Geology of Gravel Creek (105B/10) and Irvine Lake (105B/11) Map Areas, Southeastern Yukon

The Irvine Lake and Gravel lake map-areas (NTS 105 B/10,11) lie within the northern Omineca Belt, west of the Tintina-Northern Rocky Mountain Trench (NRMT) fault. The eastern part of the area is underlain by Proterozoic to early Paleozoic meta-sedimentary rocks of Cassiar terrane (Monger, 1984), a fragment of the North American miogeocline which has been displaced northward on the Tintina-NRTM fault. The western part of the area is underlain by basaltic meta-volcanics, serpentinized ultramafic rocks, meta-gabbro, and cherty and calcareous meta-sediments of the Slide Mountain terrane (Monger, 1984). Unfoliated to weakly foliated granitic intrusives (Marker Lake and Cassiar batholiths and Cabin Creek and Gravel Creek stocks) occur throughout the area intruding both the Cassiar and Slide Mountain terranes. Slide Mountain and Cassiar terranes are juxtaposed by an east-verging thrust referred to in this area as the Zak fault. Southwest of Irvine Lake, the thrust places serpentine, basaltic meta-volcanics, and an undeformed dioritic intrusion onto a footwall consisting of the Proterozoic Tsaydiz Formation and older units. Northwest of Irvine Lake, near Shootamook Creek, the thrust places cherty meta-sediments of the allochthon onto marble and quartzite inferred to be lower Cambrian Rosella and Boya Formations, respectively. The northern end of the Cassiar batholith extends into the southwestern corner of Irvine Lake map-area. Its northeastern contact with rocks of Slide Mountain terrane is a sub-vertical, northwest-southeast trending mylonite zone several tens of metres wide. Mesoscopic structures including S-C fabrics and shear bands (Berthe' et al, 1979) prove dextral displacement parallel to a variably plunging, but commonly sub-horizontal stretching lineation. The mylonite zone lies along a pronounced topographic lineamnet which extends from the trace of the Cassiar fault south of the Alaska Highway northwestwardly into the Irvine Lake map-area rather than veering to the west as previously mapped (Poole et al, 1960). Mineral occurrences in this area are primarily near the contact of granitic intrusions and carbonate rocks. Carbonate rocks hosting the deposits belong to the upper Proterozoic Ingenika Group (Swannell, Tsaydiz and Espee formations) rather than the Lower Cambrian Atan Group as has been inferred for nearby deposits in the Rancheria district. Other, non-carbonate-hosted mineral occurrences include a porphyry Mo prospect and Ag, Pb, Zn veins.