Petrography of the Mountain Lake Pipe, Grande Prairie Area, Alberta, Canada

Twelve samples rich in altered juvenile olivine from the Mountain Lake Pipe (holes ML95-1 and ML95-3) were studied for texture and mineralogy. Contrary to an earlier interpretation that Mountain Lake Pipe rocks are entirely extrusive crater-facies products, all of the samples studied show olivine pellets and evidence of pelletal nucleation, both of which are characteristic of diatreme-facies kimberlite. Rocks of the Mountain Lake Pipe show a mixed origin and have been tentatively classified as hybrid alkaline ultramafic rocks, with some petrological affinities to alnoitic magmas. Despite the occurrence of such diamond indicator minerals as garnet (G5, G9, G11), chrome diopside, chromite and picro-ilmenite, which indicate contribution of a kimberlitic component, the economic potential of the rocks as a diamond source is estimated as low.