TELESCOPING OF THE AG-PB-ZN-CU-SN KEG DEPOSIT, CENTRAL YUKON: A PRODUCT OF DELAMINATION TRIGGERED REGIONAL EXHUMATION?

Petrographic, fluid inclusions and stable isotope studies of mineralization from the 39.8 million tonne Ag-Pb-Zn-Cu-Sn Keg deposit, central Yukon, show juxtaposition of an early deep skarn and later, shallower, fracture hosted epithermal style of mineralization (telescoping). 3D modelling shows that the spatial distribution of the mineralization is controlled by a Jura-Triassic thrust fault. Re-Os dating of arsenopyrite in the epithermal mineralization gives a late Albian age (101±0.5Ma).

Telescoping of the Keg deposit is attributed to a rapid period of exhumation in the Albian and Cenomanian periods of around 10km in 10Ma as indicated by low temperature geochronology combined with pressure/time constraints from regional igneous bodies. The trigger for the exhumation is not certain. However, an isostatic calculation combined with a regional seismic survey of crustal thickness indicate the possibility of a 2500 m.a.s.l Cretaceous plateau in the area. Bulk rock geochemistry of igneous rocks in the region suggests the collapse of the plateau possibly occurred due to unstable lithospheric delamination and asthenospheric upwelling. This mechanism would also explain the anomalous regional crustal heat flow required to generate the Keg deposit mineralization.

The genesis of the Keg deposit therefore is intimately linked to Cordilleran structures, syn-mineralization tectonic events and igneous activity in this region of the Yukon.