KIMBERLITES IN THE SLAVE CRATON, NORTHWES T-TERRITORIES, CANADA (A PRELIMINARY REVIEW)

The 400 by 750 km Slave Structural Province of the Northwest Territori es, Canada, an integral part of the North American Craton has remained stable since the end of the Archean. It is composed of granites, gnei sses and supracrustal rocks of the Yellowknife Supergroup. The Acasta gneisses, the oldest rocks in the world, have been dated at 4.0 Ga and occur in the western part of the Slave Province. In addition, four sw arms of Proterozoic diabase dikes, ranging from 2.23 to 1.27 Ga, intru de the older sequences. Over the period 1991 to 1995, more than one hu ndred pipes have been discovered in the central Slave Province. Most o f these kimberlites do not crop out and have been identified using a c ombination of heavy mineral sampling, geophysical techniques and drill ing. Most of the pipes are characterized by either high or low magneti c anomalies and resistivity lows. The principal diamond-bearing pipes cluster in a northwest-trending zone in the Lac de Gras region. Crater , diatreme and massive hypabyssal facies kimberlites are present. Macr ocrysts of olivine (up to 1 cm in size), chrome diopside (>2 cm), garn et (to 0.5 cm), chromite and ilmenite are ubiquitous. Preliminary radi ometric age determinations indicate that pipe emplacement occurred at 86-81 Ma and 52 Ma. Paleontological studies on mudstone chips and wood fragments in volcaniclastic breccia and crater facies deposits lend f urther support for these young ages. A second group of pipes outline a n east-northeasterly trending zone which overlaps the southern end of the Lac de Gras cluster. Diatreme and hypabyssal facies are characteri stic of these pipes and they tend to contain less abundant olivine mac rocrysts, fewer indicator minerals and more country rock fragments tha n the Lac de Gras cluster. Wood fragments and mudstone chips have not been reported. The age of these pipes has not been established. In con trast, few kimberlites have been discovered outside of the central Sla ve Province. Those that have been found in the southwestern part of th e Province differ in xenocryst suite from the Lac de Gras pipes. As ex ploration continues, it is likely that more kimberlites will be found and a better understanding of their ages, petrology and diamond potent ial will be gained.