Helium in soil and overburden gas as an exploration pathfinder — an assessment

Orientation surveys were conducted over five deposits to test the potential of determining helium in overburden gas as a pathfinder for uranium mineralization and other deposits containing uranium or thorium. Samples were collected via fixed tubes emplaced at depths of 6 m in backfilled holes drilled for this purpose. Compared to the atmospheric background value of 5.24 ppm v/v He, a variable weak anomaly (maximum 5.45–5.65 ppm He) was found over part of the Angela uranium deposit, N.T., in an arid area where mineralization is mostly at a depth of 60–90 m, at or below the water-table. Helium contents were mostly at background levels over a uranium deposit in the Officer Basin, W.A., where mineralization is at the water-table at 30–35 m, although radon gave a marked anomaly. Neither helium nor radon indicated the Manyingee deposit, W.A., which has uranium mineralization in a confined aquifer at 60–110 m. Similarly, no helium anomalies were found over the uranium- and thorium-rich Mt. Weld carbonatite or mineral sands at Eneabba.There appeared to be no correlation between helium distributions shown by groundwater and overburden-gas sampling at Manyingee or Mt. Weld. At Mt. Weld, groundwaters contained 0.06 to 13.60 μ/l He and overburden gases 5.24–5.47 ppm He, with the higher gas concentrations over country rock, where waters had background helium contents. It is presumed that equilibration between overburden gas and the atmosphere is far more rapid than that between overburden gas and groundwater, so that any helium released from the water is quickly dispersed.Overburden-gas helium concentrations were found to vary according to overburden type, being 5.24–5.32 ppm where sandy and porous and 5.30–5.50 ppm where clay-rich and less permeable. These background variations, which are greater than the total background-anomaly contrasts reported in the literature for shallow soil gases, have not been accounted for in most trial surveys, nor has the possibility of similar variations being due to analytical error. From the data obtained, there is little evidence that helium can be considered an effective pathfinder for blind or concealed deposits using soil gas or overburden gas as sample media. Previous work on the use of soil and soil-gas helium determinations in uranium exploration is reviewed in the light of these findings and the concept and techniques assessed.