In nature, radon does not come out of solid rock but...
- When looking for the source of a radon-in-soil-gas anomaly, a nearby glacial erratic boulder or boulder field of
granite or pegmatite is not the answer.
Even though these two rock types will often have a gamma-ray anomaly, easily found by the scintillometer, radon does not come out of solid rock,
and neither do alpha particles (alpha radiation),
and these boulders will not have a radon anomaly because radon is an alpha emitter.
With a half-life of only 3.8 days, radon, a noble gas, does not have time to migrate through solid for any distance, even a milimeter.
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 a layer of 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.
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.
An experiment designed to fail
As a test of radon in soil gas in exploration my friend got some chunks of hi-grade uranium ore,
covered them with a few feet of glacial till, waited a week or two for equilibrium and measured radon in soil gas in the till.
At this point the reader will not be surprised to learn my friend did not find a radon anomaly, just background values like in any normal till.
Radon does not come out of solid rock, even ore grade.
However, fractures can be useful
- The Radiore deposit, near Rayrock, NWT lies beneath a hilltop with almost 100% outcrop
and we were puzzled as to how to use radon in soil gas to explore through solid rock.
Soils suitable for radon exploration are confined to gullies,
which represent fractures in the rock which in turn are loci of uranium mineralization.
The strongest radon anomalies occur in areas where where uranium mineralization was intersected diamond drill holes.
In the lab, a different story
In my Ph.D. thesis work at
Queen's University and the Geological Survey of Canada (1970, pp 55-57) I compared the radon emanating from dissolved and undissolved splits of
different grain size fractions of clastic stream sediments. In general,
these clastic samples released half or more of their radon without dissolving, even for the coursest fraction, +10 mesh (greater than 2 milimeters).
As a practical application we use this ready escape of radon from sand grains
as a quick and inexpensive method of radium analysis of clastic sediments in dry stream beds.
Put the dry sample in a jar, fill the jar with water, seal it, wait two weeks or so for equilibrium and measure radon in water.
We tried this in a dry stream, bed downstream from the Mary Kathleen mine in Australia and found that anomalous radon (meaning radium) was detectable
some distance downstream.
No doubt, prospectors looking for uranium have tried walking up dry streams measuring gamma radiation with a geiger counter or scintillometer,
but radon (radium) analysis is much more sensitive to the low levels found.