Volcanological and structural aspects of the Venetia kimberlite cluster - a case study of South African kimberlite maar-diatreme volcanoes

Abstract

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.

Geology of a complex kimberlite pipe (K2 pipe, Venetia Mine, South Africa): insights into conduit processes during explosive ultrabasic eruptions

“…All observations taken from 60-108 m bps a c d b (massive volcaniclastic kimberlite in this work; after Sparks et al 2006). Kurszlaukis and Barnett (2003) documented volcanological and structural aspects of the Venetia kimberlite cluster. They reported a sequence of layered breccias in K2 west with variable country rock contents and interpreted them as talus deposits derived from subsidence of the pipe walls.…”

“…Shale, rhyolite and basalt clasts occur as inclusions in the pipe-filling deposits and indicate that Early Proterozoic Waterberg Group rocks covered the basement at the time of kimberlite emplacement but have since been eroded away. About 300-600 m of post-emplacement erosion has been estimated for the Venetia area (Kurszlaukis and Barnett 2003). Preexisting structures in the basement had a major control on the position and shape of the Venetia kimberlite pipes: many appear to be elongated along faults and shear zones in the metamorphic basement (Kurszlaukis and Barnett 2003).…”

“…About 300-600 m of post-emplacement erosion has been estimated for the Venetia area (Kurszlaukis and Barnett 2003). Preexisting structures in the basement had a major control on the position and shape of the Venetia kimberlite pipes: many appear to be elongated along faults and shear zones in the metamorphic basement (Kurszlaukis and Barnett 2003). Pipe growth during eruptions is inferred to have been influenced by joint-controlled tensional failure of the pipe walls along pre-existing structural weaknesses and by regional stress orientations, accounting for the elongate and straight rectilinear shapes of the pipes ( Fig.…”

“…Sparks et al (2006) argued that kimberlite eruptions are volatile-driven and that they may comprise numerous phases of excavation and infill. The duration of kimberlite eruptions is considered to range from an hour (e.g., Wilson and Head 2007) through to several days-to-months (Lorenz 1975;Kurszlaukis and Barnett 2003;Sparks et al 2006). Much of the controversy stems from an incomplete understanding of the properties of kimberlite magma.…”

Overview of Diamond Resources in Africa

“…K04 is the only other kimberlite of significance at just over 4 ha in size, but with appreciably lower diamond grades. The regional structure exerted a significant control on emplacement, resulting in a prominent west-northwest orientation in pipe shapes (Kurszlaukis and Barnett, 2003). The kimberlites have been dated at 519 Ma (Phillips et al, 1999) and they intrude the 2.0 Ga Central Zone of the Limpopo Mobile Belt which comprises gneiss, amphibolite, schist, quartzite and marble.…”