Structural and fluid-flow evolution at the Fosterville gold deposit in central Victoria was influenced by three distinct stress regimes that each played a role in localising the sulfide-hosted gold deposits. Major gold mineralisation at Fosterville is younger than at the majority of auriferous quartz-reef-hosted deposits in the Bendigo Zone, which formed at around 440 Ma, during regional east-west shortening of the host Ordovician turbidites. In contrast, wall-rock-hosted disseminated, fine-grained auriferous pyrite-arsenopyrite mineralisation at Fosterville post-dated both the development of chevron-shaped upright folds and fold-related faults and the emplacement of Early to Middle Devonian felsic dykes. Here we show that the primary structural control on the distribution of the auriferous sulfides was related to local reactivation of the west-dipping Fosterville Fault and associated low-displacement splay faults that truncate a south-plunging syncline. Analysis of slickenline lineations on slip surfaces indicates most faults initiated as dip-slip, reverse structures during regional east-west compression, and were subsequently reactivated with sinistral-reverse displacement during later northwest-southeast compression, coincident with gold mineralisation. Kinematic analysis of conjugate sets of quartz-carbonate veins that are temporally and spatially associated with the auriferous sulfides at Fosterville also indicates mineralisation occurred during northwest-southeast shortening. A third stress regime, characterised by north-south shortening, is inferred to have been active during the late stages of mineralisation at Fosterville. Late stibnite mineralisation associated with bedding-discordant faults at Fosterville is similar to that associated with many Devonian-age gold deposits in the Melbourne Zone to the east. The absence of quartz reefs, the scarcity of visible free gold in primary ore and the wall-rock-hosted disseminated character of the sulfide ores at Fosterville are atypical of gold deposits in the Bendigo Zone. However, due to the important role of fold and fault relationships in localising mineralisation the geometry and structural setting of the Fosterville deposit is similar to many other goldfields of the Bendigo Zone.
- Lachlan Fold Belt