- © 2003 Geological Society of South Africa
The paper summarises an investigation of the volcanology of the Venetia kimberlite cluster and the country rock structures in the immediate vicinity of the pipes. Our study shows that the pre-existing country rock structure and the stress field prevailing during emplacement control the position and morphology of the pipes. The pipes were formed along pre-existing zones of weakness, and the pipe shapes are elongated in the direction of fractures and faults. The highest degree of vertical and lateral pipe shape irregularity is found in the vicinity of country rock structural weaknesses. The understanding of this relationship between pipe geometry and country rock structure can significantly affect mineral resource models.
The most important volcanological feature of the cluster is the presence of bedded fragmental rocks filling the main pipes K1 and K2. These rocks are readily interpreted as volcaniclastic in origin, yet are petrographically identified as typical Southern African tuffisitic kimberlite breccias (“TKB”). These observations contradict the hypothesis that “TKB’s” are intrusive rocks, which are expected to be massive and unstructured in nature.
The presence of accretionary (armoured) lapilli, soft sediment deformation, and signs of reworking within the volcaniclastic deposits suggest a high moisture content of the volcaniclastic material during and after emplacement of the pipe. Monolithic talus fans as well as contact breccias represent significant facies types within the Venetia kimberlite cluster. The contact breccias show signs of downward movement along the pipe walls, suggesting a considerable volume deficit at depth. These volcanological observations need to be accounted for in a comprehensive emplacement model for the Venetia kimberlite pipes.
The observed structures, internal textures and facies suggest that the Venetia pipes were emplaced in several violent volcanic phases, separated by periods of relative inactivity.