Implementing Terrestrial Cosmogenic Nuclide dating in provenance tracing of mineralized glacial erratics, pilot study of the method in Kaarestunturi, Sodankylä, Finland
Résumé
The Central Lapland Greenstone Belt in Northern Europe is an area of active ore exploration. The belt has been glaciated on multiple occasions and a great portion of it is masked by till. The till cover is often used in geochemical exploration in e.g., boulder/erratic sampling because the glacial erratics can resemble the underlying bedrock quite well. However, this is not always the case. For example, if a till unit is deposited on top of an older one it may bear no resemblance to the local bedrock.
In Kaarestunturi, Sodankylä, gold bearing quartz erratics have been found in the surficial till. Many of the erratics are underlain by multiple till units and local excavations have not revealed the source of the boulders. In this study, two of the mineralized quartz vein erratics and two bedrock outcrops were dated using the terrestrial cosmogenic nuclide (TCN) technique with the aim of finding their initial dislodgement or exposure event. The method is based on the accumulation of nuclides that form in minerals when they are
irradiated with high energy secondary cosmogenic radiation, in this study 10Be and 26Al are used. The radiation flux typically attenuates within 2–3 meters of the Earth’s surface, therefore cosmogenic nuclides are most abundant at the surface. Accumulating in the surface makes TCN dating a prominent geochronological tool for dating geomorphology, e.g., glacial erosion, that can reset the “clock
The dated erratics yielded apparent 10Be exposure ages of 39.2 ± 1.3 ka and 30.6 ± 1.0 ka, and they are thus significantly older than the recent Late-Weichselian deglaciation. Likewise, the outcrop exposure ages predate the Late-Weichselian; the lower elevation bedrock outcrop yielded an apparent 10Be age of 98.0 ± 2.8 ka and the higher one an age of 53.0 ± 1.8 ka. Neither the outcrop nor the erratic samples indicate strong erosion or glacial plucking during the Late-Weichselian. For provenance tracing this means multi staged erratic transportation. 26Al/10Be ratios, indicative of reburial after exposure are not uniform in the sampling. The older erratic shows signs of long reburial, linking it to older erosion than what its’ apparent age suggests. Plotting the apparent ages of the erratics on the local glacial history model allows picking till fabric orientations that
correspond with the time of erratic dislodgement. The interpreted time of dislodgement paired with till fabric analyses suggests that the provenance of erratics of this age could lay NW to SW of their current location