Integrating structural geology and geotechnics at Venetia diamond mine

At the Venetia diamond mine in northern South Africa, owned by De Beers, the country rock structural geology has been under investigation since before the mine was opened in 1994. This includes academic research alliances to better understand the complex structural deformation history of the mine area. These alliances are with the University of Johannesburg, Tect Consulting and more recently the University of Pretoria.

It is located within the Central Zone of the Limpopo Belt, which is a multiply-deformed orogenic zone between the Kaapvaal and Zimbabwean cratons, formed during the Archean and Paleoproterozoic. At least four phases of ductile deformation and an unconstrained number of brittle deformation events are now recognised. The dominant S2 foliation in the gneissic rocks has been the focus of structural investigations because of its strong influence on the anisotropic strength characteristics of the rock mass within the open pit and the effect of this on the slope stability and slope angles.

The S2 foliation is deformed by two phases of folding that complicate interpretation (see stereonet inset). The most obvious structural geometry is an eastward plunging synform, the southern limb of which dips between 35o and 55o towards the north. The southern open pit slope design is heavily dependent on the dip angle and projected dip behind the slope, and has been the subject of advanced analyses in probabilistic risk analysis by De Beers and SRK Consulting (South Africa).

More recently, the focus has shifted to underground mine design and the objective of trying to interpolate structural geometries, including foliations and lithological domain contacts, to depths of 1km. A database of over 950 drill holes, with at least 108 orientated cores, currently exist that aid the construction of three-dimensional computer models representing structure and lithologically controlled geotechnical domains.

De Beers logging standards include detailed structural data, with mineral lineations. Comprehensive pit mapping by Tect Consulting combined with drill hole data has allowed De Beers and SRK to construct a 3D model that predicts the dominant structural geometries at depths below the pit.

In particular, a 3D representation of the lineation trends and plunges maps out the dominant fold geometries or foliation patterns that need to be considered in geotechnical design and numerical strain models. SRK continues to be involved in open pit and underground design, and in developing the structural understanding of the country rock in order to reduce geotechnical risk.

Wayne Barnett: wbarnett@srk.com


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