An innovative seismic and statistical approach to understand 3D magmatic structures and ore deposits in the western Bushveld Complex, South Africa
Graphical abstract
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
References (71)
- et al.
The bushveld complex
- et al.
Tectonothermal history of the western part of the Limpopo Belt: tectonic models and new perspectives
J. Afr. Earth Sc.
(1999) - et al.
An intrusive origin of some UG-1 chromitite layers in the Bushveld Igneous Complex, South Africa: insights from field relationships
Ore Geol. Rev.
(2017) - et al.
The Bushveld Complex was emplaced and cooled in less than one million years – results of zirconology, and geotectonic implications
Earth Planet. Sci. Lett.
(2015) Potholes of the Merensky Reef at the Brakspruit Shaft, Rustenburg Platinum Mines; primary disturbances in the magmatic stratigraphy
Econ. Geol.
(1988)Seismic attributes past, present, and future
Volatile fluid overpressure in layered intrusions and the formation of potholes
Aust. J. Earth Sci.
(1992)- Campbell, I.H., 1986. A fluid dynamic model for the potholes of the Merensky Reef. Economic Geology, 81(5), 1118 –...
- Campbell, G., Crotty, J.H., 1990. 3-D seismic mapping for mine planning purposes at the South Deep Prospect. In:...
- Campbell, G., 1994. Geophysical contributions to mine-development planning: A risk reduction approach. In: Anhaeusser,...
Controls on the distribution of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex, South Africa: implications for disruptions of the layering and pothole formation in the Complex
S. Afr. J. Geol.
The petrogenesis of Merensky Reef potholes at the Western Platinum Mine, Bushveld Complex: Sr-isotopic evidence for synmagmatic deformation
Miner. Deposita
The Bushveld complex, South Africa
Emplacement and crystallization time for the bushveld complex
J. Petrol.
Predictability of pothole characteristics and their spatial distribution at Rustenburg Platinum Mine
J. Southern African Inst. Min. Metal.
Dip and azimuth displays for 3D seismic interpretation
First Break
Seismic exploration for Merensky Reef: the way ahead
S. Afr. J. Geol.
Platinum group metals: past and present
JOM
Pockmarks in Spitsbergen fjords
Norw. J. Geol.
Convective heat transfer as a function of wavelength: implications for the cooling of the Earth
J. Geophys. Res.
Formation of pockmarks by pore-water escape
Geo-Mar. Lett.
Pockmarks on the Scotian shelf
GSA Bull.
The fluid dynamics of a basaltic magma chamber replenished by influx of hot, dense ultrabasic magma
Contr. Mineral. Petrol.
The Sr-isotopic stratigraphy of the western Bushveld Complex
S. Afr. J. Geol.
Filling the Bushveld Complex magma chamber: lateral expansion, roof and floor interaction, magmatic unconformities, and the formation of giant chromitite, PGE and Ti-V-magnetitite deposits
Miner Deposita
How 3D seismic can help enhance mining
First Break
Field evidence for the in situ crystallization of the merensky reef
J. Petrol.
A note on the erosive nature of potholes in the Bushveld Complex
S. Afr. J. Geol.
Cited by (13)
Challenges and applications of digital technology in the mineral industry
2023, Resources PolicyDry laboratories – Mapping the required instrumentation and infrastructure for online monitoring, analysis, and characterization in the mineral industry
2023, Minerals EngineeringCitation Excerpt :Among the significant milestones in mining extraction technology are continuous mining for cutting coal, rock bolts for ground support, open-pit mining for mining massive low-grade deposits, longwall coal mining, and in-situ and automated mining (Moosavi and Gholamnejad, 2016; Pekol, 2019; Ikeda et al., 2020). Several geological issues can arise during the mining process, such as local thinning or thickening of the deposit, deposit loss, unanticipated dikes and faults (and other structures), and the intersection of gas and water reservoirs (Mkhabela and Manzi, 2017; Wagner, 2019; Sehoole et al., 2020). Even with detailed and advanced exploration at closely spaced intervals, mining operations have been compromised by a range of issues, which can result in personnel injuries, equipment and production losses (Zhi and Elsworth, 2016).
The minerals industry in the era of digital transition: An energy-efficient and environmentally conscious approach
2022, Resources PolicyCitation Excerpt :The mining and manufacturing industry in the 21st century is facing a society-wide digital and algorithmic transformation, colloquially referred to by some as the 4th industrial revolution (or Industry 4.0, Indri et al., 2018), which essentially describes further automation through human-machine interaction. In almost all mining companies, changes in the characteristics of ore deposits and socio-economic trends have driven a myriad of technological initiatives (e.g., Massinaei and Doostmohammadi, 2010; Makokha and Moys, 2012; Anderson et al., 2014; Jahedsaravani et al., 2014, 2016; Mkhabela and Manzi, 2017; Sehoole et al., 2020). The widespread technological advances have also given prominence to concepts such as artificial intelligence, robotics, the Internet of Things (IoT), and big data (Lukichev and Nagovitsyn, 2018).
Framework components for data-centric dry laboratories in the minerals industry: A path to science-and-technology-led innovation
2022, Extractive Industries and SocietyCitation Excerpt :A repurpose is different from reuse in that the purpose of the usage of the data is different from the original intent, whereas reuses simply use the data in the same manner that it was designed for, but usually with improvements or corrections to the usage methodology. A timely example of this type of reuse includes re-analyses of historical seismic data using modern algorithms and computers (e.g., Manzi et al., 2015; Sehoole et al., 2020). There are also recent examples of data repurpose in the realms of machine learning and data analytics (Nwaila et al., 2020; Ghorbani et al., 2021a; Zhang et al., 2021).
Lithospheric structures of the central Solonker-Xar Moron-Changchun-Yanji Suture (Inner Mongolia) revealed by a deep seismic reflection profile
2021, TectonophysicsCitation Excerpt :Therefore, we speculate that there may be tectonically weak zones where vast quantities of molten rock were brought from the earth's mantle to the surface, and differentiated lopolithic intrusions might have occurred during this process (Kruger, 2005). The effects of these injections of molten rocks over time combined with the crystallization of different minerals at different temperatures may be the reason for strong reflection clusters composed of several near horizontal reflections (Sehoole et al., 2020). (3) Mid-crustal weak reflectivity at the Songliao-Xilinhot Massif in this profile (CMP: 6200 to 7400) is interpreted as the Cretaceous granite which massively crops out in the Great Xing'an Range to the northwest of the study area (Wu et al., 2011).
3D reflection seismic imaging of natural gas/fluid escape features in the deep-water Orange Basin of South Africa
2023, Marine Geophysical Research