Computational Modelling Group

Investigation of a Magmaclast-Rich Intrusive Pipe Structure and its Relationship to Late Stage Magmatic Intrusions, within the K1 Kimberlite Pipe, Venetia Mine, South Africa

Started
30th September 2011
Ended
1st July 2012

The deposit is located in Northern South Africa, at the Venetia Mine (De Beers Group)

Undergraduate Master of Geology Thesis

This was a geochemical and petrological investigation of an unusual volcanic vent structure within a diamond-bearing, "kimberlite" volcano in South Africa. The structure and texture of certain clasts of material within the ancient vent suggested a new type of volcanic eruption style (previously hypothesised as responsible for other volcanic features) was responsible for the vent and its unusual, magma-coated clasts. By looking at the geochemistry of the minerals along with their physical features, it was possible to confirm that the deposit had been "fluidised", where material is suspended is a volcanic gas flow and coated with magma. This is a novel eruption style intermediate between volcanic explosions and liquid lava flows. The geochemical results imply that the gas necessary for this to occur comes from a very slight, volatiles-only melting of lower crustal material that can only occur below continents.

What follows is a more technical description.

This study within the Venetia Kimberlite cluster, South Africa, examines the relationships between a magmaclast-rich intrusive pipe structure within the K1 diatreme, the diatreme fill itself, and late stage dykes observed in the vicinity. Analysis of the Rare Earth Element contents of the dykes across the Venetia cluster reveals enrichment in light REE’s and lower heavy REE values consistent with small degrees of partial melting. Trends suggest varying degrees of melting and later fractionation between different dykes and diatremes. The magmaclast-rich facies phlogopites, Analysed using LA-ICP-MS, are enriched in compatible elements and depleted in incompatible elements to the same extent as the intrusive dykes surrounding the facies. The diatreme fill phlogopites by contrast are heavily enriched in light REE’s and incompatible elements. Spinel geochemistry, investigated using SEM and X-ray analysis, reflects the enrichment trends of the phlogopites. The magmaclasts are known to form through fluidisation and spray granulation of particles in an upwards gas flow. The magmaclast-rich facies and intrusive pipe structure were generated by magma similar in composition to the late-stage dykes, significantly this late stage magma retained enough CO2 and H2O to fluidise the facies. Such phenomena may be significant contributors to systems involving carbonatitic or alkaline, low viscosity magmas.

Supervisors;

Dr Tom Gernon (Ocean and Earth Sciences, University of Southampton)

Dr Rex Taylor (Ocean and Earth Sciences, University of Southampton)