11/21/2016

Science Blog: High-resolution X-ray computed tomography: A 3D quantitative and visualization approach to resolve complex geological problems at the microscale

Text:
Muhammad Sayab, Senior Researcher

Dr. Sayab has developed new methods to study microstructures in 3-D and this blog highlights some of his latest findings by use of  X-Ray computed tomographic on research of orogenic gold of the Suurikuusikko deposit, Northern Finland.

Much of our knowledge about the Earth’s crust and mantle geodynamics is based on the study of mineral species and associated textures. Crystal abundances, chemistry, sizes, shapes, and spatial orientations represent the specific paragenesis of magmas during transport in the crust, whereas metamorphic minerals are key to investigating the mountain-building orogenic process. The vast majority of this research is based on the study of petrographic thin sections with the optical microscope, electron microscope, microprobe or ICP-MS. Although all these techniques are important to characterize different mineral species or compositions, an important limitation of these instruments is their inability to directly visualize mineral textures in three dimensions (3D), which can lead to incomplete interpretation of the mineral textural relationships and crystal abundances. Recent technical advancements have added a promising new tool to existing microtextural methods: computed tomography (CT) with high-energy X-rays. The main advantage of this technique is that it allows mineral textures to be directly visualized in 3D at high resolution, thereby eliminating the interpretative procedures associated with conventional 2D methods. Moreover, no specific sample preparation is required before X-ray imaging analysis. This rapidly emerging technique is non-destructive, versatile, and provides detailed 3D spatial imagery of the internal architecture of a rock by measuring the attenuation of X-rays as they pass through different mineral phases. In addition to 3D spatial images, an unlimited number of serial cross-sections can be generated as a new type of virtual petrographic section.

For this blog, I would like to briefly highlight the outcomes of our recent article published in the Geological Society of America, Geology (2016, see reference below), on the 3D textural and quantitative analysis of the Suurikuusikko orogenic gold deposit in northern Finland. We first analyzed microscale textures in oriented drill cores using lab-based X-ray computed microtomography, hosted by the University of Helsinki, Department of Physics. The technique revealed a kinematic history and a number of in situ 3D quantitative aspects, including the size, shape, spatial distribution, and geometrical orientation of arsenopyrite and pyrite in a highly altered host-rock matrix (Figure 1). For 3D nanotomography, the experimental procedure known as holotomography was adopted. Individual arsenopyrite crystals were separated and scanned with voxel sizes ranging from 50 nm to 150 nm using the X-ray nanoprobe beamline (ID16B) at the European Synchrotron Radiation Facility, France (Figure 1). This ultrahigh-resolution technique illustrated the 3D distribution of micron- to nanoscale gold inclusions, mostly associated with primary rutile or along secondary microfractures inside arsenopyrite (see Figure 1). The workflow, from micro- to nanotomography, outlined in our article, offers an indispensable new technique for quantifying and characterizing the 3D textural settings of ores, which is otherwise impossible with conventional 2D imaging devices.

As a part of the RAMI (RawMatTERS Infrastructure) project, the Geological Survey of Finland has advertised a tender to purchase a state-of-the-art micro-CT instrument. RAMI has received a Finnish Research Infrastructure (FIRI) grant financed by the Academy of Finland, and the micro-CT instrument will be installed at the Otaniemi campus. We hope that the new instrument will arrive around mid-2017. The instrument will not only fulfill our “in-house” applied geoscience needs and research in material sciences, but will also serve rock mechanics engineering, building materials and mining, and metallurgy. Moreover, the micro-CT instrument will open new possibilities for academic and industrial partners to perform high-resolution scans on dense and relatively large samples without sample destruction.

Figure. 1. Three-dimensional (3D) reconstructions of X-ray computed microtomography data from part of an oriented drill-core sample from the Kittilä mine. A: A 3D micro-CT image showing the random distribution of arsenopyrite (Apy) and pyrite in yellow, with the rock matrix rendered transparent. B: A photomicrograph showing intense carbonate alteration; opaque black grains are arsenopyrite and pyrite (Py). Partial cross-polarized light. C: Surface rendering of arsenopyrite along with two orthogonal grayscale slices showing gold (Au) and rutile (Rt). D and E: Inclusions of gold and rutile showing the preferred alignment within an arsenopyrite crystal. F and G: Volume renderings of an arsenopyrite crystal showing pyrite (Py) and rutile inclusions.
Figure. 1. Three-dimensional (3D) reconstructions of X-ray computed microtomography data from part of an oriented drill-core sample from the Kittilä mine. A: A 3D micro-CT image showing the random distribution of arsenopyrite (Apy) and pyrite in yellow, with the rock matrix rendered transparent. B: A photomicrograph showing intense carbonate alteration; opaque black grains are arsenopyrite and pyrite (Py). Partial cross-polarized light. C: Surface rendering of arsenopyrite along with two orthogonal grayscale slices showing gold (Au) and rutile (Rt). D and E: Inclusions of gold and rutile showing the preferred alignment within an arsenopyrite crystal. F and G: Volume renderings of an arsenopyrite crystal showing pyrite (Py) and rutile inclusions.

Reference:

Sayab, M., Suuronen, J-P., Molnar, F., Villanova, J., Kallonen, A., O’Brien, H., Lahtinen, R., Lehtonen, M., 2016. Three-dimensional textural and quantitative analyses of orogenic gold at the nanoscale. Geology 44(9), 739-742.

See also the geoscience laboratory services provided by the GTK, and

the Finnish Geoscience Research Laboratory.

Muhammad Sayab

Text: Muhammad Sayab

Dr. Muhammad Sayab is a Senior Researcher at the Geological Survey of Finland in the Espoo office. He is a structural geologist and obtained his PhD degree from the James Cook University, Australia. He has published many articles on the tectonics of the Mt. Isa Inlier, Australia, northwest Himalayas, Pakistan and Betic Cordillera, southern Spain. Recently his research is concentrated around the Fennoscandian tectonic evolution. Dr. Sayab has developed new methods to study microstructures in 3-D.