ~50 m wide NNE-dipping fault zone. Deep within the Conrad zone, Au dominantly occurs within the anticline hinge zone, suggesting that ore fluid indeed may have risen along the fold hinge zone. Relatively more Au occurs at shallower levels, perhaps because the (now eroded) juxtaposed anticlinal fine-grained siliciclastic rocks and fault zone presented a natural trap that slowed upward fluid migration. If fluid upwelling was restrained in this zone, it could have prolonged the interaction time between ore fluid and the host rock that may have led to increased Au deposition.;Fluid-rock interaction within the Conrad Au zone changed equilibrium conditions in both the ore fluid and the host rock. The change in conditions destabilized certain components of both the host rock and the ore fluid that resulted in an elemental flux between the two. This flux is preserved within the host rock where certain elements show enrichments or depletions in the presence of Au. Within the Conrad zone, Ca, Mg, and Na are depleted in Au-bearing samples suggesting that the fluid that brought in Au also facilitated a loss of carbonate minerals. (Abstract shortened by UMI.).