The Fosterville Goldfield is located within the Bendigo Structural Zone in the Lachlan Fold Belt. The deposit is hosted by an interbedded turbidite sequence of sandstones, siltstones and shales. This sequence has been metamorphosed to sub-greenschist facies and folded into a set of upright, open to closed folds.
Mineralization at Fosterville is controlled by late brittle faulting. These late brittle faults are generally steeply west dipping reverse faults with a series of moderately west dipping reverse splay faults formed in the footwall of the main fault. There are also moderately east dipping faults which have become more significant footwall to the anticlinal offsets along the west dipping faults. Primary gold mineralization occurs as disseminated arsenopyrite and pyrite forming as a selvage to veins in a quartz–carbonate veinlet stockwork. The mineralization is structurally controlled with high-grade zones localized by the geometric relationship between bedding and faulting. Mineralized shoots are typically 4m to 15m thick, 50m to 150m up/down dip and 300m to 1,500m+ down plunge.
Antimony mineralization, in the form of stibnite, occurs with quartz and varies from replacement and infill of earlier quartz-carbonate stockwork veins, to massive stibnite-only veins of up to 0.5m in width. The stibnite-quartz event occurs in favorable structural locations, such as the Phoenix, Eagle and Lower Phoenix structures. There are also occurrences of primary visible gold (≤3mm in size) that have a spatial association with stibnite in fault related quartz veins. The occurrence of visible gold is becoming increasingly significant and is being observed more frequently with depth and down plunge within the Lower Phoenix and Harrier Mineralized Zones.