Resurgence in Au exploration during the past decade has been
characterised by a considerable emphasis and dependence on
geochemical procedures. This has been made possible, firstly, by
marked improvements in analytical sensitivity over those
previously available and secondly, of particular importance to
exploration in Australia, by a better understanding of the
distribution of Au in the regolith, which has led to the
optimisation of sample media. These developments have given
exploration companies the ability to seek and detect subtle and
widespread geochemical signatures of concealed Au
mineralisation and provided an enhanced capability to interpret
the data. As a consequence, several new discoveries have been
made, even in areas that have been intensively explored in the
past. In the deeply weathered Yilgarn Craton (WA), and in
equivalent areas elsewhere in Australia and overseas, Au has
accumulated in the upper lateritic gravels and duricrusts of the
regolith, probably during humid climatic periods in the Tertiary.
The laterites may not only be Au ores in themselves, but research
by CSIRO and CRC LEME has shown them to be very important
sample media, (eg Anand and Smith, 1992). In many areas,
however, the laterite has been eroded and hence is unavailable as
a sample medium and chemical modification under more and
conditions since the Tertiary has leached Au from the upper
saprolite. This has greatly increased the difficulties of
exploration, particularly where reliance was placed on sampling
between 2 and 15 m of the surface. Further research, however,
has demonstrated that, despite this leaching, recent remobilisation
of An has caused it to concentrate in a specific soil component,
pedogenic carbonate, usually in the top I - 2 m, thereby giving
surface expression even to mineralisation that is concealed by
over 20 in of leached overburden. Secondary carbonates, commonly referred to as calcrete, may
precipitate as calcite and/or dolomite in regoliths where the
average annual rainfall is less than about 600 mm. Pedogenic
carbonates are those that form in unsaturated (vadose) soil
horizons. They are widely distributed in the Gawler and Yilgarn
Cratons. In the Yilgarn Craton south of about 30S ('the Menzies
Line'), they are seemingly more abundant over more basic rocks
and towards the south east (Figure 1). In contrast, groundwater
calcretes form in saturated (phreatic) environments, typically in
the axes of major drainages north of the Menzies Line. These are
known to accumulate economically-significant grades of U but
not, apparently, An.