The late orogenic timing of mineralisation in some slate belt gold deposits, Victoria, Australia

Abstract

Gold mineralisation in classic Australian slate belt gold deposits at Ballarat, Bendigo, St. Arnaud and Inglewood occurred very late in the orogenic history of these rocks rather than during formation of the main “slaty cleavage”. This has been revealed through the examination of microstructural relationships in gold-bearing quartz veins and their host rocks from these deposits, which has established a D1 to D4 deformation-stage history and consistent timing for gold mineralisation over a wide area. The gold was deposited synorogenically but during the fourth deformation stage (D4) of the orogeny, a relatively weak event occurring two deformations after the main “slaty cleavage” producing event, D2. Previously, D2 had been regarded as both the source and control of gold mineralisation as most of the quartz veins that occur in these deposits formed before or during this deformation event. However, most gold is hosted in breccia veins that formed during D4. The wallrock clasts within these breccia veins contain a young rotated foliation and the breccia veins are spatially associated with a paragenetically consistent alteration of the host rocks in the deposits. This alteration both crosscuts and preferentially mineralises wallrock S4 allowing the timing of the breccia veins, alteration and gold deposition to be defined as syn-D4 in age.