Abstract

Field relations, petrography and bulk rock geochemistry constrain the emplacement mechanisms and petrogenesis of a kimberlite sill complex exposed in water tunnels at Wesselton Mine, Kimberley. The sill complex, together with associated dykes, is precursor to the main Wesselton pipe and contains:

  1. aphanitic kimberlite with a carbonatitic groundmass of calcite, perovskite, spinel, serpentine and phlogopite, with variations from rocks rich in olivine microphenocrysts to rocks with little or no olivine;

  2. macrocrystic kimberlite containing entrained harzburgite microxenoliths, olivine macrocrysts and strongly reacted orthopyroxene; and,

  3. a ‘green’ aphanitic kimberlite.

The sills were emplaced as multiple intrusions into the upper Dwyka shales immediately below a Karoo dolerite sill in a zone several metres thick. Emplacement of the sill complex was governed by the rigidity of the dolerite sill, which prevented the magma from breaching the surface. Feeder dykes to the sills were influenced by a regional crustal stress system with the principal vector of horizontal compression being between eastwest and northwest-southeast. Serpentinisation has pseudomorphed olivine, replaced calcite, apatite and phlogopite in the groundmass and thereby changed the bulk compositions. Geochemcal and mineral chemistry data on spinels and phlogopites indicate intrusion of several magma batches with different petrogenetic histories. The magmas are transitional to carbonatite with compositional variations linked to partial melting, assimilation of xenocrysts, entrainment of xenoliths, xenocrysts and macrocryst, fractional crystallization and flow differentiation during ascent and emplacement.

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