Boulder tracing and geological mapping of the bedrockBasal till and bedrock chip sampling
Basal till and bedrock chip sampling are undertaken, for example, when an interesting area has been identified with the help of geological mapping, geophysical surveying and/or boulder discoveries.
A smaller drill rig, often on tracks, is driven to the drill site (figure 5). The best available route is used, preferably on hard ground and in consultation with the land owner. Certain areas, such as wetlands, can only be drilled during the winter when the ground is frozen. Sometimes the snow is packed down by driving, for example, a snowmobile or tractor along the transport route before the drill rig is located at the drilling position. The drill rig samples the basal till, usually at a depth of 1-20 metres. Most often samples are also taken of the upper surface of the rock, rock fragments that are called rock chips or drill rock chips (figure 6). In certain cases some trees are cut down to be able to move the drill rig into drilling position.
The soil samples and rock samples are placed in containers and transported away to an archive for geological mapping and then sent further to a laboratory for analysis of the metal content.
Basal till and surface rock sampling are, as well as diamond drilling, relatively expensive exploration methods. Drilling usually costs SEK 200 to 300 per metre. The distance between the samples usually varies from 10-400 metres. The goal of this type of sampling is to test a larger area, in which there are indications of mineralisation. Raised concentrations of interesting metals can lead to the next step in exploration, usually diamond drilling.
Fig 5. An example of a drill rig.
Fig 6. Basic diagram illustrating bedrock chip sampling and basal till sampling.