Structural Interpretation and Implicit Modelling of Major Structures for Lift II Sub-Level Caving

Palabora Mine, which has been operational since its incorporation in 1956 and is South Africa’s major producer of refined copper, contracted Tect in 2012 to create the first detailed 3D Geomodel of large-scale structures associated with intrusive phases and mineralization at their operation. Ongoing infrastructure development and block caving, as part of their new Lift II Project, required a significant increase in the resolution and understanding of major structural discontinuities at depth.

3D implicit modelling of these structures by Tect incorporated over 30 years of historical data, including: 1) Geological and Geotechnical drilling programs; 2) Macrostructural logging within Lift II drillholes, wherein major faults or shear zones and their attributes were identified; 3) Previous 3D models of foskorite, banded carbonatite and transgressive carbonatite; 4) Hard-copy and digital mapping data over the last 30 years, which includes line, face and window mapping from 1984, 1998 and 1999 by PMC staff and contractors, notably Piteau Associates and Golder and Associates and; 5) Hard-copy reports, faxes, emails, maps, images, tables, appendices and stereonets, particularly from Piteau Associates, Golder and Associates and PMC since c. 1984 were imported, georeferenced where possible and used to extract joint and fault set data. Data from these programs and reports were digitized and georeferenced in 3D modelling space, capturing fields such as structure type, lithology type, traverse bearing, infill type, rock strength/rating, dip direction and dip, and structurally- or geotechnically-related comments.

3D implicit modelling revealed that the Phalaborwa deposit occurs at the intersection of at least five major, coinciding structures or shears, many of which show trends that are characteristic of long-lived structures elsewhere in the Kaapvaal Craton. Available data demonstrated that almost all of these structures consist of several non-contiguous segments, with variable offsets, suggesting that the VOI preserves evidence for rotational (torsional) deformation that would have contributed to a severely deformed or fractured volume, possibly analogous to the ‘‘structural preparation” suggested for kimberlite intrusion. Five of these structures were pivotal in the emplacement of banded carbonatite, transgressive carbonatite and very late-stage, narrow, E-W trending, sulphide veinlets with short down-dip and along-strike extensions.

The results of this study were used in downstream analysis: 3D discrete element modelling, caving simulation and hydrogeological studies all benefitted from the accurate modelling of delineated major structures. Tect has a policy of showcasing its work, in collaboration with colleagues in the industry, in international, peer-reviewed journals:

Structural analysis and 3D modelling of major mineralizing structures at the Phalaborwa copper deposit by Ian Basson and Sukey Thomas of Tect and Paulien Lourens, Hans-Dieter Paetzold, Robert Brazier and Pontsho Molabe of Phalaborwa, Ore Geology Reviews 83 (2017).

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