Kimberlite Sills and Dykes Associated with the Wesselton Kimberlite Pipe, Kimberly, South Africa

Author(s): 
J.L. White, R.S.J. Sparks, K. Bailey, W.P. Barnett, M. Field, L. Windsor
Date: 
Wednesday, March 16, 2016
First presented: 
Geological Society of South Africa
Type: 
Published paper
Category: 
Geology

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.

Feature Author

Dr. Wayne Barnett

Wayne Barnett has collected, managed, and interpreted exploration and mining data throughout his 18-year career in the mining and exploration industry. In 1999, he built the first known integrated ODBC database that used geotechnical mapping and production drilling sensor data to automatically calculate rock mass classification information for immediate use in mine planning and slope design.

A principal structural geologist, Wayne undertakes and provides best-practices training in structural and oriented drill core logging and mapping, geological data management, data QA/QC, and the statistical analysis of structural data. He is an expert in geological applications of ArcGIS and has developed customized analytical tools for exploration purposes. Wayne is driven to improve the geological mapping workflows for mining operations through integrating the benefits of old-school interpretative paper-based mapping with new mapping technologies.

Mining Data Management Specialist
PhD, PrSciNat
SRK Vancouver
SRK Turkey