DIAMOND-BEARING VOLCANICLASTIC KIMBERLITE S IN CRETACEOUS MARINE-SEDIMENTS (SASKATCHEWAN, CANADA)

The main host rock of diamond, kimberlite, is found typically in ancie nt continental terrains (cratons) as vertical diatremes or ''pipes'' t hat are the erosional remnants of deep-seated volcanoes. Some have pre served crater sediments above the diatreme but none has been reported with the superstructure intact. However, the first occurrence of wides pread layered kimberlite deposits ejected outside the craters have bee n found in boreholes in the Fort a la Corne area of central Saskatchew an, Canada. They are interstratified with shallow marine sediments of the Lower Cretaceous (100 Ma) Westgate Formation, which were deposited in a shallow epicontinental sea and subaerial coastal swamps. The kim berlites consist of primary pyroclastic ashfall deposits of probable p hreatomagmatic origin, up to 4.5 m thick, and marine reworked pyroclas tic sediments (epiclastics) in which kimberlite heavy minerals, includ ing diamond, have been concentrated by up to a factor of three.