Structural Setting of the Cantung W-Cu-Au Deposit: An Interaction Between Regional Deformation and Pluton EmplacementThursday, November 22, 2018 - 10:00 to 10:19 Theatre 1
The Cantung W-Cu-Au skarn hosts one of the most significant sources of tungsten in the world. Since 1962, ~7.68 Mt of ore has been extracted at a grade of ~1.4 % WO3. Cantung is located within the Selwyn basin of the Canadian Cordillera, in the southwest corner of the NWT. The deposit is hosted in the folded sequence of carbonate units belonging to the Cambrian Sekwi Formation and is spatially and genetically associated with 94 to 98 Ma monzogranite intrusions of the Tungsten plutonic suite. The E-Zone underground orebody is developed along the lower, overturned, limb of a large near-recumbent antiform (the ”Mine Fold”), which exerts a first-order control on the distribution and grade of mineralization. This fold has a strongly curved hinge line, which is largely oblique to the regional NW-SE structural trend, suggesting a complex deformation history.
Beyond the immediate surrounds of the Cantung deposit and the Mine Stock, the lithological units are macroscopically folded as part of a regional NW-SE trending fold event thought to be related to the Cordilleran Orogeny. This regional set of upright F1 folds and corresponding axial planar S1 foliation are a product of a major crustal shortening event, D1. Geological mapping in the area around Cantung has not identified any correlative structures to the curvilinear, near-recumbent, Mine Fold antiform. An interpretation is that the Mine fold formed from sub-vertical shortening and northward flow of Vampire and Sekwi Formation rocks during emplacement and upward “ballooning” of the Mine Stock. This would have allowed thermal weakening to localize shortening but only within the contact metamorphic aureole. The presence of limestone units immediately above the Mine Stock also favoured ductile flow. The development of a gently dipping S2 crenulation cleavage suggests that shortening may have been initiated by emplacement of the larger parent magma body prior to final emplacement of Mine Stock.
The apparent lack of macroscopic F1 folds in the zone above the Mine Stock at Cantung indicates that emplacement of the intrusion might have initiated during D1, with the region above the stock forming a D1 strain shadow. The switch to sub-vertical D2 shortening reflected a local change in the strain field associated with pluton emplacement, and may not be associated with any change in the far-field kinematic reference frame. Similar sub-vertical shortening appears to have occurred in the thermal aureoles of other Cretaceous intrusions in the region. Post-D2 cooling of the aureole may have facilitated the formation of steep brittle fractures and faults that focused the emplacement of late dykes and the upward flow of late magmatically-derived hydrothermal ore-fluids.