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

Abstract

Understanding the range of transport styles recorded by kimberlite deposits is key to describing the type and style of eruptions. Building a clear picture of the processes that shape deposits is essential for selecting exploration targets and evaluating the grade and value of diamond-bearing kimberlites. Variations in grade reflect differences in the diamond content of different magma parcels erupted during the lifetime of the kimberlite volcano, sorting during transport of eruption products, or re-working of diamonds during crater growth, cone collapse and erosion.The form of the kimberlite volcano is largely determined when the magma arrives near the surface. If magma comes into contact with external water, transport will be driven by a combination of magmatic gases + steam. From a diamond exploration perspective, the resulting deep diatremes make the most attractive targets because they survive erosion and form large geophysical anomalies. If water is too abundant, a tuff cone or tuff ring with no diatreme or a shallow one will form. On the other hand, if external water is very limited or if the conduit is rapidly sealed by chilled melt, the transport system will be driven by magmatic gases alone. The result will then be a spatter cone or cinder cone underlain by a dike, possibly with a related lava flow, but with no diatreme.

Maar-diatreme volcanoes: A review

“…These magmas have a wide range of viscosities and inferred volatile contents, but there are few systematic differences in diatreme deposits or structures beyond those of the juvenile particles themselves (e.g., juvenile fragment shape, vesicularity, crystallinity). We previously listed features observed in diatremes formed by all these magma types in Table 5 (see also McClintock et al, 2009), and reiterate here that despite the unusual petrography and xenocryst abundance of kimberlite, the diatreme structures scarcely vary across this range of magma compositions.…”

Influence of the substrate on maar–diatreme volcanoes — An example of a mixed setting from the Pali Aike volcanic field, Argentina

“…We also summarize what is currently known about maars emplaced in different types of substrates. Better understanding the influence of the external environment on eruptive processes and deposits of basaltic monogenetic volcanoes has the added benefit of potentially shedding light on kimberlite emplacement processes (e.g., McClintock et al, 2009). …”

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

“…Lorenz and Kurszlaukis 2007); deposition into the vent, either incrementally or en masse, from a collapsing eruption column (Porritt et al 2008a); in-vent deposit evolution, either through fluidisation driven by gas streaming (e.g. Gernon et al 2009c), or recycling through suppressed debris-jet mixing (McClintock et al 2009). …”

“…It is therefore not considered to have formed through phreatomagmatic processes which have been proposed in several kimberlite eruption models (cf. Lorenz et al 1999;Lorenz and Kurszlaukis 2007;McClintock et al 2009). …”

“…Sparks et al 2006) or phreatomagmatic (e.g. Lorenz et al 1999;Lorenz and Kurszlaukis 2007) fragmentation mechanism, and the roles played by in situ fluidisation (Hetman 2006;Gernon et al 2009c), full or partial eruption column collapse (Moss et al 2008;Porritt et al 2008a), and debris jets (McClintock et al 2009) . Coherent kimberlite is commonly found at depth, beneath MVK, and a textural gradation between the coherent and pyroclastic deposits is observed in some pipes.…”