Subsidence breccias in kimberlite pipes—an application of fractal analysis

Abstract

A particular variety of volcanogenic country rock breccia is described; a contact breccia that has been identified at Venetia, River Ranch and Wimbledon kimberlite pipes. The contact breccia is clast supported with no juvenile kimberlite component, has tightly packed, angular fragments (with occasional rounding of smaller particles), and has a shear-fabric dipping towards the center of each kimberlite pipe or volcanic event. Clasts have preferred orientations parallel to the fabric. Photographs of the breccia in the open pit face and measured data from drill core are analyzed specifically to quantify the clast size distributions and clast shapes by means of fractal analysis. The fractal dimension is one means of characterizing the breccia because the dimension can be specific to a fragmentation mechanism. Clast size distribution fractal dimensions in the coarser particles (greater than circa 3 cm) range from greater than 3 for nonsheared breccia, down to circa 2.3 for the sheared breccia. Breccia characterization based on this fractal analysis suggests that fragmentation occurred initially from confined high-energy explosions, followed by collapse and abrasion by subsequently gravity-induced rockmass subsidence. All studied contact breccias produced a distinctive fractal signature in the finer particles (<3 cm) of circa 1.6 that can be explained by a comminution fragmentation process in that particular particle size range. It is suggested that these subsidence breccias require a substantial volume deficit at depth within the volcanic pipe in order to explain their origin and size. The methodology used in this study could be used to characterize any other volcanic breccia and further model their origins.