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The Magdala Lode System, Stawell, Southeastern Australia: Structural Style and Relationship to Gold Mineralization across the Western Lachlan Fold Belt

John McL. Miller and Christopher J. L. Wilson

School of Earth Sciences, The University of Melbourne, Victoria 3010, Australia

Corresponding author: email, jmm{at}unimelb.edu.au

Gold mineralization in the Magdala deposit at Stawell is hosted within deformed turbidites and the underlying mafic volcanic successions. This setting is atypical when compared to the slate-hosted gold deposits elsewhere in the Lachlan orogen. The deposit is located at a regional plunge reversal (culmination) and gold lodes have developed on the western flank of a large, doubly plunging basalt dome (the Magdala antiform) during northeast-southwest and east-west shortening. The irregular shape of the dome and the contacts between different stratigraphic packages are the key controls on the location of the gold lodes. Early deformation is defined by bedding-parallel mica fabrics (S₁) that are folded by a series of upright fold closures (F₂). These upright folds are syn- to postpeak regional metamorphism and are overprinted by another folding event associated with a shallow-dipping crenulation cleavage (S₃). Gold mineralization postdates all of the ductile deformation and occurred after the formation of faults associated with large laminated and massive quartz veins that are barren of gold. In the turbidites the mineralized structures are defined by laminated quartz veins (with arsenopyrite and pyrite). Within the volcanic units gold lodes are sulfide rich (mainly pyrite, pyrrhotite, and arsenopyrite) reflecting the primary sulfide-rich stratigraphy. Later gold lodes at Stawell developed as a result of the reactivation of the system during sinistral oblique-slip. Field and geochronological data suggest deformation initiated in the Stawell region and migrated to the east resulting in dramatically different structural evolution and/or complexity in the Stawell and Bendigo-Ballarat zones. Despite this, the absolute age of major gold mineralization is the same in both zones.

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