Radon does not come out of solid rock ...
When looking for the source of a radon-in-soil-gas anomaly, a nearby glacial erratic boulder or boulder field granite or pegmatite is not the answer. Even though these two rock types will often have a gamma-ray anomaly, found by the scintillometer, radon does not come out of solid rock, and they will not have a radon anomaly because radon is an alpha emitter.
In the Bancroft area I measured radon in snow over a highly radioactive granite pegmatite which had been stripped and trenched. It gave off high gamma radiation, even through the snow. The snow over the stripped and trenched area had no radon, but as soon as I moved a little downhill into a tree-covered area (indicating soil under the snow) I found anomalous radon in the snow. This was no surprise. The radium (immediate precursor of radon) has been washed downhill from the outcrop, either in solution or in ground up uranium minerals and deposited in soil. Being fine grained this soil can release some radon.
We did a similar survey over the Faraday mine. In this case there was some soil over the radioactive pegmatite and we also collected soil samples and analysed for radium and uranium. As expected, the radium and uranium anomaly was shifted downhill, and the radon-in-snow anomaly followed the radium in the soil, not the radioactive pegmatite, so it too was shifted downhill. See http://www.finderschoice.com/radon/faraday.php
The same applies to a lesser extent to gamma surveys. If you dig into the soil at a gamma hot spot and find a radioactive boulder or pebble of any kind and it's the only radioactive thing in the hole, then it's probably glacially transported.
but still finds subsurface uranium mineralization in areas of over 99% outcrop. See http://www.finderschoice.com/radon/radiore.php
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