Geophysical Characteristics of the Tertiary Palaeochannels in the Yilgarn Block, Western Australia

The precise geometric definition of the Tertiary palaeochannels in the Yilgarn Block of Western Australia is important in the exploration for uranium and for secondary and placer gold which occur in the channel sediments. The positions of the palaeochannels normally show considerable displacement from the positions of the present-day drainages. Physical property contrasts which exist between the channel sediments and the underlying Archaean bedrock can be differentiated by geophysical methods to locate the best parts of the channels for follow-up drilling. The low densities of the channel-fill sediments in many areas give rise to 1 to 2 mGal gravity lows and the gravity technique can be used to define the broad shape and depth of the channel. The deepest parts of the channel, which correspond to the zones of highest salinity, can then be delineated using time-domain electromagnetics. Downhole gamma logs are useful in identifying and quantifying prospective uranium zones.