An evaluation of the use of high-resolution RGB and multispectral imaging as a potential tool for drill-core logging of the rocks of the Bushveld Complex

Abstract

Core recovery drilling remains a fundamental component of mineral exploration in the Bushveld Complex of South Africa, which hosts the world’s largest platinum-group element reserves within the world’s largest layered intrusion, the mafic to ultramafic Rustenburg Layered Suite. Manual logging of extensive drill-core intervals is time-consuming and inherently subjective, often limiting the precision of lithological and mineralogical interpretations. This study assesses spectroscopic drill-core logging techniques, such as RGB to-grayscale and visible to near-infrared (VNIR) multispectral imaging (400-842 nm), applied to the Rustenburg Layered Suite drill-core as potential tools for cost-effective drill-core logging of the major lithologies and minerals of the Rustenburg Layered Suite.

High-resolution RGB images were converted to grayscale using the luminosity method, and grayscale digital number (0-255) statistics, median and skewness were analysed for lithological classification. The grayscale approach distinguishes chromitite and anorthosite through contrasting histogram skewness, with median-based validation correctly classifying 3% of anorthosite samples and 0% of chromitite, the latter is due to limited representation in the training dataset and that training and validation chromitite samples differ in the proportion of dark and lighter crystals. In contrast, gabbronorite and pyroxenite proved more challenging to differentiate, reflecting their broad compositional ranges and overlapping plagioclase pyroxene assemblages, with 19% gabbronorite validation sites and 44% pyroxenite validation sites being either a gabbronorite or pyroxenite. Additionally, the weighting of the green channel in the luminosity-based grayscale conversion can shift darker pixels toward higher grayscale values, potentially leading to the misclassification of dark lithologies as lighter-coloured lithologies. Thus, this approach proved less effective for reliable lithological classification across the dataset.

VNIR multispectral imaging (475-842 nm) enables classification of major minerals such as pyroxene, plagioclase, and chromite, with spectral library correlations improving accuracy by identifying minerals prone to misclassification. The results show that 40% of pyroxenite samples contained over 90% pyroxene, 12% of anorthosite samples contained over 90% plagioclase, 71-75% of gabbronorite samples exhibited mixed plagioclase-pyroxene proportions within the 10-90% compositional range, and all chromitite samples contained more than 45% chromite. Results also reflect expected mineralogical trends, with mafic rocks showing higher mafic mineral content and anorthositic rocks dominated by felsic minerals.