Yellowknife Geoscience Forum

Early Jurassic to mid-Cretaceous accretion of the Yukon-Tanana terrane on the margin of ancestral North America resulted in the formation of eastward propagating compression that generated extensive folding and thrusting – the Cordilleran Orogeny. Between the orogenic core and foreland fold-and-thrust of the Mackenzie platform lies the Selwyn Basin, a slope-to-basin facies strata that represents passive margin sedimentation from the Neoproterozoic to the Paleozoic. Selwyn Basin rocks are intruded by 98-89 Ma Tombstone-Tungsten Belt (TTB) intrusions. In the northern Selwyn Basin, TTB intrusions cut ductile deformation fabrics and are considered post-tectonic, with crustal shortening in the northern Selwyn Basin ending before 98 Ma. Cooling ages of mica from thrust faults indicate that deformation had probably ended by about 100 Ma. In the central Selwyn Basin, un-deformed ~98-97 Ma South Fork volcanics also suggest deformation had stopped there by a similar time.

TTB intrusive belt in the southeast Selwyn basin is dominated by 99 to 95Ma Tay River suite and 98 to 94 Ma Tungsten suite. The latter intrusions are associated with significant W mineralization, including the world-class Cantung W-Cu-Au skarn, which formed above the 98 Ma Mine Stock. There has been limited detailed analysis of the relative timing of deformation to the emplacement of Tungsten suite plutons and skarn formation in the Cantung region. Here we present results of an analysis of the timing of regional deformation relative to pluton emplacement using a combination of regional fold and foliation geometry, overprinting foliation patterns, and porphyroblast-matrix microstructural relationships.

Early Cambrian to Mississippian strata in the Cantung region form a NW-SE trending upright fold train, F1, and corresponding axial planar foliation, S1; the product of NE-SW crustal shortening. In the Cantung region, F1 folds above Tungsten Suite intrusions, including the mine stock, are more open with a lower amplitude to wavelength ratio than in the areas between the stocks. The geometry indicates that plutons were emplaced during the formation of F1 folds and that shortening continued post-emplacement.

A variably-developed crenulation cleavage, S2, is well-developed in the thermal aureoles of Tungsten suite plutons and developed synchronously with cordierite and andalusite porphyroblasts that overgrow and preserve S1 as inclusion trails. S2 is less well-developed outside contact aureoles. Regionally, S2 has a sub-horizontal to shallowly inclined orientation. Above the Mine Stock, F1 folds are rotated to a shallower dip and this is attributed to vertical shortening in the thermal aureole. The same shortening is responsible for the formation of S2. Locally, S2 is crenulated by a sub-vertical S3 foliation. S1 and S3 are geometrically indistinguishable from each other and represent a continuum of NE-SW shortening that was only locally interrupted by periods of vertical shortening in the thermal aureoles of the intrusions.

Data collected in this study indicates that the emplacement of Tungsten suite plutons at Cantung occurred during regional NE-SW shortening and are not post-tectonic as previously considered. Rather deformation in the SE Selwyn Basin continued after deformation had ceased further to the west, highlighting the diachronous nature of the Cordilleran orogeny.