The Cache Creek (CCT) and Slide Mountain terranes (SMT), including the Seventymile Terrane in Alaska of the Northern Cordillera consist of oceanic assemblages that have been tectonically emplaced. The CCT has been enclosed by a series of arc terranes, whereas the SMT has been thrust onto pericratonic North America. Detailed studies of ultramafic rocks in CCT, SMT, Livengood Terrane and the Kluane metamorphic assemblage across Yukon, Alaska and British Columbia were conducted at nine site locations. The most common type of ultramafic rock present at these localities is serpentinized harzburgite. The ultramafic rocks from the CCT and SMT have been interpreted as the lower layers from dismembered ophiolite complexes. Samples collected from the CCT are consistently harzburgite, whereas samples from the SMT are both mantle-derived harzburgite and lherzolite. The variety of ultramafic rock present in the SMT suggests they were generated within contrasting geological settings.