Sulfide gold mineralization at Fosterville is relatively homogenous with only one deposit type present. There are minor variations in the host rock type and structural setting. Fosterville-type deposits form a sub-group of orogenic gold deposits that are typified by gold occurring in fine grained arsenopyrite and/or pyrite disseminated in country rocks as a selvage to faults or veins. Fosterville-type deposits and classic vein-hosted deposits are effectively end members with many orogenic gold deposits displaying features of both.

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/downdip and 300m to 1,500m+ downplunge. These sulfide bodies are the primary targets for exploration activities, especially where there is potential for grades in excess of 3 g/t Au (i.e. above underground resource cut-off gold grades).

Gold is presently mined in two forms at FGM: Sulfide-Hosted Gold and Visible Gold.

Sulfide-Hosted Gold:

Sulfide Gold Mineralization at FGM occurs mainly as gold atoms trapped within the crystal lattice of disseminated arsenopyrite and pyrite (sulfides). These sulfide minerals precipitate in the wall rock as selvage alteration proximal to veins that penetrate the host rock. Associated alteration mineralogy within veins is predominantly euhedral to amorphous quartz-carbonate, with minor amorphous albite-chlorite-epidote.

Arsenopyrite crystals occur as 0.05-6mm long acicular needles in random orientations. The disseminated pyrite associated with gold mineralization occurs as crystalline pyritohedrons 0.1mm to 2mm in size. Electron microprobe analyses and metallurgical test work indicates that the arsenopyrite contains 100 g/t Au to 1,000 g/t Au and the auriferous pyrite 10 g/t Au to 100 g/t Au (Roberts et al., 2003). Approximately 80% of sulfide-hosted gold occurs in arsenopyrite, with the remaining 20% hosted by pyrite.

The quartz–carbonate veining forms in several styles that range from isolated veins through to stockwork veining. The quartz–carbonate veining is barren of sulfide gold. Broad zones of sulfide selvedge altered zones are located where stockwork veining occurs. This can allow a pervasive body of sulfide mineralization in the wall rock around that stockwork veining to form, with widths up to several meters.

Visible Gold:

Visible gold is observed within quartz carbonate veins, with a noticeable increase in recent years as underground mining and diamond drilling has advanced deeper. Visible gold particles are predominantly specks (up to 3mm), however, they can be up to 5mm in size, and are observed in drill core, underground development face/wall mapping, and stope sampling. The width of quartz-carbonate veining that contain visible gold is variable, with widths ranging from a few millimeters to several meters (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. Alteration mineralogy associated with veins that host visible gold includes quartz - carbonate (ankerite), with minor occurrences of fibrous boulangerite (Pb5Sb4S11) as inclusions in euhedral quartz or as fibrous growths within void spaces. Selvedge sulfide alteration can be present, proximal to veins hosting visible gold.

The visible gold has a spatial association with stibnite (Sb2S3). However, the stibnite mineralization can occur without visible gold (Henderson, 2014). The rationale for the one way correlation is likely due to the stibnite mineralization occurring in different events, but utilizing the same structurally favorable locations. Stibnite mineralization has also been observed in all areas of the underground workings at FGM and has been observed in some open cut pits within MIN5404.

Since the completion of the Harrier Open Cut Mine in early December 2007, the sole source of ore had been the underground operations until Q2 2011 when ore feed became available from a series of open pit cut backs on the Harrier Pit, John’s Pit and O'Dwyer's South Pit. Since the completion of O'Dwyer's South cut back in Q4 2012, the sole source of ore has been from the underground operations. The current Life of Mine (LOM) plan contains ore sourced from underground operations only.

The underground mine commenced declining in March 2006 with production first recorded in September 2006. Development and stoping have been conducted in the Phoenix, Falcon, Ellesmere, Kink, Vulture, Raven, Robin and Harrier ore bodies since that time. As at January 1, 2017 works are planned to continue in the Phoenix, Central, Harrier and Robin ore bodies. All areas are planned to be extracted using open stoping techniques with the application of Cemented Rock Fill (CRF) 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. A standard level interval of 20 vertical meters can be applied across all mining areas. However, this can be varied as is required to maximize the extraction of the economic material. The Phoenix to 4240rl, Harrier below 4500rl, Central and Robin ore bodies are accessed from a footwall decline position while the Phoenix below 4240mRL and Harrier ore body above 4500mRL are accessed from the hangingwall.

Underground mining is conducted using a conventional fleet including jumbos, production drills, loaders, trucks and ancillary equipment. Current mining is undertaken as owner miner.

The process plant incorporates the following unit operations:
- Single stage crushing with a primary jaw crusher;
- Open stockpile with reclaim tunnel;
- Semi-autogenous grinding (SAG) mill;
- Flotation circuit to produce a gold bearing sulfide mineral concentrate and a barren residue;
- A gravity circuit recovering coarser gold from the flotation concentrate. Gravity circuit concentrate is direct smelted;
- A Bio-oxidation circuit consisting of BIOX® reactors to oxidize the flotation concentrate, releasing gold from the sulfide mineral matrix;
- A three-stage CCD circuit to separate the gold bearing oxidized solid residue from the solubilized acid oxidation products;
- A liquor neutralization circuit to neutralize acid and precipitate arsenic as stable basic ferric arsenate and sulphate as calcium sulphate (gypsum) using both ground limestone and lime slurries;
- A limestone grinding facility comprising a single wet ball mill operated in closed circuit with hydrocyclones to produce ground limestone slurry for neutralization of sulfuric and arsenic acids produced from oxidation of gold bearing sulfide minerals;
- Carbon-in-leach (CIL) circuit, with a pH adjustment tank at the head of the circuit, to leach gold from oxidized material and load the cyanide soluble gold onto activated carbon;
- Heated Leach (HL) circuit to combat preg robbing capabilities of the non-carbonaceous carbon always present in the Mill feed. Specialized in-house technology unique to Fosterville; and
- Pressure Zadra elution circuit to remove gold from carbon, followed by recovery by electrowinning and smelting to doré.

