Site selection

The field sites were selected to showcase the range of geology in Ontario. We used publicly available field guides, locations of field sites used by the University of Toronto and McMaster University in Undergraduate courses, as well as satellite images (Google Earth) to determine suitable field sites. Our selection criteria included land access, clarity of the outcrop (i.e. how obvious or hidden were the features), and how easy was it to access the full outcrop to collect the data needed.

Site  data

Locations were found and recorded by using a Garmin etrex GPS and on Google Maps using personal smartphones. Structural measurements of linear and planar features were determined using Brunton geological compasses and recorded using hand-written field notes. Other outcrop data was also recorded by hand into field notebooks, including site access, lithologies, mineralogy, and sampling.

LiDAR scanning – Scaniverse

Each outcrop was scanned using LiDAR (Light Direction and Ranging) built into the 2020 and newer models of the iPad Pro (also available on iPhone 12 Pro and iPhone 12 Pro Max). LiDAR uses a similar theory to SONAR or RADAR – it bounces a light beam (in the form of a laser) off of a surface and records the time taken for the reflection to be detected. This time is converted into a distance measurement, and from this a 3D model of the surface can be recorded. At the same time, a camera records the texture of the surface, and places this onto the 3D model. We used the app ‘Scaniverse’ to create the 3D models, using the highest possible quality resolution.

Sampling

In addition to the 3D scans, we also collected representative samples at each outcrop. Samples were chosen with the following criteria – samples that would be represent the geology present, were free from weathering or alteration and were large enough to be scanned (see below). In the field, samples were placed in large plastic sample bags, and were labelled with a code for the location (both on the sample, and on the bag).

Cutting rock

Rock samples were cut with a diamond edged saw into ~5x2x0.5cm slabs (known as billets). The rocks were usually cut perpendicular to any structure (such as foliation), though sometime cut parallel to structures to capture changes in mineralogy and mineral orientations. The billets were then sent to Vancouver Petrographics (British Columbia) and Brock University (Ontario) to be made into thin sections. These thin sections were 30 microns thick, polished (to be used for SEM analysis) and without a cover slip.

Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM EDX – also known as SEM EDS) and Electron Probe X-Ray Microanalyzer (EPMA)

A scanning electron microscope fires a beam of electrons at a sample, and records the way that the electrons are scattered from the sample. This is a commonly used methods to take ‘photographs’ of microscopic objects in very high detail. Energy Dispersive Spectroscopy is a type of SEM that instead uses the X-rays which are given off by the sample during the electron bombardment. As elements absorb, and then release, the electrons during the bombardment, they also release X-rays, the wavelengths of which correlate to the element number. The X-rays are then collected and analysed, and elemental maps are produced. SEM EDX is often presented as a series of colour coded maps of the sample, where each colour represents a separate element, and the brighter the colour indicates a higher amount of that element present. For this project, we used the thin sections for SEM EDX with the Hitachi SU3500 SEM with Bruker EDX in the Department of Chemical and Physical Sciences at the University of Toronto Mississauga. SEM EDX represents a low cost, simple method of collecting visual geochemical data.

Some samples were also run on the EPMA unit on the JEOL-JXA8320 5-WDS (wavelength-dispersive spectroscopy) Electron Microprobe at the University of Toronto St George Campus, department of Earth Science. None of these EPMA results are included in this pressbook, but are available upon request. EPMA analyses are very similar to SEM with added chemical analyses capabilities. Sometimes called a “microprobe”, an EPMA uses a microbeam of electrons to liberate energy and matter from the sample. The electron beams liberate heat, derivative electrons, and X-rays. The secondary electrons and back-scattered electrons are the most useful for imaging a thin section surface or obtaining average chemical composition. Just like SEM, EPMA is non-destructive in that there is no volume loss of the sample. A sample can be re-anaylsed more than once.

Photographic scanning of thin sections

Full thin section photographs were taken with a Keyance polarising microscope in the Department of Chemical and Physical Sciences at the University of Toronto Mississauga. This provided a high resolution, automatically stitched photo of the entire thin section. To create the smaller, zoomed-in rotation videos of the slide a down-scope camera mounted to a Leica petrographic microscope was used. The video sites were selected to highlight representative mineralogy and structures in the sample. The stage was rotated 360 degrees under plane ploarised light and again under crossed polarised light, highlighting the changes in optical properties of the minerals in the sample.

Photographic 3D scanning of rock hand samples

The hand samples collected were also photographed in high resolution, and were scanned using a Creaform Academia 20 handheld 3D scanner in the Department of Chemical and Physical Sciences at the University of Toronto Mississauga. The handheld scanner works in a similar fashion to the LiDAR, but is able to calculate smaller objects at higher resolutions of up to 0.1mm. Multiple scans were taken of the samples, which were compiled into a single 3D scan using the associated Creaform Academia software.