by Rowena Duckworth, Founder & Petrographer at Mintex Petrological Solutions
Exploration drilling plays a fundamental role in identifying and evaluating potential mineral deposits. It follows that analysis of drill core samples from exploration projects provides valuable insights into the geological composition and potential economic viability of mineral resources. This article delves into the various scanning techniques commonly used in the study of drill core samples, namely, hyperspectral imaging, X-ray scanning and laser-induced breakdown spectroscopy (LIBS). These techniques all deliver comprehensive mineralogical data that help with mineral identification, lithological characterization, elemental composition determination, alteration mapping, and ultimately, make or break decision-making processes in mineral exploration.
Paul Ashley neatly summarized the benefits of petrographic analysis on drill core sample in Issue 23 of Coring (May, 2023; p. 46). Petrographic analysis involves the detailed microscopic examination of thin sections of rock samples, and provides essential information about the rock’s mineralogy, texture, and structure. In an ideal world, thin section petrographic studies should always be conducted prior to, or complementary with, other geochemical analysis.
Hyperspectral core scanning
Hyperspectral imaging captures the reflectance or emission spectra of rock at numerous narrow and contiguous spectral bands. It enables the identification and mapping of minerals and alteration minerals based on their unique spectral signatures.
Imaging can be conducted on half core samples using core scanning machines or handheld field portable spectrometers. The spectra provide information about the mineral composition, chemical bonds, and structural features of the minerals present in the drill core.
Hyperspectral core scanning instruments consist of a spectrometer, a hyperspectral camera, and a motorized stage to move the drill core sample. The spectrometer measures the intensity of reflected or emitted light at different wavelengths. The hyperspectral camera captures the spatial information and generates high-resolution images of the drill core.
Most hyperspectral core scanners cover a broad spectral range, typically spanning from the visible to the near-infrared (VNIR) or shortwave infrared (SWIR) regions. The spectral resolution can range from a few nanometers to tens of nanometers, allowing fine discrimination of spectral features.
Drill core is placed on the stage, and the hyperspectral scanner moves along the length of the core, capturing data at regular intervals. Hyperspectral data can be processed using various techniques, such as spectral unmixing, classification algorithms, and mineral libraries. Spectral unmixing helps identify and quantify the abundance of different mineral components within the core. Classification algorithms classify the spectra into different lithological or mineralogical classes based on their spectral properties. Mineral libraries compare the acquired spectra with known reference spectra to identify specific minerals present in the core (Figure 1).
Hyperspectral core scanning is especially good for the various mica and clay phases. For example, different types of mica minerals, such as muscovite, biotite, and phlogopite, have distinct spectral features related to their chemical composition and crystal structure. Similarly, clay minerals – including kaolinite, illite, smectite, and chlorite – also exhibit specific spectral signatures due to their mineral structure and composition. This technique is particularly effective in mapping alteration patterns and zoning within the drill core as hydrous alteration minerals often exhibit distinct spectral signatures.
The main limitations of hyperspectral core scanning are in detecting minerals that lack unique spectral features or occur in low abundances. The technique is sensitive to surface conditions and requires careful handling of the drill core to avoid contamination or alteration. Data analysis can be computationally intensive and requires expertise in spectral analysis and mineralogy. Chemistry of the cores is not obtained.
XRF core scanning
XRF core scanning operates on the principle of X-ray fluorescence spectroscopy. The technique involves irradiating the drill core sample with high-energy X-rays, which causes the atoms in the sample to emit characteristic fluorescent X-rays. These emitted X-rays are then measured, and their intensity is analyzed to identify the elements present and quantify their concentrations.
XRF scanning can be conducted on half core samples using core scanning machines or handheld devices (pXRF). It provides valuable information about major, minor, and trace elements present in the sample, aiding in mineral identification and characterization, which help with identifying mineral zoning, alteration patterns, and lithological changes, facilitating geological interpretations (Figure 2).
The drill core samples are typically cut into half and positioned within the scanning machine. The samples may be placed on a moving conveyor belt or mounted on a rotating stage for analysis. The XRF core scanning machine emits high-energy X-rays which penetrate the outer layers of the sample, interacting with the atoms in the material. The emitted fluorescent X-rays are then detected by a solid-state detector, such as a semiconductor or scintillation detector, within the scanning machine. The detector captures the energy and intensity of the X-rays. The XRF core scanning machine processes the detected X-ray signals and generates elemental data for each point or interval along the drill core. This data is often presented in the form of elemental concentration profiles or elemental maps, providing insights into the distribution of elements within the sample.
It is important to remember that XRF scanning is influenced by the matrix effect, which refers to interactions between the elements in the sample and the surrounding matrix. The presence of certain elements or minerals can affect the accuracy and precision of the measurements, leading to potential errors in quantification. XRF scanning therefore requires calibration and standardization to ensure accurate and reliable results. Calibrating the instrument for different elements and matrices can be time-consuming and requires expertise. Changes in sample composition or matrix may necessitate recalibration, adding to the complexity. Also, as this is an X-ray technique, relevant permits need to be obtained to comply with radiation safety regulations.
