Elsevier

Ore Geology Reviews

Volume 126, November 2020, 103784
Ore Geology Reviews

An innovative seismic and statistical approach to understand 3D magmatic structures and ore deposits in the western Bushveld Complex, South Africa

https://doi.org/10.1016/j.oregeorev.2020.103784Get rights and content

Highlights

  • 3D seismic imaging of platinum-bearing horizon and pothole structures.

  • High-definition imaging and size quantification of ore disrupting features.

  • Potholes are randomly distributed in 2D, with some clustering.

  • Potholes are the result of a widespread extension regime in the magma chamber.

Abstract

In this paper, we demonstrate the effectiveness of a novel approach to identify and quantify structural features that disrupt ore-bearing horizons using 3D seismic attributes and a horizon-surface processing technique (difference-of-two-surfaces) on high-resolution 3D reflection seismic data from the Lonmin Platinum Mine in the Bushveld Complex, South Africa. We demonstrate that seismic attribute analysis is an effective tool to enhance the intelligibility of the UG-2, a PGE-enriched chromitite layer, as well as potholes (slump structures), faults, iron-rich ultramafic pegmatite bodies (IRUPs) and other subtle geological features. Furthermore, the horizon-surface processing technique is able to quantify the dimensions and geometry of potholes. In total, we identify 66 potholes and show that they are probably randomly distributed in 2D on the UG-2 horizon with some extent of clustering. Roughly two classes of potholes are identifiable by size. The larger potholes are clustered around a major normal fault (the Marikana Fault) and its branches in the area. Using a first-order pothole structure/geometry statistical analysis, we demonstrate that: (1) the diameters and surface areas of the potholes are strongly related to their depths; and (2) the volume of the potholes is substantially log-normally distributed. We interpret these findings to implicate a coherent and pervasive pothole formation mechanism. Furthermore, we show that statistically-derived pothole structure/geometry knowledge can be used to constrain the validity of several hypotheses. Our findings indicate the most probable pothole formation hypothesis is a widespread extensional regime in the Bushveld Complex magma chamber. Therefore, our study provides important, large-scaled and highly data-driven insights with an approach that can be applied to a wide range of magmatic structures. Furthermore, relationships identified between various pothole characteristics, in addition to their academic value, also allows for predictive modelling that can guide mine planning and resource extraction.

Introduction

The Paleoproterozoic-aged (~2.055 Ga) Bushveld Complex, located in the northern part of South Africa, hosts the world’s largest platinum and chromium deposits (Free, 2001, McCarthy and Rubidge, 2005, Zeh et al., 2015). A majority of platinum-rich deposits are found in the Critical Zone of the Rustenburg Layered Suite (RLS) in the Bushveld Complex. The Lower Critical Zone is characterized by orthopyroxenites and chromitites, whereas the Upper Critical Zone is characterized by variably cyclical units of chromitite to pyroxenite, to norite and anorthosite. The economic resources such as the platinum-group elements (PGEs), are found in stratiform horizons, which are locally referred to as ‘reefs’. The Merensky and the Upper Group 2 (UG-2) chromitite layers are two of the major economic platinum-bearing horizons of the RLS.

The extraction of ore from these economic horizons is complicated by the presence of geological features such as faults, dykes, potholes and iron-rich ultramafic pegmatite bodies (IRUPs). Potholes are one of the least-understood geological features found in the RLS, and may amount to ~15% ore loss (Viljoen and Hieber, 1986, Trickett et al., 2009). The characterization and prediction of these geological features is therefore of primary concern. Numerous definitions of potholes exist in geological, geophysical and mining literature (e.g. Carr et al., 1999, Mukherjee et al., 2017 and references therein; Manzi et al., 2020). Potholes seem to exhibit a broad range of sizes that span from the meter scale in depth and diameter (Mukherjee et al., 2017) to hundreds of meters (Carr et al., 1999, Manzi et al., 2020). In addition, the distinction between a slump and a pothole is not universal and depends on the discipline and techniques applicable to their identification (e.g. Mukherjee et al., 2017, Manzi et al., 2020). We adopt a definition similar to Manzi et al., 2020, and define potholes as variable-geometry, meter- to hectometer-scale slump structures that are distinguishable from planar horizons by vertically downward slumps. We do not adopt arbitrary size limitations in our definition. In addition to the definition of potholes, their origin remains debated (e.g. Schmidt, 1952, Elliott et al., 1982, Campbell, 1986, Boudreau, 1992, Carr et al., 1999, Mukherjee et al., 2017). The most recent hypothesis suggests that the formation of potholes is genetically related to magma emplacement through thermochemical erosion of the footwall rocks (e.g. Latypov et al., 2019). Studies on the large-scale distribution of potholes has received little attention, and therefore predicting their occurrence remains challenging from both statistical and first-principles perspectives.

