South Africa - Inkaba.org

Some aspects of late magmatic replacement phenomena
developed in the layered sequence exposed at Northam
Platinum mine
S. Jack 1 , DL Reid 2 , C. Van Jaarsveld 3
1. University of Cape Town, Department of Geological Science, South Africa, jcksiy001@uct.ac.za
2. University of Cape town, Department of Geological Science, South Africa, David.reid@uct.ac.za
3. Northam Platinum Pty Ltd
ABSTRACT
Iron rich ultramafic pegmatites (IRUP) are due to a post magmatic event that affected the cumulate horizons of the
Rustenburg Layered Suite in the Bushveld Complex. It has been proposed that the IRUP bodies are products of a
complex replacement process perhaps involving a ferrobasaltic melt derived by liquid immiscibility in the Upper
Zone. Transport of ferrobasaltic melt from the Upper Zone to parts of the Upper Critical Zone lead to disruption of
the layered sequence that hosts PGE mineralization in the Merensky Reef and UG2 Chromitite. Large distributions
of IRUP in some mine sequences are suggested to have migrated through weak zones (e.g. faults and fractures), but
also by reaction infiltration through certain lithologies such as plagioclase rich cumulates (norites and
anorthosites).
The present study is being conducted at Northam Platinum Mine, where access to underground exposures and drill
core was made possible by the management. Good exposures of IRUP development occur in the footwall drives of
many levels, where the noritic footwall of the UG1 chromitite has been extensively replaced. The aim of the study
is to characterize IRUP petrographically, mineralogically and geochemically, so we can understand their nature,
distribution controls and origin, as well as their role in mobilization of Platinum Group Elements (PGE). Major and
trace element analysis were obtained using X-ray Fluorescence (XRF) and mineral analysis data obtained from the
Electron Microprobe (EMP).
KEYWORDS: ultramafic; pegmatite; Bushveld.
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