Evolution of deep‐lead palaeodrainages and gold exploration at Ballarat,Australia

Ballarat in western Victoria hosts substantial hard‐rock and palaeoplacer gold deposits. The most famous placers are the deep leads—channel deposits of a middle Cenozoic drainage system that were buried by voluminous basalt flows over the past few million years. The basalt has also shielded large areas of the highly prospective bedrock from exploration for more of the hard‐rock gold deposits. Although difficult to explore for, such deposits could express themselves as geochemical plumes in the major aquifer system hosted by the deep leads. Groundwater sampling may provide a vector to such deposits, but around Ballarat debate has long surrounded the distribution and flow directions of the deep leads, which are critical for this exploration methodology. The present landscape around Ballarat began to develop in the Early Cenozoic when a pre‐existing Mesozoic landscape was severely dissected during Australia‐Antarctica breakup. Several cycles of erosion left several generations of fluvial placer deposits scattered across the present landscape. New data from regional mapping, boreholes and compilation of historical records elucidates the positions and flow directions of the deep leads. The distribution and flow directions of the deep leads beneath the basalt are different to, and cannot be inferred from, the present drainage upon the basalt. The deep‐lead drainage divide runs beneath the city of Ballarat with divergence of up to 30 km between the deep lead and the present drainage divides. The divide was shifted northward to its present position by the process of drainage diversion because the basalt eruptions built new topography to greater heights than along the pre‐existing deep‐lead divide.