The crushing circuit has the capacity to operate 24 hours per day, 7 days/week, at the design availability of 80%.

Hydrocyclone overflow from the SAG mill gravitates to the flotation circuit where the gold containing sulfide minerals are concentrated into a flotation concentrate containing about 8% to 10% of the feed mass with a barren flotation residue, which is rejected from the process.

Flotation reagents such as the following are added to the hydrocyclone overflow launder and flash flotation feed:
• Copper sulphate – as Activator;
• Potassium amyl xanthate (PAX) – as Collector; and
• Frother.

Flotation residue gravitates to a tailings hopper where it is combined with the products from neutralization of the BIOX® liquor and the combined product is pumped to the flotation residue storage facility.

Flotation concentrate is reground to 80% passing 20µm and is thickened in a high-rate thickener prior to feeding the BIOX® circuit.

With recent changes in the ore body showing increased occurrences of visible gold, a gravity recoverable gold circuit was constructed in Q1 2016 and commissioned in April 2016.

The gravity recoverable gold circuit is installed in the flotation concentrate regrind circuit and continuously processes 100% of the recirculating load. A Knelson concentrator is used as the primary concentrating device, with Knelson concentrate passing to a surge tank. On a day shift basis only, gravity concentrate is removed from the day surge tank and processed over a secondary concentrating Gemini GT1000 table. GT1000 concentrate is then tertiary processed over a GT250 Gemini table. All table tails are passed directly back to the regrind mill recirculating load where they pass back through the Knelson concentrator.

Final shaking table concentrate is calcined in an oven with oven exhaust being wet scrubbed. Calcine concentrate is direct smelted to doré bars.

Due to the different design availabilities between the milling/flotation circuits and BIOX® circuit, and the need for steady operation of the BIOX® circuit, a surge tank with a live capacity of about 48 hours acts as a buffer between the circuits.

The BIOX® bacteria are sensitive to chloride levels in the water, and management of BIOX® feed dilution water quality to <1,000ppm Cl is critical for the health of the BIOX® circuit. Likewise, cyanide and thiocyanate species are also toxic materials to the bacteria, hence the Flotation and Neutralization waters, plus CIL decant liquors are managed separately at the Fosterville operations to eliminate any processing risks.

Nutrient solution is dosed to the feed splitter box to maintain the correct levels of nitrogen (N), potassium (K) and phosphorous (P) levels in the BIOX® reactors.

The BIOX® culture is kept active in the reactors by controlling the slurry conditions within specific ranges. The oxidation reactions are exothermic and it is necessary to constantly cool the slurry. The reactors are equipped with cooling coil baffles through which cooling water is circulated to control the slurry temperature at about 43ºC in each reactor.

Oxygen requirements for sulfide oxidation are significant and medium pressure air is injected into each of the reactors.

The slurry pH in each of the reactors is controlled between 1.0 and 1.6 by addition of ground limestone. Hence the corrosive nature of the BIOX® slurry and the potential risk for elevated chloride levels resulted in selection of SAF 2205 stainless steel for equipment in the BIOX®, CCD, and neutralization circuits.

The oxidized product discharged from the final secondary BIOX® reactor gravitates to a product hopper from where it is pumped to the first of three CCD thickeners.

During bio-oxidation iron, sulfur and arsenic is solubilized and is washed from the solid oxidized gold containing residue in the series of three CCD thickeners. A three-stage CCD circuit with a wash ratio of 4.0 is used to ensure cyanicides, soluble arsenic and acid is reduced to levels acceptable in the oxidized concentrate prior to the CIL process. Process water is used as wash water in the CCD circuit and is added to the feed tank ahead of the third (last) CCD thickener. The underflow from the last CCD thickener (washed product) is pumped to an agitated pH adjustment tank ahead of the CIL circuit.

The acidic solution overflowing the first CCD thickener is pumped to the first of six agitated neutralization tanks in series and the solution flows from tank to tank via launders.
By-pass launders allow tanks to be taken off line for cleaning and maintenance. In the neutralization circuit the majority of the sulfuric acid is neutralized and precipitated as calcium sulphate (gypsum) and the soluble arsenic and iron precipitated as stable basic ferric arsenate.

The neutralized effluent gravitates to a flotation residue hopper and is pumped with flotation residue to the residue storage facility.

Six adsorption tanks are identical in size at 190m3 with a total circuit residence time of about 48 hours at a 30% pulp density. Test-work indicates that the leaching of the oxidized residue plateaus at 36 to 48 hours. Underflow from the last CCD thickener is pumped to the pH adjustment tank and lime slurry is used to neutralize residual acid and raise the pH of the pulp to 11 prior to cyanide addition.

Carbon concentrations (20g/L–30g/L) are maintained in all tanks to ensure high gold adsorption efficiency and achieve a low soluble tail. The last CIL tank can be used as tails retreat feed tank.

CIL discharge is fed to heated leach circuit, which was commissioned in April 2009. The process utilizes heat from steam injection and caustic to facilitate gold release from native carbon.

The heated leach circuit consists of 6 x 75m3 tanks with a residence time of 8 to 12 hours. The first three tanks are heated. The last three tanks are cooled to avoid loss of gold in solution. The heated leach process is effective in destroying WAD cyanide to < 50ppm and has replaced the former detoxification circuit.

The elution and gold room areas operate up to seven days per week, with the loaded carbon recovery on nightshift and the majority of the elution occurring during dayshift. The 3.5t pressure Zadra elution circuit consists of separate rubber lined acid wash and stainless stee