Exploration for gold using calcrete - Lessons from the Yilgarn Craton, Western Australia

This paper describes and reviews the use of calcrete (caliche) for Au exploration in the southern Yilgarn Craton of Western Australia. There are very strong correlations between Au and the alkaline earth metals, particularly in residual soils. However, where transported overburden exceeds about 10m, the use of calcrete to indicate underlying Au mineralization is severely limited. Even where transported overburden is less than 10 m, other factors, such as thick clay, may restrict geochemical signals from reaching the calcrete. In some cases, gold in calcrete occurring over mineralization may have been sourced from upslope. Several lessons are to be learnt from calcrete sampling in the Yilgarn WA. Some of the more important being: (1) Calcrete is an excellent sample medium for Au exploration because its physico-chemical properties appear to mimic those of the Au species found in soil, so that Au and Ca commonly accumulate in the same part of the soil profile. (2) Calcrete consists of a variety of materials and morphologies derived from the parent material on and within which it develops, although dominated by calcite and dolomite. (3) Gold found in calcrete may have been transported laterally up to several hundred metres - that is, its presence does not necessarily indicate Au mineralization directly below. (4) Regolith classification (including mapping and profiles) is essential to correctly interpret Au anomalies developed in calcrete. (5) If the thickness of transported overburden exceeds 10m and/or if lacustrine sediments are present, experience to date suggests that calcrete will probably be ineffective as a sample medium.