Gold mineralization under cover in southeast Australia: A review of an exploration initiative for Stawell-type deposits

Exploration for orogenic Stawell-type deposits in the northern extension of the Stawell zone under the Tertiary Murray Basin sediments, in southeastern Australia, has produced the discovery of at least two new deposits at Wildwood and Kewell. Previous research on the Stawell gold deposit has highlighted the importance of litho-facies, structural evolution and hydrothermal alteration in the localization of fluid flow at the time of mineralization. Application of geophysical tools including magnetic and gravity surveys at both regional and more detailed scales delineated a number of significant coincident magnetic and gravity anomalies, which upon further investigation through aircore drilling proved to be basalt. Subsequent drilling on the margins of the Wildwood and Kewell basaltic domes delineated areas of prospective litho-facies, hydrothermal alteration, quartz veins and mineralization akin to that found in the Stawell deposit. Analysis of multi-element geochemistry revealed significant gains in Fe in the transition from unaltered to altered unmineralized (< 1 ppb Au) host rock, and gains in As, Sb and Ag with subsequent transition to mineralized (> 1 ppm Au) altered host rock. Magnetic remanence properties of the various lithologies within the Stawell Zone showed that high fluid flow can result in the precipitation of sulfides with fixed stable remanent magnetization the orientation of which may not be parallel to the present day magnetic field, whereas zones of low fluid flow have soft magnetization that can be easily modified. The implication of variation in remanence orientation for exploration is that regional aeromagnetic anomalies, which have opposite polarities, may coincide with zones of high fluid flow and are potential target areas for mineralization.