Structural Setting of the Gold Mineralization at Stawell, Victoria, Australia
R. B. Watchorn, C. J. L. Wilson, 1989. "Structural Setting of the Gold Mineralization at Stawell, Victoria, Australia", The Geology of Gold Deposits: The Perspective in 1988, Reid R. Keays, W. R. H. Ramsay, David I. Groves
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The Stawell gold field, on the western edge of the Lachlan fold belt, is dominated by regional northwest-trending tectonostratigraphic units upon which are imposed shear zones that display repeated reactivation within weak and highly strained zones. Gold mineralization was preceded by regional deformations D, to D3 and low-grade regional metamorphism. Two distinct types of shear zone-hosted mineralization associated with deformations D4 to D6 exist in the area: vein deposits superimposed on the regional metamorphic sequence in the Magdala mine, and hydrothermal alteration along a fracture system within the contact metamorphic aureole of the Stawell granite in the Wonga open pit.
In the Magdala mine, gold mineralization can be related to late-stage D4 and D5 movements in a west-dipping fault system with initial reverse and subsequent oblique strike-slip movements. Three types of mineralization associated with arsenopyrite and pyrite are found in the Magdala mine: (1) laminated and massive quartz veins occurring as flat veins in dilational openings within shear zones or in deformed west-dipping quartz sheets within shear zones, (2) gash and stockwork veins superimposed on the D4 shear zones, and (3) sulfide lodes where shear zones are superimposed on syngenetic pyrrhotite concentrations. All gold mineralization occurs within or above a footwall volcanic sequence. Mineralization and metasomatic alteration are retrogressive with respect to, and postdate, the peak of regional metamorphism and are controlled by fluid ingress during shearing.
In the Wonga open pit, an east-dipping reverse shear system is contemporaneous with initial gold deposition and is synchronous with the very late stages of contact metamorphism produced by the emplacement of the Stawell granite. Movement in the shear system changes to a major sinistral strike-slip system which becomes the locus for sericite-carbonate metasomatic alteration. The Wonga quartz-carbonate vein deposits are spatially and temporally associated with fluids accompanying the felsic plutonism.