Potential field methods are 2D in nature and have been successfully used in the RLS to delineate dykes, faults and potholes that disrupt the economic horizons. However, these methods cannot resolve platinum-bearing horizons, pothole and fault geometries with the accuracy required for optimal mine operations. In contrast, the reflection seismic method provides a superior trade-off between resolution and depth. This method and especially 3D seismics, is now well-established worldwide for deep exploration and mine planning (Malehmir et al., 2012). Between 1985 and 1986, Northam Platinum Mine conducted a 2D reflection seismic survey in the western limb of the Bushveld Complex and the results showed that the seismic reflection method could be used to map platinum-bearing horizons, the extent of potholes, and other geological features (faults and dykes) to optimize mine planning and design (Stevenson et al., 2003, Trickett et al., 2009). However, 3D seismic surveys are more valuable to mine operations than 2D surveys. Hence, in 1993, Lonmin Platinum Mine conducted it’s first-ever high-resolution 3D reflection seismic survey covering the Karee Mine. This survey was followed in 2008 by a Lonmin 3D seismic survey, which overlaps with the 1993 3D seismic survey. The survey successfully imaged platinum-bearing horizons (~2 m thick) such as the Merensky and UG-2 chromitite layers (hereon referred to as horizons) at depths between 800 m and 1.5 km, as well as faults, pothole structures and IRUPs that affect these horizons.

Three-dimensional reflection seismics is useful for deep ore deposit targeting as it is able to delineate the present-day geometry of economic horizons, image the geometry of faults, potholes and IRUPs, and place constraints on the timing of fault activity and magmatic intrusions. Interpretation of the UG-2 horizon is of particular interest in this study because of its greater economic significance and strong seismic reflectivity, and from the fact that it is more affected by potholes as compared to the Merensky horizon (Lomberg et al., 1999). In this study we integrate advanced seismic attributes with statistical methods to: (1) enhance detection of geological structures that affect the platinum-bearing horizons within the 1993 seismic cube covering the Lonmin Platinum Mine; (2) analyse the correlation between the size and distribution of potholes within the seismic cube; and (3) quantify the differences in pothole size, examine the relationship between potholes and other geological features (faults), as well as to examine the distribution of the potholes. The results presented here clearly demonstrate the benefits of using multi-seismic and statistical techniques in the identification of potholes in 3D reflection seismic data, and in unravelling the fascinating spatial characteristics of potholes. In addition, this study further demonstrates the value of 3D reflection seismic data, by virtue of its massive areal coverage and 3D nature, to produce insightful information on outstanding geological and mining issues.

Section snippets

Geological background

The Bushveld Complex has been the subject of over a century of research, which has been succinctly summarized by Eales and Cawthorn, 1996, Cawthorn, 2015. In summary, the Bushveld Complex is divided into three major plutonic suites, the: (1) Rashoop Granophyre Suite, (2) Rustenburg Layered Suite (RLS); and (3) Lebowa Granite Suite (Fig. 1). The RLS is dated at 2.055 Ga using U-Pb zircon dating (Maier et al., 2013 and references therein; Zeh et al., 2015, Mungall et al., 2016). It attains a

Seismic data acquisition and processing

The 3D seismic data covering the Lonmin Platinum Mine were acquired and processed by the Compagnie Générale de Géophysique (CGG) in 1993 and covered the Karee Mine (Fig. 1b). The data were obtained from the mine as a prestack time-migrated (PSTM) volume with an east–west extent of 6 km, a north–south extent of 4 km, reaching a depth of 9 km (Fig. 3). The acquisition and processing parameters of the seismic volume, as initially obtained from observers’ reports, are summarized by Larroque et al.

Structural enhancement through seismic data

Processed 2D seismic sections, extracted from the seismic cube, clearly shows several important reflections associated with the Merensky and UG-2 horizons (Fig. 4). A weak seismic reflection associated with the Merensky horizon is caused by a lower acoustic impedance contrast between orthopyroxenites (compressional seismic velocity ~6400 m/s and density ~2.9 g/cm3) and norite (compressional seismic velocity ~5700 m/s and density ~2.7 g/cm3). On the other hand, the UG-2 horizon is imaged as a

Seismic attributes and surfaces-based methods for pothole detection

By using seismic horizon-based attributes, we were able to better detect a thin deep-seated PGE-enriched horizon (i.e. UG-2) as well as important structural features such as faults and potholes, which can be used to optimize mine planning and design. Specifically, the horizon-based seismic attributes were the most useful at the detection and quantification of the rough size and shape of structural features (Fig. 5, Fig. 8). The dip-azimuth attribute enabled the identification of a branching

Conclusions

Using 3D seismic attribute analysis and a novel difference-of-two-surfaces approach in combination with statistical methods, we identified up to 66 potholes within the 3D seismic data from Lonmin Platinum Mine in the western limb of the RLS, Bushveld Complex. Seismic attributes show better detection of a thin deep-seated platinum-bearing horizon (UG-2) and other subtle geological features (faults, IRUPs and potholes) than conventional seismic amplitude displays. Results from integrated seismic

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

The authors would to thank Dennis Hoffmann and Lonmin Platinum Mine for giving us access to the 3D seismic data. We also thank DUG Insight and Schlumberger for access to seismic interpretation software packages. This research was sponsored by the Wits Seismic Research Centre and the Thuthuka National Research Funding. The support of the DST-NRF Centre of Excellence for Integrated Mineral and Energy Resource Analysis (DST-NRF CIMERA) towards this research is also acknowledged. Opinions expressed

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