Summary:
The Fosterville goldfield is located within the eastern Bendigo Zone, which is bound by the Avoca Fault to the west and the Heathcote Fault Zone to the east. The Bendigo Zone contains Ordovician turbidite sequences of sub-greenschist to greenschist metamorphic grade.
The Fosterville goldfield is hosted by Lower Ordovician Lancefieldian (approximately 486 to 488Ma) turbidites within the Ordovician Castlemaine Group rocks. This sequence has been weakly metamorphosed to sub-greenschist facies and folded into a set of upright, northnorthwest trending and shallowly south plunging open to closed folds. The folding resulted in the formation of a series of bedding parallel laminated quartz veins and bedding parallel thrust faults.
Gold and associated sulphide mineralization at the Fosterville mine is controlled by late brittle faulting and fracturing. These 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 faults. There are also less abundant, moderately southeast and southwest-dipping faults which govern high grade visible gold mineralization along the Eagle and Swan zones. Two main styles of gold mineralization occur at the Fosterville mine; a sediment-hosted sub-micron refractory style where gold is locked in disseminated arsenopyrite and pyrite crystals which form selvages to quartz-carbonate vein stockworks throughout the nine kilometre long fault system, and a gold-in-vein mineralization style where visible gold is hosted in quartz-carbonate veins that show laminated and stylolitic vein textures as well as brecciation. Gold mineralization is structurally controlled with high-grade zones localized by the geometric relationship between bedding-parallel and oblique faults. Mineralized shoots are typically 4 to 15 metres thick and show down-dip and down-plunge dimensions of 50 to 150 metres and 300 to over 2,000 metres, respectively.
Antimony mineralization, mainly in the form of stibnite, occurs within quartz-carbonate stockwork veins, to massive stibnite-only veins up to 0.5 metres in width. The stibnite-quartz mineralization occurs in favourable structural locations, such as the Phoenix, Eagle and Swan vein and fault structures and therefore shows a spatial association with visible gold. The occurrence of visible gold has become increasingly significant at the Fosterville mine and is observed more frequently at greater depth within the Lower Phoenix System. Visible gold particles are predominantly specks (less than or equal to 3 mm), however they can be greater than 5 mm. The width of quartz-carbonate veining that contain visible gold is variable, with widths ranging from a few millimetres to several metres (true thickness). The veins usually have incomplete infill with druse quartz within those voids. Visible gold can be found as specks in narrow linear trends as well as isolated specks without a clear trend.
The Swan Fault exists as an oblique structure cross-cutting the eastern limb of the anticline and is bounded by the Eagle Fault down-dip and the Kestrel Syncline at its upper margin. Swan is the highest grade mineralized zone defined at Fosterville to date and contributes 588,915 ounces of gold at an average grade of 31.56 g/t gold (580,374 tonnes) to the updated December 31, 2022 mineral reserve estimate making up 35% of the total Fosterville mineral reserves. Extremely high grades in Swan are coincident with the intersection of the Eagle and Swan Splay Faults.
Continued drill definition of Lower Phoenix structures over 2019-2022, in combination with ore development and production exposure and reconciliation performance has reaffirmed the significance of footwall structures to the Lower Phoenix (Benu) Fault. Furthermore, mineralization on these structures is open along plunge, providing encouraging future mineral resource and mineral reserve growth potential for the Fosterville operation.
The Harrier Base structure exhibits reverse thrust movement of approximately 60 metres. Visible gold is hosted within a laminated quartz carbonate vein assemblage, which may contain minor amounts of stibnite. In the strongest mineralized zones, a broad halo of sulphide mineralization surrounds quartz structures bearing visible gold. The high-grade visible gold mineralization was first recognized at approximately the 4480mRL, a comparable elevation to where visible gold occurrences in the Lower Phoenix System became more prominent.