Sub‐basaltic deep‐lead systems and gold exploration at Ballarat, Australia

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Abstract
The Cenozoic sub‐basaltic river systems (deep leads) at Ballarat were one of the richest goldflelds in Australia during the second half of the nineteenth century. The precise courses of the deep‐lead systems have never been completely clarified, although drilling across various suggested courses of the deep leads has progressively eliminated some earlier suggestions for these courses. Two groups of interpretations remain. The currently favoured interpretation advocates a northerly outlet of the Ballarat deep leads and tens of kilometres of post‐basaltic migration of the drainage divide as a result of tectonic warping, drainage disruption by basaltic eruptions and/or river capture. We use detailed data from borehole and mine records to argue that the alternative interpretation, incorporating broad coincidence between the pre‐basaltic and modern divides, is certainly possible and probably more likely. In this interpretation, the Golden Point Lead, the principal and richest deep lead in the Ballarat area, flowed to the south. The best prospects for exploration would therefore be based on persistence of drainage lines rather than drainage rearrangement. The Ballarat goldfield provides an excellent illustration of the ways in which mineral exploration is theory‐dependent, and driven by paradigms.