XRF core scanning generates large volumes of elemental data along the entire drill core. When combined with other geological data, such as hyperspectral imaging or geophysical surveys, it enhances the comprehensive understanding of subsurface geology, enabling more informed decision-making in exploration and mining projects.
Laser-Induced Breakdown Spectroscopy (LIBS)
LIBS is an elemental analysis technique that utilizes a high-energy laser pulse to vaporize and excite a small portion of the sample’s surface. The resulting plasma emits light, which is collected and analyzed to determine the elemental composition of the sample. The LIBS core scanner collects the emitted light using a spectrometer, which disperses the light into its different wavelengths. The spectrometer may employ various methods to separate the wavelengths. The dispersed light is captured by a detector which measures the intensity of the light at different wavelengths. This data is referred to as the LIBS spectrum.
The LIBS spectrum is then analyzed to identify and quantify the elements present in the drill core sample. Each element emits characteristic wavelengths, enabling their identification based on the wavelengths detected in the spectrum. The LIBS core scanner processes the spectral data and generates elemental concentration information for each point or interval along the drill core. This data can be presented as elemental profiles or maps, providing insights into the elemental distribution within the sample. Mineral maps can also be generated allowing for a complete mineralogical and textural picture of the drill core (Figure 3).
Laser scanning analyses multiple elements simultaneously. The multi-elemental analysis capability of LIBS scanning allows a first pass look of the drill core before any assay samples are chosen, which can streamline the exploration process and cost. Whole core, as well as half core, can be analyzed due to the wide focusing range of the laser, allowing analysis prior to core cutting if necessary.
One of the key advantages of LIBS scanning is its ability to analyze the light elements, including lithium, which XRF cannot see, making it a perfect tool for lithium exploration. LIBS scanning is also a minimally destructive testing method, as only a thin 30-micron layer is ablated form the sample, allowing sample preservation for other analytical methods. LIBS core scanning provides real-time elemental analysis on-site with no sample preparation required. This allows exploration teams to obtain immediate results, facilitating faster decision-making and minimizing operational delays.
LIBS provides rapid and simultaneous analysis of multiple elements from hydrogen to uranium, making it suitable for real-time, on-site measurements. In mineral exploration, LIBS analysis of drill core samples can quickly provide elemental data, aiding in mineral identification, mapping alteration zones, and assessing potential mineralization. LIBS offers advantages such as portability, non-destructiveness, and the ability to analyze samples without extensive sample preparation.
Discussion
Petrographic, whole rock geochemical, hyperspectral, XRF and LIBS analysis of drill core samples are powerful mineralogical and geochemical tools in exploration projects. These techniques offer complementary information on mineralogy, lithology, elemental composition, alteration patterns, and potential mineralization.
Hyperspectral, XRF, and LIBS core scanning techniques offer distinct advantages and capabilities. All provide rapid and non-destructive analysis of drill core samples, enabling further investigations or additional analysis to be conducted on the same sample if needed. Table 1 summarizes their pros and cons.
Hyperspectral scanning employs infrared spectroscopy to identify and quantify minerals and geological constituents and provides detailed mineralogical and textural information, allowing for comprehensive analysis of mineral composition and geological features.
XRF scanning used X-rays to offer rapid elemental analysis, enabling quick assessment of major, minor, and trace elements in the sample, aiding in mineral identification and characterization. However, like traditional XRF, the technique struggles with the lighter elements in the periodic table.
LIBS scanning uses lasers to generate plasma, enabling rapid identification of elements and minerals in the sample. It is an excellent tool for rapid and visual characterization of whole core rock texture and chemistries. Scanning a whole core tray at high resolution can be done in an hour or so, and element maps are automatically generated. Mineral libraries and maps can be created on a project-by-project basis to add further understanding to the element deportment and textural observations seen in the high-resolution scan photos. Reliable detection of mineral containing the light elements, such as hydrogen, oxygen, lithium and sodium, as well as the heavier elements, such as base metals and REE elements, makes LIBS an excellent first pass scanning methods for all deposit styles. A main selling point is the ability to distinguish lithium and other light element that are not detectable by traditional XRF-based scanning technologies, and the LIBS system is also a more rapid scanning system. The laser-based system also means less permitting than X-ray based scanning systems in many states/countries.
Overall, each technique adds another layer of information to the exploration toolkit and like all tools, selecting the right one for the job at hand is important. By utilizing these scanning methodologies, geologists and mineral exploration companies can make informed decisions and optimize exploration strategies, reduce costs and increase the chances of discovering economically viable mineral deposits.
For more information visit: mintexpetrologicalsolutions.com
References
Ashley, P., 2023. Power of the microscope: petrographic analysis and mineralogy in the exploration and mining industry. Coring Magazine, 23, 46-49.
Duckworth, R. and Narbey, M., 2023. LIBS automated techniques for the mineralogical and elemental characterisation of critical mineral deposits. Critcon 2023 – Discovery, characterisation and processing of critical minerals. Abstract Volume, 22-26 May 2023, The University of Adelaide. Page 28.
Michaux, S., and O’Connor, L., 2019. How to set up and develop a geometallurgical program. GTK Open File Work Report 72/2019. Geological Survey of Finland, Economic Minerals Unit, Espoo. 245 pages.
Minalyze website: https://minalyze.com/portfolio/geochemical-core-logging.
