2D3DFusion – Spectroscopic mineral-chemical analysis of drill cores: Development of fast 2D LIBS, EDXRF and hyperspectral scanning with application to outstanding problems of the Bushveld Complex layered intrusion

Country / Region: South Africa

Begin of project: July 1, 2021

End of project: June 30, 2024

Status of project: May 10, 2022

The application of large area, high-spatial-resolution imaging techniques such as laser-induced breakdown spectroscopy (LIBS), hyperspectral image analysis (HSI) and energy-dispersive micro-X-ray fluorescence analysis (μEDXRF) in the investigation of drill cores opens up new possibilities for the quasi-nondestructive provision of information for the interpretation of magmatic layered intrusions. The combination of these analytical methods will be applied to the continuous mineralogical, chemical and textural characterization of mono-mineralic layers of Upper Zone magnetite and Critical Zone chromite of the Bushveld Complex (Fig. 1). Another focus of this study is the influence of residual melts and fluids on the formation of late magmatic and secondary minerals in the intrusive body.

Fig. 1: Simplified geologic map of the Bushveld Complex (left) and stratigraphic column of the Eastern lobe (right). The coverage of donated cores available to this project is shown in black, the proposed new drilling section in blue Source: Trumbull et al., 2015

The development of a spectral database provides the basis for an objective evaluation of long drill core sections. Visualization of the modal inventory with accessory minerals, from microstructure to mineral chemical changes in solid solutions on a LIBS or μEDXRF basis, enables pattern recognition at the mesoscale level in advance of microscopic or submicroscopic investigations. In addition, the chemical variation of magnetites, chromites and accompanying phases can be revealed over the entire drill core section investigated. Of particular importance is the LIBS-based information on the distribution of light trace elements such as Li, Be, B, etc. in the drill core as a possible indicator of hydrothermal alteration, in order to define its effect on mineralogical and chemical changes. Many phases can be identified by hyperspectral imaging (HSI) using VNIR, SWIR and LWIR. It is a central goal to provide the information up to this point largely automated via artificial intelligence and supervised classification and evaluation.

By means of optical microscopy, electron microprobe, scanning electron microscopy, Raman spectroscopy chemical properties are investigated in detail and used for validation of LIBS, HSI and μEDXRF. Trace element distribution patterns, textural features, and neighborhood relationships are used to test known genetic models for the formation of the mono-mineral layers and, if necessary, discarded or modified. The influence of hydrothermal overprinting on the genesis of mono-mineral layers is considered separately here. Here, the estimation of the effects of hydrothermal alteration on bulk and mineral chemistry with special focus on the trace element range is in the focus. Using the example of this investigation on drill core sections, the applicability and the potential of the quasi non-destructive scanning methods LIBS, HSI, μEDXRF, etc. shall be demonstrated for the already existing drill cores, but especially for the planned drilling of the Bushveld Complex.

Literature:

Meima, J.A., Rammlmair, D., Junge, M., 2022, The use of Laser Induced Breakdown Spectroscopy for the mineral chemistry of chromite, orthopyroxene and plagioclase from Merensky Reef and UG-2 chromitite, Bushveld Complex, South Africa, Chemical Geology 589, 120686. https://doi.org/10.1016/j.chemgeo.2021.120686

Nikonow, W., Rammlmair, D., Meima, J.A. et al., 2019, Advanced mineral characterization and petrographic analysis by μ-EDXRF, LIBS, HSI and hyperspectral data merging. Mineralogy and Petrology 113, 417–431. https://doi.org/10.1007/s00710-019-00657-z

Trumbull, R. B., L. D. Ashwal, S. J. Webb and I. V. Veksler, 2015, Drilling through the largest magma chamber on Earth: Bushveld Igneous Complex Drilling Project (BICDP). Sci. Dril. 19: 33-37.

Partner:

• Leibniz Universität Hannover
• Mineralogical State Collection Munich

Promotion / document number:

DFG 457479532