Gold was first discovered in the Fosterville area in 1894 with mining activity continuing until 1903. Aside from a minor tailings retreatment in the 1930’s, the field lay dormant until 1988. Between 1988 and 2001, a total of 240koz of gold were poured from heap leaching ore derived from shallow oxide open pits. The present-day Fosterville Mine commenced underground mining operations in 2005 and, during its initial years, produced gold from near-surface, low-grade mineralization.
A major inflection point was achieved in 2015 with the intersection of high-grade visible-gold bearing mineralization at depth, leading to the discovery of the Eagle Zone and, in 2016, the ultra-high-grade Swan Zone. The discovery of these zones, particularly Swan, resulted in a significant improvement in the mine's overall Mineral Reserve grade, production profile and unit-cost performance. In 2016, exploration work documented similar visible-gold bearing zones at depth in the Harrier zone. Additional exploration progress early in 2017 resulted in new Mineral Reserve and Mineral Resource estimate for the operation being released in June 2017, which more than doubled underground reserves.
With continued exploration success over the remainder of 2017, an additional Mineral Reserve and Mineral Resource estimate was released in December 2017, with Mineral Reserves increasing another 65% from the June 2017 estimate. Continued drilling in the Lower Phoenix system during 2018 resulted in another significant increase in the Fosterville Mine Mineral Reserves to 2,720,000 ounces at an average grade of 31.0 g/t.
Mineral Reserves as at December 31, 2020 totalled 1,970,000 ounces, including 1,790,000 ounces at an average grade of 15.4 g/t in the Lower Phoenix and Harrier systems (including 1,250,000 ounces at an average grade of 30.6 g/t in the Swan Zone) and 180,000
ounces at an average grade of 5.3 g/t at Robbin’s Hill.
The mine is located in an area with well-developed infrastructure and is accessible by paved roads. Fosterville’s ore is processed at the Fosterville Mill, a 2,275 tonne per day processing facility.
Geology and Mineralization
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, and have average grades of 5 10 g/t Au, with individual assays up to 60 g/t Au.
Primary gold also occurs as visible gold at Fosterville, where it variably overprints sulphide mineralization, and is found as disseminated fine specks (>1 mm) of gold within host quartz veins. The visible gold is spatially associated with antimony mineralization, in the form of stibnite that 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.
The occurrence of visible gold is becoming increasingly significant at depth and is observed more frequently below an approximate 800m with depth, down-plunge within the Lower Phoenix and Harrier Gold Systems.
Examples of the visible high-grade gold zones include the Eagle and Swan Zones in the Lower Phoenix area. The Eagle Zone is 0.5 6m in width, 50 80m in dip length and has a down-plunge extent of 700m. The Swan Zone, currently the highest-grade mineralized zone at Fosterville, is 2-5m in width, dips west and is presently defined over a 275m strike and a 200m vertical extent. The Eagle and Swan Zones remain open down-plunge for potential Mineral Resource expansion.
Mining
Mining activities at Fosterville are currently focused on the Central, Phoenix and Harrier underground ore zones. All areas are extracted using open stoping techniques with the application of Cemented Rock Fill where applicable and practical. Selection of the specific mining method within the open stoping regime is based upon previous experience at the Fosterville mine and expectations of ore zone geometry and geotechnical conditions. In 2020, work on a twin exploration drive, capable of supporting production activities, commenced to connect the existing Fosterville Mine to Robbin’s Hill, which is expected to be established as a second mining operation to feed the Fosterville Mill over the next few years. The twin drive had advanced over 7,800 metres as at December 31, 2021 and remains on track for completion in mid-2022.
Underground mining is conducted using a conventional fleet including jumbos, production drills, loaders, trucks and ancillary equipment. Open Cut mining (when required) is conducted using a conventional fleet including excavators and trucks with the mining fleet and workforce being contract.
Processing
The processing path for the ore involves crushing and grinding followed by flotation, bacterial oxidation and CIL circuits. The modern sulphide treatment plant is one of the world’s leading BIOX systems and has achieved record recoveries. In addition, the Company installed a gravity gold circuit in 2016. Fosterville commenced production in April of 2005 and produced its one millionth ounce of gold in December 2015, representing an important milestone of safe and sustainable production at the mine. In 2020, the operation produced a record 640,467 ounces of gold at an average grade of 33.9 g/t Au and average recoveries of 98.9%.
Exploration
The Fosterville property includes approximately 1400 km2 of additional land package with numerous brownfield and greenfield exploration targets which are forming a key aspect of the Company's ongoing exploration efforts. Exploration programs to date have traced only 20km of potential gold bearing structures, highlighting the exceptional exploration potential of this premier gold district. Drilling continues to intersect high-grade gold results in multiple zones and reaffirm increasing grade profiles with depth.
Exploration results at Fosterville in FY 2021 continued to demonstrate the size and scale of the gold systems at Lower Phoenix, Cygnet, Robbin’s Hill and Harrier and confirmed the presence of quartz with visible gold within each of these systems. Exploration expenditures at Fosterville in the first nine months of 2021 totalled $63.2 million with full-year expenditures targeted at $85 – $95 million, the mine’s largest budget ever.