Lonmin Marikana: Structural Interpretation of 3D Seismic Cube and Integration with Underground Mapping for 2# Siting

In 2008, Tect were approached by Lonmin Platinum Mine to provide a structural interpretation, in collaboration with Impulse Geophysics, of a new 3D seismic survey that had been commissioned over the 26km long Marikana Mine area. This occurs within the E-W trending southern portion of the Western Lobe or Limb of the Bushveld Complex, which is transected by the NNW-trending Crocodile River Fault to the west of Brits. An extensive suite of underground mapping and detailed reef traces, recorded in CADSMine, were to be combined with the 3D volume to guide Mine Planners in the siting of 2 Shaft (2#). A volume, which would include the prospective shaft pillar, had to be identified and delineated: it should contain as few as possible or no major structures, limited post-magmatic deformation, and few or no iron-rich ultramafics (IRUPs), potholes, slumps, dykes or sills.

Modelling required the creation of 3D triangulated surfaces for both the Merensky Pyroxenite and UG2 Reef horizons. These were based on reef positions derived from the interpretation of a 3D seismic cube in Datamine Studio 3, surface reef trace mapping, aeromagnetic data interpretation, stope and development mapping and drillhole intersections. The following were delivered to the client: modelled major fault planes and dykes that intersect the Merensky and UG2 Reefs, outlines of IRUPS, and domains wherein significant strike and dip changes of are present on one or both reefs. The delineation of IRUPs was particularly important. as these bodies, which are identifiable from surface geophysics, occur throughout the cumulate pile and tend to pool and spread laterally below thick, relatively chemically-inert chrome layers, including the Merensky and UG2. IRUPs, together with their vertical to subvertical feeders, displace PGE grade – an important economic consideration when siting a shaft and reef drives – and also contribute to poor ground conditions and high methane contents within fractures and faults.

Similarly, slumps, potholes and stacked potholes take a variety of forms across the Bushveld Complex. Consequently, their catalysts may have been highly variable and operative over a great variety of time scales and intervals, from syn-intrusion to late-cooling deformation. Identification of these areas in the Marikana area had to take into account a variety of causative factors, including the syn-magmatic composition of the cumulate pile; the energy and composition of the Merensky magma pulse; deformation, compaction and devolatilization of the underlying cumulate sequence; and the dip and topography of footwall units to the Bushveld Complex. Comprehensive 3D analysis of the large volume that encompasses Marikana Mine revealed that the previously-planned position of 2# occurred at the intersection point of two major interpreted faults which, collectively, comprise a regional conjugate fault system caused by an almost due N-S oriented maximum principal stress. The analysis delineated several, more appropriate prospective sites for 2# and subsequent development.