Kimberlite Intrusions

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Taken from http://www.geology.ukzn.ac.za and Hydrogeology of the Main Karoo Basin: Current Knowledge and Future Research Needs WRC Report Project K860 (2001).

Introduction

Following Gondwana breakup, there was a period of unique igneous activity. This formed numerous volcanoes of kimberlite rock, commonly known to contain diamonds.Kimberlites occur as clusters of linear or arcuate swarms of dykes and fissures associated with several enlargements, blows or pipes.  Parallel regional jointing often accompanies the fissures.  They do not contain any igneous material, except for a few indicator minerals or traces of mica.   Blows and enlargements frequently occur along the fissure and can be easily recognized on aerial photographs due to the presence of well-developed calcrete.  Kimberlite diatremes are unevenly distributed.  They are not very common and vary in diameter from only 10 to 400m in the western Karoo, Sutherland, Victoria West, Britstown, Prieska and East Griqualand areas.  They contain a large amount and a wide variety of mantle and crustal xenoliths as well as megacrysts.

Contact between basalt (right) and kimberlite (left)

Geohydrology

The intrusion of the kimberlites did not result in the intensive thermal metamorphism of the Karoo sediments, as did the dolerites, and thus they did not significantly alter the hydrological properties of the sediments.  On a regional scale, however, clusters of kimberlites may represent important fractured domains.

On a local scale, the thin kimberlite dykes (< 3m) are generally only weakly jointed and thus have a very low permeability, especially within the highly decomposed upper section of the dyke.  However, the strong regional jointing and reactivation of existing structures that accompanies the emplacement of kimberlite swarms may be important for the occurrence and movement of groundwater.  Kimberlite blows or enlargements (diameter of 4 to 10m) may represent more permeable zones along the dykes, as they are always more heterogeneous in texture, more deeply weathered and marked by dense bush growth.

Large kimberlite pipes or diatremes are more heterogeneous and brecciated.  There is thus a possibility that high-yielding boreholes can be sited alongside or within these features, similar to the breccia plugs.

Boreholes sited into kimberlite fissures commonly yield very small amounts of groundwater, mainly due clogging of near-surface joints by clays produced by the weathering and decomposition of the kimberlite.