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

Abstract

K2 is a steep-sided kimberlite pipe with a complex internal geology. Geological mapping, logging of drillcore and petrographic studies indicate that it comprises layered breccias and pyroclastic rocks of various grain sizes, lithic contents and internal structures. The pipe comprises two geologically distinct parts: K2 West is a layered sequence of juvenile-and lithic-rich breccias, which dip 20-45°inwards, and K2 East consists of a steep-sided pipe-like body filled with massive volcaniclastic kimberlite nested within the K2 pipe. The layered sequence in K2 West is present to > 900 m below present surface and is interpreted as a sequence of pyroclastic rocks generated by explosive eruptions and mass-wasting breccias generated by rock fall and sector collapse of the pipe walls: both processes occurred in tandem during the infill of the pipe. Several breccia lobes extend across the pipe and are truncated by the steep contact with K2 East. Dense pyroclastic rocks within the layered sequence are interpreted as welded deposits. K2 East represents a conduit that was blasted through the layered breccia sequence at a late stage in the eruption. This phase may have involved fluidisation of trapped pyroclasts, with loss of fine particles and comminution of coarse clasts. We conclude that the K2 kimberlite pipe was emplaced in several distinct stages that consisted of an initial explosive enlargement, followed by alternating phases of accumulation and ejection.

The importance of the transport system in shaping the growth and form of kimberlite volcanoes

“…The fill of some large diatremes can be subdivided into a number of discrete phases distinguished by variation in componentry or diamond grade (in kimberlites) that are bounded by steep contacts (e.g., White, 1991;Ross and White, 2006;Brown et al, 2008a;van Straaten et al, 2008). These different rock units were emplaced adjacent to each other in stages rather than during a major, more or less continuous pipefilling event (cf.…”

The influence of complex intra- and extra-vent processes on facies characteristics of the Koala Kimberlite, NWT, Canada: volcanology, sedimentology and intrusive processes

“…It also explains how a large volume of pyroclastic debris is deposited in the vent after initial vent opening and clearing. Brown et al (2008b) argued that the proportion of ejecta which can be trapped by the vent during column collapse is in general not sufficient to account for the volume of MVK, although it may be plausible in the case of layered deposits seen at Venetia (Brown et al 2009). Porritt et al (2008b) demonstrated that a single column collapse event involving variable density material from the conduit flow, gas thrust and convective uprise zones could easily account for the volume of the MVK in the Fox kimberlite.…”

Kimberlite wall-rock fragmentation processes: Venetia K08 pipe development

“…Analogous examples from the Venetia K02 kimberlite geology are described by Kurszlaukis and Barnett (2003) and Brown et al (2009). The K02 sidewall fan structures are zones up to about 20 m wide of country rock breccias, which have internal facies contacts dipping up to 34°(the angle of repose) away from the contact and fragment sizes that increase from sub-millimetre size to mega-lithic blocks several metres in diametre towards the contact.…”

“…Concepts discussed in the paper are presented, such as (i) explosive preconditioning of the rock prior to collapse, (ii) under-cutting and gravitational collapse of the hanging wall, (iii) zones of breccia shearing and re-fragmentation, (iv) a possible link to a source of volcaniclastic material in K02 and (v) multiple phases of magmatic kimberlite intrusion. The image does not represent the final state of the K02 pipe; see Brown et al (2009) for sections through K02 matrix by hydrothermal circulation. The explosions are likely to have been centreed on magmatic dykes or dykes of preceding volatile fluids.…”

“…The pipes are mined by DeBeers and have provided much data and insight into kimberlite emplacement processes (e.g. Seggie et al 1999;Kurszlaukis and Barnett 2003;Barnett 2004;Medlin 2005;Tait et al 2006;Brown et al 2009). The Venetia cluster is unusual in that (at least) 16 individual pipes have been identified within an area no larger than 2.5 km 2 .…”