Carta Exploration Ltd

Importance of This Case Study
This Case Study illustrates the detailed interpretation of deposit geology, sediment transport direction and glacial history made possible with the use of air photos.  The steps taken to complete an air photo interpretation study are explained.

Mineral Exploration
Air photo interpretation is useful for planning geochemical soil and sediment surveys. Glacial till is the best material to sample for diamond indicator minerals and other geochemical anomalies.  Skilled air photo interpretation can identify till deposits over large areas for a minimal cost, and plays a crucial role in the understanding the transport history of an area. 

---

                      

Background

Modern drift prospecting methods take samples of unmodified glacial till and analyze them for kimberlite indicator minerals. Positive trends are traced in an up-ice direction to locate the kimberlite source.

The Problem
A junior diamond exploration company completed a till sampling program for kimberlite indicator minerals. Survey results returned gaps in a glacial dispersion train, some gaps corresponding to a change in geochemistry. Detailed till mapping was needed in the area of the gaps to identify suitable target areas in a follow-up till sampling program (Fig. 1).

Project Tasks
Based on a preliminary look at topographic and surficial geology maps, it was obvious that the reason for the gaps in the dispersion train was because of the reworked nature of the sampled sediment. In other words, the gaps were locations where ground sediments were covered by fluvial or lacustrine deposits instead of unmodified glacial till.

The next step was to investigate available data sources for the area in order to choose the best data set suited to project objectives and budget. Budget constraints and the absence of high-resolution satellite images meant that air photographs were the best source of detailed ground information.

The most recent air photos for this part of Canada were taken in 1956 at a nominal scale of 1:60 000. Air photos were ordered to provide stereoscopic coverage of the study area, with expected delivery within two weeks. In the meantime, available satellite and air photo images over the area were reviewed along with relevant maps and reports to understand the regional geologic and glacial context.

Supporting Information
Regional maps of bedrock and surficial geology (Fig. 2) were available for the study area. A number of articles from the Geological Survey of Canada described the glacial terrain and provided valuable ground photographs of terrain features (Fig. 3). A topographic map of sample locations and a database of sample descriptors and geochemical results were also provided.

Air Photo Mapping
Once the air photographs arrived, they were organized, oriented, and the exact scale was determined. Photographs were labelled with north arrow and scale.

Clear mylar overlays were cut to size and attached to every other photo for mapping (stereoscopic overlap means that only every second photograph need be mapped in order to cover the entire area). The photograph’s fiducial marks were drawn on the mylar overlay to register the correct position should the overlay shift.

Now the photographs were ready for systematic interpretation following the air photo interpretation checklist as a guide:

• Streams, lakes and rivers were traced with a blue line and labelled by name with the direction of flow indicated by an arrow;
• Man-made features (roads, airstrips or settlements) and land cover (tundra bush, peat or unvegetated slopes) were noted qualitatively;
• Identifiable landforms, areas of erosion and other interesting land features were noted qualitatively and cross-referenced to data in published maps and reports;
• Regional glacier flow direction was determined by careful examination of asymmetric eroded or deposited glacial features;
• To denote if soil samples contained kimberlite indicator minerals or were barren, sample locations were mapped by hand onto the mylar overlays by reference to the supporting topographic maps of the sample program. A check mark indicated a positive count of kimberlite indicator minerals; a cross indicated a sample barren of the key minerals.

Interpretation Methodology
The goal of air photo interpretation was to identify all mappable areas of glacial till. A number of interpretive steps were required to produce the desired final map of till terrain suitable for sampling.

• Meltwater channels were mapped on the mylar overlays using a different blue line type with arrows pointing down-flow. These areas were unfavourable for till sampling. Meltwater channels were almost always dry so this step did not duplicate the modern drainage blue line network;
• Light-toned deposits of sand and gravel located above modern stream levels and along hillsides were interpreted to be beaches and delta deposits formed at the margins of glacial lakes. Careful mapping of these deposits allowed the former extent of glacial lakes and flooded terrain to be determined. Terrain within these margins represented unfavourable sites for till sampling;
• Other areas with textures indicative of current-directed flow or intense solifluction were identified. These areas represented terrain unsuitable for till sampling;
• Resistant protruding hills and ridges of glacial till were identified as areas suitable for till sampling. These areas had little evidence of post-glaciation modification, except perhaps for slight solifluction.

Through a combination of interpretation and elimination, terrain elevated above the level of glacial lakes and devoid of stream reworking was classified as undisturbed glacial till and outlined by polygons. Till polygons were subdivided into 2 groups: (1) probable and (2) possible, based on the level of confidence that they contained undisturbed primary till.

The overlays and photographs were scanned and placed into a geographic information system (GIS). Once the image was registered to map coordinates, and distortion removed as required, sample locations could be chosen for the follow-up program. As much as possible, sample sites were chosen from the centre or slightly down-ice position in polygons labeled ‘1’. Locations were saved as UTM coordinates in a GPS-ready list.