CHARACTERIZATION OF PALEOCENE TO EOCENE MAGMATISM AND TECTONICS IN THE NORTHERN CANADIAN CORDILLERA AND THEIR ROLE IN REGIONAL METALLOGENY

During the Paleocene to Eocene voluminous amounts of igneous material was intruded along a northwest axis from southern British Columbia into southwest Yukon. In Yukon, early Tertiary magmatic rocks are represented by the Ruby Range suite. The suite is composed of a variety of granitoid compositions including quartz diorite, granodiorite, tonalite, granite and minor diorite and gabbroic phases. Ages from the suite range from ca. 64 Ma to 52 Ma. The character of the suite changes through time, with older phases generally located near the base of the intrusion where they are often weakly to strongly foliated. Progressively younger phases intrude the older phases and show evidence for shallower depth of intrusion. At the highest crustal levels the plutonic rocks intrude into their own volcanic cover where andesite, rhyolite and rare basalt are preserved in age equivalent volcanic complexes.

The Ruby Range suite intrudes along the tectonic boundary between metamorphic rocks of the Yukon-Tanana terrane and the Kluane schist. This tectonic boundary is nowhere exposed but is assumed to be a northeast-dipping thrust contact. The strongly foliated nature of the oldest and structurally lowest part of the Ruby Range suite suggests this thrust contact may have been active into the earliest Paleocene. Younger phases of the suite cross-cut foliations and are predominantly found within the Yukon-Tanana terrane in the hangingwall.

A period of exhumation after ~64 Ma is suggested by ca. 55-53 Ma biotite cooling ages from metamorphic rocks in the Kluane schist. The exhumation mechanism is not well established but may be related to an extensional reactivation of the initial thrust contact between the Yukon-Tanana terrane and the Kluane schist, or to a series of poorly characterized faults that may be kinematically linked to regionally significant dextral strike-slip faults such as the Denali fault.

Several intrusion-related mineral occurrences and deposits are directly associated with the Ruby Range suite. These include the structurally controlled Mt. Skukum epithermal gold deposit in southern Yukon and several other porphyry and epithermal showings. Vein-hosted gold occurrences within the Kluane schist show some similarity with orogenic gold deposits found near Juneau, including their age (ca. 57 Ma) and tectonic setting.