EXPERIMENTAL MODELLING OF THE EFFECT OF COUNTRY ROCK COMPETENCE ON KIMBERLITE ERUPTIVE STYLE

This work investigates the effect of country rock competence on the emplacement style of erupting kimberlites. Sandbox experiments have been used to test the Field & Scott-Smith (1999) model that pipe shape and infill are related to country rock lithology and competency, producing 3 major styles of kimberlite found worldwide. Kimberley style pyroclastic kimberlites (KPK) in southern Africa consist of large, steep sided, deep pipes emplaced in crystalline rocks and competent sedimentary rocks. Fort-a-la -Corne style kimberlite pipes (FPK) in the Canadian prairies consist of wide shallow craters emplaced in loose sediments. Lac-de-Gras style kimberlite pipes (LDG) on the Slave craton in northern Canada consist of small steep sided pipes emplaced in crystalline basement with a thin veneer of loose sediments. The experiments used dry sand as an analogue for loose sediments, sand partially saturated with 10 wt.% ethanol as an analogue for competent sediments, and sand partially saturated with 10 wt.% water as an analogue for crystalline rocks. Adding small amounts of liquid of varying surface tension to sand generates analogue materials with cohesion values that span 3 orders of magnitude. These relative differences in cohesion between analogue materials are comparable to the differences between real country rock types. For both real rock types and our model analogues, the increase in strength between loose sediments and competent sediments is a factor of 30-40, and between competent sediments and crystalline rock is a factor of 2-3. With these 3 analogue materials we represent the country rock environment present at the three major pipe types and modeled kimberlite eruptive energy with compressed air at constant pressure. Our KPK eruption analogue results in large steep sided pipes with minimal excavation while our FPK eruption analogue results in wider, shallower craters with quicker and more thorough excavation. These experiments generally agree with Field and Scott-Smith (1999), that the nature of the country rock into which these magmas are emplaced is a dominant controlling factor determining kimberlite eruptive style and resulting deposit textures.