Interpretation of aerial gamma-ray surveys - adding the geochemical factors

Aerial gamma-ray surveying reflects the geochemical variations of potassium, uranium and thorium in the upper 30 cm of the Earth's surface. This thin layer is subject to the effects of weathering, which leads to loss of K in all rock types and, for felsic rocks, loss of U and Th as well. The extent of the loss depends on many factors, but is typically 20-30% for all three radioelements. Intermediate and basic rocks show little change in radioelement concentrations during initial weathering, but pedogenesis can result in soils with 2-3 times the U and Th content of the parent rock. However, wide ranges in radioelement compositions occur for a given rock type and its weathered products. Mineralising processes can also affect radioelement contents. For example, K is increased in altered rocks at the Copper Hill and Goonumbla porphyry Cu deposits in central NSW. Thorium concentration shows both depletion and enrichment during hydrothermal alteration, as illustrated by the Au prospects at Bimurra, Queensland. Uranium is even more erratically affected by alteration and is generally not a useful indicator of alteration. Regolith processes can affect these alteration signatures. Transported soils may disguise or change rock signatures often in unexpected ways - the Mt Leyshon Au deposit, Queensland, is seen in the aerial survey as a K-rich area because its signature is not contaminated by material weathered from late-Silurian dolerites.