Anglo Platinum: Amandelbult Mine, Moddergat Section: 3D modelling of disrupted Merensky and UG2 Reef in a gap area

In 2008, Tect were approached by Amandelbult-Tumela Mine of Anglo Platinum, South Africa to undertake an update and structural re-interpretation of its Moddergat Area, which occurs on the edge of a “gap area” in the western limb of the Bushveld Complex. Gap areas result from the erosion of Main Zone rocks by a massive, localized influx of Upper Zone magmas. The resultant geometry is a series of very large-scale unconformities wherein the mineralized part of the Upper Critical Zone (UCZ), which contains the Merensky and UG2, is lost or poorly developed. The UCZ in these areas display characteristics and geometries commensurate with slumping and infill of a major elongate depression.

Superimposed on this macro-scale geometry are potholes, slumps, iron-rich ultramafic pegmatoids (IRUPs) and aplites/aplitic pegmatoids, which are generally ascribed to either primary or syn-magmatic processes, as well as deformation of the Bushveld Complex and the underlying footwall rocks during the latter stages of cooling and crystallization. IRUPs, slumps and potholes are, broadly speaking, syn-magmatic structures, although they are often located above deep-seated, long-lived structures like normal faults or collections of normal faults such as those in the Moddergat Area. The delineation of these features, or areas wherein these features are likely to occur, is essential for mine planning. It relies on underground mapping and drilling, but also involves a degree of prediction based on their mode of occurrence and documented relationships to structures.

A 3D seismic survey over the Moddergat Area, by RDR (Rock Deformation Research) formed the basis of the revised interpretation by Tect, using Datamine. From this, predicted reef positions and geometries could be extracted, along with major ductile to brittle-ductile structures that disrupt the reefs. Zones that contain a particular combination of features which denote IRUP development and other disruptions were ring-fenced. The result of the study was an updated 3D model of the Merensky and UG2 Reefs, which allowed for several parameters to be extracted, including triangulation dip and reef middling. The 3D model permitted a greater degree of footwall drive planning, as these developments needed to be locally re-directed to account for progressive down-faulting to the SW, into the gap area that is adjacent to relatively undisrupted reef. Furthermore, elongated, fault-parallel zones of coherent, relatively flat-lying, IRUP-free reef were delineated for stope planning, thereby extending the available resource well into the flanks of the gap area.