Factors controlling the location of gold mineralisation around basalt domes in the stawell corridor: insights from coupled 3D deformation – fluid-flow numerical models

Abstract

We present coupled 3D deformation – fluid-flow models which place constraints on the importance of basalt dome shape and interpreted synmineralising shortening direction in localising gold mineralisation around basalt domes in the Stawell corridor, western Victoria. Gold mineralisation in the Magdala orebody at the Stawell mine occurs predominantly within a thin metasomatised unit named the Stawell Facies which blankets the basalt domes and also occurs close to parasitic fold-like basalt lobes on the basalt domes. In dome-scale models that do not contain basalt lobes, areas with the maximum fluid-flow rates occur on the tops of the flanks of the domes where there is a dramatic change in dip of the basalt, and a change from contraction to dilation which creates a significant pore-pressure gradient. In models that contain basalt lobes, the location of high fluid-flow rates is strongly controlled by the presence of these lobes. High fluid-pressure gradients are created between the contracting Stawell Facies in the area between the lobe and the main domes and those areas dilating above. Areas of significant dilation occur on the shallow-dipping portion at the top of the dome and cause fluid to flow towards them. Areas that have significant dilation are also areas of tensile failure in some cases and are coincident with areas of known quartz vein-associated mineralisation. In the Magdala Dome models, only the east-northeast – west-southwest- and east – west-shortened models record high fluid-flow rates in areas of known mineralisation, which is consistent with the interpreted synmineralisation-shortening directions. Therefore in this situation, fluid-flow rates during east-northeast – west-southwest- and east – west-shortening can be used to indicate the potential location of gold mineralisation. In numerical models of the Kewell Dome (a prospect to the north), the position of areas of high fluid-flow rate when shortened in the east-northeast – west-southwest and east – west direction, combined with information from limited drilling, indicated the potential for gold mineralisation at the southwest end of the dome. Diamond drillholes in this area yielded significant gold values.

Key words:

• basalt domes
• deformation
• fluid flow
• gold mineralisation
• numerical modelling
• Stawell
• Victoria

Acknowledgements

We acknowledge the continued support of the pmd * CRC throughout this project, and thank Paul Roberts, Alison Ord and Jamie Robinson for their contributions during the initiation of this project. John McLellan and David Giles provided constructive reviews that helped to improve the manuscript.